Backdrivability
The property of a drive train that allows an external force on the output to rotate the input. Low-ratio, high-efficiency gear trains are backdrivable; high-ratio, low-efficiency gear trains (especially worm gears) are not. Backdrivable joints absorb impact forces by compliance; non-backdrivable joints transmit impact as impulsive shock to the motor.
Defined in 2 GAGE programs, which carry 4 distinct definitions of it. The wording above is taught in Foundations of Robotics and Physical AI.
How each discipline defines it
The same term does different work depending on who is using it. These are the definitions as each program teaches them, unedited.
The property of a drive train that allows an external force on the output to rotate the input. Low-ratio, high-efficiency gear trains are backdrivable; high-ratio, low-efficiency gear trains (especially worm gears) are not. Backdrivable joints absorb impact forces by compliance; non-backdrivable joints transmit impact as impulsive shock to the motor.
The property of a transmission that allows an external force applied at the joint to move the actuator in reverse, rather than the transmission locking up against that external force; relevant because a joint that is not backdrivable cannot meaningfully yield to contact regardless of what compliance element is present elsewhere in its design.
The property of an actuator that allows an external force at the output to move the mechanism without encountering resistance from the drive train or gearing. A backdrivable joint can "feel" and yield to external contact forces.
The property of a mechanism that allows external forces applied at the output to move the mechanism and be felt at the input (typically the motor). Low mechanical advantage (low gear reduction, direct drive) generally increases backdrivability; high mechanical advantage generally reduces it.
Terms it appears with
Not an alphabetical neighbourhood: these are the terms taught in the same lessons, ranked by how often they appear together.