Homing / Zero-Offset Calibration

Aligning each joint's encoder reading with the model's ‘zero degrees’ so the robot has a correct angle reference.

Robot models such as URDF define one specific pose as the configuration where every joint angle equals zero, but when an encoder is physically mounted, its own zero point is arbitrary. Zero-offset calibration measures and stores the fixed offset between the raw encoder reading and the model's angle so later readings can be corrected; homing is the process of moving a joint to a reference point to establish that offset. Joints with incremental encoders (which track only relative rotation) lose their position when powered off, so they must be re-homed at every power-up: the joint first creeps slowly toward a limit switch or hard stop, then uses the encoder's index pulse to find an exact reference — a routine that CiA 402-compliant servo drives can run on their own. Absolute encoders know their position as soon as they power on, so they usually need calibrating only once. Safety features like soft limits are defined relative to this zero point — if it drifts, the end-effector develops a systematic offset, and a policy trained on one robot may not transfer well to another.

ExampleXiaomi's CyberGear micro-motor has a ‘set mechanical zero’ command (communication type 6) that sets the current position as zero, but this is lost on power-off; Dynamixel servos instead use a persistent Homing Offset parameter, where reading = actual position + offset.

Also called

Homing, Zero Calibration, Homing Offset, Joint Zero Calibration

Related

Joint Zero-Position Calibration (Homing / Offset Calibration) · Joint Zero Calibration (Homing) · Incremental Encoder · Absolute Encoder · Soft Limits (Software Joint Limits) · Forward Kinematics (FK)

Sources

LinuxCNC Docs: Homing Configuration
ROBOTIS e-Manual: XM430-W350 (Homing Offset)
Xiaomi CyberGear 微电机说明书（英译版，含零位设置与控制模式） (Chinese)