Miniature non-contact displacement sensors

Displacement can be measured in various ways by using different physical measuring principles. Some years ago displacement sensors were still relatively large, with separate, discrete electronics. However, new technologies and production systems are now enabling miniature sensors with integrated electronics to be manufactured by companies such as Micro-Epsilon.

Established in Hanover, Germany in 1968, Micro-Epsilon was originally a manufacturer of strain gauges. But by the mid-1970s it was apparent that the future for Micro-Epsilon was in non-contact displacement measurement technology. Non-contact sensors are wear-free and so provide more reliable results over longer periods. Modern production systems require lower cycle times and, therefore, require very rapid acquisition times from displacement sensors - which, in turn, can only be guaranteed by utilising non-contact measurement techniques.

In the case of sensitive objects that are adversely affected by contact, non-contact sensors are ideal as they measure the distance to the object from a safe range and without deflecting the part being measured. The requirements for the performance and reliability of the displacement sensors are very high. Important application criteria are measurement frequency, accuracy, temperature stability and resolution.

Since the start of its own development efforts, Micro-Epsilon has attempted to establish as comprehensive a portfolio as possible in the field of displacement measurement. In the non-contact displacement measurement area, the company's range today includes the traditional electromagnetic methods: capacitive, inductive and eddy current. Laser triangulation, time-of-flight and confocal sensors are provided for optical displacement measurement.

Smaller and more intelligent

Current trends in displacement measurement technology indicate that smaller, more intelligent sensors with integrated electronics are now required. In mechanical engineering, the requirement for extremely compact sensors is always an important factor, especially if installation space is restricted or if the sensor needs to be lightweight. This is also the case in terms of integration of more electronics and intelligence in the sensor. This means that sensors are more frequently required to perform the signal conditioning directly in the sensor, therefore reducing component count, whilst offering faster measuring speeds.

Eddy current sensors can be used with all electrically conductive materials. As eddy current penetrates insulator materials, even metal behind an insulating layer can be used as a measuring object. A special coil winding means that very compact sensor designs are possible, yet these can still be used in high temperatures. All eddy current sensors are insensitive to dirt, dust, moisture, oil and pressure.

Micro-Epsilon's miniature eddy current sensors are available with diameters from 2mm and cable diameters of just 0.5mm; these are claimed to be the smallest standard manufactured eddy current sensors in the world today.

New eddy current technology

Micro-Epsilon's Embedded Coil Technology (ECT) is said to represent a technological breakthrough in eddy current sensor design and manufacture, enabling the previous limitations of eddy current sensors to be overcome. Due to its ultra-compact design and by using new inorganic materials in its construction, the new eddyNCDT ECT sensors provide almost unlimited scope in terms of the external design and geometrical shape of the sensor. This means sensors can be adapted to suit virtually any application requirements.

EddyNCDT ECT sensors offer extreme mechanical robustness, resulting in longer service intervals and higher temperature stability. The electronics can be integrated into the sensor, providing an even more compact measurement device for OEMs and machine builders. The sensors are also suitable for use in harsh operating environments, including high vibration, impact shocks and operating temperatures as high as 350degC. Sensors have been produced with extremely low thermal drift and with temperature errors of less than 20 parts per million per degree Kelvin.

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

Capacitive sensors are designed as guard ring capacitors. In practice, almost ideal linear characteristics are achieved by using these sensors. However, a constant dielectric constant between sensor and target is required for a constant measurement; the system reacts sensitively to dielectric changes in the measuring gap. As thermally induced conductivity changes have no influence on the measurement, the principle is also reliable where there are strong fluctuations in temperature.

Versatile capacitive sensor designs

Micro-Epsilon's new 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.

By integrating smart electronics in the sensor itself, laser triangulation sensors are an excellent example of how much smaller sensor systems are becoming. Most conventional sensors require separate electronics as well as the sensor itself. Micro-Epsilon's optoNCDT 1302 and 1402 sensors have a very small housing in which the electronics are integrated without sacrificing sensor performance. The two series include twelve different measuring ranges between 5mm and 600mm. Other sensors in the range can measure up to 2m.

The real advantage of using this measuring principle is the relatively large stand-off distance from the target that it offers. For hot or moving targets, it is advantageous to be able to measure from a large stand-off distance. Using the optoNCDT laser sensors means that very small spot sizes can be achieved, which is often critical to the application. The spot size can be in the range of a few micrometres and so can also be used for targets of similar size.

Nanometre resolution

Extremely high resolutions are possible when using confocal chromatic measurement technology. Resolutions in the nanometre range are typically achieved by expanding the colour spectrum. As the colour, which is in the focal point, is used for distance information, confocal sensors have a very small measuring spot that enables measurements to be made on particularly small objects. Therefore even fine scratches on a surface can be measured reliably.

The beam path of the sensor is compact and concentric, which means that measurements inside bore holes or test tubes, for example, are possible. For measurements such as these, the confocal miniature optoNCDT 2402 sensors, which have a sensor diameter of just 4mm, are suitable. Five sensor models cover a measuring range from 0.4mm up to 6.5mm and achieve a resolution of 0.016um. These sensors are claimed to have been unrivalled since their launch in 2007. With the launch of the optoNCDT 2402 sensor, a reduction of the diameter from 23mm to 4mm was made in one step. Thickness measurement of transparent films, boards or layers is possible using these sensors. In contrast to other methods, the system only requires one sensor for a measurement of this type. As the measurement is only performed using white light, no laser safety regulations apply. The sensors can also be used in potentially explosive areas and in systems that are susceptible to EMC.

For more information on Micro-Epsilon's range of non-contact miniature displacement measurement sensors, go to www.micro-epsilon.co.uk or email [email protected].