Troubleshooting Guide: Incorrect Sensor Readings – Interference, Grounding and Cable Degradation

1. Problem Description and Scope

This diagnostic guide is aimed at maintenance technicians, reliability engineers, and plant managers dealing with erratic or unstable readings from industrial sensors. These anomalies can manifest themselves as unexpected fluctuations, out-of-specification values, intermittent signals or total absence of signal, compromising the reliability of control systems and operational safety.

Commonly affected sensors include, but are not limited to: inductive/capacitive/optical proximity sensors, pressure transducers, resistance thermometers (RTDs) and thermocouples, rotary encoders, flow sensors, and level sensors. The equipment typically involved is numerically controlled (CNC) machine tools, industrial robots, automated assembly lines, material handling systems and continuous processes in the mechanical manufacturing sector.

The severity classification of these anomalies is as follows:

• Critical: Unplanned machine shutdown, production of non-compliant parts, potential damage to people or equipment. Requires immediate intervention.
• Major: Degradation of machine performance, reduction of product quality, need for frequent manual intervention. Requires urgent planned intervention.
• Minor: Occasionally unstable readings with no direct impact on production or security, but indicating an evolving underlying problem. Requires monitoring and intervention planning.

2. Safety Precautions

WARNING: Before any diagnostic or maintenance work on electrical or mechanical systems, it is essential to adopt rigorous safety precautions to prevent serious or fatal injuries. Compliance with UNI EN ISO 14118 and CEI EN 60204-1 regulations is mandatory.

• LOCKOUT/TAGOUT - LOTO: Ensure that all energy (electrical, hydraulic, pneumatic, mechanical) of the machine or system is isolated and locked in the non-operating position. Verify the absence of voltage with a certified multimeter before touching any component.
• STORED ENERGY: Discharge capacitors, depressurize hydraulic and pneumatic systems, block live mechanical components (e.g. springs, counterweights).
• PERSONAL PROTECTIVE EQUIPMENT (PPE): Always wear PPE appropriate to the specific risk: insulating gloves (according to CEI EN 60903), safety glasses, safety shoes, fireproof clothing for hot work.
• ELECTRICAL RISKS: Work with open or live circuits only if strictly necessary and with the necessary training and insulated equipment.
• UNEXPECTED MOVEMENT: Even with LOTO applied, pay attention to possible residual movements of actuators or robotic arms due to internal faults or programming.

3. Necessary Diagnostic Tools

The use of calibrated and appropriate instrumentation is critical for an accurate diagnosis.

4. Initial Assessment Checklist

Before starting any in-depth diagnosis, it is essential to gather preliminary information. This systematic approach reduces downtime and prevents unnecessary interventions.

5. Systematic Diagnostic Flow Chart

Follow this logical path to isolate the root cause of erratic sensor readings.

