1) The motor couples to the output through a drivetrain with low reflected inertia and low friction (e.g. quasi-direct-drive, low gear ratio). 2) An external force at the output easily turns the motor backwards (transparency). 3) Output torque is estimated from motor current/torque (proprioceptive actuator), without a separate force sensor. 4) The controller implements torque/impedance control (compliance), enabling safe contact, impact absorption and kinesthetic guiding of the arm.
Self-locking drives (high gear ratios) are stiff, opaque and unsafe on contact and hard to force-control. Back-drivable actuation provides compliance, torque control and safe physical interaction, often without a dedicated force sensor.
Typically a BLDC/PMSM motor capable of torque control without a high gear ratio.
Official
Low gear ratio and an efficient transmission minimizing reflected inertia and friction (back-drivability).
Official
Sensing output torque from motor current, without a dedicated force sensor.
Official
A control loop implementing compliance and the desired output torque.
Official
A low gear ratio improves back-drivability but requires a larger/heavier motor for a given torque.
Friction, torque ripple (cogging) and temperature introduce errors in sensorless force estimation.
A back-drivable drive is not self-locking - on power loss the joint may drop under load.
Pratt and Williamson introduce series elastic actuators - an early approach to compliant, force-controlled actuation.
Wensing et al. describe the proprioceptive actuator design in the MIT Cheetah - a highly back-drivable quasi-direct-drive for dynamic locomotion and physical interaction.
Back-drivable actuators (often quasi-direct-drive) become standard in modern humanoids and cobots for safety and data collection.
Time complexity: Nie dotyczy (wlasciwosc mechatroniczna napedu, nie algorytm). Space complexity: Nie dotyczy.
Back-drivability itself is a mechanical property; what matters computationally is a fast (kHz) torque/impedance control loop and current-based torque estimation.
A lower gear ratio means more back-drivability and transparency but lower output torque.
Quasi-direct-drive/planetary (back-drivable) vs worm/harmonic with high friction (less back-drivable).
Current-based estimation (proprioceptive) vs a dedicated torque sensor.
This describes the actuator control loop; back-drivability itself is a mechanical property, not a compute paradigm.
Each actuator/joint has its own torque control loop, easy to parallelize in hardware.
A mechatronic actuator property, independent of the compute accelerator type; the control loop typically runs on the drive's microcontroller/DSP.
