1. Problem Description and Scope

This guide has been developed for maintenance technicians and reliability engineers who are confronted with critical error messages on servo-controlled industrial machines. A tracking error occurs when the deviation between the calculated target position and the actual position of the axis exceeds the defined threshold value. Loss of position indicates an inconsistency between the system null model and the physical location of the actuator.

Symptoms:

Sudden emergency stop with error code "Following Error Limit Exceeded".
Drift in position accuracy after a series of cycles.
Unstable behavior or oscillations when standing still or accelerating.
Inconsistent accuracy with varying loads.

Severity: This issue is classified as Critical as it directly leads to downtime, potential product damage, and safety risks.

2. Safety regulations

WARNING: Strictly follow the Lockout/Tagout (LOTO) procedure when working on servo drives. Servo drives contain capacitors with a residual charge that can cause life-threatening electric shocks even after the power supply has been switched off. Wait at least 10 minutes after switching off before working on the DC bus. Wear personal protective equipment (PPE), including safety shoes, insulating gloves and goggles.

3. Required Diagnostic Tools

Tools	Specification / Model	Measuring range	Goal
Multimeter	True RMS, CAT III/IV	0.1 Ohm - 1000V AC/DC	Checking earth resistance and cable integrity
Oscilloscope	Minimum 100 MHz, 4 channels	50mV - 50V	Analyzing encoder signals (A/B/Z channels, Hall)
Vibration meter	ISO 10816-3 compliant	0.1 - 50 mm/s	Detecting mechanical binding or bearing faults
Thermal imaging camera	Resolution > 160x120 pixels	-20°C to +250°C	Locating thermal overload in couplings/motors

4. Initial Assessment Checklist

Action	Observe/Record	Check
Check recent changes	Have mechanical parts been replaced or parameters adjusted?	[ ]
Log analysis	Do the errors occur at fixed times or at specific positions?	[ ]
Environmental conditions	Temperature, humidity, vibration sources in proximity.	[ ]
Visual inspection	Traces of wear on coupling, cables or motor housing.	[ ]

5. Systematic Diagnosis Flowchart

Symptom: High tracking error at standstill
1. Check mechanical friction: Is the shaft rotatable by hand?
  - NO: Mechanical binding (blocked guide/bearing defect).
  - YES: Go to step 1.2.
2. Check tuning: Is the I-gain (Integral term) set correctly?
  - NO: Adjust tuning (Bode plot analysis).
  - YES: Go to step 1.3.
Symptom: High tracking error on acceleration
1. Check inertia matching: Is the load inertia calculated correctly?
  - NO: Perform inertia identification again.
  - YES: Go to step 2.2.
2. Check mechanical coupling: Is there any play in the connection?
  - YES: Inspect/replace mechanical coupling.
Symptom: Intermittent loss of position
1. Check encoder cabling: Is shielding properly grounded?
  - NO: Restore grounding (single-point grounding).
  - YES: Check for electromagnetic interference (EMI).

6. Fault-Cause Matrix

Symptom	Probable Cause	Test	Expected Result
Tracking error	Mechanical bond	Vibration measurement/Manual resistance	> 2.8 mm/s or high resistance
Loss of position	Encoder cable shielding	Oscilloscope (noise on signal)	High noise floor on A/B signals
Tracking error	Tuning (PID setting)	Step response test	Surplus (overshoot) > 10%
Loss of position	Clutch slip	Marking check (axle/load)	Markers move

7. Root Cause Analysis

Mechanical Binding

Friction caused by worn bearings or lack of lubrication on linear guides forces the motor to draw higher current. With servo drives this results in a lower bandwidth of the control system and unavoidable tracking errors. If this is not corrected, it will lead to premature failure of the motor windings.

Encoder Interference

If the encoder cable shield is not properly grounded, EMI spikes from the motor power cable can affect the pulse counts. This causes "ghost glitches" where the position suddenly jumps, leading to catastrophic position loss and possible collisions.

Tuning Parameters

Aging of mechanical parts (e.g. timing belt wear) changes system dynamics. When the PID parameters do not evolve, the system loses its ability to tightly track the reference position, which manifests as a cumulative tracking error.

8. Step-by-Step Resolution Procedures

Restore Mechanical Bond
1. Disconnect the motor from the load.
2. Clean and lubricate the linear guides according to OEM specifications.
3. Measure the friction resistance with a torsion meter. Compare with datasheet.
4. Verify after installation.
Encoder Grounding
1. Check continuity between encoder shield and PE rail (resistance < 0.5 Ohm).
2. Separate power cables from signal cables in cable ducts (minimum 100mm distance).
Tuning
1. Perform an automatic inertia identification via the drive software.
2. Adjust P, I and D parameters.
3. Validate via a Bode plot; aim for a phase reserve of at least 45 degrees.

9. Preventive Measures

Root Cause	Prevention strategy	Monitoring	Interval
Mechanical Binding	Periodic lubrication	Vibration analysis (ISO)	Monthly
Encoder Interference	Periodic grounding check	Visual inspection	Quarter
Tuning	Parameter backup	Software monitoring (Log)	Annually

10. Spare Parts and Components

Description	Specification	When to replace	UNITEC Category
Servo clutch	Nm rating, shaft diameter	In case of backlash (backlash)	Links
Encoder cable	Shielded, twisted pair	In case of cable breakage/insulation damage	Cabling
Linear lubricant	NLGI class, ISO VG	Planned maintenance	Maintenance products

For specific replacement parts, please consult our catalogue: https://www.unitecd.com/e-catalog/

11. References

ISO 2372 / ISO 10816: Vibration standards for rotating machines.
IEC 61800-3: EMC requirements for variable speed drive systems.
NEN-EN-IEC 60204-1: Safety of machines - Electrical equipment of machines.