Diagnosis and Remedy of Tracking Error and Position Loss in Servo Drives

1. Problem Description & Scope

This manual covers the diagnosis and recovery of tracking error (following error) and position loss in servo drive systems. These faults manifest themselves when the actual position or speed of a mechanical system deviates from the value commanded by the servo control. This can lead to inaccurate operations, production downtime, damage to machines and product loss. The manual applies to all industrial servo drives used in mechanical engineering, automation, robotics and handling systems within the Benelux.

1.1 Symptoms

• Continuous tracking error: A consistent, albeit small, deviation between the commanded and actual position or speed during operation. Often an indication of insufficient system stiffness or suboptimal tuning.
• Intermittent tracking error: Sporadic or temporary tracking errors, often related to varying loads, external disturbances, or incipient defects.
• Abrupt position shifts: Sudden, unexplained jumps in position, indicating problems with the encoder feedback or mechanical linkage.
• Inaccurate positioning: The machine does not consistently reach the target position or has repeatability errors.
• Overshoot/Oscillation: The system responds too aggressively to commands, overshooting the target position and/or oscillating around it.
• Alarm codes: The servo control reports specific error codes related to tracking error (e.g. F8001, AL.F01), encoder error, overcurrent or overload.

1.2 Severity classification

• Critical: Immediate production stoppage, risk of serious machine damage, safety risks. Immediate action required.
• Major: Significant loss of quality, drastic reduction in production speed, accelerated wear of components. Quick recovery necessary.
• Minor: Small, inconsistent deviations, slight efficiency reduction. Planned maintenance required.

2. Safety measures

WARNING! Always perform diagnostic work on electrical drive systems with the utmost caution. Ignoring safety procedures can result in serious injury or death and irreparable damage to equipment.

• Lockout/Tagout (LOTO): Ensure that all energy sources to the machine and the servo drive are shut off and locked in accordance with NEN 3140 and EN ISO 14118 before commencing inspection or maintenance. Check the absence of voltage with a suitable voltage meter.
• Personal Protective Equipment (PPE): Always wear the required PPE, including safety glasses (EN 166), safety gloves (EN 388, EN 60903 for electrical work) and safety shoes (EN ISO 20345).
• Opgeslagen Energie: Condensatoren in de servodrive kunnen na uitschakeling nog gevaarlijke spanningen bevatten. Wacht de aanbevolen ontlaadtijd van de fabrikant af (doorgaans 5-10 minuten na uitschakeling) voordat u interne componenten aanraakt. Check the tension.
• Bewegende Delen: Voorkom contact met bewegende machineonderdelen. Schakel de machine uit en activeer LOTO voordat u mechanische componenten inspecteert. Bij functionele tests (bijv. tuning) met deuren gesloten werken en noodstops binnen handbereik.
• Electrical Hazards: Only work on electrical installations if you are qualified and authorized in accordance with the applicable regulations (NEN 3140).

3. Required Diagnostic Tools

4. Initial Assessment Checklist

Before beginning detailed diagnosis, a systematic visual inspection and collection of basic data is essential.

