3D digital model based CMM surface detection (3)

3. Tip compensation

At present, the most commonly used coordinate measuring machine is the mechanical trigger type probe, which is equipped with a ruby ​​stylus, which will inevitably bring about the problem of tip compensation.

For standard features such as planes and circles, the probe can be automatically compensated by means of integral offset. For continuous scan curves, it can also be processed automatically in the same way. But how do you solve the tip compensation problem for single-point measurements that are often encountered in surface measurement?

To compensate for a single point, you must know the direction vector of the compensation, which is the direction of the normal vector at the point of contact. In order to find the normal direction, it is more accurate to measure a micro-plane around the measuring point, and the normal of the micro-plane is regarded as the normal of the curved surface at the measuring point, thereby completing the tip compensation.

If the workpiece measurement point has a small change in its curvature, or if the deviation between the workpiece and the digital model itself is small, if the requirement is not high, in order to reduce the number of points, the micro-plane may not be measured, and the software directly pierces the measurement point. The direction vector of the digital model is used for the tip compensation, that is, the normal vector at the digital model is used instead of the normal vector at the actual measurement on the workpiece as the direction of the tip compensation. However, if the curvature deviation between the workpiece and the digital model itself is large, the tip compensation will be inaccurate, resulting in unreliable measurement data.

For non-contact probes, there is no tip compensation problem.

4. Theoretical value capture

After solving the import and alignment of the digital model, the capture of the theoretical value is relatively simple. For standard features such as circles, the software only needs to be able to select and identify the feature from the CAD model to extract the theoretical value directly from its characteristics. For automatic measurement, it can be programmed directly according to the digital model features to guide the machine to the vicinity of the theoretical position of the feature for measurement.

For points on curved workpieces, they are usually divided into surface points and edge points, and some software is more finely divided. For the points on the surface, the theoretical points can be obtained by direct measurement and the measurement points are projected onto the surface along the normal surface of the digital model surface. But the edge points are different, the edge is the boundary of the CAD surface, for example, the edge of the sheet metal, the simplest such as the edge of the cube. If the point on the edge is to be detected, since the stylus cannot be directly and accurately measured, and the compensation direction of the probe cannot be determined, it cannot be directly measured, and only the indirect measurement method can be used. Generally, the processing principle is as shown in Fig. 3. In order to measure the P point on the edge, points can be measured on both sides. In this example, the first 3 points are used to determine the above, the 4th and 5th points determine the boundary direction, and the last point 6 determines the position of the target point, which is projected to the point generated by the previously determined edge, which is regarded as the edge measurement point. The value is the closest to the edge of the surface in the digital model.

In the above way, edge point detection can be realized. Specific to different software, there may be different treatment methods.

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