--- title: "Arc welder noise coupling in robotic arm controller" subject: "Instrumentation II" chapter: 6 key: "semester_5/instrumentation_ii:arc-welder-noise" source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/arc-welder-noise site: BCT Engineering --- # Arc welder noise coupling in robotic arm controller Instrumentation II · Chapter 6: Grounding And Shielding - Asked in: 2068 Baisakh (old course), 4 marks - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/arc-welder-noise - Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-6 ## Question An arc welder on the end of the robotic arm generates noise interference in the local embedded controller. The welder produces 120A at 12V. What could be the coupling mechanism for noise interference? How this can be minimized? ## Short answer Source impedance is 0.1 Ω (low), so magnetic (inductive) coupling dominates: large di/dt in welder cables induces voltage V = M di/dt in controller loops. Radiated and conductive coupling also occur. Minimize by twisting welder cables, shielding the controller, using optical isolation, separating grounds, and increasing distance. ## Answer The welder carries a very high current (120 A) at low voltage (12 V), so the source impedance is Z = V/I = 12/120 = 0.1 ohm, far below 377 ohm. This is a **low-impedance, high-current source**, so the main coupling is **inductive (magnetic field) coupling**: V = M·dI/dt, as the cables carrying the large and rapidly changing current create strong magnetic flux linking the controller's wires. Because arc welding produces sparks and broadband RF, **radiated (EM) coupling** and **conductive coupling** via common ground/power supply are also present. #### Minimising - Keep the welder cables **twisted/coaxial** (go and return together) to reduce the loop area, and route them away from controller wiring. - Use **shielded, twisted-pair cables** for the controller signals, with a magnetic shield (steel conduit) where needed. - Put the controller in a **grounded metal enclosure**; filter all its inputs/outputs and power lines (LC filters, ferrite beads, TVS). - Use **optical isolation** or fibre-optic links to the controller's I/O. - Separate the welder supply and controller supply; single-point ground with a proper star arrangement. - Increase distance, and cross cables at right angles. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the source impedance of the welder? A. 10 ohm B. 377 ohm C. 0.1 ohm (12 V / 120 A) D. 1440 ohm Answer: C. Z = V/I = 12/120 = 0.1 ohm, far below 377 ohm. (Key point: Source impedance) 2. Which coupling is the main mechanism? A. Capacitive coupling B. Inductive (magnetic field) coupling C. Optical coupling D. Thermal coupling Answer: B. Low impedance and high current means a magnetic-field source. (Key point: Main mechanism) 3. How should the welder cables be run to reduce noise? A. Twisted or coaxial, with go and return together, away from controller wiring B. Widely separated go and return wires C. Parallel and tight against the controller wiring D. Single wire with chassis return Answer: A. This reduces loop area and the flux linking the controller. (Key point: Cable remedy) 4. Which measure protects the controller I/O? A. Long unshielded cables B. Common supply with the welder C. Higher input impedance only D. Grounded metal enclosure, filtering and optical isolation Answer: D. These block fields and break conductive/radiated paths. (Key point: Controller protection) ## Related in Instrumentation II - [Identify noise coupling mechanism by calculation](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aswitching-noise-calc) — Instrumentation II, chapter 6 - [Noise coupling types and checking predominance](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Anc-predominance) — Instrumentation II, chapter 6, asked 2× - [Inductive noise and capacitive coupling shielding](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ainductive-noise-capacitive-shield) — Instrumentation II, chapter 6 - [Noise coupling mechanisms with pseudo impedance](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Anc-pseudo-impedance) — Instrumentation II, chapter 6 - [Types of noise coupling mechanisms](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Anc-types) — Instrumentation II, chapter 6, asked 3× - [Filtering for conductive noise coupling](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aconductive-noise-filtering) — Instrumentation II, chapter 6, asked 2× ## Related in other subjects - [Noise vs interference; minimizing effects; need for modulation](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Anoise-interference-minimize-modulation-need) — Communication Systems, chapter 1 - [Noise vs interference vs distortion](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Anoise-interference-distortion) — Communication Systems, chapter 1 - [Distortion vs noise vs interference](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adistortion-noise-interference-diff) — Communication Systems, chapter 1 - [Noise, distortion, interference; internal and external noise](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Anoise-types-internal-external) — Communication Systems, chapter 1