Root locus negative gain

Hi, I'm not an expert so I might be wrong, correct me in case. So, when I plot a root locus' function, specifing the positive and negative gain locus I use this command: rlocus(G,-G) I'm assuming there will be a label on the positive locus plot with, of course, positive gain (and so it is) as well as a label with negative gain on root locus plot for negative gain(since I've issued '-G' as parameter of rlocus command). Here's the problem: the label positioned on the negative root locus plot always gives me a positive gain where I'm assuming to have a negative gain instead. How is this possible? I thought to have always a negative gains, for a negative gain plot. Where am I wrong? Maybe my assumptions are incorrect... Thanks in advance.

Answer 1

[Jatin Waghela]

Suggestions:

In his case, you are actually plotting two root locus plots on the axes. One is the original of the system, the other of the negative of that system. The plot will end up looking like it is plotting both positive and negative gains for the original system, but actually, they are two separate systems and all gains plots are will still be positive for both systems.

If you want positive and negative gains for a system, you can pass in a specific vector of gains as the 2nd argument to 'rlocus'.

For example:

h = tf([2 5 1],[1 2 3]);
rlocus(h,-10:.0001:10)

Also, please note that for certain ranges of gains, you will need a really fine resolution to make the curve look smooth.

Please refer to the below documentation link which provides more information about 'rlocus' https://www.mathworks.com/help/control/ref/rlocus.html