Question concerning safety light curtains logic

Hi everyone!
I'm working on the controls design of a semi-automatic pneumatic vertical press. The machine will consist of two double-acting cylinders with 5/3 closed center valves and pilot-operated check valves on cylinders ports to keep them in position when no power is applied to the solenoids. Everything controlled by a non-safety PLC to automate the operation. An e-stop, light curtains and a dual dump valve hard-wired to safety relays (all dual channel) with auxiliary force-guided relays will be the safety devices. If the e-stop is pressed or light curtains are blocked, the dump valve will exhaust the air and cut the power of the plc outputs off. However, I don't want this to happen every time curtains are blocked since interrupting them a couple of times to load parts during machine cycle will be part of the process. I only want this behavior if the cylinders are retracted (home position) and (maybe) during upstroke. Blocking curtains during downstroke will definitely stop all motion and cut all power.
Is this logic allowed? If so, how can I do it? I’ve seen many presses working this way, but I’m a bit confused how to do it in a safe way.
I know the muting modules but is there any other way? How about to place a safety limit switch to detect when cylinder is retracted so that if the curtains are blocked and the cylinder is in other position than home the power and air will be cut off. Is this approach safe?
I’ve attached some diagrams illustrating my idea and the safety circuit i got so far. Any help will be appreciated.

I think your basic design concept is valid. You need to make sure you check all the details related to stop time and light curtain offset but the idea of bypassing the light curtain with a safety device that detects the hazard in a safe position is valid. Just make sure that if the light curtain is blocked and the ram starts to move that you can get it stopped when it comes off the limit switch before it becomes a hazard.

Keith

kamenges said:

I think your basic design concept is valid. You need to make sure you check all the details related to stop time and light curtain offset but the idea of bypassing the light curtain with a safety device that detects the hazard in a safe position is valid. Just make sure that if the light curtain is blocked and the ram starts to move that you can get it stopped when it comes off the limit switch before it becomes a hazard.

Keith

Click to expand...

Thank you very much. I'll check those details to make sure the press will be able to stop quickly enough once the limit switch is deactivated.
I've attached my safety circuit in my latest post. Could you check it out and give me your opinion?

Since you are working on the design, several questions have to be asked on my part.


1. how far apart are the cylinders?
2. (2) solenoid valves?
3. size of the cylinders?
4. force required for your operation?
5. size of the opening for the press?


it has been my experience that cylinders spaced apart will bind on the way up/down. the seal friction will not be the same for both cylinders.


it is better to use 1 valve, not 2 for these cylinders. place the valve in the middle of the cylinders with the same amount of airlines for each cylinder. put check valves on each side of the stroke. air valves do not operate at the same speed.


can you use an air over oil cylinder to get the required force using only 1 cylinder?


we had a vertical operation for a weight and used a lexan sliding door with reed switches.


hope this helps,
james

James Mcquade said:

Since you are working on the design, several questions have to be asked on my part.


1. how far apart are the cylinders?
2. (2) solenoid valves?
3. size of the cylinders?
4. force required for your operation?
5. size of the opening for the press?


it has been my experience that cylinders spaced apart will bind on the way up/down. the seal friction will not be the same for both cylinders.


it is better to use 1 valve, not 2 for these cylinders. place the valve in the middle of the cylinders with the same amount of airlines for each cylinder. put check valves on each side of the stroke. air valves do not operate at the same speed.


can you use an air over oil cylinder to get the required force using only 1 cylinder?


we had a vertical operation for a weight and used a lexan sliding door with reed switches.


hope this helps,
james

Click to expand...

James, thanks for your answer.


I forgot to mention this, but there are two cyilinders because two different press operations are going to be made. Operator places part A and B on nest of first press. First press actuates, forming assembly C. Operator takes assembly C and places it aswell as part D on nest of second press. Second press actuates, forming assembly E.
Both cylinders will never be activated at the same time, thats why there are going to be 2 cylinders with 2 valves.

1. how far apart are the cylinders?
40 cm (16")

2. (2) solenoid valves?
Yes two 5/3 closed center solenoid valves

3. size of the cylinders?
100 mm (4") diameter x 100 mm (4") stroke, 50 mm (2") diameter x 160 mm (6.2") stroke.

4. force required for your operation?
3800N (854 lb-f) max. for the first press (100 mm diameter cylinder), 800 N (179 lb-f) max. for the second one.

5. size of the opening for the press?
I'm sorry I don't know what you mean.

Please see the picture I just attached.