5. Systematic Diagnosis Flow Chart

1. Symptom: Tracking Error or Loss of Position
   1. Is there an alarm code on the servo drive?
      1. YES: Refer to the servo drive diagnostic tools and manufacturer's manual for the specific alarm code. Focus on encoder errors (e.g. “encoder disconnect”, “encoder overspeed”, “encoder cyclic communication error”) or tracking error limit exceeded. Continue with specific steps for that alarm code.
      2. NO: Continue with generic diagnosis.
   2. Visual inspection of mechanical coupling and shafts:
      1. Check the coupling between motor and load:
         1. Is it properly aligned (EN ISO 14686)?
         2. Is there any noticeable play when turning the shaft manually (check for torsional play and radial/axial play)? Acceptable play in couplings for servo applications is generally < 0.05 degrees.
         3. Are the clamping screws of the coupling properly tightened (see NEN EN ISO 898-1 for bolt classes)?
      2. Check the motor bearings and load:
         1. Is there any noticeable radial or axial play in the motor bearings or machine shafts? Acceptable radial clearance in unloaded motor bearings < 0.02 mm (depending on size).
         2. Are there unusual noises or elevated temperatures (measure with thermal camera: > 80°C is critical) in the bearings?
      3. Check the mounting of the servo motor and encoder:
         1. Are all mounting bolts of the motor and encoder firmly tightened and provided with locking compound if applicable?
      4. Mechanical Inspection Result:
         1. PROBLEM FOUND (e.g., play, loose coupling): Go to Section 8: “Step-by-Step Troubleshooting Procedures – Mechanical Play” and Section 7: “Root Cause Analysis – Mechanical Play.”
         2. NO PROBLEM FOUND: Proceed with electrical check.
   3. Electrical check of encoder feedback:
      1. Check the encoder cable:
         1. Perform a continuity test with a multimeter on all cores of the encoder cable. An interruption (> 10 Ω) indicates a cable break.
         2. Check the shielding for proper grounding on both sides (if applicable) and for damage.
         3. Check the connectors for dirt, corrosion or loose pins.
      2. Check the encoder output (with oscilloscope or encoder tester):
         1. Incremental encoder (TTL/HTL): Measure the A, B and Z tracks. Expect square waves with equal duty cycle (approx. 50%) and a 90° phase shift between A and B. Z trace is a single pulse per revolution. TTL levels 0-5V, HTL levels 0-10/24V. Signal amplitude must not drop below 4V (TTL) or 15V (HTL) under load.
         2. Sine/Cosine encoder: Measure the Sin and Cos traces. Expect sinusoidal signals with a peak-to-peak amplitude of approximately 1Vpp. Signals must be phase shifted by 90°.
         3. Absolute encoder (e.g. EnDat, Hiperface, BiSS): Use the servodrive software or a specific encoder tester to verify communication and read the position value. Check for CRC errors or communication errors.
      3. Encoder Feedback Check Result:
         1. PROBLEM FOUND (e.g. no signal, distorted signal, communication error): Go to Section 8: “Step-by-Step Troubleshooting Procedures – Defective Encoder/Cable” and Section 7: “Root Cause Analysis – Defective Encoder/Cable”.
         2. NO PROBLEM FOUND: Continue with drive and engine check.
   4. Check Servo Drive and Motor:
      1. Check the drive's supply voltage: Measure the incoming voltage. Check that it is within the specified range (e.g. 3x400V ±10%).
      2. Check the motor winding resistances: Switch off the drive (LOTO!) and measure the resistance between the U, V, W phases of the motor (multimeter). The values ​​must be identical (< 1% deviation). Typical values ​​are between 0.1 and a few Ohms. A significant difference indicates a defective winding or loose connection.
      3. Check the insulation resistance of the motor: Using an insulation tester (500V DC), measure the resistance between each winding and the motor frame. A value < 1 MΩ (according to IEC 60034-27) is critical and indicates insulation damage.
      4. Cooling: Are the motor and drive not overheated (> 80°C on the housing measured with a thermal camera)?
      5. Drive/Motor Check Result:
         1. PROBLEM FOUND (e.g. defective winding, overheating): Go to Section 8: 'Step-by-Step Troubleshooting Procedures – Faulty Motor/Drive'.
         2. NO PROBLEM FOUND: Continue with tuning parameters and load.
   5. Control of Tuning Parameters and Load:
      1. Read out the tuning parameters: Use the servo drive software to read out the current PID parameters (P-gain, I-gain, D-gain).
      2. Perform a step response test: Command a small position or velocity step and analyze the response via the oscilloscope function of the drive software.
         1. System oscillates/overshoot: P-gain too high.
         2. Slow response, large tracking error: P-gain too low, I-gain too low.
         3. Response with long brewing time: D-gain too low or mechanical play.
      3. Perform a load analysis: Monitor motor current and torque via the servo drive software.
         1. High continuous current/torque: Indicates constant motor overload. Check that the average current does not exceed the rated current (IEC 60034-1).
         2. Peak current/torque exceeds limit values: Indicates short-term overload, e.g. during acceleration or stalling.
      4. Tuning/Load Check Result:
         1. PROBLEM FOUND (e.g. sub-optimal tuning, overload): Go to Section 8: “Step-by-Step Troubleshooting Procedures – Tuning Optimization” or “Tackling Overload” and Section 7: “Root Cause Analysis – Improper Tuning/Overload”.
         2. NO PROBLEM FOUND: Consult OEM for specialist support or consider external factors such as mains pollution.

   6. Error Cause Matrix

   This matrix categorizes the symptoms, likely causes, diagnostic tests and expected results to speed diagnosis. The likely causes are listed from most to least likely.

