First order control system for proportion​al valves

Question

I am working on a proportional valve control program. We have four gas lines and it is desired to send four different gases through a combustion chamber sequentially. The close time for each valve is determined by the pressure measurement inside the combustion chamber. For instance, firstly, we let the nitrogen gas flows to the chamber, and the pressure inside the of it reaches 5 bars, we closed the nitrogen valve, and open the argon gas valve, and hold it opened until the pressure is reached at 15 bars inside the chamber and so on.
I was thinking on utilizing PID for control of these valves, however, one author from my institute said that you can not do this with a PID program, you can do that with a  first order control system. Because in the system,  it is not allowable to gases exceed their set points (set pressure values). PID controls the system with osilations around set point and after it provides the set value, but in this case, we are not allowed to exceed the set point (we may reach the set point slowly and asymptotically, but never exceed it).
I have not background on controlling hence I need your help. Could I do that with Labview? Is there a first order system control module in Control & Simulation Toolkit? And also where can I find examples, tutorials, etc. -if there are- about first order system controlling? Thanks in advance.
Egemen

Answer 1

what you are trying to do can be done with PID, but you would need to tune it to avoid overshot. But, most importantly, the use of the closed-loop PID will guarantee that your system will follow the setpoint, which can be a ramp going from the 0 to 100 % of the valve. The ramp on setpoint is a typical control technique to avoid overshoot of the system.
Now, if you look in the PID palette under Control and Simulation, you will see the function called "Lead-Lag". This function is a 'first order' system and you can define a pole and a zero for your system. If your objective is just make the change smooth, you can make the Lead-Lag to work as a "Lag" (make the lead zero) and apply to your system. here is an example:
C:\Program Files (x86)\National Instruments\LabVIEW 2014\examples\control\PID\Simulation - Lead-Lag.vi
If you need something more complicated than Lead-Lag, than I would you advice to use the LabVIEW Control Design and Simulation Module. There you can really develop more advanced controllers, like higher order filters or Model Predictive Control. But this also requires you to have a better knowledge of control theory...
Barp - Control and Simulation Group - LabVIEW R&D - National Instruments