Learn Automation with MINATA #49: Mitigating Noise on Signals — Start at the Source and the Cable Route
Mitigating Noise on Signals: Start at the Source and the Cable Route
Dealing with noise starts with two questions: where is it generated, and which route does the sensitive signal take. Raising a filter in the PLC is the last step, not the first. Most noise problems are solved in the routing of the cable.
Typical noise sources on a machine
- Drive motor cables, with their fast switching and high currents.
- Contactor coils, solenoid valves and brakes as they pick up and drop out, producing inductive spikes.
- Switched-mode supplies, lighting and transformers mounted close to signals.
- Power runs travelling a long way parallel to signal cable.
Knowing the sources lets you route trunking and cable so sensitive signals stay away from them, instead of defending against noise after everything already shares a route.
Separating the routes is the cheapest measure
Do not run motor or contactor cable in the same trunking as analogue or encoder signals without proper separation. Where they must cross, let them cross at right angles rather than run parallel for metres. Keep signal cable as short as possible and as far from the sources as possible, and use screened twisted pair for analogue and encoder circuits.
Diagnosis by symptom
Each symptom points somewhere different:
| Symptom | Where to look first |
|---|
| Analogue value jittering or jumping | Cable route, screening, ground loop |
| Lost encoder counts | Cable, line driver type, frequency against length |
| Communication dropouts | Termination, noise on the bus, contact quality |
| Inputs turning on or off falsely | Parallel runs, leakage current, the 0V reference |
Measure and record the symptom before changing anything. "It fails occasionally" needs data before you can tell noise from a real fault.
A worked engineering situation
An input reports ON at exactly the moment a contactor drops out. The team traces the contactor coil and its inductive spike, the signal wire running parallel to the coil wire, and the 0V reference point — before adding any debounce in the PLC. After separating the wires and fitting suppression across the coil, the input is stable. The software did not have to carry a physical problem.
Separation, screening and suppression all have to follow the real layout and the equipment manuals.
Common mistakes
- Running analogue or encoder cable in the same trunking as motor cable, unseparated.
- Letting signal cable run parallel to power for a long distance.
- Adding a digital filter before checking the physical route.
- Leaving contactor and valve coils near signals without suppression.
- Not recording the symptom, and guessing at an intermittent fault.
Noise mitigation checklist
- [ ] The noise sources are identified and laid out away from signals.
- [ ] Analogue and encoder circuits use screened twisted pair in their own trunking.
- [ ] Power and signal cross at right angles where crossing is unavoidable.
- [ ] Coils near signals have suppression fitted.
- [ ] The symptom was measured and recorded before any digital filter was raised.
Good noise immunity is the result of layout, not of software. #50 turns to inspecting panel wiring before the first energisation.
Read more automation knowledge at MINATA: https://minatavn.com/en/blog/industrial-automation
Previous — #48: Earthing and screening against noise: https://minatavn.com/en/blog/automation-48-grounding-shielding
Next — #50: Inspecting panel wiring before power-up: https://minatavn.com/en/blog/automation-50-panel-wiring-inspection
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