System and method for adaptive control of process

A system and method for the adaptive control of a process in which a feed forward control signal corresponding to a process demand is calculated according to a predetermined algebraic function, while a feedback correction signal is calculated on the basis of an error of a process feedback signal indicative of an error of a controlled variable from a predetermined setting, and the controlled variable of the process is controlled on the basis of the sum of these two signals. The adaptive control is such that, when a set point of the function deviates from the actual process demand, a value corresponding to the error appears in the feedback correction signal, and this value is used for automatically modifying the function itself to ensure the adaptive control of the process. A determination is made whether or not the process is in the steady state and when steady state operation is determined, the function of the feed forward control signal is modified.

BACKGROUND OF THE INVENTION 
This invention relates to a process control system which controls a process 
by the combination of feed forward control and feedback control, and more 
particularly to a process control system suitable for the adaptive control 
of a process whose dynamic characteristics are variable depending on the 
factors such as secular variations. 
A process control method is well known in which a feed forward control 
signal is determined as a function of a demand for a specific process and 
is modified on the basis of a process feedback signal applied from a 
feedback loop of a controlled variable for determining by calculation the 
desired value of the control variable of the process. Such a control 
method is commonly employed for the control of various processes. 
Such a control method is disclosed in, for example, U.S. Pat. No. 
3,894,396. In this U.S. patent, the desired values of flow rates of feed 
water, fuel, air and recirculation gases supplied to a boiler in a heat 
power plant are arithmetically calculated according to individual 
predetermined rates so that they meet the plant load demand. A feed 
forward control signal is produced on the basis of these calculated flow 
rate values to be used for the control of the opening of a feed water 
control valve, a fuel control valve, an air flow control damper and a gas 
recirculation flow control damper which control actually the flow rates of 
feed water, fuel, air and recirculation gases supplied to the boiler. The 
feed forward control signal is modified by individual feed forward 
modifying signals obtained by arithmetic calculations based on a main 
steam pressure error, a main steam temperature error, an O.sub.2 error and 
a gas recirculation flow error, so that the modified feed forward control 
signal can be used to control the opening of the feed water control valve, 
fuel control valve, air flow control damper and gas recirculation flow 
control damper. The rates used for the determination of the feed forward 
control signal are calculated according to algebraic functions which are 
so pre-selected that the main steam pressure error, main steam temperature 
error, O.sub.2 error and gas recirculation flow error are zero in the 
steady state of the process. Therefore, when these functions are always 
appropriate, the feedback correction signals act to absorb solely 
transient variations in the controlled variables so that the entire plant 
can be controlled to operate with a quick response. 
However, the performance of such a power plant varies gradually due to 
deposit of soot in the boiler, contamination of pipe inner walls and other 
secular variations. Thus, even when the functions used for the arithmetic 
calculation of the feed forward control signal were originally properly 
set, these functions would become unfit for the purpose of control of the 
controlled variables after an extended period of continuous operation, and 
the resultant errors would be absorbed in the feedback correction signals. 
Since the feedback control system would not respond until an error appears 
in the parameter of one or more of the controlled variables, the process 
control with the quick response as in the initial stage of plant operation 
would become impossible in such a situation. 
In order to prevent the undesirable reduction in the response of the 
process control system due to such secular variations of the 
characteristics of the controlled variables, it has been a common practice 
to periodically re-set the functions used for the determination of the 
feed forward control signal. However, this re-setting has required a great 
deal of costs and labors. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an improved process 
control method and system according to which the functions used for the 
determination of the feed forward control signal generated from the feed 
forward control system can be automatically modified so that the process 
can be continuously controlled to make a quick response to variations. 
Another object of the present invention is to provide a process control 
method and system according to which the functions used for the 
determination of the feed forward control signal generated from the feed 
forward control system can be modified at a proper time. 
Still another object of the present invention is to provide a process 
control method and system according to which the functions used for the 
determination of the feed forward control signal can be properly modified 
during progression of the process even when the initial set points of the 
functions are not appropriate. 
It is a first feature of the control method and system according to the 
present invention that whether the process is in its steady state or not 
is detected, and when the process is proved to be maintained in the steady 
state, the feedback correction signals appearing from the feedback 
correction system in the steady state of the process are successively 
transferred to the feed forward control system so as to modify the 
functions used for the determination of the feed forward control signal. 
Thus, the feed forward control system which generates the feed forward 
control signal determines or modifies the functions in quick response to 
variations of the process demand so that any appreciable error inputs may 
not be applied to the feedback correction system. Therefore, the response 
of the feedback correction system itself can be improved, and the response 
of the entire control system would not be reduced regardless of secular 
variations in the characteristics of the controlled variables in the 
process. 
