Abstract
A system for monitoring a patient on a hospital bed includes load
cells and a controller for detecting patient movement on, exit from,
and impending exit from the hospital bed, including movement across
or outside a virtual boundary or region located on the bed. The controller
is configured to detect changes in the distribution of patient weight
among the load cells. The controller is also configured to accommodate
the installation or removal of medical equipment or other tare weights
on the bed, head incline adjustment, and bed frame deformation or
other changes in the system causing redistribution of weight among
the load cells, while minimizing false alarms. A caregiver may select
between various patient monitoring modes and remotely monitor patient
movement relative to a reference load cell distribution, impending
patient exit from the bed and patient exit from the bed.
Claims
1. A method of monitoring a patient in a hospital bed having at least
three load cells positioned about a periphery of the bed, each load
cell producing a signal corresponding to a distribution of patient
weight on that load cell, the method comprising the steps of: establishing
a plurality of sets of exit conditions about a periphery of the bed,
each of the plurality of sets of exit conditions defining threshold
values for each of the load cells, determining a current distribution
of patient weight on each of the load cells, and determining that
the patient is about to exit the bed if the current distribution of
weight on at least some of the load cells exceeds corresponding ones
of the threshold values of one of the sets of exit conditions.
2. The method of claim 1 further including the step of activating
an alarm local to the bed if the patient is about to exit the bed.
3. The method of claim 1 further including the step of activating
an alarm remote from the bed if the patient is about to exit the
bed.
4. The method of claim 1 wherein the step of establishing a plurality
of sets of exit conditions includes: placing an object having a
predefined calibration weight near one edge of the bed, measuring
a current distribution of the calibration weight on each of the
load cells, defining as one of the plurality of sets of exit conditions
a set of normalized threshold values corresponding to the current
distribution of the calibration weight on each of the load cells,
and moving the calibration weight about the periphery of the bed
and executing the measuring and defining steps at each discrete
placement of the calibration weight.
5. The method of claim 4 wherein the step of establishing a plurality
of sets of exit conditions further includes forming a data table
populated by the plurality of sets of exit conditions with each
of the plurality of sets of exit conditions defined by a corresponding
set of the normalized threshold values for each of the load cells.
6. The method of claim 4 wherein the step of determining that the
patient is about to exit the bed includes: determining from the
current distribution of patient weight on each of the load cells
a total patient weight on the bed, multiplying selected ones of
the normalized threshold values of one of the plurality of sets
of exit conditions by a ratio of the total patient weight and the
calibration weight to define computed threshold values, comparing
the current distribution of patient weight on the load cells that
correspond to the selected ones of the normalized threshold values
with corresponding ones of the computed threshold values, and deciding
that the patient is about to exit the bed if the current distribution
of weight on the load cells that correspond to the selected ones
of the normalized threshold values exceed the corresponding ones
of the computed threshold values.
7. The method of claim 6 wherein the step of determining that the
patient is about to exit the bed further includes: selecting a subset
of the plurality of sets of exit conditions; and executing the multiplying,
comparing and deciding steps for each set of exit conditions in
the subset of the plurality of sets of exit conditions.
8. The method of claim 6 wherein the step of determining a total
patient weight on the bed includes: determining a zero weight distribution
on each of the load cells when the bed is empty, determining a current
distribution of weight on each of the load cells when the patient
is supported by the bed, determining the current distribution of
patient weight on each of the load cells by subtracting the current
distribution of weight on each of the load cells when the patient
is supported by the bed from the zero weight distribution on each
of the load cells when the bed is empty, and computing the total
patient weight as a function of the current distribution of patient
weight on each of the load cells.
9. The method of claim 1 wherein the bed includes four load cells
each positioned near a different corner of the bed, and wherein
the plurality of sets of exit conditions includes a first plurality
of sets of exit conditions defined along a first side of the bed,
a second plurality of sets of exit conditions defined along a second
side of the bed opposite to the first side, a third plurality of
sets of exit conditions defined along a head end of the bed and
a fourth plurality of sets of exit conditions defined along a foot
end of the bed, and wherein the method further includes the steps
of: processing the current distribution of patient weight on some
the load cells to determine only one of the first and second plurality
of sets of exit conditions to monitor; and monitoring the one of
the first and second plurality of sets of exit conditions to determine
whether the patient is about to exit the bed.
10. The method of claim 9 wherein the processing step includes:
determining from the current distribution of patient weight on the
some of the load cells the one of the first and second sides of
the bed that patient exit is most likely to occur, and selecting
the one of the first and second plurality of sets of exit conditions
associated with the one of the first and second sides of the bed
that patient exit is more likely to occur.
11. The method of claim 9 wherein the step of determining that
the patient is about to exit the bed includes: comparing the current
distribution of patient weight on the load cells at the head and
foot ends of the one of the first and second sides that patient
exit is most likely to occur with corresponding ones of the threshold
values of each of the selected one of the first and second plurality
of sets of exit conditions, and deciding that the patient is about
to exit the bed if the current distribution of weight on the load
cells at the head and foot ends of the one of the first and second
sides that patient exit is most likely to occur exceed corresponding
head end and foot end threshold values of one of the selected one
of the first and second plurality of sets of exit conditions.
12. The method of claim 9 wherein the step of establishing a plurality
of sets of exit conditions includes: placing an object having a
predefined calibration weight near one edge of the bed, measuring
a current distribution of the calibration weight on each of the
load cells, defining as one of the plurality of sets of exit conditions
a set of normalized threshold values corresponding to the current
distribution of the calibration weight on each of the load cells,
and discretely moving the calibration weight about the periphery
of the bed and executing the measuring and defining steps at each
discrete placement of the calibration weight.
13. The method of claim 12 wherein the step of determining that
the patient is about to exit the bed includes: determining from
the current distribution of patient weight on each of the load cells
a total patient weight on the bed, for one of the selected one of
the first and second plurality of sets of exit conditions, multiplying
the normalized threshold values that correspond the head and foot
end load cells of the one of the first and second sides of the bed
that patient exit is most likely to occur by a ratio of the total
patient weight and the calibration weight to define computed head
and foot end threshold values, comparing the current distribution
of patient weight on the load cells at the head and foot ends of
the one of the first and second sides that patient exit is most
likely to occur with corresponding ones of the computed head and
foot end threshold values, deciding that the patient is about to
exit the bed if the current distribution of patient weight on the
load cells at the head and foot ends of the one of the first and
second sides that patient exit is most likely to occur exceed the
corresponding ones of the computed head and foot end threshold values,
and executing the multiplying, comparing and deciding steps for
remaining ones of the selected one of the first and second plurality
of sets of exit conditions.
14. The method of claim 9 wherein the step of determining that
the patient is about to exit the bed includes: comparing the current
distribution of patient weight on the load cells at the head and
foot ends of the one of the first and second sides of the bed that
patient exit is most likely to occur with corresponding head and
foot end threshold values of each of the selected one of the first
and second plurality of sets of exit conditions, of the selected
one of the first and second plurality of sets of exit conditions,
determining a first set of exit conditions where the current distribution
of patient weight on the load cell at the head end of the one of
the first and second sides of the bed that patient exit is most
likely to occur exceeds a corresponding head end threshold value,
of the selected one of the first and second plurality of sets of
exit conditions, determining a second set of exit conditions where
the current distribution of patient weight on the load cell at the
foot end of the one of the first and second sides of the bed that
patient exit is most likely to occur exceeds a corresponding foot
end threshold value, identifying a third set of exit conditions,
among the selected one of the first and second plurality of sets
of exit conditions, that is defined between the first and second
sets of exit conditions, and deciding that the patient is about
to exit the bed as a function of the current distribution of weight
on each of the load cells and of each of the third set of exit conditions.
15. The method of claim 14 wherein the deciding step includes:
computing a first difference between the distribution of patient
weight on the load cell at the head end of the one of the first
and second sides of the bed that patient exit is most likely to
occur and the distribution of patient weight on the load cell at
the head end of the other of the first and second sides of the bed,
computing a second difference between the head end threshold value
of the third set of exit conditions that corresponds to the load
cell at the head end of the one of the first and second sides of
the bed that patient exit is most likely to occur and the head end
threshold value of the third set of exit conditions that corresponds
to the load cell at the head end of the other of the first and second
sides of the bed, computing a third difference between the distribution
of patient weight on the load cell at the foot end of the one of
the first and second sides of the bed that patient exit is most
likely to occur and the distribution of patient weight on the load
cell at the foot end of the other of the first and second sides
of the bed, computing a fourth difference between the foot end threshold
value of the third set of exit conditions that corresponds to the
load cell at the foot end of the one of the first and second sides
of the bed that patient exit is most likely to occur and the foot
end threshold value of the third set of exit conditions that corresponds
to the load cell at the foot end of the other of the first and second
sides of the bed, and determining that the patient is about to exit
the bed if a sum of a difference between the first and second differences
and a difference between the third and fourth differences is greater
than zero.
16. The method of claim 15 further including: determining a fifth
difference between the head end threshold value of the third set
of exit conditions that corresponds to the load cell at the head
end of the one of the first and second sides of the bed that patient
exit is most likely to occur and the distribution of patient weight
on the load cell at the head end of the one of the first and second
sides of the bed that patient exit is most likely to occur, determining
a sixth difference between the distribution of patient weight on
the load cell at the foot end of the other of the first and second
sides of the bed that patient exit is most likely to occur and the
foot end threshold value of the third set of exit conditions that
corresponds to the load cell at the foot end of the other of the
first and second sides of the bed, computing an exit adjust value
as a ratio of the fifth and sixth differences, computing an exit
adjust clamp value as a function of the exit adjust value and one
or more of a total patient weight on the bed, a total weight on
the bed and a location of the third set of exit conditions relative
to the one of the first and second plurality of sets of exit conditions,
and multiplying the second and fourth differences by the exit adjust
clamp value prior to the step of determining that the patient is
about to exit the bed.
17. The method of claim 9 further includes the steps of: processing
the current distribution of patient weight on each of the load cells
to determine only one of the third and fourth plurality of sets
of exit conditions to monitor; and monitoring the one of the third
and fourth plurality of sets of exit conditions to determine whether
the patient is about to exit the bed.
18. The method of claim 17 wherein the processing step includes:
determining from the current distribution of patient weight on each
of the load cells the one of the head and foot ends of the bed that
patient exit is most likely to occur, and selecting the one of the
third and fourth plurality of sets of exit conditions associated
with the one of the head and foot ends that patient exit is more
likely to occur.
