root locus code from scilab book
s=%s; syms k H=syslin('c',(k*(s+1)*(s+2))/(s*(s+3)*(s+4))); evans(H,5)
I found this code from a scilab control systems online book and tried executing it in the cloud. No good! I see that in this example there is a feedback gain, k, but searching for syms in scilab search, I can find no reference to it. I ran evans before using a transfer function without the k and it works ok (right out of the evans example code definition). Is this code right out of one of your books wrong? The k does look like a good idea, What is the meaning of "syms k" and what is wrong with this example?
There is nothing wrong with this example except that it uses 'syms' to declare a symbolic variable. 'Syms' command is no longer supported in the latest scilab versions. syms k will simply declare a symbolic variable k, just like you declare s=%s. You may simply replace k with its correct value and the example will work. You can attach the example url next time to help us help you better. Just a note, if I am not wrong, the k here is "forward" gain and not "feedback" gain.
When doing a root locus, one starts with the form 1 + k(Q(s)/P(s)) and the roots are plotted for k values from 0 to inf thus forming a root locus graph. Which brings up 2 questions: in the above code k would have to be scanned over 0 to inf, so putting in a constant seems contrary to forming a root locus graph, and as evans with k a constant would seem not to show a root locus, but it does, somehow the evans function inserts a k parameter to make a 1 + k(Q(s)/P(s)) function for evans to analyze the above function. Any comments welcome.