Troubleshooting Guide: Tracking Error and Loss of Position of Industrial Servomotors

1. Description of the Problem and Scope

This diagnostic guide addresses critical servo motor system malfunctions manifested by excessive tracking error (failure of the servo motor to reach or maintain the commanded position, speed or torque) or unexpected loss of position (uncommanded deviation from setpoint). These failures can lead to production stoppages, manufacturing defects and safety risks.

Affected equipment includes AC and DC servo motors, their feedback systems (incremental, absolute encoders, resolvers), servo drives (amplifiers), gearboxes and mechanical coupling elements. This guide is relevant for critical applications in the Aerospace (NC machine tool positioning servos, dynamic test benches) and Energy (solar panel orientation systems, positioning of control valves, turbine drives) sectors, where precision and reliability are imperative.

The severity classification of these incidents is generally critical due to their direct impact on product quality, operator safety and equipment availability, potentially causing irreversible damage to the machine or product.

2. Safety Precautions

CRITICAL SAFETY WARNING: Before any intervention, it is imperative to comply with the lockout/tagout procedures (Lockout/Tagout, LOTO) in accordance with standard NF C18-510 for electrical installations. Make sure all power sources (electrical, pneumatic, hydraulic) are isolated, checked and padlocked. Failure to follow these instructions could result in serious injury or death.

Stored Energy: Servo drive capacitors can retain a dangerous charge even after the main power is turned off. Wait for the discharge time specified by the manufacturer (generally 5 to 10 minutes) and check the absence of voltage with a calibrated multimeter.

Personal Protective Equipment (PPE): Always wear insulating gloves, safety glasses complying with the EN 166 standard, safety shoes and non-conductive work clothing during any electrical or electrical work. mechanical.

