Noise Resetting Safety Controller (SLX)

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1

I’ve seen several issues with our safety circuit resetting due to our servo and/or VFD drives. I’m curious if other people have had similar issues.
Our system usually consists of a X20CPU with and SLX module on the same rack. We have ACOPOS and/or multi drives, and sometimes VFDs.
Often, when the motor cables aren’t properly terminated at the drives, the SLX will reset - usually when first enabling the servo or VFD. Properly terminating the shields of these motor cables usually fixes any issues we have.
It just seems a little odd what the ‘noise’ does to our SLX module, and if anyone else has seen this.

2

Just as a follow up - when we first started experiencing this issue, I contacted B&R support. They did explain that noise, could, indeed, cause our safety controller to ‘reset.’

3

I’ve seen this happen before as well. If they aren’t properly separated and terminated, high voltage (i.e. motor) cables can cause noise in lower voltage signal lines. In non-safe cases, this can result in invalid data. You can “see” this if you cause noise in a serial communication (e.g. RS232) line for example. However, safety systems are quite sensitive to small disturbances because they’re designed to trigger a failsafe condition when they don’t see what they’re expecting.

It’s always important to make sure your wires are properly shielded and grounded. Additionally, it’s a good idea to keep high voltage wires separated from low voltage wires as much as possible (i.e. don’t run them right next to each other).

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4

I suppose there’s no setting in the safety program that can ‘filter’ out any ‘discrepancies?’

5

There are some settings available in SafeDesigner (see this safety input module as an example). However, in my opinion, these settings are not a replacement for proper wiring for a couple of reasons:

  • I’m not sure if you can filter out noise issues completely with these settings. However, the less sensitive the safety system is, the less effective it is. As an extreme example, consider the case where the system reacts to an Emergency Stop a whole second after it is pressed. A lot can happen to hurt someone in that second
  • Noise can cause other issues unrelated to the safety system (e.g. in my serial communications example above). Changing the safety settings is a band-aid solution that won’t fix these other issues. If you have a reliable method for removing noise from your machines, that’s a much more robust solution
6

The exact behaviour should be known before testing workarounds.
To see if it detects e.g. a low signal on the input (which could be filtered) or a stuck at high (which is a critical error which can’t be filtered) or …

There might be logger messages which provide more details about that.

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