Electronic Design

Noncontact Displacement Sensors Bring Quality To Auto Assembly Lines

As stringent quality control practices become commonplace in automotive manufacturing processes, capacitive noncontact displacement sensors are finding use in a number of locations. By helping engineers test parts precisely before using them in cars and other vehicles, these sensors are raising the quality and efficiency levels of auto assembly lines worldwide.

For instance, Capacitec Inc. has developed a new line of disc-brake wear-analysis sensors capable of taking high-temperature (500°C) dynamic brake measurements both in laboratory dynamometers and on vehicles at test-track facilities. This noncontact displacement sensor can exactly measure rotor runout, thickness variation, coneout, thermal expansion, plate-to-plate orientation, wobble, and ovality on brake rotors in motion.

To see if sensors could perform on-board measurements of this phenomenon and survive the high G forces of high-speed rotation, tier-1 supplier Robert Bosch GmbH installed Capacitec's noncontact high-temperature brake probes on a truck. The sensor wires were routed through wheel drums and linked to special electronics mounted to the outside conference of the wheel. Both the sensors and electronics outlived the rigors of automotive testing.

Making measurements in the presence of magnetic fields or when immersed in engine oil is even more challenging. The Capacitec sensors are being used to fine-tune the closure position of injection nozzles in large engines, which is critical in boosting engine efficiency and reducing engine noise. According to Capacitec, because these sensors are immune to high magnetic fields created by generator coils, they have performed well in the diesel engines in which they were tested.

Caterpillar Tractor additionally uses these sensors to measure the minute motion of oil-cooled engine pistons. Also, researchers at Austria's AVL List GmbH and Germany's Federal Mogul are employing such capacitive probes to study piston-ring dynamics of a utility wagon diesel engine. The aim of this study is to reduce the burning of engine oil and the concomitant exhaust gas emissions by accurately tracking piston-ring motion.

Consistent and precise measurement of various gaps around a car's exterior is equally arduous. Capcitec has developed both contact and noncontact gap and flushness sensors for this application. A good example where this can find use is in controlling the flushness between a mounting bracket and the glass in an automobile's sunroof. In this application, the sensors control the amount of epoxy that must be distributed between the glass and the bracket to ensure a consistent geometry for each sunroof. General Motors is applying this technique to ensure that the finished sunroofs are consistently flushed against the top of their respective vehicles.

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