Backlash

The small bit of free play where the input reverses direction before the output actually starts moving.

Backlash is the gap left between meshing teeth in a transmission: when the input reverses direction, it has to travel through this gap before the output starts following it. It's usually measured in arcminutes. Backlash creates a small “dead zone” at every direction reversal, which lowers positioning accuracy and control stability — especially noticeable during fine manipulation or force control, and it can also cause oscillation in a control loop. Backlash varies a lot by gearbox type: a harmonic drive has close to zero, an RV reducer has very little, and an ordinary planetary gearbox has comparatively more. Whether the encoder sits on the motor side or the output side (a dual-encoder setup) also affects whether backlash can be detected and compensated for.

ExampleIf an arm's end effector sweeps back and forth over a small range and a joint has significant backlash, the end effector pauses briefly at each direction change before catching up, showing up as a small step in the trajectory.

Also called

Gear Play, Lost Motion

Related

Speed Reducer / Gearbox · Strain Wave Gear (Harmonic Drive) · Planetary Gearbox · Dual Encoder · Pose Repeatability

Sources

Wikipedia: Backlash (engineering)