Find system rise time with matlab

[erezb84]

Homework Statement

I need to find a system rise time (from 10% to 90%) using matlab.
Is this code right?

The Attempt at a Solution

sys = tf(num, den);
y= step(sys, t);
tr1 = max(find(y<0.1));
tr2 = min(find(y>0.9));
rise_time = t(tr2) - t(tr2);

Is this seems to be right?

 

[RoshanBBQ]

No. What makes you think that 10% of the rise is always 0.1 and that 90% of the rise is always 0.9? Have you considered a system whose step response settles to a negative value such as
num = [1 0 -1];
den = [1 4 6 4];
?
What if it doesn't settle to 1?

 

[erezb84]

Yes.. You are right...
Any ideas how can i do it?

 

[RoshanBBQ]

Yes.. You are right...
Any ideas how can i do it?

Have you tried the built-in function "stepinfo()"?

http://www.mathworks.com/help/toolbox/ident/ref/stepinfo.html