PROCESS CONTROL SYSTEM AND METHOD

Abstract: A process control system has a timer circuit (1) which fixes operational periods by issuing an instruction for starting each of a plurality of periods in such a manner that the computation of control blocks will be repeated in periods of fixed duration. A periodic operation selection circuit (2) selects control blocks to be computed within each period in response to an instruction from the timer circuit (1). Control blocks selected by the periodic operation selection circuit (2) and an instruction for completion of the computation of control blocks for each period are stacked in an operation instruction circuit (3), which issues an instruction for consecutively executing the operation of the control blocks and an instruction for effecting an idle operation after completion of the operation of control blocks for each period. When an instruction for starting an ensuring period is issued from the timer circuit (1) at a time when computation of a control block has not been completed within a predetermined duration period, an operation completion determining circuit (5) issues an instruction for starting the computation of the ensuring period beginning with computation of the remaining control blocks in the predetermined period and an instruction for stacking control blocks in the ensuing period in the operation instruction circuit (3) by means of the periodic operation selection circuit (2) by cancelling the instruction for completion of the predetermined period, thereby making periodical and continuous operations possible.

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to a process control system and a method for controlling plants of various kinds by applying predetermined operational signals to various items of equipment within the plant over which control is to be maintained. DESCRIPTION OF THE PRIOR ART Hitherto, process control systems have been known which are adapted to control plants of various kinds by periodically repeating the computation of various control factors. Among such known systems, one type sets the operational periods within which control factors are computed at fixed short periods in order to improve the response characteristics (i.e., setting is made in such a manner that the computation to be undertaken in one period will be completed within a short period of time), and the control factors are classified in accordance with their degree of importance; the computation of control factors of low priority is not effected when it becomes impossible to complete the computation of all scheduled control factors within each operational period. Another system is known which is adapted to reduce the length of each operational period by refraining from performing the computation of any unnecessary operational factors during a period, thereby improving the response characteristics. These systems are disclosed in, for instance, Japanese Patent Unexamined Publication Nos. 65301/1984 and 98480/1979. The aforementioned prior arts, however, have a drawback in that there are cases where such systems cannot suitably be used for controlling a plant which is complex and large in size and hence requires a correspondingly complicated system of control. In other words, in a control system used for controlling a steam turbine in a nuclear or thermal power plant, for instance, higher response characteristics and a more stabilized level of control have come to be necessary in conjunction with the tendency for turbines and generators to become large in size in consequence of the recent increase in the generating capacity of single units, and cases are therefore encountered where the aforementioned prior arts are incapable of meeting such requirements. Namely, with the former type of prior art, there is no continuity in operations since, when it has become impossible to complete the operations of all the scheduled control factors during a predetermined operational period, the arrangement is such that the operation proceeds to the next period without performing the computation of the remaining operational factors during that period. There is therefore a possibility of losing control stability or giving rise to erroneous outputs. Also, the latter type of prior art is not suited either to applications which require continuity in computation, as described above. US-A-4 205 380 discloses a process control system with the features included in the first part of claims 1 and 4. This document discloses a concrete method of effecting a periodic operation or computation by a digital control system. Just as in the prior-art systems described above, no means are provided to effect the control when the scheduled computation is not completed within a predetermined period of time. SUMMARY OF THE INVENTION It is an object of the present invention to provide a process control system and method which ensure stability of the control, at the same time achieving fast response. This object is met by the process control system and method set forth in claims 1 and 4. The invention is particularly applicable to the control of steam turbines of nuclear or thermal power plants, as set forth in claims 2 and 3. In accordance with the process control system and method of the invention, operational periods with fixed short durations are established, and predetermined operational factors are continuously computed. When it is impossible to complete the computation of all the control factors or control blocks within each operational period, the non-completed computing routines are resumed in the subsequent period, so that the continuity of computation and control will never be lost. The other objects, features, and advantages of the present invention will become apparent from the detailed description of the preferred embodiments when read in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic diagram illustrating a process control system in accordance with an embodiment of the present invention which is arranged as a system for controlling a steam turbine of a nuclear power plant; Fig. 2 is a diagram illustrating an operation processing unit in the control system; Fig. 3 is a diagram illustrating the operation of the operation processing unit; Fig. 4 is a diagram illustrating the operation of a periodic operation selection circuit of the operation processing unit; and Fig. 5 is a diagram schematically illustrating a mode in which the operation of control blocks is carried out periodically and continuously in the control system. