PID with Ramp and pressure control

Ok heres my question:

I need a PID control for heat, I need to heat the vessel to 100 C once I reach that temp I want the pressure to take over control of the heater.

Any body can give me some help with this one??

FP0-T32CT PLC and FPWINPRO software,

really confused about the switch to pressure control, Ramp ? not sure on that but could figure that one out, but transfer of control is a sticky point for me.

Search for low select

http://www.plctalk.net/qanda/showpost.php?p=247663&postcount=14

ok that helped a little, once I got it translated. but I am still somewhat confused. I want to be able to control the temp with either the temperature set point OR the pressure set point. I also want the heater to respond to temp to ramp to set point then to switch to pressure control. Is this even possible and if so howa/

Arlie said:

I want to be able to control the temp with either the temperature set point OR the pressure set point.

Click to expand...

This usually isn't possible. Normally you can control one or the other or limit both. It is usually impossible to control both at the same time.

Do you have a formula that provides temperature as a function of pressure? If not then doing what you want to do will not be possible.

I get the same question about controlling position and pressure on hydraulic presses.

I also want the heater to respond to temp to ramp to set point then to switch to pressure control. Is this even possible and if so howa/

Click to expand...

The ladder will turn on the heater until the temperature limit or the pressure limit which ever comes first.

The reason why you are confused is because you don't know the relationship, the formula, between the temperature and pressure and you somehow think a control system can make up for the lack of a formula. You need to understand the physics of what you are controlling. So far you have told us nothing.

Ok I have no idea what formula you are talking about, this is all new to me.

I ask for a push in the right direction.

I want to ramp to temp then switch to pressure control of the PID heater control.

so instead of belittling me how about you ask me what it is you need to know to help me.

Thank you

Arlie, is this an autoclave application?

No its not an autoclave what it is, is well kinda hard to put,but I will try to explain,

I have a vessel, I need to heat it to a specific temp. but the temp will affect the delivered gas, (I am heating a liquid to gas) so once I reach the temperature I need to keep the pressure at a specific press (usually in torr) but since press affects temp, I need to control the heater with pressure after I reach temperature.

Does that make any sense at all?? I hope it does

Arlie
This is a perfect example of why you need to tell us what the machine is and what it does.

If you are working with steam you only need to control pressure or temperature.

The reason is that the two are interrelated. Look at a steam table you will see that pressure rises as temperature rises. At approx 100 psig steam will have a temperature of approx 300F (off top of my head)

The other thing that comes to mind is WHAT does the process do
heat something
or
pressurize something

IF heating is the process -- control temperature
IF pressurizing is the process -- control pressure.

Dan Bentler

What happens if temperature drops back below 100? Does control switch back to controlling with temperature?

Initially this really sounds like a goood fit for a low select (what was posted earlier). Basically, you run two loops side by side. One is temperature, it generates a control variable (CV_1) to command the heater. The other is pressure, and it also generates a control variable (CV_2) that also is intented to command the heater. Based on some criteria (usually whichever PID had the lowest CV) you then select which CV you actually send to the output that drives the heating element. The other PID (the one that was not selected) then needs to be updated so that its output is the same as what was selected, so you write the selected CV back into the other PID's CV - this helps so that when you transiton control from one to the other you don't get a big bump.

Since this isn't an autoclave the more information you can provide the more help we can be. This is why Peter was looking for an equation. Anything, something to describe the temperature/pressure relationship and the process will help immensely.

edit: An equation doesn't have to be numbers at first - variables and text describing the rough relationship you think exists between the variables and what you expect to do with them is sufficient for a starting point - from there things can be refined.

I'm guessing here but I suspect all Arlie is really worried about is thermal shock. Otherwise he could just control pressure and let the temperature do what it needs to do to get the required pressure.
Peter's point about a the system model is well taken. It is hard to make considered decisions about how to design your control system if you don't fully understand how the process will respond to control system inputs. A model will tell you this.

It really doesn't sound like you are looking for limit control. It sounds like you know you can get to the temperature you want without hitting the pressure setpoint. So you may simply be able to heat to temperature, preload the pressure loop with the output from the temperature loop and then let the pressure loop take over. But we won't know that without a decent system model.

Keith

Sounds like a vaporizer application to me.

I did a Chlorine/Steam Vaporizer application a long, long time ago.
Basically you warm the thing up steadily until it reaches the point at which the liquid feed starts to vaporize then switch to pressure control to trim the outlet pressure and keep that steady using the temperature.
Sounds like a classic cascade loop scenario.
PID with a ramp section/output rate-of-change limit for the intial temperature control then switch it to cascade once it meets its ramp endpoint.
In that way two things CAN control the same valve (sort of I guess...).

Hopefully we'll find out what liquid it is and if its steam or something else thats being used as the heating medium.

Either way that'll help us find a useful solution.

ok the process is more pressure based than temperature, however I want to ramp the temperature up to the set point (aprox 100C) and then let pressure take over. If however a jump in temperature is required then again PID would revert to temp control untill set point reached.

this is a vapor generation system, if that helps, We start with a liquid, heat it til we generate a vapor , that vapor is now pressurized (by the heating process) and delivered to the point of use.I need to keep it at that pressure while maintaining the heat to keep it a vapor.So if Pressure drops I need the heater to come on to maintain the vapor state.

Yes SLCintegrator it is a vaporize of sorts, exactly, I use an immersion heater not steam but what you have said matches what I am doing

does that help?? best way i can think of to describe what I am doing

Two PID loops. Inner (slave) loop for temperature control with external ramp/soak control. Outer (master) loop for pressure control.

Initialize inner loop in manual,use external (to PID) ramp/soak logic to heat up vessel near SP, switch slave loop to auto with local SP, once vessel reaches SP, switch to remote SP (from master loop). Monitor actual temp vs. SP and if it falls below SP, switch slave loop back to local until SP reached, go back to remote for pressure control.

Not complicated, but tedious, this type of arrangement is often used for thermally initiated chemical reactions.

Ken thats great thanks but since I am a newbie to PID loops I will have to experiment with the controls. Thanks for the info