Measurement tasks in industrial environments and laboratory research require sensors with high resolution and signal stability. Non-contact measurement of displacement, distance and position in these circumstances can be accomplished by capacitive sensors.

Two types of capacitive sensors for displacement measurement are coaxial and triaxial sensors. These types of sensors can be used in a variety of applications including measurement of tool position and displacement in machines, as well as monitoring the roller gap and precise thickness measurement in production control.

Capacitive displacement sensors

Capacitive displacement sensors operate based on the principle of the ideal plate-type capacitor. As the distance between two plates — in this case, the sensor and the measurement target — varies, the total capacitance changes. If there is a flow through the sensor capacitor of an alternating current of both constant amplitude and frequency, the distance to the target (which acts as a ground electrode) is proportional to the amplitude of the alternating voltage on the sensor. The value of the measurement is then output by the controller as the displacement detected between the target and the sensor changes.

For a stable measurement, a steady dielectric constant is required between the sensor and the target. This is because the sensing system reacts to dielectric changes in the gap in addition to changes in the distance between the sensor and the target. The highest measurement precision is achieved in a clean and dry environment, but even in industrial environments, capacitive sensors from Micro-Epsilon achieve micrometer-range resolutions and enable successful use in varied applications.

Figure 1. Triaxial sensor design provides a homogeneous measuring field for high signal stability. Source: Micro-EpsilonFigure 1. Triaxial sensor design provides a homogeneous measuring field for high signal stability. Source: Micro-Epsilon

The capacitive measuring principle provides a number of benefits including non-contact measurement that applies no interfering or abrasive forces to the target. The principle allows high bandwidth for quick measurements, delivers unequaled stability and accuracy, and is robust against run out of measurement objects that conduct electricity like metals, carbon fiber-reinforced polymers, silicon and graphite. Even surfaces that are reflective can be measured with the highest precision since there is no interference caused by optical properties of the target.

Highly precise at extreme temperatures

Micro-Epsilon has developed an innovative capacitive displacement sensor that delivers highly precise measurements even in tough environmental conditions with an operating temperature range of -50° C to 800° C. The capaNCDT 6228 sensors provide a measuring range from 1 mm to 20 mm and come integrated with a high temperature coaxial cable that is resistant to interference due to outside magnetic or electric fields. High measurement accuracy and stable measurement values are obtained even when temperatures fluctuate because changes in conductivity of the target that are thermally induced do not affect the measurement.

Figure 2. Micro-Epsilon's capaNCDT 6228 capacitive measuring system delivers high temperature stability and linearity at ambient temperatures from -50° C to 800° C. Source: Micro-EpsilonFigure 2. Micro-Epsilon's capaNCDT 6228 capacitive measuring system delivers high temperature stability and linearity at ambient temperatures from -50° C to 800° C. Source: Micro-Epsilon

The capaNCDT 6228 series is ideal for a number of high temperature displacement measurement applications including thickness measurements of brake discs and level monitoring of molten glass. Measuring the thickness of brake discs occurs during the braking process when the temperature of the disc can reach 700° C. Two sensors are placed on either side of the disc for disc thickness variation measurements in car repair shops, road tests and on test benches. In level monitoring of molten glass during the production of flat glass, measurement of the thickness of the glass is achieved by measuring the difference between inline thickness correction on the surface of the glass and on the surface of the tin bath with sensors. The glass reaches temperatures of 600° C, requiring sensors capable of operating at high temperatures.

Coaxial versus triaxial

The triaxial sensor design of capaNCDT 6228 capacitive displacement sensors has several advantages compared to the coaxial arrangement of other sensors. The triaxial design consists of a guard ring electrode, ground and a measuring electrode on the sensor’s front edge. High signal stability for precise measurements is enabled by a homogeneous measuring field ensured by the guard ring electrode.

The triaxial design not only allows the sensors to be mounted flush in electrically conductive materials but also permits contact between multiple sensors for multi-channel measurements without the danger of a short circuit. The extremely low noise sensor cable provides an impermeable electrical shield. In contrast, competing coaxial capacitive displacement sensors are prone to larger measurement error due to signal noise caused by an inhomogeneous measuring field, are sensitive to interference fields, have reduced measurement range, and rely on outdated controller technology.

Micro-Epsilon

Capacitive displacement sensors are essential tools for non-contact measurement of displacement, distance and position. For the highest signal stability, linearity and resolution in industrial environments with ambient temperatures up to 800° C, the capaNCDT 6228 capacitive displacement sensors are unmatched. With a triaxial sensor design for increased linearity and measurement accuracy, the capaNCDT sensors are immune to interference, provide a measurement range up to 20 mm, and are available with a multi-channel controller.

Learn more about Micro-Epsilon's capaNCDT 6228 capacitive sensor system for high-temperature measurement tasks.