It is a second feature of the control method and system according to the 
present invention that, when variance of any one of the process parameters 
is less than a predetermined reference value within a predetermined period 
of time, the process is judged to be maintained in its steady state, and 
the functions used for the determination of the feed forward control 
signal generated from the feed forward control system are modified only 
when the process is proved to be in its steady state. 
It is a third feature of the control method and system according to the 
present invention that, among the set points of any one of the functions 
used for arithmetically calculating the feed forward control signal on the 
basis of the value of the process demand, those corresponding to the 
demand values close to an instantaneous value of the process demand are 
only modified or corrected when modification or correction is required.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a block diagram showing connections between controlled units and 
control units in an embodiment of the present invention which is applied 
to a control system for a heat power plant. 
Referring to FIG. 1, an electric generator 3 is mechanically connected to a 
high pressure turbine 1 and a low pressure turbine 2 to be driven by these 
turbines. Steam discharged from the low pressure turbine 2 is condensed in 
a condenser 4. A feed water pump 5 driven by a feed pump drive turbine 7 
pumps the condensate from the condenser 4 to supply it as feed water to a 
boiler. The flow rate of boiler feed water is controlled depending on the 
opening of a steam flow control valve 8 which controls the flow rate of 
steam supplied to the feed pump drive turbine 7. The feed water supplied 
from the feed water pump 5 is heated in a pre-heater 9 by extraction steam 
extracted from the low pressure turbine 2 and is then further heated in an 
economizer 10. The feed water is then turned into steam by a water wall 11 
of the boiler. The steam, from which non-vaporized water is separated in a 
water separator 12, is further heated in a primary superheater 13 and 
passes through an attemperator 14 into a secondary superheater 16 to be 
further heated therein. The main steam thus obtained is admitted into the 
high pressure turbine 1 through a turbine control valve 17, and the steam 
having been used for driving the high pressure turbine 1 is heated again 
in a reheater 18 to provide reheat steam which drives the low pressure 
turbine 2. A fuel feed pump 20 supplies fuel to the boiler, and a fuel 
flow control valve 19 controls the flow rate of fuel supplied from the 
pump 20 to the boiler. The flow rate of spray supplied into the 
attemperator 14 is controlled by a spray flow control valve 15. 
Air for combustion is supplied to the boiler by an air fan 21, and the flow 
rate of air supplied from the fan 21 to the boiler is controlled by an air 
flow control damper 22. A portion of combustion gases having successively 
passed through the water wall 11, secondary superheater 16, reheater 18, 
primary superheater 13 and economizer 10 is supplied to the boiler again 
by a gas recirculating fan 23, and its flow rate is controlled by a gas 
recirculation flow control damper 24. 
A pressure detector 101 detects the pressure of main steam and generates an 
output signal indicative of the main steam pressure. A temperature 
detector 102 detects the temperature of main steam and generates an output 
signal indicative of the main steam temperature. An O.sub.2 detector 103 
detects the concentration of O.sub.2 in gases discharged from the boiler 
and generates an output signal indicative of the concentration of O.sub.2. 
Another temperature detector 104 detects the temperature of reheat steam 
and generates an output signal indicative of the reheat steam temperature. 
The output signals of these detectors 101 to 104 are applied to a master 
controller 100. The master controller 100 produces a plant demand signal 
applied to various sub-loop controllers 40, 50, 60, 70, 80 and 90 on the 
basis of a final load demand issued from an operator console 30. 
The sub-loop controllers 40, 50, 60, 70, 80 and 90 receive output signals 
of a generator output detector 401, a feed water flow rate detector 501, a 
primary superheater outlet temperature detector 601, a fuel flow rate 
detector 701, an air flow rate detector 801 and a gas recirculation flow 
rate detector 901 respectively so as to control the individual sub-loops 
on the basis of these inputs. 
The master controller 100 and the sub-loop controllers 40, 50, 60, 70, 80 
and 90 are each in the form of a digital controller and are connected to 
buses 200 respectively. 
FIG. 2 is a block diagram or a control flow diagram showing the control 
functions of the individual controllers shown in FIG. 1. 
An economic load dispatch signal (ELD) generated from a load dispatch 
station (not shown) is applied to the operator console 30 which comprises 
an auto/hand switch 31, an Ld setter 32, a transmitter 33 and a steady 
state detector 34. The Ld setter 32 includes therein a memory which stores 
the value of ELD which is variable. The memory holds the ELD value 
supplied in the "hand" position of the auto/hand switch 31, and the ELD 
value stored in this memory can be increased or decreased as desired by 
manipulation by the operator. 
