Controlling power in a prosthesis or orthosis based on predicted walking speed or surrogate for same
In some embodiments of a prosthetic or orthotic ankle/foot, a prediction is made of what the walking speed will be during an upcoming step. When the predicted walking speed is slow, the characteristics of the apparatus are then modified so that less net-work that is performed during that step (as compared to when the predicted walking speed is fast). This may be implemented using one sensor from which the walking speed can be predicted, and a second sensor from which ankle torque can be determined. A controller receives inputs from those sensors, and controls a motor's torque so that the torque for slow walking speeds is lower than the torque for fast walking speeds. A controller determines a desired torque based on the output, and controls the motor's torque based on the determined desired torque.
1. An ankle-foot prosthesis or orthosis apparatus comprising:
a shank member;
a foot member that is operatively configured with respect to the shank member so as to support walking and permit the foot member to plantarflex and dorsiflex with respect to the shank member;
a motor configured to plantarflex the foot member with respect to the shank member;
a series elastic element connected between at least one of (a) the motor and the shank member and (b) the motor and the foot member;
a first sensor having an output serving as a surrogate for walking speed; and
a controller configured to:
determine, based on the output of the first sensor, a control value corresponding to at least one of: an angular rate ω x of the shank member at a start of controlled dorsiflexion, an angular rate of a leg section above a knee of a user of the apparatus, a knee linear moving velocity in a stance phase, or a Cartesian trajectory of an ankle joint of the apparatus or the knee, and
control the motor's torque, based on the determined control value, so that the motor's torque for slow walking speeds is lower than the motor's torque for fast walking speeds.
2. The apparatus of claim 1 , wherein the motor is also configured to dorsiflex the foot member with respect to the shank member.
3. The apparatus of claim 1 , wherein the first sensor comprises at least one of an angular rate sensor or an inertial measurement unit (IMU).
4. The apparatus of claim 1 , wherein the controller is configured to (i) determine, based on the output of the first sensor, a control gain that varies with walking speed, wherein the control gain at slow walking speeds is lower than the control gain at fast walking speeds and (ii) determine a desired motor torque based on the control gain and a determined ankle torque, wherein controlling the motor's torque comprises driving the motor to achieve the desired motor torque.
5. The apparatus of claim 1 , wherein the controller is configured to
(i) determine, based on the output of the first sensor, the angular rate ω x of the shank member,
(ii) determine a control gain Kv(ω x ) that is a function of the angular rate, wherein the control gain at low angular velocities is lower than the control gain at high angular velocities, and
(iii) determine a desired motor torque based on the equation
Motor Torque= Kv (ω x )× pff ×(normalized_Torque) n ,
where pff is a constant and n is between 2 and 7, wherein controlling the motor's torque comprises driving the motor to achieve the desired motor torque.
6. The apparatus of claim 5 , wherein Kv(ω x )=0 when ω x =0, Kv(ω x )=1 when ω x exceeds a threshold ω TH , and Kv(ω x ) is a monotonically increasing function between ω x =0 and ω TH .
7. The apparatus of claim 6 , wherein the motor is also configured to dorsiflex the foot member with respect to the shank member.
8. The apparatus of claim 1 , further comprising a second sensor having an output from which ankle torque can be determined, and wherein the controller is configured to control the motor's torque based in part on the output of the second sensor.
9. The apparatus of claim 8 , wherein the second sensor measures ankle torque directly.
10. The apparatus of claim 8 , wherein the second sensor has a second sensor output from which a deflection of series elastic element can be determined, and the controller computes the torque based in part on the second sensor output.
11. The apparatus of claim 8 , wherein the second sensor senses a position of the motor, the apparatus comprises a third sensor that senses an angle of the foot member with respect to the shank member, and the controller computes the torque based on the sensed position of the motor and the sensed angle.
12. The apparatus of claim 8 , wherein the second sensor senses a position of the motor, the apparatus comprises a third sensor that senses an angle of the foot member with respect to the shank member, and the controller determines a torque component Γ S based on the sensed position of the motor, the sensed angle, and a torque vs. deflection characteristics of the series elastic element.
13. The apparatus of claim 12 , further comprising a bumper that is compressed when the foot member is sufficiently dorsiflexed with respect to the shank member,
wherein the controller determines a torque component FB based on the sensed angle and a torque vs. deflection characteristics of the bumper, and
wherein the controller determines a total torque based on Γ s and Γ B .
14. The apparatus of claim 1 , wherein control of the torque of the motor is based on non-linear positive feedback according to the output of the first sensor.
15. The apparatus of claim 14 , wherein the non-linear positive feedback involves adjustment of a parameter of a power control gain function, the parameter comprising at least one of a power control gain or a power exponent.
16. A method, comprising:
receiving a sensor output associated with an ankle-foot prosthesis or orthosis, the sensor output serving as a surrogate for a walking speed of a user of the prosthesis or orthosis;
determining, based on the sensor output, a control value corresponding to at least one of: (i) an angular rate w of a shank member at a start of controlled dorsiflexion, the shank member operatively connected to a foot member of the prosthesis or orthosis so as to support walking and permit the foot member to plantarflex and dorsiflex with respect to the shank member, (ii) an angular rate of a leg section above a knee of a user of the prosthesis or orthosis, (iii) a knee linear moving velocity in a stance phase, or (iv) a Cartesian trajectory of an ankle joint of the prosthesis or orthosis or the knee; and
transmit a torque control signal to a motor of the prosthesis or orthosis, the torque control signal being calculated based on the determined control value and configured to control the motor's torque so that the motor's torque for slow walking speeds is lower than the motor's torque for fast walking speeds.
17. The method of claim 16 , wherein the determining comprises determining a control gain that varies with walking speed, wherein the control gain at slow walking speeds is lower than the control gain at fast walking speeds, and further comprising:
determining a desired motor torque based on the control gain and a determined ankle torque, wherein
the control signal is configured to drive the motor to achieve the desired motor torque.
18. The method of claim 16 , further comprising:
(i) determining, based on the sensor output, the angular rate ω x of the shank member,
(ii) determining a control gain Kv(ω x ) that is a function of the angular rate, wherein the control gain at low angular velocities is lower than the control gain at high angular velocities, and
(iii) determining a desired motor torque based on the equation
Motor Torque= Kv (ω x )× pff ×(normalized_Torque) n ,
where pff is a constant and n is between 2 and 7, wherein the control signal is configured to drive the motor to achieve the desired motor torque.
19. The method of claim 18 , wherein Kv(ω x )=0 when ω x =0, Kv(ω x )=1 when ω x exceeds a threshold ω TH , and Kv(ω x ) is a monotonically increasing function between ω x =0 and ω TH .