“ num”和“ den”属性的值必须是行向量或行向量的单元格数组，其中每个向量均为非空

Question

I am quite new to Matlab . I want to plot for the following equation in Matlab .

G(s)= 1* (e-0.5s) /(s+1) (s+5)^2

When, I type in Matlab like this,

clear all

clc

syms s

Gnum=exp(-0.5*s);

Gden=((s+1)*((s+5)^2));

G= tf (Gnum,Gden)

H=1

T= feedback (G,H)

step(T)

But I encountered error as per below.

??? Error using ==> tf.tf at 239

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

Error in ==> Untitled4 at 7

G= tf (Gnum,Gden)

Please help. Thanks in advance.

Answer 1

You are mixing things very badly. You can't make s syms and then make transfer function using it. Also, you do not plot a transfer function. You should use Bode on it to show the frequency and the magnitude of the transfer function for that. Just forget about syms all together when using Matlab control systems.

s=tf('s');
sys=exp(-0.5*s)/((s+1)*(s+5)^2)    
sys =     
                           1
  exp(-0.5*s) * ------------------------
                s^3 + 11 s^2 + 35 s + 25

Continuous-time transfer function.

sysf= feedback (sys,1);
step(sysf)

Answer 2

If all you are wanting to do is plot that function, then something along the lines of this should suffice:

s = -10:.5:10;

G = 1 .* (exp(1)-0.5.*s) ./ (s+1) .* (s+5).^2;

plot(s,G)

Answer 3

You can use ezplot too if you want to plot G(s) where:

G(s)=exp(-0.5*s)/((s+1)*((s+5)^2));

You can use ezplot as follows:

ezplot('exp(-0.5*s)/((s+1)*((s+5)^2))');

to generate the following image: