Coordinate Positioning Calibration with Pivot Pose and 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, such as a ballbar, to minimize overall error, and an adaptor with a part-spherical bearing surface for precise measurement, allowing the tool to remain in place during calibration.
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 replaces expensive laser trackers with inexpensive ballbar measurement devices. The ballbar is a simple mechanical device with balls at each end that can be measured by the robot's own sensors, eliminating the need for costly external measurement equipment while achieving sufficient calibration accuracy for industrial applications.
Solution Approach 2:
The robot calibrates itself by using its own motion capabilities to position measurement targets and its own sensors to measure the ballbar. The system uses the robot's existing actuators and encoders to perform calibration without requiring external measurement equipment or additional actuators, making the calibration process self-contained and eliminating the need for expensive external devices.
2Measurement precision
If traditional calibration methods are used, then measurement precision is improved, but device complexity increases
Solution Approach 1:
The patent extracts only the essential measurement function needed for calibration - measuring the distance between two points - and implements it through a simple ballbar device. This eliminates the complexity of laser trackers and other sophisticated measurement systems while retaining the core measurement capability needed for calibration.
Solution Approach 2:
Instead of using complex external measurement devices to measure the robot, the patent inverts the approach by having the robot measure a simple passive artifact (the ballbar). The robot's own motion system becomes the measurement tool, and the calibration artifact becomes a simple passive object with known geometry, reversing the traditional master-measured relationship.
3Measurement precision
If multiple measurement points are used for calibration, then measurement precision is improved, but the number of measurements and user input increase
Solution Approach 1:
The patent combines multiple calibration measurements into a single integrated ballbar measurement process. By measuring the distance between two balls at opposite ends of the bar, the system obtains multiple geometric constraints simultaneously, reducing the number of separate measurement operations and user inputs required compared to traditional point-by-point calibration methods.
Data Source
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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(s). An error value for that pose is determined based on the known separation (s) 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 (S) between the target point and the pivot point is measured and an error value for that pose is determined based on the measured separation (S) and a separation expected for that pose from the existing model parameters. An overall error measure is determined from the error values, and a new parameter set is determined that would result in a lower overall error measure than for the existing parameter set. Also described is a ball adaptor and a ballbar extension piece.