1. Symptom: Incorrect/Unstable Sensor Readings
   1. Perform Initial Checklist (Sec. 4):
      • IF obvious damage (cable cut, connector loose, sensor dirty/damaged): THEN
        1. Clean/Repair/Replace the damaged component.
        2. Check operation. IF problem solved: END. ELSE: Proceed to point 1.a.ii.
      • IF no visual anomaly but unstable sensor power supply voltage (e.g. fluctuations >10% or ripple >50mVpp):
        1. Diagnosis Power supply: Check the power supply dedicated to the sensor or I/O module.
        2. Test: Measure the voltage output from the power supply under load with DMM and oscilloscope.
        3. IF defective power supply: THEN Replace the power supply. Check. END.
        4. ELSE: Proceed to point 1.a.iii.
      • IF no visual anomalies and stable sensor power: THEN Proceed to step 2.
2. Isolation Source of Problem: Sensor, Cable or Environment?
   1. Directly Measure Signal from Sensor (without transmitter):
      • Disconnect the sensor from the field wiring and connect it directly to a clean external power supply and the oscilloscope/DMM.
      • IF the signal is clean and stable: THEN the problem is not the sensor itself. Proceed to step 3.
      • IF the signal is still erratic: THEN the sensor is defective. Replace the sensor. Check. END.
3. Cable Diagnosis and Grounding
   1. Cable Continuity and Insulation Check:
      • Disconnect the sensor cable from both ends (sensor and I/O module).
      • Test: Use a DMM to check the continuity of each conductor (<1Ω) and a Megger for the insulation between conductors and towards the screen (>20 MΩ at 500V).
      • Test: Check the continuity of the shield and its connection to the PLC/I/O module end (grounded on one side only).
      • IF no continuity or degraded insulation: THEN the cable is defective. Replace the cable. Check. END.
      • ELSE (cable OK): Proceed to point 3.b.
   2. System Grounding Verification:
      • Test: Use a DMM to measure the resistance between the grounding point of the I/O module and the general grounding point of the electrical panel/machine (<0.5Ω).
      • Test: Measure the potential difference between the grounding points of the various interconnected equipment (e.g. sensor, machine, PLC). Ideal values ​​are <50mV AC.
      • IF high resistance or significant AC potential: THEN grounding problem. Repair/optimize the grounding system.
      • Check. END.
      • ELSE (grounding OK): Proceed to step 4.
4. Diagnosis of EMI/RFI Interference
   1. Signal Analysis with Oscilloscope:
      • Connect the oscilloscope directly to the input terminals of the PLC I/O module (or to the transmitter, if present).
      • Observe: Abnormal waveforms, high frequency spikes, noise superimposed on the useful signal.
      • IF significant noise is observed: THEN EMI/RFI interference. Proceed to point 4.b.
      • ELSE (clean signal, but still erratic): THEN the problem could be the I/O module or the PLC. Proceed to step 5.
   2. Identification Source of Interference:
      • Method: Sequentially turn off nearby equipment (motors, inverters, welding machines, neon lighting, radio frequencies) and observe if the noise on the oscilloscope decreases.
      • Test: Use a magnetic/electric field probe to detect emission sources.
      • IF source identified: THEN apply preventive measures (shielding, filters, separation). Proceed to step 7.
      • ELSE: Proceed to step 6.
5. Diagnosis of the I/O/PLC Module
   1. Check I/O Module Functionality:
      • Test: If possible, connect a known working sensor to the same or adjacent input of the I/O module.
      • Observe: The readings.
      • IF the known sensor works correctly: THEN the problem is not the I/O module. Return to step 2 or 3 for a more in-depth review.
      • IF known sensor also produces erratic readings: THEN the I/O module is faulty. Replace the I/O module. Check. END.
6. Transmitter/Converter Diagnosis (if present)
   1. Open Loop Test:
      • Disconnect the transmitter input and power it directly. Apply a known and stable signal (e.g. with signal generator) to its input.
      • Measure: The output signal from the transmitter (e.g. 4-20mA, 0-10V) with DMM and oscilloscope.
      • IF the output is erratic or out of specification: THEN the transmitter is defective. Replace/Calibrate the transmitter. Check. END.
      • ELSE: Return to step 4 for a more in-depth EMI/RFI check on the transmitter output wiring.

6. Fault-Cause Matrix

This table identifies common symptoms and their likely causes, providing specific diagnostic tests and expected results for confirmation.

7. Root Cause Analysis for Each Fault

7.1 Electromagnetic Interference (EMI) / Radio Frequency (RFI)

Explanation: EMI (ElectroMagnetic Interference) and RFI (Radio Frequency Interference) are electrical disturbances that can degrade the performance of electronic circuits, including those of sensors. This interference can be generated by internal sources (e.g. relays, contactors, motors, VFD inverters) or external sources (e.g. radio transmitters, mobile phones, arc welders, electrostatic discharges). They propagate through conductive coupling (on power lines), inductive coupling (magnetic fields), capacitive coupling (electric fields), or radiative coupling (electromagnetic waves).

How to Confirm: The oscilloscope is the most effective tool. A signal disturbed by EMI/RFI will exhibit high-frequency noise superimposed on the useful signal, transient spikes, or unexpected level variations. Confirmation is often done by turning off potential noise sources one at a time and observing whether the noise on the sensor signal disappears or is reduced. VFD (Variable Frequency Drive) inverter-induced noises are often recognizable by their PWM carrier harmonic frequency. A thermal imager can detect hot-spots in electrical panels or motors that indicate insulation problems or overloads, which in turn can be sources of EMI.

Damage if left unresolved: False or unstable readings leading to incorrect PLC decisions, unwarranted machine shutdowns, inefficient operation, waste production, stress on electronic components and, in the long term, premature failures. Failure to comply with the CEI EN 61000 standards on Electromagnetic Compatibility (EMC) may also result in sanctions.

