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Multi-sensor systems

The right sensor for every measuring task

The use of different sensors (multi-sensor systems) can replace many single-purpose measuring devices. Various of the sensors described above are therefore used in combination in multisensor coordinate measuring machines. Depending on the basic properties of the sensors, there are different application focuses (Fig. 60). The main application-related distinguishing features are the size of the object features that can be probed, the type of object features (edge, surface), the required accuracy and the suitability for fast capture of many points (scanning). In order to be able to solve complex measurement tasks comprehensively, it is often necessary to use several sensors in one measurement sequence. If several sensors are arranged on a coordinate measuring machine, the usable common measuring range is reduced by the distance between the sensors. A larger basic machine may therefore be required. Sensors that combine several functions in one position, such as the Werth Fiber Probe® 3D (see Measuring tactile-optical sensors, p. 45 ff.), are of benefit here. With the Werth multi-sensor system, additional sensors can be automatically inserted in front of the beam path of the image processing sensor. Alternatively, the various sensors can be distributed across independent sensor axes to prevent mutual restrictions or collisions with the workpiece.

<p>Fig. 60: Multi-sensor systems: typical applications of different sensors: a) conventional stylus, b) Werth Stylus®, c) laser, d) image processing, e) autofocus, f) Werth 3D Patch, g) Werth Stylus® 3D, h) X-ray tomography sensor</p>

Multi-sensor systems in practice

The ability to switch between different sensors means that the surface of a part can be captured almost completely, even with complex forms. The set-up times for sensor changes are completely eliminated and the entire measuring task can be completed in a single set-up. The arrangement of several sensors of the same type with different parameters such as scanning ball diameter, measuring range or resolution or optical magnification may also make sense. A common example is the combination of two image processing sensors with high magnification for precise measurement and low magnification (or zoom) for an overview.

Optically "capture" – tactile-optical measurement

Multi-sensor arrangements are also suitable, for example, for "capturing" the position of a bore, which is only known to a limited degree of accuracy, by optical measurement with low magnification and then measuring the diameter with high magnification or the axis direction and form of the bore with a stylus. Another option is to measure the spatial position of parts tactilely, align the workpiece coordinate system accordingly and then measure small, complicated features optically.

Multi-sensor systems and tomography scans

In coordinate measuring machines with X-ray tomography, multi-sensor systems are also used to increase accuracy through the auto-correction procedure[8] or to shorten the measuring time through combined measurement. For example, the measuring object can be qualified with a tactile or optical sensor and then only the zones of interest on the workpiece can be scanned tomographically at high magnification. It is also possible to scan the entire workpiece tomographically at a lower-resolution and measure closely toleranced features with other sensors. In both cases, the relatively time-consuming Raster Tomography at high resolution is bypassed.

Multi-sensor systems offer many benefits

To summarise, multisensor coordinate measuring machines offer the following key benefits:

  • A wide variety of measuring tasks can be solved with one machine.
  • Measuring objects with features that require the use of different sensors can be measured in one set-up in common datum systems.
  • The measuring time can be reduced by selecting the optimum sensors with adapted accuracy.

The benefits mentioned have led to a growing spread of this technology in recent years. Multisensor coordinate measuring machines are used in many industries. The focus is on the automotive supply industry, toolmaking, the consumer goods industry and medical technology.