I think there are some possibilities here in identification. the gradient of y(control system output) is persistent in some cycles. u(control system input) never be changed. delay time of calculation is negative. gain of the control system is 0 or response time is negative or delay time is negative. In particular, the temperature control system is a nonlinear system with a large delay time and a large response time, and it is difficult to identify it. So it is hard to calculate if we use the linear transfer function to fit or if the data quantity is worse.
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