7.2 Grounding Problems

Explanation: Insufficient or incorrect grounding (GND) is one of the most common causes of noise in control systems. An inadequate grounding system can create ground loops, where potential differences between different ground points induce eddy currents in signal cables, generating noise. The high impedance to ground or the absence of a low-impedance path for fault currents and high-frequency disturbances prevents noise from being effectively dissipated.

How to Confirm: Use a DMM to measure the resistance between the ground point of the sensor, that of the I/O module and the reference ground of the main electrical panel. Values ​​above 0.5 Ω indicate inadequate grounding. Measure the AC potential differences between the different ground points with the oscilloscope when the machine is running: values ​​higher than 50mV AC p-p indicate the presence of ground loops. Visually check the integrity of the earth conductors and the connections to the terminals, which must be clean and well tightened, in compliance with the CEI regulation EN 60204-1.

Damage if not resolved: Continuous noise on signals, damage to I/O module input circuits due to overvoltages, risk of electrocution for personnel (in case of ground faults not properly managed), intermittent malfunctions that are difficult to diagnose. A non-compliant grounding system is a serious violation of electrical safety.

7.3 Degradation of the Signal Cable

Explanation: Signal cables in an industrial environment are subject to mechanical (bending, torsion, abrasion), chemical (oils, solvents), thermal (high temperatures) and environmental (humidity, dust) stress. This can lead to degradation of the insulation, partial or total breakage of the conductors, damage to the shielding or corrosion of the contacts in the connectors.

How to Confirm:

• Visual inspection: Look for signs of pinching, cuts, abrasions, exposed or hardened sections of cable, deformation, or discoloration. Check the connectors for oxidation or loose wires.
• Continuity and resistance testing: Use a DMM to test the continuity of each conductor and the shield. Excessive resistance (>1Ω per conductor for short cables) indicates a partial break or poor connection.
• Insulation test (Megger): Measure the insulation resistance between conductors and between conductors and shielding. Values ​​below 20 MΩ (at 500V) for in-service cables indicate compromised insulation.
• Dynamic testing: Perform continuity and insulation tests while the cable is slightly moved or flexed, particularly in areas where damage is suspected. A change in values ​​indicates an intermittent fault.

Damage if not resolved: Total cable breakage with loss of signal, short circuits between conductors or to ground, introduction of noise due to compromised shielding, damage to the sensor or I/O module in case of short circuits. Damaged cords also pose a fire or electrocution risk.

7.4 Defective Transmitter/Converter

Explanation: Transmitters and converters have the task of transforming the raw sensor signal into a standardized signal (e.g. 4-20mA, 0-10V, RS485) that can be read by the PLC or control system. They can fail due to faulty electronic components (e.g. capacitors, resistors, amplifiers), power surges, thermal stress, incorrect calibration, or internal drift due to component aging.

How to Confirm:

• Bench/Open Loop Test: Disconnect the transmitter from the field system. Power it with a clean power source and apply a known, stable signal (simulating the sensor output) to its input. Measure the output signal with a DMM and/or oscilloscope. Compare with manufacturer's specifications.
• Calibration: Perform a complete calibration procedure according to the manufacturer's instructions. If the transmitter fails to maintain calibration or shows significant offset, it is defective.
• Replacement: If all other tests are negative, replace the transmitter with a new or known good one. If the problem goes away, the original transmitter was the cause.

Damage if left unresolved: Consistently incorrect readings (offsets), signal fluctuations, total loss of signal, inaccurate or inefficient machine operation, producing quality defects, need for frequent manual adjustments.

8. Step-by-Step Resolution Procedures

8.1 EMI/RFI Interference Resolution

1. Source Identification:
   • Use the oscilloscope to monitor the sensor signal. Sequentially turn nearby equipment (VFDs, motors, switching power supplies, etc.) off and on again to isolate the source.
   • Threshold value: A reduction in noise (>50% p-p amplitude) on the sensor signal when a source is turned off is a confirmation.
2. Cable Re-Routing:
   • Separate low voltage signal cables from high voltage power cables. Maintain a minimum distance of 300 mm.
   • Avoid parallel paths; prefer 90 degree intersections to minimize inductive coupling.
3. Installing Additional Shielding:
   • Ensure that the signal cables are shielded and that the shielding is grounded on one side only (normally the PLC/receiver side) to avoid induced ground loops.
   • Consider installing closed metal conduit or copper braiding for signal cables in high-noise environments.
   • Install ferrite filters (chokes) on the signal and power cables near the sensor or I/O module input.
4. Mains/Power Filters:
   • Install EMI/RFI filters on the power lines of equipment that generates noise (e.g. VFD).
   • Use switching power supplies with low ripple (typically <20mVpp).
5. Final Check: Monitor the sensor signal with the oscilloscope to confirm noise reduction or elimination.