   Symptom	Probable Causes (likelihood)	Diagnostic Test	Expected Result if Cause Confirmed
   Continuous/High Tracking Error	Incorrect PID tuning (high), Mechanical backlash (medium), Overload (medium), Insufficient drive power (low)	Step response test (drive software), Manual backlash test, Current/torque monitoring, Drive specification check	Oscillation/slow response, Sensible backlash, Current/torque above nominal, Demand peaks above drive limit
   Intermittent Tracking Error/Position Loss	Loose/damaged encoder cable (high), External electrical fault (medium), Initial encoder failure (medium), Varying load (low)	Cable continuity/tensile test, EMC analysis, Oscillogram encoder, Load profile monitoring	Signal interruptions/spikes, External noise on encoder line, Sporadic signal distortion, Load spikes correlate with interference
   Abrupt Position Shifts	Defective encoder (high), Loose mechanical coupling (high), Fixed motor shaft/machine (medium), Communication bus failure (low)	Encoder tester/oscilloscope, Manual backlash test, Motor torque test, Communication bus diagnosis	No/wrong signal, Palpable play, Stuck when switching on, Bus error messages on drive
   Inaccurate Positioning	Mechanical backlash (high), Encoder resolution too low (medium), Incorrect calibration/reference (medium), Thermal drift (low)	Manual clearance test, Encoder specification check, Software calibration, Thermal camera	Tactile play, Delivered resolution < required, Position offset after calibration, Position varies with temperature
   Overshoot/Oscillation	P-gain too high (high), D-gain too low (medium), Low mechanical stiffness (medium)	Step response test, Frequency response analysis, Stiffness physical inspection	Fast, unstable response, Slow damping of oscillation, Visible machine flex/vibration

   7. Root Cause Analysis for Each Error

   7.1 Defective Encoder or Cable

   Explanation: The encoder is the 'eye' of the servo system, providing precise position and/or speed information to the servo drive. Defects can range from mechanical damage (bearing wear, shock damage), contamination of the optical drive, internal electronic failures, to cable breaks, bad connectors or insufficient shielding. Electromagnetic Interference (EMI) can also disrupt encoder pulses, especially with long, unshielded cables.

   Confirmation: The oscilloscope is a critical instrument here. A distorted, missing, or intermittent encoder signal confirms the defect. Use an encoder tester to verify that the encoder functions correctly when disconnected from the drive. Error messages on the drive, such as 'encoder fault' or 'loss of communication', are direct indicators.

   Consequences if left unresolved: Permanent position losses, unpredictable machine behavior, risk of collisions, production of rejected products, and eventual machine downtime. The drive may go into overcurrent when attempting to reach the desired position without proper feedback.

   7.2 Mechanical Backlash (Backlash)

   Explanation: Mechanical backlash refers to lost motion in a mechanical system due to loose components, wear, or improper fits. This includes play in clutches, gearboxes, ball screws, timing belts, belts and bearings. Backlash forces the motor to move before the load begins to move, leading to tracking error and reduced positioning accuracy.

   Confirmation: Manual inspection is often the first step. Manually rotate the shaft and feel for a delay before the load responds. Use a dial gauge to quantify backlash in ballscrews or gearboxes. Vibration analysis can also detect bearing wear. Error messages such as too high a tracking error when reversing the direction of movement are typical.

   Consequences if left unresolved: Reduced repeatability and accuracy, accelerated wear of mechanical components (due to shock loading), increased noise production, and in extreme cases, structural damage to the machine.

   7.3 Incorrect Tuning Parameters (PID)

   Explanation: The servo drive's tuning parameters (P-gain, I-gain, D-gain) determine how the drive responds to a tracking error. Suboptimal tuning can lead to overshoot (P-gain too high), oscillation (P-gain too high, D-gain too low), slow response (P-gain too low), or constant tracking error (I-gain too low). Tuning is critical and must be matched to the mechanical stiffness and inertia of the loaded system.

   Confirmation: Analysis of the step response via the drive software is the primary method. An undamped or too slowly damped response indicates suboptimal tuning. The 'tracking error' graph in the drive software will show a consistent deviation or oscillation that does not disappear after settling time.

   Consequences if unresolved: Unstable machine behavior, overheating of motor and drive (due to constant corrections), increased energy consumption, reduced component life, and poor process quality.

   7.4 Overload

   Explanation: Overload occurs when the requested force or torque exceeds the rated capacity of the servo motor or drive. This can be the result of a mechanically seized part, a process that is too heavy, too high acceleration/deceleration profiles, or friction in the machine. The drive will attempt to maintain the commanded position, resulting in increasing tracking error and high motor currents.

   Confirmation: Monitoring motor current and torque via the servo drive software will show peaks or consistently high values ​​that exceed nominal values. A thermal camera can confirm engine overheating. Alarm codes such as 'overcurrent' or 'overtorque' are direct indicators.

   Consequences if left unresolved: Overheating and premature failure of the servo motor and/or drive, damage to mechanical transmission components (gearboxes, shafts), and immediate production downtime due to drive shutdown.

