Note: Descriptions are shown in the official language in which they were submitted.
21 84957
HUMAN LIMB LOAD SENSING DEVICE
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for
monitoring the amount of weight applied to the limb of a patient, particularly
5 useful during rehabilitative therapy.
BACKGROUND
After illness or injury an individual may require rehabilitative
therapy to help regain the use of an injured or atrophied limb. It is often
very beneficial during therapy, particularly in the case of individuals who are
10 having problems walking or standing, to have an aid for determining the
amount of weight being applied to the limb, as well as an aid for achieving
the proper weight distribution between the limbs.
Devices which attach to the limb of the individual during
therapy and provide audio feedback to the individual are known. These
15 devices include devices of the type having a sensor which attaches to the
limb for detecting the load applied to the limb. The device communicates an
audio signal to the individual indicating the load being applied. This audio
signal helps the individual to apply the correct amount of weight.
Some problems are associated with this type of device. The
20 known devices often indicate when weight is applied to the limb up to a
preset amount. At that level, the feedback signal cuts off. Consequently, it
is possible for an individual unintentionally to apply an excessive load on a
limb and become unbalanced.
The known devices also require calibration for each individual
25 before use. This often requires weighing the individual and manually
adjusting settings on the device. This is time consuming and makes it
difficult for an individual to use the device without assistance.
21 ~d 4957
SUMMARY
According to one aspect of the present invention there is
provided a method of producing feedback signals representative of the
magnitude of a load applied to a limb of a user, said method comprising:
attaching a load sensor to the limb;
applying a load to the sensor through the limb;
monitoring a load sensor output representative of the load
applied to the limb;
generating a load signal representative of the load signal
1 0 output;
generating a load threshold signal representing a desired
maximum load to be applied to the limb;
generating an overload signal representing an excessive load
applied to the limb;
comparing the load signal with the load threshold signal;
comparing the load signal with the overload signal;
generating a load feedback signal variable with the difference
between the load signal and the load threshold signal when the load signal is
less than the load threshold signal; and
generating an alarm feedback signal when the load signal is
greater than the overload signal.
The feedback thus includes both a normal load signal for
feedback when the load is within the desired range, and an overload alarm
to warn when an unbalanced condition is being approached. Where the
load threshold signal is less than the overload signal, no feedback signal is
generated when the load is between the desired maximum load and the
excessive load. The user is not then confronted with a sudden transition
21 84957
from the normal feedback signal to an alarm condition.
The two feedback signals are preferably an audible signals.
The load feedback signal may vary in frequency and magnitude with
differences between the load signal and the load threshold signal, while the
alarm feedback signal is a constant tone.
According to another aspect of the present invention there is
provided a method of monitoring a load applied to a limb of a user and
producing a signal that is variable in response to variations in the magnitude
of the load, said method comprising:
attaching a load sensor to the limb of the user for generating a
load sensor output representative of loads applied to the limb;
monitoring the load sensor output;
generating a load signal representative of the load sensor
output;
applying a peak load to the limb;
detecting a peak load signal;
recording the peak load signal;
selecting a proportion of the peak load for use as a load
threshold;
generating a load threshold signal equivalent to said selected
proportion the peak load signal;
comparing the load threshold signal with the load signal; and
generating a feedback signal representative of the difference
between the load threshold signal and the load signal.
This provides for automatic calibration simply by applying the
peak load to the sensor. The selection of the load threshold is based on the
activity to be carried out. It may be the maximum load, or 100% for
- 21 8~9~7
walking, or less for activities where loads are carried by both limbs at all
times.
According to another aspect of the present invention, there is
provided a limb load sensing apparatus for producing feedback signals
5 representative of the magnitude of a load applied to a limb of a user, said
apparatus comprising:
a load sensor adapted to be attached to the limb of the user for
producing a load sensor output representative of the load applied to the
limb;
load signal generating means coupled to the load sensor for
monitoring the load sensor output and for generating a load signal
representative of the load signal output;
load threshold signal generating means for generating a load
threshold signal representing a desired maximum load to be applied to the
1 5 limb;
overload signal generating means for generating an overload
signal representing an excessive load applied to the limb;
load feedback generator means for comparing the load signal
with the load threshold signal and generating a load feedback signal that
varies with the difference between the load signal and the load threshold
signal when the load signal is less than the load threshold signal; and
overload feedback generator means for comparing the load
signal with the overload signal and generating an alarm feedback signal
when the load siynal is greater than the overload signal.
