Dual Encoder

A joint with one encoder at the motor and another at the output, reading both the motor's angle and the joint's true angle.

A dual encoder is a joint-module configuration: one encoder (a sensor that measures rotation angle) sits on the motor shaft, measuring how far the motor has turned, while a second sits at the gearbox's output, directly measuring where the joint actually is. With only a motor-side encoder, the joint angle has to be inferred as “motor angle divided by gear ratio,” and errors creep in from the gearbox's backlash and the flexspline's elastic deformation. Adding an output-side encoder lets the system read the real joint position directly, so it also knows its absolute position the instant it powers on, without needing to home first. The difference between the two readings can also be used to estimate transmission deformation and load torque. This improves positioning accuracy and safety, at the cost of more expense, bulk, and wiring. High-end collaborative arms and many humanoid-robot joint modules use dual encoders.

ExampleA harmonic-drive joint might use a high-resolution incremental encoder at the motor for its velocity loop, and an absolute encoder at the output to read the true joint angle.

Related

Rotary Encoder · Absolute Encoder · Joint Actuator Module · Backlash · Strain Wave Gear (Harmonic Drive) · Homing / Joint Zero-Offset Calibration

Sources

Rotary encoder - Wikipedia