   8. Step-by-Step Resolution Procedures

   Perform these procedures only after strict compliance with LOTO and PPE regulations (Section 2).

   8.1 Defective Encoder or Cable Repair/Replacement

   1. WARNING! Perform LOTO on the entire machine before working on the encoder or cables.

   2. Identify the location: Determine whether the problem is in the cable, the connector, or the encoder itself based on the diagnosis (Section 5.3).
   3. Cable Inspection: Replace damaged encoder cables with cables of the correct specification (shielded, twisted pair, suitable AWG). Ensure proper shield grounding in accordance with manufacturer specifications to minimize EMI.
   4. Connector Repair: Clean dirty connectors. Replace corroded or damaged connectors. Check for proper crimping of pins.
   5. Encoder replacement:
      • Document the current orientation and mounting position of the encoder.
      • Carefully remove the defective encoder.
      • Install a new encoder of the same type and resolution. Ensure correct mechanical mounting (e.g. tightening torques in accordance with NEN EN ISO 898-1) and alignment (optical encoders are very sensitive to this).
      • Perform a zero point adjustment or calibration via the servo drive software if required for absolute encoders.
   6. Verification: Perform a jog test with the machine in safe mode. Monitor the encoder signal with an oscilloscope and the tracking error in the drive software. The signal must be clean, the tracking error minimal.

   8.2 Eliminating Mechanical Backlash

   1. WARNING! Run LOTO on the machine. Support any loads that may fall when loosening couplings or bearings.

   2. Locate the play: Use a dial gauge and manual manipulation to determine the exact location and extent of the play (coupling, bearing, gearbox).
   3. Check tightening torques: Check all bolts and screws on the mechanical transmission (coupling, motor flange, gearbox mounting) and tighten them to the correct torque (consult OEM manual and NEN EN ISO 898-1).
   4. Replacing the coupling: If the coupling is worn or damaged, replace it with a new, backlash-free servo coupling from UNITEC with the correct torsional stiffness. Ensure accurate alignment between the motor and load shafts (< 0.05 mm radial, < 0.1 mm axial).
   5. Replacing bearings: Replace worn motor or shaft bearings. Use industrial quality bearings (e.g. SKF, FAG) and pay attention to correct mounting (heating, press fit).
   6. Gearbox overhaul/replacement: If the backlash in the gearbox is unacceptable, consider overhaul or replacement with a low-backlash gearbox.
   7. Verification: After repair, perform another manual clearance test. Monitor the tracking error during movements with reversals and high accelerations. The tracking error must remain stable and within tolerance.

   8.3 Tuning Optimization

   1. WARNING! Tuning can lead to unexpected machine movements. Ensure a safe working environment and emergency stops within easy reach.

   2. Initialization: Reset the drive parameters to factory settings or a last known good configuration if the current tuning is completely unstable.
   3. Auto-tuning: Use the auto-tune function of the servodrive software. This is often a good starting point. Follow the manufacturer's instructions.
   4. Manual PID adjustment (if auto-tune is insufficient):
      • P-gain (Proportional gain): Increase gradually until the system starts to oscillate, then decrease by 10-20%. This affects the stiffness and direct response.
      • I-gain (Integral gain): Gradually increase to eliminate residual tracking error (steady-state error). Too high leads to slow oscillation.
      • D-gain (Differential gain): Increase to dampen overshoot and stabilize response. Too high can amplify noise and cause instability.
   5. Step Response Analysis: Perform a step response test after each adjustment and analyze the tracking error and motion profiles. Aim for a fast response with minimal overshoot and fast damping (settling time).
   6. Frequency response analysis: Some drives provide the ability to generate a Bode plot for accurate frequency response analysis and identification of resonances.
   7. Verification: Test the machine with realistic loads and motion profiles. The tracking error must remain within the specifications of the application (often < 1-5 pulses or < 0.01-0.05 mm).

   8.4 Tackling overload

   1. WARNING! Perform LOTO. Overloaded mechanical systems can be under stress.

   2. Identify cause:
      • Mechanical bond/friction: Check all moving parts for blockages, seized bearings, dirt buildup, or improper lubrication.
      • Process related: Analyze the process. Are the acceleration/deceleration times too short? Is the movement of a load too heavy?
      • Incorrect sizing: Is the servo motor or gearbox correctly sized for the application? Please refer to the sizing calculations.
   3. Reducing friction: Lubrication, alignment of shafts and guides, cleaning of guides.
   4. Adjust process parameters: Decrease the acceleration/deceleration profiles. Reduce the mass of the load if possible.
   5. Resizing components: If the motor or gearbox is structurally undersized, consider upgrading to a more powerful model via UNITEC-D.
   6. Verification: Monitor motor current and torque during operation. These values ​​must remain within nominal limits, with peaks within the short-term overload capacity of the drive/motor. The engine temperature must remain stable.

