Absolute encoders measure absolute or true angular position. This contrasts with incremental encoders which measure the change in angular position. The test as to whether an encoder is absolute or incremental is what happens at power up. If the encoder has to move or go through some ‘wake and shake’ process to determine its position then it is incremental. If it outputs its true position without motion then it is absolute.
Absolute encoders typically output either a digital data stream, such as serial synchronous interface (SSI) or an analogue signal such as 0-10V or 4-20mA. Incremental encoders typically output pulses which are often described as A/B pulse or ‘A quad B’ encoders.
Traditionally, the cost of an absolute encoder is more than a comparable incremental encoder. Whilst this remains true, the difference is diminishing as the use of absolute encoders is increasingly preferred over incremental encoders.
Unfortunately, some incremental encoders are marketed by some less scrupulous encoder manufacturers as absolute – because the required motion at start up is small. In reality, they are incremental encoders with sharp marketing.
Optical encoders are the most common form of encoder on the market and they can offer good measurement performance. They are not well suited to harsh environments with extreme temperatures, shock, vibration, fluids or foreign matter. In more aggressive environments, an increasingly popular choice for absolute encoding is the inductive encoder or incoder.