Optical Extensometer Calibration for Accurate Strain Measurement
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Solution Overview
Problem
Conventional camera-based vision systems in materials testing suffer from measurement inaccuracies due to imaging and positional discrepancies between perceived and actual reference positions, leading to distorted readings and calibration burdens on operators.
Innovation Solution
The system employs a kinematic mount for the imaging device, pre-calibrates intrinsic properties during manufacturing, and uses a simpler verification plate for extrinsic calibration, reducing the need for frequent recalibration by maintaining consistent position and orientation, and enabling easier adjustment of working distances.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional camera-based vision systems are used to capture images for measuring specimen elongation and strain, then the system can collect images synchronized with test signals, but imaging and measurement differences between perceived reference position and actual position lead to distorted readings and inaccurate measurements
Solution Approach 1:
The patent introduces a calibration plate with known geometric features as an intermediary reference object between the camera and the specimen. This calibration plate serves as a mediator to establish accurate correspondence between image coordinates and physical dimensions, eliminating the distortion caused by direct imaging of the specimen without proper reference framing.
Solution Approach 2:
The patent applies parameter changes by determining calibration parameters that transform image coordinates to physical coordinates. These parameters (including translation, rotation, and scaling factors) are calculated based on the known geometry of the calibration plate and applied to correct the imaging measurements, thereby changing the coordinate system parameters to achieve accurate measurements.
2Measurement precision
If conventional calibration procedures are used, then calibration parameters can be determined, but the process requires extensive operator intervention and is time-consuming
Solution Approach 1:
The patent implements self-service calibration by enabling the system to automatically capture images of the calibration plate, identify its features through image processing algorithms, calculate calibration parameters, and apply corrections without requiring manual measurement or extensive operator intervention. The system performs the entire calibration process autonomously, significantly reducing calibration time while maintaining accuracy.
3Adaptability or versatility
If the imaging device position or orientation changes, then the system can be repositioned for different measurements, but recalibration is required to maintain accuracy
Solution Approach 1:
The patent applies preliminary action by performing calibration before actual specimen measurements are taken. The calibration plate is imaged and calibration parameters are determined in advance, establishing a reference coordinate transformation that remains valid as long as the camera position and orientation remain unchanged. This preliminary calibration step ensures that subsequent measurements are accurate without requiring repeated calibration during the testing process.
Data Source
AI summary
Disclosed example video extensometers include: a load string configured to secure a test specimen; an imaging device configured to capture images of a surface of the test specimen when secured in the load string; a storage device configured to store a plurality of first calibration parameters corresponding to intrinsic properties of the imaging device; and control circuitry configured to: perform a verification process using the first calibration parameters to verify that a plurality of second calibration parameters correspond to an arrangement of the test specimen with respect to the imaging device; and perform an optical strain measurement process to measure displacement of the test specimen based on the first calibration parameters and the second calibration parameters.