19. The method of claim 17 wherein the step of determining that
the patient is about to exit the bed includes: comparing the current
distribution of patient weight on the load cells at the one of the
head and foot ends of the bed that patient exit is most likely to
occur with corresponding ones of the threshold values of each of
the selected one of the third and fourth plurality of sets of exit
conditions, and deciding that the patient is about to exit the bed
if the current distribution of weight on the load cells at the one
of the head and foot ends of the bed that patient exit is most likely
to occur exceed corresponding threshold values of one of the selected
one of the third and fourth plurality of sets of exit conditions.
20. The method of claim 17 wherein the step of establishing a plurality
of sets of exit conditions includes: placing an object having a
predefined calibration weight near one edge of the bed, measuring
a current distribution of the calibration weight on each of the
load cells, defining as one of the plurality of sets of exit conditions
a set of normalized threshold values corresponding to the current
distribution of the calibration weight on each of the load cells,
and moving the calibration weight about the periphery of the bed
and executing the measuring and defining steps at each discrete
placement of the calibration weight.
21. The method of claim 20 wherein the step of determining that
the patient is about to exit the bed includes: determining from
the current distribution of patient weight on each of the load cells
a total patient weight on the bed, for one of the selected one of
the third and fourth plurality of sets of exit conditions, multiplying
the normalized threshold values that correspond the load cells on
either side of the one of the head and foot ends of the bed that
patient exit is most likely to occur by a ratio of the total patient
weight and the calibration weight to define computed first and second
side threshold values, comparing the current distribution of patient
weight on the load cells at the first and second sides of the one
of the head and foot ends of the bed that patient exit is most likely
to occur with corresponding ones of the computed first and second
side threshold values, deciding that the patient is about to exit
the bed if the current distribution of patient weight on the load
cells at the first and second sides of the one of the head and foot
ends of the bed that patient exit is most likely to occur exceed
the corresponding ones of the computed first and second side threshold
values, and executing the multiplying, comparing and deciding steps
for remaining ones of the selected one of the third and fourth plurality
of sets of exit conditions.
22. The method of claim 1 wherein the step of determining that
the patient is about to exit the bed if the current distribution
of weight on at least some of the load cells exceeds corresponding
ones of the threshold values of one of the sets of exit conditions
includes determining that the patient is about to exit the bed only
if the current distribution of weight on at least some of the load
cells exceeds corresponding ones of the threshold values of one
of the sets of exit conditions for a predefined time period.
23. The method of claim 1 wherein the threshold values of each
of the plurality of sets of exit conditions are threshold weight
values.
24. The method of claim 1 wherein the threshold values of each
of the plurality of sets of exit conditions are threshold weight
percentage values.
25. A system for monitoring a patient, comprising: a patient support
surface configured to support the patient, at least three load cells
each configured to produce a signal indicative of weight impressed
on that load cell via the patient support surface, and a controller
responsive to the signals produced by each of the at least three
load cells to determine a current distribution of patient weight
on each of the load cells, the controller determining that the patient
is about to exit the bed if the current distribution of weight on
at least some of the at least three load cells exceeds corresponding
threshold values comprising one of a plurality of sets of exit conditions.
26. The system of claim 25 further including a memory having stored
therein the plurality of sets of exit conditions defined about a
periphery of the patient support surface, each of the plurality
of sets of exit conditions defining threshold values for each of
the load cells.
27. The system of claim 25 wherein the patient support surface
is a hospital bed.
28. The system of claim 25 further including an alarm local to
the bed, the controller activating the alarm if the patient is about
to exit the bed.
29. The system of claim 25 further including an alarm remote from
the bed, the controller activating the alarm if the patient is about
to exit the bed.
30. A method of monitoring a patient in a hospital bed having at
least three load cells positioned about a periphery of the bed,
each load cell producing a signal corresponding to a distribution
of patient weight on that load cell, the method comprising the steps
of: establishing a first plurality of sets of excessive patient
movement conditions, each of the first plurality of sets of excessive
patient movement conditions defining first threshold values for
each of the at least three load cells, determining a total patient
weight on the bed, for each of the first plurality of sets of excessive
patient movement conditions, computing weight change values for
each of the at least three load cells according to a patient movement
model as a function of the total patient weight on the bed, and
determining that patient movement is excessive if at least some
of the weight change values exceed corresponding ones of the first
threshold values for at least one of the first plurality of sets
of excessive patient movement conditions.
31. The method of claim 30 wherein the step of computing weight
change values further includes computing weight change values for
each of the at least three load cells according to the patient movement
model further as a function of a patient movement sensitivity value.
32. The method of claim 30 further including the step of activating
an alarm remote from the bed if the patient movement is excessive.
33. The method of claim 30 wherein the first threshold values are
percentage weight values and the weight change values are percentage
weight change values.
34. The method of claim 30 wherein the bed includes four load cells
each positioned near a different corner of the bed, and wherein
the step of determining that patient movement is excessive includes
determining that patient movement is excessive if at least three
of the weight change values exceed corresponding ones of the first
threshold values for at least one of the first plurality of sets
of excessive patient movement conditions.
35. The method of claim 30 further including the steps of: establishing
a second plurality of sets of excessive patient movement conditions,
each of the second plurality of sets of excessive patient movement
conditions defining normalized threshold values for each of the
at least three load cells, determining a current distribution of
patient weight on each of the at least three load cells, for each
of the second plurality of sets of excessive patient movement conditions,
computing second threshold values for each of the at least three
load cells each as a function of a corresponding one of the normalized
threshold values and the total patient weight on the bed, and computing
weight differential values for each of the at least three load cells
as a function of the current distribution of patient weight on that
load cell and a reference weight for that load cell, and determining
that patient movement is excessive if at least some of the weight
differential values exceed corresponding ones of the second threshold
values for at least one of the second plurality of sets of excessive
patient movement conditions.
36. The method of claim 35 wherein the step of computing second
threshold values further includes computing the second threshold
values for each of the at least three load cells further as a function
of a patient movement sensitivity value.
37. The method of claim 35 wherein the second threshold values
are percentage weight values and the weight differential values
are percentage weight differential values.
38. The method of claim 35 wherein the bed includes four load cells
each positioned near a different corner of the bed, and wherein
the step of determining that patient movement is excessive includes
determining that patient movement is excessive if at least three
of the weight differential values exceed corresponding ones of the
second threshold values for at least one of the second plurality
of sets of excessive patient movement conditions.
39. The method of claim 35 wherein the bed includes four load cells
each positioned near a different corner of the bed, and wherein
the step of determining that patient movement is excessive includes
determining that patient movement is excessive if at least two of
the weight differential values exceed corresponding ones of the
second threshold values for at least one of the second plurality
of sets of excessive patient movement conditions.
40. The method of claim 35 further including the step of determining
that patient movement is excessive if at least one of the weight
differential values is less than a corresponding one of the second
threshold values while at least another of the weight differential
values exceeds a corresponding one of the second threshold values
for at least one of the second plurality of sets of excessive patient
movement conditions.
41. A system for monitoring a patient, comprising: a patient support
surface configured to support the patient, at least three load cells
each configured to produce a signal indicative of weight impressed
on that load cell via the patient support surface, and a controller
responsive to the signals produced by each of the at least three
load cells to determine a total patient weight on the patient support
surface, the controller computing weight change values for each
of the at least three load cells according to a patient movement
model as a function of the total patient weight on the bed for each
of a plurality of sets of excessive patient movement conditions,
the controller determining that patient movement is excessive if
at least some of the weight change values exceed corresponding ones
of the first threshold values for at least one of the plurality
of sets of excessive patient movement conditions.
42. The system of claim 41 further including a memory having stored
therein the plurality of sets of excessive patient movement conditions,
each of the plurality of sets of excessive patient movement conditions
defining threshold values for each of the load cells.
43. The system of claim 41 wherein the patient support surface
is a hospital bed.
44. The system of claim 41 further including an alarm remote from
the bed, the controller activating the alarm if the patient is about
to exit the bed.
45. A method of monitoring a patient in a hospital bed, the method
comprising the steps of: determining whether the patient is within
a safe arming zone of the hospital bed, and enabling monitoring
of the patient in the hospital bed only if the patient is within
the safe arming zone.
46. The method of claim 45 wherein the hospital bed has at least
three load cells positioned about a periphery of the bed, each load
cell producing a signal corresponding to a distribution of patient
weight on that load cell, and wherein the determining step includes:
determining a current distribution of patient weight on each of
the load cells, and determining that the patient is not within the
safe arming zone of the hospital bed if the current distribution
of patient weight on each the load cells exceed corresponding load
cell threshold values of one of a plurality of sets of arming conditions
defined about a periphery of the bed.
47. The method of claim 46 wherein the method of monitoring a patient
includes a patient exit mode configured to determine whether the
patient is about to exit the hospital bed, and wherein the enabling
step includes enabling activation of the patient exit mode only
if the patient is within the safe arming zone.
48. The method of claim 47 wherein the patient exit mode includes:
establishing a plurality of sets of exit conditions about a periphery
of the bed, each of the plurality of sets of exit conditions defining
threshold values for each of the load cells, determining a current
distribution of patient weight on each of the load cells, and determining
that the patient is about to exit the bed if the current distribution
of weight on at least some of the load cells exceeds corresponding
ones of the threshold values of one of the sets of exit conditions.
49. The method of claim 48 further including determining the one
of a plurality of sets of arming conditions by scaling a corresponding
one of the plurality of sets of exit conditions by a first predefined
scaling factor.
50. The method of claim 48 wherein the method of monitoring a patient
includes a patient movement mode configured to determine whether
the patient movement relative to a reference distribution of weight
on the load cells is excessive, and wherein the enabling step includes
enabling activation of the patient movement mode only if the patient
is within the safe arming zone.
51. The method of claim 50 wherein the patient movement mode includes:
establishing a first plurality of sets of excessive patient movement
conditions, each of the first plurality of sets of excessive patient
movement conditions defining first threshold values for each of
the at least three load cells, determining a total patient weight
on the bed, for each of the first plurality of sets of excessive
patient movement conditions, computing weight change values for
each of the at least three load cells according to a patient movement
model as a function of the total patient weight on the bed, and
determining that patient movement is excessive if at least some
of the weight change values exceed corresponding ones of the first
threshold values for at least one of the first plurality of sets
of excessive patient movement conditions.
