Achieving high precision with non-contact measurement Capacitive displacement sensors are still deemed to be primarily suited to clean, dry environments. However, in the form of industry-optimized models such as those designed by Micro-Epsilon, capacitive sensors achieve peak performances in harsh industrial applications. They also measure with the highest precision down to sub-micrometer accuracy in industrial environments. With temperature resistance, robustness and flexibility, capacitive sensors can also be used in environments where fluctuating temperatures or magnetic fields occur.
Capacitive displacement sensors are still deemed to be primarily suited to clean, dry environments. However, in the form of industry-optimized models such as those designed by Micro-Epsilon, capacitive sensors achieve peak performances in harsh industrial applications. They also measure with the highest precision down to sub-micrometer accuracy in industrial environments. With temperature resistance, robustness and flexibility, capacitive sensors can also be used in environments where fluctuating temperatures or magnetic fields occur.
Capacitive displacement sensors mainly determine changes in displacement and position of conductive objects. Here, Micro-Epsilon applies its expertise in the development of innovative capaNCDT sensors from 50 years of company history. This is how these sensors, which originally were assigned to clean environments, have become stable, high precision measuring systems for industrial applications.
Capacitive sensors measure changes of an electrical property described as capacity. This is a body’s or 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 is why the amplitude of the resulting voltage is proportional to the distance of the two objects. C = ϵ × Ad C = Capacity ϵ = Dielectric constant A = Surface of capacitor plates d = Distance between capacitor plates
Capacitive capaNCDT sensors are available in different models. The cylindrical version is the most common design. New, cylindrical displacement sensors from Micro-Epsilon have a mounting thread and can be easily screwed in or fixed in place using a nut.
In addition to the cylindrical design, flat sensors are available which require significantly less installation space. The PCB model of capacitive sensors is ideally suited to installation in narrow, confined spaces.
Capacitive sensors cover measuring ranges of 50 µm to 10 mm and are therefore suitable for multiple measurement tasks. The sensors are either equipped with plug-in connection or integrated cable. Different housing materials and manufacturing technologies are used for sensor construction. As well as the standard stainless steel/Invar design, sensors made from titanium, which are suitable for measurements in vacuums, are also available.
Unlike conventional capacitive sensors, sensors from Micro-Epsilon do not appear as with coaxial but as triaxial designs. This means that the capacitor is not simply surrounded by a housing but, in addition, an extra-fed guard ring is 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 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.
Micro-Epsilon also provides a suitable solution for particularly restricted installation situations. A patented miniature triaxial connector is used together with the PCB sensors that are only 14 mm to 25 mm thick. Cable, sensor and connector are almost the same size here, which means in simple terms: where the cable fits through, the connector also fits through. There is no need for time-consuming drilling of large holes. At only 3.6 mm, the connector is about three times smaller than conventional connectors. This greatly simplifies integration of the sensor in machines and systems with particularly narrow installation spaces. The spiral sheathing for the cables used offers additional protection against damage. The PCB sensors of the CFS series cover measuring ranges from 2 mm to 6 mm and achieve a linearity of 4 µm.
The advantages of industry-optimized sensors become evident, among other things, with large electric motors such as those found in rock grinding mills for cement production or for mining. The runout of these systems must be permanently monitored. With diameters of more than 10 m, the motors are badly damaged when the rotor touches the stator. This is why the air gap, i.e. the distance between the 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 onto the sensor or measurement object which, provides long-term stability. Different sensor designs, including flat sensors, also enable easy installation in these harsh environments, which only offers a small mounting space requiring sensors with a max. height of 2.5 mm. Another challenge is the large cable length which should usually be about 8 m.
In addition to the standard sensors, Micro-Epsilon offers the possibility to modify sensors and controllers according to the customer’s requirements, among other things, with respect to cable length, sensor design or modified measuring ranges. In areas where sensors are to a great extent exposed to shocks, thread sensors offer an ideal installation option. These are firmly screwed to the respective machine and prevent the sensor from slipping due to strong vibrations. This is how the measurement results remain precise on a long-term basis even in challenging environments. They are specially designed for machine building applications where the sensors must be mounted at a defined distance, e.g., on walls, in bore holes or even very deep drill holes. Capacitive thread sensors can also be mounted on thin walls using two lock nuts on both the inside and outside. Due to their integrated plug-in connection, thread sensors are easy to install. This prevents the cable from twisting. First, the sensors are screwed in and only afterwards connected with the cable via the integrated plug-in connection.
