Pneumatic controller damping discussion wanted

realolman

I have been working on an old recording Taylor fulscope pneumatic controller with sensitivity adjustment. I can't get it to trace a nice temperature line like it used to. 

It is used to control a pneumatic wafer valve that modulates the combustion air to a dual burner gas HX.  The modulated combustion air modulates  gas regulators keeping the ratio pretty steady throughout the entire range of control.



When the process is started up, the process temperature will oscillate slowly (probably every minute or two ) above and below the set point (380 oil circulating through the HX )  for about 10 minutes. The oscillations  decrease in amplitude so that the trace rather looks like an arrowhead.  A the end of these oscillations the process temp is held pretty good ( within a couple degrees... always changing ), but it used to draw an line around the circular chart that looked like it was done with a compass.



It is a long story, but there have been several modifications done to just about everything involved with this process... and the controller is pretty old... even it has been modified over the years.



All in all the process temperature control is not terrible, but I am in humble admiration of the people who engineered and built this

thing.    I would like to understand it well enough to make it draw circles like it used to.



What effect would making the flame leaner have?  What if I adjusted the

linkage to the wafer valve so that the airmotor had to move farther to

make a change in the combustion air?

 

If anyone would be so kind as to offer some discussion or pointers on servo systems and damping in general as they relate to something like this, and/or specifics on this particular type of controller, I'd appreciate it.  I have adjusted just about everything I can think of on this thing, and it always seems to act pretty much the same.