Glossary of Terms


Absolute Encoder

An absolute encoder is a device that reports the shaft angle within a 360 deg. range. An absolute encoder does not lose its position after a power-down and provides the absolute position upon power-up without requiring a home cycle or any shaft rotation. A traditional absolute encoder has one track per encoder bit, and provides one fixed resolution. Thus, a 1024 position encoder would have 10 tracks (one bit per track). US Digital absolute encoders use a single barcode track that is precisely read by an internal microcontroller or DSP. This enables the resolution, the zero point, and the incrementing direction to be set by nonvolatile configuration parameters in the field. Absolute encoders do not provide quadrature outputs. Rather, they report their position as a digital word or as an analog voltage. An incremental encoder can serve as a pseudo-absolute encoder by providing a battery backup power supply to the encoder or by executing a home cycle after power-up. A true absolute encoder does not need a backup power supply or a home cycle.



A channel is an electrical output signal from an incremental encoder. Channels are designated A and B for the two quadrature outputs and I or Z for the index output. See Quadrature.


Cycles Per Revolution. The number of full quadrature cycles per full shaft revolution (360 mechanical degrees). A 200 CPR encoder can provide 200, 400 or 800 positions per revolution depending on whether x1, x2 or x4 quadrature decoding is done. See Resolution.


One complete four-state quadrature cycle. One quadrature cycle is generated by one line and one window, called a line pair on the encoder disk.

Each cycle is divided into 360 electrical degrees (°e) and can be decoded into 1, 2, or 4 counts, referred to as x1, x2, or x4 resolution multiplication. See Resolution.

Cycle Error

The difference between the actual shaft position and the position indicated by the encoder cycle count. An indication of cycle uniformity. The difference between an observed shaft angle which gives rise to one electrical cycle, and the nominal angular increment of 1/N of a revolution.


Duty Jitter

Variation from pulse width to adjacent pulse width.


Electrical Degree (°e)

It is usually obvious whether the "degree" unit refers to an angular, mechanical degree (e.x. the position of a shaft in degrees) or the phase delay/position in degrees of an electrical signal (e.x. the 90 degree phase difference between the A and B quadrature channels). If the context could be unclear, the term "electrical degree" is used to refer to a degree of phase difference between 2 signals or the phase position in one cycle of a signal (e.x. the 90 electrical degree phase difference between the A and B quadrature signals.). There are 360 electrical degrees in one cycle of a signal.

Eccentricity Error

Measuring error of a rotary encoder caused by an eccentricity in the mounting of the encoder disk.


Home Cycle

Adding an index channel to an incremental encoder allows the encoder to provide absolute position. This is done by executing a home cycle after each power-up. A home cycle is completed by rotating the shaft after power up until the index is detected, then resetting an external position counter. See Pseudo-Absolute Encoder.


A Hubdisk assembly consisting of an aluminum hub and an optical encoder disk.



A device which reports the angle of an object with respect to gravity. Also known as a "tilt sensor".

Incremental Encoder

An incremental encoder is made up of 2 major parts: the disk and the sensor. The disk of an incremental encoder is patterned with a single track of repeating identical lines near the outside edge of the disk. The number of line pairs on the disk determines the encoder resolution (CPR). Virtually all incremental encoders produce quadrature outputs which indicate the speed, angle and direction of the shaft. Incremental encoders are commonly used as feedback devices for motor controllers. They also serve as operator interfaces and are sometimes called rotary pulse generators. An operator can turn the knob of a front panel encoder to control a parameter or motor. These are also referred to as "soft" controls, since they have no stops, and the limits and controlled parameter can be set by software rather than hardware. An incremental encoder can serve as a pseudo-absolute encoder by providing a battery backup power supply to the encoder or by executing a home cycle after power-up. See Pseudo-Absolute Encoder.

Index (Ch. I)

Also referred to as the Z-channel. The index channel is normally a once per revolution pulse sent from the encoder. This output can be used to indicate a zero or home position on the encoder. The index outputs of US Digital encoders are internally gated to coincide with the low states of channels A and B. This provides a precise index position that is 1/4 of one quadrature cycle wide. Normally, the index output goes high once per revolution. Custom disks can be made to provide multiple indexes per revolution. Adding an index channel to an incremental encoder allows the encoder to provide absolute position. See Home Cycle.


This is an analog method of multiplying the resolution of an encoder by some integer factor. Interpolation can only be done on analog (not digital) output encoders that provide sine and cosine analog outputs. The output from an interpolator can be analog (sine and cosine) or digital quadrature signals.


Multiturn Rotary Encoder

Absolute rotary encoder which determines the angular position of the shaft and the number of shaft rotations.


Optical Encoder

A rotary encoder is a sensor that uses light to sense the speed, angle and direction of a rotary shaft. A linear encoder reads a linear strip instead of a disk to provide the same information for linear motion. Optical encoders use light instead of contacts to detect position, so they are inherently free from contact wear and the digital outputs are bounceless (no contact bounce). The accuracy of an optical encoder is as good as the codewheel. US Digital makes the codewheels for all of our encoders. The code wheel patterns are created using precision digital plotters and cut using either a punching system or a laser, each guided by closed loop precision vision systems. The light source used for all US Digital encoders is a point source LED, rather than a conventional LED or filament. Most optical encoders are transmissive type. The light is collimated light into parallel light rays and passes through the disk (or strip) pattern. The image of the pattern is detected using a phased array monolithic sensor and converted to TTL digital quadrature outputs. Reflective type encoders (such a US Digital E4 and S4 series) bounce collimated off a patterned reflective code wheel. Fitting all of the electronics of a reflective encoder onto one side of the code wheel makes it a more compact design than transmissive types. See Absolute Encoder and Incremental Encoder.



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The delay in time or degrees between the rising edge of channel A and the rising edge of channel B. Also defined as the delay between the center of the high state on channel A to the center of the high state on channel B.

Phase Jitter

Variation from rising edge on channel A to rising edge on channel B.

Phased Array

Unlike traditional encoders that use an aperture plate and two discrete photo sensors, US Digital incremental encoders feature a monolithic array of interleaved photodiodes and use a single track to generate both channels A and B. There are two photodiodes for each bar and two for each window of the disk. They are arranged in an A, B, A-not, B-not pattern over a disk segment ranging from 70 to 100 mils. All A outputs are summed together (likewise for B's, etc.). Thus, a relatively large number of disk line pairs are averaged together and fed into differential amplifiers, all on a monolithic chip. Phased array encoders offer a number of advantages over traditional encoders, such as a wide gap tolerance (typically 25 mils rather than ≤ 5 mils), excellent tolerance to mechanical variations and disk misalignment, excellent quadrature and symmetry, high tolerance to disk flaws and contaminants, immunity to supply voltage variations and immunity to aging and batch variations.

Pseudo-Absolute Encoder

An incremental encoder can serve as a pseudo-absolute encoder by providing a battery backup power supply to the encoder or by executing a home cycle after power-up. Adding an index channel to an incremental encoder allows the encoder to provide absolute position after a home cycle. If the encoder has a single index, the home cycle could require nearly one full revolution. There is a way to shorten the home cycle to a fraction of a revolution. This is done by using a special disk that has multiple indexes spaced so that a unique number of quadrature cycles is between each index. The shaft then needs to rotate only enough to encounter two indexes. The external controller can count the number of cycles between the two indexes and determine the absolute position.



Virtually all incremental encoders provide quadrature output signals. A single track optical disk encoder can provide speed information only. Adding a second track to the optical disk and offsetting it by 1/4 cycle from the first track provides two advantages.

1) Rotation direction can be determined by examining the relative phase of the two channels (i.e. whether A-leads-B or B-leads-A)

2) The encoder resolution is effectively increased since there are now 4 transitions for each line pair, instead of 2.

The phase lag or lead between channels A and B is nominally 90 electrical degrees. Quadrature signals are easily converted to step/direction signals or up/down clock signals with a single chip such as the LSI Chips.



Standard unit of angle: the angle at which the subtended arc of a circle has the same length as the radius. There are 2*pi radians in 360 degrees.


The number of full quadrature cycles per full shaft revolution (360 mechanical degrees). Note that each cycle can provide 1, 2, or 4 counts, called x1, x2, or x4 decoding or multiplication, depending on how it is decoded. See Quadrature.


One complete rotation of the encoder shaft, 360 mechanical degrees.



The word "stiction" is a combination of the words stick and friction. Stiction exists when the static (starting) friction exceeds the dynamic (moving) friction. An example of stiction would be a shaft sticking when small changes are attempted, requiring a larger input force to initiate movement. The result being that the force required to start the shaft moving is more than is needed to go to the desired shaft position, causing the movement to be jerky.


When an incremental encoder is rotated at a constant speed, the output of each quadrature channel should be a square wave with a 50% duty cycle. The ratio between the high time and low time (nominally unity) is a measure of how close the output is to 50% duty cycle.


x1, x2 & x4 Decoding

The quadrature signal produced by incremental encoders has 4 state changes per quadrature cycle. A 500 CPR encoder has 500 cycles (2000 quadrature states) per revolution. x1 decoding means that the external electronics counts only state per quadrature cycle, so there will be 500 counts per revolution. x2 counts two states per quadrature cycle (1000 counts per revolution). x4 counts every quadrature state (2000 counts per revolution). A properly implemented quadrature decoder can reduce noise by examining the current quadrature state and ignoring the signal if the next detected quadrature state is invalid. If this is not done, shaft vibrations (often caused by stepper motor ringing) may accumulate counts in one direction when the counter should be oscillating up and down. Quadrature signals are easily converted to step/direction or up/down clock signals using the LSI Chips.