Excellent PLC
Author: Daniel Harper – Field Maintenance Engineer (11 years in rotating machinery & industrial HMI systems)
Last month I was called into a gas compression plant where the operators were dealing with a frustrating issue on a Bently Nevada 120M8155-01 touchscreen display. The screen was showing normal graphics, but the touch input behaved like it had a mind of its own.
Below is a breakdown of the failure symptoms, root cause, and what actually fixed it.
Observed Symptoms
The operator described it as “the screen is haunted,” which wasn’t far off:
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Touch points shifted 1–2 cm to the side
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Buttons required multiple presses
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Random “self-touching” in the lower corners
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No display freeze, no rebooting, no alarms
The issue did not appear on day one; it developed gradually within 10–12 days.
Initial Diagnostic Assumptions
I initially suspected:
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Calibration loss
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Faulty touch sensor
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Outdated HMI firmware
None of these ended up being the culprit.
Calibration was intact, sensor passed basic tests, and firmware was already current.
Root Cause: Local Electromagnetic & Grounding Interference
After isolating the unit from the cabinet and testing it in a clean office environment, the display worked flawlessly for 48 hours. That immediately pointed to the site’s electrical environment.
We later confirmed the cabinet contained:
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High-power VFDs (Variable Frequency Drives)
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Multiple pump motors
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Poor bonding & mixed grounding
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High humidity in the enclosure
All of these factors contributed to EMI/EMC interference and capacitive static buildup, which caused touch drift and ghost touches.
Corrective Actions That Solved the Problem
These were the adjustments that stabilized the display:
1. Reworked cabinet grounding
Created proper bonding and separated protective earth from signal ground.
2. Improved EMI shielding on signal lines
Rerouted the touchscreen signal cable away from VFD cabling and added an additional shield sleeve.
3. Removed screen film & cleaned surface
Dust + humidity were increasing capacitive noise on the plastic film.
After these adjustments, the screen behavior returned to normal and remained stable for the following week of observation.
Preventive Recommendations for Industrial HMIs
From an engineering standpoint, these will avoid similar failures on Bently and other industrial HMIs:
✔ Keep HMI wiring away from VFDs, contactors, and high-tension lines
✔ Maintain proper single-point grounding architecture
✔ Avoid mixed bonding between control & motor cabinets
✔ Keep dust/humidity in check (capacitive touch is sensitive)
✔ Perform enclosure EMI checks during commissioning
Most “haunted touchscreen” cases are not hardware failures — they’re environmental.
Closing Thoughts
The Bently Nevada 120M8155-01 is generally reliable, but it is sensitive to electrical noise. If you see touch drift, ghost taps, or dead zones, always consider EMI and grounding before replacing hardware.