3 Incremental and absolute encoders

Incremental encoders typically use ABZ digital or analog unidirectional interfaces, where two quadrature-encoded digital pulse train signals (A and B) or two analog Sin/Cos signals (A and B), allow for a low latency relative angle measurement with high resolution from approximately 10 bits up to 28 bits. An optional index (Z or I) enables absolute mechanical angle information. Incremental encoders do not provide an absolute angle at startup, and need to turn up to one revolution before the index occurs. Therefore, these encoders fit well in speed-variable applications that need very low latency (<1µs) but do not need an absolute angle at startup.

Conversely, absolute single- or multiturn rotary encoders offer an absolute angle position at startup. They offer a bidirectional RS-485 interface with vendor-specific protocols, and enable time-triggered angle measurement as well as information such as rotary speed and number of rotary turns. The angle resolution typically goes from 10 bits to >30 bits, with a latency as low as 10µs to meet a wide range of industrial applications. The position resolution is typically the data format, which transmits through the digital interface. For example, an angle with a 20-bit integer format has a resolution of 360/2²⁰; 0h = 0 degrees and 0xFFFFF = 360 degrees-360/2²⁰. Overall system noise is significantly higher than quantization noise; the effective number of bits (ENOB) characterizes this effect.

Equation 1 calculates the ENOB of the angle with the standard deviation of the angle measured in degree:

The root mean square of the angle noise signal equals the standard deviation (1 sigma). Figure 1 illustrates the angle accuracy; the related angular error is larger than the standard deviation. The angle accuracy not only depends on the peak noise, which often uses the 6-sigma value, but also the nonlinearity over one revolution.