Resolution is a fundamental specification and one of the most important factors when selecting an encoder. For linear encoders, resolution designates the number of measuring units per distance (inch or millimeter), whereas for rotary encoders, it refers to the number of measuring units per revolution (also called pulses per revolution, or PPR) or per degree of angle. The resolution of an absolute encoder is often given in bits, which are binary units: a 16 bit absolute rotary encoder can measure 65,536 (216) positions per encoder revolution.
It’s important not to confuse resolution with accuracy or repeatability. Accuracy indicates the discrepancy between where the encoder says it is and where it actually is. The primary influence on an encoder’s accuracy is how precisely the measuring units are spaced, although other machine parameters, such as deflection or backlash in the motion components, can also have a detrimental effect on accuracy. Repeatability is the consistency with which the encoder makes the same measurement, and is an important specification for applications that perform repetitive tasks, such as assembly or pick-and-place processes.
Magnetic encoders use permanent magnets placed on the edge of a rotor, with hall sensors that detect changes in the magnetic field as the alternating poles of the magnet pass by. The more magnetic poles and sensors an encoder has, the higher its resolution.
For optical encoders, a disk (or strip, in the case of linear versions) is patterned with transparent and opaque areas that are sensed by a photodetector as a light source shines through. The number and pattern of opaque and transparent sections determines the resolution. Optical encoders typically have between 100 and 6000 segments per revolution, which means they can provide between 3.6 and 0.06 degrees of resolution.
Incremental encoders produce one or two square wave pulses, termed A and B. When only one pulse is produced, the encoder can detect position. For detecting both position and direction, encoders use quadrature output, which produces two pulses—A and B—that are 90 degrees out of phase. Direction is determined by which channel is leading and which is following. Some incremental encoders also produce a third channel (often termed Z) with a single pulse, which is used as the index or reference position for homing.
Quadrature output allows three types of encoding: X1, X2, and X4. With X1 encoding, either the leading (aka rising) or the following (aka falling) edge of channel A is counted. If channel A leads channel B, then the rising edge is counted and the movement is forward, or clockwise. If channel A follows channel B, then the falling edge is counted, and the movement is backwards, or counterclockwise.
With X2 encoding, both the leading and trailing edges of channel A are counted. This doubles the number of pulses counted for each rotation or linear distance increment, which provides twice the resolution.
X4 encoding counts both the leading and following edges of both channels A and B, which quadruples the number of pulses and provides a four-fold increase in resolution.
For rotary encoders, position is calculated by dividing the number of edges counted by the product of the number of pulses per revolution and the encoding type described above (1, 2, or 4), and then multiplying the result by 360 in order to get degrees of motion.
x = type of encoding (X1, X2, or X4)
N = number of pulses generated per shaft revolution or distance
For linear encoders, position is calculated by dividing the number of edges counted by the product of the number pulses per distance and the encoding type. This result is then multiplied by the inverse of the pulses per millimeter (or per inch).
PPM = pulses per millimeter
PPI = pulses per inch
Absolute encoders have multiple concentric rings, or tracks, of opaque and transparent segments on the encoder disk. These tracks start in the middle of the disk, and as they go outward, each track has double the number of segments than the previous track. The first track has one transparent and one opaque ring, the second has two of each, the third track has four of each, and so forth. The number of tracks determines the encoder’s resolution. For example, an absolute encoder with 12 tracks is a 12-bit encoder, which has a resolution of 4096 (212) increments per revolution.
Absolute rotary encoders are further distinguished by whether they are single-turn or multi-turn. A single-turn encoder uses one code disk, and the digital values for position are repeated for each revolution of the encoder. When the measurement is conducted over more than one revolution, a single-turn encoder has no way to determine how many turns the encoder has completed.
When the application requires measurement over multiple encoder turns, a multi-turn version is needed. Multi-turn encoders do not repeat the digital position value until the maximum number of encoder turns—typically 4096—is reached. The most common type of multi-turn encoder is an optical version that uses multiple disks that are geared together. Resolution for this type of encoder is the sum of the output of each disk. So if the primary disk gives 12 bit output, and two secondary disks give 4 bits of output each, the total encoder resolution will be 20 bits, or 1,048,576 unique digital position values.