Method and Apparatus for Controlling Acid Concentration for Pickling in Cold Rolling

An acid concentration control method for pickling in a cold rolling, comprising:

• providing three acid-filling tanks which are interconnected with each other on an acid circulating tank of a pickling production line;
• concatenating an acid concentration measuring device to an acid solution back-flow pipeline between each acid-filling tank and the acid circulating tank;
• obtaining an actual acid concentration of acid solution in the acid tank of a production line by the acid concentration measuring device;
• analyzing the actual acid concentration by an analyzer and feedbacking to a multi-variable controller;
• comparing the actual acid concentration, in the multi-variable controller, with a specific acid concentration required by a processing, and setting a difference between the actual acid concentration and the specific acid concentration as an input value for a model of the multi-variable controller;
• as the acid concentrations of the three acid-filling tanks are affected by each other, coupling relationship of the acid concentrations at three measuring points must be found out to build a mathematical model for the acid circulating tank of the production line, and perform multi-variable decoupling calculation so that multi-variable control system is converted into single-variable control system, wherein specific steps are as follows:
• Step 1, setting a multi-variable mathematical model for the acid circulating tank of a controlled object in the pickling production line,
• wherein a formula of a transfer function matrix G(s) for the acid-filling tank of the controlled object in the pickling production line is:
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• and a formula of an inverse function G−1(s) of the transfer function matrix G(s) for the acid-filling tank is:
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• wherein:
• a1, a2, a3 are respectively the sectional areas of the three acid-filling tanks, and the three acid-filling tanks have a same volume;
• β1, β2, β3 are allowable deviations of acid-filling amount;
• Step 2, designing a transfer function matrix Kp(s) for a pre-compensator based on the transfer function matrix G(s) for the acid-filling tank obtained in Step 1, to make Kp (s) and G(s) become the diagonally dominant matrix, wherein a specific method is as below:
• 1) operating an existing software of plotting Gussie-Collins belt of the transfer function G(s), inputting the transfer function G(s) into a pop-up interface, and clicking an operation after completion of input, then obtaining a Nyquist plot attached with Gussie-Collins belt displayed on the interface, in which Kp(s) and G(s) become diagonally dominant matrix;
• 2) calculating the transfer function matrix Kp(s) for a pre-compensator by using an existing diagonal dominance software;
• plotting a Gussie-Collins belt of Q(s) and then reaching a diagonal dominance for an open-loop system compensated;
• Step 3, designing a single-loop compensation for the transfer function matrix G(s) of the acid-filling tank;
• since Q(s) has reached the diagonal dominance, applying a single-variable design method to complete a compensation design for three single loops, taking Kci(i=1, 2, 3) as PI adjuster and obtaining value of transfer function matrix Kc(s) of dynamic compensator through several times of trial and errors;
• Step 4, plotting a Nyquist plot attached with Gussie-Collins belt for G(s)Kc(s)Kp (s), ensuring close-loop system to be stable according to a Nyquist stability criterion and obtaining a feedback gain value F(s) of the acid concentration.