Measuring tactile sensors

In a measuring touch probe, the sensor has displacement measuring systems (scales, inductive sensors, optical sensors), usually in all three coordinate axes. If the stylus tip is deflected in any orientation on contact with the measuring object, the magnitude of this deflection can be determined from the information from these displacement measuring systems (Fig. 28). The measurement points are obtained by superimposing the sensor deflection on the sensor position in the machine's Coordinate system. In addition, there is the above-described probe ball correction according to the vectorial position of the surface to be probed and the probe deflection.

Due to the measuring principle of the touch probe, permanent measurement points can be captured during the entire probing process (deflection and return movement). From this, averaged and therefore reproducible measurement points can be determined. The complete probing process can also be recorded and the probing point for an assumed zero deflection (probing with 0 N probing force) can be extrapolated from this. This is useful for measuring flexible workpieces, for example.

In combination with an appropriate control system, measuring touch probes can be used to automatically scan the surfaces of the objects to be measured. With this method, many surface points can be measured in a relatively short time. The coordinate axes of the measuring machine are controlled in such a way that the sensor always remains within its measuring range. When moving tangentially along the workpiece surface, the stylus follows its course (Fig. 29a). The orientation of the surface tangent can be determined from the vector of the probe deflection, as this always points approximately perpendicular to the surface. This applies if friction is neglected and the probing force is isotropic (the same in all orientations). This process can be visualised as a movement of the sensor in a virtual coordinate system with its origin at the point of contact between the stylus tip and the measuring object. Rotating this sensor coordinate system along the surface normal during the tangential movement results, for example, in the scanning of a circle (Fig. 29b).

Spatial scanning in predefined planes can be carried out in a similar way. For three-dimensional surfaces, however, the problem of scanning sphere correction on unknown surfaces must always be taken into account (see Fig. 25). Scanning can also be carried out using predefined paths (e.g. from CAD data). This makes it possible to scan much faster, as the control process according to the probe deflection becomes simpler or can be omitted completely. The real algorithms for scanning take other influencing factors into account according to the task and are therefore more complex. Another operating mode of the scanning probes is the self-centring measurement of gaps in gears and of flutes or similar (see Fig. 31f, p. 47 Measuring tactile-optical sensors).