Two kinds of autotuning are possible: autotuning at a warm and cold system start

The first phase of autotuning applies for both kinds of tuning: this involves a sound and stability test of the control process lasting 0.5 * tmax with constant outputs. Subsequent phases depend on the kind of tuning.

Autotuning at a cold start is referred to when the deviation between the actual and setpoint values exceeds 40% and the process value is less than 30%. In this case the TRI output of the function block is admitted with two actuator pulses of the same kind. Each actuator pulse has duration tmax. When autotuning ends, there is a smooth return to the previous operating mode for the servo loop:

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If the conditions for autotuning at a cold start are not fulfilled, tuning at a warm start takes place: the output is admitted with an actuator pulse, followed by an actuator pulse in the opposite direction. Each stage has duration tmax. When autotuning ends, there is a smooth return to the previous operating mode for the servo loop:

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