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MRMC Turntable — False Home / 45° Position Error / Sporadic Move Failure (EMI from Stage Dimmers)

CONFIRMED TURNTABLE UNI BOARD

Summary

The MRMC small turntable (harmonic drive, Uni board running a stepper) exhibits two related faults on sets with stage dimmers or HMI lights: (1) false home — the turntable homes to an obviously wrong position (up to ~45° off), inconsistently; (2) sporadic move failure — the turntable randomly won't execute a move with no trip, no warning, no error. Both are caused by EMI (Electro-Magnetic Interference) from lighting dimmers or HMI lamps entering the ethernet cable or creating a noise differential between the ethernet and the Uni board power. Fix: shorten the ethernet cable, route it away from power lines, and power the Uni board from the same source as the control box.

Symptoms

False Home (wrong position after zeroing)

  • After a Zero Home, the turntable ends up ~45° from the expected zero position
  • The error is not consistent — it doesn't happen every time
  • Mechanical inspection finds nothing wrong — harmonic drive is solid and sealed

Sporadic Move Failure (no trip, no error)

  • The turntable occasionally refuses to run a programmed move
  • No trip, no axis warning, no error message — Flair shows the move as ready to run but nothing happens
  • Jogging the axis before running the move may help temporarily
  • Issue is intermittent — it works fine other times without any change

Root Cause

Simon Wakley (MRMC) confirmed this is an EMI issue, not a mechanical one:

"Ethernet cables can be affected by Electronics noise and interference. Possibly a better (short?) cable would help and also do not run it parallel to A/C or power lines if possible. MRMC can provide more data concerning EMC (Electro-Magnetic Compatibility). Also Noise differential between the Ethernet and the power for the Uni-board can cause issues. Power the Board from the same source as the control-box rather than from a nearby socket etc. And yes, dimmers make a LOT of electronic noise especially when they are being dimmed up or down. HMI Lights are also bad sources of EMI (Electro-Magnetic Interference)."

The MRMC small turntable uses a Uni board driving a stepper motor through a harmonic drive. Ethernet to the Uni board is vulnerable to interference from: - Stage lighting dimmers (especially when dimming up or down) - HMI lamps (strong EMI source) - Long ethernet cable runs running parallel to AC power cables - Powering the Uni board from a different circuit than the control box (ground reference difference)

Fix

[CONFIRMED] Reduce EMI exposure

2023-03-18 — Simon Wakley (MRMC) / Josh M

  1. Shorten the ethernet cable to the turntable Uni board — the longer the run, the more EMI it picks up
  2. Route the ethernet cable away from AC power cables and lighting dimmer feeds — never run ethernet parallel to mains/dimmer wiring
  3. Power the Uni board from the same source as the Flair control box — not from the nearest stage outlet, which may be on a dimmer feed or different phase
  4. Use shielded (STP/FTP) ethernet cable if runs must be long
  5. When zeroing: jog the turntable before running a move to confirm communication is live
  6. If an HMI lamp is nearby, extend the ethernet further from it or route via a conduit
  7. Flag to MRMC Support if persistent — MRMC can provide EMC guidance for your specific setup (contact Peter Rush for advanced EMC advice)

confidence_score: 0.85

WhatsApp Excerpts

[18/3/2023, 2:26:33 pm] ~ Josh M: Our small MRMC turn table has also been acting a bit odd at times. Def not mechanical. Right now my main suspicion is packet loss over a long ethernet cable. It'll work, but it's either sporadically delayed, gets a false home signal, or loses it's position. By quite a bit, like 45deg

[18/3/2023, 2:35:56 pm] ~ Josh M: I cannot for the life of me get it to be repeatable though and at this point I'm blaming the gaffers dimming up their 20k. The big problem for me is that when I home it, it homes in a obv non-zero position, but again, not consistently.

[18/3/2023, 2:36:20 pm] ~ Josh M: On a bit of a side note. The turntables harmonic drive is a thing of beauty. Took the whole thing apart to try and figure out if there was something mechanical going on... Nope. This thing is rock solid and well sealed.

[18/3/2023, 2:50:35 pm] ~ Simon Wakley: Pretty sure that IS a "Uni" board running a stepper through that Harmonic drive. Ethernet cables can be affected by Electronics noise and interference. Possibly a better (short?) cable would help and also do not run it parallel to A/C or power lines if possible. MRMC can provide more data concerning EMC (Electro-Magnetic Compatibility). Also Noise differential between the Ethernet and the power for the Uni-board can cause issues. Power the Board from the same source as the control-box rather than from a nearby socket etc. And yes, dimmers make a LOT of electronic noise especially when they are being dimmed up or down. HMI Lights are also bad sources of EMI (Electro-Magnetic Interference). Peter Rush may be able to help out and knows more about this than I.

Official Documentation

Flair Classic Manual — Ch.5 Setups: Axis Setup