I am trying to find the integral gain (ki) of the following transfer function.

This is my code,

syms ki

sys = tf([30.672 175.694*ki],[1 38.672 175.694*ki]);

S = stepinfo(sys);

subplot(2,1,1)

step(sys)

However it's showing the error:

Error using tf (line 303)

The values of the "Numerator" and "Denominator" properties must be row vectors or cell arrays of row vectors, where each vector is nonempty and containing numeric data. Type "help tf.num" or "help tf.den" for more information.

Answer 1

[VBBV]

syms ki
ki = 10 % e.g numeric values , not symbolic 

ki = 10

sys = tf([30.672 175.694*ki],[1 38.672 175.694*ki]);
S = stepinfo(sys);
subplot(2,1,1)
step(sys)

As the error states num and den vectors should be numeric, and not symbolic as you defined.

Answer 2

[Star Strider]

Symbolic variables are not permitted in Control System Toolbox objects, however anonymous functions are.

Try this —

sys = @(ki) tf([30.672 175.694*ki],[1 38.672 175.694*ki]);
ki = 42;
S = stepinfo(sys(ki))

S = struct with fields:
         RiseTime: 0.0130
    TransientTime: 0.1931
     SettlingTime: 0.1931
      SettlingMin: 0.7501
      SettlingMax: 1.5173
        Overshoot: 51.7266
       Undershoot: 0
             Peak: 1.5173
         PeakTime: 0.0333

subplot(2,1,1)
step(sys(ki))

The value of ‘ki’ can be whatever you want, and you can use ‘sys’ here as an anonymous function as an argument to integration and optimisation functions.

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