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. l shows a process control system in accordance with an embodiment of the present invention. This system is arranged as a control system 30 for a steam turbine of a nuclear power plant. In the illustrated nuclear power plant, steam generated from a nuclear reactor 2l, i.e., the source of generating steam for operating a turbine, is introduced into a high-pressure turbine 25 via a main steam stop valve (MSV) 23 and a steam control valve (CV) 24. The steam which has done work in the high-pressure turbine 25 is led to a medium- and low-pressure turbine 27 via an intermediate steam stop valve (IV) 26, and the steam which has finished work at the medium- and low-pressure turbine is led to a condenser 28, thereby constituting the so-called Rankine period. In addition, a generator 40 generates electric power on receiving the rotational force of the turbine 27. This plant is provided with a bypass line 4l which has a turbine bypass valve (TBV) 29 and directly leads the steam generated from the nuclear reactor into the condenser 28. Incidentally, the power plant per se of the type described above is already known. A steam turbine controller 30 of the illustrated embodiment is provided with the following: a speed and load control circuit 32 into which a turbine speed signal detected by a speed pick-up (SPU) and a generator output signal are input; a pressure control circuit 3l into which a steam pressure signal is input via a pressure transmitter (PX); a turbine bypass valve (TBV) flowrate control circuit 34 into which an output signal of the pressure control circuit 3l and an output signal of a low value gate (LVG) 33 are input; a CV/IV opening control circuit 35 into which an output signal of the low value gate 33 is input, this circuit 35 being adapted to generate an output signal for controlling the opening of the steam control valve (CV) 24 and the intermediate steam stop valve (IV) 26; and a TBV opening control circuit 36 into which an output signal from the TBV flowrate control circuit 34 is input, this circuit 36 being adapted to produce an output signal for controlling the opening of the turbine bypass valve (TBV) 29. Accordingly, this controller 30 is adapted to detect the turbine speed, generator output, and steam pressure and control the opening of the turbine bypass valve 29, the steam control valve 24, and the intermediate steam stop valve 26 through the computation of various control factors of circuits, thereby controlling the turbine speed, generator output, and steam pressure. Incidentally, the above-described control per se of a steam turbine with the aforementioned control factors or circuits is already known. In effecting the above-described control, it is desirable from the viewpoint of maintaining stability of the control thereof for signals representing speed and the like to be input at fixed periods as practically as possible so as to adjust the opening of valves. Meanwhile, with respect to any rise in the turbine speed resutling from interruption or the like of the load or fluctuations of steam pressure in a nuclear power plant, it is extremely important to cope with such situations with high response in order to ensure prevention of turbine over-speed and protection of the nuclear reactor and so forth. For this reason, an operation processing unit is adopted in the present invention which is capable not only of ensuring control stability by fixing periods and effecting continuous computation but also of improving the response characteristics by shortening those periods. Fig. 2 shows the operation processing unit of the control system shown in Fig. l. Specifically, this processing unit includes the following: an operational period timer circuit l for controlling the periods of computation; a period operation selection circuit 2 for selecting control factors or blocks to be computed within a given period; an operation instruction circuit 3 for stacking the control factors to be computed and imparting an instruction; an operation processor 4 for actually performing computation on the basis of that instruction; an operation completion determining circuit 5 for determining whether or not all the scheduled operations have been completed within the period; an operation program 6 divided to accommodate each control factor and adapted to store the content of computation; and an idle operation program 7 which exerts no influence on control and effects time adjustment. It should be noted that the various phases of control of the nuclear power generator turbine are effected as a result of executing the operating program 6. Referring now to Figs. 3 to 5, description will be made of the operation of the above-described operation processing unit. Referring first to Fig. 4, description will be made of the operation of the periodic operation selection circuit 2 for consecutively instructing and stacking control factors or control blocks to be computed into the operation instruction circuit 3. In other words, if an instruction for starting the operation of a predetermined period