JPH0312703A - Process controller - Google Patents

Abstract

PURPOSE:To compensate the dead time and to improve the controllability by outputting a manipulated variable accordant with the disturbance to a control subject via an arithmetic part when a prescribed time passed before the controlled variable is affected after detection of the disturbance. CONSTITUTION:A comparison element 15 compares the set value SV of a drum level with the measured value PV' obtained in consideration of the disturbance Dn and outputs the control deviation (e). A PID arithmetic element 11 operates the increase value DELTACn of this time to the preceding manipulated variable Cn-1 and outputs it to an addition element 17. The element 17 adds the output DELTACn to the output K.DELTAn of a static characteristic compensation element 23 and outputs DELTACn+K.DELTADn to a manipulated variable adder 12 as MVn. The adder 12 adds the decrease value DELTAMVn of this time to the preceding manipulated variable MVn-1 and outputs MVn. An addition element 18 adds the output En given from a blind sector setting element 32 to the output MVn and outputs the final manipulated variable MVn+En to a service water flow rate control system 13. Thus the system 13 controls the flow rate of water supplied to a boiler based on the variable MVn+En.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a process control device, and particularly to one having a feedforward control function.

(Prior art) Regarding drum level control in boiler plants,
This will be explained with reference to FIG. Water 2 is contained inside a boiler 1, and steam that is heated and evaporated by a furnace 6 passes through a duct 4 in the direction of the arrow. Water is supplied to the boiler 1 from a water supply vibrator 3, and the water level in the drum 1 (hereinafter referred to as drum level) is adjusted. In order to keep this drum level constant, it is necessary to not only directly measure the drum level and supply water to a constant level, but also use the duct 4
Feedforward control is performed in which the main steam flow rate within the main steam flow rate is fixed at n1 and water is supplied in response to changes in the main steam flow rate.

This main steam flow rate is determined by measuring the pressure inside the duct 4 with the pressure gauge 5.

However, as shown in FIG. 4, there is a time lag between the change in the main steam flow rate and the change in the drum level.

After a disturbance occurs and the main steam flow rate changes, a time delay (hereinafter referred to as dead time) occurs before the drum level as a control variable changes due to this influence. this is,
This is due to the fact that there is a distance for the steam to circulate inside the process and return to the drum, and depending on the mounting position of the pressure gauge 5, there is normally a wasted time of several seconds to several tens of seconds.

Conventionally, as shown in Figure 5, when a disturbance occurs, the amount of operation is calculated by capturing this disturbance, and is immediately added to the output result of feedback control without considering this wasted time to calculate the final operation. The amount was controlled. As a result, unnecessary operations are performed before the drum level as a control variable changes, which in turn causes a change in the control variable as shown in Figure 5, disrupting controllability and delaying the stability of the system. It was the cause.

(Problem to be solved by the invention) In this way, in the past, feedforward control was performed without considering the wasted time from when a disturbance was detected to when the control amount appeared, which actually worsened controllability. was.

In view of the above circumstances, it is an object of the present invention to provide a process control device that compensates for wasted time and improves controllability.

[Structure of the invention]

(Means for Solving the Problems) The process control device of the present invention takes measures to deal with the influence of the disturbance on the control amount before a change in the control gaO occurs due to the influence of the disturbance occurring in the process control system. In a process control device including a feedforward control unit that outputs a manipulated variable to control a controlled object, the feedforward control unit performs a pre-measurement process that is required from when the disturbance is detected to when an effect appears on the controlled variable. means for storing the time during which the disturbance was detected (hereinafter referred to as "dead time"); The present invention is characterized by comprising a wasted time compensator that causes the manipulated variable to be output to the process control system at a time point delayed until the time elapses.

(Function) When the time has elapsed after the disturbance is detected by the detection unit until the controlled variable changes due to its influence,
In order to cause the calculation unit to output a manipulated variable corresponding to the disturbance to the controlled object, control is performed at appropriate timing.

(Embodiment) A process control device according to an embodiment of the present invention will be described with reference to FIG. 1 showing the configuration of its calculation elements.

As in the conventional case, control N controls drum 1 shown in FIG.
drum level, and the disturbance is the main steam flow rate.

First, a calculation element that performs feedforward control on the drum level using a change in the main steam flow rate as a disturbance will be described. This control includes one that performs static characteristic compensation through proportional operation and one that performs dynamic characteristic compensation through differential operation. First, control using static characteristic compensation will be explained. A change in disturbance is taken as a difference Dn with respect to the equilibrium state. This Dn is determined by a main steam flow rate calculating section (not shown) using the steam pressure measured by the pressure gauge 5 in FIG. In wasted time compensation element 21,
The amount of time required for this disturbance change Dn to affect the process 14 and cause a change in the drum level is obtained from the wasted time storage section 24, and the amount of time required for calculation is subtracted from this wasted time. is delayed and outputs Dn to the proportional action element 22. The proportional action element 22 multiplies the variation Dn of the disturbance by a proportionality constant K and outputs the result as -Dn to the static characteristic compensation element 23. In the static characteristic compensation element 23,
The current increase/decrease K·(Dn −Dn−1) with respect to the previous change mK−D n−1 is determined and given to the addition element 17 as the static characteristic compensation operation amount K·ΔDn.

