Capacitive displacement sensors 'provide unique benefits'

Micro-Epsilon's capaNCDT sensor system is claimed to be unique in several areas. The system uses a 'guard ring' principle that is said to provide three specific technical advantages for customers when compared to competing sensors currently on the market.

First, it enables earthing through the sensor rather than the target, which means the target does not have to be electrically grounded. This means it is easier to install, especially on rotating or fast-moving components. Also, the customer only needs one sensor. Some sensor manufacturers have to use two sensors - one on the top and one at the bottom of the target - to achieve this. In addition, the sensor can be used to measure plastic sheets or film, although the plastic must have a stable and consistent electrical resistivity. In fact Micro-Epsilon has already supplied complete sensor systems to manufacturers of blown film, for measuring to single-micron accuracies and at high speed.

Second, it means the measurement output has a very high linearity, so users do not have to calibrate the sensor against the target when it is delivered from the factory. 'Plug-and-play' really is the case here, with no calibration required at all.

And third, if the customer has to replace a damaged sensor or plug in another sensor with a different measurement range - for R&D, for example - users can do this without having to recalibrate or make any adjustments to the sensor electronics.

Versatile

The capaNCDT sensors can be used to measure a variety of features and characteristics, including: vibration; amplitude; clearance; run-out; displacement; distance or position; elongation; deflection; deformation; waviness; and tilt angle. They can also be used for dimensional inspection, parts recognition, tolerance checking and sorting.

Micro-Epsilon's capaNCDT (capacitive Non-Contact Displacement Transducer) 6300/6310 series is a compact, single-channel sensor system that comprises a capacitive displacement sensor, a sensor connecting cable and signal conditioning electronics. For metals, the system is calibrated at Micro-Epsilon's factory and can then be used by customers without any recalibration or optimisation, which saves much time and, potentially, operator error. The system is also user-configurable for customers measuring against insulating (non-metallic) materials, using simple three-point linearisation, although a constant relative dielectric strength of the material is necessary for accurate measurement.

Because of their capacitive operating principle, the sensors exert no interference force on the target and operate independent of variations in the conductivity of electrically conductive (metals) target materials; changes in the conductivity of the target material do not affect either sensitivity or linearity. The sensors therefore offer excellent zero point stability and accuracy and operate virtually independent of temperature.

Variants

The capaNCDT 6300 comes with a preamplifier, which is integrated into the controller. With the 6310 series, an external preamplifier is used, enabling cable extension of up to 20m.

Both the CapaNCDT system electronics and sensor are protected to IP54 and the sensor can operate in ambient temperatures of between -50 to 200degC. Static (up to 2kHz) resolution is 0.1um for the CS10 sensor and down to 0.0005um for the S601 sensor. Measuring range for metals is from 10mm down to 0.05mm and from 20mm down to 0.4mm for insulating materials.

The high-speed version of the sensor is the capaNCDT 6350, which offers advanced performance via an integrated DSP (digital signal processor). The DSP achieves up to 50kHz (-3dB) bandwidth on analogue output. Fast-moving production and inspection environments involving difficult materials can be measured using the calibrated DSP.

Capacitive sensors versus lasers

Chris Jones, managing director at Micro-Epsilon UK, comments: "The capacitive measurement principle is by far the most accurate and thermally stable method of non-contact measurement. Laser triangulation has become increasingly popular in recent years and claims to offer accuracies down to single microns. But when you introduce a non-ideal surface (ie most metal or strip products) and a temperature deviation of just a few degrees Centigrade, laser sensors will deviate dramatically.

"Capacitive sensors from Micro-Epsilon do not suffer these types of 'real world' issues and quite often deliver sub-micron performance. The sensor measures over a defined surface area, which minimises errors due to surface roughness and the thermal drift is 100 to 200 times smaller than optical devices. Our task as application engineers is to educate the customer as to the correct sensor selection, based upon their application requirements. I feel that more knowledge is needed with regard to helping users to understand the key advantages of the capacitive sensor principle - something we at Micro-Epsilon are very happy to do."

Operating Principle

The operating principle of the capaNCDT system is based on the 'ideal parallel plate' capacitor, with the two plate electrodes 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.

Micro-Epsilon's capaNCDT system evaluates the reactance of the capacitor, which changes strictly in proportion to the distance. This theoretical relationship is put into practice by constructing the sensors as 'guard ring' capacitors.

Applications

Micro-Epsilon supplies capacitive displacement sensors to a wide variety of customers for a diverse range of applications.

Take brake disc manufacturers, for example, who require a very robust measuring principle to measure the deformation of the disc in extreme temperatures. In one particular application for a customer, Micro-Epsilon's capaNCDT sensor is positioned only 2mm from the brake surface during measurement, with the temperature of the brake reaching 800degC.

Capacitive sensors are also used extensively in the semiconductor and silicon wafer manufacturing industry. Micro-Epsilon has worked closely with Carl Zeiss, for example, for ultra-precise lens alignment in silicon wafer processing.

Other applications include measuring the wear of bore holes in extruder machines, or for the positioning of metal plate detectors used in large CAT scanner machines in the medical sector. Each CAT scanner has hundreds of small plates that are used to precisely position and read information. These plates have to be positioned precisely, typically to within nanometre accuracy.

Measuring the run-out of air bearings is another good application for capacitive sensors. Micro-Epsilon is working with manufacturers of air bearings to provide ultra-precise measurement of run-out of the bearing in hard disk drives. As hard disks are being developed with even greater storage capacity, it is not acceptable to have any run-out of the disk. Micro-Epsilon has therefore developed a special version of its capaNCDT sensor that measures down to 0.1nm resolution.

Micro-Epsilon has also developed custom sensors, including credit-card sized electronics that help OEMs reduce installation size and cost of the system. Micro-Epsilon's custom DT6019 capacitive sensor is a good example of this. Also, the company can custom design special sensing heads for OEM applications, where standard sensors are not suitable or just do not fit easily.

More information and datasheets are available at www.micro-epsilon.co.uk/uk-en/Sensors/Capacitive/.