light curtain muting photo eyes getting bumped

[unsaint32]

I think the problem here is that the sensors are NC, so when they are misaligned they give a signal to the muting system. If the muting system can have the input logic reversed, change the sensors to NO operation so that when they are misaligned they don't fool the muting system.

JRoss,
our UE410-MU needs two 24V inputs for its muting to begin. Muting is designed to be in effect when a part breaks the cross beams, right? So, I need the sensors to send 24V to the UE410 when a part breaks the beams. With the sensors on NO operation, breaking the beams will send 0V inputs to the UE410.

 

[JRoss]

Normally Open (NO) means that when the sensor sees the reflector (or receiver if it's throughbeam), the output is "open", which means 0V. This is a holdover from limit switches, which have a contact inside that is "normally open" until the limit switch is made, at which point it closes. So this is how your sensors are already set up.

You have the sensors set up correctly, I just wasn't thinking straight when I responded. Of course the sensors act as though they are blocked when they are misaligned... but it sounds like the muting device needs to be set up differently. What is the full part number of the part, and the settings of the configuration dials? You should be able to turn on the safety timers so the system can't be muting for too long, but I think there's also a setting to pause the timers if the conveyor is stopped. This would protect against misaligned sensors, as well as allow you to stop the conveyor with the pallet in the curtain.

 

[JRoss]

Gary - I still don't understand. What's the logic you use? I'm used to two sensors that either are crossed and have to be blocked nearly simultaneously, or are parallel to the curtain at different distances and have to be blocked in the correct order. Either way, a single person can't fool it, but it takes at least two sensors.

Applying that logic, a single sensor can be fooled by a person. All that have to do is block the sensor before entering the light curtain. So you must be doing something else?

 

[isiah_s]

stupid question but can you put the photo eyes above and below the pallet instead?

 

[osmanjdt]

how about guide the pallet with roller or steel plat

 

[JRoss]

In the muting systems I'm used to, you actually need sensors on both sides of the curtain, and you need redundancy, so either 2 sensors crossed as in this system, or four sensors parallel, two in front and two behind. These have to be placed so the pallet makes them in sequence, and so that it isn't possible for a human to trigger muting by just walking in. Having sensors on both sides is instead of using the curtain itself to know when the pallet is clear. It also allows bi directional muting, if the pallet use the same opening for entry and exit.

Again, this is for off the shelf muting logic. Custom setups can be developed using a programmable safety controller, but then the onus is on the programmer to make sure it's fail proof.

 

[GaryS]

I hope I can make things a little clearer
First as I look at the drawing I see flow in both directions if that is true it would be a very unusual system. But not something that we can’t deal with.
On any light curtain system you want to have the curtain muted as little as possible for as short a time a possible. And any failures cause the protected area to shut down in a safe mode.
Anytime the curtain faults it must require that an operator check the area and clear the problem and verify that no personnel and inside the danger zone before resetting the system.

What I see in the drawing is a mute window that’s way too long. I know the drawing is not to scale so it is hard to tell.
If you want to set it up that both of the crossed beams have to broken simultaneously you have a problem.
With the crossed beams for mute you have to depend on the pallets being on the center of the conveyor and be exactly square when they enter the eyes. Now we all know that’s not the real world. So you are going to have a lot of false trips causing lost production time. Then on the exit they would both have to clear the beams at the same time again not realistic.
With the mute point so far from the curtain it is possible that a person could just step through before or after the pallet while the curtain is muted. That not what you want
If you set it up that either of the mute eyes blocked will mute the curtain the problem I even worse. You could just block one eye and walk through on the other side.
I have found over the years that if you have too many false trips they will just disable the curtain to keep production up.
First we need to determine the direction of flow from the machine or to the machine or both.
When I mount the mute eye it’s just a few inches in from of the curtain on the in feed side the orientation doesn’t matter. Across the conveyor, vertical up from the floor or at a vertical angle. In all cases it must detect the pallet as close to the curtain as possible. Now in some cases if you have product overhanging the leading edge of the pallet then you have to move the mute eye back to accommodate this. All the mute eye is doing I tell the control when to mute the curtain. If you are running the conveyor in both directions then you will need a mute eye on both side of the curtain.
To ensure that a pallet is actually moving into the curtain and not a person that just trips the mute eye and walks in I look to see that the conveyor is on and moving in feed direction at the time the mute eye is activated. If you just active the mute eye without the conveyor running the controller will treat it like a fault and shut everything down in safe mode.
The plc controls the mute input to the curtain as well as the safety input and the reset
I monitor the curtain status ( Blocked / Clear ) when the in feed mute eye signals the PLC to mute the e curtain then the mute will stay active as long as the conveyor remain running, and the curtain status input shows first blocked and then clear. That’s the signal that the pallet has passed completely through the curtain and the mute is turned off. The trailing edge of the pallet would be within few inches of the curtain
The only time the curtain is muted is when the pallet in moving through the curtain or within a few inches of the curtain. The pallet would at that time block any personnel from passing through. Unless they actually clime over the pallet while it is moving through not a good move.
There is also a timer on the mute if it either the mute eye or the curtain status remain blocked for an extended time it will fault the system and shut down. Something is misaligned or otherwise not right.
At no time can the curtain remain blocked, like a full conveyor causing the pallet to stop in the curtain. You have to make sure that the program does not allow a pallet to stop while it is in the curtain.
Is it possible to somebody to get on the conveyor and pass through the curtain like a pallet? Yes but to do that they will have to be on a moving conveyor and that would be a safety violation in any plant I have worked in.
you can bypass or get around any guarding system if you try hard enough. It is not possible to prevent stupid people from doing stupid things. All you are really trying to do is prevent somebody from accidently entering a danger area.

