What Is a Dual Absolute Encoder?

A dual absolute encoder configuration is the encoder architecture of high-end joint motors: one absolute encoder on the motor rotor (input side, before reduction) and one on the output flange (output side, after reduction), measuring angle directly at both ends.

Why Two Encoders?

With only a motor-side encoder, output angle must be inferred as "motor angle ÷ reduction ratio" — and gearbox backlash, elastic deformation, and assembly tolerance all pull that estimate away from the true joint angle. An output-side encoder measures the real post-reduction angle directly, so the control loop closes on ground truth:

• Higher precision: angle errors from backlash and deformation are eliminated.
• Zero-calibration startup: absolute encoders retain position through power loss, so the robot knows every joint pose at power-on with no homing routine.
• Redundancy: input and output angles cross-check each other, catching anomalies such as gearbox slip.

Implementation

The BXI 85/70/50-series joint motors pair a magnetic encoder on the input with an inductive encoder on the output — two sensing principles that don't interfere — achieving true dual-encoder closed loop in a compact package.