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Safety·7 min read·

Muting a light curtain without defeating it

Why muting exists, the sensor arrangements that make it safe, and the timing rules that stop a person walking through behind a pallet.

Short answer

Muting temporarily and automatically disables a light curtain so material can pass through while people still cannot. It is made safe by muting sensors arranged so that only an object of the expected size, moving in the expected direction, at the expected time can satisfy them. Two sensors crossed at the curtain, or four in sequence, with a maximum muting time, is the standard arrangement.

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A light curtain across a conveyor opening has an obvious problem: the pallets have to get through and the people must not.

Muting is the answer, and it is the safety feature most often implemented in a way that quietly defeats the thing it is attached to.

What muting is

Muting suspends the protective function of the curtain, automatically, for the duration of a specific expected event. It is not a bypass switch and it is not under anybody's control.

The safety of the arrangement rests entirely on one question: can a person produce the same signal pattern as the expected object? If yes, the muting is defeatable and the curtain is decoration.

The sensor arrangements

Two sensors, crossed. Two beams cross at or near the plane of the curtain. Both must be broken within a short window for muting to begin. A person walking through breaks them sequentially rather than together, because the crossing point is at a height and geometry a body cannot satisfy simultaneously.

Four sensors, sequential. Two before the curtain and two after, broken in a defined order. This proves direction of travel as well as presence, and it lets you check the object's length against the time between sensors.

The four sensor arrangement is preferred on pallet conveyors because it can tell a pallet going in from a person walking out, and because the length check catches somebody riding the pallet.

The timing rules

Three timers do most of the work, and all three exist because of a specific defeat.

  • Simultaneity. The two muting sensors must be broken within a short window, typically under four seconds. Break one, wait, break the other, and muting is refused. This is what stops a person triggering both sequentially.
  • Maximum muting time. Muting ends after a fixed period whatever the sensors say, typically thirty to one hundred and twenty seconds depending on the process. A stalled pallet or a stuck sensor cannot hold the curtain off forever.
  • End of muting. Muting must end as soon as the object has passed, not when the sensors happen to clear. On a four sensor arrangement, the trailing edge past the last sensor ends it.

Muting lamps are not optional

A muted curtain looks exactly like an active one. IEC 61496 and the machinery standards require a visible indication that muting is active, positioned where somebody approaching can see it.

It is a small lamp and it is genuinely load bearing: it is the only thing telling a person that the protective device in front of them is currently not protecting anything.

Where it goes wrong

Every one of these has been found on a real machine during a validation.

  • Both muting sensors mounted at the same height on the same side, so a hand across both mutes the curtain.
  • Muting time set to the maximum the device allows, because a pallet occasionally stalled and somebody made the alarm go away.
  • Muting driven from the standard PLC rather than the safety device, so a program change can disable the curtain.
  • A muting lamp fitted, wired, and mounted inside the panel.

The honest alternative

Muting adds complexity and complexity is where defeat hides. On new designs, look first at whether the geometry can remove the need.

An L shaped or U shaped conveyor path means a person cannot walk in a straight line to the hazard, which sometimes lets a simpler arrangement do the job. It is cheaper to bend a conveyor at the design stage than to validate a four sensor muting array for the life of the machine.

Common questions

What is the difference between muting and blanking?
Blanking permanently ignores certain beams of the curtain, for a fixture that is always there. Muting temporarily ignores the whole curtain while a specific expected event happens. Blanking reduces detection capability permanently; muting suspends it briefly and automatically.
How many muting sensors do I need?
Two as an absolute minimum, arranged so that a person cannot satisfy both. Four, in a sequence with defined order and timing, is the common arrangement for pallet conveyors because it also proves direction of travel and object length.
Why is there a maximum muting time?
So that a fault which leaves the muting condition satisfied cannot disable the curtain indefinitely. If a pallet stalls, or a sensor sticks, muting times out and the curtain becomes active again, which stops the conveyor rather than leaving an opening.

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