Servo Troubleshooting: Tracking Error and Position Loss

1. Description of the Problem and Scope of Application

This diagnostic manual is designed to systematically identify and troubleshoot following errors and loss of position in servo drive systems. Servo system failures can cause significant production disruptions, product quality degradation, and equipment damage if not diagnosed and corrected in a timely manner.

1.1. Potential Malfunction Symptoms

• Following Error: The servo cannot accurately reach or maintain a given position or speed. This may manifest itself as a constant or periodic deviation between the set and actual position.
• Loss of Position: The servo system completely loses its orientation to its zero position or cannot return to it after power failure.
• Poor positioning accuracy: Inability to reposition the mechanism with the required accuracy, resulting in missing or inconsistent dimensions.
• Oscillations and vibrations: Unwanted oscillations of the drive axis, which may be related to system instability.
• Motor or drive overheating: Excessive heat generation, often accompanied by increased current consumption.
• Abnormal noises: Squealing, knocking or other abnormal sounds from the servo motor, gearbox or mechanical connection.
• Faults on the servo display: Displays specific error codes related to the encoder, tracking limit or overload.

1.2. Types of Equipment That May Be Involved

Servo drives are an integral part of high-precision industrial equipment. This manual is applicable to the following machine types:

• CNC machines: milling, turning, grinding machines.
• Industrial robots: manipulators, welding robots, assembling robots.
• Packaging machines: forming, filling, sealing units.
• Printing and textile machines.
• Automatic assembly lines.
• Transportation and positioning systems.

1.3. Malfunction Severity Classification

Correct assessment of the severity of the malfunction helps to prioritize diagnostic and repair work.

• Critical: A complete shutdown of the production process, an immediate threat to the safety of personnel or the risk of significant damage to expensive equipment. Requires immediate intervention.
• Significant: Decrease in productivity or product quality, frequent unscheduled shutdowns, increased component wear. Requires intervention as soon as possible.
• Minor: Periodic, irregular issues that do not directly affect production or security, but may progress. Requires diagnostic and repair planning.

2. Security measures

WARNING! All standard safety procedures must be strictly followed before starting any diagnostic or repair work on servo systems. Failure to follow these precautions could result in serious injury or death, or damage to the equipment.

• LOCKOUT / TAGOUT (LOTO): Always perform the LOTO (Lockout/Tagout) procedure in accordance with DSTU EN 1037:2003 and the equipment manufacturer's instructions. Ensure that all power sources (electrical, hydraulic, pneumatic) are disconnected and locked out before accessing electrical or mechanical components.
• Residual Energy: Capacitors in servos can store a dangerous high voltage charge even after the power is turned off. Wait for it to fully discharge, which can take up to 10 minutes. Use a suitable voltmeter (safety class III or IV according to DSTU EN 61010-1:2014) to check the absence of voltage. Mechanical systems can also have stored energy (springs, lifted loads). Ensure their fixation.
• Personal Protective Equipment (PPE): Use appropriate PPE: protective glasses, dielectric gloves, protective clothing, protective footwear.
• Hot Surfaces: Servo motors and drives can heat up to dangerous temperatures during operation. Allow them to cool before touching.
• Rotating Parts: Never attempt to diagnose or adjust a servo system while it is operating without proper protective covers. There is a risk of clothing or body parts being pulled.
• Electrical Safety: Only work with electrical components if you are qualified and authorized (eg Electrical Safety Approval Group III or IV). Make sure that the equipment is properly grounded according to DSTU EN 60204-1:2019.

3. Necessary Diagnostic Tools

For effective diagnostics of servo systems, specialized tools are required:

4. Initial Assessment Checklist

Before starting a systematic diagnosis, it is necessary to conduct an initial examination and collect as much information as possible about the malfunction. This will help narrow down the potential causes.

5. Systematic Diagnostic Algorithm

Below is a sequential algorithm for diagnosing tracking errors and loss of position in servo systems. Go step by step, isolating potential causes.

1. Initial Score and Error Codes

   1. Perform an initial assessment: Complete all items in the Initial Assessment Checklist (Chapter 4).
   2. Check the servo error codes:
      • If there are specific error codes (such as "Encoder Fault", "Over-current", "Following Error Limit"):
        1. Refer to the drive manufacturer's manual to interpret the code.
        2. Go directly to "7. Root Cause Analysis" for the corresponding fault.
      • If the error codes are missing or general ("Following Error"): Continue diagnostics.

