# PID controller - order

## Main Question or Discussion Point

I'm a physics student so don't do much in the way of electrical engineering, pardon my ignorance. However I'm looking at using a PID controller with a resistance thermometer sensor and heating element plant, with a reference point of some resistance on the thermometer. That is heat up a device to a particular temperature. However when considering generally whats going on here, I get a bit confused. That is considering the stability of the system, the PID controller seems to be 2nd order, surely there is no unstable point for a 2nd order transfer function? I know I can simply use the Ziegler–Nichols method to come up with tuning parameters, what I don't get is how there is a point at which the system starts to become unstable?

Separately does anyone know of any digital tuning scripts that would work with labview, well free ones anyway? (Ignore this really, it's a long shot to a separate issue).

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Bystander
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how there is a point at which the system starts to become unstable?
You are controlling a real system with thermal conductivities, and diffusivities between the error sensor and the response heating.

jim hardy
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That is considering the stability of the system, the PID controller seems to be 2nd order, surely there is no unstable point for a 2nd order transfer function?
Please excuse if i missed the question here...

Remember stability is for the closed loop
If transfer function of process is G and controller is H
you have to make G/(1+GH) stable.

donpacino
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I'm a physics student so don't do much in the way of electrical engineering, pardon my ignorance. However I'm looking at using a PID controller with a resistance thermometer sensor and heating element plant, with a reference point of some resistance on the thermometer. That is heat up a device to a particular temperature. However when considering generally whats going on here, I get a bit confused. That is considering the stability of the system, the PID controller seems to be 2nd order, surely there is no unstable point for a 2nd order transfer function? I know I can simply use the Ziegler–Nichols method to come up with tuning parameters, what I don't get is how there is a point at which the system starts to become unstable?

Separately does anyone know of any digital tuning scripts that would work with labview, well free ones anyway? (Ignore this really, it's a long shot to a separate issue).
To add to what others have said, do some research on phase and gain margin
http://en.wikipedia.org/wiki/Phase_margin

the PID controller seems to be 2nd order, surely there is no unstable point for a 2nd order transfer function? I know I can simply use the Ziegler–Nichols method to come up with tuning parameters, what I don't get is how there is a point at which the system starts to become unstable?
What is your model for the plant (thermal system) you are trying to control? The closed loop response is a combination of the controller transfer fucntion and the plant transfer function. So, you can't predict stability or optimim PID gains without consideration of your plant. If you dont know your plant model, then you must tune experimentally (e.g. Your Z-N method). Keep in mind that these tuning methods only get you in the ball-park of a good solution, and you should manually tune it from there.

Typically thermal systems require only a PI controller and the D part is usually not needed.

CalcNerd