Controlling Pneumatics

Hi all,

I'm programming a Toshiba T1 PLC to control several pneumatic actuators that perform some forming and pressing operations. These pneumatics do not currently have any limit switches for position feedback.

I successfully created a program to achieve the desired result but the problem is that the program does not "know" when the pneumatic actuator has reached full stroke or park position. Thus I made extensive use of single shot functions and delay functions. There are a few problems with this method-

The single shot functions that turn on the pneumatics must have timer values that are at least as long as the stroke time to insure that they reach full extension. If the stroke times(set at pneumatic) and air pressure is not changed there should not be any problem but if they do everything goes to hell. Should a pneumatic fail (ie. air hose failure) the elements controlled by the pneumatics could collide. Also, the sequence is somewhat slower than desired since there needs to be some "cushion" built in to insure full stroke.

The program is real ugly since 6 pneumatics have to operate in a specific sequence and often there has to be some overlap in operation or a delay between actions.

I know every application is different but in general do most systems using pneumatics employ limit switches for position feedback? The machine designer I'm working with has planned for the park limit switches but I'm not sure he planned for full stroke switches.

For what I'm doing the park switches would be less useful than the full stroke switches but it's not obvious to me how the full stroke switches would be implmented since switches cannot easily be placed at the "business end" of the actuator. It seems to me that the actuator manufacturers would provide these switch outputs but I have no knowledge of pneumatics.


Thanks for any advice.

I would say that most systems do use position feedback signals on cylendars. Do you know if the cylenders are magnetic whereas you can use reeds switch mounted on the cyl's instead of external limits switches?

Magnet proximity sensors

hi,

as others suggested, if your pneumatic cylinder has a magnet (usually a round piece of magnet in the seal cap) then, you will be able to use micro proximity sensors I Know FESTO makes them, have a look at FESTO they are so tiny and they get installed on the exterior of your cylinder.

this way when the cylinder extends you'll have a digital input to your PLC telling it it has reached its limit.

hope this helps.

Without question, your application demands position sensors on the cylinders. It's just a matter of specifying them when you order the air cylinders. This is a very common option and should not be a problem. The position sensors I have encountered on air cylinders are usually 24vdc.

here is the sensor from FESTO SME-10-KQ-LED-24
and the link: https://xdki.festo.com/xdki/xDKI.asp

Thanks all,

I found the sensors - the body of the ISO cylinders is made to accept the sensors.

Can you recommend a pneumatics and accessories distributor?
automationdirect does not have a lot of pneumatics.

I can't find a dist listing on the festo site.

Thanks

try this link: http://sensors-transducers.globalspec.com/datasheets/180/Festo/FA46D2B7-11A0-4687-AC0C-7790ABD1418C they have a sensor.

if you go to www.festo.com then type proximity sensor you will be able to find it.

hope this help

try this direct link: http://www.festo.com/pnf/en-us_us/p...en-us&country=US&key=proximity+sensor&submit=

I dislike festo
Look here http://www.smcusa.com/
or here http://www.numatics.com/
Numatics valves have no O-rings, but need good filtering.
Awesome stuff.

If you ever deal with a project like this again I think it would be a good idea to specify to the client to buy actuators that have built in positioning limits.Saves alot of hasles later and I think cost aswell.

I can't even imagine working on a project with cylinders like that. You're currently sacrificing so much control/reliability/safety just because you don't have a cheap reed switch on there.

If a cylinder never makes it into position because it: loses air somehow, hits something etc, your program will just continue on.

Always get both rod end and blind end reed switches, or get limit switches on both ends. This goes for both single acting and double acting cylinders.