2. Diagnostics of Encoder Feedback

   Reliable encoder feedback is critical for precise servo control.

   1. Visual inspection of the encoder and cable:
      • Check the reliability of the encoder attachment to the motor shaft.
      • Inspect the encoder cable for damage, kinks, fraying, correct connection of connectors.
      • Make sure the encoder cable is routed separately from the power cables to minimize electromagnetic interference.
   2. Encoder power check:
      • ATTENTION! Before measuring voltage, make sure you are qualified and use safe methods.
      • Measure the supply voltage at the encoder with a multimeter.
      • Expected result: Nominal voltage (eg 5V DC or 24V DC) according to encoder specification.
      • If the voltage is absent or incorrect:
        1. Check the integrity of the encoder power cable and the corresponding power outputs on the servo.
        2. Possible malfunction of the servo drive or power supply.
   3. Analysis of the encoder signals using an oscilloscope:
      • Connect the oscilloscope to the A, B and Z signal outputs of the encoder.
      • Carefully turn the engine shaft slowly by hand or run the engine at very low speed (if safe and permitted).
      • Expected result:
        • Incremental encoders (TTL/HTL): Clear rectangular pulses shifted by 90° between channels A and B. The Z pulse (reference mark) should be one per revolution.
        • Sine encoders (Sin/Cos): Clear sine and cosine signals shifted by 90°, without distortion or noise.
      • If the signals are missing, distorted, noisy or not as expected:
        1. Probable malfunction of the encoder or encoder cable.
        2. Diagnostic test: Use the Encoder Test Kit (if available) to test the encoder in isolation.

3. Diagnostics of Mechanical Connections and Loads

   Mechanical problems are a common cause of tracking errors.

   1. Coupling inspection:
      • WARNING! Ensure full lockout and tagout before handling mechanical parts.
      • Check the coupling connecting the servo motor with the load (reducer, lead screw) for backlash, cracks, wear, and deformation.
      • Make sure that all coupling fixing screws are tightened to the recommended torque (use a torque wrench).
      • Expected result: No signs of play or wear, reliable connection.
      • If play or wear is found: Replace or repair the coupling.
   2. Checking the mechanical load system:
      • Disconnect the engine from the load (if possible without damage).
      • Try to manually move the load. It should move smoothly, without jamming, excessive friction or backlash.
      • Check load bearings, guides, ball screws, etc.
      • Measure the force required to move the load (if appropriate tools are available).
      • Expected result: Low movement resistance, no backlash (>0.05 mm - alarm for precision systems).
      • If excessive friction or backlash is found: Identify and eliminate the source of mechanical resistance (lubricant, bearings, alignment).
   3. Misalignment diagnosis:
      • Use the Laser Centering Meter to check the alignment of the motor shaft and the load shaft.
      • Expected result: Offset no more than 0.02 mm (for parallel) and 0.05 mm/100 mm (for angular).
      • If misalignment is detected: Perform a fine axis alignment.

4. Analysis of Servo Setting (Tuning) Parameters

   Incorrect PID controller parameters (P, I, D) can cause instability and tracking errors.

   1. Accessing the parameters via software:
      • Connect the PC to the servo using the dedicated software.
      • View the current values ​​of PID coefficients, filter coefficients, speed gain and current.
   2. Executing auto-tuning (if available):
      • Many modern servos have an auto-tuning function. Run it following the manufacturer's instructions.
      • CAUTION! During auto-tuning, the motor shaft can make sudden movements. Make sure there are no obstacles and the safety of personnel.
      • Expected result: The system stabilizes, the tracking error decreases.
   3. Manual tuning (if auto-tuning did not help or is not available):
      • Sequentially adjust the coefficients P, I, D, observing the system's response to test movements (step function).
      • Critical observations:
        • P is too high: oscillation, overshoot, instability.
        • P too low: sluggish response, large tracking error.
        • I too high: overshoot, slow stabilization.
        • I too low: large static error.
        • Too high D: sensitivity to noise, vibration.

5. Engine / Drive Overload Analysis

   Exceeding the permissible load leads to tracking errors and overheating.