The apparatus is preferably configured as a sensor pad, a small
control unit that can be worn on a belt and a set of headphones to provide
an audible feedback. A control knob on the unit allows the overload signal
21 84q~7
to be set to represent a load applied to the limb that is a set amount above
the desired maximum load.
The load signal generating means preferably vary both the
frequency and amplitude of the load feedback signal.
The apparatus may also include a calibrator having a peak load
responsive means for recording a peak load signal generated in response to
a peak load applied to the sensor and a load threshold signal generator for
selecting a proportion of the peak load signal as the load threshold signal.
This calibrates the control to a peak load applied to the sensor, recording
the peak sensor output.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings, which illustrate an exemplary
embodiment of the present invention:
Figure 1 is an illustration of the components of the apparatus;
Figure 2 is a top view of an alternative insole sensor pad;
Figure 3 is a cross sectional view of the alternative sensor pad
taken along line 3-3 of Figure 2;
Figure 4 is a block diagram showing the main components of
the apparatus;
Figure 5 is a partial circuit diagram showing a signal processing
circuit;
Figure 6 is a partial circuit diagram showing an alarm signal
enable/disable circuit;
Figure 7 is a partial circuit diagram showing a volume control
and audio circuit;
Figure 8 is a partial circuit diagram showing a multiplexer and
related circuitry;
21 84~57
Figure 9 is a circuit diagram of an indicator light control; and
Figure 10 is a circuit diagram of a calibrator reset circuit;
DETAILED DESCRIPTION
Referring to Figure 1, a limb load sensing device 10 has three
5 basic components. These are a sensor 12 for attachment to the foot of a
user, a control unit 14 for receiving and processing an output signal from
the sensor 12, and a set of headphones 16 for delivering an audible
feedback to the user. The control unit 14 may be carried or worn by the
user, for example on a belt.
The sensor 12 is responsive to an applied load and provides an
output that varies with the load applied to it by the limb. The sensor 12
comprises spaced apart electrically conductive plates 18 and a layer of non-
conducting, resiliently compressible material 20 between the plates 18. The
spaced apart plates 18 and compressible material 20 act as an electrical
15 capacitor. As a load is applied to the sensor 12 the resiliently compressiblelayer 20 is compressed an amount that varies with the applied load. This
varies the spacing between the plates 18, varying the capacitance of the
sensor 12, which is the sensor output. The sensor plates are connected to
the respective conductors of a two conductor cable 24. A jack 26 is
20 connected to the cable end for electrically connecting the sensor to the
control unit 14.
The sensor 12 is placed under an user's foot for use. In the
embodiment of Figure 1, the sensor is equipped with straps 28 that engage
around the user's foot or shoe to hold the sensor in place. The straps have
25 fasteners 30, for example the face to face fastener material sold under the
trade mark "Velcro" for coupling the straps. An alternative embodiment,
without straps and sized to fit within the user's shoe is shown in Figure 2.
21B4q57
The control unit 14 is a battery operated processor that
requires low voltage and current. The control unit has a housing 32 of
relatively small dimensions, e.g. 150mm x 75mm x 50mm. The unit has a
number of external controls, including an on/off switch 34, a volume control
5 36, an overload setting control 38, a threshold load setting control 40, a
sensor input connector 42, an audio output connector 44, and a battery
recharger connector 46. The unit also has a battery condition LED window
47.
As shown in the block diagram of Figure 4, the output of
10 sensor 12 is input to an oscillator and divider 48. The oscillator produces an
output signal with a frequency that is inversely proportional to the
capacitance of the sensor 12. This output goes to a low pass filter and DC
converter 50. The filter and converter attenuates the output from the
oscillator and converts it to a DC output with a voltage that varies with the
15 load applied to the sensor 12. The filter and converter output is supplied toa calibrator circuit 52. This stores the highest voltage that the converter
can produce.
The calibrator voltage is applied to a percentage load
adjustment circuit 54 to adjust the threshold load setting using the control
20 40 on the control unit 14.
The outputs from the percentage load adjustment circuit 54 and
the DC converter 50 are supplied to a feedback control comparator 56. The
output of this comparator goes negative when the output voltage from the
DC converter is greater than or equal to the voltage from the threshold load
25 adjustment circuit 54. The output of the feedback control comparator 66 is
used to switch the audio feedback tone on and off as will be discussed more
fully in the following.
