The muting deadlock that locked a cart inside its own safety field

A light curtain has one job: stop the machine when a person is somewhere they shouldn’t be. On a muting setup I worked on, it did that job. It also trapped the cart it was specifically built to let through.

What we were protecting

A transport cart ran back and forth through a guarded zone. The guard was a SICK deTec4 light curtain with a DMM4 muting controller feeding the safety signal into a Siemens PLC.

A light curtain in front of a moving cart has an obvious problem. The cart breaks the beams every time it passes. If the curtain treats the cart like a person, the machine trips on every cycle and nothing moves. The curtain has to let the cart through and still catch people. That is what muting does.

Why muting exists

Muting is the deliberate, automatic, temporary suspension of a safety function so that an expected object can pass through a protective device without stopping the machine, while a person still can’t. It is a defined function with standards behind it. The light curtain itself falls under IEC 61496, the application of muting is covered by IEC 62046, and the performance level the whole safety function has to hit comes from ISO 13849.

Telling a cart apart from a person is done with muting sensors. Separate sensors, arranged so that only the expected object, arriving the expected way, can satisfy them. Right pattern in the right order and the controller decides it’s the cart and mutes. A person walking up can’t reproduce that pattern, so the curtain stays live for them.

Ours was partial muting. The lower beams were muted so the cart could roll through underneath. The upper beams stayed live the whole time. The muting itself was fired by two photosensors on a flag, an L-muting arrangement, tripping as the cart came in.

The deadlock

Someone broke the upper beams while the cart was still sitting in the field. That is the curtain working. The upper beams exist for exactly this. The OSSD outputs, the safety outputs that tell the drives they’re allowed to move, tripped off. Correct behavior.

The DMM4 drops muting the instant it trips. But the cart is still there, still blocking the lower beams. With muting gone, the curtain sees the cart as an obstruction too, and the whole field reads as blocked. It won’t take a reset either. A reset wants the entire field clear first, and the curtain has to show flashing yellow before it accepts an acknowledge.

So the cart can’t move, because the drives need a safety OK to run. The safety can’t be acknowledged, because the curtain isn’t clear. The curtain can’t clear, because the cart is parked in the middle of it. The cart sat stuck between the emitter and the receiver, and no button on the panel gets it out.

Edge, not level

This took me a while to actually understand rather than just brute-force.

The L-muting on the DMM4 is edge triggered, not level triggered. Level-triggered logic acts on where a signal is: as long as the input is high, the condition holds. Edge-triggered logic acts on the change itself, the moment the signal goes 0 to 1 or 1 to 0.

The DMM4 arms muting on a rising edge of the muting sensors, the moment they go from no cart to cart arriving. Normally that is exactly what you want. It proves a fresh, sequenced approach rather than something just sitting in the beam.

In our situation the cart never left. The muting sensors went high when it arrived and stayed high the whole time. To re-arm, the DMM4 needs to see a rising edge, and a signal that is already high has no edge to give. No edge, no muting, no way out.

I proved it the crude way. Pulled the cart out of the field by hand, then drove it back in. Fresh approach, fresh rising edge, and muting came straight back. That confirmed it. The DMM4 was doing exactly what edge-triggered logic does. It had no path back to a muted state without a transition the deadlock made impossible.

The one-button fix

The obvious fix looked clean. In SICK Safety Designer, flip the existing reset input over to a reset/override input. One button, and the DMM4 decides which one it is from the current state. Override the stuck condition, get the cart moving, done.

Then I looked at what override actually does. An override holds the OSSD green for as long as the muting sensor stays blocked. It drops when the sensor goes 1 to 0, the cart finally leaving. Fine, if the only thing that ever blocks that sensor is a real cart on its way out. But nothing forces that. Hold anything in front of the muting sensor and the override keeps the OSSD green basically forever (or at least for an extended period of time, I never actually tested whether it times out). The whole curtain, upper beams included, bypassed with one blocked sensor and one button press.

Why the override is the wrong fix

I called SICK to talk it through and never found an answer that closed the hole without reintroducing some version of the original problem. A visible stop that needs someone to physically clear the field is annoying. A silent bypass of a guard that protects people, one you can hold open, is a different category of problem. Not worth designing in to save a few seconds of recovery time.

Where we landed

We never fixed the muting. We gave the operators a way out instead. There’s a bypass on the HMI that enables the drive for the cart only, whatever the OSSD or the rest of the safety is doing. It moves the cart, slowly, and nothing else. That’s enough to walk it out of the field and let the curtain clear on its own.

Not elegant. But it’s bounded, and that’s the part that matters. It can’t hold the guard open and it can’t keep production running. The only thing it moves is the thing that’s stuck.

Muting is a deliberate hole in your safety function. That is the whole point of it, and the standards allow it. The job is keeping the hole tightly bounded, by the sensors, the sequence, the geometry and the timing, so it can’t be faked open and can’t be held open. The override solved a real annoyance by quietly removing that bound.

The rest of it is a lesson about knowing your kit. Not what it does when everything goes to plan, but what it does underneath. Edge or level decides how your logic behaves in the situation you never planned for. Edge-triggered logic strands you when the transition you’re waiting on can’t happen. Level-triggered logic holds a dangerous state open as long as something keeps it high. Know that going in and you see this coming. Don’t, and you find out with a cart parked in the middle of a light curtain.

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Read more at tianengineering.com — https://tianengineering.com/articles/muting-deadlock-light-curtain/