Pneumatic safety

[Eric Nelson]

* If light curtain is tripped: air remains on, PLC outputs remain on (or off), PLC logic has input for light curtain and prevents valves from shifting in logic. Essentially, safety is handled in the PLC.

I wouldn't be comfortable with it ONLY in logic. There may be an unforeseen scenario that is overlooked. A future programmer may make changes to the logic. He might not realize that the logic is providing personnel safety.

How about this?... For the cylinders in question, use double-solenoid valves, wire these cylinders to outputs that share the same common, and hardwire that common supply through the light curtain safety relays. This way, even if the PLC tries to fire the output, there will be no voltage to energize the solenoid.

This is safer than just using logic, but I would STILL prefer that all air got dumped.



-Eric

 

[JesperMP]

What about the 5/3 way solenoid I suggested in post # 2.
Also see this thread: http://www.plctalk.net/qanda/showthread.php?t=50952

 

[Eric Nelson]

What about the 5/3 way solenoid I suggested in post # 2.
Also see this thread: http://www.plctalk.net/qanda/showthread.php?t=50952

With an 'open-center' valve, and neither coil energized, air is relieved to atmosphere on both sides of the cylinder. I don't see how this is any different than relieving the main air supply via the dump valve...



-Eric

 

[leitmotif]

With an 'open-center' valve, and neither coil energized, air is relieved to atmosphere on both sides of the cylinder. I don't see how this is any different than relieving the main air supply via the dump valve... -Eric

I still like either a block under the rod or the air bled off the cylinder. What happens if the actuator is stuck?

Bear in mind I am answering from a maintenance trouble shoot view point where I am in there trying to find out what is wrong (or right?) Machine or parts of it may be operational for me to figure it out. Wood blocks have saved me several times.

For operators clearing jams etc where they gotta go thru the curtain or pull a guard I think a full shut down and everyting in zero mechanical energy state is best.

Dan Bentler

 

[OkiePC]

With an 'open-center' valve, and neither coil energized, air is relieved to atmosphere on both sides of the cylinder. I don't see how this is any different than relieving the main air supply via the dump valve...



-Eric

The main air dumps everything. 3 position open center valves let you pick and choose what gets dumped.

 

[jdbrandt]

My opinion - and it is only that - is that as long as the cylinders cannot change state when the light curtain is triggered, it will be correct.

I work with several robots and the last thing you want is products falling from the pickup head - especially when they are 50kg sacks.

I would also try to make tripping the light curtain a PITA to persuade the operator to press the control stop rather than using the curtain "Because it's easier".

There is a line in (I think) NFPA 79 that says, to paraphrase, "EStopping a machine, in itself, should not introduce a hazard.". Its almost like a Get Out Of Jail Free card, and causes one to think for a moment, about implementing onerous estop circuits that take forever to reset, because of 'introduced' trouble.

$0.02

 

[JesperMP]

With an 'open-center' valve, and neither coil energized, air is relieved to atmosphere on both sides of the cylinder. I don't see how this is any different than relieving the main air supply via the dump valve...

No big difference. It is achieving the same in a slightly different way.

Small difference, you could have it so only the few cylinders that are relevant for when the light curtain is activated are affected, rather than the entire machine.

/what OkiePC said already !

 

[muttleydoo]

I've used the solution of selective dumping in the past, where only the cylinders that were deemed hazardous by the risk assessment were de-energised. Splitting the air system to keep all the non hazardous air supplies active such as vacuums and general air blows is perfectly acceptable. I use specific dump valves controlled by safety gate relays for the hazardous actuators with the safe air only being dumped by the e-stop safety relays. Hazardous actuators could be controlled by reducing the pressure but there is always the risk that the pressure could be increased by personnel unaware of the safety risk. It's a difficult one!