Root Cause Analysis 1 min read

Root Cause Analysis: Encoder signal loss and degradation of connections in industrial drives

Technical analysis: NG6 ;symbol 012;24v WV06P1V1012WS024/00-D0

Аналіз першопричин відмови: Втрата сигналу енкодера та деградація з’єднань у промислових приводах - UNITEC-D Industrial MRO
Аналіз першопричин втрати сигналу енкодера в промислових приводах. Досліджено вплив електромагнітних перешкод (EMI), деградації екранування кабелів та окиснення контактів роз'ємів M12. Наведено практи

1. Introduction

During a planned startup of the hot rolling line at one of the heavy machinery enterprises in Ukraine, a critical stoppage of the main drive occurred. The control system recorded an emergency stop due to an error code indicating loss of feedback from the incremental encoder. Analysis of the controller logs showed periodic signal disturbances in phases A and B over the last 72 hours, which escalated into complete communication failure. Downtime of the equipment caused significant financial losses to the plant. As the Head Engineer of UNITEC-D GmbH, I conducted a judicial-technical expert examination of this incident in order to determine the exact causes of the signal degradation and to develop a prevention protocol for similar emergencies in the future.

2. Component Overview

The feedback system is based on heavy-duty industrial incremental encoders integrated into multi-loop electric drives with hydraulic distributors, including valves of type Bosch NG6 (specifically models marked WV06P1V1012WS024/00-D0, operating at 24 V DC voltage).

  • Working environment: High level of electromagnetic interference (EMI) caused by frequency converters with power over 250 kW.
  • Temperature range: from -15 °C to +65 °C on the sensor housing (within the permissible design range up to +85 °C).
  • Вібраційні навантаження: до 2.5 g у діапазоні частот 10-500 Гц.
  • MTBF indicator (mean time between failures): 45 000 hours under ideal operating conditions.

3. Failure Evidence

On-site inspection of the component and surrounding infrastructure revealed the following data:

  • Signal Oscillogram: Measurements using a digital oscilloscope showed significant distortion of pulse fronts (jitter over 120 ns) and a drop in amplitude of RS-422 differential signals below the threshold value of 2.0 V (actually recorded 1.4 V).
  • Screening status: The cable shield had a circular connection on both sides (ground loop), as well as mechanical damage to the outer polyurethane sheath due to friction against the cable duct.
  • Контактна група: М12 роз'єм мав сліди окиснення на пінах живлення 24V та сигнальних жилах, оскільки ступінь захисту IP67 був порушений через відсутність динамометричної затяжки гайки згідно з вимогами виробника (рекомендований момент — 0.6 Нм).

4. Root Cause Analysis

For determining the causes of failure, the Fault Tree Analysis method and the Ishikawa analysis (fishbone diagram) were applied. The main factors are divided into three categories:

  1. Electromagnetic Compatibility (EMC): Laying of signal cables in the same tray with power cables of frequency converters without separating metallic partitions.
  2. Mechanical factors: Vibration of the technological equipment caused micro-shifts and loosening of the threaded connections of the connectors.
  3. Installation errors: Incorrect grounding of the shielded cable's screen, creating a grounding loop with a potential difference of 1.2 V between the control cabinet and the working machine.

5. Identified Root Causes

Primary CauseProbability (%)Verification Evidence
Degradation of contacts through corrosion and loose connections40%Oxidation of M12 contacts, voltage drop down to 21.2 V at the sensor input.
Electromagnetic Interferences (EMI) through poor shielding35%Presence of grounding loops, high level of high-frequency noise on the oscillogram.
Mechanical damage to the cable from cyclic vibrations25%Break in the conductor in the bend zone of the cable duct, insulation wear.

6. Corrective Actions

Emergency repair (emergency maintenance)

  • Replaced cable assembly with a new one featuring double shielding (tinned copper braid + aluminum foil).
  • Contact groups are cleaned with a special tool for electrical contacts, and the connectors are torqued using a torque wrench to 0.6 Nm.
  • The screen of the cable is earthed on one side in the control cabinet in accordance with the requirements EN 50170.

Long-term prevention

  • Full reconfiguration of signal lines in separate metallic grounded tubes at a distance of no less than 300 mm from power cables.
  • Periodic thermal imaging control and insulation resistance measurement according to the standards ISO 13374.

7. Quick Diagnostic Checklist for Technicians

Use this list during scheduled technical maintenance on the production floor:

  • [ ] Check the torque of M12 connectors (norm: 0.6 Nm).
  • [ ] Measure the supply voltage at the sensor terminals (normal: 24 V ± 5%).
  • [ ] Check the integrity of the cable screen with a multimeter (resistance less than 0.1 Ohm).
  • [ ] Assess the condition of cable entries for cracks and leakage of lubricant/coolant fluid.
  • [ ] Perform an oscilloscope check for noise on RS-422 differential lines.
  • [ ] Check for the absence of a grounding loop (resistance between the cabinet and the mechanism).
  • [ ] Make sure there are no cable duct vibrations exceeding ISO 10816-3.
  • [ ] Analyze the controller logs for the presence of an encoder error counter.

8. Prevention Strategy

To ensure uninterrupted operation of Ukrainian industrial enterprises, the following measures need to be integrated:

  • Регламентні інтервали: Перевірка контактних з'єднань кожні 2000 годин напрацювання.
  • Condition monitoring: Continuous control of feedback signal amplitudes using PLC with a warning bit generated when the signal voltage drops below 2.2 V.
  • Modernization: Replacement of obsolete connector types with industrial components of protection class IP68 and oil resistance according to DIN EN 60529.

9. Conclusion

Loss of encoder signal is a result of the combined influence of the surrounding environment and violation of the installation schedule. Compliance with shielding standards (EN 50170) and technical maintenance rules allows avoiding emergency stoppages. For selection of certified components, connectors, cables and related industrial equipment complying with European norms and standards DSTU, we recommend visiting UNITEC-D E-Catalog.

10. References

  • DSТU EN 60204-1:2015 Machinery safety. Machinery electrical equipment.
  • ISO 13374 Machine Condition Monitoring and Diagnosis.
  • EN 50170 General requirements for industrial networks and cable connections.
  • Technical documentation Bosch Rexroth on hydraulic distributors and related electronics.
  • Handbook on Analysis of Differences and Faults in Industrial Automation (MRO Handbook, UNITEC-D GmbH, 2023).

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