On the other hand, dynamic characteristic compensation is performed using the following calculation elements. As in the case of static characteristic compensation, the wasted time compensation element 21 delays the time in consideration of the wasted time and applies the change in disturbance ffi D n to the proportional action element 22, where the proportional constant K is multiplied. -Dn is output. -Dn is given to this output to the dynamic characteristic compensation element 31,
The differential time is Td.

Letting the Laplace operator be S, ra −S/ (1, +ra
・S) Calculation by differential operation is performed. Letting this calculation result be En, this En is given to the dead zone setting element 32. Here, if the absolute directivity of En is within a predetermined value, the value of zero is zero, and if the absolute directivity of En exceeds the predetermined value, the value of En is the addition element 1.
given to 8.

Next, the calculation elements that perform feedback control on the drum level and the calculation of the final operation amount using the feedback control and the feedforward control described above will be explained. The drum level measurement value PV is output from the process 14, but this value is based on the change in the main steam flow rate (disturbance Dn).
varies depending on the influence it has on the process 14, so the value obtained by subtracting this influence bone by the subtraction element is the comparison element 15.
given to. The comparison element 15 compares the set value Sv of the drum level with the measured value Pv' in which the bones affected by the disturbance are taken into consideration, and outputs a control deviation e which is the difference between the two. P.I.
The D calculation element 11 calculates the current increase/decrease ΔCn with respect to the previous operation amount Cn-1 and outputs it to the addition element 17. The addition element 17 calculates a value Δ which is the sum of this output ΔCn and the above-mentioned output K・ΔDn of the static characteristic compensation element 23.
Manipulated amount adder 12 with Cn +K・ΔDn as ΔMVn
Output to. The manipulated variable adder 12 adds the current increase/decrease ΔMVn to the previous manipulated variable M V n-1 and outputs M V n. The addition element 18 further adds the output En from the dead zone setting element 32 described above to this output MVn, and outputs MVn+En to the water supply flow rate control system 13 as the final manipulated variable. The water supply flow rate control system 13 controls the flow rate of water supplied to the boiler 2 based on this operation ffiMVn+En.

Control characteristics when such control is performed will be explained using FIG. 2. Unlike conventional methods, the manipulated variable is given to the process control system after a dead period of time has elapsed after a disturbance is detected, which minimizes disturbances in the controlled variable, shortens the time required for stabilization, and improves control. Improves sex.

The embodiments described above are merely examples and do not limit the present invention. For example, in this embodiment, static characteristic compensation by proportional operation and dynamic characteristic compensation by differential operation are performed as feedforward control, but it is not necessary to provide both. Further, in this embodiment, not only feedforward control but also feedback control is performed, but it is also possible to have no feedback control.

〔Effect of the invention〕

As explained above, the process control device of the present invention outputs the manipulated variable corresponding to the disturbance to the process control body when the time elapses from the detection of the disturbance until the time when the influence appears on the controlled variable, thereby wasting time. Therefore, controllability can be improved by operating at the optimal timing when the control amount is affected.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing the configuration of calculation elements of a process control device according to an embodiment of the present invention, Fig. 2 is an explanatory diagram showing control characteristics when the same device is applied, and Fig. 3 is an explanatory diagram showing the control characteristics when the same device is applied. A block diagram showing a possible boiler plant process, Figure 4 is a time chart showing the relationship between disturbance and controlled variable, and Figure 5 is a relationship between disturbance and controlled variable when a conventional process control device is applied. This is a time chart showing the following. 1...Drum, 2...Water, 3...Water supply pipe, 4
...Duct, 5...Pressure gauge, 6...Furnace, 11.
... PID calculation element, 12 ... operation amount adder, 13.
...Water supply flow rate control system, 14...Process, 15...
Comparison element, 16... Subtraction element, 17.18... Addition element, 21... Waste time compensation element, 22... Proportional operation element, 23... Static characteristic compensation element, 24... Waste Time storage section, 31... dynamic characteristic compensation element, 32... dead zone setting element.

Claims (1)

[Scope of Claims] Before a change in the controlled variable appears due to the influence of a disturbance occurring in the process control system, the controlled object is controlled by outputting a manipulated variable corresponding to the influence of the disturbance on the controlled variable. In a process control device including a feedforward control unit, the feedforward control unit calculates a pre-measured time (hereinafter referred to as dead time) required from when the disturbance is detected until the control amount is affected. a storage unit; a calculation unit that calculates a manipulated variable according to the detected disturbance and outputs it to the controlled object; A process control device comprising: a waste time compensator that outputs the manipulated variable to the process control system.