Approximately three times as many incremental sensors are sold as absolute sensors. A major reason for this is an incremental sensor is usually cheaper than an absolute sensor of otherwise comparable performance.
Things change and nowadays absolute sensors are not as expensive as many might believe. A change to (non-contact) absolute position measurement can offer better performance, better accuracy and lower overall costs. This is because there can be practical problems with the incremental sensor approach. The most obvious one is that every time power is lost the system must perform a calibration step, which slows system performance and may have safety implications if power is lost suddenly.
Secondly, position is calculated by counting from a reference mark. In some instances – notably voltage supply variation or high speed position changes – count can be lost. This has a potentially catastrophic effect on operation which, if unchecked, can lead to prolonged out-ofsynch operation. Most incremental sensors are optical and in order to provide high resolution readings, very fine features on the optical grating must be used – sometimes the features measure just a few microns across. Whilst such fine features increase sensitivity, it also means that they become more delicate and susceptible to foreign matter. Moisture, grease or dirt can cause an optical device to stop working – or, worse still, produce incorrect readings.
The price differential between absolute and incremental sensors has reduced in recent years partly because of the greater use of absolute sensors but, more importantly, the introduction of new, absolute sensing techniques. Whilst optical sensors still remain the automatic choice for some engineers, new generation inductive devices now offer accurate, absolute position sensors that are unaffected by harsh environments.
Rather than a grating and optical sensor, these inductive devices use printed, laminar windings and their fundamental operating principles are similar to those of a transformer or resolver. The fundamental physics enables absolute, compact, lightweight, high resolution sensors, which are not dependent on optical features passing a light source. As well as being fundamentally absolute, they also have other advantages over optical sensors. First, they are unaffected by foreign matter such as dirt or moisture. Second, their measurement performance is generally unaffected by offsets or generous mounting tolerances. This means that they do not require their own precision housings or bearing assemblies but can be simply fixed to the host system’s mechanical parts e.g. a motor or gearbox housing. In turn, this enables radical simplification, size and weight reduction of the surrounding mechanical parts through eradication of bearings, shafts, couplings, seals. Advantageously these new generation inductive devices can be arranged with a generously sized through bore to allow the passage of the host equipment’s shaft, cables or slip-rings. From the design engineer’s perspective this new approach means that absolute measurement performance to be offered at roughly the same price as a traditional incremental device.