   9. Preventive Measures

   A proactive approach prevents recurring failures and extends the lifespan of your servo drive systems.

   Root Cause	Prevention strategy	Monitoring Method	Recommended Interval
   Defective Encoder/Cable	Regular inspection of encoder cables for wear and correct strain relief, checking connectors for corrosion. Ensure optimal EMC environment (separate cable ducts).	Visual inspection, Cable insulation test, Oscillogram encoder signal.	Every six months or at every planned downtime.
   Mechanical Backlash	Regular checking of tightening torques of couplings and mountings. Vibration analysis of bearings and gearboxes. Follow lubrication schedules.	Momentsleutelcontrole, Trillingsanalyse (mm/s RMS), Visuele inspectie op beweging. (Acceptable: < 2.8 mm/s RMS for motors < 15kW, ISO 10816-3).	Quarterly for critical axes, annually for others.
   Incorrect Tuning Parameters	Periodic check of the tuning parameters and step response, especially after significant changes in load or mechanical configuration.	Step response test via drive software, Monitoring tracking error.	Annually or after any change to the mechanical system/load.
   Overload	Continuous monitoring of motor current and torque. Check for binding and friction in the machine. Analyzing process data for unexpected peaks.	Servo drive software (trend logging), Motor/drive thermography, Process data analysis. (Nominal current may not be structurally exceeded, motor temperature < 80°C).	Continuously via SCADA/DCS, monthly for trend analysis.

   10. Spare Parts & Components

   Keeping critical spare parts on hand minimizes downtime. UNITEC-D GmbH supplies a wide range of quality parts for servo drive systems.

   Part description	Specification	When to Replace	UNITEC Category
   Incremental Encoder	Pulse/revolution (e.g. 1024, 2048, 4096), Type (TTL/HTL), Flange size, Shaft diameter	Bij defect of signaaldegradatie, Preventief na 5-7 jaar afhankelijk van bedrijfsuren.	Sensors & Feedback
   Absolute Encoder	Type (EnDat, Hiperface, BiSS), Resolution (e.g. 18-bit), Multi-turn/Single-turn, Flange size, Shaft diameter	In case of defect or communication error. Preventive after 5-7 years.	Sensors & Feedback
   Servo coupling (backlash-free)	Torsional stiffness (Nm/rad), Max. torque (Nm), Shaft diameters, Length, Type (bellows, slats)	In case of visible wear, play, or after extreme overload. Preventive after 3-5 years.	Mechanical Transmission
   Motor bearings	Type (e.g. 6205-2RS), C3/C4 clearance, Manufacturer (SKF, FAG)	In case of audible wear, vibrations above normal, or temperature increase. Preventively every 20,000-30,000 operating hours.	Bearings & Seals
   Servo motor	Vermogen (kW), Nominaal koppel (Nm), Nominale snelheid (RPM), Flensmaat, Encoder type	In case of irreparable damage to windings, bearings or stator/rotor.	Drive motors
   Servo Drive Module	Power (kW), Supply voltage, Communication interface (EtherCAT, PROFINET), Software version	In case of irreparable electronic malfunctions, internal errors.	Drive systems
   Shielded Servo Cable (Encoder)	Length, Number of cores, Shielding efficiency, Connector type	In case of cable breakage, insulation damage, or EMI-related problems.	Cabling & Connectors

   For specific product information and orders, visit our UNITEC-D e-catalogue.

   11. References

   • NEN 3140: Operation of electrical installations - Low voltage.
   • EN ISO 13849: Safety of machinery – Safety-related parts of control systems.
   • EN ISO 14118: Safety of machines – Prevention of unexpected start-up.
   • EN ISO 10816-3: Mechanical vibrations – Evaluation of machine vibrations by measurements on non-rotating parts – Industrial machines with rated powers above 15 kW and rated speeds between 120 rpm and 15 000 rpm when measured in situ.
   • IEC 60034-1: Rotating electrical machines – Part 1: Ratings and performance.
   • IEC 60034-27: Rotating electrical machines – Part 27: Off-line partial discharge measurements on stator windings of alternating current rotating machines.
   • BS EN ISO 898-1: Mechanical properties of fasteners – Part 1: Bolts, screws and studs of carbon steel and alloy steel.
   • OEM manuals of servo drive and motor systems (Siemens, Bosch Rexroth, Rockwell Automation, etc.).
   • UNITEC-D GmbH technical documentation for specific components.