U.S. patent application number 10/399374 was filed with the patent office on 2007-04-12 for method of and system for controlling the ratio of a variable lead parameter and an adjustable lag parameter for a lag-lead process.
This patent application is currently assigned to Coventry University. Invention is credited to Keith J. Burnham, Marcus Paul Grant.
Application Number | 20070082304 10/399374 |
Document ID | / |
Family ID | 9901301 |
Filed Date | 2007-04-12 |
United States Patent
Application |
20070082304 |
Kind Code |
A1 |
Burnham; Keith J. ; et
al. |
April 12, 2007 |
Method of and system for controlling the ratio of a variable lead
parameter and an adjustable lag parameter for a lag-lead
process
Abstract
In a method of and system for controlling the air/gas ratio in a
lag-lead combustion plant the lead and lag parameters are monitored
to provide lead and lag signals representative of the values of the
parameters. These are compared to provide an error signal
representative of the deviation of the ratio of the lead and lag
parameters from a preselected ratio. The lag parameter is then
adjusted to reduce the deviation in response to the deviation
exceeding a preselected deviation.
Inventors: |
Burnham; Keith J.;
(Coventry, GB) ; Grant; Marcus Paul; (Coventry,
GB) |
Correspondence
Address: |
FULWIDER PATTON LLP
HOWARD HUGHES CENTER
6060 CENTER DRIVE, TENTH FLOOR
LOS ANGELES
CA
90045
US
|
Assignee: |
Coventry University
Priority Street
Conventry
GB
CV1 5FB
|
Family ID: |
9901301 |
Appl. No.: |
10/399374 |
Filed: |
October 15, 2001 |
PCT Filed: |
October 15, 2001 |
PCT NO: |
PCT/GB01/04586 |
371 Date: |
December 5, 2005 |
Current U.S.
Class: |
431/1 |
Current CPC
Class: |
F23N 5/022 20130101;
F23N 2235/06 20200101; Y02T 10/30 20130101; F02D 19/023 20130101;
F23N 2005/185 20130101; F23N 1/022 20130101; F23N 2005/181
20130101; G05D 11/131 20130101; F23N 2235/16 20200101; F02D 19/027
20130101; F23N 2225/16 20200101 |
Class at
Publication: |
431/001 |
International
Class: |
F23C 15/00 20060101
F23C015/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 14, 2000 |
GB |
0025231.2 |
Claims
1. A method of controlling the ratio of a variable lead parameter
and an adjustable lag parameter for a lag-lead process, the method
comprising: monitoring said lead parameter and providing a lead
signal representative of the value of said lead parameter;
monitoring said lag parameter and providing a lag signal
representative of the value of said lag parameter; comparing said
lead and lag signals and providing an error signal representative
of the deviation of the ratio of said lead and lag parameters from
a preselected ratio; and adjusting said lag parameter to reduce
said deviation in response to said deviation exceeding a
preselected deviation.
2. A method as claimed in claim 1 comprising comparing said error
signal with a preselected threshold value and adjusting said lag
parameter in response to said error signal exceeding said
preselected threshold value.
3. A method as claimed in claim 1 comprising comparing said error
signal with an error range defined by a first, upper preselected
threshold value and a second, lower preselected threshold value and
adjusting said lag parameter in response to said error signal
falling outside said error range.
4. A method as claimed in claim 3 comprising: adjusting said lag
parameter to reduce said ratio in response to said error signal
being above said error range; and adjusting said lag parameter to
increase said ratio in response to said error signal being below
said error range.
5. A method as claimed in claim 4 wherein: said first, upper
preselected threshold value is a positive value and said second,
lower preselected threshold value is a negative value; a positive
error signal indicates an increase in said ratio from said
preselected ratio and a negative error signal indicates a decrease
in said ratio from said preselected ratio; and the method
comprises: adjusting said lag parameter to reduce said ratio in
response to said error signal being positive and exceeding said
first, upper preselected threshold value; and adjusting said lag
parameter to increase said ratio in response to said error signal
being negative and exceeding said second, lower preselected
threshold value.