The ELD signal is transmitted from the transmitter 33 to the master 
controller 100 as a final load demand (FLd) when the auto/hand switch 31 
is in its "auto" position. On the other hand, the value of ELD stored in 
the memory in the Ld setter 32 is supplied to the master controller 100 as 
the final load demand (Fld) when the switch 31 is in its "hand" position. 
The signals indicative of the main steam pressure (MSP), main steam 
temperature (MST), O.sub.2 concentration (O.sub.2) and reheat steam 
temperature (RST) are also applied from the respective detectors 101 to 
104 to the master controller 100. The inputs described hereinbefore are 
the same as those in a prior art control system for such a heat power 
plant. The present invention differs from the prior art control system in 
that a steady state signal generated from the steady state detector 34 is 
additionally applied to the master controller 100, so that the algebraic 
functions stored in the master controller 100 can be modified depending on 
the level of the steady state signal. 
FIG. 3 shows the circuit structure of the steady state detector 34. An 
output signal of "1" level appears from an AND gate 341 when the "hand" 
position of the auto/hand switch 31 is selected to apply an "H" select 
signal to the AND gate 341, and when no change occurs in the Ld set signal 
applied from the Ld setter 32 to the AND gate 341 through an inverter 342. 
When this AND gate output signal of "1" level lasts over more than a 
predetermined length of time, this is detected by a timer composed of an 
integrator 343 and a comparator 345, and the steady state signal of "1" 
level appears from the steady state detector 34. An inverter 344 is 
connected between the AND gate 341 and the reset terminal of the 
integrator 343. When the operator selects the "auto" position of the 
auto/hand switch 31 or when the Ld set signal changes due to manipulation 
by the operator, the integrator 343 is immediately reset, and the steady 
state signal turns into its "0" level from the "1" level. 
An Ld set system 110 in the master controller 100 shown in FIG. 2 acts to 
limit the rate of change of FLd thereby producing an instantaneous load 
demand Ld. Numerals 121, 122, 123 and 124 in FIG. 2 designate the set 
points of MSP, MST, O.sub.2 and RST respectively. Subtractors 131, 132, 
133 and 134 subtract the detected values of MSP, MST, O.sub.2 and RST from 
their set points to provide output signals corresponding to the main steam 
pressure error (MSPe), main steam temperature error (MSTe), O.sub.e error 
(O.sub.2 e) and reheat steam temperature error (RSTe) respectively. A 
first proportional adjuster 141 carries out a proportional operation on 
the error signal indicative of MSPe, while a first integrating adjuster 
151 carries out an integrating operation on the output signal of the 
proportional adjuster 141, and the sum of the output signals of these 
adjusters 141 and 151 provides a feedback correction signal for correcting 
the boiler input demand (BId). A first value determining means 171 
determines the then existing value 175 of BId by introducing the value of 
Ld into a first algebraic function f.sub.1 (x). The set point of the first 
algebraic function f.sub.1 (x) used for the determination of the then 
existing value of BId is modified by the function of a first adapter 161 
only when the plant or process is in its steady state. A first adder 181 
provides the sum of the then existing value of BId and the BId feedback 
correction signal to modify the boiler input demand (BId). 
A second proportional adjuster 142 carries out a proportional operation on 
the error signal indicative of MSTe, while a second integrating adjuster 
152 carries out an integrating operation on the output signal of the 
proportional adjuster 142, and the sum of the output signals of these 
adjusters 142 and 152 provides a feedback correction signal for correcting 
the firing ratio demand (FRd). A second value determining means 172 
determines the then existing value 176 of FRd by introducing the value of 
BId into a second algebraic function f.sub.2 (x). A second adder 182 
provides the sum of the then existing value 176 of FRd and the FRd 
feedback correction signal to modify the firing ratio demand (FRd). 
Similarly, the sum of output signals of a third proportional adjuster 143 
and a third integrating adjuster 153 making individual operations on the 
error signal indicative of O.sub.2 e and the output signal of the adjuster 
143 provides a feedback correction signals for correcting the air flow 
demand (AFd), and the then existing value 177 of AFd determined by a third 
value determining means 173 is added in a third adder 183 to the AFd 
feedback correction signal to modify the air flow demand (AFd). Similarly, 
the sum of output signals of a fourth proportional adjuster 144 and a 
fourth integrating adjuster 154 making individual operations on the error 
signal indicative of RSTe and the output signal of the adjuster 144 
provides a feedback correction signal for correcting the gas recirculation 
flow demand (GRFd), and the then existing value 178 of GRFd determined by 
a fourth value determining means 174 is added in a fourth adder 184 to the 
GRFd feedback correction signal to modify the gas recirculation flow 
demand (GRFd). 
