Open loop transfer function to closed loop

Thread Starter

Kestutis Savickis

Joined May 2, 2017
8
Hi everyone,
I'm new to this forum, I can't sorting simple thing. I got open loop transfer function from my heater project. Measured data from waveform graph are with step point 60/420s+1. How add negative fedback for further pid calculations. I have mason formula but can't get right answer.
 

MrAl

Joined Jun 17, 2014
11,587
Hi everyone,
I'm new to this forum, I can't sorting simple thing. I got open loop transfer function from my heater project. Measured data from waveform graph are with step point 60/420s+1. How add negative fedback for further pid calculations. I have mason formula but can't get right answer.
Hi,

You will have to show your schematic or other drawing, and also what result you got so far.
It's not possible to calculate anything without the information that goes with it.
 

Thread Starter

Kestutis Savickis

Joined May 2, 2017
8
Hi,

You will have to show your schematic or other drawing, and also what result you got so far.
It's not possible to calculate anything without the information that goes with it.
It is a simple heating plate controlled by NI cRio-9012 and Labview. I run that in open loop until got steady state output 60 degree in about 1000seconds, then calculating time constant - 420sec. And time delay is 30sec.
So 1 step input and funcion 60/420s-1. Now I have to that open loop add fedback node and then calculate parameters for pid controller. Add time delay a think no problem. That situation, or I misunderstood, I know that's are simple but can't get right way.
 

MrAl

Joined Jun 17, 2014
11,587
Hello again,

If you dont take the time to show the details then you are going to end up spending time answering 100 questions, so one way or the other, you have to take the time to explain things carefully if you expect expert assistance.

What is see so far is that your forward transfer function is:
H(s)=60/(420*s-1)

Even if that's not correct, you see how i took the time to write it out carefully and correctly so other's know what is being said exactly.

So question #1 is: Is that function correct?

Also, your end goal is to create a PID controller?
 

Thread Starter

Kestutis Savickis

Joined May 2, 2017
8
Hi,
There we are, attached graph with transport delay 30 sec. step input 10 sec. after start, amplitude (has to be Temperature) from 19.5 degree up to 82 degree and time constant ~420 sec. That run at open loop and heating plate max producing heat are 80-84 degree.
Now I have to add feedback get closed loop transfer function then calculate pid parameters.
 

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MrAl

Joined Jun 17, 2014
11,587
Hi,

Ok so you should probably start with an integration. Do you know how to add an integrator/error amplifier?
I assume you have a temperature sensor of some kind, and can provide the function for that sensor here too.
 

MrAl

Joined Jun 17, 2014
11,587
Hello again,

We seem to have some kind of communication problem. I ask a question but you dont seem to answer it. I ask these questions to better understand what you are doing and what you are capable of doing on your own.

You say you dont have the function of the sensor, and now you say it connects to a pid controller.
Do you mean it WILL connect to the pid controller later once it is designed?
Also, if you dont have or can find the function of the sensor then you will have to assume some function about the operating point, and that it is reasonably linear about that point. Does this sound reasonable for your application? You might find information on your sensor on the web too. We at least have to know what kind of signal it puts out though, like a voltage, current, or change of resistance for example, and this would tell us what kind of interface it needs.

I'll ask again:
Do you know how to add a simple error amplifier/integrator to your system as the first step in creating a feedback system? Note this could come in the form of additional hardware or additional software.
 

Thread Starter

Kestutis Savickis

Joined May 2, 2017
8
Hi,
As mentioned before system input are pt100 sensor with linear parameters at 0 degree - 100 ohm 100 degree - 138 ohm and connected to converter with output voltage range 0-10v at 0-200 degree input. In others words Celsius to voltage. In Labview have plant and controller with manual pid settings. All this can run in real time or simulated.
I don't know how converting signal from like ambient temperature sensor and add to comparator.
 
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