6. A method as claimed in claim 2 wherein the or each said
threshold value is a fixed value.
7. A method as claimed in claim 2 wherein the or each said
threshold value is a function of the value of said lag
parameter.
8. A method as claimed in claim 7 wherein the step of comparing
said error signal with the or each threshold value comprises
adjusting said threshold value in dependence on the value of said
lag parameter and comparing said error signal with said adjusted
threshold level.
9. A method as claimed in claim 8 comprising: providing a look up
table for storing a plurality of threshold values; and the step of
adjusting said threshold value comprising selecting one of said
threshold values in dependence on the value of said lag
parameter.
10. A method as claimed in claim 7 wherein: the or each threshold
value comprises a plurality of threshold levels; and the step of
comparing said error signal with the or each threshold value
comprises selecting a threshold level for said threshold value in
dependence on the value of said lag parameter and comparing said
error signal with said selected threshold level.
11. A method as claimed in claim 10 comprising: providing a look up
table for storing said plurality of threshold levels; and the step
of adjusting said threshold value comprising selecting one of said
threshold levels in dependence on the value of said lag
parameter.
12. A method as claimed in claim 1 wherein: said lead and lag
parameters are lead and lag fluid flow rates for said industrial
process; and said method comprises: providing lag valve means for
controlling flow of said lag fluid; monitoring the position of said
lag valve means during opening or closing of said valve means;
monitoring the flow rate of said lag fluid during opening or
closing of said valve means; comparing the change in position of
said valve means with the change in said flow rate of said lag
fluid; and adjusting said threshold value in response to said
comparison indicating a change in the characteristic of said valve
means.
13. A method as claimed in claim 29 wherein the step of adjusting
said threshold value comprises adjusting the threshold values or
levels in said look up table.
14. A method as claimed in claim 1 wherein: said lead parameter is
the flow rate of a combustion gas; said lag parameter is the flow
rate of air; and said process is a combustion process.
15. A control system for providing lag-lead control of a process
having a variable lead parameter and an adjustable lag parameter,
the system comprising: lead monitoring means for monitoring said
lead parameter and providing a lead signal representative of the
value of said lead parameter; lag monitoring means for monitoring
said lag parameter and providing a lag signal representative of the
value of said lag parameter; comparator means for comparing said
lead and lag signals and providing an error signal representative
of the deviation of the ratio of said lead and lag parameters from
a preselected ratio; and adjusting means for adjusting said lag
parameter to reduce said deviation in response to said deviation
exceeding a preselected deviation.
16. A control system as claimed in claim 15 further comprising:
threshold value means for providing a preselectable threshold
value; comparator means for comparing said error signal with said
preselectable threshold value; and wherein said adjusting means is
operable to adjust said lag parameter in response to said error
signal exceeding said preselectable threshold value.
17. A control system as claimed in claim 16 wherein: said threshold
value means comprises a first, upper threshold value means for
providing a first, upper preselected threshold value and a second,
lower threshold value means for providing a second, lower
preselectable threshold value, thereby to define an error range;
said comparator means is operable to compare said error signal with
said upper and lower preselectable threshold values; and said
adjusting means is operable to adjust said lag parameter in
response to said error signal falling outside said error range.
18. A control system as claimed in claim 17 wherein: said adjusting
means is operable to adjust said lag parameter to reduce said ratio
in response to said error signal being above said error range and
to adjust said lag parameter to increase said ratio in response to
said error signal being below said error range.
19. A control system as claimed in claim 18 wherein: said first,
upper preselected threshold value is a positive value and said
second, lower preselected threshold value is a negative value; a
positive error signal indicates an increase in said ratio from said
preselected ratio and a negative error signal indicates a decrease
in said ratio from said preselected ratio; and said adjusting means
is operable reduce said ratio in response to said error signal
being positive and exceeding said first, upper preselected
threshold value and to increase said ratio in response to said
error signal being negative and exceeding said second, lower
preselected threshold value.
20. A control system as claimed in claim 16 wherein the or each
said threshold value is a fixed value.