2 1 8~
A delay circuit 58 is associated with the overload setting
control knob 38 on the control unit 14. The control adjusts the delay circuit
to provide a selectable overload on the sensor 12 in the range from zero to
45 Ibs.. Each quarter turn of the knob provides an increment corresponding
5 to approximately 15 Ibs.
The output from the delay circuit 58 and the output from
feedback control comparator 54 are summed using a summer 60. The
summed signal is applied to an alarm comparator 62 with the output of the
DC converter 50. When the signal from the summer is less than or equal to
10 the output from the DC converter, the comparator provides a positive output
that is used for turning on an alarm tone to indicate that the load on the
sensor is too high.
A multiplexer circuit 64 is a switching circuit for turning on and
off the feedback signals. The feedback signals from the oscillator are
15 delivered to the multiplexer. Alarm and load feedback signals are output to
an audio amplifier 66.
The multiplexer circuit 64 provides an alarm output signal when
the output from the alarm comparator 62 is positive. A load feedback
output signal is provided when the output from the load feedback control
20 comparator 56 is positive. The multiplexer will pass only one of the alarm
signal and the load feedback signal at a time, with the alarm signal
overriding the load feedback signal.
The audio amplifier 66 receives the alarm and load feedback
signals from the multiplexer 64 and provides audible alarm and load
25 feedback signals. The circuit also receives input from an audio volume
control 68 coupled to the DC converter 50 to modulate the load feedback
volume according to the output of the DC converter.
21 84~5;7
The audio amplifier circuit provides output to the headphones
16.
The control unit 14 includes an indicator light control circuit 70.
This indicates the battery charge level to the user using red and green
5 indicator lights.
The detailed circuit diagrams are shown in Figures 5 to 10.
Referring to Figure 5 a signal processor circuit is shown
generally at 72. This includes an RC sensor input circuit 74 consisting of
the sensor capacitance C3 connected in series with two resistors R11 and
10 R12. The sensor capacitance is connected to an input pin 9 of a 4060
integrated circuit oscillator 48. The resistors are connected to pins 10 and
11 respectively of the oscillator. The oscillator produces at pin 4 an output
signal with a frequency that is inversely proportional to the capacitance C3
of the sensor 12. Consequently, as the load applied to the sensor 12
15 increases and the capacitance C3 is reduced, the frequency of the signal
generator output increases.
The oscillator IC 48 also provides a pulsed output at pin 2, an
audio feedback tone signal at pin 14, and an alarm tone signal at pin 5.
The variable frequency output from oscillator pin 4 goes to the
20 positive input of a square wave generator 76 consisting of amplifier U1 and
resistors R13 and R14, which constitute a voltage divider 78. This
generator converts the oscillator output to a square wave form that is
delivered to a low pass filter 80. The low pass filter 80 includes resistors
R15 and R16, an amplifier U2 and feedback capacitor C6. This filter
25 attenuates the square wave output from the square wave generator 76 to
produce an output with an amplitude that varies inversely with its
frequency. Thus, the amplitude of the output from the low pass filter 80
21 84957
- ~o -
increases as the load applied to the sensor 12 increases.
The signal from the low pass filter is rectified and filtered by
the DC converter 82. This includes rectifier diode D4, resistor R17 and
capacitor C4. The DC converter 82 coverts the square wave output from
the low pass filter 80 to a solid non-oscillating DC signal output with a
voltage that varies with the load applied to the sensor 12.
The DC converter output is supplied to the calibrator circuit
52. This includes diode D5, a high capacitance capacitor C5 and an output
resistor R18. The output from the DC converter charges the capacitor,
10 which stores the highest voltage that the converter can produce, less a
small voltage drop across the diode.
The calibrator voltage is applied to the positive input of the load
adjustment circuit 54. This is a unity gain follower 84, including amplifier
U4, feedback resistor R19, and variable resistor R1A. The variable resistor
is used to adjust the threshold load setting using the control 40 on the
control unit 14.
The outputs from the load adjustment circuit 54 and the DC
converter 82 are supplied to the feedback control comparator 56. The load
adjustment circuit output is delivered to the positive input and the converter
output is delivered to the negative input. The output of this comparator
goes negative when the output voltage from the DC converter 82is greater
than or equal to the threshold voltage set by the load adjustment circuit 54.
The output of the feedback control comparator 56is used to switch the
audio feedback tone on and off as will be discussed more fully in the
following.