52. The method of claim 51 further including determining the one
of a plurality of sets of arming conditions by scaling a corresponding
one of the plurality of sets of exit conditions by a second predefined
scaling factor.
53. The method of claim 45 wherein the method of monitoring a patient
includes a patient exit mode configured to determine whether the
patient is about to exit the hospital bed, and wherein the enabling
step includes enabling activation of the patient exit mode only
if the patient is within the safe arming zone.
54. The method of claim 45 wherein the method of monitoring a patient
includes a patient movement mode configured to determine whether
the patient movement relative to a reference distribution of weight
on the load cells is excessive, and wherein the enabling step includes
enabling activation of the patient movement mode only if the patient
is within the safe arming zone.
55. The method of claim 45 further including determining a total
patient weight on the bed, and wherein the enabling step is further
conditioned upon the total patient weight being less than a maximum
total patient weight.
56. The method of claim 45 further including determining a total
patient weight on the bed, and wherein the enabling step is further
conditioned upon the total patient weight being greater than a minimum
total patient weight.
57. The method of claim 56 wherein the enabling step is further
conditioned upon the total patient weight being less than a maximum
total patient weight.
58. A method of monitoring a patient in a hospital bed having a
head section that may be controllably elevated, the method comprising
the steps of: determining a total patient weight on the bed, and
enabling monitoring of the patient in the hospital bed if the total
patient weight is greater than about 50 pounds and the head section
of the bed is elevated in excess of 45 degrees relative to bed flat.
59. The method of claim 58 wherein the head section of the hospital
bed may be controllably elevated to a maximum head angle of at least
about 65 degrees relative to bed flat, and wherein the enabling
step includes enabling monitoring of the patient in the hospital
bed if the total patient weight is greater than about 50 pounds
and the head section is elevated anywhere between bed flat and the
maximum head angle.
61. The method of claim 58 further including the step of determining
whether the patient is within a safe arming zone of the hospital
bed, and wherein the enabling step is further conditioned upon the
patient being within the safe arming zone of the hospital bed.
62. A method of monitoring a patient in a hospital bed according
to a patient movement mode configured to determine whether patient
movement relative to a reference distribution of weight on the bed
is excessive, the bed having a head section that may be controllably
elevated to a maximum head angle of at least about 65 degrees relative
to bed flat, the method comprising the steps of: enabling monitoring
of the patient according to the patient movement mode regardless
of elevation of the head section of the bed relative to bed flat,
and continuing to monitor the patient according to the patient movement
mode without activating an alarm if the head section is articulated
to an elevation, between the maximum head angle and bed flat, that
is different than the elevation of the head section when the patient
movement mode was enabled.
63. The method of claim 62 further including determining a total
patient weight on the bed, and wherein the enabling step is conditioned
upon the total patient weight being greater than a minimum total
patient weight.
64. The method of claim 62 further including determining a total
patient weight on the bed, and wherein the enabling step is conditioned
upon the total patient weight being less than a maximum total patient
weight.
65. The method of claim 62 further including determining whether
the patient is within a safe arming zone of the bed, and wherein
the enabling step is conditioned upon the patient being within the
safe arming zone of the bed.
66. The method of claim 62 further including: determining a total
patient weight on the bed, and determining whether the patient is
within a safe arming zone of the bed, and wherein the enabling step
is conditioned upon the total patient weight being greater than
a minimum total patient weight, on the total patient weight being
less than a maximum total patient weight and on the patient being
within the safe arming zone of the bed.
67. The method of claim 62 wherein the continuing step is conditioned
upon articulating the head section via a control panel mounted to
the bed.
68. The method of claim 62 further including the step of activating
an alarm if the head section is articulated to the elevation that
is between the maximum head angle and bed flat via a patient control
pendant.
69. A method of monitoring a patient in a hospital bed according
to a patient movement mode configured to determine whether patient
movement relative to a reference distribution of weight on the bed
is excessive, the method comprising the steps of: enabling monitoring
of the patient according to the patient movement mode, and continuing
to monitor the patient according to the patient movement mode without
activating an alarm after further weight, less than a maximum further
weight, is added to the bed regardless of total patient weight on
the bed prior to adding the further weight.
70. The method of claim 69 further including determining the total
patient weight on the bed, and wherein the enabling step is conditioned
upon the total patient weight being greater than a minimum total
patient weight.
71. The method of claim 69 further including determining the total
patient weight on the bed, and wherein the enabling step is conditioned
upon the total patient weight being less than a maximum total patient
weight.
72. The method of claim 69 further including determining whether
the patient is within a safe arming zone of the bed, and wherein
the enabling step is conditioned upon the patient being within the
safe arming zone of the bed.
73. The method of claim 69 further including: determining the total
patient weight on the bed, and determining whether the patient is
within a safe arming zone of the bed, and wherein the enabling step
is conditioned upon the total patient weight being greater than
a minimum total patient weight, on the total patient weight being
less than a maximum total patient weight and on the patient being
within the safe arming zone of the bed.
74. The method of claim 69 wherein the maximum further weight is
about 30 pounds.
75. A method of monitoring a patient in a hospital bed according
to a patient exit mode configured to determine whether the patient
is about to exit the bed, the method comprising the steps of: determining
a bed zero weight without the patient supported by the bed, determining
a total patient weight on the bed as a function of the bed zero
weight when the patient is supported by the bed, enabling monitoring
of the patient in the bed according to the patient exit mode, and
continuing to monitor the patient according to the patient exit
mode without activating an alarm after further weight, less than
a maximum further weight, is added to the bed after determining
the bed zero weight.
76. The method of claim 75 wherein the maximum further weight is
about 30 pounds.
77. The method of claim 75 wherein the bed includes a head end
and a foot end, and wherein the further weight is added to one of
the head and foot ends.
78. The method of claim 75 wherein the enabling step is conditioned
upon the total patient weight being greater than a minimum total
patient weight.
79. The method of claim 75 wherein the enabling step is conditioned
upon the total patient weight being less than a maximum total patient
weight.
80. The method of claim 75 further including determining whether
the patient is within a safe arming zone of the bed, and wherein
the enabling step is conditioned upon the patient being within the
safe arming zone of the bed.
81. The method of claim 75 further including determining whether
the patient is within a safe arming zone of the bed, and wherein
the enabling step is conditioned upon the total patient weight being
greater than a minimum total patient weight, on the total patient
weight being less than a maximum total patient weight and on the
patient being within the safe arming zone of the bed.
82. A system for controlling air pressure in a number of different
zones of an air mattress supporting an occupant, comprising: a plurality
of load cells each configured to produce a signal indicative of
weight impressed upon that load cell via the air mattress, and a
controller responsive to the signals produced by the plurality of
load cells to determine a current distribution of occupant weight
on each of the load cells, the controller controlling air pressure
within any one or more of the number of different zones of the air
mattress based on the current distribution of occupant weight on
at least some of the plurality of load cells.
Description
CROSS-REFERENCE TO RELATED U.S. PATENT APPLICATIONS
[0001] This patent application claims priority to and the benefit
of U.S. Provisional Patent Application Ser. No. 60/599,558, filed
Aug. 9, 2004, and U.S. Provisional Patent Application Ser. No. 60/615,031,
filed Oct. 1, 2004, the disclosures of which are each incorporated
herein by reference.
FIELD OF THE INVENTION
[0002] This invention relates generally to systems for monitoring
and detecting movement of a mass on a platform, and more specifically
to methods and systems for detecting patient movement on, exit from,
or impending exit from a patient support system such as a hospital
bed.
BACKGROUND
[0003] When a patient is required to stay in a hospital bed at
a hospital or other patient care facility, it is desirable for a
caregiver to be able to monitor the presence, absence, and movement
of the patient on the bed platform, generally a mattress, and to
monitor the patient's activity level. Caregivers are generally responsible
for a number of patient-related activities, examples of which include
monitoring the presence or absence of patients on their hospital
beds and/or monitoring patient movement relative to their hospital
beds.
[0004] One system for monitoring patient movement on a hospital
bed is disclosed by U.S. Pat. No. 5,276,432, issued Jan. 4, 1994,
to Travis. The disclosed system calculates the center of gravity
of a patient within a two-dimensional Cartesian coordinate-based
region defined relative to the patient-supporting surface of the
bed mattress. The center of gravity of the patient relative to the
region is determined using data from load cells coupled to the hospital
bed frame. Patient movement relative to the region is detected by
monitoring movement of the center of gravity of the patient, and
by determining the location of the patient's center of gravity relative
to the region.
[0005] Alternative methods and systems for monitoring patient movement
on, exit from and/or impending exit from, a hospital bed are desirable.
SUMMARY
[0006] The present invention may comprise one or more of the features
recited in the appended claims and/or one or more of the following
features or combinations thereof. A system for monitoring a patient
on a patient support, such as a hospital bed or stretcher having
a support surface, may comprise a plurality of load cells positioned
to weigh the support surface and the patient supported thereon.
In one exemplary embodiment, four such load cells may be mounted
at or near each of the four corners of the support, one at each
right and left side at or near the head of the support and one at
each right and left side at or near the foot of the support. Output
signals of the load cells, which may typically be voltage or current
level outputs, may be digitized for processing by a control system
computer.
[0007] Various algorithms may utilize various combinations of the
load cell output signals to determine the weight of the patient
on the support surface, exit of the patient from the support surface,
impending exit of the patient from the support surface and/or movement
of the patient relative to reference load cell distribution values.
[0008] One such algorithm, for example, may be configured to determine
when the patient is in process of exiting the support surface. When,
for example, the sum of output signals of all four cells is substantially
less than, e.g., by 30 pounds to 60 pounds, the established weight
of the patient, this may be indicative that the patient has transferred
at least some of the patient's total weight off the support surface
onto some other support surface or structure that supports the missing
weight.
[0009] Another algorithm, for example, may determine when a patient's
movement on the support surface exceeds any of a number of predetermined
load cell thresholds. A collection of weight distribution threshold
percentages for each of the four cells RH, LH, RF and LF is stored
in memory and continually compared against the current load cell
values after arming of the system. When the distribution of weight
among two or three of the four cells changes by more than one of
the corresponding stored collection of weight distribution threshold
percentages as a result of excessive patient movement subsequent
to the system being armed, an alarm is triggered.
[0010] Yet another algorithm, for example, will determine when
a patient is about to exit the support surface. A collection of
load cells thresholds for each of the four cells RH, LH, RF and
LF is stored in memory, and the current load cell values are compared
to selected portions of the load cell threshold collection. When
the measured distribution of weight among the four cells RH, LH,
RF and LF exceeds a set of load cell thresholds forming the collection
of load cell thresholds as a result of impending patient exit from
the support surface, an alarm is triggered.
