Measuring Tactile Sensors

In a measuring probing system, the sensor is equipped with path measuring systems (scales, inductive sensors, optical measuring systems, etc.), usually in all three coordinate axes. If, upon contacting the workpiece, the stylus sphere is deflected in any direction, the magnitude of this deflection can be read off from the path measuring system (Fig. 20). The measured point is generated by superimposing the sensor coordinates over the coordinates of the coordinate measuring machine (similar to “measuring on the image”). The stylus sphere correction corresponding to the vector position of the surface being probed described above and the elastic deflection of the stylus are then also taken into account.

Fig. 20: Principle of the 3-D scanning probe: Stylus change is implemented via a magnetic interface. Interconnected parallelogram springs permit movement in three axes. The complete movement is transmitted to a compact 3-D measuring system and thus measured via a continuous pin. The probing force depends on the deflection (according to Renishaw).

It is not necessary to remove the stylus from the workpiece in order to measure several different surfaces of the same object. When combined with the appropriate control software, this principle provides the basis for automatic scanning of object surfaces (Fig. 21). The coordinate axes of the measuring machine are controlled so that the sensor always remains within its measuring range, thus following the surface of the workpiece when executing tangential movements. The control software moves the sensor in a virtual coordinate system whose origin is located at the point of contact between the probing element and the workpiece. If carried out as a control process, this results in the scanning of a circle in the example shown. Similarly, 3-D scanning can also be performed in predefined planes. However, in connection with three-dimensional surfaces, the problem of performing stylus sphere corrections on unknown surfaces must also be considered (see Fig. 17).

Fig. 21: Scanning with a measuring probing system; a) 1-D: The stylus is moved in direction X and readjusted (controlled) to nominal deflection z’s in direction Z. b) 2-D: The stylus is moved in direction X” and readjusted to nominal deflection y”s in direction Y”. The coordinate system X”, Y” for the control loop rotates along with the scanning path. The measured points are acquired in probe coordinate system X’, Y’ and added to the machine coordinates.