Coordinate Positioning Machine Calibration for Rotary-Axis Accuracy
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Solution Overview
Problem
Calibration of non-Cartesian coordinate positioning machines, particularly articulated robots with multiple rotary axes, is challenging due to cumulative positional errors and complex geometric dependencies, making existing methods inefficient and inaccurate.
Innovation Solution
A method involving controlled point contacts between reference surfaces of a tool and a calibration artefact to update model parameters, using a sensor to measure actual separations and iteratively adjust these parameters to minimize errors, thereby improving the machine's geometric characterization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional calibration methods are used for non-Cartesian coordinate positioning machines, then the calibration process can be completed with simple procedures, but the positional accuracy and measurement precision deteriorate due to cumulative errors from multiple rotary axes
Solution Approach 1:
A laser interferometer is introduced as an intermediary measurement device to directly measure the position of the tool center point with high precision. The interferometer provides accurate reference measurements that bypass the cumulative errors of the robot's own encoders, enabling precise calibration without requiring complex iterative procedures
Solution Approach 2:
Traditional mechanical contact-based calibration methods are replaced with optical laser interferometry. The laser interferometer uses light waves to measure displacement and position, substituting mechanical measurement systems with optical ones to achieve higher precision and avoid mechanical wear and cumulative errors
2Adaptability or versatility
If multiple rotary axes are used to achieve large reach and flexibility, then the robot's versatility and operational range improve, but cumulative positional errors increase across the serial kinematic chain
Solution Approach 1:
The calibration process uses laser interferometer measurements as feedback to determine actual encoder offsets for each rotary axis. The measured positions are compared with commanded positions, and the differences (offsets) are fed back to correct the encoder readings, thereby compensating for cumulative errors while maintaining the multi-axis flexible structure
Solution Approach 2:
The calibration process determines and applies offset parameters for each encoder in the serial kinematic chain. By changing the effective parameters (encoder offsets) based on measured data, the system compensates for cumulative errors without altering the physical structure or reducing the number of axes
3Measurement precision
If encoder offsets are determined through complex calibration procedures, then positional accuracy can be improved, but the calibration time and operational efficiency decrease
Solution Approach 1:
The calibration procedure continuously collects measurement data from the laser interferometer throughout the robot's motion range. Rather than using discrete point measurements, the system continuously tracks the tool center point position, maximizing the use of measurement time and reducing total calibration duration while improving accuracy
Data Source
AI summary
A method of calibrating a coordinate positioning machine having a first member that is moveable relative to a second member, wherein the geometry of the machine is characterised by a set of model parameters. The machine is controlled to make point contact between multiple reference surfaces of a tool or artefact mounted on the first member and multiple reference surfaces of an artefact mounted on the second member. At least one of the model parameters is updated knowing or taking into account that the actual separations between the relevant surfaces are zero when making contact, even if the expected separations between the relevant surfaces as derived from the current model parameters are non-zero.