1. Description of the Problem and Scope
Following Error occurs when the difference between the commanded position and the actual position of the motor axis exceeds the limit configured in the servo drive. Position loss occurs when the system cannot maintain the origin reference, resulting in collisions, defective products or emergency stops in CNC machinery, robotic arms and automated assembly lines. This document focuses on closed-loop control systems that use incremental or absolute encoders. The severity is classified as critical, as it directly affects the safety of the operation and the mechanical integrity of the equipment.
2. Safety Precautions
WARNING: Before performing any intervention, execute the Lockout/Tagout protocol. Residual energy in DC bus capacitors can be lethal; Wait at least 10 minutes after disconnection before touching the power terminals. Wear appropriate personal protective equipment (PPE), including insulated gloves and eye protection.
3. Diagnostic Tools
| Tool | Specification/Model | Range/Accuracy | Purpose |
|---|---|---|---|
| digital oscilloscope | Minimum 2 channels, 100 MHz | 0 - 500V | Noise analysis in encoder signals |
| True RMS Multimeter | Class III/IV | 0 - 1000 VAC/VDC | Verification of control and power voltages |
| vibration analyzer | FFT (Fast Fourier Transform) | 0 - 10kHz | Detection of misalignment or mechanical looseness |
| thermal camera | Min resolution 160x120 | -20°C to 300°C | Identification of hot spots in couplings |
| torque wrench | Range 5 - 50 Nm | +/- 3% accuracy | Tightening verification of couplings and motors |
4. Initial Checklist
| Observation | Action |
|---|---|
| Alarm history | Record the last 5 error codes and timestamp. |
| Operating conditions | Check if the load is higher than the nominal (overload). |
| Recent changes | Confirm if there have been firmware updates or mechanical changes (replacements, adjustments). |
| Around | Check for the presence of metal chips, oil or excessive humidity. |
5. Systematic Diagnostic Flow
- If dynamic tracking error occurs (during motion):
- Check current/torque limits: Does the motor reach its torque limit before following the trajectory? If yes, investigate mechanical load (friction or inertia).
- Check PID tuning: Is there oscillation? If yes, adjust gains (Kp, Ki, Kd) or notch filters.
- If static tracking error occurs (at rest or low speed):
- Check feedback: Does the position fluctuate without movement? If yes, diagnose noise in the encoder wiring or encoder failure.
- Check mechanical coupling: Does the motor rotate but the load does not? If yes, inspect flexible couplings or cotter pins.
6. Matrix of Failures and Causes
| Symptom | Probable Cause | Diagnostic Test | Expected Result |
|---|---|---|---|
| F error in acceleration | Excessive inertia / insufficient gain | Monitor % current vs nominal torque | Current < 150% of nominal |
| Position drift | Encoder noise | Oscilloscope in A/B/Z signals | Clean signal, without high frequency peaks |
| Error after stop | Mechanical play (Backlash) | Compare motor encoder vs load encoder | Difference within tolerance (e.g. <0.05mm) |
7. Root Cause Analysis
Encoder failure
Signal degradation occurs due to electromagnetic interference (EMI) or pollution. If the signal arrives distorted, the drive calculates an incorrect position. Damage: Premature bearing wear if the motor compensates for high frequency vibrations.
Mechanical Clearance
Wear on loose couplings or keys creates a delay between motor and load. The tuning of the system is destabilized. Damage: Mechanical fatigue from shock loads when closing the clearance gap.
Improper PID Tuning
Too high gain values induce instability; Too low causes delay. Damage: Overheating of the motor winding due to excessive compensation currents.
8. Resolution Procedures
- Mechanical Readjustment: Check the tightening of the coupling screws. Standard torque: 12 Nm for M6 class 12.9 screws. Check laser alignment (angular misalignment < 0.2°).
- Electrical Validation: Ensure shielding of the encoder cable connected to ground at only one end (drive chassis). Check encoder supply voltage: it should be 5V +/- 5%.
- PID recalibration: Perform auto-tuning at idle, then apply load. Adjust integrator (Ki) to eliminate static error, ensuring it does not cause overshoot.
9. Preventive Measures
| Root Cause | Strategy | Monitoring | Interval |
|---|---|---|---|
| EMI noise | Separate power/signal cables | Visual inspection | Semester |
| Clearance | Check torques | Vibration analysis | Monthly |
| Encoder wear | Preventive replacement | Hours of operation | According to OEM |
10. Spare parts and components
| Description | Specification | When to Replace | UNITEC Category |
|---|---|---|---|
| Flexible Coupling | Shaft diameter 14mm-20mm | If it has axial play | Mechanical/Couplings |
| Absolute Encoder | 20-bit multiturn | Frequent communication error | Servo/Electronics |
| power cables | Armored, 4x1.5mm2 | Insulation damage | Cables/Accessories |
For specific spare parts, visit our online catalog: https://www.unitecd.com/e-catalog/
11. References
- ISO 230-1: Test codes for machine tools.
- EN 60034-1: Rotating electric machines.
- Specific user manuals from the drive manufacturer.
