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Qualifying the sensors

Measuring in a Coordinate system

Certain parameters of sensors vary from specimen to specimen. The sensors must therefore be calibrated by the manufacturer or, if the parameters change, by the user.

Universal qualification standard

The parameters are determined by measuring special calibration standards with the sensors installed in the coordinate measuring machine. This is done simply and economically using a universal calibration standard that is suitable for many sensors (Fig. 61).

Universal qualification standard
<p>Fig. 61: Universal calibration sphere with Werth fiber probe® 3D</p>

Determining the diameter and position of the sensing sphere

For tactile sensors, the diameter of the stylus tip must be determined so that it can be taken into account in the measurements. During the calibration process, the diameter of the calibration sphere (stylus calibration sphere or universal calibration sphere) is measured automatically using a predefined strategy and the deviation from the calibrated value is used to calculate the diameter of the stylus sphere. Not only the ball diameter is captured, but also the deflection of the stylus shaft, for example. For very accurate measurements, the form of the stylus tip and the directional dependence of the stylus are also determined with many measurement points on the surface of the stylus tip and corrected during the measurement. The calibration process for probe balls must be carried out for each probe tip to be used.

Qualifying characteristic lines and magnification

For measuring sensors with tactile or optical probes, the course of the deflection characteristics of the sensor axes (dependence of the measuring signal on the deflection) must be captured. Qualification is usually carried out by the manufacturer by qualifying many points step by step over the entire measuring range of the sensor axes. This is also realised by automatic programmes using the universal calibration sphere and can thus also be carried out by the user if necessary (e.g. when replacing complete styli and special accuracy requirements). For sensors with less than three dimensions, the sensor characteristics can also be determined on flat surfaces or ring-shaped structures. The magnification and any imaging errors of image processing sensors are often measured using chrome structures on glass plates, for example.

Special surface for many sensors

Due to the use of changeover devices, tilt heads or multiple probe arrangements, different probe positions often have to be taken into account for tactile sensors. The resulting offset is determined by qualifying the sphere centre of the calibration sphere. In principle, the same procedure is also used when different sensors are used in multisensor coordinate measuring machines. For this purpose, the calibration sphere must be accessible with all sensors in a fixed position. To ensure measurement of the sphere position with the various sensors, a specially manufactured surface is used (Fig. 61). The software automatically takes the distance between the sensors into account according to the qualification. All measurement points appear in the common machine coordinate system. The different sensors can thus be used in combination within a measurement sequence.