   1. Motor Current Monitoring:
      • Use the servo software to monitor the RMS (root mean square) current of the motor during operation.
      • Expected result: The peak current values ​​should not exceed the rated current of the motor by more than 150% (short-term) and the average RMS current should not exceed 80% of the rated current.
      • If the current exceeds the norm: Probable overload.
   2. Temperature measurement:
      • Use a thermal imaging camera to measure the temperature of the motor housing and servo heat sink during operation.
      • Expected result: The temperature of the engine body should not exceed 80°C, the drive radiator – 60°C.
      • If the temperature is excessive: This may indicate an overload, cooling problem or motor/drive failure.
   3. Mechanical load analysis:
      • Review the design of the machine. Were new components added that increased mass or friction?
      • Check the friction conditions in the guides, bearings.
      • Assess adequacy of motor size for current load.

6. Matrix "Failure-Cause"

This matrix provides a quick overview of typical symptoms, likely causes, and diagnostic tests for confirmation.

7. Root Cause Analysis for Each Malfunction

A deep understanding of the root causes helps not only to eliminate the current malfunction, but also to prevent it from reoccurring.

7.1. Encoder and Cable malfunctions

Explanation: The encoder provides feedback on the current position and/or speed of the motor. Any distortion, loss, or inaccuracy of its signal directly affects the servo's ability to accurately control an axis.

• Common causes: Mechanical wear of the encoder bearings, contamination of the optical elements (for optical encoders), damage to the magnetic disk (for magnetic ones), electrical interference, cable damage (break, short circuit, loss of shielding), failure of the encoder electronics.
• How to confirm: Analysis of signals with an oscilloscope (absence of pulses, distorted waveforms, noise), checking the resistance and ringing of the cable with a multimeter, using a specialized encoder tester.
• Possible damage: Left unchecked, this can lead to uncontrolled axis movement, collisions, damage to the tool and workpiece, mechanical parts of the machine. Loss of position is critical for multi-coordinate systems.

7.2. Problems of Mechanical Connections and Transmission

Explanation: Even a perfectly functioning servo cannot compensate for mechanical deficiencies in the motion transmission system.

• Common causes: Backlash in the coupling that connects the motor to the load (eg, worn splines, loose screws), worn gears in the gearbox, backlash or binding in slide/roller bearings, misalignment of shafts, increased friction in guides.
• How to confirm: Visual inspection, manual clearance check, use of torque wrench to check tightening, laser shaft alignment, vibration analysis, friction force measurement.
• Possible damage: Increased wear of couplings, bearings, reducers, servo motor, increased energy consumption, vibrations, noise, reduced positioning accuracy, component failure.

7.3. Incorrect adjustment (tuning) of the servo drive

Explanation: The parameters of the PID controller of the servo drive must be optimally selected for the dynamic characteristics of the motor and the inertia of the load. Improper tuning can result in instability, slow response, or overshoot.

• Common reasons: Incorrect gain (P), integration (I) and differentiation (D) coefficients, incorrectly configured filters, incorrectly set load inertia, using standard settings for non-standard mechanics.
• How to confirm: Analysis of tracking and speed oscillograms using servo software (observed oscillations, slow response to command, significant tracking error), execution of auto-tuning function and further analysis of results.
• Possible damage: Engine overheating due to constant oscillations, increased wear of mechanical components, vibrations, acoustic noise, low machining accuracy, unstable operation.

7.4. System overload

Explanation: If the mechanical system requires more torque or power than the servo/drive can provide, a tracking error will occur because the drive will not be able to maintain the specified motion profile.

• Common causes: Excessive load weight, increased friction in mechanical parts (e.g. lack of lubrication, damaged guides), change in motion profile requiring more acceleration/deceleration, wrong servo motor for the current task.
• How to confirm: Motor current monitoring (RMS and peak values) via drive software, motor and drive temperature measurement by thermal imaging camera, manual measurement of the force required to move the load.
• Possible damage: Overheating and failure of the motor windings, overloading of the power modules of the servo drive, damage to the gearbox or couplings, reduction of the resource of the entire system.

8. Step-by-Step Troubleshooting Procedures

Perform these procedures only after the root cause has been identified and with full safety precautions (Chapter 2).

8.1. Restoration of Encoder Functionality

If the encoder or its cable is faulty:

1. Security: Apply LOTO. Переконайтеся у відсутності залишкової енергії.
2. Disconnect: Disconnect the old encoder from the servo motor and from the cable. Pay attention to the marking of the wires.
3. Visual cable inspection: Carefully inspect the encoder cable. If damage is found, replace the cable completely or repair (only if the repair ensures reliability and shielding). Make sure the new cable has adequate shielding and connectors.
4. Installing the new encoder:
   • Install the new encoder, making sure it is securely and accurately mounted on the motor shaft (if it is a rotary encoder). For enclosure mount encoders, ensure proper clearance and alignment.
   • Use a torque wrench to tighten the mounting screws to the torque recommended by the manufacturer (usually 1-5 Nm).
5. Wire connection: Connect the cable to the encoder and servo strictly following the manufacturer's wiring diagram. Check the correct connections.
6. Checking and setting:
   • After connecting, turn off LOTO.
   • Run the servo in test mode.
   • Use the drive software to check the encoder signals and correct position reading.
   • For incremental encoders, it may be necessary to perform a homing procedure (search for a reference point).