The set point of the second algebraic function f.sub.2 (x) used for the 
determination of the then existing value of FRd is also modified by the 
function of a second adapter 162. Similarly, the set point of a third 
algebraic function f.sub.3 (x) used for the determination of the then 
existing value of AFd and the set point of a fourth algebraic function 
f.sub.4 (x) used for the determination of the then existing value of GRFd 
are also modified by the functions of a third adapter 163 and a fourth 
adapter 164 respectively. 
Referring to FIG. 2, the signal representative of Ld appearing from the Ld 
set system 110 in the master controller 100 is applied together with the 
output signal of the generator output detector 401 to the sub-loop 
controller 40 which makes a proportional plus integral (PI) operation on 
the error between these two inputs and provides an output signal which 
controls the opening of the turbine control valve 17. Similarly, the 
sub-loop controllers 50, 70, 80 and 90 make proportional plus integral 
(PI) operations on the error between the BId signal and the output signal 
of the feed water flow rate detector 501, the error between the FRd signal 
and the output signal of the fuel flow rate detector 701, the error 
between the AFd signal and the output signal of the air flow rate detector 
801, and the error between the GRFd signal and the output signal of the 
gas recirculation flow rate detector 901, respectively. The output signals 
of these sub-loop controllers 50, 70, 80 and 90 control the feed water 
control valve 8, fuel control valve 19, air flow control damper 22 and gas 
recirculation flow control damper 24 respectively. The BId signal and the 
MSTe signal are applied together with the output signal of the primary 
superheater outlet temperature detector 601 to the sub-loop controller 60. 
In this sub-loop controller 60, the then existing value of the opening of 
the spray valve 15 obtained by introducing the value of BId into a fifth 
algebraic function f.sub. 5 (x) representing the static balance between 
the flow rate of feed water and the opening of the spray valve 15, the 
corrected value of MSTe and the then existing value of SHT corrected on 
the basis of the error between the SHT set point and the output signal of 
the primary superheater outlet temperature detector 601 are summed, and 
the sum is applied to a proportional adjuster to provide an output signal 
which controls the opening of the spray valve 15. 
FIGS. 4A and 4B are flow charts showing the sequence of processing in the 
master controller 100 shown in FIGS. 1 and 2. 
In step B#1 shown in FIG. 4A, the applied inputs including the MSP signal, 
MST signal, O.sub.2 signal, RST signal, FLd signal and steady state signal 
are stored to be read. 
In step B#2, the blocks 131, 132, 133 and 134 shown in FIG. 2 make their 
subtracting operation. That is, the values of MSP, MST, O.sub.2 and RST 
and the MSP set point, MST set point, O.sub.2 set point and RST set point 
are read out, and the former are subtracted from the latter to calculate 
MSPe, MSTe, O.sub.2 e and RSTe respectively. 
In step B#3, the Ld set system 110 shown in FIG. 2 performs its function. 
That is, the presence of a change in the value of Fld is checked to 
determine the load demand Ld. The detail of this step B#3 is shown in FIG. 
5. 
In step B#30 shown in FIG. 5, the value of FLd and the value of the pre-set 
load change rate limit .DELTA.Ld/.DELTA.t are read. In step B#31, the 
length of time .DELTA.t lapsed after the previous determination of Ld 
(which time .DELTA.t is equal to the control period of the master 
controller 100) is multiplied by the load change rate limit 
.DELTA.Ld/.DELTA.t to obtain the product 
.epsilon.=(.DELTA.Ld/.DELTA.t).times..DELTA.t. In steps B#32 and B#33, the 
difference between the value of FLd and the previously determined value of 
Ld is compared with the value of .epsilon.. When FLd-Ld.gtoreq..epsilon., 
(Ld+.epsilon.) is selected as a new value of Ld in step B#34, while when 
FLd-Ld.ltoreq.-.epsilon., (Ld-.epsilon.) is selected as a new value of Ld 
in step B#36. When neither of the above relations holds, the value of FLd 
is selected as a new value of Ld. 
In step B#4 shown in FIG. 4A, the blocks 141 and 151 shown in FIG. 2 make 
their individual operations for calculating the BId feedback correction 
signal on the basis of the error MSPe. More precisely, a proportional 
signal P.sub.i and an integral signal I.sub.i are calculated according to 
the following equations (1) and (2) respectively: 
EQU P.sub.i =K.sub.1 .multidot.MSPe (1) 
EQU I.sub.i =I.sub.i-1 +K.sub.2 P.sub.i (2) 
where K.sub.1 and K.sub.2 are predetermined coefficients, and I.sub.i-1 is 
an integral signal previously calculated or obtained. Thus, the integral 
signal I.sub.i represents the sum of the previously calculated integral 
signal I.sub.i-1 and the product obtained by multiplying the proportional 
signal P.sub.i by the coefficient K.sub.2. 
