Estimation of Input to Transfer Function

Hello,

I want to control 220V 100 Watt "Band Heater", to apply 160 degree celsius on load and control it using PID Controller using on/off Relay, so in order to get Transfer function for this system, i applied 220V to the

heater and record the output values in array and send them to MATLAB, but in order to get transfer function i have to specify input/output for the system, so i specify the output(temperature), but i couldn't specify the exact input to the system. So my question is:

What is the exact input to this system in order to identify it in MATLAB?

Answer 1

[Ameer Hamza]

Since you applied a constant voltage of 220V, so you should create a vector with all values 220 and the same size and y2. Then use it as input for transfer function estimation.

x2 = 220*ones(size(y2));

define this in MATLAB workspace, and input x2 in the input field.

[Omar Khalifa]

I thought that, but why not say that the Input is 100 Watt?

[Ameer Hamza]

It depends on what is your input. Are you controlling the voltage or power?

[Omar Khalifa]

I think i am controlling power, since i am using ON/OFF Relay to switch On/Off Power.

[Ameer Hamza]

I guess in that case, you can use the 0 and 1 signal of relay switch as input. Input is anything that can control be controlled. So you can just use input value equal to when switch is ON and zero when it is OFF.

[Omar Khalifa]

I already did that, but PID gains were too small values which make the system's controller response very slow, otherwise i let the power is input and resulted that PID gains were reasonable numbers.

What do you suggest ?

[Ameer Hamza]

I guess the number is small because, in the case of switching, the input variable will only vary between 0 and 1. Whereas, in the case of power, it can vary from 0 to 100. I am sure, apart from that, the final solution should be the same. Can you share the response of your system in a .mat file?

[Omar Khalifa]

I appreciate your help.

[Ameer Hamza]

Omar, the first problem I can see in your recorded temperature array Temperature_Output is that it contains insufficient data to estimate a transfer function. You should at least record data to the point that the curve becomes horizontal like shown in the figure below

[Omar Khalifa]

Alright, i understand that i should give Band Heater enough time to make the temperature curve becomes horizontal. I hope this will fix my problem.

Once again, thank you very much for your help.

[Ameer Hamza]

Correct. That will give enough data to estimate an accurate transfer function.