Rotary Encoder

A position sensor mounted on a motor or joint that converts shaft angle into an electrical signal.

A rotary encoder is an electromechanical device that converts a shaft's angular position or amount of rotation into an analog or digital signal; a robot has at least one on every joint, telling the controller ‘how many degrees have I turned right now.’ There are two output types: incremental encoders only report how much they've turned, usually via two channels A and B offset by 90° in phase to determine direction, and they lose absolute position when power is cut, needing a homing routine afterward; absolute encoders directly report the current angle and keep it through a power loss. The underlying principle can be optical, magnetic, capacitive, or inductive, with resolution given as pulses per revolution or in bits. Joint modules often mount one encoder on the motor side and another at the reducer's output — a dual-encoder setup — so the true output-side angle is measured directly, canceling out the error introduced by gear backlash (the small play between gear teeth). The joint-angle information in a policy's proprioceptive input comes from encoder readings like these.

ExampleA 14-bit absolute magnetic encoder divides a full turn into 2^14 = 16,384 parts, giving an angular resolution of about 360° ÷ 16,384 ≈ 0.022°.

Also called

Joint Encoder, Angle Encoder

Related

Absolute Encoder · Incremental Encoder · Magnetic Encoder · Dual Encoder · Proprioception · Joint Actuator Module

Sources

Wikipedia: Rotary encoder