Slim, lightweight position encoder is flexible and low cost

Unique position encoder technology is now available that offers slim, lightweight, low cost designs suitable for a variety of industrial applications. Dean Palmer reports A company from the heart of Cambridge's Silicon Fen has developed a unique, compact design of position encoder that is based on a new electromagnetic resonance technology. At only 50 microns thick, two of the encoders could comfortably fit within the thickness of a typical piece of paper. The technology's main components are based on simple printed circuit boards (PCBs) and, when conformally coated, require no mechanical housing. The sensors can be sealed to IP68 and are rated from -40°C up to 125°C. If flexible PCBs are used, the technology can be extended to carry power and signals, therefore avoiding the use of cables and connectors. With the application of contact adhesive on one side of the PCB, mechanical fixings can also be avoided when connecting to fascia panels. The company responsible for developing the new technology is Zettlex, a technology start up firm comprising of four engineers with bags of experience in sensor technologies. According to Zettlex's managing director Mark Howard, target applications include non-contact displacement encoders where cost, space, dirt or weight issues mean that traditional sensors cannot be used. Other applications include rotary knobs or linear encoders for user interfaces in tough environments such as off-road vehicles, medical devices and domestic appliances. In the Nineteenth Century, Michael Faraday discovered that a current flowing in a first conductor could induce a current to flow in a second conductor. Zettlex's technology is based on this physical phenomenon of electrical inductance. Engineers may be familiar with inductive (non-contact) proximity sensors, where the presence or absence of a metallic disc is sensed to provide a simple switch output. But while Zettlex uses this same physical phenomenon, the method of operation is very different to proximity sensors. Rather than a conductive disc, a resonant 'target' is used, made from an inductor and capacitor in series. The target is attached to the object whose position is to be measured. A (normally) stationary 'antenna' is supplied with an AC signal to produce a low power AC-electromagnetic field. As the target enters the antenna's field, currents are induced to flow in the target which, in turn, produce their own electromagnetic field. The interaction of the two fields is sensed by the antenna so that the identity and position of the target can be determined. The technology has been patented for eight years now but was originally purchased from a Japanese company that, said Howard, "clearly didn't understand the potential of what it had discovered". Zettlex spent all of last year in its laboratories, refining the technology. At the end of 2004, the company had a finished product. There are currently more than 50 product design variants. The encoder can be adapted to almost any application. The linear version of Zettlex's technology is the simplest and ranges from 1mm up to 2,850mm. Here, a target's position along an axis can be measured and an electrical analogue of the position provided to a host system. Zettlex has used this basic technique to construct a wide variety of geometrical arrangements including rotary, 2D, 3D, curvi-linear, 2D + rotation, and roll/pitch/yaw. Unusually, the sensors can be designed so that measurement performance is generally unaffected by mechanical variations in other axes. Howard explained: "This is an important feature of the technology and means that high performance sensors can be constructed even with generously-toleranced [low cost] mechanical housings or mouldings, as might be bulk produced in the Far East. Zettlex has developed four versions of the technology to suit different geometrical or functional requirements, ranging from small scale deflections of around 0.1mm through to complex multi-axis loci of multiple targets over several cubic metres. Howard said that often the company uses PCBs as the basis of the 'targets' and 'antennae' because they are readily available and are an inexpensive method of producing accurate patterns of conductors. But, he explained, PCBs are not strictly necessary as the key element is the pattern of electrical conductors in the target and antenna. Such patterns can be formed by a variety of means, including conductive inks, wire forms and metal pressings. Zettlex's measurement technique is fundamentally absolute. Howard explained: "A measurement resolution of 12 bits of full scale is typical, although 32 bit measurements have been achieved. Temperature or humidity variations are generally negligible and the temperature ranges are usually determined by the grade of electronics used." Another unusual feature of the technology is that a single set of electronics can control a number of antennae and targets. Also, the electronics can be placed some distance from the antennae so that the antennae may be placed in harsh environments and the electronics in a more benign location. For this reason, units can be sealed to IP68 and can operate under water or salt spray. Although the technology is still being refined, some UK companies have already agreed to try out the encoder in their own products. Liverpool-based instrumentation manufacturer ITI is one of Zettlex's early adopters. It has chosen Zettlex's encoder technology as the basis of its new 'EGAUGE' range of products. EGAUGE transforms mechanical, analogue gauges into electronic transducers capable of providing both local and remote indication of a parameter's magnitude. A Zettlex target is attached to the gauge's pointer and an antenna forms part of the gauge's glass assembly. The pointer's position can be measured and an electrical output provided. But the really clever thing is that because of the sensor's measurement accuracy, there is negligible loss in overall accuracy and any mechanical misalignments of variations do not affect the gauge's performance. EGAUGE is a system that is readily fitted to a wide range of gauges and the electronic outputs can be configured to the user's requirements (eg. 4 to 20mA, 0 to 5V, PWM, as well as programmable levels of hysteresis, specific switching positions, etc). Joe Murray, managing director at ITI told Eureka: "You can probably imagine that our gauges have to operate in some pretty arduous environments for many years. Given this and our requirements for measurement accuracy without affecting the gauge's performance, it's a pretty tough call for any sensor. I'm pleased to say that Zettlex has lived up to its promises." The technology has also been used in audio mixing studios, to replace potentiometers on the sliders on mixing desks. Dirt often gets between the contacts on the potentiometers underneath the sliders, so recordings can often get messed up. Zettlex used a linear encoder to replace the existing potentiometers. Currently, the most widespread non-contact sensor technology is Hall Effect sensing, where the position of the magnet is sensed relative to a Hall chip. Unlike Zettlex's technology, Hall Effect is fundamentally relative and requires precise mechanical arrangements in order to ensure good measurement performance. Also, Hall Effect is not very well suited to long, linear or complex measurement geometries and can be 'upset' by DC or low frequency AC fields. Dr Darran Kreit, chief technologist at Zettlex, concluded: "We hope the market will find the slim, lightweight form of the sensor attractive, but I suspect the slim price tags will be the key to our success." ointers * The technology is a low cost, non-contact sensing and identification technique * The operating principle is inductive resonance, based on mutual inductance between an antenna and a target * The technology could replace Hall Effect sensing where DC or low frequency AC fields are a problem or for long, linear or complex measurement geometries applications Eureka Says: As well as offering technical advantage to users, the real success factors for this technology is that it's also low cost and can be readily adapted to almost any industrial or consumer product sensing application.