rotation of a spray head using analog porportional valve

Have a question that I hope someone may have a answer to.
Have a project where we have a machine that has a spray head that is spraying chemicals on cloth that varies in width.
The spray rotates from 0-45 degrees depending upon the width of the cloth. At the moment we are experimenting with using a stepper motor and appears to work but still still needs some more work.
Our boss has done a lot of autoclave and steam chamber type projects in the past and is wanting us to see if we could this this same type of rotation of a spray head using a pneumatic rotary type 4-20ma porportional valve. I have done a couple of projects in the past using porportional steam valves on autoclave machines to control the temperature in the vessel depending upon the pressure.
I have done some searching on the internet and see where you can get rotary pneumatic proportional valves with positioner units that accept a 4-20 ma signal.
Has anyone on this site ever did a project similar to this, and if so would be willing to share a little information.e
Our boss keeps saying he thinks there is a better way than using a stepper drive, which to him just seems way too complicated, and you know how it is, the boss may not always be right, but he is the boss.
Thanks, hope the question makes sense

How quickly does this thing need to cycle through the rotation? My guess is you may have bandwidth limitations with a pneumatically actuated force balanced system.

Keith

not something that will have to happen very quickly. will be a menu in the operator screen where you select the width and it will rotate to match the width. Have that code working with a stepper motor, but been asked to see if a pnuematic porportional valve could be used and controlled with a 4-20ma signal

Forget pneumatics. Sounds like something can can be easily done with a simple, small, linear positioner (ELECTRICAL), and mounting the head on a pivot.

JMHO. I'm starting to work hard at eliminating as many pneumatic controls as possible. (Okay, so to that extent, I'll be 'Green').

You won't get good position control with a pneumatic system with a PLC Because air has such a low bulk modulus (it compresses easily) it is difficult to accurately control position with it. It can be done, but it will require a good motion controller.

I'll second rdrast's suggestion, use a simple self contained electrical linear actuator.

Originally posted by Alaric:

You won't get good position control with a pneumatic system with a PLC Because air has such a low bulk modulus (it compresses easily) it is difficult to accurately control position with it.

Click to expand...

I think what irondesk40 is looking to do is tear the guts out of a process valve and use that as the head positioner. So it is a pressure controlled force balance system. Quite a bit of care is taken in the construction of those valves to make sure that the command you give them results in a (reasonably) deterministic output. So I would think his positioning accuracy would be OK. I think it will just take him forever to get there. He also may need some damping if he tries to move quickly and the spray head is "heavy".

That being said, I think he should stick with what he has right now unless he has a specific issue he needs to fix.

Keith

Keith -
Not exactly. Every pneumatic control valve we have that is used in an actual positioner application has an attached, pneumatic position controller as well. Just taking the bell and stem won't work, as the bell and the spring are very non-linear devices.

Running a control LOOP using a control valve without positioner is possible, but only because you don't care where the stem is, just that the final (and external) PV is controlled.

Besides that, jeez... even a small control valve, (assuming he just wants to use the bell and stem) is certainly not an inexpensive item. I've got baby, low pressure Research (brand) pneumatic valves here that are over $5000 each.