The packaged shaft encoder is a common format, in which the encoder’s shaft is mechanically connected to the host system. The encoder’s shaft runs in a bearing assembly and carries an optical disk which, in turn, runs in close relationship to the optical detectors. Electrically, the connection is usually a multicore cable which supplies DC power and carries the encoder’s position output data. The simple electrical interface combined with their widespread availability makes them easy to specify and deploy. Their major weakness is that they simply cannot cope with harsh environments that might include vibration, shock, foreign matter or extreme temperatures. There is little or no warning of imminent failure which may result in a worst case scenario of incorrect position data output or – in a best case – an error message. Typically, the reporting of the wrong position (with no error message) is a much more serious failure mode than no reading at all, since the result can be catastrophic.
With larger diameter or ring encoders, the datasheet small print often specifies extremely tight tolerances for the position of the read head to the optical disc or grating in order to achieve the stated measurement performance. Such unpackaged ring encoders are particularly susceptible to foreign matter given the tiny size of the optical features compared with the similarly sized dust or dirt particles.
Unsurprisingly, optical encoders are typically not a preferred choice for high reliability or safety related applications.
What’s an Inductive Encoder?
An inductive encoder – often referred to as an incoder – uses inductive or transformer principles to measure the position of a target or rotor relative to a stator. Incoders use the same fundamental physics as traditional inductive devices such as brushless resolvers or LVDTs but their electrical interface is similar to an optical encoder – a simple DC power supply and digital electrical signal as an output.
Most traditional resolvers look rather like an electric motor – with copper windings on the stator which cooperate with a metal rotor or target. The inductive or transformer coupling between the stator’s windings varies according to the position of the rotor. Rather than wound transformer constructions, incoders use printed circuit boards for their rotor and stator, making them less bulky, more accurate and less costly to manufacture.
Since their use in military aircraft in WWII, resolvers and LVDTs have established a well-earned reputation for accuracy, robustness and reliability, thus often making them the automatic choice for high-reliability and safety related applications. This is because transformer principles of operation are generally unaffected by harsh environmental conditions including the presence of dirt, water and ice.
Incoders are as easy to specify and deploy as optical encoders, because they too only require a DC supply and output a digital signal representing position. This means that incoders have all the advantages of resolvers but with none of their disadvantages.
Because incoders do not use delicate optical components, they are not susceptible to foreign matter and do not only operate in limited temperature ranges. Further, accurate position measurement does not depend on the accurate alignment of the moving and stationary elements – allowing generous installation tolerances and a ‘bearingless’ approach. The eradication for the need for bearings has led to thin annular constructions with low axial height and a large bore – making them easy to integrate in to equipment with tight size or weight constraints such as gimbals, robotic arms and actuators.