IP Valve

Today I did a last minute upgrade on an oven system to convert from
an old mercury temperature controller to regulate air to a Steam servo
valve to a Digital controller with an IP valve.

I have a problem with the IP valve oscillating when the controller
wants more than 10 LBs of air to open the servo valve

Now I noticed if I shut it down to 0 output and then call for
lets say a 15 LB output, It then some times will get up there smoothly
with out oscillating..

One other observation, the unit I pulled out of stock is used where
some one obviously exchanged the main unit with a used one but leaving
the new cover.. Is it possible the valve is defective? We don't have
another in stock at the moment.

P.S.
I checked the output signal from the controller, it holds a steady
20 ma current while it's trying to open the IP valve to max position
and the incoming is also regulated at the 40 LBs of supply as per
labeled on the IP valve.

Is it possible this valve is defective ? since it works fine under 10
Lbs and appears that it was already in service at one time.
The rated output is 3..15 Lbs normal., but we have these same valves
on other equipment where they don't oscillate and even run higher output
LBS..

 

You should post this over on sci.engr.control -- the group's a lot
quieter, but that's because the discussion is nearly all on topic. Even
the acrimony is about control systems, not Republicrat vs. Demoblican.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html