3. Required Diagnostic Tools

4. Initial Assessment Checklist

5. Systematic Diagnostic Process

1. Initial Symptom: Tracking error or loss of servomotor position.
1. Checking alarms and error messages:
1. View the servo drive display and controller (PLC/CNC) event log.
2. SI specific error codes (e.g., 'Encoder Fault', 'Overtemperature', 'Overcurrent', 'Position Error Limit Exceeded') are present:
1. Isolate the code.
2. Refer to the drive manual for code-specific troubleshooting.
3. Proceed directly to section 7 for associated root cause analysis.
3. ELSE (no specific alarm or generic 'Monitoring' alarm): Go to step 2.
2. Initial Mechanical Inspection:
1. DEPOSIT (LOTTO) REQUIRED.
2. Check the motor-load coupling:
1. Disconnect the motor from the load. Attempt to turn the motor shaft by hand.
2. IF the motor shaft turns freely and without excessive play: The problem is probably not a motor seizure. Go to step 3.
3. IF the motor shaft is difficult to turn or has noticeable play:
1. Check the condition of the coupling (wear, play, misalignment, failure of elastic elements).
2. Check the motor bearings (axial/radial play, noise).
3. PROBABLE CAUSE: Mechanical engine or coupling problem.
4. Go to section 7 (Mechanical Coupling/Motor Bearings).
3. Check the load (after uncoupling the motor):
1. Attempt to move the load by hand or with a suitable tool.
2. IF the load is difficult to move, exhibits excessive friction or seizing:
1. PROBABLE CAUSE: Mechanical problem with the load (guides, reducer, rack).
2. Go to section 7 (Excessive Loading/Friction).
3. OTHERWISE (the load moves freely and without excessive friction): Go to step 3.
3. Feedback System Diagnosis (Encoder):
1. DEPOSIT (LOTTO) REQUIRED.
2. Physical inspection of the encoder and its wiring:
1. Check that the encoder is correctly mounted and tightened on the motor shaft (torque according to OEM specification).
2. Inspect the encoder cable along its entire length for damage (cuts, crushing).
3. Check connections at both ends (encoder and drive) for bent pins, corrosion or loose clamps.
3. Testing encoder signals (Multimeter/Oscilloscope):
1. SI incremental encoder (TTL/HTL):
1. With the motor off and the drive powered on (if secured), turn the motor shaft slowly by hand.
2. Measure the voltage on channels A, /A, B, /B, Z, /Z. Signals should alternate between high and low (e.g., 0V and 5V for TTL, 0V and 24V for HTL) with a clear transition.
3. Use the oscilloscope to check the waveform and the 90° phase shift between A and B.
4. THRESHOLD: High signal typically > 4.5V (TTL), > 18V (HTL); Low signal typically < 0.5V (TTL), < 5V (HTL). A-B phase shift: 90° +/- 10°.
2. SI sin/cos encoder:
1. Measure the AC voltages on the sin/cos channels. They must vary sinusoidally.
2. Use the oscilloscope to check sine and cosine signals with a phase shift of 90°.
3. IF signals are absent, weak, noisy or incorrect:
1. PROBABLE CAUSE: Faulty encoder or damaged cable.
2. Go to section 7 (Encoder/Wiring Fault).
4. Checking feedback in the drive:
1. Connect the diagnostic software to the drive.
2. Access feedback settings and position/velocity monitor.
3. Manually turn the motor shaft (if secured and uncoupled).
4. IF the drive reads no position, erratic position, or does not detect movement:
1. PROBABLE CAUSE: Faulty encoder, incorrect wiring, or faulty drive encoder input board.
2. Go to section 7 (Encoder/Wiring/Driver Fault).
5. OTHERWISE (the drive reads the position/speed correctly): Go to step 4.
4. Servodrive and Adjustment Parameters Diagnosis (Tuning):
1. Checking configuration settings:
1. Compare the drive parameters (motor type, number of poles, encoder resolution, direction of rotation) with the motor and application documentation.
2. PROBABLE CAUSE: Incorrect settings after replacement or initial configuration.
3. Proceed to section 7 (Inconsistent Configuration Settings).
2. Checking power supply and DC bus:
1. Measure the three-phase (phase-to-phase) input voltage and the DC bus voltage in the drive.
2. THRESHOLDS: Three-phase input voltage within OEM tolerances (e.g., 400V +/- 10%). DC bus voltage nominally 1.414 x V_peak_line (e.g., ~560V for 400V AC).
3. SI voltages out of specification, unstable or with excessive ripple (oscilloscope):
1. PROBABLE CAUSE: Mains power problem, defective rectifier, or degraded DC bus capacitors.
2. Go to section 7 (Power Supply/Drive Problem).
3. Analysis of adjustment parameters (Tuning):
1. Use the drive diagnostic software to view the performance curves (setpoint, actual position, tracking error, motor current).
2. Perform autotuning (if possible and safe) or manually adjust the PID gains.
3. IF the tracking error is high and unstable during movement, the motor vibrates or oscillates:
1. PROBABLE CAUSE: PID adjustment unsuitable for the inertia of the load or the mechanical characteristics.
2. Go to section 7 (Inadequate Dimmer Setting).
5. Engine Diagnosis:
1. DEPOSIT (LOTTO) REQUIRED.
2. Measurement of winding resistance (Multimeter):
1. Uncouple and disconnect the motor from the drive.
2. Measure the resistance between phases (U-V, V-W, W-U).
3. THRESHOLDS: The values ​​must be very close (< 5% difference) and correspond to the manufacturer's specifications. Values ​​of a few ohms to a few tens of ohms are typical.
4. SI significant differences or open circuit:
1. PROBABLE CAUSE: Damaged motor winding (internal short circuit or open circuit).
2. Go to section 7 (Motor Winding Fault).
3. Insulation test (Megger):
1. Measure the insulation resistance between each phase and the motor ground.
2. THRESHOLDS: Generally > 100 MΩ. A value less than 1 MΩ indicates faulty insulation.
3. SI low insulation resistance:
1. PROBABLE CAUSE: Deterioration of motor insulation (humidity, overheating, aging).
2. Go to section 7 (Motor Insulation Fault).
4. Bearing inspection:
1. Listen for abnormal noises (squealing, clicking) when rotating the shaft manually.
2. Use the vibration analyzer to confirm the condition of the bearings.
3. THRESHOLDS: RMS speed level > 4.5 mm/s (ISO 10816-3 standard, category II) is an alarm threshold for bearings.
4. IF excessive noise or vibration:
1. PROBABLE CAUSE: Worn or damaged motor bearings.
2. Go to section 7 (Faulty Motor Bearings).

