Distance Measurement – Capacitance

Non-contact displacement measurement instrumentation using a capacitance technique

Applications include semiconductor processing, assembly of precision equipment such as disc drives and machine tools, metrology, assembly line testing

Two conductors in close proximity form a capacitance, which is inversely proportional to the distance between them. It’s possible to use this in a distance measurement system where one of the conductors is a measurement probe and the other is the “target”. The electronic signal conditioning connected to the probe can then provide the distance between probe and target.

Probes are typically concentric tubes with a small inner conductor and a larger outer “guard” which isolates the inner conductor from external influences. The inner conductor then only “sees” the target.

The inner conductor’s size for the probe typically sets the distance which can usefully be measured. As a rule of thumb the inner conductor diameter equals the measurement range, so a probe with a 5 mm inner diameter can measure a 5 mm distance.

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WHAT IS DISTANCE MEASUREMENT BY CAPACITANCE?

WHAT FYLDE PRODUCTS ARE AVAILABLE?

Both probes and capacitance signal conditioning are available. Probes are available in sizes from 1 mm to 10 mm with varying cable lengths. (See FE-925-CDT Transducers).
Signal conditioning for these probes are the FE-419-CDT and FE-420-CD amplifiers. When the cable length from amplifier to probe is short (2m or less) the ideal choice is usually the FE-420-CD. The FE-419-CDT can operate with longer cable lengths of 10 m or more.
For large systems where the senor count is high FE-549-CDT amplifiers are recomended. Up to 14 modules (56 channels) may be fitted into a 42HP Eurocard frame, which contains a common power supply and master oscillator. The unit is driven from a common oscillator utilising individual power buffers, and the problems of beating which may be associated with multichannel CDT systems are thus avoided.

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