Excellent PLC
Written by: Elaine Brook – Maintenance Superintendent, rotating equipment monitoring division.
Introduction
Over the past quarter, our plant experienced a recurring usability issue involving the Bently Nevada 120M8155-01 touchscreen displays. These panels form part of the operator interface layer for machine protection systems. They normally perform well, but several units showed progressive touch response delay and occasional mis-triggered screen presses.
This record summarizes what was observed, what testing was conducted, and what resolved the issue. It may serve as reference material for other facilities using the same hardware.
Operating Environment
The affected displays were installed inside MCC-level instrument enclosures with the following conditions:
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Ambient temperature: 27–32°C
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Relative humidity: 55–70%
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Intake air filtered, no active cooling
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Duty cycle: 24/7 operation
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Installed on vibration-critical turbines and compressors
No major environmental anomalies were present, which initially ruled out heat or condensation.
Observed Behavior
Operators reported three specific symptoms:
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Touch delay of approximately 0.3–1.2 seconds after screen press
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Intermittent double-activation of adjacent UI elements
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Reduced accuracy in the lower-right quadrant of the screen
No image distortion or LCD defects were present. Display refresh and machine data updates continued normally.
Evaluation and Testing
To verify the origin of the issue, technicians performed the following checks:
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Screen calibration routines
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Visual inspection of overlay and bezel
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Cleaning of touch surface and edge areas
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Measurement of supply power to the display
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EMI survey of surrounding devices
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Connector integrity checks
None of these actions resolved the latency problem.
The breakthrough came when one unit was temporarily relocated from the compressor area to an isolated test bench inside a quieter auxiliary building.
Touch accuracy immediately improved.
On that basis, we conducted further EMI measurements in the original installation area. The readings confirmed that nearby VFD cabinets were emitting interference in close proximity to the touchscreen cabling path.
Identified Cause
The root cause was classified as electromagnetic interference affecting the touch controller signaling lines. The interference did not affect the video data path, only the touch overlay.
Contributing factors included:
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Touch controller using unshielded signaling
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Cabling routed parallel to VFD output cables
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Lack of ferrite or line filtering
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Continuous EMI exposure from high switching frequency drives
Corrective Measures
Once confirmed, the following changes eliminated the latency and mis-touch behavior:
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Re-routing of touchscreen signal cable away from VFD conduit bundles
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Addition of two ferrite cores at both ends of touch controller harness
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Installation of grounded shielding braid along cable run
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Updating operator procedures to avoid cable pinch points during panel servicing
Units were retested under operating load and showed normal performance.
Plant-Level Takeaways
Three conclusions were recorded for future deployment of similar hardware:
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EMI effects can manifest as usability issues rather than communication losses
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Shielding and routing need attention even on short internal cables
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Touch systems are more sensitive to VFD emissions than display logic
These findings have been added to our installation standards for instrumentation cabinets.
Closing Remarks
The 120M8155-01 itself was not defective. The interference originated from installation practices rather than equipment failure. With corrected cable routing and proper shielding, the units have operated without further latency for six consecutive weeks.
— Elaine