21. A control system as claimed in claim 16 wherein the or each
said threshold value is a function of the value of said lag
parameter.
22. A control system as claimed in claim 21 further comprising
adjusting means for adjusting said threshold value in dependence on
the value of said lag parameter and wherein said comparator means
is operable to compare said error signal with said adjusted
threshold value.
23. A control system as claimed in claim 22 wherein: said threshold
value means comprises a look up table for storing a plurality of
threshold values; and said threshold value adjusting means is
operable to adjust said threshold value by selecting one of said
threshold values in dependence on the value of said lag
parameter.
24. A control system as claimed in claim 21 further comprising:
adjusting means for adjusting said threshold value in dependence on
the value of said lag parameter; and wherein: the or each threshold
value comprises a plurality of threshold levels; said threshold
value adjusting means is operable to adjust said threshold value by
selecting one of said threshold levels in dependence on the value
of said lag parameter; and said comparator means is operable to
compare said error signal with said selected threshold level.
25. A control system as claimed in claim 24 wherein: said threshold
value means comprises a look up table for storing a plurality of
threshold values; and said threshold value adjusting means is
operable to adjust said threshold value by selecting one of said
threshold values in dependence on the value of said lag
parameter.
26. A control system as claimed in claim 15 wherein: said lead and
lag parameters are lead and lag fluid flow rates for said process;
and said control system comprises: lag valve means for controlling
the flow of said lag fluid; position monitoring means for
monitoring the position of said lag valve means during opening or
closing of said lag valve means and providing a position signal
representative thereof; wherein: said lag monitoring means is
operable to monitor the flow rate of said lag fluid during opening
or closing of said lag valve means and provide a flow rate signal
representative thereof; and said control system further comprises:
storage means for storing sampled values of said position and flow
rate signals representing a preselected characteristic of said lag
valve means; second comparator means for comparing the valve
position of said lag valve means and lag fluid flow rate during
movement of said lag valve means with the stored values; and second
adjusting means for adjusting said threshold value in response to
said comparison indicating a change in the characteristic of said
lag valve means.
27. A control system as claimed in claim 15 wherein: said lead and
lag parameters are lead and lag fluid flow rates for said process;
and said control system comprises: lag valve means for controlling
the flow of said lag fluid; position monitoring means for
monitoring the position of said lag valve means during opening or
closing of said lag valve means and providing a position signal
representative thereof; wherein: said lag monitoring means is
operable to monitor the flow rate of said lag fluid during opening
or closing of said valve means and provide a flow rate signal
representative thereof; and said control system further comprises:
storage means for storing sampled values of said position and flow
rate signals representing a preselected characteristic of said lag
valve means; second comparator means for comparing the lag fluid
flow rate during movement of said valve means with stored value
corresponding to the monitored position of said lag valve means;
and second adjusting means for adjusting said threshold value in
response to said comparison indicating a change in the
characteristic of said valve means.
28. A control system as claimed in claim 15 wherein: said lead
parameter is the flow rate of a combustion gas; said lag parameter
is the flow rate of air; and said process is a combustion
process.
29. A method as claimed in claim 8 wherein: said lead and lag
parameters are lead and lag fluid flow rates for said industrial
process; and said method comprises: providing lag valve means for
controlling flow of said lag fluid; monitoring the position of said
lag valve means during opening or closing of said valve means;
monitoring the flow rate of said lag fluid during opening or
closing of said valve means; comparing the change in position of
said valve means with the change in said flow rate of said lag
fluid; and adjusting said threshold value in response to said
comparison indicating a change in the characteristic of said valve
means.
Description
[0001] The present invention relates to a method of and system for
controlling the ratio of a variable lead parameter and an
adjustable lag parameter for a lag-lead process and particularly,
but not exclusively, to apparatus for controlling the air/gas ratio
in a-gas combustion plant.
