New bidirectional sensor measures the shape and roughness of shafts inline

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The Fraunhofer Institute for Laser Technology (ILT) in Germany has developed an interferometric distance sensor that can measure the geometric features of shafts – such as camshafts and crankshafts – with sub-micrometer accuracy. The ‘bd-1’ sensor head can be integrated into shaft measuring machines and measures surface roughness as well as geometric features. The device will be demonstrated Control 2014 trade fair in Stuttgart, Germany between 6-9 May.

Crankshafts, driveshafts, and camshafts are found in all internal combustion engines. The automotive industry sets extremely high standards on the manufacturing accuracy and surface characteristics of these shafts. For example, camshafts must work with microsecond precision in controlling valve opening times synchronous to piston movement. Even the tiniest manufacturing errors can greatly impair engine performance, fuel consumption, and the service life of components. Deviations from the specified roundness or roughness values, for instance, can lead to increased wear, unwanted noises, and malfunctions.

For the most part, shaft measuring machines still use tactile distance sensors and laser triangulation sensors that measure a variety of characteristics such as cam shape, cam lift, base circle radius, roundness, eccentricity, angular position, and straightness of bearings. Surface roughness is usually measured separately using perthometers.

Now scientists at Fraunhofer ILT have developed the bidirectional optical sensor, bd-1, which can measure both the shape and roughness of shafts inline and needs only a fraction of the installation space that triangulation sensors take up. Its name alludes to the fact that the laser beam moves back and forth along a bidirectional single path. This does away with adjustment problems, as the transmitter and receiver no longer have to be aligned with each other.

Bd-1 can measure surfaces including finely polished, shining, and reflective surfaces, which can be difficult to measure with other optical sensors. It not only measures surfaces positioned at steep angles and drill holes with high aspect ratios; it also records surface roughness while measuring deviations in shape on rotating shafts – eliminating the need for a separate process step and equipment specifically for this purpose.

The sensor also recognises deviations in shape and the microscopic surface structure of shafts at speeds of several thousand revolutions per minute with an accuracy in the 100nm range. This is made possible by high-speed data acquisition and processing and distance measurement frequencies of up to 70kHz.

Fraunhofer ILT developers created the distance sensor primarily for manufacturers of shafts or of high-precision cylinder coordinate measuring machines (CCMMs) for components such as camshafts and crankshafts. The sensor has been made suitable for the 100 per cent inline inspection of geometric features in accordance with the requirements of the automotive industry. In field tests, bd-1 has proved how effectively it can measure the thickness of rolled strips and blown films inline, or carry out roundness and distance measurements during the manufacture of turned parts in machine tools.