Measuring tactile-optical sensors

Conventional tactile sensors have in common that the signal is transmitted from the stylus via a rigid shaft to the actual sensor (switch, piezo element). As any deflection of the stylus has an effect on the measurement result, efforts are made to use styli that are as rigid as possible. In conjunction with the sensors used, this leads to relatively large dimensions and probing forces (see p. 102 ff. Probing force). In practice, the lower limit for the stylus diameter is a few 0.1 mm and for the probing force approx. 10 mN. Such touch probes are therefore only suitable to a limited extent for measuring small or sensitive geometric features. Even reducing the size while retaining the principle does not solve these problems, which is why such micro-probes have not become established in practice.

The disadvantages described above are avoided with measuring tactile-optical sensors because the stylus shaft is only used to position the stylus tip. With the Werth Fiber Probe® (WFP: Werth Fiber Probe®), the actual measurement of the position of the stylus tip is carried out directly by optical sensors integrated into the system. The deflection of the shaft is therefore not included in the measurement result.

The deflection of the stylus tip in the lateral orientation to the shaft (x, y) is determined using an image processing sensor (Fig. 30 left). For this purpose, light is supplied to the probing element (glass sphere) via the glass fiber shaft. This allows measurements to be taken in self-illumination mode (Fig. 31). It is also possible to operate the fiber probe in transmitted light mode. A mark positioned close above the actual stylus tip (e.g. second sphere) can be used to avoid shadowing of the image processing beam path by the measuring object (Fig. 30 right).

By holding the thin stylus shaft in a metal tube, for example, a two-dimensional fiber probe is created that is rigid in the shaft direction. This also ensures that the stylus tip can be positioned well despite the low stiffness of the shaft (Fig. 32). Three-dimensional measurements can also be carried out with a fiber probe of this type, provided that the object surfaces to be probed form a sufficiently small angle (optimally up to 45°) with the fiber probe axis.

Due to the difference between static and dynamic friction, the stick-slip effect can occur during scanning, especially with very long and thin stylus shafts. This causes the stylus tip to move along the surface with uneven speed and point density. An integrated piezo element moves the stylus along the shaft direction at a low frequency during the scanning movement. This avoids the occurrence of static friction and ensures a uniform scanning process.

Typical applications for the fiber probe are bores and slots with sizes of less than 0.5 mm to a few 10 µm, fiber optic connectors, micro-gears (module approx. 0.1 mm, see Fig. 32), spinneret geometries, dental implants and - in conjunction with a tilt head - cooling holes on parts of aircraft engines (Fig. 17). For the measurement of nozzle geometries of injection systems for engines, the workpieces are positioned in the measuring volume on a rotary/tilt axis with micrometre precision. The approximately 0.1 mm small bores are "captured" using image processing. The actual measurement of the bores is then carried out using the fiber probe. This determines both the form of the drill holes and their spatial position. Similarly, micro-thread pitches can also be measured in a normal section. The fiber probe is also suitable for roughness measurements[6].

The principle of self-centring measurement with measuring touch probes mentioned above is imaged in Figure 31f using the example of a tooth gap measurement with the fiber probe. If the calibrated stylus tip is positioned in a tooth space, the measured value of the fiber probe and the sensor position result in the position of the centre of the ball and thus the space on the gear. Several positions in different tooth spaces can then be used to determine the two-ball dimension or the pitch deviation of the gear, for example.

The Werth Fiber Probe® 3D (Fig. 33 left) can be used in all operating modes (e.g. point-by-point measurement, scanning with and without predefined paths) that are also available for conventional scanning probes. Applications include the measurement of micro-optics (lenses for mobile phones) and moulded rubber parts as well as the scanning of helical micro-gears with a rotary axis.

The fiber probe is placed in a stylus changer. A machine equipped in this way can be used as an optical tactile multisensor coordinate measuring machine without additional sensors. There is no need to offset the measuring ranges between the different sensors. Due to its operating principle, the fiber probe is currently one of the most accurate sensors for coordinate measuring machines, alongside the image processing sensor. Its probing error (as of 2019) is just 0.1 µm (Fig. 34).