is issued from a timer circuit for setting operational periods, the periodical operation selection circuit 2 operates as shown in the following steps: A ring counter is updated (Step 50), and determination is made from the count position of the ring counter as to whether or not the first or starting bit of control block No. 1 is set to '1' (Step 51). If YES is the answer, the operation proceeds to Step 52 to stack the content of control block No. 1 in the operation instruction circuit 3, and the control block No. is updated in Step 53. On the other hand, if the starting bit of control block No. 1 is not '1' in Step 51, the operation proceeds to Step 53 to update the control block No. Determination is made in Step 54 on the basis of that result as to whether or not the stacking of the entire blocks has been completed. If the stacking of the entire blocks has not been completed, the operation returns to Step 51 again, and the above-described operation is repeated until the stacking of the entire blocks is completed. When the stacking of the entire control blocks has been completed, a completion instruction is stacked in the operation instruction circuit 3 in Step 55, and the operation of the periodical operation selection circuit 2 is thereby completed. Incidentally, in the illustrated embodiment, the control blocks shown in Fig. 5 include a speed/load control block in block No. 1, a pressure control block in block No. 2, a TBV flowrate control block in block No. 3, a CV/IV opening control block in block No. 4, and a TBV opening control block in block No. 5. Their contents are consecutively stacked in the operation instruction circuit 3, as described above. Referring now to Fig. 3, description will be made of the overall operation of the above-described operation processing unit. First, if an instruction for starting an operation in a predetermined period, e.g., a second period, is issued from the timer circuit 1 in a normal state, determination is made in Step 61 as to whether whether or not a completion flag in an operation completion determining circuit 5 has been set to '1' by an instruction for completion of the computation in the preceding period, i.e., the first period. If the completion flag is '1', selection of a periodic operation in the second period is effected in Step 61. In other words, all the control blocks to be computed in the second period and a completion instruction are stacked consecutively in the operation instruction circuit 3 by the periodic operation selection circuit 2. An operating program is thus executed for corresponding control blocks on the basis of instructions stacked in advance in the operation instruction circuit 3 (step 62). The instruction of the operation instruction circuit 3 is updated in Step 63. Thus, the operation of control blocks in the second period is effected consecutively. If it is judged (Step 66) that the execution a completion instruction is executed (Step 64) after execution of a control operation, and the completion flag in the operation completion determining circuit 5 is set to '1', the operation process to Step 68, where an idle operation is carried out. In other words, a control operation is not effected and adjustment of time is carried out until the timer circuit gives an instruction which indicates the ensuing period. In the event that the computation of all scheduled control blocks is not completed during the predetermined period, the following operation takes place. As shown in Fig. 5, the computation of control blocks is started when a short time duration t has elapsed after the timer circuit l issued an instruction for starting each period, and the arrangement is such that control blocks are stacked in the operation instruction circuit 3 by the periodic operation selection circuit 2 during that time duration t. Accordingly, while an operation of control block No. 5 (Fig. 5) of the second period, for instance, is being carried out, if the timer circuit l issues an instruction for starting the ensuing period, i.e., the third period, the execution of the operating program 6 is suspended temporarily (it is suspended for a time duration corresponding to the aforementioned time duration t). At that time, since judgment is made in Step 60 that the completion flag of the previous period, i.e., the second period, has not been set to 'l', the operation completion determining circuit 5 saves the content of the program counter (PC) which indicates the position of the operating program being executed, so that a restarting position of the remaining operations is instructed to the operation instruction circuit 3. Subsequently, the last completion instruction of the previous period, i.e., the second period, stacked in the operation instruction circuit 3 is canceled by the operation completion determining circuit 5. Thus, the control blocks and completion instruction of the third period are consecutively stacked by the periodic operation selection circuit 2, as described referring to Fig. 4. Consequently, the operating program 6 restarts the aforementioned remaining operations from the point where it had suspended the computation temporarily, jumps the idle operating program, and executes the computation of the control blocks of the third period (Step 62). It should be noted that the aforementioned save is also effected when there occurs, for instance, a man-machine interface interrupt instruction from the outside in the midway of the second period, as in the case shown in Fig. 5, and the operation of the operation instruction circuit 3 (i.e., an operation concerning pressure control in Fig. 5) is retained. A program based on the interrupt instruction is executed in the operation processor 4, and in response to the occurrence of the instruction for terminating the interrupt processing