In steps B#5, B#6 and B#7, the block 161 shown in FIG. 2 executes its 
function. More precisely, in step B#5, whether the steady state signal 
appearing from the steady state detector 34 is in its "1" level or "0" 
level is checked. When it is proved that the steady state signal is in its 
"1" level, the function f.sub.1 (x) is modified in step B#6, while when it 
is proved that the steady state signal is in its "0" level, a jump to step 
B#8 occurs. 
FIG. 6 is a characteristic diagram to illustrate how the function f.sub.1 
(x) is modified in step B#6. FIG. 7 is a flow chart showing in detail the 
sequence of processing in step B#6. 
Referring to FIG. 6, the function f.sub.1 (x) representing the static 
balance between the load and the flow rate of feed water is initially set 
by the solid curve of broken pattern. Actually, a plurality of values 
f.sub.1 (1), f.sub.1 (2), . . . , f.sub.1 (j), . . . , f.sub.1 (j.sub.max) 
of the set point corresponding to a plurality of values Ld(1), Ld(2), . . 
. , Ld(j), . . . , Ld(j.sub.max) of the load demand Ld are stored in the 
memory. 
In step B#61 shown in FIG. 7, the value of Ld determined in step B#3 is 
read. Then, steps B#62, B#63, B#64 and B#65 judge the location of the 
value of the instantaneous load demand Ld between the plural values Ld(1), 
Ld(2), . . . , Ld(j), . . . , Ld(j.sub.max) of the load demand Ld. When it 
is proved that the value of the instantaneous load demand Ld lies within 
the range Ld(j).ltoreq.Ld&lt;Ld(j+1) in step B#63, the set point values 
f.sub.1 (j) and f.sub.1 (j+1) are modified in steps B#66, B#67 and B#68 so 
as to renew or modify the function f.sub.1 (x). 
This manner of modification of the function f.sub.1 (x) is based upon the 
concept described presently. According to the function f.sub.1 (x) before 
being modified or adapted, the then existing value of the boiler input 
demand f.sub.1 (Ld) corresponding to the load demand Ld is given by the 
following equation (3): 
##EQU1## 
where Ld(j).ltoreq.Ld&lt;Ld(j+1). 
Suppose that f*(j) and f*(j+1) are the ideal values of the set point values 
f.sub.1 (j) and f.sub.1 (j+1) corresponding to the load demand values 
Ld(j) and Ld(j+1) respectively. Then, the ideal value f.sub.1 *(Ld) of the 
then existing value of the boiler input demand BId corresponding to the 
load demand Ld is expressed by the following equation (4): 
##EQU2## 
Subtraction of the equation (3) from the equation (4) provides the 
following equation (5): 
##EQU3## 
When {f.sub.1 *()-f.sub.1 ()} is the equation (5) is represented by 
{f.sub.1 *()-f.sub.1 ()}=.DELTA.f.sub.1 (), and the equation (5) is so 
transformed, the following equation (6) is obtained: 
##EQU4## 
This .DELTA.f.sub.1 (Ld) is the steady state error of the then existing 
value of the boiler input demand BId due to secular variations, and this 
error is considered to be equivalent to the value of the integral signal 
I.sub.i included in the BId feedback correction signal. Therefore, this 
integral signal I.sub.i can be used for modifying the function f.sub.1 
(x). Suppose that the two set point values f.sub.1 (j) and f.sub.1 (j+1) 
among the set point values of the function f.sub.1 (x) include errors 
whose ratio is represented by the following equation (7): 
##EQU5## 
Introduction of the equation (7) into the equation (6) to find the values 
.DELTA.f.sub.1 (j) and .DELTA.f.sub.1 (j+1) provides the following 
equations (8) and (9): 
##EQU6## 
Theoretically, an ideal function can be obtained by a single modification 
when the errors given by the equations (8) and (9) are directly added to 
the non-modified set point values f.sub.1 (j) and f.sub.1 (j+1) 
respectively to provide the set points of the new function f.sub.1 (x). 
However, in a practical application to the heat power plant, an excessive 
modification may be given rise to due to the factors including plant 
noise. Thus, in the present embodiment, the set point values f.sub.1 (j) 
and f.sub.1 (j+1) are gradually modified according to the following 
equations (10) and (11) respectively: 
EQU f.sub.1 (j)=f.sub.1 (j)+K.sub.3 .multidot..DELTA.f.sub.1 (j) (10) 
EQU f.sub.1 (j+1)=f.sub.1 (j+1)+K.sub.3 .multidot..DELTA.f.sub.1 (j+1) (11) 
where K.sub.3 is a coefficient satisfying the relation 0&lt;K.sub.3 &lt;1. 
