Kinematic Calibration

运动学标定Advanced

Measuring a robot's actual end-effector positions to correct the model's link lengths, joint zero offsets, and other geometric parameters.

A controller computes end-effector position from a nominal set of kinematic parameters — such as DH parameters: link length, twist angle, offset, and joint zero position — but manufacturing tolerances and wear cause the real parameters to drift from the drawings, so the robot ends up with good repeatability (returning to the same point) but poor absolute accuracy (reaching a specified coordinate). Kinematic calibration corrects these parameters: the robot is driven through many poses, an external device such as a laser tracker or camera measures the true end-effector position, the measurements are compared against the model's predictions, and an optimization such as least squares solves for the parameter errors, which are then written back into the controller as a correction. Calibration is usually split into three levels: level one calibrates only the joint zero offsets (the gap between encoder readings and true angles), level two calibrates all the geometric parameters, and level three adds non-geometric errors such as joint flexibility and friction. According to Wikipedia, calibrating a six-axis industrial robot can improve absolute accuracy by roughly an order of magnitude, usually bringing the error down to under 1 millimeter.

ExampleOffline-programmed trajectories, or a single trajectory shared across several arms of the same model, will land in different places on an uncalibrated robot. One approach mounts a calibration board on the end effector, photographs it with a single camera across a few dozen poses, and optimizes the corrected DH parameters against pixel reprojection error (arXiv:2005.08420).

Also called: DH Parameter Calibration, Robot Calibration
Related: Denavit-Hartenberg (DH) Parameters · Modified Denavit-Hartenberg Parameters · Joint Zero-Position Calibration (Homing / Offset Calibration) · Absolute Positioning Accuracy · Pose Repeatability · Hand-Eye Calibration
Sources: Wikipedia: Robot calibration
A Visual Kinematics Calibration Method for Manipulator Based on Nonlinear Optimization (arXiv:2005.08420)
Bayesian Optimal Experimental Design for Robot Kinematic Calibration (arXiv:2409.10802)