Inductive Versus Capacitive Position Sensors

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1 T E C H N I C A L W H I T E P A P E R Inductive Versus Capacitive Position Sensors Author: Darran Kreit, Technical Manager, Zettlex UK Ltd File ref: technical articles/inductive vs. capacitive_rev2.0 w w w. z e t t l e x. c o m

Introduction Inductive and capacitive position sensors can look rather similar. Both use a non-contact, AC electrical technique to measure position and both can be built using printed circuit boards.

2 Introduction Inductive and capacitive position sensors can look rather similar. Both use a non-contact, AC electrical technique to measure position and both can be built using printed circuit boards. Nevertheless, the fundamental physics on which the sensor rely are very different and this means that each technology is suited to particular geometries and applications. This paper describes the physics behind each technology and outlines the consequent strengths and weaknesses of each approach. Operating Principles Capacitive Sensors When German scientist Ewald Georg von Kleist suffered an electric shock from his laboratory apparatus in 1745, he suddenly learnt 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. A simple mathematical model of a capacitor is shown n Figure 1:- Figure 1. A simple capacitor model T E C H N I C A L W H I T E P A P E R / 1

3 Capacitors are used various sensors, notably in the touch sensors of portable devices such as mobile phones and tablet computers. These capacitive sensors detect the absence or presence of a person s finger, acting 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. As can be seen by the mathematical formula in Figure 1, capacitance varies in proportion to the distance between the plates (d) as well as their overlap (A). Displacement can be measured axially (variation in d) or in the planar direction of plate overlap (variation in A). Advantageously, capacitor plates can be generated using printed circuit boards as illustrated in Figure 2. Figure 2. Capacitor plates on a printed circuit board In order to store any significant amount of charge, the separation dimension d must be small compared to the area of the plates. Dimension d is usually <<1mm. Hence, such a technique is well-suited to load or strain measurement which 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 A, 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. As the two elements displace relative to each other, A varies. 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 there is the possibility of ingress of foreign matter or large temperature swings. Unsurprisingly, capacitive sensors are not suitable for environments where condensation may occur at lower temperatures. Given the inherent physics, the distance between the sensor s plates must be kept small relative to the size of the capacitor plates and set within tight limits. This can require extremely precise mechanical installation of the sensor and this may not be practical or T E C H N I C A L W H I T E P A P E R / 2

4 economical, as differential thermal expansion, vibration or mechanical tolerances of the host system will cause the separation distance to vary and hence distort measurement. Further, the measured effect in a capacitive sensor is inherently 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 it will report a credible but incorrect reading. In some instances earthing of the hosts system 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 etc. Operating Principles Inductive Sensors In 1831, Michael Faraday, discovered that an alternating current flowing in one conductor could induce a current to flow in an 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 basic theory can be explained by considering two coils - a transmit coil (Tx), with an alternating current applied to it, and a receive (Rx) coil, in which a current will be induced: FIGURE 3. Faraday s Induction Law T E C H N I C A L W H I T E P A P E R / 3

The voltage signal in the receive coil is proportional to the relative areas, geometry and displacement of the two 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

Unlike capacitive methods, inductive techniques are much less affected by foreign matter such as water or dirt.

5 The voltage signal in the receive coil is proportional to the relative areas, geometry and displacement of the two coils. However, as with capacitive techniques, other factors can also affect the behaviour 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. Figure 4. Examples of typical harsh environments for inductive sensors Inductive sensors provide a robust, reliable and stable approach to position sensing, and are thus the preferred choice in applications where harsh conditions are common, such as defence, aerospace, industrial and the oil & gas sectors. 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 accurately in order to achieve accurate position measurement. A significant number of coils must be wound in order to achieve strong electrical signals. This wound spool construction makes traditional inductive position sensors bulky, heavy and expensive. T E C H N I C A L W H I T E P A P E R / 4

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.

6 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 is the approach taken by Zettlex. It means that the windings can be produced from etched copper or by printing on a wide variety of substrates such as polyester film, paper, epoxy laminates and even 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. Figure 5. Example of an inductive sensor with printed laminar construction Electromagnetic noise susceptibility is often cited as a concern by engineers 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. T E C H N I C A L W H I T E P A P E R / 5

7 Summary The benefits of each of the three approaches are summarised in table below. It can be seen that of the three, the non-traditional inductive approach using printed laminar coils, used by Zettlex, provides the greatest number of advantages: Capacitive Inductive (Traditional coils) Inductive (Printed Coils) High Resolution High Repeatability High Accuracy Resilience to Dirt, Water or Condensation Resilience to electrostatic effects Robust EMC Operation Low Thermal Drift Easy to Install? Compact Lightweight Economical? Figure 7. Table listing the strengths of each approach T E C H N I C A L W H I T E P A P E R / 6

8 Further Information / Contact For more information about Zettlex inductive position sensing technology, or to discuss your application with a position sensor expert, please contact Zettlex directly or speak with your nearest local representative. UK Head Office Zettlex UK Ltd Newton Court, Newton, Cambridge, CB22 7ZE, United Kingdom Sales Contacts: Mark Howard or Darran Kreit info@zettlex.com Telephone: Web: International Zettlex has a worldwide network of resellers and distributors. To find your local representative please visit T E C H N I C A L W H I T E P A P E R / 7

A Comparison of Inductive and Capacitive Position Sensors

T E C H N I C A L W H I T E P A P E R A Comparison of Inductive and Capacitive Position Sensors Author: Mark Howard, General Manager, Zettlex UK Ltd File ref: technical articles/inductive vs. capacitive_rev4.0