[0011] The above briefly described algorithms of the system do
not determine the center of gravity of a patient. Nor do the algorithms
determine the actual position of a patient relative to a reference
position. Nor do the algorithms require a measured length or width
of the support surface. Nor do the algorithms determine or use any
information relating to the physical locations of the various load
cells relative to a reference position, or any information relating
to distances between such load cells. The actual locations of the
various load cells are arbitrary, and the locations of the load
cells shown in the illustrated embodiments are provided only by
way of example. The locations of the load cells may therefore be
different for different applications.
[0012] The briefly described algorithms monitor the distribution
of patient weight supported by each of the four cells RH, LH, RF
and LF, and compare the resulting load cell weights to empirically
determined and/or model-based collections of load cell threshold
data. Changes in patient weight distribution among two, three or
four of the load cells relative to the one or more collections of
load cell threshold data are then used in a decision process to
detect excessive patient movement and/or impending exit from, and/or
exit from, the support surface. The algorithms accomplish this without
reference to a patient center of gravity, an actual patient position
relative to a reference position or a coordinate axis, actual locations
of one or more of the load cells relative to a reference position
or distance between any such load cells.
[0013] These and other features of the present invention will become
more apparent from the following description of the illustrated
embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1A is a perspective view of a hospital bed including
an exemplary embodiment of a system for monitoring patient movement
on, exit from, and impending exit from, the bed.
[0015] FIG. 1B is a block diagram illustrating exemplary locations
of a number of load cells relative to the bed of FIG. 1A.
[0016] FIG. 1C is a perspective view illustrating the hospital
bed of FIG. 1A with the foot end of the bed shown in an extended
position.
[0017] FIG. 2 is a perspective view of one of the siderails of
the bed shown in FIG. 1A having a control and display panel mounted
thereto.
[0018] FIG. 3 is a partial cutaway view of the circled portion
3 of FIG. 1A illustrating control circuitry and one of the load
cells carried by the bed.
[0019] FIG. 4 is a block diagram of one exemplary embodiment of
a monitoring system for monitoring patient movement on, exit from,
and impending exit from, the bed illustrated in FIGS. 1A-3.
[0020] FIG. 5 is a combined flowchart and state diagram illustrating
one exemplary embodiment of a software algorithm for monitoring
patient movement on, exit from, and impending exit from, the bed
illustrated in FIGS. 1A-3.
[0021] FIG. 6 is a flowchart illustrating an exemplary embodiment
of the state machine preparation routine forming part of the algorithm
of FIG. 5.
[0022] FIGS. 7A-7C show a flowchart illustrating an exemplary embodiment
of a software routine for executing the PM Off State that forms
part of the state machine of FIG. 5.
[0023] FIG. 8 is a flowchart illustrating an exemplary embodiment
of a software routine for executing the PM Zero State that forms
part of the state machine of FIG. 5.
[0024] FIG. 9 is a flowchart illustrating an exemplary embodiment
of the zero capture software routine called by the PM Zero State
routine of FIG. 8.
[0025] FIGS. 10A-10B show a flowchart illustrating an exemplary
embodiment of a software routine for executing the PM Movement/Exit
Transition State that forms part of the state machine of FIG. 5.
[0026] FIGS. 11A-11C show a flowchart illustrating an exemplary
embodiment of a software routine for executing the PM Active State
that forms part of the state machine of FIG. 5.
[0027] FIG. 12 is a flowchart illustrating an exemplary embodiment
of a software routine for executing the PM Active State that forms
part of the state machine of FIG. 5.
[0028] FIG. 13 is a block diagram illustrating one example construction
of an exit condition threshold table for use by a patent exit routine
called by the PM Active STATE software routine of FIGS. 11A-11C.
[0029] FIGS. 14A-14C show a flowchart illustrating an exemplary
embodiment of the exit mode routine called by the PM Active State
software routine of FIGS. 11A-11C.
[0030] FIG. 15 is a diagram illustrating a mathematical model of
vertical movement of a patient in the hospital bed of FIGS. 1A-1C.
[0031] FIG. 16 is a diagram illustrated a mathematical model of
horizontal movement of a patient in the hospital bed of FIGS. 1A-1C.
[0032] FIG. 17 is a block diagram illustrating one example construction
of a movement condition threshold table for use by a patent movement
routine called by the PM Active STATE software routine of FIGS.
11A-11C.
[0033] FIG. 18 is a flowchart illustrating an exemplary embodiment
of the movement mode routine called by the PM Active State software
routine of FIGS. 11A-11C.
[0034] FIG. 19 is a flowchart illustrating an exemplary embodiment
of the out-of-bed mode routine called by the PM Active State software
routine of FIGS. 11A-11C.
DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS
[0035] For the purposes of promoting an understanding of the principles
of the invention, reference will now be made to one or more illustrative
embodiments shown in the drawings and specific language will be
used to describe the same. It will nevertheless be understood that
no limitation of the scope of the invention is thereby intended.
[0036] Referring now to FIGS. 1A-1C, one illustrative embodiment
of a hospital bed 50 is shown. The bed 50 is implemented in the
context of a hospital bed and patient monitoring apparatus generally
of the type described in U.S. Pat. No. 6,208,250, issued Mar. 27,
2001, and entitled Patient Position Detection Apparatus for a Bed,
which is assigned to the assignee of the present invention, and
the disclosure of which is incorporated herein by reference. In
the exemplary embodiment shown and described herein, the hospital
bed 50 may illustratively be a VersaCare.RTM. hospital bed, which
is commercially available from Hill-Rom Company, Inc. of Batesville,
Ind. It will be appreciated, however, that this implementation and
the illustrated embodiment is provided only by way of example, and
that the concepts illustrated and described herein are applicable
to other patient support systems, including for example, but not
limited to, stretchers, wheelchairs, or other patient-supporting
apparatus. Any such other systems utilizing the concepts illustrated
and described herein are contemplated by this disclosure.
[0037] The exemplary hospital bed 50 includes a stationary base
54 coupled to a weigh frame 56 that is mounted via frame members
57a and 57b to an adjustably positionable mattress support frame
or deck 58 configured to support a conventional foam mattress 60.
The mattress 60 defines a patient support surface 65 bounded by
a head end 60a positioned adjacent to a headboard 62 mounted to
the mattress support frame 58 at a head end 62 of the bed 50, a
foot end 60b positioned adjacent to a footboard 64b mounted to the
mattress support frame 58 at a foot end 64 of the bed 50, a left
side 60c and a right side 60d. A pair of siderails 66a and 66c are
mounted to the mattress support frame 58 adjacent to one side 60c
of the mattress 60, and another pair of siderails 66b and 66d are
mounted to the mattress support frame 58 adjacent to the opposite
side 60d of the mattress 60. The siderail 66a supports a patient
monitoring control panel 70, and the siderail 66b supports a mattress
position control panel 70. The bed 50 is generally configured to
adjustably position the mattress support 58 relative to the base
54.
[0038] Conventional structures and devices may be provided to adjustably
position the mattress support 58, and such conventional structures
and devices may include, for example, linkages, drives, and other
movement members and devices coupled between base 54 and the weigh
frame 56, and/or between weigh frame 56 and mattress support frame
58. Control of the position of the mattress support frame 58 and
mattress 60 relative to the base 54 or weigh frame 56 is provided,
for example, by a patient control pendant (not shown), a mattress
position control panel 69, and/or a number of mattress positioning
pedals 55. The mattress support frame 58 may, for example, be adjustably
positioned in a general incline from the head end 62 to the foot
end 64 or vice versa. Additionally, the mattress support 58 may
be adjustably positioned such that the head end 60a of the mattress
60 is positioned between minimum and maximum incline angles, e.g.,
0-65 degrees, relative to horizontal or bed flat, and the mattress
support 58 may also be adjustably positioned such that the thigh
area 60f of the mattress 60 is positioned between minimum and maximum
bend angles, e.g., 0-35 degrees, relative to horizontal or bed flat.
Those skilled in the art will recognize that the mattress support
frame 58 or portions thereof may be adjustably positioned in other
orientations, and such other orientations are contemplated by this
disclosure.
[0039] A number of load cells are positioned between the weigh
frame 56 and the base 54, wherein each load cell is configured to
produce a voltage or current signal indicative of a weight impressed
on that load cell from the weigh frame 56 relative to the base 54.
In the illustrated embodiment, four such load cells are positioned
between the weigh frame 56 and the base 54; one each near a different
corner of the bed 50. Two such load cells 68a and 68c are shown
in FIG. 1A, and all four are shown in FIG. 1B. Some of the structural
components of the bed 50 will be designated hereinafter as "right",
"left", "head" and "foot" from the
reference point of an individual lying on the individual's back
on the support surface 65 of the mattress with the individual's
head oriented toward the head end 62 of the bed 50 and the individual's
feet oriented toward the foot end 64 of the bed 50. For example,
the weigh frame 56 illustrated in FIG. 1B includes a head end frame
member 56c mounted at one end to one end of a right side weigh frame
member 56a and at an opposite end to one end of a left side frame
member 56b. Opposite ends of the right side weigh frame member 56a
and the left side weigh frame member 56b are mounted to a foot end
frame member 56d. A middle weigh frame member 56e is mounted at
opposite ends to the right and left side weigh frame members 56a
and 56b respectively between the head end and foot end frame members
56c and 56d. The frame member 57a is shown mounted between the right
side frame member 56a and the mattress support frame 58, and the
frame member 57b is shown mounted between the left side frame member
56b and the mattress support frame 58. It will be understood that
other structural support is provided between the weigh frame member
56 and the mattress support frame 58, although only the frame members
57a and 57b are shown in FIGS. 1A-1C for ease of illustration.