The dotted curve shown in FIG. 6 represents the new function f.sub.1 (x) 
obtained by modifying the set point values f.sub.1 (j) and f.sub.1 (j+1) 
according to the equations (10) and (11). It will be seen in FIG. 6 that 
the amount K.sub.3 .multidot..DELTA.f.sub.1 (j) added to modify the set 
point f.sub.1 (j) and the amount K.sub.3 .multidot..DELTA.f.sub.1 (j+1) 
added to modify the set point f.sub.1 (j+1) are respectively inversely 
proportional to the difference between the instantaneous load demand 
values Ld and Ld(j) and to the difference between the instantaneous load 
demand values Ld and Ld(j+1). In the embodiment of the present invention, 
all of the set points f.sub.1 (1), . . . , f.sub.1 (j), . . . , f.sub.1 
(j.sub.max) of the function f.sub.1 (x) are not modified, but the set 
points f.sub.1 (j) and f.sub.1 (j+1) corresponding respectively to the 
load demands Ld(j) and Ld(j+1) close to the actual instantaneous load 
demand Ld are only modified. Thus, the function f.sub.1 (x) can be finely 
modified, and even when the function f.sub.1 (x) pre-set before the 
initiation of the plant operation is inadequate, it can be precisely 
modified during the continuous plant operation. 
The explanation of the sequence of modification of the set points of the 
function f.sub.1 (x) in the embodiment of the present invention will be 
continued with reference to FIG. 7 again. In step B#66, the integral 
signal I.sub.i included in the BId feedback correction signal is read. In 
step B#67, the values of .DELTA.f.sub.1 (j) and .DELTA.f.sub.1 (j+1) used 
for the ideal modification of the set points f.sub.1 (j) and f.sub.1 (j+1) 
are arithmetically calculated according to the equations (8) and (9) 
respectively. In step B#68, the set points f.sub.1 (j) and f.sub.1 (j+1) 
are modified according to the equations (10) and (11) respectively. 
Referring to FIG. 4A again, the integral signal I.sub.i in the BId feedback 
correction signal is modified in step B#7. As described hereinbefore, the 
set points of the function f.sub.1 (x) are modified in such a manner as to 
minimize the value of the BId feedback correction signal. Thus, in step 
B#7, the integral signal I.sub.i is modified according to the following 
equation (12): 
EQU I.sub.i =(1-K.sub.3)I.sub.i (12) 
In step B#8 shown in FIG. 4A, the block 171 shown in FIG. 2 executes its 
function for determining the then existing value f.sub.1 (Ld) of BId on 
the basis of the function f.sub.1 (x) and the instantaneous load demand 
Ld. More precisely, this f.sub.1 (Ld) is arithmetically calculated 
according to the following equation (13): 
##EQU7## 
The meaning of this equation (13) is that linear interpolation is applied 
to the set points f.sub.1 (j) and f.sub.1 (j+1) of the function f.sub.1 
(x) so as to calculate the then existing value f.sub.1 (Ld) of BId 
corresponding to the instantaneous load demand Ld. 
In step B#9 shown in FIG. 4A, the function of the block 181 shown in FIG. 2 
is executed. That is, in this step, the then existing value of BId 
obtained in step B#7 is added to the BId feedback correction signal to 
arithmetically calculate the boiler input demand BId according to the 
following equation (14): 
EQU BId=f.sub.1 (Ld)+P.sub.i +I.sub.i (14) 
In step B#10 shown in FIG. 4A, the blocks 142 and 152 execute thier 
functions to arithmetically calculate the FRd feedback correction signal 
on the basis of the value of MSTe. 
Steps B#11, B#12 and B#13 shown in FIG. 4A execute the function of the 
block 162 shown in FIG. 2. In step B#11, judgement is made as to whether 
the steady state signal is in its "1" level or not. When it is proved that 
the steady state signal is in its "1" level in step B#11, the set point of 
the function f.sub.2 (x) representing the static balance between the flow 
rate of feed water and the flow rate of fuel is modified in step B#12, and 
then, the integral signal I.sub.i in the FRd feedback correction signal 
calculated in step B#10 is modified in step B#13. The practical manner of 
this modification is entirely similar to that described in detail with 
reference to the steps B#6 and B#7, and its detailed description is 
therefore unnecessary. 
In step B#14, the block 172 shown in FIG. 2 executes its function to 
arithmetically calculate f.sub.2 (BId), which is the then existing value 
of FRd, on the basis of the value of BId obtained in step B#9 and the 
function f.sub.2 (x) modified in step B#12. The manner of this calculation 
is also similar to that described with reference to step B#8. 
