Encoder glossary

Absolute

An absolute position is one which is complete in itself and is defined independently of any other position or value. The main benefit of an absolute encoder is that after the power is turned on, the information about the current position of the movable system is instantly obtained. There is no need for any initial movement and contrary to incremental encoders it is not necessary to move to a reference position.

Accuracy

Accuracy is a measure of how close the output is to where it should be – the deviation between actual position and the encoder’s reported position.

CPR (counts per revolution)

Counts per revolution is the number of changes of state on both channels in one revolution.

Distance coded reference mark (DCRM)

Most commonly found in linear or ring incremental encoders, the distance coded reference mark (DCRM) scale allows a measuring system to re-establish absolute position after traversing a short travel.

Encoder

An encoder is a device, circuit, transducer, software program, algorithm or person that converts information from one format or code to another, for the purpose of standardization, speed or compression.
Read more: https://en.wikipedia.org/wiki/Encoder

Gray code

Gray code is a special ordering of the binary numerical system. The difference between Gray code and natural binary is the method of incrementing to the next number. In Gray code only one digit may change states for every increment.

Hysteresis

Hysteresis is the difference in reported position when coming to the same position from either direction of rotation or linear movement.

Incremental

Incremental encoders provide position with zero value at point of switch on. Position is reported in the form of two signals shifted for quarter period (90°). Usually these signals are known as A quad B. Signals can be analogue (sin/cos) or digital. The resolution of the system is defined as distance between two neighbouring transitions of both signals. They provide a specific number of equally spaced pulses per revolution of scale. As the encoder is moved it generates a pulse stream. For each movement of 1 unit of resolution (increment), the encoder outputs a change of state on the output.

Interpolation

Interpolation is a process which divides one cycle of analogue input signal (sin/cos) to many cycles of digital output signals (A quad B).

Interpolation rate

Interpolation rate is defined as number of output measuring steps inside of one input signal period.

Minimum edge separation

Minimum edge separation means the smallest possible time between two adjacent edges of the quadrature signal. It is given as a unit of time (μs). Typical selections are 0.07 μs through 20 μs. When specifying a magnetic incremental linear encoder, you want to the select a minimum edge separation value that is slower than the speed of your counter but faster that the highest velocity of your application, when taking resolution into account.

MTTF (Mean Time To Failure)

MTTF (M) = p × t / n

PPR (pulses per revolution)

Pulses per revolution is the number of pulses in one revolution.

Resolution

Resolution is the smallest movement detected by the encoder. It is measured differently according to the type of encoder.

Repeatability

• Uni-directional repeatability is the difference in reported position of the same physical point from multiple measurements by travelling in the same direction and under the same conditions.
• Bi-directional repeatability is the difference in reported position of the same physical point from multiple measurements by travelling in the opposite direction and under the same conditions.
• Bi-directional repeatability = Uni-directional repeatability + hysteresis.