Why I get different result at Simulink? (Matlab, Simulink)

Why I get different results? (Matlab, Simulink)

My problem is that I get different results with Matlab (Transfer Function) and Simulink at closed loop.

Amplitude is different and when I zoom at Simulink's Plot I get this odd irregularity.

Matlab:

Simulink:

Answer 1

[Stefan Raab]

Hello,

I suppose that depends on the solver. The documentation of the "step" function describes in the "More About/Algorithms" section, that the sample time is chose automatically, fixed step if I remember correctly. In Simulink however, you might have used a variable step solver (probably ode45), which is more accurate generally. You can also execute the "step" function with a specified sample time, try a really small sample time as 1e-6 or 1e-7 as this should increase the accuracy. The command then would be:

t = 0:1e-6:10;
step(YourSYS,t);

For the documentation on the step function, execute the following command:

web(fullfile(docroot, 'control/ref/step.html'))

Does this yield more accurate results?

Kind regards, Stefan

[Bob]

Thanks for your answer,

This fixed my problem. Now my results is correct.

%%Simulink Model
t = 0:1e-6:10;
sim('Simulink_Model',t);

[Stefan Raab]

I'm glad it works now. But I would use the t for the step function, not the Simulink model. The variable step solvers in Simulink are more powerful and now you force to simulate with a fixed step solver. This cannot adjust it's step size in order to achieve better accuracy and it cannot react to zero-crossings.