How capacitive sensors shine in industrial environments
Capacitive displacement sensors are still deemed to be primarily suited to clean, dry environments. However, in the form of industry-optimised versions, capacitive sensors can also achieve peak performance in harsh industrial applications, explains Glenn Wedgbrow, Business Development Manager at Micro-Epsilon UK.
Capacitive sensors measure changes of an electrical property described as ‘capacity’. This is a body’s or a conductor arrangement’s ability to store electrical charge. Capacitive sensors from Micro-Epsilon are based on the principle of the ideal plate capacitor. The electric field is only located between the two conductive objects in the active measuring range, i.e. between the sensor and the conductive measurement object. The sensor electrode is fed with alternating current of constant frequency and amplitude, which means the amplitude of the resulting voltage is proportional to the distance between the two objects.
More precision with triaxial design
Unlike conventional capacitive sensors with coaxial designs, sensors from Micro-Epsilon have a triaxial design. This means that the capacitor is not simply surrounded by a housing but also has an extra-fed guard ring located between the capacitor and the housing, which generates an electric field. This creates a homogeneous field between the measurement electrode and the object surface. The protective field around the measurement electrode prevents this field from spreading over other close conductive objects or other areas of the target. It also inhibits other objects from influencing the measurement. This triaxial design makes the sensors more robust and considerably more accurate. Linearity is significantly improved and interferences of the measuring field are reliably prevented. In addition, these sensors can be flush mounted in conductive materials without generating a measuring error.
Capable of sensing a wide range of materials, capacitive sensors are at their most accurate when the target material is electrically conductive. The electric field in this case does not penetrate into the target, so even thin films such as 10µm thick conductive paint, will provide accurate measurement results. Many semiconductors such as silicon wafers can also be measured accurately. With certain modifications such as lowering the sensing frequency, good results can also be obtained for semiconductors with weak conductivities, e.g. gallium arsenide.
Capacitive sensors can even be used to sense electrical insulators. Non-conductive material allows sensor electric field lines to pass through, but special electronic circuitry and specific factory calibrations compensate for this behaviour, enabling distance measurements on insulators, albeit at reduced accuracy. The thickness of insulators can be measured by backing them with a conductive material. The measured reactance changes with the varying thickness of the insulator as the field lines penetrate the insulator to join with the conductor.
The material in the gap between a capacitive sensor and its target affects the reading of the sensor. This is because the measured capacitance is directly correlated with the dielectric constant of the gap material. As the dielectric constant rises, so does the capacitance reading of the sensor.
This has implications for the design of a capacitive sensing system. Maximum measurement accuracy will be obtained in laboratory clean room environments that are free from contaminants. In such locations, capaNCDT sensors from Micro-Epsilon can deliver resolutions down to 0.0375 nm, linearities of 0.1 µm and reproducibility of 0.0003 per cent FSO (full scale output). However, sub-micron precision can also be achieved in industrial environments. The probe can be fitted with a protective cap in these demanding environments to protect the measuring mechanism. In this case, a simple constant offset adjustment will correct for the dielectric effects of the cap on the capacitance measurement.
Industrial application examples
Air gap measurement in large electric motors
With large electric motors such as those found in rock grinding mills for cement production or mining, the runout must be permanently monitored. With diameters of more than 10m, the motors are badly damaged when the rotor touches the stator. This why the air gap i.e. the distance between rotor and stator is monitored using capacitive displacement sensors. Due to the triaxial design and the non-ferromagnetic materials used such as titanium or stainless steel, the changing magnetic fields have no influence on the measurement result. Due to the non-contact measuring principle, there is no physical impact on the sensor or measurement object, which provides long term stability. Different sensor designs, including flat sensors, also enable easy installation in these harsh environments, where a small mounting space is available for the sensor (maximum sensor height of 2.5mm). Another challenge is the cable length, which should normally be around 8m.
Monitoring alignment of rollers and roller gap
The capaNCDT MD6 handheld measuring device from Micro-Epsilon detects gaps in industrial environments to micron accuracy and is used, for example, in the adjustment of rolls. Used in commissioning servicing tasks, the double-sided flat sensor determines the roller gap. The measuring system provides high accuracy, versatile application possibilities and intuitive operation. Applications include plant and machine building, wind turbines, construction and maintenance of turbines and engines, as well as test benches.
Capacitive flat sensors are operated with multi-channel controllers for permanent monitoring of the roll gap. This means that the measured values can be transferred to the control system in order to precisely readjust the lowering of the rolls.
Measurement of roll wear via the bearing gap
Capacitive displacement sensors are used to determine the wear of rollers. Roll wear is measured indirectly via the change in the bearing gap of the drive shaft. Due to the capacitive sensors, the measurement is continuous and with high precision. As a result, wear is permanently determined and detected at an early stage, which means that maintenance intervals can be scheduled in a targeted way. The capacitive sensors can also be used where strong temperature fluctuations occur and provide high signal stability.
Thickness measurement using capacitive sensors
Two capacitive sensors mounted opposite each other enable two-sided thickness measurement of electrically conductive materials. Strip thickness in the micrometre range can be measured using this method. Each of the two capacitive displacement sensors provides a linear distance signal which is calculated by the controller as a thickness measurement value. The measuring spot of the sensors is larger than that of the optical methods, which averages out any structures and anomalies on the surface. Using the capaNCDT multi-channel controllers from Micro-Epsilon enables processing of several sensor pairs with just one controller.
Capacitive-type displacement sensors are those that measure the gap between the stator and rotor (generator air gap) so that the rotor does not touch the stator. The sensors offer high resolution and operate on the principle that electrical capacitance exists between any two conductive surfaces that are near to each other. When this distance changes, the capacitance also changes and the sensor measures the distance, which is extremely important to the safety and maintenance of the wind turbine. These sensors can operate at high temperatures and in the electromagnetic fields present in any motor or generator.
Measurements in the particle accelerator
Capacitive sensors are also at the heart of a custom-engineered hydrostatic levelling system for particle accelerators. The sensors ensure the exact alignment of the kilometres-long accelerator track, achieving accuracies in the sub-micrometre range to ensure the tiny particles are guided to pinpoint high-energy collisions. This harsh, radioactive, moisture-filled environment would likely render standard, unmodified capacitive sensors incapable of providing accurate results. However, the capaNCDT sensors in this specially designed system feature a protective housing equipped with a ceramic heating element that keeps them dry by elevating the temperature by a few degrees Centigrade above the surroundings.
Brake disc variation
Protective measures guarantee high accuracy even in harsh environments where capaNCDT sensors have proven their robustness in a variety of situations. In an innovative setup designed by Micro-Epsilon to measure brake disc variation, four capacitive sensors are enclosed by a special ceramic substrate that shields them from high temperatures and mechanical stress. The system delivers precise measurements on test benches, in road tests and in automotive repair shops.
For more information on capacitive displacement sensors, please call the Micro-Epsilon sales department on +44 (0)151 355 6070 or email email@example.com