I my tried to use for loop to find a transfer function and root louces

Im trying to find the transfer function which included in the code and plot the root louces for TF3 so, i need to find them at a different value of k which is in PitchRateSen transfer function so when i run the code the transfer function and root louces plot just the for loop run for the first value of k and i dont know why

[A,B,C,D]=linmod('controlpr');
[num,den]=ss2tf(A,B,C,D);
g=tf(num,den);

for k = [2,1,3,4]
    VehiicleDy_tf=tf([-0.0125,-0.054375],[1,1.456 ,0.2948 , 0.020787 ]);
    ElevatorAcu_tf=tf([2],[1,2]);
    PitchRateSen_tf= -k*tf('s');
    Pitchgain=-5;
    TF1=series(ElevatorAcu_tf,VehiicleDy_tf);
    TF2=feedback(TF1,PitchRateSen_tf);
    TF3=series(TF2,Pitchgain);
    TF4=feedback(TF3,1);
    rlocus(TF3);
end

Answer 1

[Paul]

I think that rlocus(TF3) clears the figure and then overwrites, so I think you're actually getting the root locus for the last value of k.

If you want four separate figures then open a new figure each time through the loop (it doesn't look like the value of k does very much)

for k = [2,1,3,4]
VehiicleDy_tf=tf([-0.0125,-0.054375],[1,1.456 ,0.2948 , 0.020787 ]);
ElevatorAcu_tf=tf([2],[1,2]);
PitchRateSen_tf= -k*tf('s');
Pitchgain=-5;
TF1=series(ElevatorAcu_tf,VehiicleDy_tf);
TF2=feedback(TF1,PitchRateSen_tf);
TF3=series(TF2,Pitchgain);
TF4=feedback(TF3,1);
figure
rlocus(TF3);
end

[ahmad salahat]

The value k affect on how much the poles which behind the orgin close to each other.

the plot is done but why when i write in command window TF3 to calculate the transfer function it calculate just for the fist value?

thank you

[Paul]

If I understand this follow-up question correctly ....

When the loop is finished, TF3 will be the transfer function computed for the last value of k, i.e., k = 4.

Do you want to save four different instances of TF3, one instance for each value of k?

[ahmad salahat]

yes this what i want

[Paul]

You have several choices depending on your needs. One option would be to save of each instance of TF3 and its associated value of k to a container array, like a struct.

% slight change to structure of the loop
k = [2, 1, 3, 4];
for ii = 1:numel(k)
    VehiicleDy_tf=tf([-0.0125,-0.054375],[1,1.456 ,0.2948 , 0.020787 ]);
    ElevatorAcu_tf=tf([2],[1,2]);
    PitchRateSen_tf= -k(ii)*tf('s');
    Pitchgain=-5;
    TF1=series(ElevatorAcu_tf,VehiicleDy_tf);
    TF2=feedback(TF1,PitchRateSen_tf);
    TF3=series(TF2,Pitchgain);
    TF4=feedback(TF3,1);
    % no need to plot the root locus for this discussion
%    figure
%    rlocus(TF3);
    TF3a(ii).k   = k(ii);
    TF3a(ii).TF3 = TF3;
end

Then to see one of the TF3, for example for k = 1

TF3a(2).k

ans = 1

TF3a(2).TF3

ans =
 
                  0.125 s + 0.5437
  ------------------------------------------------
  s^4 + 3.456 s^3 + 3.232 s^2 + 0.7191 s + 0.04157
 
Continuous-time transfer function.

Or, you can stack up each instance of TF3 in an array

clear
k = [2, 1, 3, 4];
for ii = 1:numel(k)
    VehiicleDy_tf=tf([-0.0125,-0.054375],[1,1.456 ,0.2948 , 0.020787 ]);
    ElevatorAcu_tf=tf([2],[1,2]);
    PitchRateSen_tf= -k(ii)*tf('s');
    Pitchgain=-5;
    TF1=series(ElevatorAcu_tf,VehiicleDy_tf);
    TF2=feedback(TF1,PitchRateSen_tf);
    TF3(:,:,ii) = series(TF2,Pitchgain);
    TF4=feedback(TF3,1);
    % no need to plot the root locus for this discussion
%    figure
%    rlocus(TF3);
end

Using this approach you have to remember the correspondence of each value of k to each entry in TF3. Access TF3 that corresponds to k = 1

TF3(:,:,2)

ans =
 
                  0.125 s + 0.5437
  ------------------------------------------------
  s^4 + 3.456 s^3 + 3.232 s^2 + 0.7191 s + 0.04157
 
Continuous-time transfer function.

Yet another approach is to use a tunable, or parametric, model. Here, we build the model with k as a parameter, and then sample the model for specific values of k afterwards

clear
k = realp('k',2);  % default value of 2 corresponding to first element of k 
% No need for a loop
    VehiicleDy_tf=tf([-0.0125,-0.054375],[1,1.456 ,0.2948 , 0.020787 ]);
    ElevatorAcu_tf=tf([2],[1,2]);
    PitchRateSen_tf= -k*tf('s');
    Pitchgain=-5;
    TF1=series(ElevatorAcu_tf,VehiicleDy_tf);
    TF2=feedback(TF1,PitchRateSen_tf);
    TF3=series(TF2,Pitchgain);
    TF4=feedback(TF3,1);

Now, get TF3 for the value of k = 1

tf(sampleBlock(TF3,'k',1)) % tunable models are ss, so convert to tf if that is what is really desired

ans =
 
                  0.125 s + 0.5438
  ------------------------------------------------
  s^4 + 3.456 s^3 + 3.232 s^2 + 0.7191 s + 0.04157
 
Continuous-time transfer function.

The display of the last coefficient in the numerator in rounded to 0.5438, but if you check the actual value it's the same as in the previous case down to the last digit. There will, of course, be very slight diiferences between this method and the previous ones because the underlying numerical computations are different.