In step B#15, the block 182 shown in FIG. 2 executes its function. That is, 
f.sub.2 (BId) which is the then existing value of FRd is added to the FRd 
feedback correction signal to obtain the value of FRd. 
In step B#16, the blocks 143 and 153 shown in FIG. 2 execute their 
functions. In steps B#17, B#18 and B#19, the block 163 shown in FIG. 2 
executes its function. In step B#20, the block 173 shown in FIG. 2 
executes its function, and in step B#21, the block 183 shown in FIG. 2 
executes its function. In these steps, the then existing value of the air 
flow demand AFd is arithmetically calculated on the basis of the value of 
FRd obtained in step B#15 and the function f.sub.3 (x) representing the 
static balance between the flow rate of fuel and the flow rate of air, and 
the AFd feedback correction signal is arithmetically calculated on the 
basis of the value of O.sub.2 e, so that the value of AFd is 
arithmetically calculated as the sum of the then existing value of AFd and 
the value of the AFd feedback correction signal. The manner of 
modification of the set point of the function f.sub.3 (x) in step B#18 and 
the manner of modification of the integral signal I.sub.i in the AFd 
feedback correction signal in step B#19 are also entirely similar to those 
described with reference to steps B#6 and B#7. 
Referring to FIG. 4B, the blocks 144 and 154 shown in FIG. 2 execute their 
functions in step B#22 and the block 164 shown in FIG. 2 executes its 
function in steps B#23, B#24 and B#25. Then, the block 174 shown in FIG. 2 
executes its function in step B#26, and the block 184 shown in FIG. 2 
executes its function in step B#27. In these steps, the value of f.sub.4 
(FRd), which is the then existing value of the gas recirculation flow 
demand GFRd, is arithmetically calculated on the basis of the value of FRd 
and the function f.sub.4 (x) representing the static balance between the 
flow rate of fuel and the flow rate of recirculated gases, and the GRFd 
feedback correction signal is arithmetically calculated on the basis of 
the value of RSTe, so that the value of GRFd is arithmetically calculated 
as the sum of the then existing value of f.sub.4 (FRd) and the value of 
the GRFd feedback correction signal. The manner of modification of the set 
point of the function f.sub.4 (x) in step B#24 and the manner of 
modification of the integral signal I.sub.i in the GRFd feedback 
correction signal in step B#25 are also entirely similar to those 
described with reference to steps B#6 and B#7. 
In the embodiment described hereinbefore, the steady state signal of "1" 
level appears from the steady state detector 34 when the auto/hand switch 
31 associated with the Ld setter 32 setting the load demand Ld is changed 
over to the "hand" position, and when the process is proved to be 
maintained in its steady state because of no change in the Ld set signal 
over more than a predetermined length of time. When the steady state 
signal of "1" level appears, the set points of the functions for 
determining the feed forward control signal are modified in the 
aforementioned embodiment. However, the manner of judgment of the steady 
state of the process is in no way limited to that described hereinbefore. 
For example, the process may be judged to be maintained in its steady state 
when the value of the final load demand FLd for the process does not 
change over more than a predetermined length of time. That is, in the case 
of the aforementioned embodiment applied to the control system for the 
heat power plant, the process is proved to be maintained in its steady 
state when the value of FLd does not change over more than a predetermined 
period of time regardless of whether the auto/hand switch 31 associated 
with the Ld setter 32 setting the load demand Ld is in its "hand" position 
or not. 
When a more accurate judgment of the steady state is desired to more 
accurately modify the set points of the functions used for the 
determination of the feed forward control signal, the variance of any one 
of the process parameters within a predetermined period of time may be 
arithmetically calculated, and the process may be judged to be maintained 
in the steady state when the value of the variance is less than a 
predetermined reference value. 
FIG. 8 shows a modification of the aforementioned embodiment of the present 
invention applied to the control system for the heat power plant. In the 
modification shown in FIG. 8, the variance, within a predetermined period 
of time, of each of the errors calculated from the detected values of the 
main steam pressure MSP, main steam temperature MST, O.sub.2 concentration 
O.sub.2 and reheat steam temperature RST is detected, and the variance, 
within the predetermined period of time, of each of the values of the 
demands used as the reference values for the calculation of the then 
existing values of the demands is also detected, so that, when both the 
variance of the former and the variance of the latter are less than their 
predetermined reference values, the individual sub-loop control systems 
are proved to be maintained in the steady state, and the set points of the 
functions are then modified. 
Referring to FIG. 8, a first steady state detector 191 detects whether the 
variance of each of MSPe and Ld is less than a reference value, and a 
second steady state detector 192 detects whether the variance of each of 
MSTe and BId is less than a reference value. A third steady state detector 
193 detects whether the variance of each of O.sub.2 e and FRd is less than 
a reference value, and a fourth steady state detector 194 detects whether 
the variance of each of RSTe and FRd is less than a reference value. 
