by Mark Howard, Co-founder, Zettlex UK
When it comes to inductive and capacitive position sensors, each is built quite differently, meaning that each technology is suited to particular geometries and applications.
When German scientist Ewald Georg von Kleist suffered an electric shock from his laboratory apparatus in 1745, he suddenly learned that it was possible to store a substantial electrical charge. Arguably, he had built the world’s first capacitor. A capacitor is a collector of electrical charge and typically comprises two conductive plates separated by a relatively small thickness of non-conductive material, or dielectric. The dielectric is typically air, plastic or ceramic.
Capacitors are used in various sensors, notably in the touch sensors of mobile phones and tablet computers. These capacitive sensors detect the absence or presence of a person’s finger, as an alternative to a push button switch.
Another type of capacitive sensor is the capacitive displacement sensor, which works by measuring change in capacitance from the change in dimensions of the capacitor.
To store any significant amount of charge, the distance between the plates must be small compared to their surface area. Such a technique is suited to load or strain measurement that might cause relatively large changes in this small dimension. Similarly, capacitive linear or rotary sensors can be arranged so that displacement causes a variation in the effective overlap of the plates. In other words, one set of plates is on the moving element of the sensor while the other set is on the stationary element.
Unfortunately, capacitance is also sensitive to factors other than displacement. If the capacitor’s plates are surrounded by air, then its permittivity will also vary with temperature and humidity, because water has a different dielectric constant to air.
A nearby object, which varies the permittivity of the surrounding area, will also vary the capacitance. With a touch sensor, it is the water in the finger that causes a change in local permittivity, changing the capacitance and thus triggering a switch. This is why the operation of unresponsive touch sensors can be improved by wetting the end of the operator’s finger.
Unless the surrounding environment can be sealed or tightly controlled, capacitive sensors are not suited to harsh environments where condensation may occur at lower temperatures; there is the possibility of ingress of foreign matter or large temperature swings.
The inherent physics between the plates require precise mechanical installation of the sensor, which may not be practical or economical, as differential thermal expansion, vibration or mechanical tolerances of the host system will cause the separation distance to vary and distort measurement.
Furthermore, the measured effect in a capacitive sensor is linked to the storage of electrical charge on a capacitor’s plates. If the host system can generate electrostatic energy while it is in motion — from rubbing, sliding or rotating parts — this energy can disturb the sensor. In extreme cases the sensor will not work at all or, worse still, the electrostatic disturbance will report a credible but incorrect reading. In some instances, earthing of the hosts system’s component is required to dissipate the electrical energy away from the sensor’s plates. This is frequently necessary in capacitive angle sensors where a rotating shaft generates static charge from relative motion of bearings, gears, pulleys and so on.
Operating principles—inductive sensors
In 1831, Michael Faraday discovered that an alternating current flowing in one conductor could “induce” a current to flow in the opposite direction in a second conductor. Since then, inductive principles have been widely used as a basis for position and speed measurement in devices such as resolvers, synchros and linearly variable differential transformers (LVDTs).
The voltage signal in the second conductor is proportional to the relative areas, geometry and displacement of the two coils. However, as with capacitive techniques, other factors can also affect the behavior of the coils. One such factor is temperature, but this effect can be negated by the use of multiple receive coils and by calculating position from the ratio of the received signals (as in a differential transformer). Accordingly, if temperature changes, the effect is cancelled out since the ratio of the signals is unaltered for any given position.
Unlike capacitive methods, inductive techniques are much less affected by foreign matter such as water or dirt. Since the coils can be a relatively large distance apart, precision of the installation is less of an issue, and the principal components of inductive position sensors can be installed with relatively relaxed tolerances. This not only helps to minimize costs of both sensor and host equipment, but also enables the components to be encapsulated, allowing the sensors to withstand environmental effects, such as long-term immersion, extreme shock, vibration or the effects of explosive gaseous or dust-laden environments.
Inductive sensors provide a robust, reliable and stable approach to position sensing, making them the preferred choice in applications where harsh conditions are common.
Despite their robustness and reliability, however, traditional inductive sensors have some downsides, which have prevented their uptake from becoming more widespread. Their construction uses a series of wound conductors or spools, which must be wound properly in order to achieve accurate position measurement. A significant number of coils must be wound to achieve strong electrical signals. This wound spool construction makes traditional inductive position sensors bulky, heavy and expensive.
A different approach to inductive sensing
Another approach to inductive sensors uses the same physical principles, but laminar printed constructions are used instead of wire wound spools. This means windings can be produced from etched copper or by printing on a variety of substrates, such as polyester film, paper, epoxy laminates and ceramics. Such printed constructions can be made much more accurately than windings. Hence a far greater measurement performance is attainable at less cost, bulk and weight — whilst still maintaining the inherent stability and robustness of the inductive technique.
Engineers often cite electromagnetic noise susceptibility as a concern considering inductive position sensors. The concern is misplaced given that resolvers have been used for many years within the harsh electromagnetic environments of motor enclosures for commutation and speed control. As with temperature stability, robustness in harsh electromagnetic environments can be achieved using a differential approach whereby the electromagnetic energy entering different parts of the sensor is effectively self-cancelling. This is why inductive sensors such as resolvers and LVDTs have been the preferred choice for civil aerospace applications for many years.