[0002] It is known that the air/gas ratio (AGR) in a gas combustion
plant should be maintained substantially constant to achieve
optimum combustion efficiency of the plant. Air/gas ratio
controllers are used in the plant to maintain the air/gas ratio
when the gas flow rate is increased or decreased. To achieve this,
the air/gas ratio controller monitors the gas flow rate and adjusts
the air flow rate accordingly, usually by adjusting a valve in an
air supply line.
[0003] A problem with existing air/gas ratio control is the
difficulty in adjusting the air flow rate to match accurately the
gas flow rate. The present invention aims to provide an improved
method and system for air/gas ratio control.
[0004] Accordingly, the present invention provides a method of
controlling the ratio of a variable lead parameter and an
adjustable lag parameter for a lag-lead process, the method
comprising: monitoring said lead parameter and providing a lead
signal representative of the value of said lead parameter;
monitoring said lag parameter and providing a lag signal
representative of the value of said lag parameter; comparing said
lead and lag signals and providing an error signal representative
of the deviation of the ratio of said lead and lag parameters from
a preselected ratio; and adjusting said lag parameter to reduce
said deviation in response to said deviation exceeding a
preselected deviation.
[0005] In a preferred form of the invention said error signal is
compared with a preselected threshold value and said lag parameter
is adjusted in response to said error signal exceeding said
preselected threshold value. Advantageously, the error signal is
compared with an error range defined by a first, upper preselected
threshold value and a second, lower preselected threshold value and
said lag parameter is adjusted in response to said error signal
falling outside said error range.
[0006] The present invention also provides a control system for
providing lag-lead control of a process having a variable lead
parameter and an adjustable lag parameter, the system comprising:
lead monitoring means for monitoring said lead parameter and
providing a lead signal representative of the value of said lead
parameter, lag monitoring means for monitoring said lag parameter
and providing a lag signal representative of the value of said lag
parameter; comparator means for comparing said lead and lag signals
and providing an error signal representative of the deviation of
the ratio of said lead and lag parameters from a preselected ratio;
and adjusting means for adjusting said lag parameter to reduce said
deviation in response to said deviation exceeding a preselected
deviation.
[0007] Advantageously, the system further comprises threshold value
means for providing a preselectable threshold value and comparator
means for comparing said error signal with said preselectable
threshold value. The adjusting means is operable to adjust said lag
parameter in response to said error signal exceeding said
preselectable threshold value.
[0008] Preferably, said threshold value means comprises a first,
upper threshold value means for providing a first, upper
preselected threshold value and a second, lower threshold value
means for providing a second, lower preselectable threshold value,
thereby to define an error range; said comparator means is operable
to compare said error signal with said upper and lower
preselectable threshold values; and said adjusting means is
operable to adjust said lag parameter in response to said error
signal falling outside said error range.
[0009] The present invention will now be described, by way of
example only, with reference to the accompanying drawings in
which:
[0010] FIG. 1 is a schematic block diagram showing a typical gas
combustion plant;
[0011] FIG. 2 is a schematic block diagram of an air/gas ratio
controller used in the plant of FIG. 1;
[0012] FIG. 3 is a schematic block diagram of a control system
having a preferred form of air/gas ratio controller according to
one aspect of the present invention;
[0013] FIG. 4 is a schematic block diagram of a preferred form of
air/gas ratio controller according to another aspect of the present
invention;
[0014] FIG. 5 is a schematic block diagram of a modification to the
controller of FIG. 4;
[0015] FIG. 6 is a graph shoving Me change in valve position with
applied control voltage
[0016] FIG. 7 is a graph showing the derivative of the valve
characteristic of FIG. 6; and
[0017] FIG. 8 is a graph showing the relationship between the valve
derivative and total deadband value.
[0018] A typical gas combustion plant 10 is shown in FIG. 1. The
plant 10 consists of three main parts, a temperature controller 12,
an air/gas ratio control system 20 and a burner 40 within, for
example, a kiln or furnace 41.
