If you’ve never really understood the difference between incremental and absolute measurement, don’t worry – you’re not alone. Plenty of engineers have never really got to grips with this terminology. Furthermore, sensor manufacturers have confused matters by claiming absolute measurement when what they really offer is incremental.
Some definitions may help. First, we will use the generic term ‘sensor’ to cover encoders, transducers and detectors. The distinguishing feature of an incremental position sensor is that it reports an incremental change in position. In other words, when an incremental sensor is powered up, it does not report its position until it is provided with a reference point from which it can measure.
An absolute sensor unambiguously reports its position within a scale or range. In other words, when an absolute sensor is powered up it will report its position without the need for any reference information. ‘What happens on power up?’ is a good acid test to differentiate the two types of sensor. If the sensor has to go through some form of calibration step – it’s incremental; if it doesn’t – it’s absolute.
Some sensor manufacturers claim absolute measurement performance because a battery stores position information from the incremental sensor when power is lost. All very well – but what happens when the battery runs out? Similarly, some sensor manufacturers claim absolute measurement performance when an incremental sensor needs to move only a small amount at power up to gain the reference information. These are incremental sensors, marketed and priced as absolute sensors.
Potentiometers are still the most common form of position sensor, but over the past 25 years the use of non-contact sensors has grown significantly. This continuing trend towards noncontact devices is due to the problems associated with potentiometer wear and reliability – especially in harsh environments (notably vibration) or extended life-times. Potentiometers are almost always absolute but a common form of non-contact sensor is the optical encoder. These devices 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 to act as a reference mark.
The absolute optical device is similar but uses a different kind of scale, whereby absolute position is determined at power up – without the need for a reference mark. Typically, these sensors 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.