Expressions of feedback resolution for absolute encoders

Unlike the relative values output by an incremental encoder, every shaft position output by an absolute encoder corresponds to a unique digital value or word. This is in part how these encoders track shaft position even if there’s a power interruption.

Multiturn absolute rotary encoders and sensors report two aspects of shaft position — the angular single-turn (ST) position over one revolution and the overall multiturn (MT) count — how many full revolutions have occurred. So, an encoder’s ST resolution is the number of angular positions it can track over one rotation. An encoder’s MT resolution is the number of full revolutions it can uniquely count before the digitally reported value resets to zero.

Confusing matters is that for certain binary counter-based encoder technologies (including some using optical or inductive operation) these two values are expressed with the same units of bits and can even be equal. For example, the data sheets for some suppliers’ optical encoder product lines list ST and MT values together as in:

Product version one = 11 bit ST + 12 bit MT
Product version two = 12 bit ST + 12 bit MT
Product version three = 13 bit ST + 12 bit MT
Product version four = 14 bit ST + 12 bit MT
Product version seven = 17 bit ST + 12 bit MT
Product version nine = 19 bit ST + 12 bit MT

A given bit can represent one of two states (0 or 1) so combining bits makes possible unique values exponentially grow. The resulting bit count is how many distinct numerical states (binary digits) can be output.

Such expressions of bit depth — determined by the electronics’ memory depth, counter width, and interface word length — reflect how counter-based encoders’ circuits (independent of sensing physics) process, code, and store counts of partial and full rotation increments.

Consider product version four from the example lineup having 14-bit ST and 12-bit MT resolution. This encoder can discern 2¹⁴ or 16,384 different angular positions around a single revolution. It can also keep count of up to 2¹² or 4,096 turns of the shaft.

The enduring prevalence of 12-bit resolution in feedback systems derives from how it was and remains a “sweet spot” balancing practical design tradeoffs between signal hardware memory, power consumption, and cost as well as physical system needs and limitations.

Counting past the published limit

As mentioned, typical multiturn absolute encoders that can track (for example) 4,096 revolutions of the encoder wheel provide accurate position information only until the encoder makes more than 4,096 turns. Then the digital position values begin to repeat. But some drives and controllers offer a special type of position tracking called modulo positioning that can store any overflow movement (rotations or partial rotations that occur after 4,096 turns) and use this information to provide accurate positioning even beyond the maximum number of encoder turns.

Other multiturn-resolution expressions

In contrast with MT resolutions expressed in bits, MT resolutions expressed in degrees or turns that aren’t some whole power of 2 (for example, 44 turns) may obliquely indicate that the sensor is using some other technology besides a digital counter.  Some such sensors include those using magnetic pole patterns, phase-shifted tracks, or other coding geometry. For these structures, total MT is determined by their combined periodicities and the interpolation algorithms used to resolve them.