[0019] The temperature controller 12 is able to control the
temperature of the furnace 41, either by following a predetermined
temperature profile or by allowing a user to define the desired
temperature profile. To increase the temperature of the furnace,
for example, the controller 12 adjusts the valve in the gas supply
line to increase the flow rate of the gas supplied to the burner
and the air/gas ratio control system 20 adjusts the air flow rate
to attempt to maintain the ratio between the flow rates of the air
and the gas supplied to the burner substantially constant. A
typical configuration for an air/gas ratio control system is shown
in FIG. 2.
[0020] The system 20 includes a gas valve 22 connected to the gas
supply line 24 for varying the gas flow rate along the line. A gas
flow measurement sensor 26 is positioned downstream of the gas
valve 22 for monitoring the gas flow rate along the line.
Similarly, an air valve 28 is positioned at a point in the air
supply line 30 for varying the air flow rate along the line and an
ail flow measurement sensor 32 is positioned downstream of the air
valve 28 for monitoring the air flow rate along the air line.
[0021] The gas valve 22 is connected to receive an input signal
from the temperature controller 12 to adjust the flow rate of the
gas. The air valve 28 is connected to receive an input from an
air/gas ratio controller 34 to adjust the flow rate of the air in
dependence on the gas flow rate. The air/gas ratio controller 34
receives an input from both of the gas and air measurement sensors
26, 32 and compares the flow rates of the gas and the air and
adjusts the air valve to maintain the required air/gas ratio.
[0022] It can be seen that if the combustion process is to function
with the maximum possible efficiency, the air/gas ratio controller
34 must control the air valve to follow changes in the gas valve as
closely as possible.
[0023] Such a system is commonly known as a lag-lead system. In a
lag-lead system when a lead parameter (the gas flow rate) varies, a
lag parameter (in this case the air flow rate) is adjusted to
maintain the ratio of the parameters substantially constant.
[0024] The flow rate of the air and the gas are monitored by the
measurement sensors of the air/gas ratio control system 20. These
preferably sample the flow rate at a predetermined sampling rate.
The lead parameter (ere the gas flow rate) and the lag parameter
(here the air flow rate) are sampled at regular time intervals. The
lead parameter is sampled usually at a faster rate than the lag
parameter and can be sampled as fast as once every 20 ms. The
sample rate of the lag parameter would be adjusted to suit the lead
parameter sample rate and in this instance would be typically once
every 120 ms. A typical sample range for the lag parameter would be
between 100 ms and 500 ms. In a natural gas combustion system the
air/gas ratio is typically required to be maintained in the order
of 10:1, known as the stoichiometric/gas ratio. Changes in the
temperature reference signal result in the gas valve being adjusted
by the temperature controller 12. This changes the gas flow rate
and thus the air/gas ratio from the desired value. The change in
the gas flow rate is monitored by the controller 34 which acts to
adjust the air valve 28 to return the air/gas ratio to the desired
value.
[0025] If a change in the air/gas ratio is detected (i.e. the
air/gas ratio moves away from the desired value) by the air/gas
ratio controller during a particular sampling of the air and gas
flow the controller will move the air valve in the required
direction (either towards its fully open or fully closed position)
until the next sample is taken.
[0026] However, if the error in the air/gas ratio is smaller than
the change in the air/gas ratio effected by the air valve movement
over one sample interval (the period between one sample time and
the next) the valve will overshoot the desired position and the
desired air flow rate will not be achieved. At the next sampling,
the controller 34 will detect a reverse error and will move the
valve in the opposite direction i.e. it will move the valve towards
its closed position if the previous error caused the valve to be
moved towards its open position, and vice versa. Again, the valve
will be moved too far the reverse direction during the sample
interval and will stop at or close to its initial position i.e. the
position from which it was first moved in response to the
originally monitored error in the air/gas ratio. This opening and
closing of the valve, known as hunting, will repeat for as long as
the error in the air/gas ratio remains substantially the same as or
smaller than the chance effected by movement of the air valve over
one sampling interval. The air valve and consequently the air flow
rate will thus oscillate about the level required to achieve the
desired air/gas ratio. These oscillations are known as limit
cycles.
