hey. I keep getting an error for this code. If anyone could help me it would be greatly appreciated.

clear;

clc;

a = [1,12];

b = [1,15,54];

rt = tf(a,b);

% b) Determine the steady state errors for the open-loop systems below with the following inputs: 8𝑢(𝑡), 8𝑡𝑢(𝑡), and 8𝑡2𝑢(𝑡).

syms s;

num = a;

den = poly2sym(b,s);

gs = num/den;

es = 8/(1+lim(gs)); % Steady-state error with step input

er = 8/(lim(s*gs)); % Steady-state error with ramp input

ep = 8/(lim(s^2*gs)); % Steady-state error with parabola input

fprintf('b) Determine the steady state errors:\n\t\t with step input = %.3f,\n\t\t with ramp input = %.3f,\n\t\t with parabola input = %.3f\n', es, er, ep)

%% User-defined functions

function y = lim(f)

syms s;

y = limit(f,s,0);

if isnan(y)

y = inf;

end

end

Answer 1

[Chetan Bhavsar]

It was because value of (1+lim(gs)) was

a=(1+lim(gs))
 
a =
 
[55/54, 11/9]

clear;
clc;
a = [1,12];                                           
b = [1,15,54];                                        
rt = tf(a,b);                                        
% b) Determine the steady state errors for the open-loop systems below with the following inputs: 8𝑢(𝑡), 8𝑡𝑢(𝑡), and 8𝑡2𝑢(𝑡).
syms s;                                               
num = a;
den = poly2sym(b,s);
gs = num/den;                                        
es = 8./(1+lim(gs));                                     % Steady-state error with step input
er = 8./(lim(s*gs));                                     % Steady-state error with ramp input
ep = 8./(lim(s^2*gs));                                   % Steady-state error with parabola input
fprintf('b) Determine the steady state errors:\n\t\t with step input = %.3f,\n\t\t with ramp input = %.3f,\n\t\t with parabola input = %.3f\n', es, er, ep)
%% User-defined functions
function y = lim(f)
syms s;
y = limit(f,s,0);
if isnan(y)
y = inf;
end
end

Is output as per expectation?

Answer 2

[Paul]

Should num also use poly2sym? Otherwise, gs is 1 x 2 transfer function matrix, which may be what you want, but maybe not.

If you really do want gs to be 1 x 2, then you need to change the divide to dot-divide for es and er and ep.

es = 8./(1+lim(gs));  % was es = 8/(1+lim(gs));