SiteIt's 3am. The flow wrapper has stopped with an encoder alarm on the cross-seal axis. Someone resets it. It runs for twenty minutes and trips again. By the third trip there's talk of fitting the spare servo motor from stores.
If there is a spare, it gets fitted. The alarm comes back on the first shift. Now you've lost four hours, the old motor is in the bin, and the axis needs homing from scratch.
Most encoder alarms are the drive telling you it's lost the signal. Far more often than not, the encoder itself is fine. The signal died somewhere between the encoder and the drive.
Read what the drive is telling you
"Encoder fault" covers a lot of ground. Before anyone touches reset, photo the alarm and the drive's fault history. Then work out which kind of fault you've got.
- Signal lost or comms error. The drive can't hear the encoder at all, or the data is garbled. Think cable, plug, screen and supply.
- Following error or position deviation. The drive can hear the encoder, but the axis isn't where it was told to be. Look at the mechanics first. Our guide to servo drive alarms covers it.
- Count or data mismatch on an absolute encoder. The position it reports doesn't match what the drive expects. Think noise, a slipping coupling or a lost reference.
Ask the operator what the axis was doing. Starting from rest? At full speed? Only when the machine is warm? If the manual's gone, our piece on reading drive fault codes will get you started.
Start with the cable
Nine times out of ten, in our experience, an intermittent encoder fault is the cable or the plug. Feedback cables carry low-level signals, and they live a hard life.
Walk the cable from the motor to the drive. Look hardest where it moves. A cable in a drag chain flexes thousands of times a shift. The cores fracture inside the sheath, and you can't see it. It works when the chain is straight and drops out when it bends. If the fault comes and goes with the axis position, the drag chain is your first suspect.
Then the plugs. Look for bent or pushed-back pins, green corrosion and water or oil inside the connector. Washdown lines and machines with coolant are the usual ones. Check the plug is fully home and locked. A connector that's half done up will work for days and then fail on a hot afternoon.
Isolate before you unplug anything. Plugging and unplugging feedback connectors on a live drive can damage the encoder or the drive input, and the motor plug often sits right beside it.
Then the screen
Encoder signals are small. Motor power cables are noisy. Put them together badly and the drive sees rubbish.
Look at how the screen is finished at both ends. It should be clamped all the way round at the gland or the drive's screen clamp, not twisted into a pigtail. Check the feedback cable isn't tied tight against the motor cable for a long run. Ask what's changed. A new inverter in the same panel, a cable rerouted after a breakdown or a welding set plugged in nearby can all start noise trouble.
If the fault only happens when something else starts, such as a big motor or a heater bank, that's a strong hint the problem is noise.
Rule out the mechanics
On motors with the encoder built in, mechanical faults are less common. They still happen. Bearing wear, a knock to the back of the motor or heat can all damage the encoder.
On axes with a separate encoder, look at the mounting and the coupling. A loose grub screw lets the encoder slip on the shaft. The drive then thinks the axis is somewhere it isn't. Mark the shaft and the coupling with a paint pen and see if they move apart.
Some drives let you watch the encoder count with the power stage disabled. Where the axis can be moved safely by hand, turn it slowly and watch the count. Jumps, stalls or a count that drifts back point to a damaged encoder or a slipping coupling. A count that's smooth until you flex the cable points back at the cable.
The axis can still move
This is where encoder work gets dangerous. Swapping a motor or encoder can lose the commutation offset and the home position. Get the offset wrong and the axis can lurch or run off when it's enabled. Follow the drive maker's procedure for setting it up again, and keep people clear for the first move.
Vertical axes are worse. Release the brake to change a motor and the load comes down. The PUWER code of practice says potential energy can't be isolated from the equipment. There should still be a way to stop it hurting anyone, by shielding, barriers or restraint (L22, paragraph 199)*. Pin it, prop it or lower it to the stops before the motor comes off.
The drive itself holds a charge after it's isolated. HSE's guidance on the Electricity at Work Regulations warns that energy may be stored in capacitors within equipment (HSR25, paragraph 195)*. Wait for the time on the drive label and prove it dead. Our guide to safe isolation has the full sequence.
Why the wrong engineer costs you
HSE's electrical safety guidance makes the point that electrical accidents often happen during fault-finding after a breakdown, when there's pressure to get the plant going (HSG85, paragraph 38)*. Encoder faults are a good example. They're intermittent, the line keeps restarting, and the temptation is to swap parts until it stops.
Here's our view. A good fitter who's never set up a servo axis shouldn't be the one left alone with this at 3am. The PUWER code of practice says maintenance should only be done by people competent to do it (L22, paragraph 79)*. On a servo machine that means someone who can read the drive's diagnostics, check a screen termination and home an axis properly.
The cost of getting it wrong adds up fast. A motor that didn't need changing. Hours of homing and test runs. Servo motors can be on long lead times, so the spare you used may not be back for a while. Meanwhile the fault is still there. If it's not found in half an hour, the 30-minute rule applies.
What to do on Monday
- Back up the parameters on every servo drive, including the encoder settings, and keep a copy off the machine.
- Check the feedback cables in your drag chains for wear, and look at the screen clamps at both ends.
- Write down the homing and set-up steps for each servo axis, and keep them with the machine.
- List your vertical axes and agree how each one is pinned or propped before a motor change.
* Paragraph numbers correct as of 20 October 2026 and refer to L22, fourth edition (2014, amended 2018), HSR25, third edition (2015), and HSG85, third edition (2013). Check HSE's HSG85 page for the latest.