Coordinate Positioning Machine Calibration With Pivot-Pose Ballbar
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Solution Overview
Problem
Calibrating articulated robots is challenging due to cumulative positional errors from multiple joints, and existing methods like laser trackers are expensive and slow, while other techniques require complex setups and multiple measurements.
Innovation Solution
A method involving a pivot pose and target poses to determine new model parameters using a length-measuring device like a ballbar, with an adaptable system that allows for quick and accurate calibration by varying only a subset of parameters, and an adaptor with a part-spherical bearing surface for precise measurements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If laser trackers are used for calibration, then measurement precision is improved, but cost and calibration time increase significantly
Solution Approach 1:
The patent extracts the essential calibration function from complex laser tracker systems by using a simplified ballbar mechanism that measures only the distance between two points (pivot point and tool center point), eliminating the need for comprehensive laser scanning while maintaining sufficient accuracy for robot calibration
Solution Approach 2:
The patent employs inexpensive ballbar measurement devices and simple spherical targets instead of expensive laser trackers, accepting that the measurement system itself is relatively simple and can be easily replaced or recalibrated without significant cost
2Measurement precision
If traditional calibration techniques are used, then comprehensive machine geometry characterization is achieved, but device complexity and setup complexity increase
Solution Approach 1:
The patent extracts only the essential calibration parameters (tool center point position and orientation) using a simplified measurement approach with ballbar, eliminating the need for complex multi-point scanning and comprehensive geometric characterization while focusing on the most critical calibration needs
Solution Approach 2:
The calibration system uses the robot's own motion capabilities to perform self-calibration by moving the tool between known positions and comparing measured distances with expected distances, reducing the need for external complex measurement equipment and manual setup
3Measurement precision
If multiple measurements and user inputs are required, then calibration accuracy is improved, but ease of operation deteriorates
Solution Approach 1:
The patent merges multiple calibration measurements into a unified ballbar distance measurement process, where a single measurement between the pivot point and tool center point incorporates information about tool position and orientation, reducing the number of separate measurements and user inputs required
Solution Approach 2:
The calibration process uses feedback from the robot's own position data (provided by its control system) combined with ballbar measurements to automatically calculate and adjust calibration parameters, reducing manual user input while maintaining accuracy through iterative refinement
Data Source
AI summary
A method of calibrating a coordinate positioning machine is described. The machine is controlled into a pivot pose in which a target point associated with a moveable part of the machine and a pivot point associated with a fixed part of the machine are separated from one another by a known separation. An error value for that pose is determined based on the known separation and a separation expected for that pose from the existing model parameters of the machine. The machine is controlled into a plurality of different target poses, and for each target pose a separation between the target point and the pivot point is measured and an error value for that pose is determined based on the measured separation and a separation expected for that pose from the existing model parameters.