unwrapped phase in bode plot

For an iwpt system, I am getting bode phase plot like in the attached image (Blue one)

1) Why we get this kind of phase plot?

2) How to convert it into -180 to +180 degree phase plot?

3) How to proceed with controller design for such systems?

Answer 1

[Paul]

If using bodeplot to make the plot, then use the plotoptions returned from bodeoptions with PhaseWrapping 'on',

For example

h = zpk([],-1,1); h = h^8; % transfer function with lots of phase change
plotoptions = bodeoptions;
plotoptions.PhaseWrapping='on';
bodeplot(h,plotoptions);

If not using bodeplot then we'll need to know how that plot is being constructed to get it into the desired form.