It should be noted that the curtain status and mute eyes have time buffer on the inputs to prevent nuisance trips.

Just a side note as you are aware that as a programmer once you touch it you own it and you inherit any existing problems as well so you have to make sure you CYA. They will blame you if you worked on that part of the code or not.

 

[JRoss]

Yeah, I misspoke on the sensors getting tripped simultaneously. Trouble with memory I guess. Always good to check the manuals! They actually have to be tripped in sequence, MS1, then MS2, the the reverse when leaving. The OP copied the sample drawing out of the literature, so the scale is odd, but you can see MS1 is rotated at a different angle so the pallet breaks it first and clears it last.

Gary, what do you do if the conveyor is a chain-slot conveyor running in a concrete floor? I've seen these in distribution and automotive facilities, where it's expected the personnel will be walking across it constantly. Do you use a safety PLC for the logic?

 

[GaryS]

for the chain link on the floor the mute eye sits on the floor if they are walking that close to the curtain than they will be tripping I anyway.
even with the sequence of the 2 mute eyes they still depend on the pallet being square and aliened the same every time and never have anything overhanging form or back
ideal conditions but hardly the real work site.
what are you going to do when the pallet is setting on the conveyor at an angle and it hits the eyes in the wrong order

 

[JRoss]

Sorry for the delay in response. I've been getting ready for an install this week.
Are you using a safety controller or just regular PLC logic for that setup? To get to the highest SIL, you need the redundancy of two sensors.
The crossed sensor positions can be adjusted so that the pallet will always break them in the proper order. I've attached a sketch with a slightly more real-world setup.

 

[GaryS]

two words to never use in a control are always and never things happen
the plant manager or engineer are the last call on safety whatever they except
the mute eyes are not a safety device and each of them only has 1 output
I was dong it this way long before they even had SIL or safety PLC's
you say you could adjust the mute eyes for a misaligned pallet but to do that you will need to have somebody stationed there to adjust them in the event a pallet get knocked out of line.
the curtain itself is a safety device and yes it should be connected to a safety PLC depending on how you want to rate the system. the way to do a safety evaluation is to do the whole system and all devices and how they are connected that's not just the curtain.
in the end you are the one that must make the call if you don't thing it's safe then it is not and you need to do whatever you need to get it there.

 

[JRoss]

"never say never" ;-)
But you're right. Given the above drawing, there is the slight possibility of the pallet breaking MS2 first. However, it would have to be so rotated that it wouldn't make it through the available space, in which case faulting out the safety system and stopping the conveyor is a good thing. As far as product hangover, generally the sensors are placed to trigger on the pallet, not the product, so product hangover is only a problem if it's so bad that it gets into the light curtain before the muting is enabled. If this is normal, then the safety system would need to be adjusted accordingly. If it's abnormal, then there's an issue upstream and stopping the conveyor would avoid issues in the work cell.

But your point is taken. Every system is different, and the real world is much messier and more complicated than a piece of paper with lines on it. Each system must be evaluated as such, and it is the responsibility of the safety designer and programmer to make sure that the system operates to the level of safety required by the application.