The use of non-contact displacement technologies in precision measurement is growing rapidly. Customers need to measure more accurately, to sub-micron or nanometre resolutions, and against difficult surfaces or materials that cannot be touched during measurements.
Non-contact displacement sensors come in a wide variety of shapes, sizes and measurement principles, so it is critical that engineers have a greater understanding of the strengths and limitations of each principle when selecting the most appropriate one for the application.
The Eddy Current Principle
The eddy current measurement principle is an inductive measuring method. A coil is supplied with an alternating current, which causes a magnetic field to form around the coil. If an electrically conducting object is placed in this magnetic field, eddy currents are induced, which form an electromagnetic field according to Faraday’s Induction Law. The controller calculates the change in energy transferred from the sensor coil to the target material and converts this into a displacement measurement.
The advantages here are that this method can be used on all electrically conductive, ferromagnetic and non-ferromagnetic metals. The size of the sensor is relatively small compared to other technologies and the temperature range is high due to the resistance measurement of the sensor and cable. The technology is high accuracy and is immune to dirt, dust, humidity, oil, high pressures and dielectric materials in the measuring gap.
However, output and linearity depend on the electric and magnetic features of the target. Therefore, individual linearisation and calibration is required. Maximum cable length is 15m and the diameter of the sensor increases as the measuring range increases.
The Capacitive Principle
With the capacitive principle, sensor and target operate like an ideal parallel plate capacitor. The two plate electrodes are formed by the sensor and the opposing target. If an AC current with constant frequency 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.
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