The benefits of using Capacitive Sensors for displacement measurement

Measuring distance or displacement is a common requirement in many industry sectors, but selecting the most suitable sensor for an application is never easy. Micro-Epsilon discusses the principle of capacitive displacement sensors, how they function and the technical advantages they provide.

Capacitive sensors provide the highest precision of any non-contact displacement sensor technology. The latest electronics make it possible to offer resolutions in the picometre (one trillionth of a metre) range. Generally, these sensors are used to measure against conductive targets, but certain insulators can also be measured.

With the capacitive principle, sensor and target operate like an ideal parallel plate or guard ring capacitor. The two plate electrodes are represented by the sensor and the opposing measurement object (target). If a constant alternating (AC) current flows through the sensor capacitor, the amplitude of the AC voltage on the sensor is proportional to the distance between the capacitor electrodes. An adjustable compensating voltage is simultaneously generated in the amplifier electronics. After demodulation of both AC voltages, the difference is amplified and output as an analogue signal.

Because the sensor is constructed like a guard ring capacitor, almost ideal linearity and resolution against metal targets is achieved. The technology also offers high temperature stability, as changes in the conductivity of the target have no effect on the measurement.

The technology is sensitive to changes in the dielectric constant in the sensor-to-target gap and so operates most effectively in clean, dry applications. Cable length is also relatively short, typically 1-2m, due to the effect of cable capacitance on the oscillating circuit tuning. However, a small preamplifier circuit can be added after this short cable, which enables sensor cable lengths of up to 10m.

Capacitive displacement sensors from Micro-Epsilon are available in different designs and versions. The sensors are differentiated by their measuring range, their design and by the technology used to manufacture them. Capacitive sensors are available in a cylindrical design (with integrated cable or socket) or as flat sensors (with integrated cable). These displacement sensors can be exchanged without recalibration; sensor replacement is easy typically taking around 5 seconds, unlike conventional sensors that require time-consuming calibration and linearisation. The majority of capacitive sensors can be used in clean rooms and often in ultra high vacuums.

Micro-Epsilon’s new generation capaNCDT CSH sensor utilises a special ceramic substrate that provides extremely high temperature stability. Virtually unlimited sensor geometries can be developed using this technology. For example, an extremely flat sensor has been produced with an installation height of just 4mm. This technology overcomes the previous limitations of cylindrical sensor designs. To date, a maximum resolution of 0.037nm has been achieved using these sensors.

For more information on Micro-Epsilon’s range of capacitive displacement sensors, please email info@micro-epsilon.co.uk or telephone the sales department on 0151 355 6070.