Robotics
Backdrivability
Definition
Backdrivability is the ability of an external load applied at a mechanism’s output to drive motion back through its transmission. In a robot joint, it describes how readily an outside force can move the joint and its actuator.
Also known as: Backdrivability of an actuator, Backdriveability
Updated
Moving the joint from the outside
If a person pushes a robot's hand and the joint transmission moves the motor in response, the mechanism is being backdriven. Friction, gear reduction, rotor inertia, and the load all influence how much force is required and how the motion develops.
Modern Robotics explains why the apparent motor inertia increases with the square of the gear ratio in an ideal transmission. This is one reason low-ratio designs can feel different from heavily geared joints when moved externally.
Why it matters for interaction
Gealy and colleagues connect backdrivable actuation with force-controlled manipulation. Katz's actuator work likewise uses low-ratio transmissions to support torque control in dynamic robots. Such mechanisms can be useful for teleoperation and for responding to contact without a large commanded position error.
Passive mechanics and active behavior differ
A mechanically backdrivable joint can still resist motion when its controller commands a high stiffness. Conversely, feedback can make a less backdrivable mechanism yield actively. A compliant spring also permits some output motion without requiring the gearbox to turn. Describe whether backdrivability was measured with power off, gravity compensation, or an active controller; these conditions do not represent the same property.
Sources
Related terms
Quasi-direct drive
Quasi-direct drive is an actuation approach that combines a torque-capable motor with a relatively low transmission reduction to preserve useful backdrivability and force-control behavior. It differs from direct drive because it still uses a transmission.
Series elastic actuator
A series elastic actuator places an elastic element in the force-transmission path between the drive and its load. Measuring the element’s deflection can support force or torque feedback while the elasticity changes the actuator’s response to impacts.
Torque control
Torque control regulates the turning effort delivered by an actuator or robot joint. It provides an actuation interface from which motion, force, and impedance controllers can produce the joint torques their tasks require.
Teleoperation
Teleoperation is the control of a robot by a human operator from a separate location or interface. The operator supplies commands while feedback, such as camera images or the robot's motion, helps them guide the task.