PID with zero output for decreasing

[8bitbanger]

Hi Folks,

Due to some historical reasons, we're using a mass flow controller to maintain pressure within an enclosure for leak testing (not much at all, 2"WC). Obviously in order for the PID to decrease the process value, it would need to set the flow controller to zero and turn on a 'vent' flow controller. I know this is really backwards, but what are your thoughts on properly implementing this with the standard PID vi? I've tuned the PID for one particular enclosure (which leaks, without using a vent controller) but am afraid of the PID response if we have an incredibly well-sealed enclosure. Overshoot is acceptable, up to about 4"WC.

The main problem is getting the PID to drop to zero if it needs to decrease the pressure. I hacked this by monitoring the last value through a shift register, then if the value changed, set flow to zero. This didn't lend itself to a very stable PID either.

Any thoughts?

Jamie

Message Edited by 8bitbanger on 04-29-2008 01:43 PM

[VGA_CD-ROM]

Hi 8bitbanger,
Have you experimented with decreasing your response time to the system in order to lessen the likelihood of a sudden overpressure? And when you say you set mass flow control to zero upon (I'm assuming) whatever condition determines that the vent flow controller should come on, are you placing a zero into the PID setpoint for the mass flow controller, or are you directly replace its output with a zero value (i.e. bypassing PID response and going straight to zero output)?

I'm also assuming the vent flow controller can also be PID controlled and is not a simple On/Off device which cannot be dialed up or down. If that's so, is it possible to strike a balance between the mass flow controller and the vent flow controller together simultaneously using PID with both of them, or should the vent controller not come on except when overpressure is likely?