Other numerals designate the same parts as those appearing in FIG. 2 since 
FIG. 8 is a modification of FIG. 2, and in FIG. 8, the steady state 
detector 34 shown in FIG. 2 is eliminated. 
FIG. 9 shows the sequence of processing in the first steady state detector 
191 shown in FIG. 8. In a series of steps B#111 to B#115, judgment is made 
as to whether n sampled data used for the calculation of the variance have 
already been obtained. More precisely, in step B#112, k=1 is set in a 
counter when the data sampling is proved to be initial sampling in step 
B#111. Step B#113 is a counting-up step to set k=k+1 in the counter. The 
number n of samples is determined by the process dynamics. 
In a series of steps B#116 to B#118, the previously sampled and stored data 
are renewed. In step B#119, new data of Ld and MSPe are read. In step 
B#120, judgment is made as to whether n sampled data have been read. When 
the judgment in step B#120 proves that k=n, the mean values of Ld and MSPe 
are arithmetically calculated in step B#121. When, on the other hand, it 
is proved that the n sampled data have not yet been read, the steady state 
signal is turned into its "0" level in step B#126. In step B#122, the 
variance .sigma..sub.L.sup.2 of the n sampled data of Ld and the variance 
.sigma..sub.e.sup.2 of the n sampled data of MSPe are arithmetically 
calculated on the basis of the means values Ld and MSPe of Ld and MSPe 
calculated in step B#121 and the n sampled data Ld(i) and MSPe(i). In step 
B#123, judgment is made as to whether the variance .sigma..sub.L.sup.2 
calculated in step B#122 is less than a predetermined reference value 
.epsilon..sub.L or not. In step B#124, judgment is made as to whether the 
variance .sigma..sub.e.sup.2 calculated in step B#122 is less than a 
predetermined reference value .epsilon..sub.e or not. When both of 
.sigma..sub.L.sup.2 and .sigma..sub.e.sup.2 are proved to be less than 
their reference values .epsilon..sub.L and .epsilon..sub.e respectively, 
the steady state signal of "1" level appears from the steady state 
detector 191 in step B#125 since the process is in the steady state in 
such a case. On the other hand, when one of .sigma..sub.L.sup.e and 
.sigma..sub.e.sup.2 is more than its reference value, the steady state 
signal is turned into its "0" level. 
The overall sequence of processing in the master controller 100 in the 
embodiment shown in FIG. 8 differs from the control flow shown in FIG. 2 
in the points described presently. In the first place, the steps of 
arithmetic calculation carried out by the steady state detector 191 are 
interposed between the steps B#4 and B#5 shown in FIG. 4A. In the second 
place, the steps of arithmetic calculation carried out by the steady state 
detector 192 are interposed between the steps B#10 and B#11 shown in FIG. 
4A. In the third place, the steps of arithmetic calculation carried out by 
the steady state detector 193 are interposed between the steps B#16 and 
B#17 shown in FIG. 4A. In the fourth place, the steps of arithmetic 
calculation carried out by the steady state detector 194 are interposed 
between the steps B#22 and B#23 shown in FIG. 4B. The arithmetic 
calculations carried out by these steady state detectors 192, 193 and 194 
are essentially the same as that carried out by the steady state detector 
191 except that different data are processed for the calculation of the 
variance. Therefore, any detailed description of the sequence of 
arithmetic calculations in these detectors is unnecessary. 
In the step B#6 in FIG. 4A, the function f.sub.1 (x) has been modified on 
the basis of the then existing value of the load demand Ld. In the 
embodiment shown in FIG. 8, however, the function f.sub.1 (x) is modified 
on the basis of the mean value Ld of Ld calculated in the step #121 in 
FIG. 9. That is, the mean value Ld is read instead of reading the value of 
Ld as in the step B#61 shown in FIG. 7, and in lieu of the equations (8) 
and (9) used for the arithmetic calculations of the ideal modifying values 
in the step B#67 shown in FIG. 7, the following equations (15) and (16) 
are used: 
##EQU8## 
According to the embodiment described with reference to FIGS. 8 and 9, the 
variance of the process parameter is arithmetically calculated for each of 
the control loops or blocks arithmetically calculating the demand signals 
such as the BId, FRd, AFd and GRFd signals so as to judge whether the 
process is in the steady state or not, and only when the process is proved 
to be maintained in the steady state, the functions used for the 
arithmetic calculations of the then existing values of these demand 
signals are modified. Therefore, the function modification timing can be 
accurately detected, and the functions can be modified more quickly than 
in the embodiment described with reference to FIGS. 2 to 7.