This block detects when a PID tuning period should start and
end. Specifically, the tuning period is triggered to begin when
either tOn or tOff becomes greater than
0. The tuning period is triggered to end when either
tOn or tOff changes after the tuning
period starts, as illustrated below:

J. Berner (2017). "Automatic Controller Tuning using Relay-based Model Identification." Department of Automatic Control, Lund University.