the retaining state of the operation instruction circuit 3 is cancelled and the operating program (Step 62) receives from the operation instruction circuit 3 the instruction on the restarting position of the operation which has been saved. Thus, computation is resumed based on the stacked instructions of the second period. Fig. 5 clearly shows that computation is carried out periodically and continuously. In other words, in the example of Fig. 5, the computation of control block No. l to No. 5 as well as an idle operation during the subsequent predetermined time duration is effected consecutively in the normal procedure. In the second period, however, the computation is interrupted temporarily owing to an interrupt for the man-machine interface, such as a request for display of the operational result, and the computation is subsequently resumed from the point where the operation was interrupted. Accordingly, the computation of control block No. 5 is not terminated in the second period. As is apparent from the foregoing description, in this case, an instruction for the position of restarting the computation is set in the operation instruction circuit 3 at the time of starting the third period, the completion instruction of the second period is canceled, and the control blocks of the third period are stacked. Consequently, the computation of the third period is started from the remaining portion of control block No. 5 of the second period. Subsequently, the operations of control blocks Nos. l - 5 of the third period are effected consecutively without carrying out an idle operation. As is apparent from the above, in the present invention, operational periods are fixed by providing a timer circuit, and in a case where the scheduled control operations have been terminated during, for instance, a first period, an idle operation is performed until the ensuring period sets in, thereby making time adjustment. In addition, for instance, in a case where the scheduled control operations have not been completed during, for instance, a first period, the remaining control operations are performed at the beginning of the second period, and the control operations scheduled for the second period are effected continuously without performing an idle operation. Accordingly, since control operations can be carried out continuously during fixed periods, high response characteristics and stable control can be ensured.

Claims: Process control system, comprising a timer circuit (1) for issuing a sequence of signals each starting a period of fixed duration, and a periodic operation selection circuit (2) for selecting, on receipt of a signal from said timer circuit (1), control routines to be executed within each period, characterized by an operation instruction circuit (3) for storing said control routines to be executed followed by an instruction indicating the completion of the computation of all said selected control routines, and for imparting said routines and said completion instruction sequentially to an operation processor (4) which executes said control routines and, on receipt of said completion instruction, executes an idle routine, and an operation completion determining circuit (5) which, on receipt of a signal from said timer circuit (1), (a) determines whether a control routine is currently being executed and if so suspends it in such a way that it may be resumed later, (b) causes said periodic operation selection circuit (2) to select control routines for the ensuing period and to add them to any control routines remaining unexecuted in said operation instruction circuit (3) ahead of the said completion instruction, and (c) either causes said suspended control routine to be resumed or, if there was no interrupted control routine, starts the execution of the first control routine in the operation instruction circuit (3). The system of claim 1 for controlling a steam turbine (25, 27) in an electric power plant, the system being adapted to receive signals of the steam pressure and rotational speed of said steam turbine and an output of a generator (40), and to generate an output signal for adjusting the opening of a valve means (24, 26) for adjusting a steam flow in said steam turbine (25, 27). The system of claim 2, wherein said control routines (30) include a steam turbine speed-load control routine (32), a steam pressure control routine (31), a turbine bypass valve flowrate control routine (34), a routine (35) for controlling the opening of a steam control valve (24) and an intermediate steam stop valve (26), and a turbine bypass valve opening control routine (36). Process control method, comprising issuing a sequence of timing signals each starting a period of fixed duration, and selecting, on receipt of a timing signal, control routines to be executed within each period, characterized by storing said control routines to be executed, followed by an instruction indicating the completion of the computation of all said selected control routines, executing said stored control routines sequentially and, on reaching said completion instruction, executing an idle routine, and, on receipt of a timing signal, (a) determining whether a control routine is currently being executed and if so suspending it in such a way that it may be resumed later, (b) selecting control routines for the ensuing period and adding them to any stored control routines remaining unexecuted ahead of the said completion instruction, and (c) either resuming said suspended control routine or, if there was no interrupted control routine, starting the execution of the first control routine.

Inventors: IIJIMA, TADAHIKO, KAWAI, TAKUMI, TENNICHI, YASUHIRO, YANADA, SADAO
Applicants: HITACHI, LTD.
Application year: 1987
Main classification: G06F9
IPC codes: G05B15, G05B9, G06F9, F01K13