Chris Jones, managing director, Micro-Epsilon
When selecting a capacitive displacement sensor, a number of key factors need to be considered, including target size and shape, guarding method, and bandwidth. As well as clean environments, these sensors can operate in dirty, dusty industrial areas.
The capacitive displacement measuring principle is one of the traditional methods used for distance, displacement and position measurement. Considered as one of the most reliable and thermally stable of the non-contact displacement measuring techniques, capacitive displacement sensors achieve resolutions well below 1 nm.
Contrary to what many engineers may think, non-contact capacitive displacement sensors are not only suitable for use in clean environments such as laboratories, clean rooms and operating theatres. The latest sensors are designed to also operate in dirty, dusty industrial environments. Modular designed sensors featuring threaded bodies are also available, which simplifies their mounting in industrial or process manufacturing environments.
Exceptional precision
Capacitive displacement sensors operate on a non-contact, wear-free basis. In practice, they achieve excellent results in terms of linearity, reproducibility and resolution. While sub-micrometre precision is reached in typical industrial environments, high-precision sub-nanometre measurements are possible in clean environments, where dirt, dust, oil or moisture are not present.
Capacitive displacement sensors can be used for the detection of fast-moving objects and in dynamic high-speed processes, thus enabling fast, reliable measurements of motion sequences. In addition, they are suitable for use in vacuums and ultra-high vacuum applications.
Micro-Epsilon DT6220 capaNCDT (capacitive non-contact displacement transducer) sensor system, comprising a capaNCDT DT6220 controller and DL6220 demodulators.
Capacitive displacement measuring principle
Capacitive displacement sensors operate by measuring changes in electrical capacitance. Capacitance describes how two conductive objects with a space between them respond to a voltage difference applied to them. When a voltage is applied to the conductors, an electrical field is generated between them, which causes positive and negative charges to collect on each object. Capacitive sensors use an alternating voltage that causes the charges to continually reverse their positions. This creates an alternating electric current that is detected by the sensor. The capacitance is directly proportional to the surface area of the objects and the dielectric constant of the material between them, and inversely proportional to the distance between them.
The capacitive displacement measurement principle is based on how an ideal plate-type capacitor operates. The distance displacement of the plates (sensor and measurement object) leads to a change in the total capacity. If an alternating current of constant frequency and constant amplitude flows through the sensor capacitor, the amplitude of the alternating voltage on the sensor is proportional to the distance to the target (ground electrode). The distance change between the measurement object and the controller is detected, processed and output as a measurement value by the controller via different outputs.
To ensure stable measurements, a continuous dielectric constant between sensor and target is required, as the system not only depends on the distance between the electrodes but also reacts to dielectric changes in the measuring gap. Therefore, in order to achieve the highest possible measurement precision (i.e. in the nanometre range), the operating environment needs to be relatively clean and dry. For example, oil or moisture in the air gap can affect measurement performance by causing sensor drift and changes in the output signal. That said, some dust or dirt in the air gap is acceptable as the sensors typically operate at such high resolutions that the effects of some dust particles are quite low.
Material types
In an electromagnetic process, a capacitive measuring system measures electrically conductive objects with constant sensitivity and linearity as standard. It evaluates the reactance of the plate capacitor, which changes in proportion to the distance. As there is no interference caused by the optical characteristics of the target, even transparent or reflective surfaces can be measured at high precision. Examples of conductive measurement objects are metals, silicon, graphite and water.
Capacitive displacement sensors can also measure insulating materials such as adhesives, ceramics, glass, oils and plastics. Here, the sensor guard ring acts as a ground electrode and the insulating material as a coupling medium. An almost linear output signal for insulators is also achieved by using special electronic circuitry. Capacitive displacement sensors are typically used on metals, but advice and guidance should be sought from the sensor supplier when measuring on insulators.
Capacitive displacement measuring principle.
Active guarding
In most capacitive displacement sensing applications, the sensor is one of the conductive objects and the target object is the other. For accurate measurements, the electric field from the sensing area needs to be contained within the space between the sensor and the target. If the field is allowed to spread to other items or areas on the target, a change in the position of the other item will be measured as a change in the position of the target. Guarding is therefore used to prevent this from happening.
Most sensor suppliers use a guard ring principle, although some suppliers offer a guard ring principle combined with a special triaxial cable (double guarding). Extremely small measuring distances cause an equally small change in the signal. This means there are only a few electrons between sensor and controller for an indicated change in distance. If very small leakage (parasitic) currents flow on the path from the sensor to the controller, the distance measurement is no longer accurate. Therefore, very complex radio frequency (RF) triaxial cables are required.
Micro-Epsilon uses RF triaxial cables that possess excellent shielding characteristics and thus actively guard the field in order to ensure consistently high signal quality and low noise. The company has developed cables up to 8 m long that do not require a preamplifier and yet afford full exchangeability between the sensor and controller; these are specifically intended for industrial environments requiring longer cable lengths.
High stability
As thermally induced conductivity changes of the measuring object have no influence on measurements, the capacitive principle is reliable even with fluctuations in temperature. As well as temperature stability, the long-term stability guarantees reliable operation over many years without parts or components needing to be replaced.
Calibration
In an experimental environment where the measuring range will typically vary from test to test, the user requires a capacitive displacement measurement system that allows this. A system that needs re-calibrating each time a different measuring range is required would be costly and time-consuming. It is therefore important to select a sensor supplier that can offer a system whereby various capacitive sensors can be exchanged without having to send the sensor back to the supplier’s factory for recalibration each time.
Electrical runout and bandwidth
Electrical runout occurs in rotating ferrous targets. It shows up as a very repeatable error once per revolution on the system output and is caused by small variations in the permeability and conductivity along the circumference of, for example, a rotating shaft. Whilst eddy current displacement sensors are affected by electrical runout, capacitive displacement sensors are completely unaffected by this problem.
As they are based on an analogue circuit, capacitive displacement sensors measure using a measurement frequency or bandwidth rather than a measuring rate. This means they are suitable for measuring vibration, amplitude, oscillation and shaft runout. Some suppliers offer capacitive sensors that measure up to 20 kHz (-3 dB) bandwidths, making them ideal for high-speed measurements on rotating shafts.
Micro-Epsilon capaNCDT (capacitive non-contact displacement transducer) sensor diagram.
Target size and shape
The target size is a key consideration when selecting a sensor for a specific application. The further the sensor is from the target, the larger the minimum target size. If the target area is too small, the electric field tends to wrap around the sides of the target, which results in the electric field extending further than it did during factory calibration and the target being measured as further away. In general, capacitive displacement sensors require a target size ratio of 1:1 with the size of the measurement electrode. Smaller or narrower targets can be measured but require some special adaptation from the supplier.
The shape of the target is also important. As most sensors are calibrated to a flat target, measuring a target with a curved surface will cause errors. As the sensor will measure the average distance to the target, the gap at zero volts will be different to when the sensor was calibrated. The electric field behaves differently on a curved surface compared with a flat surface. If a curved or non-flat surface must be measured, the measurement system can be factory calibrated to the final target shape. However, it is recommended that advice is sought from the sensor supplier as some customisation will be required.
Micro-Epsilon
Common capacitive displacement sensor diagram.