Biomimetic joint actuators
In a powered actuator for supplying torque, joint equilibrium, and/or impedance to a joint, a motor is directly coupled to a low-reduction ratio transmission, e.g., a transmission having a gear ratio less than about 80 to 1. The motor has a low dissipation constant, e.g., less than about 50 W/(Nm) 2 . The transmission is serially connected to an elastic element that is also coupled to the joint, thereby supplying torque, joint equilibrium, and/or impedance to the joint while minimizing the power consumption and/or acoustic noise of the actuator.
1. A powered human augmentation device comprising:
a low revolutions-per-minute (RPM) motor having an RPM less than 1500 configured to apply torque to a joint of the human augmentation device during a phase of a gait cycle to modulate at least one of joint augmentation torque, joint impedance, or joint equilibrium during the phase of the gait cycle;
a transmission coupled directly to the low RPM motor; and
a controller configured to short leads of the motor to exert a viscous damping torque on the motor proportional to a motor dissipation constant given by R/k t 2 , where R refers to a stator winding resistance and k t refers to a motor torque constant, the dissipation constant being less than 50 W/(Nm) 2 .
2. The device of claim 1 , wherein the motor comprises a high-torque motor supplying motor torque of at least about 0.06 Nm/kg.
3. The device of claim 1 , wherein the device is adapted to be backdrivable.
4. The device of claim 1 , wherein the transmission has a gear ratio less than about 80:1.
5. The device of claim 1 , wherein:
the motor comprises an external rotor; and
the transmission comprises a cable and a joint output pulley, the cable coupling the external rotor and the joint output pulley.
6. The device of claim 1 , further comprising:
a motor encoder adapted to measure angular displacement of a rotor of the motor with respect to a stator of the motor; and
a joint encoder adapted to measure angular displacement of the joint about a joint pivot.
7. The device of claim 6 , wherein at least one of the motor encoder and the joint encoder is selected from a group comprising: an absolute encoder, or a magnetic encoder having at least 13-bit resolution.
8. A method for augmenting joint function using a powered human augmentation device, the method comprising:
energizing a low revolutions-per-minute (RPM) motor having an RPM less than 1500 to apply torque to a joint of the human augmentation device during a phase of a gait cycle to modulate at least one of joint augmentation torque, joint impedance, or joint equilibrium during the phase of the gait cycle, the motor coupled directly to a transmission; and
shorting leads of the motor to exert a viscous damping torque on the motor proportional to a motor dissipation constant given by R/k t 2 , where R refers to a stator winding resistance and k t refers to a motor torque constant, the dissipation constant being less than 50 W/(Nm) 2 .
9. The method of claim 8 , wherein the viscous damping torque is exerted on the motor during a loss of battery power.
10. The method of claim 8 , wherein the dissipation constant is about 10 W/(Nm)2.
11. The method of claim 8 , further comprising energizing the motor to apply stiffness to the joint, the elastic element configured to store energy and to release the stored energy as power, the motor applying power to augment the power of the elastic element to achieve a positive torque feedback response.
12. The method of claim 8 , further comprising:
energizing the motor to apply the torque to achieve a desired joint equilibrium position; and
shorting leads of the motor during a stance phase of the gait cycle to substantially maintain the joint equilibrium position during a portion of the stance phase.
13. The method of claim 8 , further comprising:
measuring an angular displacement of a rotor of the motor with respect to a stator of the motor;
measuring an angular displacement of a structure with respect to the joint;
determining, using a hardware controller, a state of the elastic element based, at least in part, on both the angular displacement of the rotor and the angular displacement of the structure;
computing, based at least in part on the state of the elastic element and the angular displacement of the motor, a torque contribution of the motor using the hardware controller; and
adjusting the modulating, based at least in part on the computed contribution of the motor torque.
14. A powered human augmentation device comprising:
a low revolutions-per-minute (RPM) motor having an RPM less than 1500 configured to apply torque to a joint of the human augmentation device during a phase of a gait cycle to modulate at least one of joint augmentation torque, joint impedance, or joint equilibrium during the phase of the gait cycle;
a transmission coupled directly to the low RPM motor;
an elastic element serially coupled to the transmission, the elastic element coupled to the joint; and
a controller configured to short leads of the motor to exert a viscous damping torque on the motor.
15. The device of claim 14 , wherein the elastic element comprises a spring, the device further comprising a cable and a joint output pulley, the cable being coupled to both the spring and the joint output pulley.
16. The device of claim 14 , wherein the transmission comprises a ball-screw transmission having a ball nut coupled to the motor rotor and the screw coupled to the elastic element.
17. The device of claim 14 , wherein the motor comprises a high-torque motor supplying motor torque of at least about 0.06 Nm/kg.
18. The device of claim 14 , wherein the device is adapted to be backdrivable.
19. The device of claim 14 , wherein the transmission has a gear ratio less than about 80:1.
20. The device of claim 14 , further comprising:
a motor encoder adapted to measure angular displacement of a rotor of the motor with respect to a stator of the motor; and
a joint encoder adapted to measure angular displacement of the joint about a joint pivot.