Steel plant control device

Abstract

A steel plant control device includes a subtractor, a PI controller, a first-order lag controller, and an adder. In a band that is lower than a first frequency f1, a gain of the PI controller is higher than a gain of the first-order lag controller; in a band from the first frequency f1 to a second frequency f2, the gain of the first-order lag controller is higher than the gain of the PI controller; and in a band that is higher than the second frequency f2, the gain of the PI controller is higher than the gain of the first-order lag controller.

Claims

1. A steel plant control device, comprising: a subtractor that outputs a deviation between a target value and a control variable, the control variable being fed back from a control object; a PI controller that receives an input of the deviation and outputs a first manipulated variable for reducing the deviation by using a proportional action and an integral action; a first-order lag controller that outputs a second manipulated variable, the second manipulated variable being obtained by multiplying the deviation by a proportional gain and further multiplying by a first-order lag element; and an adder that outputs to the control object a final manipulated variable, the final manipulated variable being obtained by adding the second manipulated variable to the first manipulated variable; wherein, in a band that is lower than a first frequency, a gain of the PI controller is higher than a gain of the first-order lag controller; in a band from the first frequency to a second frequency that is higher than the first frequency, the gain of the first-order lag controller is higher than the gain of the PI controller; and in a band that is higher than the second frequency, the gain of the PI controller is higher than the gain of the first-order lag controller, and wherein the band from the first frequency to the second frequency is a band that causes load change and characteristic change of the control object after the deviation converges to zero.

2. A steel plant control device, comprising: means for subtracting that outputs a deviation between a target value and a control variable, the control variable being fed back from a control object; PI control means for receiving an input of the deviation and outputting a first manipulated variable for reducing the deviation by using a proportional action and an integral action; first-order lag control means for outputting a second manipulated variable, the second manipulated variable being obtained by multiplying the deviation by a proportional gain and further multiplying by a first-order lag element; and means for adding that outputs to the control object a final manipulated variable, the final manipulated variable being obtained by adding the second manipulated variable to the first manipulated variable; wherein, in a band that is lower than a first frequency, a gain of the PI control means is higher than a gain of the first-order lag control means; in a band from the first frequency to a second frequency that is higher than the first frequency, the gain of the first-order lag control means is higher than the gain of the PI control means; and in a band that is higher than the second frequency, the gain of the PI control means is higher than the gain of the first-order lag control means, and wherein the band from the first frequency to the second frequency is a band that causes load change and characteristic change of the control object after the deviation converges to zero.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a control block diagram of a closed loop system according to a first embodiment.

(2) FIG. 2 is a Bode diagram that shows the frequency characteristics of a transfer function.

(3) FIG. 3 is a chart that shows the fluctuation of a strip thickness deviation on a delivery side of a finishing rolling mill in a case where a first-order lag controller is not provided (only a PI controller).

(4) FIG. 4 is a chart that shows the fluctuation of a strip thickness deviation on the delivery side of the finishing rolling mill in a case where the first-order lag controller has been added in parallel to the PI controller.

(5) FIG. 5 is a diagram that shows a system configuration around a looper on a cold rolling line.

DESCRIPTION OF EMBODIMENTS

(6) Hereinafter, embodiments of the present invention will be described in detail with reference to drawings. It should be noted that common elements in the drawings are denoted by the same reference signs to omit redundant explanations.

First Embodiment

(7) [Configuration of Closed Loop System]

(8) FIG. 1 is a block diagram of a closed loop system according to a first embodiment. A control device of a closed loop system shown in FIG. 1 includes a subtractor 2 and a PI controller 3, and in addition, a first-order lag controller 4 and an adder 5.

(9) In one example, a control object 1 is a finishing rolling mill that is provided on a hot rolling line and rolls a material to be rolled. The control object 1 receives an input of a final manipulated variable u and outputs a control variable y. The final manipulated variable u is a roll gap of a final stand of the finishing rolling mill. The control variable y is a strip thickness on a delivery side of the finishing rolling mill.

(10) The subtractor 2 outputs a deviation e (control deviation) between a target value r and the control variable y which is fed back from the control object 1. The control variable y is detected by a sensor. The sensor is a strip thickness gauge which is arranged on the delivery side of the finishing rolling mill. The target value r is a target strip thickness on the delivery side of the finishing rolling mill.

(11) The PI controller 3 (transfer function is K.sub.p1+K.sub.i/s) will be described. The PI control is a type of feedback control, and is a control method that combines a proportional action and an integral action. The proportional action is an action that changes the manipulated variable in proportion to the deviation e. The integration action is an action that changes the manipulated variable in proportion to the integration of the deviation e. The PI controller 3 receives an input of the deviation e; and outputs a first manipulated variable u1 for reducing the deviation e by using the proportional action and the integral action.

(12) The first-order lag controller 4 is arranged in parallel with the PI controller 3. The first-order lag controller 4 receives an input of the deviation e; and outputs a second manipulated variable u2 which is obtained by multiplying the deviation e by a proportional gain K.sub.p2 and further multiplying by a first-order lag element LAG (transfer function is 1/(1+T.sub.s), where T is a time constant) which exists in the control object 1.

(13) The adder 5 outputs to the control object 1 a final manipulated variable u which is obtained by adding an output of the first-order lag controller 4 (second manipulated variable u2) to an output of the PI controller 3 (first manipulated variable u1).

(14) Features of the present embodiment will be described with reference to FIG. 2. FIG. 2 is a Bode diagram that shows the frequency characteristics of a transfer function. A horizontal axis is a logarithmic axis of an angular frequency [rad/s]. A vertical axis on a left side is a logarithmic axis of a gain [dB].

