Provides a manual means of tuning a PID controller but does not attempt an automatic solution, thus its namepid_designer. Edit: renamed function torootlocus_pid_designer to distinguish it from loopshaping techniques. Partially addresses#657.

UsesSisotool to investigate the effect of adding or subtracting an
amountdeltaK to the proportional, integral, or derivative (PID) gains of
a controller. One of the PID gains,Kp,Ki, orKd, respectively, can
be modified at a time.Sisotool plots the step response, frequency
response, and root locus.

When first run,deltaK is set to 0; click on a branch of the root locus
plot to try a different value. Each click updates plots and prints
the correspondingdeltaK. To tune all three PID gains, repeatedly call
pid_designer, and select a differentgain each time ('P','I',
or'D'). Make sure to add the resultingdeltaK to your chosen initial
gain on the next iteration.