The encoder is the sensor that tells the controller the exact angular position of each axis at every moment. Without that precise, reliable feedback, the robot cannot move safely or reproduce trajectories accurately. Yet when an encoder starts to fail, the symptoms closely resemble those of other components — gearboxes, cables, batteries — making diagnosis harder and downtime longer.
What does an encoder do and why does it matter?
Every axis on an industrial robot has at least one encoder — optical, magnetic or resolver, depending on the manufacturer and range. Its role is twofold: it sends position feedback to the control loop so the servo can correct any deviation, and, in the case of absolute encoders, it retains the position even when the robot is powered off, eliminating the need to recalibrate at every start-up.
A degraded encoder does not just cause inaccuracy: it can trigger emergency stops, mechanical damage if the controller loses position reference, or unexpected collisions with tooling and structures.
Common causes of encoder failure
- Natural ageing. Optical encoders accumulate dirt on the disc and read cells; magnetic ones can lose field strength over time.
- Moisture and contamination. Environments with coolant, steam or fine dust are particularly aggressive. A single deteriorated seal is enough for moisture to penetrate and cause erroneous readings.
- Mechanical vibration. Vibrations transmitted from the gearbox or structure can loosen internal connectors or damage the optical disc. If you have already read our article on abnormal vibrations in industrial robots, you will know this symptom often appears alongside other mechanical indicators.
- Depleted backup battery. Absolute encoders need a small battery to retain position when the robot is unpowered. If that battery runs flat, the encoder loses its reference and the controller demands recalibration.
- Impacts and overloads. An accidental collision can damage the encoder directly or misalign it relative to the axis.
Symptoms that point to an encoder fault
Reproducible position drift
The robot runs the programme correctly for a while and then the end position gradually or randomly drifts. If the drift disappears after switching off and back on — or after mastering — the encoder or its battery are the first suspects.
Position-tracking alarms
ABB, KUKA and FANUC controllers generate specific alarms when the error between the commanded position and the actual encoder position exceeds a threshold. These alarms can appear during motion or even at standstill if there is electrical noise on the signal line.
Jerky or stepped movement
The axis moves in a stuttering fashion at low speeds. This indicates the controller is receiving a discontinuous signal: a damaged encoder channel, a connector with poor continuity, or interference on the signal cable.
Frequent mastering errors
If the robot requires recalibration unusually often — with no collision or mechanical intervention — the encoder battery or the encoder itself is the most likely cause.
How to distinguish an encoder fault from similar failures
- Cabling and connectors: A faulty connector between the encoder and the cabinet produces exactly the same symptoms. Inspect the motor harness and intermediate connectors; gently flex the cable in manual mode and watch for changes in readings.
- Backup batteries: Before replacing the encoder, check the battery voltage for the affected axis. In many cases the problem is solved by a simple battery swap.
- Electromagnetic interference: Power and signal cables running in parallel without adequate separation can induce noise on the encoder line. Check the routing inside the cabinet and along the arm.
- Servo or drive issues: A degraded drive can misinterpret a perfectly good encoder signal. Compare behaviour across different axes to isolate whether the fault is global or axis-specific.
Information to gather before calling the technician
- Exact robot reference (model, serial number, year of manufacture)
- Affected axis or axes
- Exact alarm text shown on the teach pendant
- Alarm log from the past few days (most controllers allow export)
- Approximate date of the last encoder battery change
- Whether the fault appears cold, hot or randomly
When to replace the encoder
Replacement is justified when:
- Cabling, connectors and batteries have been ruled out as the cause.
- Encoder readings show erroneous values or jumps even with the axis at rest.
- The encoder has suffered a physical impact or severe moisture exposure.
- The frequency of required recalibration is already affecting productivity.
On older-generation robots, availability of the original encoder can be limited. In those cases, our spare parts and exchange service provides access to reconditioned units faster than the official channel, reducing production impact.
Preventive maintenance for encoders
Including encoders in your preventive maintenance plan extends their service life and avoids unpleasant surprises:
- Check and replace backup batteries at the manufacturer-recommended interval — do not wait for the alarm.
- Inspect encoder seal integrity during periodic arm inspections.
- Log every unscheduled recalibration event in your CMMS to spot trends early.
- Verify signal cable condition during harness maintenance checks.