Hi Onn Lim,
From the documentation for function "stepinfo", the function input "SettlingTimeThreshold" should be a scalar between value 0 and 1. In your case, the value 0.0001 is within the accepted range. However, the NaN value of the settling time could be that there is not a finite time which takes for the error y(t) - yfinal between the response y(t) and the steady-state response yfinal to fall to within 0.01% of yfinal for the dynamic system that passed into the "stepinfo" function.
The following system "sys" is from an example on the Documentation page for function "stepinfo":
A = [0.68 -0.34; 0.34 0.68];
B = [0.18 -0.05; 0.04 0.11];
C = [0 -1.53; -1.12 -1.10];
D = [0 0; 0.06 -0.37];
sys = ss(A,B,C,D,0.2);
It can be seen that changing the "SetllingTimeThreshold" to 0.0001 for this case the values of "SettlingTime" are not NaN:
S1 = stepinfo(sys,'SettlingTimeThreshold',0.0001)