[0040] A right head load cell (RHLC) 68a is illustratively positioned
near the right head end of the bed 50 between a base support frame
54a secured to the base 54 near the head end 62 of the bed and the
junction of the head end frame member 56c and the right side frame
member 56a, as shown in the block diagram of FIG. 1B. A left head
load cell (LHLC) 68b is illustratively positioned near the left
head end of the bed 50 between the base support frame 54a and the
junction of the head end frame member 56c and the left side frame
member 56b, as illustrated in FIGS. 1A-1C and 3. A right foot load
cell (RFLC) 68c is illustratively positioned near the right foot
end of the bed 50 between a base support frame 54b secured to the
base 54 near the foot end 64 of the bed 50 and the junction of the
foot end frame member 56d and the right side frame member 56a, as
shown in the block diagram of FIG. 1B. A left foot load cell (LFLC)
68d is illustratively positioned near the left foot end of the bed
50 between the base support frame 54b and the junction of the foot
end frame member 56d and the left side frame member 56b, as illustrated
in FIGS. 1A-1C. It should be noted that in FIGS. 1A and 1C, the
base support frame 54b coupled to the left foot load cell 68d is
not shown so that the exemplary position of the load cell 68d relative
to the weigh frame 56, and to the bed 50 generally, can be seen.
In the exemplary embodiment illustrated in FIG. 1B, the four corners
of the mattress support frame 58 are shown extending beyond the
four corners of the weigh frame 56, and hence beyond the positions
of the four load cells 68a-68d.
[0041] In the illustrated embodiment, each of the load cells 68a-d
are weight sensors of the type having resistive strain gauges coupled
to a deflectable block (not shown), and structurally couple the
weigh frame 56 to the base 54. It will be appreciated, however,
that other weight detection devices may alternatively be used, wherein
such alternative devices may be or include, but are not limited
to, linear variable displacement transducers (LVDTS) and/or other
weight detection devices operable in accordance with known capacitive,
inductive, or other physical principles. In any case, all such alternative
weight detection devices are contemplated by this disclosure.
[0042] As shown in FIG. 1C, one exemplary embodiment of the hospital
bed 50 includes a foot end 64 that may be moved between a retracted
position, as shown in FIG. 1A, and an extended position, as shown
in FIG. 1C. The extended position of foot end 64 may be used, for
example, to accommodate varying patient sizes and/or to provide
a support surface between the foot end 60b of the mattress 60 and
the footboard 64b to accommodate placement thereon of medical or
other equipment.
[0043] Referring to FIG. 2, details of one exemplary control panel
70 mounted to the siderail 66a of the bed 50 of FIGS. 1A-1C is shown.
The control panel 70 includes various user-interface components
including, for example, a zero select switch 72, an enable or key
switch 74, a volume control switch 76, a volume strength indicator
78, a movement mode switch 80, an exit mode switch 82 and an out-of-bed
mode switch 84. The zero select switch 72 may be actuated to calibrate
an empty bed weight; i.e., with out a patient on the mattress 60,
and the enable or key switch 74 is used to enable various patient
monitoring functions as will be described in greater detail hereinafter.
The volume control switch 76 may be actuated to control the volume
of a local alarm; i.e., an audible, visual and/or other alarm (not
shown) mounted to or near the bed 50, and the volume strength indicator
78 may, for example, include a number, e.g., 3, of visual indicators,
e.g., LED's, that are selectively activated to indicate a volume
level of the local alarm. The mode switches 80-84 may be individually
actuated to select between various patient monitoring modes. For
example, actuation of the movement mode switch 80 selects a patient
movement monitoring mode that monitors certain patient movement
within the bed 50. Actuation of the exit mode switch 82 selects
a patient exit, which may also be referred to as a bed exit, monitoring
mode that monitors impending exit of the patient from the bed 50,
and actuation of the out-of-bed mode switch 84 selects an out-of-bed
(OOB) monitoring mode that monitors when at least a portion of the
patient's weight is not supported by the weigh frame 56, thereby
indicating that the patient is exiting, or has exited, the bed 50.
Further details relating to the operation of each of these patient
monitoring modes will be described in greater detail hereinafter.
The control panel 70 illustrated in FIG. 2 includes additional switches
and other components that provide for monitoring and control of
other features of the bed 50.
[0044] Referring now to FIG. 3, the right side frame member 56a
of the weigh frame 56 includes a housing mounted thereto adjacent
to the base support frame 54a. The housing is configured to carry
a processor module 86 and a logic module 96 electrically coupled
thereto. In the exemplary embodiment, the processor module 86 forms
part of a patient monitoring control system and includes a number
of executable software algorithms for controlling operation of the
system, and one illustrative embodiment of such a patient monitoring
system 75 is shown in FIG. 4. The patient monitoring system 75 includes
the processor module 86 electrically coupled to the logic module
96, the load cells 68a-68d, the control panel 70, a local alarm
98 (mounted to or near the bed 50) and a remote alarm 99. The remote
alarm 99 is located near a caregiver or other patient monitoring
individual, and is controlled by the processor module 86 to alert
the remote caregiver or other patient monitoring individual via
an audible and/or visual or other alarm (not shown) of certain patient
movement activities as will be described in greater detail hereinafter.
[0045] The processor module 86 includes a microprocessor-based
controller 88 having a Flash memory unit 90 and a local RAM memory
unit 92. The module 86 further includes an auxiliary memory unit
94, which may be an EEPROM or other conventional memory unit that
is electrically connected to the controller 88. The logic module
96 and load cells 68a-68d are electrically connected to the controller
88, and in the exemplary embodiment the logic module 96 is configured
to continually determine a height of the bed 50 via one or more
conventional sensors and to supply the bed height information to
the controller 88. Alternatively, the controller 88 may be operable
to determine the height of the bed 50 via any one or more conventional
techniques. In any case, the controller 88 is also electrically
connected to the local alarm 98 and to the remote alarm 99, and
the controller 88 is configured to control operation of such alarms
98 and 99 in a conventional manner. The control panel 70 is also
electrically connected to the controller 88 to communicate information
from the various switches and other input devices 72-76 and 80-84
from the control panel 70 to the controller 88, and to communicate
information from the controller 88 to the volume strength indicator
78.
[0046] In the illustrated embodiment, the Flash memory 90 of the
processor module 86 includes a number of software algorithms and
other data that are executable by the controller 88 to monitor patient
movement relative to a reference load cell distribution, impending
exit from the mattress 60 and/or exit from the mattress 60. An exemplary
main software algorithm 100 for managing such functions is illustrated
in FIG. 5 in the form of a combined flowchart and state machine
120. The software algorithm 100 is executed periodically by the
controller 88, e.g., once every 200 ms, to monitor patient movement
relative to a reference load cell distribution, impending exit from
the mattress 60 and/or exit from the mattress 60. Referring to FIG.
5, the software algorithm 100 begins at step 102 where the controller
88 is operable to determine whether an invalid bed zero warning
is active upon power up, wherein an invalid bed zero warning indicates
that the zero reference weight of the bed 50, e.g., the total weight
impressed upon the weigh frame 56 without a patient supported by
the mattress 60, may not be current or accurate. If, at step 102,
the controller 88 determines that an invalid bed zero warning is
active upon power up, algorithm execution advances to a Freeze State
126 of the state machine 120. The controller 88 is operable in the
Freeze State 126 to wait until a bed weight zeroing process is activated
as will be described below. If, at step 102, the controller 88 instead
determines that an invalid bed zero warning is not active upon power
up, execution of the algorithm 100 advances to step 104 where the
controller 88 is operable to determine whether the patient monitoring
system 75 was armed, i.e., whether one of the patient monitoring
modes was active, before the last power down of the system 75. The
system 75 is operable to save the current data to the memory unit
94 at power down, and to recall such data from the memory unit 94
upon subsequent power up so that the controller 88 may make the
determination illustrated in step 104.
[0047] The patient monitoring modes include a patient movement
(PM) mode wherein the system 75 is operable to monitor movement
of a patient on the mattress 60 by monitoring weight distribution
among two or three of the four load cells 68a-68b relative to a
predefined set of PM load cell threshold data, a patient exit (PE)
mode wherein the system 75 is operable to monitor impending exit
from the mattress 60 by monitoring weight distribution of the four
load cells relative to a predefined set of PE load cell threshold
data, and a patient out-of-bed (OOB) mode wherein the system 75
is operable to monitor exit of the patient from the mattress 60
by monitoring the patient weight distributed over the four load
cells relative to an armed patient weight, wherein the armed weight
corresponds to the patient weight distributed over the four load
cells when the patient monitoring mode was armed as will be described
in greater detail hereinafter. In any case, if the controller 88
determines at step 104 that the system 75 was not armed before the
last system power down, execution of the algorithm 100 advances
to step 110 where the controller 88 is operable to execute a state
machine preparation routine. If, at step 104, the controller 88
instead determines that the system 75 was armed before the last
system power down, execution of the algorithm 100 advances to an
Arming From Power Up Transition State 138 of the state machine 120
where the patient weight is processed to determine whether it is
contained within a defined armed range prior to advancing to the
PM Active State 130 of the state machine 120 to resume operation
of the patient monitoring mode that was active at the last system
power down.
[0048] After executing the state machine preparation routine at
step 110, execution of the algorithm 100 advances to the state machine
120. In the first execution of the algorithm 100, the state machine
120 is started at the PM Off State 122. In further executions of
the algorithm 100, the state machine 120 will be entered from step
110 at its current operational state. In addition to the operating
and transition states just described, the state machine 120 further
includes a PM Movement/Exit Transition State 128 that is selected
for operation following the PM Off State 122 when either a request
for Patient Monitoring (PM) Mode or Patient Exit (PE) Mode is received
via the control panel switch 80 or 82 respectively. From the PM
Movement/Exit Transition State 128, the state machine 120 advances
to the PM active state 130 where the controller 88 is operable to
actively monitor patient activity pursuant to the PM mode or PE
mode respectively. The state machine 120 also includes a PM OOB
Transition State 132, that is similar to the PM Movement/Exit Transition
State 128 and that is selected for operation following the PM Off
State 122 when a request for out-of-bed (OOB) mode is received via
the control panel switch 84. From the PM OOB Transition State 132,
the state machine 120 advances to the PM active state 130 where
the controller 88 is operable to actively monitor patient activity
pursuant to the OOB mode.
[0049] From either of the PM Movement/Exit Transition state 128
or the PM OOB Transition State 132, the state machine 120 advances
to a PM Failed Arming State 134 if either of the transition states
128 or 132 failed the arming function; i.e., failed to arm the system
75 by determining a total patient weight. From the PM Failed Arming
state 134, the state machine 120 advances back to the PM Off State
where the patient may be repositioned relative to the mattress 60,
followed by selection of the PM, PE or OOB mode.
