I understand that you want the step response of 2 input and 2 output system. You are getting the 4 plots correctly if you use MATLAB script, but same system when simulated in Simulink produces only two responses instead of four.
This is an expected behaviour. Simulink is not finding the step response of the system in the above case.The simulink solves the state-space model of a system for the given input signals (In your case, They both are step signals). In simpler words, it is plotting the outputs of the plant when both the input signals are present together. As your plant has two output ports, simulink will find both the outputs by exciting the system with both the inputs present at the input ports.
On the other hand, "step" command in MATLAB does the following:
- Exite the plant with a unit step input at only one of the input ports (
), and find the outputs 
and 
. This will giive two plots: 
- Now exite the plant with unit step input at the other input port (
), keeping 
, and find the outputs 
and 
. This will again give two plots: 
In this way, you can get four plots showing the step response of plant for each corresponding input-output pair.
Work-around:
To obtain step responses for each input-output pair in Simulink, you can follow this approach:
- Single Input Excitation: Apply a unit step input to one of the input ports while ensuring the other input remains at zero. Measure the system's outputs in response to this input.
- Repeat for Other Input: Next, apply the unit step input to the second input port, keeping the first input at zero. Again, record the system's outputs.
By following these steps, as illustrated in the accompanying image, you can effectively analyze the step responses for each input-output pair in your state-space model.
I hope it answers the question !