In industrial environments, the temperature often fluctuates very strongly. Materials expand with high temperatures and contract with lower temperatures. This is why the distance between sensor and measurement object can change too. In areas with strongly fluctuating temperatures, capacitive sensors from Micro-Epsilon offer, in the range from -270 °C to +200 °C, extremely high temperature stability of 5 ppm. Long-term stability is ±0.002 %FSO/month. For much higher temperature ranges, special sensor designs with ceramics can be provided.
Extremely temperature-stable and robust sensors are required with non-contact detection of Disc Thickness Variation of automotive brake discs. In order to achieve maximum efficiency of the braking system, the disc must have an even, consistent thickness. Unevenness, runout or abrasion on the friction surface of the disc causes the brake pads to lose contact with the rotating disc, which in turn reduces the braking effect. Providing high resolution, the capaNCDT DTV determines deviations well below 1µm in a test bench at room temperature and also during road tests when the brake disc reaches temperatures of 600 degrees Celsius. For brake disc thickness measurements, Micro-Epsilon has designed an innovative 4-channel sensor. Its robust construction makes the sensor perfectly suitable for harsh ambient conditions from production monitoring and quality control via test benches to on-road vehicle purposes. In its compact housing, four capacitive sensors detect the measurement values with each operating in standalone mode. A special ceramic substrate protects the sensors from mechanical and thermal loads, providing high temperature stability. In order to enable accurate measurements with simplified mounting, the sensors are available as mirror-inverted arrangements that can be mounted on opposite sides of the brake disc. Combined with the capaNCDT 6220 controller, the four sensor channels can be processed synchronously with dynamic measurements up to 5kHz and digital outputs via Ethernet or EtherCAT or as analog signal.
The extraordinary temperature stability of sensors is demonstrated by measurement tasks for space research. In areas away from the sun, the lowest temperatures are down to -271°C, which is just above absolute zero and therefore in the cryogenic temperature range. However, in trajectories where the components face the sun, they become extremely hot with temperatures rising rapidly to more than 200°C. In addition, rather than the Earth’s atmosphere, vacuum prevails. All components that are used in space travel missions must withstand these extreme conditions. For example, material should not expand thermally, should be as free from gas emissions as possible, and be resistant to radiation. In terms of metrology, three factors are crucial: robustness, reliability and long service life. KRP Mechatec GmbH is an engineering services company that has specialized in aerospace structural design, analysis and testing, as well as fusion research. In cooperation with Micro-Epsilon, a measuring platform for high precision thermal deformation measurement of space components has been co-developed.
The experimental verification of thermo-elastically stable structures requires a measurement platform with much higher thermal stability than can be demonstrated by the components to be investigated. In order not to falsify measurement results by deformation of the measuring platform itself, the use of extremely thermally stable sensors and ULE (ultra-low expansion) materials such as Clearceram and Zerodur are necessary. This is a glass ceramic that has extremely low thermal expansion and length stability. One of the measuring tasks is to determine the thermal expansion of star sensor carriers. Star sensors are optical measuring instruments based on CCD elements or other optical sensors. Their task is to search and determine the direction of bright celestial bodies. For aerospace, star sensors are of the utmost importance. They are necessary for determining the attitude and attitude control. For example, satellites also use the stars in the sky to position themselves in space. Star sensors must therefore provide micrometer-accurate measurement results in order to derive from them the exact positioning of satellites in space. Therefore, the supports on which star sensors are mounted must not be subjected to any thermal deformation. The behavior of the material used is therefore tested using high precision capacitive sensors from Micro-Epsilon for displacement, distance and position measurement.
Also in ultra-high vacuum, capaNCDT sensors perform with high precision. With its project “bERLinPro”, Helmholtz Zentrum Berlin [HZB] strives to develop a new technological basis for an “Energy Recovery Linac” (linear accelerator with energy recovery) in order to further develop accelerator technologies. Helmholtz Zentrum Berlin relies for this purpose on Micro-Epsilon’s precise measuring technology. A photo cathode in a superconductive high-frequency electron gun is used to optimize the source of the electron bunches that are to be accelerated. During further acceleration, the electron bunches thereby generated remain more compact than bunches from other sources. This allows them to be used for achieving higher-quality X-rays, among other things. The electron source in the bERLinPro project is aligned by using three capacitive flat sensors together with a DT6220 controller. In the process, the tilting and change of the holder’s position during cooling from room temperature to 2 Kelvin (-271 °C) is monitored. Micro-Epsilon’s DT6220 measuring system was selected because of the combination of prevailing ambient conditions (very low temperatures in the physical low limit range, ultra-high vacuum, x-ray bremsstrahlung (braking radiation) and weak radio-frequency interference) and requirements for sensor type, temperature and long-term stability. Thanks to the system’s modular design, all three measuring locations can be captured with just one controller.
Capacitive sensors from Micro-Epsilon are used in a special design as the Hydrostatic Leveling System CHLS4 in particle accelerators. The altitude of the measuring stations in the kilometer-long pipe system therefore must be inspected continuously, while achieving an accuracy in the submicrometer range in order to ensure high functional safety and efficiency with the particle collision. The concrete pillars have been constructed at a distance of about six to eight meters. The CHLS4 is at their base, with one sensor per concrete base respectively. These sensors are interconnected with a hose system. Similar to the conventional water level principle, a change in height is precisely detected via the water level. For the measurements, several systems are joined together via hoses.
The water level in the “water pot” levels out after a certain time. This water level indicates if both concrete bases are on the same horizontal plane. Due to gravity, the water levels out in the hose so that the water level is the same in all CHLS4 systems – provided that they are all on the same plane. If the height of one of the concrete pillars changes due to earth movement, the sensor detects this with high precision. The ambient conditions are a particular challenge in this measurement task. The sensor is in this case embedded in a protection housing which is equipped with a flat, heated sensor element. This is always a few degrees warmer than its environment and therefore stays dry. The height of the water level can be monitored through the sensor element and a precise distance measurement against the water surface can be carried out. With experiments under radioactive radiation, only the sensor is mounted within the radiation area. The other components such as cable and controller are outside, which is why the system is easy to maintain. Therefore, all components are interchangeable. Particularly with extreme radiation-affected applications such as the particle accelerator, it is possible to place the control and evaluation unit at safe distance from 6-8m due to its long cable length.
In addition to a wide, industry-optimized sensor range, Micro-Epsilon offers special systems which were designed for industrial requirements. The capaNCDT 61×0/IP capacitive displacement measuring system is suitable for inline quality assurance of production processes, and for control of plant and machinery. Protected to IP68, this measuring system is therefore suitable for many industrial measurement tasks. Fluctuating temperatures and magnetic fields are no match for this capacitive system. It is specially designed for industrial measurement tasks where high precision is required. It detects parameters such as displacement, distance, deflection, expansion, and deformation of conductive targets.
The capaNCDT 61×0/IP includes a sensor, sensor cable and controller. Calibrated at the factory, the system is immediately ready for use with no on site calibration necessary. Even after replacing the sensors, no recalibration is required. Data output is via current, voltage or digital RS485 interface. Its aluminum die-cast housing protects the controller from dust and humidity. As the sensor cable is extremely robust, it provides resistance to treading and high temperatures. The cable length is designed for large distances up to 8 meters. More than 15 different sensor models are available for different measurement tasks. Board sensors, cylindrical sensors and models with thread are the three basic sensor types from which the customer can choose depending on the respective installation scenario. The sensor can be replaced easily. All capacitive sensors from Micro-Epsilon are compatible with all controllers. Complex sensor calibration is not necessary.
Almost no other system is able to detect linearity deviations up to 0.005% and better across the whole measuring range. Despite comparatively high conduction capacities up to 1nF (1x10e-9 F), the measurement capacity can be resolved to approx. 0.25aF (0.25*10e-18 F), a relation of 1:4,000,000,000. This is achieved with the corresponding subtleties in the high quality components, from the sensor via the sensor connection cable to the controller. Micro-Epsilon capacitive sensors are principally passive. Therefore, there are no electronic components that are sensitive to temperature which leads to high long-term stability and extreme precision. Capacitive sensors are often used for applications which require highest precision and stability with a resolution down to the submicrometer. Thanks to their innovative technology, capacitive sensors from Micro-Epsilon provide highly accurate readings even in harsh industrial environments. They deliver measured values down to the nanometer range even at extreme temperatures and are suitable for operating conditions ranging from cryogenic temperatures or ultra-high vacuum to dusty industrial environments or clean room applications. Capacitive sensors from Micro-Epsilon also provide long-term stability as there are no integrated components that could reduce the service life. Another strength is the combination variety. Each capacitive sensor can be operated with any Micro-Epsilon controller without requiring any complex calibration. The various interfaces such as analog, Ethernet and EtherCAT enable connection to modern machines and systems. Settings can be made via the user-friendly web interface.
Your email address will not be published. Required fields are marked *
Copyright © 2022. Divya Media Publications Pvt. Ltd. All rights reserved