Switching tactile sensors

Simple touch trigger probes work according to the tripod principle (Fig. 27 left). When the stylus tip contacts the measuring object, a signal (trigger) is generated to read out the scale systems of the coordinate measuring machine. The measurement points are derived from the coordinates of the measuring machine and refer to the centre of the stylus tip. This is mounted via a rigid shaft on a three-point bearing, which is designed as a switch in each of the three supports. If the stylus is deflected from any orientation, at least one switch is opened. This is further processed as a trigger signal. The main disadvantage of this system is that the probing forces are highly dependent on the probing direction. This leads to different probe deflection and thus to a non-negligible direction-dependent probing error (triangular characteristic), which is difficult to correct.

Higher-quality touch probes use transducer elements such as piezo elements or strain gauges to convert the mechanical signal into an electrical signal (Fig. 27 right). They can be used to achieve a largely direction-independent probing behaviour. Downstream electronics ensure that it is possible to work with very low probing forces. The measurement uncertainty influenced by the sensors is lower. The tripod is usually only deflected after the probing point has been detected. This makes relatively long "braking distances" in the axes permissible. A common disadvantage of all trigger probes is that the coordinate measuring machine is brought into contact with the measuring object to determine a measuring point and then has to be moved out of contact again. This results in probing times in the range of seconds for each measurement point. The main advantages of the trigger probes over the scanning probes described below are their relatively low price and somewhat smaller dimensions. Due to the small number of measurement points, their use is limited to the measurement of features with negligible form errors (Fig. 26).