The autotuning function is used to gain time during the start-up of an installation by guaranteeing a stable adjustment.
The autotuning algorithm is based on a method of Ziegler-Nichols type:
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Analysis of the procedure for checking if this is first order with delay. The length of this analysis is 2.5 times the response time of open loops.
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Calculation of PID adjustment parameters (KP, TI, TD) or those of the model loop controller (KS, T1, T_DELAY). The range of set parameters is moduled by performance criteria in order to prioritize the response time to disruptions or stability.
The algorithm processes the following process types:
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Single input and single output processes.
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Naturally stable processes or integrators.
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Asymmetric processes in the limits allowed by the PID algorithm.
There are two possible autotuning types: warm or cold autotuning. The first phase of autotuning is the same for each of these: Noise and stability test of the process of length 0.5x AT_TMAX during which the outputs remain constant. The following phases depend on the type of autotuning. The selection is made automatically by the algorithm.
Cold autotuning is performed if the deviation between process value and setpoint exceeds 40% and if the process value is less than 30%. Two scales in the same direction are then applied to the loop controller output (OUT_MAN). Each scale has a length of AT_MAX.
When autotuning is complete, the loop assumes its previous operating mode.
If cold autotuning conditions are not fullfilled, warm autotuning is performed. A scale is applied to the loop controller output (OUT_MAN), then an inverse scale. Each scale has a length of AT_MAX.
When autotuning is complete, the loop assumes its previous operating mode.
