The encoder of an injection molding machine servo motor is the sensor at the rear of the motor that measures the position and speed of the shaft. The servo drive reads it continuously: it needs the rotor position to switch the current in the windings at the right moment, and the speed to control the motor. Without a good encoder signal the drive cannot run the motor, so an encoder or encoder cable fault stops the pump or the axis at once.
Where the encoder sits
The encoder is mounted at the rear end of the servo motor, behind the rear bearing and under a cover, as the drawing shows. Its signal cable runs from the encoder connector on the motor to the servo drive, separately from the motor power cable.
How an encoder works
Many servo motor encoders are optical: a disc with fine lines turns with the shaft between a light source and sensors.
- An incremental encoder gives two pulse trains, A and B, offset by a quarter of a pulse, so the drive can count pulses and tell the direction. A once-per-revolution index pulse (Z) gives a reference point. Many incremental encoders for servo motors also have commutation tracks that tell the drive the rotor position at start-up. Resolution is given in pulses per revolution (ppr); counting every edge of A and B gives four counts per pulse.
- An absolute encoder sends the shaft position as a digital value over a serial link, so the position is known at power-on. Multi-turn versions also count revolutions.
- A resolver is a rotary transformer with no electronics in the motor. It tolerates heat and vibration well and is used on some servo motors.
| Incremental encoder | Absolute encoder | Resolver | |
|---|---|---|---|
| Output | Pulse trains A, B and Z, often with commutation signals | Digital position value over a serial link | Analog sine and cosine signals |
| Position at power-on | Not known until the index is found | Known | Known within one revolution |
| Electronics in the motor | Yes | Yes | None |
Our AC servo motor is listed with a 2500 ppr incremental encoder.
On hydraulic machines, the positions of the mold, the screw and the ejector are usually measured by linear transducers, not by the motor encoder. On all-electric machines, those positions are usually calculated from the servo motor encoders.
Signs of encoder failure
- Encoder alarms on the servo drive: signal loss, count error, phase or communication error.
- The motor runs roughly, vibrates or is noisy at low speed.
- Overcurrent alarms as the motor starts.
- Intermittent faults when the cable moves or the machine vibrates.
- The motor jumps or runs uncontrolled at start-up. Stop the machine and do not run it again until the fault is found.
Causes of encoder faults
- A damaged, pinched or oil-soaked encoder cable, or a loose connector.
- A poorly connected cable shield, or an encoder cable run alongside power cables, which lets electrical noise into the signal.
- Heat from an overheated motor.
- Vibration and shock, or a worn rear bearing that lets the shaft move.
- Oil, coolant or dust getting under the encoder cover.
Checking and replacing the encoder
Checks on the encoder and its cable, and any replacement, are for qualified electricians, with the machine switched off at the main isolator and locked out.
- Read the alarm code on the drive and look it up in the drive manual.
- Check the cable along its whole length and the connectors at both ends, and check that the shield is connected as the wiring diagram shows.
- Where possible, test with a known good encoder cable.
- If the encoder itself has failed, remember that on a permanent-magnet servo motor it must be aligned to the rotor magnets. It is often safer to replace the complete motor, or have it repaired by a specialist, than to change the encoder on site.
- After any encoder or motor change, run the encoder alignment or auto-tuning in the drive if its manual requires it, and check the direction and smoothness of the motor at low speed in manual mode before production.
What to send for a replacement quote
- A photo of the motor nameplate, with the encoder type if it is marked.
- The model of the servo drive.
- The alarm code.
- Photos of the encoder connector and cable.
- The machine brand, model and serial number.
The servo motor guide covers the rest of the motor, the servo drive guide lists the other drive alarms, and the electrical system guide gives the overview.
Updated by the IMSPARES technical team.