First, let’s deal with the terminology. Some engineers ask about the difference between angle sensors, angle transducers, angle encoders, rotary sensors and rotary encoders. The answer: – nothing. All these devices provide an electrical signal in proportion to angle or change of angle.
For the purposes of this article we shall simply use the term ‘encoder’.
Incremental Encoders and Absolute Encoders
Encoders can be divided in to two basic types – incremental encoders and absolute encoders.
The distinguishing feature of an incremental encoder is that it reports a change in angle. In other words, when an incremental encoder is powered up, it does not report its angular position until it is provided with a reference point from which it measures.
An absolute encoder unambiguously reports its position within a scale or range. In other words, when an absolute encoder is powered up it will report its angle without the need for any reference information or movement. ‘What happens on power up?’ is a good acid test to differentiate between an absolute encoder and an incremental encoder.
Confusingly, some manufacturers are now marketing ‘pseudo-absolute’ encoders. These encoders must go through some form of ‘wake and shake’ routine at start up in order to determine absolute angle. They are more accurately described as incremental encoders that need a limited calibration step before producing absolute data.
If an angle encoder is required to go through some form of calibration step – it’s incremental; if it doesn’t – it’s absolute.
Angle Encoder Technology
Industry still uses more potentiometers than anything else to measure angle. Nevertheless, over the past 25 years, the use of non-contact techniques has grown significantly. The continuing trend towards non-contact measurement is due to the problems of potentiometer wear and reliability – especially in harsh environments (notably vibration) or extended periods.
Optical encoders are a common form of non-contact rotary encoder. They work by shining a light through or onto an optical grating and calculating position from the intensity of the returned light. Most optical devices are incremental. Typically, the position information is delivered using a series of pulses – usually in phase quadrature, so that direction of travel can be determined. These are usually referred to as A/B pulses. A separate pulse train, typically referred to as the Z reference, provides one pulse per revolution as a datum or reference mark.
Fig. 1 – Schematic of an incremental encoder with a reference pulse.
The absolute encoder is similar but uses a different type of scale. This arrangement allows absolute angle to be determined at power up – without any reference mark. Usually, absolute encoders have a digital output and their resolution is defined by the number of bits in the output. A 10-bit device will offer 1,024 counts; an 11-bit device will offer 2,048 counts and so on.
Angle Encoder Communications
Traditionally, there has been two ways for absolute encoders to report angle
– serial data or parallel data. The use of high speed serial data now dominates with parallel data seldom used nowadays. Serial data is usually provided according to the RS-422 hardware standard and in various formats. The most popular formats for absolute encoders are SSI (Synchronous Serial Interface), BiSS-C and SPI (Serial Peripheral Interface). Notably these are open standards. Some encoder manufacturers have launched and promoted their own closed communications standards in an effort to trap unwary customers in to becoming locked in to using only their products. Watch out!
The Relative Merits of Absolute Encoders and Incremental Encoders
Traditionally absolute encoders have been more expensive than incremental encoders. Although this is still true, the difference is not that great.
A change to (non-contact) absolute encoding 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-of-synch operation. Most incremental encoders are based on optical techniques and 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. Fluff, condensation, grease or dirt can cause an optical encoder to stop working – or, worse still, produce incorrect readings.
Optical Encoders & Inductive Encoders
The price difference between absolute encoders and incremental encoders 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 a frequent choice for some engineers, new generation inductive encoders (sometimes referred to as incoders) now offer accurate, absolute angle measurement which is unaffected by harsh environments
Rather than a grating and opto-detector, inductive encoders use printed, laminar windings and their fundamental operating principles are similar to a transformer or resolver. Their basic physics enables absolute, compact, lightweight, high resolution encoding. As well as being fundamentally absolute, they have other advantages:- they are unaffected by foreign matter and their measurement performance is generally unaffected by offsets or mounting tolerances. This means that they do not require their own precision housings or bearing assemblies but can be simply screwed to the host system e.g. a motor or actuator. In turn, this enables radical simplification, size and weight reduction of the local machinery by eradicating bearings, shafts, couplings, seals etc. Advantageously, these new generation inductive encoders are axially thin and can be arranged with a generously sized bore to allow passage of shaft, cables or slip-rings.
Fig 3 – New generation inductive encoders are increasing the number of absolute encoders