Measuring Tactile-Optical Sensor

The conventional mechanical sensors mentioned above all have one thing in common: the signal is transmitted from the probing element through a rigid shaft to the actual sensor (for example, a switch or piezoelectric element). Since each deflection of the shaft affects the measured result, the aim is to use shafts of maximum rigidity. In connection with the sensor technology applied, this leads to relatively high dimensions and probing forces. Practically speaking, the minimum diameter of the probing sphere is several tenths of a millimeter. Such probing systems are thus suitable for measuring small geometric features only under certain conditions.

With the Werth Fiber Probe, these disadvantages are circumvented by using the stylus shaft only in order to position the stylus sphere. The actual measurement of the position is performed by an image processing sensor integrated into the system (Fig. 22). The deflection of the shaft is therefore not included in the measured result. While the fiber probe itself is designed according to a two-dimensional principle, it may also be used to perform three-dimensional measurements, provided that the object surfaces to be probed and the axis of the fiber probe form a sufficiently small angle. The position of the probing sphere in the direction of the fiber axis can also be determined by integrating a second viewing direction of the image processing sensor (second camera or mirror) into the design of the fiber probe.

Fig. 22: Working principle of Werth Fiber Probe; a) 2-D measuring set-up, b) 3-D measuring set-up with second camera.

Fiber probes are manufactured by wire-drawing thin glass fibers and melting the spheres onto them. Good positioning of fiber probes at the location to be measured can be achieved by mounting them in a hollow metal needle (Fig. 23).

Fig. 23: Werth Fiber Probe with magnetic interface for measuring a microgearwheel.

If the glass fiber supplies light to the probing tip, measurements can be performed in the self-illuminating mode (Fig. 24). It is also possible to use the fiber probe in the transmitted-light mode. Due to its small dimensions, the resulting probing forces are negligible (up to several micronewtons). For this reason, it can be used to measure especially contact-sensitive workpieces. The fiber probe belongs to the measuring sensor group. For this reason, it is primarily suitable for scanning material surfaces.

 

Fig. 24: Examples for measuring with the Werth Fiber Probe in the self-illuminating mode; Top: diesel engine injection nozzle with diameter of 200 µm, insertion depths of 0 mm and –0.6 mm; Bottom: measurement of bore with burr.

In addition, the principle of self-centering measurement with measuring probing systems is shown here based on the example of the fiber probe (Fig. 25). A calibrated sphere is positioned in a tooth space to determine the pitch errors of a gear. The position of the sphere on the gearwheel results from the value measured by the fiber probe (position of the sphere in the image field) and the coordinates of the coordinate measuring machine. Values such as the dimension over two balls or the pitch of the gearwheel can then be determined based on several positions measured in different tooth spaces. The profile can be measured by probing the tooth flanks.

After the image processing sensor, the fiber probe is one of the most accurate sensors now available for multisensor coordinate measuring machines due to its unique method of operation.

Fig. 25: Top: Self-centering measurement of gearwheels with the Werth Fiber Probe in the transmitted light mode, diameter of stylus sphere 180 µm. Bottom: Measurement of flank angle in luminous mode.