Referring to Fi~qure 10, the calibrator 52is reset by a reset
switch circuit 86 when power to the control unit 14is turned off. The reset
21 ~49~7
- 11 -
circuit includes a transistor switch Q7. The base of the transistor is
connected to the control unit on/off switch 34 through resistor R32. The
collector is connected to capacitor C5, and the emitter is connected to
~round through resistor R31. When the on/off switch 34 is switched off the
5 base of the transistor Q7 is connected to the battery through resistor R32,
turning Q7 on to discharge the capacitor C5 through resistor R31. The
calibrator 52 is therefore reset every time the control unit 14 is turned off.
Referring to Figure 6 there is illustrated an overload alarm
circuit 88. This includes a voltage divider 90 including resistor R20 and
10 variable resistor R23. The overload setting control knob 38 on the control
unit 14 adjusts the variable resistor to provide a selectable overload voltage.
This overload voltage represents a load on the sensor 12 in the range from
zero to 45 Ibs.. Each quarter turn of the knob provides a voltage increment
corresponding to approximately 15 Ibs.
The output from voltage divider 90 and the threshold load
signal output from load adjustment circuit 54 are summed using voltage
summer 60 consisting of a resistor R21 connected to the follower output, a
resistor R22 connected to the voltage divider 90, an amplifier U5 with its
inverting input connected to the resistors R21 and R22, and feedback
20 resistor R24. The summed voltage is inverted by an inverter U6 and applied
to the negative input of the comparator 62. The output of the DC converter
82 is applied to the positive terminal of the comparator 62. When the signal
from the inverter U6 is greater than the output from the DC converter 82
the comparator provides a negative voltage output, and when the summed
25 voltage output from the inverter U6 is less than or equal to the output from
the DC converter 82 the comparator provides a positive voltage output. A
positive output is used for turning on an alarm tone to indicate that the load
21 84~S7
- 12 -
on the sensor is too high.
The multiplexer circuit 64 is shown in Figure 8. This is a
switching circuit for turning on and off the feedback signals. The circuit
includes a 4019 quad and/or select gate 94. The audio feedback tone signal
5 from pin 14 of the oscillator is delivered to pin 15 and the alarm tone signalfrom pin 5 of the oscillator is delivered to pin 3. Alarm and counter tone
outputs are at pins 12 and 13 respectively.
The gate 94 receives input at pin 14 from the overload alarm
comparator through an alarm enable/disable front end 96. This includes a
10 rectifying diode D3 and an RC filter with resistor R10 and capacitor C1. The
gate 94 receives input at pin 9 from the feedback control comparator 56
through a load feedback enable/disable front end 98. This includes a
rectifying diode D2 and an RC filter consisting of resistor R9 and capacitor
C2.
The multiplexer circuit 64 provides an alarm output signal at pin
12 when the output from the alarm enable/disable front end 96 is positive.
A load feedback output signal is provided at pin 13 when the output from
the load feedback enable/disable front end 98 is positive. The gate 94 will
pass only one of the alarm signal and the load feedback signal at a time,
20 with the alarm signal overriding the load feedback signal.
The audio volume control and amplifier circuits 66 and 68 are
shown in Figure 7. These circuits receive the alarm and load feedback
signals from the multiplexer 64 and provide audible alarm and load feedback
signals. The volume control circuit 68 includes a load signal amplifier 100.
25 A transistor Q1 receives the output of the DC converter 82 at its base
through a resistor R1B. Resistor R1B is ganged with resistor R1A of the
load adjustment circuit 54. The collector and emitter are connected across
21 849~7
the battery through resistors R2 and R3. The collector is also connected to
the base of transistor Q2 and to ground through resistor R4. The load
feedback signal from pin 13 of the multiplexer gate 94 is applied to the
collector of transistor Q2 to be modulated according to the output of the DC
5 converter 82.
As load is increased on the sensor 12 the voltage output of the
DC converter 82 increases. The increased voltage at the base of transistor
Q1 reduces the voltage drop across the transistor and hence the voltage at
the base of transistor Q2. The load feedback signal through transistor Q2 is
reduced. The inverting amplifier circuit therefore generates a feedback tone
output signal which is attenuated as the output voltage from the DC
converter 82 increases from zero to the threshold voltage. When the
threshold level is reached, the signal to pin 9 of multiplexer gate goes
negative and is cut off by diode D2, cutting of the load feedback output
from pin 13.
The output from the emitter of transistor Q2 is delivered
through diode D1 to a logarithmic audio pot R4 which serves as a volume
control. The volume control 36 on the control unit 14 allows for the
adjustment of the pot R4.
