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Slave Robot Fires Randomly When Triggered from Master

CONFIRMED RESOLVED MULTI-ROBOT SETUP BOTH

Summary

In a two-robot setup where one robot triggers the other (master/slave via hardware trigger cable), the slave robot fires randomly or at unexpected times — not reliably when the master fires. The cause is electrical noise on the trigger cable, typically caused by ground imbalance between the two robot systems, long cable runs, or unshielded connections. The fix is opto-isolation, short cable runs, and correct cable type.

Symptoms

  • Slave robot engages unexpectedly during pre-roll or between runs
  • Slave robot does not engage at the correct moment despite master firing correctly
  • Behaviour is intermittent and worsens with cable length
  • Problem may be worse on set with heavy lighting loads (noise on mains)

Cause

Trigger cables between two robots carry a low-voltage signal that is susceptible to electrical noise pickup and ground potential differences between the two systems. When the two robots are powered from different sources (or different phases), a ground imbalance creates a voltage differential on the cable that the slave system can misinterpret as a trigger pulse.

Common contributing factors: - Long trigger cable runs (especially unshielded) - Two robots on different power sources or different phases - Heavy lighting loads on the same mains circuit - Use of unbalanced (2-conductor) cable instead of balanced (3-pin XLR)

Fix

1. Use an Opto-Isolated Trigger Cable

An opto-isolated cable physically breaks the electrical connection between the two systems. The signal is transmitted optically, so ground differences cannot create spurious triggers.

Opto-isolation is the proper fix

If random slave triggers are a recurring problem on your rig, an opto-isolated interface cable is the correct long-term solution. This eliminates the ground imbalance issue regardless of power configuration.

2. Keep Cables Short

Minimise trigger cable length. Longer cables pick up more noise. Run cables away from power cables and lighting cabling where possible.

3. Use 3-Pin XLR (Balanced) Cable

Use a properly balanced, shielded 3-pin XLR cable for the trigger connection rather than unshielded or consumer-grade cable. XLR provides: - Shield/ground separation from signal conductors - Better rejection of induced noise

4. Engage Slave Only Immediately Before Running

Do not leave the slave robot in the engaged/ready state while idle. Only engage the slave immediately before running the move:

  1. Bring master to Ready to Shoot
  2. Engage slave robot
  3. Immediately run the move

This minimises the window during which a spurious trigger could cause an unexpected slave engagement.

Prevention

For permanent multi-robot installations, use opto-isolated interconnects as standard. The electrical environment on a film set — with high-power HMI and LED lighting, generators, and long cable runs — is hostile to unprotected low-voltage trigger signals.

WhatsApp Excerpts

[25/9/2021, 5:09:23 am] ~ Radu Stefan Fulga: Speaking of, I've recently shot with two Bolts, triggered by two trigger boxes, Output to Input.

The slave robot would randomly trigger. Is that normal?

[25/9/2021, 5:10:32 am] ~ Radu Stefan Fulga: We would bypass this problem by engaging the slave robot at the very last moment before running the move.

[25/9/2021, 5:15:57 am] ~ Simon Wakley: Random triggering is common when there is electrical noise. Without knowing how you had it wired I can't say but the lighting desk would fire all the time until I added the resistor. Sometimes it's easy to fix and sometimes harder. Can also be caused by ground inbalances

[25/9/2021, 5:20:07 am] ~ Dan Gottesman: we've had that happen, as well; just with trigger outputs in general.

we use 3-pin XLR cables, which are usually pretty reliable, but with longer runs, I think any cable is susceptible to noise.

as far as triggering your second robot - I'd suggest only putting in "ready to shoot" until you're actually about to shoot, just from a safety standpoint.

[25/9/2021, 5:20:10 am] ~ Simon Wakley: I have a special Flair to Flair trigger cable that uses the built in Opto isolators and I've not seen that issue. I have seen long trigger cables pick up noise

[25/9/2021, 5:25:18 am] ~ Radu Stefan Fulga: Triggers on other devices seemed fine, just the robot trigger would fire randomly. We ruined two pizzas :)) We just set it for ready to shoot just before running the move.

[7/6/2022, 8:21:50 am] ~ Radu Stefan Fulga: i don't know how relevant it is, but we did manage to greatly diminish the frequency of these random triggers by making sure that the cables that connect to the trigger box are a decent distance away from any high power cables. (hmi cables, distribution boxes, that kind of stuff) Thinking they may cause interference.

[7/6/2022, 9:43:50 am] ~ Radu Stefan Fulga: We work on one stabilised generator that we use only for the robot, and robot related equipment. We always make sure the earthing is good, the generator has no issues. Random triggers on engage, and more rarely just idling, still happen. Thats why for example, when shooting with two robots, we put the slave robot in "ready to shoot" only at the very last moment before shooting a take. We also always make sure that the model mover and camera robots have moves such that they wouldn't hit each other if one of the random triggers happen.

[7/6/2022, 4:17:43 pm] ~ Niko: In my case it happened only when powering the robot from the E-stop. I think that the powering creates some kind of high frequency that moves around all the system

References

Official Documentation

Trigger Box Quick Start Guide — printed p.22–23: Troubleshooting

Trigger Box Quick Start Guide — printed p.14: Sync Input Wiring

YouTube 11 - Camera Triggering and Synchronisation Flair Classic

▶ 00:57 — Trigger and sync wiring from Ultibox to camera (electrical noise context)