Tactile Sensors

All tactile sensors function on the principle of mechanical contact with the workpiece. The resulting signals are then derived from this contact for further processing. A distinction is made here between touch trigger and measuring probing systems. With a tactile sensor, the geometry (typically, the form and size) of the probing form element (or sphere) as well as the spatial position and geometric form of the object surface to be measured are contained in the measured result. Figure 17 shows that during tactile scanning, the position of the probing point cannot be determined without a mathematical correction based on the known coordinates of the center point of the stylus tip or sphere. In order to carry out an exact correction, the probing element must be calibrated or “qualified” carefully (stylus sphere correction). Further, it is normally necessary to probe a large number of points on any geometric feature to be measured. If this correction is not performed, the resulting error will depend on the diameter of the stylus sphere (i.e., the smaller the tip diameter, the smaller the error). In addition, spheres with a large diameter can also suppress small deviations in structure. This “mechanical filtering” can have either a favorable effect on the measured result or lead to its falsification.

Fig. 17: Influence of the stylus tip form when measuring curved surfaces: For unknown surfaces, the stylus sphere correction can be determined through closely spaced adjacent paths. The path correction can be omitted for laser sensors.
Fig. 18: Influence of the number of measured points on the measured result: Few measured points result in an incorrect associated circle. A large number of measured points yield the correct associated circle. The minimum circumscribed and maximum inscribed circles show that the Gaussian associated circle does not describe the feature exactly where form deviations are involved.

Figure 18 shows the major influence that the number of probing points has on the measured result. It is essential to probe a large number of measured points in cases involving real geometric features with form errors. However, due to the amount of time touch trigger probing requires, this is easier said than done.