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Drives & Motion·7 min read·

Encoders: incremental, absolute, and the wiring that fails

Quadrature, single turn and multi turn absolute, the difference between resolution and accuracy, and why the marker pulse exists.

Short answer

An incremental encoder outputs pulses as it turns, so position is counted from wherever it started and is lost on power down. An absolute encoder outputs a unique code for each position, so it knows where it is the moment power returns. Multi turn absolute encoders also remember how many revolutions, using gearing or a battery, which removes the homing routine entirely.

The drive loop, and the line that closes itSetpointfrom the PLCDriveV/f, vectorMotor3 phaseLoadENCCLOSED LOOP ONLYWithout the dashed line the drive is guessing at speed from current and voltage. That is the whole difference between sensorless vector and closed loop.

Encoders divide into two families and the choice between them decides whether your machine needs a homing routine every time somebody turns it off.

Incremental

A disc with lines on it and a sensor that produces a pulse per line. Count the pulses, know how far you have moved.

From where, though? From wherever the counter was zeroed. An incremental encoder measures change, not position, and the moment power is lost the count is gone.

Quadrature. Two channels, usually called A and B, ninety degrees out of phase. Which leads gives direction. Counting every edge of both gives four counts per line, which is why a 1000 line encoder is often quoted as 4000 counts per revolution.

The marker, or Z channel. One pulse per revolution, at a fixed angle. Used to establish a reference during homing, and to detect lost counts: if the marker arrives at a count you did not expect, something has slipped.

Absolute

The disc carries a pattern that is unique at every angular position, so the output is a position rather than a change.

Single turn. Unique within one revolution. Power up and it knows exactly where it is within that revolution, and nothing about which revolution.

Multi turn. Also counts revolutions, either with a gear train inside the encoder or with a battery backed counter. Power up and it knows the absolute position across the whole travel.

Multi turn absolute is what removes homing from a machine. On a system where homing means driving an axis to a limit switch, that can be the difference between a thirty second start-up and a five minute one, and on some machines homing is not safe to do automatically at all.

Reading them

Incremental encoders are read as pulses, either into a high speed counter input or directly into a drive.

Absolute encoders speak a protocol: SSI, BiSS, EnDat, Hiperface, or a fieldbus like PROFINET or EtherCAT. Which one is a compatibility question between encoder and drive, and getting it wrong is not a wiring problem, it is a different part number.

Resolution is not accuracy

Worth its own section because the datasheets encourage the confusion.

Resolution is how finely a revolution is divided. Accuracy is how close the reported position is to the true one.

The dominant error in most installations is not inside the encoder at all. It is the coupling: any eccentricity or backlash between the shaft and the encoder appears directly as a position error, and no amount of resolution corrects it.

A 20 bit encoder on a flexible coupling with 0.1 degrees of play is a machine with 0.1 degrees of accuracy and a very precise readout of the wrong number.

The wiring that fails

Encoders are low voltage, high frequency signals, usually in a panel with a drive in it. Three faults account for most of the trouble.

  • Unscreened cable, or the screen terminated at one end. Noise from the drive appears as extra counts, and the axis drifts under no load.
  • Single ended rather than differential. Differential outputs, A and A inverted, reject common mode noise. On any cable longer than a metre or two in a drive panel, use them.
  • Shared conduit with motor cable. The single most effective fix in most cases is separating the two runs by a few hundred millimetres.

Choosing

  • Speed feedback only, on a drive: incremental is fine and cheaper.
  • Positioning, where a homing routine is acceptable: incremental with a marker.
  • Positioning where homing is slow, unsafe or impossible: multi turn absolute, and it is worth the money.

Common questions

What is quadrature and why two channels?
Two pulse trains ninety degrees out of phase. Which one leads tells you the direction of rotation, and counting the edges of both gives four times the resolution of the disc's line count. One channel alone gives speed and no direction.
Do I still need a homing routine with an absolute encoder?
Not for the encoder's sake. A single turn absolute encoder knows its position within one revolution immediately; a multi turn one knows the revolution count too. You still need a one off procedure to teach the machine which absolute reading corresponds to which physical position.
What is the difference between resolution and accuracy?
Resolution is how finely the encoder divides a revolution; accuracy is how close a reading is to the true position. A cheap encoder can have very high resolution and poor accuracy, reporting a position to five decimal places that is wrong in the second. Coupling and mounting eccentricity usually dominate the error.

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