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Sensors and device axes

Measurement of points, lines, areas and volumes

Some sensors (e.g. all styluses) only work on a point-by-point basis. These point sensors are either switching or measuring.

Switching and ...

As already explained in the Sensors section (see p. 8 ff.), switching point sensors only supply a trigger signal when a measurement point is picked up. This causes the position measuring systems of the coordinate axes to be read out and the point coordinates on the surface of the measuring object to be determined. This requires movement in the axes (dynamic measuring principle). These sensors can work in one, two or three dimensions (Fig. 57).

Switching and ...
<p>Fig. 57: Sensor dimensionality: Sensors can provide measurement signals in 1, 2 or 3 orientations. They can capture points, lines, areas or volumes. There are various combinations, the most important of which are shown here.</p>

... measuring sensors

Measuring point sensors have their own internal measuring range (Fig. 3) in one, two or three dimensions. Its size can be several millimetres. The measured value is defined as the distance of the measured point of the respective axis to the internal sensor zero point. An object point is determined by superimposing the measured values of the sensor and the coordinates of the sensor position read out in the measuring machine. The prerequisite is that the object point is within the measuring range of the sensor. It is therefore also possible to determine a point when the coordinate measuring machine is stationary (static measuring principle). However, the measurement of several points required to determine features also requires movement of the machine axes.

... measuring sensors
<p>Fig. 3: Comparison of switching (left) and measuring sensors (right): A deflection, S signal course, M measuring range</p>

1, 2 or 3 sensor axes

For point sensors with two or only one switching or measuring axis, the coordinates not captured by the sensor are blocked for probing by reducing the degrees of freedom. With the 2D fiber probe (see p. 46 ff. Measuring tactile-optical sensors), for example, this is done in the shaft direction in order to measure in the position defined by this in the plane perpendicular to the shaft. With the contour sensor (see Tactile-optical contour sensor) this is reversed, the movement in the plane is blocked, the sensor measures along its guide axis. The blocked coordinates result from the previously measured position of the sensor probe point. This principle limits the applicability for three-dimensional objects, as it is not possible to probe in the blocked orientations.

Measurement "in the image"

Other sensor principles (e.g. image processing and X-ray tomography) allow the measurement of several points in a separate two- or three-dimensional measuring range practically simultaneously without moving the sensor in the machine axes. These sensors can therefore be referred to as line, flat or volume sensors. Small object features can thus be captured all at once. This is referred to as measurement "in the image" (Fig. 58). In X-ray tomography, the volume of the object is captured completely. For dimensional measurement, the material transitions (surfaces) are extracted. This can be done as a surface or in cross sections (linear) with any spatial position.

Measurement "in the image"
<p>Fig. 58: Measurement "in the image" and "on the image": The diameter a and the distance b are measured at a sensor position "in the image". The circle with the diameter c is determined from various sensor positions x, y in the machine coordinate system and thus measured "on the image"; the same applies to the distance d.</p>