[0050] From either of the PM Active state 130 or the Arming from
Power Up State 138, the state machine 120 advances to a PM Alarm
State 136 if any alarm conditions are met in the PM Active State
130 or the patient weight arming conditions are not met in the Arming
from Power Up State 138. From the PM Alarm State 136, the state
machine 120 advances to the PM Off State 122 where the alarm condition
may be remedied and/or the patient may be repositioned relative
to the mattress 60 before re-selecting the PM or PE mode. The state
machine 120 further includes a PM Zero State 124 that is selected
to compute a new zero weight bed reference upon actuation of a predefined
combination of the switches forming part of the control panel 70
as will be described below.
[0051] Referring now to FIG. 6, a flowchart is shown of one illustrative
embodiment of the state machine preparation routine called by step
112 of the algorithm 100 of FIG. 5. In the illustrated embodiment,
the state machine preparation routine begins at step 150 where the
controller 88 is operable to acquire new load cell data, LCD, by
sampling the signals produced by the load cells 68a-68d. Thereafter
at step 152, the controller 88 is operable to correct the load cell
data, LCD, as a function of the current bed height. In the illustrated
embodiment, the various structures and mechanisms used to raise
and lower the height of the mattress support frame 58 relative to
the base 54 to cause the weights measured by the load cells 68a-68d
to vary as a function of bed height. In this embodiment, the logic
module 96 is coupled to one or more sensors from which the logic
module is operable to determine bed height in a known manner, and
the logic module 96 supplies the bed height information to the process
module 86. Alternatively, the controller 88 may be operable to determine
the height of the bed 50 via any one or more conventional techniques.
In any case, the memory unit 90 illustratively includes a bed height
conversion function in the form of a lookup table or other conversion
function mapping the bed height information to load cell signal
correction information. In one embodiment, for example, the lookup
table maps discrete bed height values to corresponding offset values,
and the offset value corresponding to the current bed height is
then used to correct the load cell data, LCD, by adding the offset
value to the values produced by two diagonally opposed pairs of
the load cells 68a-68d, and subtracting the offset value from the
values produced by the remaining two diagonally opposed pairs of
the load cells 68a-68d. Those skilled in the art will recognize
other techniques for correcting the signals or values produced by
the load cells 68a-68d as a function of bed height, and such other
techniques are intended to fall within the scope of claims appended
hereto.
[0052] Following step 152, the controller 88 is operable at step
154 of the state machine preparation routine to convert the corrected
load cell data signals to load cell weight values. Hereinafter,
the term "RH" may be used to identify the bed height-corrected
weight value produced by the right head load cell 68a, the term
"LH" may be used to identify the bed height-corrected
weight value produced by the left head load cell 68b, the term "RF"
may be used to identify the bed height-corrected weight value produced
by the right foot load cell 68c, and the term "LF" may
be used to identify the bed height-corrected weight value produced
by the left foot load cell 68d. In the illustrated embodiment, the
load cell signals produced by the load cells 68a-68d are analog
current or voltage signals, and the amplitudes of these analog signals
are converted via analog-to-digital inputs of the controller 88
to corresponding discrete, raw "count" values. The controller
88 is then operable at step 152 to add or subtract an offset count
value to or from the various discrete, raw count values, as just
described, to produce bed height-corrected count values for each
of the load cells 68a-68d. At step 154, the controller 88 then multiplies
the corrected count values of each the load cell by a predefined
conversion constant to convert the corrected count values of each
load cell to a corresponding weight value, LH, LF, RH and RF.
[0053] Following step 154, the controller 88 is operable at step
156 to correct the load cell weight values, LH, RH, LF and RF based
on the current PM State of the state machine 120. For example, if
the state machine 120 is currently in the PM Off State 122, the
controller 88 is operable to correct the load cell weight values
by subtracting tare weights in the form of original zero weight
(OZ) values, and then by correcting these weight difference values
by a trend angle, or reverse trend angle, factor, TAF. In one embodiment,
the controller 88 is operable to determine a trend angle or reverse
angle, .alpha., corresponding to the angle of incline or decline
of the entire mattress 60 from the head end 62 of the bed 50 to
the foot end 64 of the bed 50, as a function of bed height difference
between the head end 62 and foot end 64 of the bed 50. In this embodiment,
the logic module 96 supplies the bed height information to the processor
module 86, and the controller 88 is operable to determine the trend
angle, .alpha., as a known function of the bed height difference
between the head end 62 and foot end 64 of the bed 50. Alternatively,
the controller 88 may be configured to determine a via conventional
techniques. In any case, with the trend angle, .alpha., determined,
the controller 88 is then operable to compute the trend angle factor
according to the relationship TAF=cos .alpha.. As illustrated in
step 156, the controller 88 is thus operable to correct the load
cell weight values when the state machine 120 is in the PM Off State
122 according to the equations LH=(LH-OZLH)/TAF, RH=(RH-OZRH)/TAF,
LF=(LF-OZLF)/TAF and RF=(RF-OZRF)/TAF.
[0054] If, at step 156, the state machine 120 is instead in the
PM Zero State 124, the controller 88 is operable to correct the
load cell weight values by the trend angle, or reverse trend angle,
factor, TAF. As illustrated in step 156, the controller 88 is operable
to correct the load cell weight values when the state machine 120
is in the PM Zero State 124 according to the equations LH=LH/TAF,
RH=RH/TAF, LF=LF/TAF and RF=RF/TAF.
[0055] If, at step 156, the state machine 120 is instead in either
of the PM Movement/Exit Transition state 128 or the PM Active State
130, the controller 88 is operable to correct the load cell weight
values by tare weights in the form of auto zero (AZ) weight values.
As illustrated in step 156, the controller 88 is operable to correct
the load cell weight values when the state machine 120 is in the
PM Movement/Exit Transition State 128 or the PM Active State 130
according to the equations LH=LH-AZLH, RH=RH-AZRH, LF=LF-AZLF and
RF=RF-AZRF.
[0056] Following step 156, the controller 88 is operable to compute
a corrected total patient weight, CTPW, corresponding to a total
bed weight impressed upon the load cells 68a-68d by a patient. In
one embodiment, the controller 88 is operable to execute step 156
by computing the total patient weight, CTPW, as an average of the
sum of the corrected load cell weight values, or CTPW=(LH+RH+LF+RF)/4.
It may also be desirable to further correct CTPW, as a function
of the trend angle or reverse trend angle, .alpha., when CTPW is
determined from load cell weight values when the state machine 120
is in either the PM Movement/Exit Transition State 128 or the PM
Active State 130. In such cases, the controller 88 may further be
operable at step 158 to compute CTPW=CTPW/cos .alpha..
[0057] Following step 158, the controller 88 is operable at step
160 to compute a number of total patient weight running averages
using any one or more conventional sample averaging techniques.
In the illustrated embodiment, for example, the controller 88 is
operable at step 160 to compute a total patient weight slow running
average, SRA, and a total patient weight fast running average, FRA,
using conventional averaging techniques, although other sample averaging
techniques may be used to provide other patient weight running average
values. At step 162, the controller 88 is operable to determine
whether the corrected total patient weight, CTPW, is greater than
a minimum total patient weight threshold, MINTPW. The minimum total
patient weight threshold, MINTPW, may be selected to fit the particular
application, and one example value of MINTPW may be, but should
not be limited to, 50 lbs. If the controller 88 determines at step
162 that CTPW is greater than MINTPW, execution of the state machine
preparation routine advances to step 164 where the controller 88
is operable to compute a sensitivity to minimum weight change, SMWC,
as a function of the corrected total patient weight, CTPW, wherein
the minimum weight change may result from adding weight to the support
surface 65 of the mattress 60 and/or to the region of the mattress
support frame 58 between the end 60b of the mattress and the foot
end 64 of the bed 50 as illustrated in FIG. 1C. The sensitivity
to minimum weight change is used, as will be described in detail
hereinafter, to determine a maximum amount of weight change in the
patient movement (PM) mode that will be tolerated before temporarily
disabling the PM mode until the change in weight settles.
[0058] In one embodiment, for example, the controller 88 is operable
to execute step 164 by computing SMWC according to the equation
SMWC=MWCHBNR+(CTPW-MINTPW)*WCHS, where SMWC is the sensitivity to
minimum weight change, MWCHBNR is a minimum weight change before
new reference value, e.g., 5 lbs., CTPW is the corrected total patient
weight, MINTPW is the minimum total patient weight threshold and
WCHS is a weight change sensitivity value, e.g., 0.05. One example
implementation of this equation may thus result in SMWC=5+(CTPW-50)*0.05,
although other values of MWCHBNR, MINTPW and WCHS may be used. In
any case, execution of the state machine preparation routine advances
from step 164 and from the "NO" branch of step 162 to
step 166 where algorithm execution is returned to step 110 of the
main algorithm 100 of FIG. 5.
[0059] Following completion of the state machine preparation routine
at step 110, execution of the algorithm 100 advances to the current
state of the state machine 120. One such state may be, for example,
the PM Off State 122. Referring now to FIGS. 7A-7C, a flowchart
is shown of one illustrative embodiment of a software algorithm
or routine for executing the PM Off State 122 of the state machine
120. The PM Off State routine begins at step 180 where the controller
88 is operable to determine whether the corrected total patient
weight, CTPW, is stable. In one embodiment, the controller 88 is
operable at step 180 to determine whether CTPW is stable by comparing
CTPW to a sum and difference of a last weight settling snapshot,
LWSS, and an arming weight settle constant, AWSC, wherein the last
weight settling snapshot corresponds to the total weight impressed
upon the load cells 68a-68d at the most recent determination that
CTPW was stable. Specifically, the controller 88 is operable in
the illustrated embodiment to determine that CTPW is unstable if
CTPW>(LWSS+AWSC) OR CTPW<(LWSS-AWSC). If the controller 88
determines at step 180 that either of these conditions is met, execution
of the routine advances to step 182 where the controller 88 is operable
to reset a weight stable timer and capture a new weight reference
snap shot. Thereafter at step 184, the controller 88 is operable
to set the last weight settling snapshot, LWSS, to the corrected
total patient weight, CTPW.
[0060] Execution of the PM Off State routine advances from the
"YES" branch of step 180, which is indicative of CTPW
satisfying neither of the above inequalities and therefore being
considered to be stable, and from step 184 to step 186 where the
controller 88 is operable to execute the Exit Mode routine, which
will be described in detail hereinafter, to determine whether the
patient weight is contained within a safe arming zone of the mattress
60. Thereafter at step 188, the controller 88 is operable to determine
whether any patient monitoring mode requests; i.e., the patient
movement (PM) mode, patient exit (PE) exit mode or patient out-of-bed
(OOB) mode, are active. If so, execution of the PM Off State routine
advances to step 190 where the controller 88 is operable to determine
whether the patient monitoring mode request corresponds to either
of the patient movement (PM) or patient exit (PE) monitoring modes.
