How can I change the "Set Point"? (Feedback)

How can I change the "Set Point"? (Feedback)

I assume "0" is the default? Can I set for example the "Set Point" to "1"

sys_cl = F*feedback(F*G1,C);

At Simulink I just change the constant value.

Answer 1

[Stefan Raab]

Hello,

I think your feedback might not be correct. I will abbreviate (X1-X2) as Y:

Y = G1(s) * (F(s)*W + C(s)*(R - Y)) ;

Y + G1(s)*C(s)*Y = G1(s)*F(s)*W + G1(s)*C(s)*R ;

Y = [G1(s)*F(s)]/[1+G1(s)*C(s)] * W + [G1(s)*C(s)]/[1+G1(s)*C(s)] * R ;

But if you apply your feedback structure, this will yield (with a general output U, which is NOT the one in you sketch)

Y = F(s)*G1(s) * (U - C(s)*Y) ;

Y + F(s)*G1(s)*C(s)*Y = F(s)*G1(s)*U ;

Y = [F(s)*G1(s)]/[1 + F(s)*G1(s)*C(s)] * U ;

Note that this transfer function does neither correspond to the transfer function from W -> Y nor the transfer function from R -> Y.

Regarding your "Set Point": If I understand you right, you want to have R also as input for your transfer function. Thus you could use a MIMO Transfer function:

web(fullfile(docroot, 'control/ug/mimo-transfer-function-models.html'))

In your case the tf would be a "row" vector tf, mapping from an input [W; R] to the output Y. It might be easier then to convert this into a state space model:

web(fullfile(docroot, 'control/ref/ss.html'))

Hope this helps.

Kind regards, Stefan

[Bob]

Thanks for your answer.

Now I can control the input W and set point R

Y = (((G1*F)/(1+G1*C)*W)+(G1*C)/(1+G1*C)*R);

In the end, why we use "feedback"? I think the way you show me is more simple.

[Stefan Raab]

feedback can be comfortable if you want a simple feedback loop, but in your case it might be better to do as I told you, yes. ;)

[Bob]

Thanks a lot.