RT - PID Gains - where and when to use them

Question

The attached is example code for controlling a temperature chamber. The question is where and when to use PID Gains? This code looks as though the PID Gain is used for the variable/parameter temperature (i.e. the temperature is the controlled variable).
So would there be just one PID Gain per controlled variable/parameter (temperature in this case) or would each device controlling temperature in the system get a PID Gain?
In this example, the fan and lamp are controlling the temperature.
Attachments:
Exercise 8-1.zip ‏171 KB

Answer 1

Russ_in_Louisville wrote:
I came here for help as this is where NI pointed me in their failure. I did not come here to get my "mental model" attacked.
Can you accept that your model of the system might be wrong, though? In this thread from a few weeks ago, multiple experienced users told you that what you had on your block diagram was an array, and yet you insisted - repeatedly - that what you had was a cluster. Your model was that the item on the block diagram was a cluster, leading to the question "how do I get an index display on it?" which made no sense, because the item was in fact an array. And, when you finally realized you were wrong, you couldn't even admit it or thank the people who pointed out your error - it just "turned out" that the constant wasn't what you thought it was. I am trying to tell you that you are making a similar error here by asking questions about the system that do not correspond to what the system actually does.
"Parameter/variable" isn't clear. There is correct terminology. A PID controller has, as inputs, a process variable and a setpoint. The process variable is the parameter you want to control, such as temperature. The setpoint is the desired value of the process variable. Let's say you do have a temperature control system where there is both a heating system and a cooling system (again, not the situation in your sample code). The two systems could both be driven by PID and share the same setpoint and process variable. Some control options for this system:
- a single PID controller with one set of gains, where a negative output drives the cooling system and a positive output drives the heater (or some variation on this), if both the heater and chiller have similar performance characteristics.
- two PID controllers, each with a set of gains, one controlling the heating and one controlling the cooler.
- a combination of these two: a single controller with gains that are selected based on the setpoint or process variable, which is known as gain scheduling. The advantage to a single controller is that you'd never have the two controllers fighting each other.
You could also have a code that controls several similar processes (for examply, multiple identical temperature chambers). In that situation you could have a single set of PID gains but multiple controllers each with a different setpoint and process variable.
So there's not a simple yes/no answer to when you need a separate set of PID gains or an additional PID controller. It depends on the characteristics and design goals of your system. Does that help?