If so, the operating state of the state machine 120 moves to the
PM Movement/Exit Transition State 128 and the controller 88 is operable
at step 192 to execute a PM Movement/Exit Transition State routine,
one example of which will be described below with reference to FIGS.
10A-10B. If, on the other hand, the controller 88 determines at
step 190 that the patient monitoring mode request corresponds to
the patient out-of-bed (OOB) monitoring mode, the operating state
of the state machine 120 moves to the PM OOB Transition State 132
of the state machine 120 and the controller 88 is operable at step
194 to execute a PM OOB Transition State routine similar to the
PM Movement/Exit Transition State routine, one embodiment of which
will be described below with reference to FIGS. 10A-10B.
[0061] In the illustrated embodiment, the PM OOB Transition State
routine may be identical to the PM Movement/Exit Transition State
routine illustrated in FIGS. 10A-10B with the exception that step
362 is omitted and the Exit mode routine is therefore not executed
to determine whether the patient weight is within the safe arming
zone of the mattress 60. In all other respects, the PM OOB Transition
State routine in the illustrated embodiment is identical to the
PM Movement/Exit Transition State routine of FIGS. 10A-10B. A flowchart
and description of the details such a PM OOB Transition State routine
would thus be repetitious of the PM Movement/Exit Transition State
routine of FIGS. 10A-10B, and is accordingly omitted from this document
for brevity.
[0062] Returning to the PM Off State routine of FIGS. 7A-7C, execution
of this routine advances from the "NO" branch of step
188 to step 196 where the controller 88 is operable to determine
whether a common zero request flag, CZR, is active. This flag is
set when a user pushes the enable switch 74 and thereafter pushes
the zero select switch 72, thereby manually requesting zeroing of
the bed weight. If the controller 88 determines at step 196 that
CZR is "true", the operating state of the state machine
120 moves to the PM Zero State 124 and the controller 88 is operable
at step 198 to execute a PM Zero State routine to zero the bed weight,
and one example of such a PM Zero State routine will be described
below with reference to FIG. 8.
[0063] Execution of the PM Off State routine advances from the
"NO" branch of step 196 to step 200 where the controller
88 is operable to determine whether the corrected total patient
weight, CTPW, is between a minimum auto-zero weight value, MINAZW,
and a maximum auto-zero weight value, MAXAZW. When the state machine
120 is in the PM Off State the controller 88 executes steps 200
and beyond of the PM Off State routine to determine whether to perform
an auto-zeroing process, or an automatic bed zeroing. MINAZW may
be in the range of -4 to -5 lbs., and MAXAZW may be in the range
of 4-5 lbs., although it will be understood that these values are
provided only by way of example, and that MINAZW and MAXAZW may
take on any desired values. The auto-zeroing process of steps 200
and beyond is conducted because the weight distribution between
the load cells 68a-68d may have shifted since the last bed zeroing
event. Such a shift may have occurred for any number of reasons
including, for example, a visitor sits on the corner of the bed
for a brief time period, someone leans against one of the siderails
66a-66d, medical monitoring equipment may have been briefly stored
on the mattress 60 or between the end 60b of the mattress 60 and
the foot end 64 of the bed 50, or the like. In any of these cases,
the total bed weight after one or more such occurrences may remain
at or near the zeroed bed weight, but the distribution of the weight
between the various load cells 68a-68d may have shifted significantly.
Such weight shifting between the load cells 68a-68d may compromise
subsequent calculations made by the system 75, and it is accordingly
desirable to carry out an auto-zeroing process whenever the state
machine 120 is in the PM Off State 122 and conditions indicate that
auto-zeroing is desirable.
[0064] In any case, if the controller 88 determines at step 200
that CTPW does not fall between MINAZW and MAXAZW, CTPW is too high
to consider auto-zeroing and execution of the routine advances to
step 254 for execution of a warning algorithm. If, on the other
hand, the controller 88 determines at step 200 that CTPW falls between
MINAZW and MAXAZW, the controller 88 is thereafter operable at step
202 to determine whether the auto-zero weight conditions are satisfied.
In one embodiment, the controller 88 is operable at step 202 to
determine whether the auto-zero weight conditions are satisfied
by determining whether a patient exiting zone flag, PEZ, is false,
thereby indicating that the patient is within the safe arming zone
of the mattress, or whether the corrected total patient weight,
CTPW is greater than a minimum auto-zero trigger weight, MINAZTW,
or that CTPW is less than a maximum auto-zero trigger weight, MAXAZTW.
As examples, MINAZTW may be in the range of -4 to -5 lbs., e.g.,
-4.1 lbs., and MAXAZTW may be in the range of 4-5 lbs., e.g., 4.1
lbs., although it will be understood that these values are provided
only by way of example, and that MINAZTW and MAXAZTW may take on
any desired values. In any case, if the controller 88 determines
at step 202 that any of these conditions is met, execution of the
routine advances to step 204 where the controller 88 is operable
to determine whether an auto-zero (AZ) timer should be reset. If,
on the other hand, the controller 88 determines at step 202 that
none of the foregoing conditions are met, the weight conditions
for auto-zeroing are not satisfied and execution of the routine
advances to step 254 for execution of a warning algorithm.
[0065] Generally, it is not desirable to reset the auto-zero timer
and to therefore avoid executing an auto-zeroing process if it appears
that CTPW has not shifted recently. In one embodiment of step 204,
if the current state of the auto-zero timer indicates that sufficient
time, e.g., 700 ms, has elapsed since the last time an auto-zero
was considered, if the bed 50 has not been articulated within a
sufficient time period, e.g., 1 second, and if CTPW has been stable
within a specified weight window, e.g., +/-2.5 lbs., for a specified
time period, e.g., 1 second, the auto-zero timer is reset. If all
of these conditions are not satisfied in this exemplary embodiment,
execution of the routine advances to step 254 for execution of a
warning algorithm. If, however, all of these conditions are satisfied
in the exemplary embodiment of step 204, execution of the routine
advances to step 206 where the controller 88 resets the auto-zero
timer.
[0066] Following step 206, the controller 88 is operable at step
208 to determine whether an auto-zero has not occurred since the
last patient exit from the bed 50, whether the corrected total patient
weight, CTPW, is less by a pound than the last auto-zero weight,
LAZW, and whether the change in the absolute sum of LF and RF since
the bed 50 was last zeroed by the user is greater than a head angle
threshold value, HATH. Generally, the angle of the head section
60e of the mattress relative to horizontal will affect the weight
distribution among the load cells 68a-68d. It is accordingly desirable
to account for the possibility that that head 60e of the mattress
60 may positioned at some angle other than what is was the last
time a bed weight zeroing process was executed. In the illustrated
embodiment, a change of the angle of the head section 60e of the
mattress 60 from zero to maximum head section elevation, e.g., 65
degrees, may significantly change the distribution of weight between
the head end load cells 68a and 68b and the foot end load cells
68c and 68. This can be detected by monitoring a change in the absolute
value of the weight impressed upon the foot end load cells 68c and
68d. In this embodiment, the head angle threshold value, HATH, is
chosen to be slightly less than one-half of this value, e.g., 8
lbs., since the head section 60e may have been at an elevated position
during the last bed zeroing process. In any case, if none of the
conditions of step 208 are satisfied, it is unlikely that the head
section 60e of the mattress 60 has moved since the last bed zeroing
event, and execution of the routine advances to step 254 for execution
a warning algorithm. If, however, the controller 88 determines that
any of the conditions of step 208 are satisfied, it is likely that
the head section 60e of the mattress 60 has moved since the last
bed zeroing event and steps 210-224 are then executed to determine
an appropriate head angle weight compensation value.
[0067] At step 210, the controller 88 tests the change in the absolute
sum of LF and RF against the head angle threshold value, HATH, to
determine if the head angle has increased since the last bed zeroing
event. If the change in the absolute sum of LF and RF exceeds HATH,
the head angle has increased since the last bed zeroing event and
execution of the routine advances to step 212 where the controller
88 is operable to determine whether the difference in the absolute
sum of LF and RF is between the head angle threshold value, HATH,
e.g., 8 lbs., and a higher head angle threshold, HHATH, e.g., 12
lbs. If so, a dynamic compensation weight, DCW, which will be added
to the bed zero weight at the end of the auto-zeroing process, is
assigned a first weight value, W1, e.g., 2 lbs., at step 214. If
not, the dynamic compensation weight, DCW, is assigned a second
greater weight value, W2, e.g., 3 lbs., at step 216.
[0068] If, at step 210, the controller 88 determines that a change
in the absolute sum of LF and RF is not greater than HATH, execution
of the routine advances to step 218 where the controller 88 tests
the change in the absolute sum of LF and RF against the negative
of the head angle threshold value, HATH, to determine if the head
angle has decreased since the last bed zeroing event. If the change
in the absolute sum of LF and RF is less than -HATH, the head angle
has decreased since the last bed zeroing event and execution of
the routine advances to step 220 where the controller 88 is operable
to determine whether the difference in the absolute sum of LF and
RF is between the negative head angle threshold value, -HATH, e.g.,
-8 lbs., and a lower head angle threshold, -LHATH, e.g., -12 lbs.
If so, a dynamic compensation weight, DCW, which will be added to
the bed zero weight at the end of the auto-zeroing process, is assigned
a negative first weight value, -W1, e.g., -2 lbs., at step 222.
If not, the dynamic compensation weight, DCW, is assigned a second
lesser negative weight value, -W2, e.g., -3 lbs., at step 224. It
will be understood that while numerical values are given above for
parameters such as HATH, HHATH, LHATH, W1 and W2, such numerical
values are provided only by way of example and each of these parameters
may alternatively be assigned different values.
[0069] Execution of the PM Off State algorithm advances from steps
214, 216, 222 and 224, as well as from the "NO" branch
of step 218, to step 226 where the controller 88 is operable to
determine the absolute weight on each of the load cells 68a-68d,
resulting in the absolute weight values ALH, ALF, ARH and ARF. In
the illustrated embodiment, the ALH, ALF, ARH and ARF values are
determined from values obtained during the state machine preparation
routine of FIG. 6, although these values may be updated at step
226 by taking new weight measurements. In any case, the controller
88 is operable following step 226 at step 228 to determine whether
a first auto-zero attempt flag, FAZA, is "false." If not,
this is the first execution of step 228 and a set of last auto-zero
weight variables may not be updated. Execution of the routine accordingly
advances from the "NO" branch of step 228 to step 230
where the controller updates the last auto-zero weight variables
with the current absolute weight values by setting LAZLH=ALH, LAZLF=ALF,
LAZRH=RH and LAZRF=ARF. The controller 88 is also operable at step
230 to set FAZA="false" so that the next execution of
step 228 results in the routine advancing to step 232. Execution
of the routine advances from step 230 to step 254 for execution
of a warning algorithm.
