Touch Trigger Tactile Sensors

Basic touch trigger probing systems function according to the “three-leg principle” (Fig. 19a). If the stylus sphere contacts the workpiece, a trigger signal is generated for read-out by the scale systems of the coordinate measuring machine. The measured point results from the coordinates of the measuring machine and is referenced to the center point of the stylus sphere. Via a rigid shaft, the stylus sphere is attached to a three-point bearing equipped with a switch at each of its three bearing points. If the stylus is deflected from a given direction, at least one of these switches will be opened. This is then further processed as a trigger signal. The major disadvantage of this system lies in the fact that the variation of probing forces according to the probing direction results in a stylus deflection of varying elasticity. This in turn results in a directional probing behavior (three corner characteristic) which is difficult to correct.

Fig. 19: Principles of touch trigger 3-D probe; a) Electromechanical contacts (threeleg principle), b) Mechanical-electrical converter.

Higher quality probing systems utilize various conversion elements (for example, piezoelectric elements or wire resistance strain gages) to convert mechanical signals to electrical ones (Fig. 19b). Non-directional probing behavior can be achieved using these elements. The downstream electronics ensure that the measurement can be performed with very low probing forces. The influence of the sensor on the measuring uncertainty is minimal. The threelegged bearing is deflected only after the probing point is detected. Relatively long “brake paths” are therefore permissible in the coordinate axes.

The disadvantage common to all touch trigger sensor systems is that the tactile element of the coordinate measuring machine first must be brought into contact with, and then retracted away from, the workpiece in order to measure a point. Probing times in the seconds range are thus required for each measured point.