(15) A gain curve 21 shows the gain characteristic of first-order lag control. A gain curve 22 shows the gain characteristic of PI control. A gain curve 23 shows a gain characteristic obtained by adding the PI control and the first-order lag control together.

(16) In a steel plant control device according to the present embodiment, the K.sub.p2 and time constant T of the first-order lag controller 4 are adjusted so that the frequency characteristic indicated by the gain curve 23 in FIG. 2 can be obtained. Specifically, as shown in the gain curve 21, the proportional gain K.sub.p2 and the time constant T of the first-order lag controller 4 are adjusted so that the gain becomes 0 or more at the angular frequency =1 or less.

(17) Therefore, as for the gain curve 23 obtained by adding the gain curve 21 and the gain curve 22 together, a gain of the PI controller 3 is higher than a gain of the first-order lag controller 4 in a band that is lower than a first frequency f1; the gain of the first-order lag controller 4 is higher than the gain of the PI controller 3 in a band from the first frequency f1 to a second frequency f2 that is higher than the first frequency f1; and the gain of the PI controller 3 is higher than the gain of the first-order lag controller 4 in a band that is higher than the second frequency f2.

(18) In other words, the gain of the first-order lag controller 4 is dominant in the band from the first frequency f1 to the second frequency f2; and in the band lower than the first frequency f1 and the band higher than the second frequency f2, the gain of the PI controller 3 is dominant. The band from the first frequency f1 to the second frequency f2 corresponds to the vicinity of a fluctuating frequency due to a load change or a characteristic change of the control object that may occur after the deviation e converges to zero. Therefore, according to the present embodiment, as indicated by the gain curve 23, only the gain in the vicinity of the fluctuating frequency is increased, and the other frequency bands are hardly affected.

(19) Thus, by adding the first-order lag controller 4 in parallel to the PI controller 3 to increase the gain only for the vicinity of a control response, tracking to the target value becomes quick. As a result, it is possible to immediately respond to a load change or a characteristic change of the control object that may occur after the control deviation converges to zero, thereby allowing an excellent response characteristic to be realized. It should be noted that in the case of hot rolling, a load change or a characteristic change of the control object includes the expansion of a machine due to the temperature or heat of a strip, roll eccentricity, and the like.

(20) Next, referring to FIG. 3 and FIG. 4, the fluctuation of the deviation e in the case of only the PI control and the fluctuation of the deviation e in the case of having added the first-order lag control to the PI control will be compared. FIG. 3 is a chart that shows the fluctuation of a strip thickness deviation e on the delivery side of the finishing rolling mill in a case where the first-order lag controller 4 is not provided (only the PI controller 3). FIG. 4 is a chart that shows the fluctuation of the strip thickness deviation e on the delivery side of the finishing rolling mill in a case where the first-order lag controller 4 has been added in parallel to the PI controller 3.

(21) In the chart of FIG. 3, after a head end of the material to be rolled reaches the delivery side of the finishing rolling mill at time 0, the strip thickness deviation e temporarily converges to within an allowable range (between an upper limit value 31 and an lower limit value 32) (zero convergence) by the PI control. However, it cannot respond to a load change or a characteristic change of the control object after the zero convergence, and deviates from the allowable range.

(22) On the other hand, in the chart of FIG. 4 in which the proportional gain K.sub.p2 and the first-order lag element LAG are added to the PI control, after the head end of the material to be rolled reaches the delivery side of the finishing rolling mill at time 0, the strip thickness deviation e converges to within a target range (between the upper limit value 31 and the lower limit value 32) mainly by the PI control. As with the gain curve 23 in FIG. 2 described above, the proportional gain K.sub.p2 and the time constant T are adjusted so that the gain between the first frequency and the second frequency is increased. Therefore, even when there is a load change or a characteristic change of the control object after the deviation e converges to zero, the convergence to the target value is quick and the deviation e can be stabilized within the target range.

(23) It should be noted that the steel plant control device according to the present embodiment performs control outside the band from the first frequency f1 to the second frequency for large changes other than the load change or the characteristic change of the control object after zero convergence, in which convergence is achieved mainly by the PI control.

(24) (Modification)

(25) Incidentally, as for the above system of the first embodiment, description has been made by using strip thickness control on the delivery side of the finishing rolling mill on the hot rolling line; however, an application object of the present invention is not limited. For example, it can also be applied to tension control on a cold rolling line. FIG. 5 is a diagram that shows a system configuration around a looper on a cold rolling line.

(26) In one application example, the steel plant control device can also be applied to ones that use as a feedback signal a control variable y which is detected by a tension meter 53 between a bridle roll 51 and a loop car 52 downstream thereof, and output a final manipulated variable u of the loop car 52 so that a deviation between the control variable y and a target value r converges to 0.

(27) In another application example, the steel plant control device can also be applied to ones that use as a feedback signal a control variable y which is detected by a tension meter 56 between a bridle roll 54 downstream of the loop car 52 and a continuous rolling mill 55 downstream thereof, and output a final manipulated variable u of the bridle roll 54 so that a deviation between the control variable y and a target value r converges to 0.

(28) The embodiments according to the present invention have been described above; however, the present invention is not limited to the above embodiments and various modifications can be made without departing from the scope of the present invention.

REFERENCE SIGNS LIST

(29) 1 Control object 2 Subtractor 3 PI controller 4 First-order lag controller 5 Adder 31 Upper limit value 32 Lower limit value 51, 54 Bridle roll 52 Loop car 53, 56 Tension meter 55 Continuous rolling mill f1 First frequency f2 Second frequency K.sub.p2 Proportional gain LAG First-order lag element r Target value T Time constant u Final manipulated variable u1 First manipulated variable u2 Second manipulated variable y Control variable Angular frequency