Function 'subsindex' is not defined for values of class 'tf'.

Hi to all.

I have try to write a simple code for step and impulse response of a transfer function.

The code is this:

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% BODE PLOT OF SUSPENTIONS TRANSFER FUNCTION
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% Suspentions Transfer Function
[num, den] = suspension(240, 16e3, 980, 36, 160e3);

% Make Suspentions Transfer Function
G_s = tf(num, den);

% Create a Bode plot with plot handle h
figure('name', 'Bode plot of Suspentions Transfer Function');
h = bodeplot(G_s);
grid;

% Create a plot options handle p
p = getoptions(h);

% Modify frequency units
p.FreqUnits = 'Hz';

% Apply plot options to the Bode plot and render
setoptions(h, p);

clear h p;

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% STEP AND IMPULSE RESPONSE
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

figure('name', 'Step and Impulse response');
subplot(211);
step(G_s);
grid;
subplot(212);
impulse(G_s);
grid;

And the output of suspension(240, 16e3, 980, 36, 160e3) is:

num = [0.0612 1]
den = [3.375e-06 0.0001057 0.0167 0.0612 1]

The problem is that in the Command Window the code works, but in my program - with no other command, no. The error is when I call 'step' and is:

Error using subsindex
Function 'subsindex' is not defined for values of class 'tf'.

Thank you in advance, Giacomo.

Answer 1

[Giacomo Alessandroni]

Thank you Wayne King, but the question is another.

My function was named:

function kinetic(v_min, v_max, step)

so... it was only my fault.

Thanks anyway.

[Wayne King]

yes, that is the problem I said, you are passing a variable called step.

Answer 2

[indah octaviyani]

i get an error for my matlab, could you help me too? Error using subsindex Function 'subsindex' is not defined for values of class 'ss'.

Answer 3

[Wayne King]

Giacomo, likely in your program you make the mistake of creating a variable called step.

Then you try to index that variable by G_s which is an object of class tf

So note that

G_s = tf(num, den);
step = randn(10,1);
step(G_s)

gives exactly the error you report. Because I make the mistake of shadowing the step() method with a variable called step.