8.2 Grounding Troubleshooting

1. Visual Inspection and Tightening:
   • Examine all earth connection points: cable lugs, clamps, copper bars, earth cables. They must be clean, free from corrosion and tightened to the specified torque (see technical data sheets).
   • Threshold value: Use a torque wrench to check tightening.
2. Measure Earth Resistance:
   • Disconnect the system from the power supply. With a DMM, measure the resistance between the various ground points and the main panel earth.
   • Limit value: Resistance must be less than 0.5 Ω. If higher, locate the high resistance connection and clean/restore it.
3. Ground Loop Elimination:
   • If the oscilloscope has detected significant AC potential differences (>50mV AC) between the ground points, a ground loop is likely.
   • Make sure that the shields of the signal cables are connected to ground only at one end (receiver/PLC side).
   • Consider the use of mass isolators or signal converters with galvanic isolation for particularly sensitive sensors.
4. Structural Earthing:
   • Check that the metal structure of the machine is correctly connected to the main earthing system of the system, according to UNI regulations.
5. Final check: Remeasure the AC potential differences between the masses and monitor the sensor signal.

8.3 Resolution of signal cable degradation

1. Replacing Damaged Cable:
   • ATTENTION: Perform the LOTO procedure before disconnecting and replacing any cable.
   • Locate the damaged section of the cable using continuity and insulation tests (as described in Section 7.3).
   • Replace the entire cable with a new one having the same specifications (shielding, conductor section, bending resistance, operating temperature). Use industrial category cables, compliant with CEI EN 50289.
   • Make sure that the new installation respects the distances from the power cables and is adequately protected from abrasion and mechanical stress.
2. Restoring Connections:
   • Clean the connector contacts from oxidation or dirt with a specific spray for electrical contacts.
   • Retighten loose contacts to manufacturer specifications.
   • Check that the connector pins are not bent or damaged.
3. Post-Repair Verification:
   • Rerun continuity, insulation and grounding tests on the replaced or repaired cable.
   • Monitor the sensor signal with the oscilloscope under operating conditions.

8.4 Troubleshooting Defective Transmitter/Converter

1. Calibration:
   • See the transmitter manual for the calibration procedure.
   • Use a calibrated signal generator to simulate the sensor input (e.g. resistor for RTD, pressure for pressure transducer).
   • Adjust the transmitter offset and span to align the output with the reference values.
   • Threshold value: Deviation from the accuracy declared by the manufacturer (<0.5% of full scale is a good reference) must be minimized.
2. Replacement:
   • CAUTION: Perform the LOTO procedure before replacing the transmitter.
   • If calibration fails or the transmitter continues to show erratic readings after bench testing, proceed with replacement.
   • Install a new transmitter, with the same technical specifications (range, signal type, power supply, accuracy).
3. Functional Check:
   • After replacement/calibration, connect the transmitter to the system and monitor the signal under operating conditions.
   • Compare PLC readings to a known reference or process expectations.

9. Preventive Measures

Implementing a preventative maintenance strategy significantly reduces the likelihood of failures due to erratic sensor readings.

10. Spare parts and components

The availability of critical spare parts is essential to reduce downtime. Always consult your OEM manual for exact specifications.

To purchase certified, high-quality spare parts, visit our e-catalog: www.unitecd.com/e-catalog/

11. References

• UNI EN ISO 14118:2018: Machinery safety – Prevention of unexpected starting.
• CEI EN 60204-1:2018: Machinery safety – Electrical equipment of machines – Part 1: General requirements.
• CEI EN 61000 (Series): Electromagnetic compatibility (EMC).
• Original Manufacturer's (OEM) Operation and Maintenance Manuals.
• International Standards for the calibration of measuring instruments (e.g. ISO/IEC 17025).