6. Fault-Cause Matrix

Symptom	Probable Causes (in order of probability)	Diagnostic Test	Expected Result if Cause Confirmed
Constant tracking error (static lag)	1. Unsuitable adjustment parameters (PID) (proportional gain too low) 2. Excessive friction in the load 3. Encoder or drive offset 4. Load too high for the motor	1. Analysis of tracking curves (drive software) 2. Measurement of the friction torque of the load (with dynamometer or by estimation of the motor current) 3. Checking offsets (drive software) 4. Measurement of motor current at nominal load	1. Actual position does not follow the setpoint with a constant offset. 2. Friction torque > 20% of the motor's nominal torque. 3. Significant offset value in the drive registers. 4. Motor current > rated current or > 80% of peak continuously.
Intermittent loss of position / Dynamic tracking error	1. Faulty encoder wiring (interference, loose connection) 2. Damaged encoder (erratic readings) 3. Loose or damaged mechanical coupling (mechanical play) 4. Electromagnetic interference (EMI) on the encoder signal 5. Aggressive PID tuning (overshoot)	1. Visual inspection and continuity test of the encoder cable. Twist test on the cable. 2. Oscilloscope on encoder signals (slow moving motor). 3. Visual inspection of the coupling, testing the play by hand. 4. Checking grounding, shielded wiring. 5. Analysis of tracking curves (integrated oscilloscope of the drive)	1. Intermittent continuity, broken pin, noisy signal during movement. 2. Distorted encoder signals, missing pulses, significant noise (peaks > 200 mV). 3. Visible or audible play, relative rotation between motor and encoder or load. 4. High frequency noise visible on encoder signals (oscilloscope). 5. Rapid and large oscillations in tracking error.
Excessive motor/machine vibration	1. Motor-load or bearing misalignment 2. Unbalance of the load or the motor shaft 3. Defective motor/load bearings 4. Mechanical resonance of the system 5. Unstable PID setting	1. Vibration analysis (peaks at 1X, 2X rpm for misalignment; 1X for unbalance). 2. Game test with comparator. 3. Vibration analysis (characteristic frequencies of bearings). 4. System frequency response test. 5. Analysis of tracking curves (variator).	1. Dominant peaks at rotational harmonic frequencies (e.g., 1X, 2X, 3X RPM). 2. Radial or axial clearance > 0.05 mm. 3. Vibration peaks at bearing fault frequencies (BPFI, BPFO, FTF, BSF). 4. Amplified response at a command frequency. 5. Self-sustained oscillations of the motor.
Motor or drive overheating	1. Continuous motor overload 2. Excessive mechanical friction 3. Motor winding fault (partial short circuit) 4. Insufficient drive cooling 5. Poor power quality (harmonics)	1. Measurement of motor current (RMS) and comparison with nameplate. 2. Measurement of the frictional torque of the load. 3. Measurement of winding resistances (multimeter) and insulation test (megohmmeter). 4. Inspection of fans, filters, ambient temperature. 5. Power quality analyzer.	1. RMS current > motor rated current. 2. Motor surface temperature > 80°C (class F). 3. Unbalanced winding resistance (>5%) or low insulation resistance (<1 MΩ). 4. Drive internal temperature > 60°C, cooling air temperature > 40°C. 5. Current/voltage Harmonic Distortion (THD) ratio > 5% (EN 61000-3-2 standard).