8.2. Troubleshooting Mechanical Connection Problems

If backlash, clutch wear or misalignment is detected:

1. Security: Apply LOTO. Make sure there is no residual energy.
2. Coupling disassembly: Remove the damaged coupling.
3. Shaft inspection: Clean the motor and load shafts from dirt and rust. Check them for damage (scratches, dents).
4. Bearing Check: Check motor bearings and load. If play, noise or damage is found, replace them.
5. Installing a new coupling:
   • Install a new coupling according to specification (type, torque, shaft diameters). Preference should be given to flexible couplings for servo systems, which compensate for small misalignment and dampen vibrations.
   • Important: Accurately align the shafts with a laser alignment meter. Permissible misalignment: parallel < 0.02 mm, angular < 0.05 mm/100 mm.
   • Tighten the coupling fixing screws with a torque wrench to the torque specified by the manufacturer (usually 20-100 Nm depending on size).
6. Check: Perform a test run at low speeds, visually and audibly check for vibrations and unusual noises.

8.3. Optimization of Servo Drive Settings

If the problem is incorrect tuning:

1. Safety: Ensure that there are no obstacles to the movement of the axis. Be prepared for sudden movements during tuning.
2. Software access: Connect the PC to the servo drive and open the software.
3. Save current settings: Save current drive settings as a backup before making changes.
4. Executing auto-tuning:
   • Start the servo auto-tuning function. Follow the software instructions.
   • After completion, analyze the obtained parameters and test movement results.
5. Manual tuning (if necessary):
   • If autotuning did not give satisfactory results, carefully adjust the coefficients of the PID regulator.
   • Start by increasing the P-factor until there are small fluctuations, then decrease it a bit.
   • Add an I-factor to eliminate static error.
   • Add a D-factor to dampen vibrations and speed up response, but avoid an excessive value that causes noise.
   • Perform test movements (stepping commands) and observe the tracking error and velocity waveforms. Optimum response – quickly reaching the set position without over-adjustment or oscillations.
6. Verification: Carry out a full cycle of the equipment with the new parameters, verifying the absence of tracking errors and stable operation.

8.4. Elimination of Overloading

If overload detected:

1. Safety: Use LOTO for mechanical work.
2. Determine source of overload:
   • Mechanical friction: Check and lubricate guides, bearings, ball screws. Assess the condition of all moving parts.
   • Mass Increase: Check if new components have been added to the moving part without corresponding servo change.
   • Changing motion profile: If speeds or accelerations have increased, consider optimizing the kinematics or upgrading the servo system.
3. Lubrication and Cleaning: Ensure proper lubrication of all moving parts according to the manufacturer's lubrication chart. Clean the mechanical components from dirt.
4. Optimization: If the source of the overload cannot be eliminated, it may be necessary to recalculate and replace the servo motor/reducer with a more powerful one.
5. Check: After removing the overload source, monitor the motor current and temperature during operation. The motor current should not exceed 80% of the rated RMS.

9. Preventive Measures

Regular maintenance is the key to long-term and reliable operation of servo systems.

10. Spare Parts and Components

Timely availability of quality spare parts reduces equipment downtime. UNITEC-D GmbH offers a wide range of components for servo systems.

To order spare parts and receive advice, refer to our electronic catalog UNITEC-D.

11. Links

• DSTU EN 1037:2003 Machine safety. Prevention of unexpected start.
• ДСТУ EN 61010-1:2014 Вимоги щодо безпечності електричного обладнання для вимірювання, керування та лабораторного застосування.
• DSTU EN 60204-1:2019 Machine safety. Electrical equipment of machines. Part 1: General requirements.
• ISO 10816-1:2010 Mechanical vibration. Evaluation of machine vibration by measuring on stationary parts.
• Instructions for operation and maintenance of servo drives and servo motors from manufacturers (Siemens, Bosch Rexroth, Fanuc, Yaskawa, Rockwell Automation, etc.).
• Other service manuals from UNITEC-D GmbH.