Step response of feedback system

Question

We have a feedback system F(s) as in this figure:

and this is the step response of G(s):

and H=10.

F(s):

We got the root locus of the system F(s) as this:

then we determined the gain at marginal stability:

which is equal to 2.5.

Then, we made H = 2.57 (slightly larger than 2.5) and we obtained the step response of the new system Fh(s). Here is it with the step response of the original system F(s):

Then, we made H = 2.43 (slightly lower than 2.5) and we obtained the step response of the new system Fl(s). Here is it with the step response of the original system F(s):

My questions:

• What can we know from the step response plot of G(s)?
• What can we know from the step response plot of F(s)?
• When we changed the gain to 2.57, how did the system F(s) change? What can we conclude by comparing the step responses of F(s) and Fh(s)?
• When we changed the gain to 2.43, how did the system F(s) change? What can we conclude by comparing the step responses of F(s) and Fl(s)?

Answer 1

• G(s) is unstable because the step-response doesn't have a steady-state value. It goes to infinity.
• F(s) is stable since its step response has a steady-state value near 0.1. It's underdamped response.
• Fh(s) is stable since its step response has a steady-state value near 0.2. It's underdamped response with more oscillations than F(s).
• Fl(s) is unstable because it doesn't have a steady-state value. It has a very large oscillations that start after some time.