Encoder
Gives precise position or speed feedback by producing pulses (or a coded value) as a shaft turns.
What it is
An encoder reports how far and how fast a shaft has turned, giving a control system precise feedback for positioning and speed control.
How it works
An incremental encoder produces a stream of pulses as it rotates; counting the pulses gives position, and their rate gives speed. An absolute encoder outputs a unique coded value for each shaft position, so it knows where it is even after a power cycle.
Encoders are sensitive to cable damage and electrical noise — both corrupt the counts. A slipping coupling makes the feedback disagree with reality.
Where it's used
Servo and motion systems, precise speed control, and position feedback on drives. Pulses-per-revolution (PPR) and encoder type must match the controller's configuration.
What it looks like when it fails
Electrically: feedback-loss faults on the drive, counts that drift or jitter (noise coupled from motor cables), and intermittent dropouts from a damaged cable or connector — usually where it flexes.
Mechanically: a slipping coupling makes the feedback disagree with reality — the drive hunts, positions creep, and every electrical test passes. Encoder bearings fail too, and on optical types the disc gets dirty.
How to test it
Start in the drive or PLC diagnostics: feedback-loss codes and the live position counter. Then — with the drive isolated and locked off — bar the shaft over slowly by hand and watch the count move smoothly, both directions: steps, jumps, or dead zones are the fault talking.
Check supply volts at the encoder end of the cable, then the grub screws and the coupling before anything else electrical — 'counts fine, position wrong' is mechanical slip, not electronics.
Gotchas
Run encoder cable away from motor/VSD cables and land the shield properly — PWM noise corrupts counts in ways that look exactly like a failing encoder.
After a replacement, PPR and signal type (differential vs single-ended, push-pull vs open-collector) must match the controller config — a mismatch runs, but every speed and position is scaled wrong or the counts only work one direction.
Safety first
Isolate and lock off the drive before barring a shaft over by hand — a feedback test is never worth a start-up with your hands on the coupling.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Related faults
Encoder feedback fault (position/speed wrong)
Position or speed feedback from an encoder is wrong — counts drift, position is lost, or a drive faults on encoder loss, so motion control misbehaves.
VSD powered up but won't start the motor
The drive is energised and the display is alive, but it won't run the motor. No fault may be shown — it just sits in 'ready' or 'stopped' and ignores the start command.
Sensor intermittent / drops out randomly
A sensor works most of the time but drops out or false-triggers intermittently, causing random stops, miscounts, or sequence faults that are hard to pin down.
Related definitions
VSD (Variable Speed Drive)
Controls the speed of an AC motor by converting the supply to a variable frequency and voltage.
Induction motor
The workhorse AC motor — a rotating magnetic field in the stator drags the rotor around with it.
Proximity sensor
Detects the presence of a target without contact — inductive, capacitive, or photoelectric.
PLC (Programmable Logic Controller)
An industrial computer that reads inputs, runs a program, and drives outputs to control machinery.