Laser Point Sensors

The measuring principle of a laser point sensor is based on the projection of a beam of light produced by a laser (usually a laser diode) onto the object to be measured. The reflected beam spot is imaged on an optoelectronic sensor. The position of the point to be measured is then determined via a suitable technique. The best-known techniques generally belong to one of two categories: triangulation and interferometric technique (see Fig. 6).

The triangulation sensors often used in automation technology function according to the following principle: the laser beam and the axis of the sensor’s imaging optics encompass an angle measuring several tens of degrees. A triangle is thus formed between the laser transmitter, the measured point and the sensor and can then be used to determine the distance via trigonometric relationships (or triangulation). The measured result depends heavily on the structure and angle of inclination of the surface. This leads to relatively large measuring uncertainties, making the use of this technique only suitable in less accurate coordinate measuring machines.

Better results can be achieved with laser sensors which function according to the Foucault principle (Fig. 15). This principle uses the aperture angle of the sensor imaging optics as the triangulation angle. Instead of a laser spot, a Foucault knife edge located in the beam path is imaged onto the object. Signal evaluation is performed via a differential photodiode. The deviations from the zero position of the laser sensor determined in this way are used for readjustment in the corresponding axis of the coordinate measuring machine. The measured result is obtained by superimposing the values measured by the laser sensor over those measured by the coordinate measuring machine.

Fig. 15: Laser sensor utilizing the Foucault principle with integrated image processing sensor (illumination system of sensor not shown).

As was the case before, the material and inclination of the surface also influence the measured result considerably with this type of sensor. A correction of these influencing variables is therefore required. However, if suitable software is used, the measuring uncertainty can be reduced to a point where it satisfies the requirements of high-precision coordinate measuring machines. In practical use, a Foucault laser sensor of this type is generally integrated in the beam path of an image processing sensor. This arrangement makes it possible to switch back and forth between the two sensors without any mechanical movement. Moreover, the laser probing process can be observed visually.

The chief advantage of the laser point sensor in comparison to the focusing technique described above is its considerably higher measuring speed. Several hundred to one thousand points per second can be measured in this way. These sensors are thus ideal for scanning surface profiles.