[0070] At step 232, the controller 88 is operable to set current
auto-zero weight values to the last auto-zero weight values by setting
AZLH=LAZLH, AZLF=LAZLF, AZRH=LAZRH and AZRF=LAZRF. Thereafter at
step 233, the controller 88 is operable to again update the last
auto-zero weight variables with the current absolute weight values
by setting LAZLH=ALH, LAZLF=ALF, LAZRH=RH and LAZRF=ARF. Thereafter
at step 234, the controller 88 is operable to determine whether
a constant head angle correction factor flag, CHACF, is "true."
If so, this indicates that a significant positive shift exists between
the head end load cells 68a-68b and the foot end load cells 68c-68d,
and that the sum of LH and RH are thus greater than their bed flat
values. The constant head angle correction flag, CHACF, is reset
("false") when a significant positive shift exists between
the foot end load cells 68c-68d and the head end load cells 68a-68b,
and the sum of LF and RF are thus greater than their bed flat values.
In any case, execution of the routine advances from the "YES"
branch of step 234 to step 236 where the current auto-zero weight
values, AZLH, AZLF, AZRH and AZRF, are computed each as a sum of
its current value, a constant head angle factor, CHAF, and the dynamic
head angle factor, DCW. The constant head angle factor, CHAF, is
a constant weight value indicative of added bed weight due to raising
of the head section 60e of the mattress 60, and an example value
may be 5.0 lbs., although it will be understood that other values
may be used. Following step 236, execution of the routine advances
to step 237 where the controller 88 is operable to determine whether
the dynamic head angle factor, DHAF, is greater than a minimum dynamic
head angle factor, MDHAF, e.g., 5. If so, the controller 88 is operable
thereafter at step 239 to set the constant head angle correction
flag, CHACF, to "false." From step 239, and from the "NO"
branch of step 237, execution of the routine advances to step 254
for execution of a warning algorithm.
[0071] If, at step 234, the constant head angle correction flag,
CHACF, is "false, a significant positive shift does not exist
between the head end load cells 68a-68b and the foot end load cells
68c-68d, and execution of the routine advances to step 238 where
the controller 88 is operable to compute a dynamic head angle factor,
DHAF, as a function of auto-zero weights, AZW, and bed flat zero
weights, BFZW. In one embodiment, DHAF=[(AZLF-BFLF)+(AZRF-BFRF)-(AZRH-BFRH)-(AZLH-BFLH)]/4,
where BFLH, BFLF, BFRH and BFRF correspond to bed flat LH, LF, RH
and RF values determined during a bed flat zero request as will
be described hereinafter with respect to FIG. 8. In any case, execution
of the routine advances from step 238 to step 240 where the controller
88 is operable to compare the dynamic head angle factor, DHAF, to
the sum of a maximum dynamic head angle factor value, MAXDHAF, and
the current value of the dynamic compensation weight, DCW. If the
controller 88 determines at step 240 that DHAF>MAXDHAF+DCW, execution
of the routine advances to step 242 where the controller is operable
to clamp the value of the dynamic head angle factor, DHAF, to MAXDHAF+DCW.
In one embodiment, MAXDHAF is chosen to correspond to a maximum
of 45 degree head section angle relative to horizontal.
[0072] If, at step 240 the controller 88 determines that DHAF is
not greater than MAXDHAF+DCW, execution of the routine advances
to step 244 where the controller 88 is operable to determine whether
the dynamic head angle factor, DHAF, is less than an unsafe maximum
dynamic head angle factor, USMAXDHAF. If so, the controller 88 is
operable thereafter at step 246 to set the constant head angle correction
flag, CHACF, to "true." If not, the controller 88 is operable
at steps 248 and 250 to clamp DHAF at zero if DHAF is less than
zero.
[0073] Execution of the PM Off State routine advances from steps
246 and 250, and from the "NO" branch of step 248, to
step 252 where the controller 88 is operable to update current values
of the auto-zero weights, AZLH, AZLF, AZRH and AZRF, each as a sum
of corresponding ones of the individual original zero weights, OZLH,
OZLF, OZRH and OZRF, and the dynamic head angle factor, DHAF. Following
step 252, the controller 88 is operable at step 254 to execute a
warning algorithm by comparing a difference between a common zero
total weight, CZTW, and the corrected total patient weight, CTPW,
to a first weight value, and to compare an absolute difference between
the corrected total patient weight, CTPW, and the frame weight,
FW, i.e., the weight of the mattress support frame 58 (see FIG.
9), to a second weight value, W2. If, at step 254 the controller
determines that (CZTW-CTPW)>W1 AND abs(CTPW-FW)>W2, the controller
88 activates a warning mechanism, e.g., visual or audible alarm,
at step 256 and thereafter at step 258 goes to the Freeze State
126 of the state machine 120 until the warning condition is addressed.
Alternatively, the controller 88 may wait or delay for some timer
period before activating the warning mechanism at step 256. In any
case, if both of these conditions are not met at step 254, execution
of the PM Off State routine advances to the return step 260. In
one embodiment, W1=4.5 lbs., W2=19 lbs., although other values may
be used. The warning algorithm of step 254 is intended to detect
removal of the mattress 60, as it activates a warning only if the
corrected total patient weight, CTPW, has decreased by at least
4.5 lbs. and the difference between CTPW and FW is more than 19
lbs. (the mattress may add at least 25 lbs. to the frame weight).
[0074] Referring now to FIG. 8, a flowchart is shown illustrating
an exemplary embodiment of a software routine for executing the
PM Zero State 124 of the state machine 120 of FIG. 5. The PM Zero
State routine begins at step 270 where the controller 88 is operable
to determine whether the routine was called pursuant to a common
zero request, CZR. A common zero request, CZR, is made manually
via pressing a specified combination of the switches forming part
of the control panel 70 (FIG. 2). In one embodiment, for example,
a common zero request is made by pressing the enable switch 74 for
at least 0.5 seconds and then releasing the enable switch 74, followed
by pressing the zero select switch 72 for at least 0.5 seconds.
It will be appreciated that other combinations of switches and switch
activation scenarios may be used to activate a common zero request,
and any such other switch combinations and/or switch activation
scenarios are intended to fall within the scope of the appended
claims. For example, the zero switch hold time may be a function
of the corrected total patient weight, CTPW, slow running average,
SRA, or fast running average, FRA, such that the time required for
the zero select switch 72 to be pressed in order to complete a common
zero request is a function of patient weight. In one embodiment,
the zero switch hold time may be directly proportional to patient
weight so that longer zero switch hold times are required as patient
weight increases, although other functional relationships between
the zero switch hold time and the patient weight may be used.
[0075] If, at step 270, the controller 88 determines that a common
zero request, CZR, is active, execution of the routine advances
to steps 272 and 274 where the controller 88 is operable to verify
that the zero switch 72 was activated for T1 seconds, e.g., T1=0.5
seconds, and thereafter released. If so, the controller 88 is operable
at step 276 to determine whether the weight impressed upon the load
cells 68a-68d is stable using one or more of the techniques described
hereinabove with respect to the PM Off State. If the controller
88 determines at step 276 that the weight is not stable, the controller
88 is thereafter operable at steps 282 and 284 to reset the weight
stable timer and set the last weight settling snapshot, LWSS, equal
to the corrected total patient weight, CTPW. Execution of the PM
Zero State routine advances from step 284 to a return step 286.
[0076] If, at step 276 the controller 88 determines that the weight
has been stable for a specified time period, execution of the routine
advances to step 278 where the controller 88 executes a zero capture
routine. An exemplary embodiment of the zero capture routine will
be described below with respect to FIG. 9. Following execution of
the zero capture routine at step 278, the controller 88 is operable
to clear the common zero request, CZR, and clear any frozen status
in case the PM Zero State routine was called when the state machine
120 was in the Freeze State 126.
[0077] If, at step 270, the controller 88 determines that a common
zero request, CZR, is not active, execution of the PM Zero State
routine advances to step 288 to determine whether a bed flat zero
request, BFZR, is active. A bed flat zero request, BFZR, is made
manually via pressing a specified combination of the switches forming
part of the control panel 70. In one embodiment, for example, a
bed flat zero request, BFZR, is made by pressing the volume switch
76 for at least 3.0 seconds, followed by pressing the OOB mode switch
84, and then releasing either the OOB mode switch 84 or the volume
switch 76 if that is also pressed. It will be appreciated that other
combinations of switches and switch activation scenarios may be
used to activate a bed flat zero request, and any such other switch
combinations and/or switch activation scenarios are intended to
fall within the scope of the appended claims.
[0078] If, at step 288, the controller 88 determines that a bed
flat zero request, BFZR, is active, execution of the routine advances
to step 290 where the controller 88 executes the zero capture routine,
an exemplary embodiment of which will be described below with respect
to FIG. 9. Following execution of the zero capture routine at step
290, the controller 88 is operable at step 292 to clear the bed
flat zero request, BFZR, and clear any frozen status in case the
PM Zero State routine was called when the state machine 120 was
in the Freeze State 126.
[0079] If, at step 288, the controller 88 determines that a bed
flat zero request, BFZR, is not active, execution of the PM Zero
State routine advances to step 294 to determine whether a frame
zero request, FZR, is active. A frame zero request, FZR, is made
manually via pressing a specified combination of the switches forming
part of the control panel 70. In one embodiment, for example, a
frame zero request, FZR, is made by pressing the volume switch 76
for at least 3.0 seconds, followed by pressing the Exit mode switch
82 for at least 10.0 seconds, and then releasing either the Exit
mode switch 82 or the volume switch 76 if that is also pressed.
It will be appreciated that other combinations of switches and switch
activation scenarios may be used to activate a frame zero request,
and any such other switch combinations and/or switch activation
scenarios are intended to fall within the scope of the appended
claims.
[0080] If, at step 294, the controller 88 determines that a frame
zero request, FZR, is active, execution of the routine advances
to step 296 where the controller 88 |