plotting nonlinear system with constant input

if I enter function in M-file like that:

function dx = suspension(t,x,Mb,Mf,Mr,I,Ksf,Ksr,Ktf,Ktr,Csf,Csr,Lf,Lr,Wf,Wr)

Mb=730;
Mf=40;
Mr=36;
I=1230;
Ksf=19960;
Ksr=17500;
Ktf=175500;
Ktr=175500;
Csf=1290;
Csr=1620;
Lf=1;
Lr=(1.8);
Wf=0.05;
Wr=0.05;
dx=zeros(8,1); %Wf and Wr is constant input as road bump

dx = [x(5);
      x(6);
      x(7);
      x(8);
      1/Mb*((-Ksf-Ksr)*x(1)+Ksf*x(2)+Ksr*x(3)-Lf*Ksf*sin(x(4))+Lr*Ksr*sin(x(4))-Lf*Csf*x(8)*cos(x(4))+Lr*Csr*x(8)*cos(x(4))-(Csf+Csr)*x(5)+Csf*x(6)+Csr*x(7));
      1/Mf*(Ksf*x(1)-(Ksf+Ktf)*x(2)+Lf*Ksf*sin(x(4))+Lf*Csf*x(8)*cos(x(4))+Csf*x(5)-Csf*x(6)+Ktf*Wf);
      1/Mr*(Ksr*x(1)-(Ksr+Ktr)*x(3)-Lr*Ksr*sin(x(4))-Lr*Csr*x(8)*cos(x(4))+Csr*x(5)-Csr*x(7)+Ktr*Wr);
      1/I*((-Lf*Ksf+Lr*Ksr)*x(1)+Lf*Ksf*x(2)-Lr*Ksr*x(3)-Lf*Lf*Ksf*sin(x(4))-Lr*Lr*Ksr*sin(x(4))-Lf*Lf*Csf*x(8)*cos(x(4))-Lr*Lr*Csr*x(8)*cos(x(4))-(Lf*Ksf-Lr*Csr)*x(5)+Lf*Csf*x(6)-Lr*Csr*x(7))];

   y=[x(2);x(3)];%output

end

>>[t,y]=ode45('suspension',[0 10],[0;0;0;0;0;0;0;0]);

>>plot(t,y(:,1));

what must I add in order to function running is true?

I was able to run your code without error. I don't understand the question you are asking. Are you able to write a little bit more explanation?

it's right if I run in command windows,but it's error if I run in M-file,..can you help me?thank you

You are going to have to show us the .m file that is producing the error, and show us the error message you are getting.

error comment "variable x undefined " and dx=x(5) error

Answer 1

You are likely running into difficulty because you are using 'suspension' (the string) as the routine for ode45 to invoke, instead of using an anonymous function. When you use a string, the search path for the function is not the same as when you use an anonymous function.

Code using the anonymous function is shown in http://www.mathworks.com/matlabcentral/answers/42590-plotting-nonlinear-system-with-constant-input