7. Detailed Root Cause Analysis

7.1. Encoder Fault or Feedback Wiring

Explanation: The encoder is the "direction" of the servomotor, providing position and/or speed to the drive. A fault can be electrical (missing, distorted, noisy pulses) or mechanical (play in internal coupling, dirty/damaged sensor, excessive vibration). Wiring may be damaged (crushed, cut, loose connection), or electromagnetic interference (EMI) if shielding is insufficient or improperly grounded. These problems alter the drive's perception of position, causing it to "overcorrect" or "lose track".

How to confirm: Using the oscilloscope to view A/B/Z (incremental) or sin/cos (absolute) signals. No signal, degraded waveform, visible electrical noise (abnormal peaks/dips in signals), or variation in encoder supply voltages. An encoder tester can simulate healthy operation to isolate the drive.

Damage if not resolved: Erratic movements, cumulative position errors, machine collisions, motor overheating (due to constant attempts to correct), premature wear of mechanical components.

7.2. Mechanical Coupling Problems

Explanation: The coupling between the servomotor and the load (or the encoder and the motor) is essential for faithful transmission of motion. Misalignment (angular, parallel, axial), excessive play (backlash), wear of elastic elements or insufficient tightening can introduce position errors. The game causes the motor to spin without the load (or encoder) immediately moving, creating a perceived tracking error.

How to confirm: Visual inspection of the coupling for wear, cracks. Using a dial indicator and feeler gauge to measure misalignment and axial/radial clearance. Manually rotating the motor shaft and observing the delay of the load or encoder. Vibration analysis for misalignments (peaks at 1X, 2X rpm).

Damage if not resolved: Increased positioning errors, vibration, accelerated wear of motor and machine bearings, catastrophic coupling failure, or even shaft breakage.

7.3. Inadequate Dimmer Tuning Parameters

Explanation: Servo drive tuning involves adjusting the PID (Proportional, Integral, Derivative) gains to optimize the system response. A proportional gain (Kp) that is too low results in a static tracking error. Too high Kp gain or poorly adjusted derivative gain (Kd) can cause oscillations, overcorrections and instability. The inertia of the system (motor + load) must be correctly entered for optimal adjustment. Incorrect adjustment for a given load results in imprecise movements and tracking errors.

How to confirm: Analysis of the response curves of the drive (setpoint, actual position, tracking error, motor current) via the diagnostic software. Autotuning may fail or give unsatisfactory results. The motor may vibrate or make abnormal noises when moving.

Damage if not resolved: Jerky movements, motor overheating, premature wear of mechanical components, excessive stress on the electronic components of the drive, degradation of application precision.

7.4. Overload or Excessive Friction of the Load

Explanation: If the mechanical load constantly exceeds the rated capacity of the servo motor, the servo motor will not be able to reach or maintain the position accurately, generating a tracking error. Excessive friction in the mechanical transmission (dirty guides, seized bearings, defective gearbox) increases the required torque of the motor, causing it to work outside of its optimal performance range and potentially leading to overheating and loss of control.

How to confirm: Measurement of the RMS current of the motor in operation and comparison with specifications. Using a dynamometer to measure friction torque. Visual inspection of guides, load bearings for condition, lubrication and clearance. High motor surface temperature.

Damage if not resolved: Motor overheating (which could damage the windings and insulation), accelerated wear of bearings and transmission elements, premature failure of the servomotor, loss of precision, production stoppages.

7.5. Internal Motor Fault

Explanation: Internal problems with the servo motor, such as worn or damaged bearings, partially shorted motor windings, or degraded insulation, can impair the motor's ability to produce the required torque or operate without vibration. Defective bearings introduce friction and vibration which can affect the encoder reading or the motor's ability to position itself. A winding fault reduces torque capacity and causes overheating.

How to confirm: Vibration analysis for characteristic bearing fault frequencies. Measurement of winding resistance (must be balanced). Insulation test (megohmmeter) to check the integrity of the insulation between phases and to ground.

Damage if not resolved: Excessive noise, vibration, catastrophic engine overheating, total engine failure, damage to adjacent equipment.

8. Step-by-Step Resolution Procedures

8.1. Encoder Replacement or Wiring Repair

1. DEPOSIT (LOTTO) REQUIRED.
2. Disconnection: Carefully disconnect the encoder cable from the drive and the encoder itself. Note the pinout or take photos.
3. Cable inspection: If the cable is damaged, replace it with a shielded cable of identical specification (diameter, materials, shielding, connectors). Ensure proper grounding of the shield at only one end (drive) to avoid ground loops.
4. Replacing the encoder:
1. Loosen the encoder fixing screws. Remove the old encoder.
2. Clean the mounting surface.
3. Install the new encoder (UNITEC Category: Feedback Components) ensuring that it is correctly engaged on the motor shaft and that there is no play.
4. Tighten the encoder mounting screws to the torque specified by the manufacturer (e.g., 3 Nm for a standard incremental encoder, check OEM manual).
5. Reconnect the encoder cable.
5. Checking: After reassembly, check the encoder signals with an oscilloscope. Carry out partial decommissioning for a functional test.

8.2. Fixing Mechanical Coupling Issues

1. DEPOSIT (LOTTO) REQUIRED.
2. Disassembly: Loosen and remove the coupling. Inspect the motor and load shafts for damage or burrs.
3. Alignment: Use alignment rulers or a laser system (accuracy < 0.02 mm angular and parallel misalignment) to precisely align the motor and the load.
4. Replacing the coupling: If the coupling is worn or damaged (UNITEC Category: Couplings & Transmissions), replace it with a model of the same type and torque capacity.
5. Tightening: Reassemble the coupling. Tighten all screws to the torque specified by the manufacturer (use torque wrench). Check the residual clearance (< 0.05 mm axial, < 0.03 mm radial).
6. Verification: Perform a vibration analysis after decommissioning to confirm the absence of residual misalignment.

8.3. Optimization of Inverter Tuning Parameters

1. Preparation: Connect the diagnostic software to the drive. Save the current settings before making any changes.
2. Autotuning:
1. Run the drive's autotuning function, if available. Carefully follow manufacturer's instructions (may require uncoupling of load or free movement).
2. If autotuning is successful, check the generated parameters and test the movement.
3. Manual tuning (if autotuning is impossible or insufficient):
1. Start with a low proportional gain (Kp), then gradually increase it until you observe an oscillation. Reduce Kp by 10-20% from the oscillation point.
2. Adjust the integral gain (Ki) to reduce static tracking error, but be careful not to introduce sluggishness or over-correction.
3. Adjust the derivative gain (Kd) to damp rapid oscillations and improve rigidity, but too high a Kd can amplify noise and vibration.
4. Test each modification with a representative movement profile. Tracking error must be minimized, without oscillation or excessive stabilization time (typically stabilization < 100 ms).
4. Backup: Save the new parameters in the drive and in a backup file.

8.4. Troubleshooting and Reducing Load/Friction

1. DEPOSIT (LOTTO) REQUIRED.
2. Identification: Locate the source of excessive friction (machine bearings, linear guides, gearbox, transmission system).
3. Maintenance:
1. Lubrication: Clean and lubricate the guides and bearings in accordance with the preventive maintenance plan. Use the specified lubricants (NF EN ISO 6743-9 grease).
2. Replacement: Replace worn bearings (UNITEC Category: Bearings), damaged gears or defective gearbox components.
3. Adjustment: Check the preloading of the guides and adjust if necessary.
4. Verification: After decommissioning, measure the motor current at no load and at nominal load. Check the motor surface temperature. The current must not exceed the rated current of the motor.

