Servo Axis Position Error Too High: How Do You Find the Real Cause?

Key Specifications

Types & Variations

Maintenance Steps

1. Capture the faulted move
   Preserve alarms and trend commanded position, actual position, following error, speed, and torque. Record recipe, payload, axis location, temperature, and whether the fault began after a crash or change.

   Keep personnel outside the safeguarded motion envelope. Use the machine builder's approved diagnostic mode and reduced motion settings.

2. Isolate and inspect the load path
   Apply lockout/tagout (LOTO), verify zero energy, bleed down pneumatic and hydraulic pressure, discharge stored electrical energy per the drive instructions, and block gravity or spring-loaded members. Inspect the brake, coupling, keys, belts, gearbox, screw, guides, bearings, and product clearance through the usable stroke.

   A disabled servo is not energy isolation. Stored pressure, suspended axes, charged capacitors, counterweights, and springs can move the mechanism after power is removed.

3. Check feedback and configuration
   Reseat approved connectors with power isolated, inspect pins and cable-flex areas, verify shield clamps and grounds, then compare motor, encoder, direction, scaling, gearing, and limits with the approved backup.

   Do not megohm-test an encoder or connected drive electronics unless the manufacturer explicitly permits it; the test voltage can destroy feedback circuits.

4. Run a controlled proof test
   Clear the work area, restore guards, remove LOTO under the site procedure, and test first at reduced speed and acceleration. Repeat the same trace unloaded and loaded where permitted, then confirm full-stroke operation and reversal behavior before production release.

   Abort on abnormal noise, rough travel, unexpected direction, rising torque, or unstable oscillation. Reapply LOTO before any further hands-on work.

Confirm What the Axis Did

Save the alarm history and trace these channels on one time base at the same sample rate: commanded position, motor feedback, load feedback where a second encoder or linear scale exists, following error, velocity command and feedback, torque command, actual current, the torque-limit-active bit, brake command, and DC bus voltage. Note where the error begins: at servo enable, during acceleration, at constant speed, near reversal, only after warm-up, only at one machine position, or in one direction of travel. Record the recipe and payload with the trace, because a comparison run without them proves little. A single sharp position step with no matching motor motion suggests lost feedback or a loose feedback coupling. A smooth, growing lag says the axis is not developing the acceleration or speed that move demands. Do not clear the history until the trace and machine state are recorded.

Most Likely Causes, in Shop-Floor Order

Start with what changed, and follow the path that change opens. After a crash, look at alignment, coupling, bent screws, and guide damage. Washdown or a coolant leak puts connector seating, cable-flex zones, and a contaminated linear scale at the top. Parameter downloads and motor or encoder swaps put motor data, feedback scaling, gearing, direction, absolute offset, and limits there instead. Heavier payload or a product jam is a question of available torque. Beyond those, the recurring causes are brake drag, a torque or speed limit that is doing its job, a loose key, pulley, or ball-screw support, damaged encoder cabling and poor shield termination, and gains or feedforward that no longer suit the load. Backlash and lost motion show their worst error at reversal. Thermal growth, dry ways, tight seals, and failing bearings worsen after repeated cycles rather than on the first move of the shift.

Separate Load, Feedback, and Control Faults

Read torque with following error, never by itself, and know which signal the screen shows: torque command, estimated torque, q-axis current, or percent of rating. Rising error with torque command and actual current both sustained near the drive ceiling: inspect the load path first, meaning brake, bearings, linear guides, ball screw, gearbox, counterbalance, and product contact. Rising error while torque sits clipped flat below that ceiling: check the torque-limit-active bit, safety functions, drive derating, DC bus and supply voltage, and the selected motor data set. Position feedback that steps, chatters, reverses briefly, or moves while the shaft sits still points to the encoder, connector seating, shield, grounding, or feedback coupling; raw counts and the drive encoder diagnostics separate a real signal fault from normal compliance between motor and load feedback. Mark the motor shaft and the driven hub before jogging. If the marks shift relative to each other, the coupling is slipping, and note that a motor-mounted encoder cannot see slip that happens after it. With energy isolated and gravity loads supported, move the mechanism by the machine maker's manual method through its usable travel and feel for a repeatable tight spot. Measure backlash at reversal and runout at suspect shafts with a dial indicator. If travel is rough or binds, stop the test, re-isolate the machine, and correct the mechanical fault. Tuning cannot repair metal-to-metal drag.

Correct the Fault Without Hiding It

Remove the obstruction, repair the brake, restore lubrication, align the coupling, set belt tension by the machine maker's method, and replace damaged bearings or feedback parts as findings require. A brake that drags needs its own numbers, not a guess: release voltage and current, release delay, air gap, rectifier or relay volt drop, and behaviour once hot. Restore the approved limit values if an unauthorized or accidental change is confirmed. Verify motor data, encoder type, absolute offset, homing datum, electronic gearing, travel units, load inertia, and direction against the approved machine backup, and check firmware revision while you are there. Retest at reduced speed and acceleration. Tune only after mechanics and feedback are sound: begin from the machine builder's stable parameter set, change one gain, feedforward, or filter item at a time, and repeat the same trace watching for oscillation as well as error. Rising oscillation at a fixed frequency is resonance, not missing gain; stop raising gain there. Widening the position-error limit belongs at the end of that list, needs the machine builder's approved range, and must be proven at the worst validated load, because a wider limit also delays the fault reaction and lengthens stopping distance. Record the old and new values.

Keep the Fault From Returning

Keep one healthy trace for the production move and compare later traces against it. Trend peak following error and peak torque by recipe; slow drift often exposes lubrication loss, bearing wear, brake drag, or growing payload before a trip stops the line. Put coupling witness marks, connector seating, shield clamps, cable-flex zones, brake release, backlash, and full-stroke drag on planned inspections. Back up drive parameters after an approved change and label the motor, encoder, and drive as one matched set. Following-error margin is not spare machine capacity. A profile that runs continuously at the torque ceiling needs a load or motion-profile review.

FAQ

References

• Rockwell Automation, Integrated Motion on the EtherNet/IP Network Reference Manual — CIP axis attributes and position error faults
• Yaskawa America, Sigma-7 and SigmaWin+ Ver.7 Servo Tuning training manual — position deviation overflow alarm, motor wiring and gain causes
• OSHA 29 CFR 1910.147, The control of hazardous energy (lockout/tagout)