Help logic problem light guard & conveyor

[rigicon]

Hi to the forum.
I have a logic problem which is confusing me. From my sketch I have seven conveyors that I control using a twido PLC via canbus.
number 4 & 5 conveyors goes up & down for the robot to pick & place boards. Input Light guard through safety relay to plc x 2 - output to halt robot.

All working fine(still commisioning) until they enclosed the machine and installed cat2 light guards. The idea is to halt the robot only (suppose if one jumps into the cell) .. now the confusion ... if a pallet is not passing through. on each conveyor is a switch at the end so one knows if there is a pallet. There is a hard wired Reset button.

I was wondering if someone could put me out of my misery... I know there is some logic that I can follow but what ?

 

[gmferg]

Need more information...

Are you trying to figure out how to override the light curtain shutdown when a pallet is passing through?

The light curtains I've used in the past had 2 photoeyes wired directly to them to override a shutdown alarm. These photoeyes are setup in a criss-cross configuration around the light curtain so a pallet coming through would break both photoeyes at the same time. This would override the shutdown. If one photoeye is blocked before the other, no override is made. It is very difficult for a human walking through to block both photoeyes.

 

[rigicon]

This is a brilliant solution if I understand you something like the drawing. maybe more criss cross. How would they be wired into the Light guard?
Not pallets - stacks of wood boards might be skew so switches won't make at same time darn.
maybe only solution is if the conveyor is stopped check the light guards.
If they are moving so be it the robot continues heaven help anyone who jumps through a moving conveyor.

 

[ojz0r]

if you have a safety relay you can check your manual cours they tend to have built in time limited bypass just for this application

 

[rigicon]

Ah ojz0r I think you are onto something. I know a pallet passes through the end switch on 2 so ignore the light guard after this switch is made for a certain time then recheck.
what happens if the human walks past this sensor then the light guard which I am ignoring. It is a good idea maybe another switch???

 

[Doug-P]

What material are the pallets made of?

 

[ojz0r]

Ah ojz0r I think you are onto something. I know a pallet passes through the end switch on 2 so ignore the light guard after this switch is made for a certain time then recheck.
what happens if the human walks past this sensor then the light guard which I am ignoring. It is a good idea maybe another switch???

the signal should be generated by a plc rather then the photoelectronic cell right infront of the light guard.
For example when the plc orders the conveyor to start and the photoelectronic cell is both activated then it should send the bypass signal.
But as with everything else ofcours a human can fool the system but its harder this way when you need a run signal aswell.

 

[rigicon]

What material are the pallets made of?

Wood

 

[rigicon]

the signal should be generated by a plc rather then the photoelectronic cell right infront of the light guard.
For example when the plc orders the conveyor to start and the photoelectronic cell is both activated then it should send the bypass signal.
But as with everything else ofcours a human can fool the system but its harder this way when you need a run signal aswell.

There is a photocell at each end of the conveyor so if thats on and the light guards are clear - ok to start.
If not on = no pallet then alarm out when light guard broken.
The plc then ignores the light guard.

 

[CharlesM]

You need to see if your light curtains can mute. Take a look here:

www.sti.com/ltr2/access.php?file=pdf/3956.pdf

 

[rigicon]

You need to see if your light curtains can mute. Take a look here:

www.sti.com/ltr2/access.php?file=pdf/3956.pdf

Brilliant that's it.

 

[rigicon]

Thank you ojz0r you were right only when CharlesM pointed it out to me did I click. Thanks to you both.
Excellent forum.

 

[Doug-P]

I have a logic problem which is confusing me. From my sketch I have seven conveyors that I control using a twido PLC via canbus.
number 4 & 5 conveyors goes up & down for the robot to pick & place boards. Input Light guard through safety relay to plc x 2 - output to halt robot.

There might be a problem here. With the safety relay feeding the PLC to stop the robot you are essentially depending on the PLC as part of the E-stop chain. Our robot cell was built with its own subpanel containing a hardwired relay circuit which operates entirely independently to stop the robot. There is a PLC but its involvement in this safety monitoring function is only to sense when the circuit is tripped and allow reset when the problem is cleared.

To allow pallets to automatically pass in and out and stop only when a human enters, several sensors were used. There are: 2 ea. metallic proximity sensors (one on each side) to detect the cart, 2 ea. retroreflective PEs at about waist height, and two more PEs about shoulder height. This setup appears at both the entry and exit.

Your mileage may vary.

 

[silva.foxx]

See attached .pdf

Page 2 conf. C will overcome skewed wood stacks.

As Doug-P stated... the lightbarrier must stop the robot via hardwire not PLC (unless a Safety PLC). If anything other than a permitted load passes a lightbarrier = e/stop robot.

 

[rigicon]

Thanks Doug-P I was impressed when you asked what's it made of because if it was steel it would of been a whole lot easier = prox. Good logical thinking.
Anyway I agree with all of you, thanks silva.foxx for the info, the light guard installation must be hard wired and have nothing to do with the controlling PLC. I have used an omron safety plc before but would rather a muting controller. Problem is we already have the schmersal light guards so I'll have to source a muting controller.