The output from the audio volume control is passed to the
audio amplifier circuit 66. The audio amplifier circuit 66 includes two
transistors Q3 and Q4 arranged as a darlington pair. The audio amplifier
circuit 66 provides output to the headphones 16.
The audio alarm signal output from the multiplexer 64 is applied
to the audio amplifier circuit 66 at the base of transistor Q3 through a
resistor R6. The audio amplifier circuit thus communicates an alarm tone of
constant volume to headphones 16.
2 1 8~19 ~7
- 14 -
The control unit 14 is powered by two 9 volt rechargeable
Nicad batteries 104 (Figure 1). A 24 volt DC transformer is used for
recharging the batteries 104. The charger plugs into the recharger jack 46
in the control unit 14.
The indicator light control circuit 70 is shown in Figure 9. This
indicates the battery charge level to the user using a green indicator light D7
and a red indicator light D8. The green indicator light D7 indicates sufficient
battery charge and the red indicator light D8 indicates low battery power
when the battery charge is below a minimum acceptable level. Both the
10 green and red lights D7 and D8 are LEDs.
Battery voltage is applied to the cathode of zener diode D6
through resistor R27. The zener provides a constant voltage on the
inverting terminal of a comparator U8. This voltage is the lowest acceptable
battery voltage. The battery voltage is also applied to a potentiometer R58
15 connected to the non-inverting input of comparator U8. The output of the
comparator is applied to the base of a PNP transistor Q5 and to the collector
of an NPN transistor Q6. The collector of transistor Q5 is connected to
battery voltage, while the emitter is connected through a current limiting
resistor R29 to the cathode of red LED D6. The base of transistor Q6 is
20 connected to the pulsed output at pin 2 of the oscillator. The emitter of
transistor Q6 is connected through a current limiting resistor R30 to the
anode of green LED D7.
When the battery voltage is above the lowest acceptable
voltage the voltage output of the potentiometer R58 is greater than the
25 constant voltage from the diode D6. This results in a positive voltage
output from the comparator U8. The transistor Q6 receives the positive
comparator output and a pulsed signal at its base. This pulses the transistor
21 849~7
Q6 on and off connecting the green LED D7 intermittently to the comparator
output and pulsing the light D7.
When the battery voltage drops to the lowest acceptable
operating voltage, the voltage output of the potentiometer R58 falls just
below the zener voltage. This results in a zero output voltage from the
comparator. This turns on the PNP transistor Q5 and applies battery voltage
to the red LED D8.
In use the device 10 operates to give the user an audio
feedback response to his/her weight shifting. The user will hear a feedback
tone of decreasing frequency and volume as weight is applied on the sensor
12. Conversely as weight on the sensor 12 is decreased the volume and
frequency of the feedback tone will increase. At the selected threshold load
setting the signal will stop.
This feature will allow the user to accurately know when any
desired percentage of full load (threshold load) has been reached. At the
threshold load setting the audio tone will stop as the user shifts his/her
weight through the threshold load. If weight continues to be applied to the
sensor 12 a predetermined overload will be reached and an alarm will sound
to indicate the overload condition to the user.
The device 10 is self-calibrating to the user's weight when
being used, and may be reset by turning device off. Calibration is achieved
by bearing the maximum load on the sensor 12 and holding it momentarily.
This feature enables the user to calibrate the device 10 quickly and
automatically.
To use the device, the control unit 14 is turned on with on/off
switch 34. If the green LED light D7 is on, this indicates that the battery is
charged sufficiently to operate the unit. If the red LED light D8 is on then
2~ 84q57
- 16-
the device must be recharged before it is to be used. The sensor 12 is
strapped to the shoe 27 of user securely enough so the sensor 12 does not
slip but not so it is over tight. The control unit 14 is strapped to the waist
of the user, the sensor 12 is plugged into the jack 42 labeled "Sensor" and
5 the headphones are plugged into the jack 44 labeled "Phones".
When the device 10 is turned on, a tone should be heard. The
user then now apply full load on the limb to which the sensor 12 is
attached. This action will calibrate the device 10 to the full weight of the
user.
The volume control dial 36 can be used to set the signal volume to
any comfortable level of audio output. The threshold load setting dial 40
may be used to set the threshold load to any percentage of full load, and the
overload setting dial 38 may be used to set the amount of weight that must
be applied to sensor 12 above the threshold load before the alarm will come
15 on.
While one embodiment of the present invention has been
described in the foregoing, it is to be understood that other embodiments
are possible within the scope of the invention. The invention is to be
considered limited solely by the scope of the appended claims.