8.5. Engine Replacement or Repair

1. DEPOSIT (LOTTO) REQUIRED.
2. Disconnection: Disconnect the motor power supply, the encoder cable and the mechanical coupling.
3. Removal: Loosen the motor fasteners and remove the faulty motor.
4. Installation:
1. Install a new servomotor (UNITEC Category: Servomotors & Motors) of the same specification (power, speed, mounting flange).
2. Tighten the motor fasteners to the specified torque.
3. Reconnect the coupling (precise alignment required, see 8.2).
4. Reconnect the power and feedback cables.
5. Verification: Perform insulation and winding resistance tests on the new motor. After decommissioning, check the direction of rotation, then carry out autotuning of the drive.

9. Preventive Measures

Root Cause	Prevention Strategy	Monitoring Method	Recommended Interval
Encoder/Wiring Fault	Correct installation of shielded cables, optimized cable paths, selection of robust encoders (IP65/IP67).	Annual visual inspection of cables and connectors. Analysis of encoder signals (oscilloscope).	Annual / During planned machine maintenance.
Mechanical Coupling Problems	Precise alignment during installation (laser). Use of suitable couplings (elastomer for minor misalignments).	Vibration analysis to detect misalignment. Visual inspection of coupling play.	Quarterly (vibration) / Annual (visual).
Inadequate Setting Parameters	Complete documentation of optimal settings. Re-autotuning after major modification of the load or the motor.	Regular backup of drive parameters. Monitoring tracking error in production.	After modification / Bi-annual (performance review).
Overload/Excessive Friction	Correct load calculation and motor selection with margin. Regular lubrication of mechanical components.	RMS motor current measurement. Engine temperature monitoring. Inspection of friction points.	Monthly (current/temperature) / Quarterly (lubrication/inspection).
Internal Motor Fault	Predictive vibration analysis of bearings. Periodic insulation test of windings.	Vibration analysis (FFT) and infrared thermography. Megohmmeter.	Annual (vibration/thermal) / Triennial (insulation).

10. Spare Parts and Components

Part Description	Key Specification	When to Replace	UNITEC category
Incremental encoder	Resolution (e.g., 1024 imp/turn), output type (TTL/HTL), mounting (solid/hollow shaft), protection (IP65).	Confirmed fault (erratic signal, no signal), physically damaged.	Feedback Components
Absolute encoder	Resolution (e.g., 13 bits), interface (SSI, BiSS, EtherCAT), mounting, protection (IP67).	Confirmed fault, damaged.	Feedback Components
Elastic coupling	Rated torque (Nm), max. misalignment (angular, axial, radial), shaft diameter.	Wear of elastic elements, excessive play, cracks, permanent deformation.	Couplings & Transmissions
Motor bearings	Type (ball/rollers), dimensions (inner/outer diameter, width), series (e.g., 6205 2RS).	Excessive noise, radial/axial play out of tolerance, localized overheating, fault detected by vibration analysis.	Bearings
Synchronous AC servo motor	Power (kW), torque (Nm), rated speed (rpm), voltage (V), flange type, integrated encoder.	Confirmed winding or insulation fault, non-replaceable bearings, irreparable damage.	Servomotors & Motors
Servo drive	Power (kW), voltage (V), interface (EtherCAT, Profinet), features (STO, autotuning).	Non-repairable internal fault (power module, control card), chronic overheating.	Power Electronics

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11. References

• NF C18-510: Operations on electrical installations.
• EN 166: Individual eye protection - Specifications.
• ISO 10816-3: Mechanical vibrations - Evaluation of machine vibrations by measurements on non-rotating parts - Part 3: Industrial machines with a nominal power greater than 15 kW and nominal speeds between 120 rpm and 15,000 rpm.
• EN 61000-3-2: Electromagnetic Compatibility (EMC) - Part 3-2: Limits - Limits for harmonic current emissions.
• NF EN ISO 6743-9: Lubricants, industrial oils and related products (Class L) — Classification — Part 9: Family X (Greases).
• User and programming manuals from servo drive manufacturers (e.g., Siemens, Rockwell Automation, Schneider Electric, Bosch Rexroth).
• AFNOR standards relating to the quality of mechanical and electrical components.