[0027] It can be seen that if the error in the air/gas ratio
exceeds a particular threshold level (being defined by the change
in the air/gas ratio effected by the air valve movement over one
sample interval) then no limit cycling will occur. However, if the
error lies below the threshold level then limit cycling will occur.
For valves with linear characteristics i.e. which exhibit a linear
response, the threshold level is constant throughout the valve's
operating range. However, many electromagnetically operated valves
exhibit a non-linear response where the air flow rate through the
valve varies non-linearly in relation to the applied control
signal. Thus, the change in the air flow through the valve during
movement of the valve towards its fully open or fully closed
position over a single sample interval will be different depending
on the position of the valve within its operating range (FIG. 6).
Consequently, the threshold level defining the area value below
which limit cycling occurs will vary over the operating range of
the valve.
[0028] Since the motor driving the valve acts as an integrator, the
change in flow over one sampling interval can be found by
differentiating the valve characteristic (FIG. 7). The differential
curve of the valve characteristic shows how much the valve moves
(and thus by how much the air flow rate will alter) during one
sample interval, depending on the initial position of the valve in
the valve operating range. Since errors in the air/gas ratio can
have negative as well as positive values, it is necessary to
establish both positive and negative derivative curves centred
around a zero value in order to establish the threshold level. As
shown in FIG. 8, this effectively produces an "error envelope"
within which limit cycling occurs (FIG. 8). Thus limit cycling will
occur where: |.epsilon..sub.(IS, u)|<|.delta.(u)| (1) where:
[0029] .delta.(u) is the derivative of the valve characteristic at
any given valve position (u) and 2*.delta.(u) represents the
deadband value; [0030] Ts is the sample time; and [0031] u is the
valve position.
[0032] Conversely, limit cycling will not occur where:
|.epsilon..sub.(Ts, u)|.gtoreq.|.delta.(u)| (2)
[0033] In order to reduce or substantially eliminate limit cycling
in the air/gas ratio controller, it is therefore desirable to
ensure that the air valve is not adjusted when the error lies
within the error envelope of the valve. In other words when
equation 1 applies. In a preferred form of the invention, this is
achieved by implementing a so-called "deadband" as described
below.
[0034] FIG. 3 is a schematic block diagram of part of a control
system 90 having a preferred form of air/gas ratio controller 100.
The controller 100 has a first comparator 102 which is connected to
receive two input signals, the first from the gas flow sensor 26
being connected to a non-inverting input of the comparator 102 and
the second from the air flow sensor 32 connected to an inverting
input. An output of the first comparator 102 is connected firstly
to a non-inverting input of a second comparator 104 and secondly to
an inverting input of a third comparator 106.
[0035] Positive and negative fixed threshold value circuits 108,
110; the purpose of which is described below, are connected to
non-inverting and inverting inputs of the second and third
comparators 104, 106 respectively. An output of each of the second
and third comparators 104, 106 is connected to a respective
operational amplifier 112, 114. An output of each operational
amplifier is connected to a respective relay 116, 118 which actuate
movement of the air valve 28.
[0036] During operation of the combustion plant 10, the flow rates
of the gas and the air supplied to the burner 40 are measured by
the flow sensors 26, 32 each of which generates a signal S.sub.g,
S.sub.a corresponding to the respective flow rate and sends the
signal to the air/gas ratio controller 100.
[0037] The gas flow signal S.sub.g and the air flow signal S.sub.a
are fed to the first comparator 102, the gas flow signal S.sub.g to
the non-inverting input and the air flow signal S.sub.2 to the
inverting input. The comparator 102 compares the two signals and
generates an error signal .epsilon. as a function of the
comparison.
[0038] The error signal .epsilon. represents the difference between
the actual air flow measured by the sensor 32 and the desired air
flow to produce a stoichiometric air/gas ratio with the current gas
flow rate. Since the sensor 32 would normally produce an air signal
S.sub.a which is a magnitude of 10 greater than the gas signal
S.sub.g produced by the gas sensor 26 for a stoichiometric ratio
(i.e. an airflow rate which is a magnitude of 10 greater than the
gas flow rate) the value of the air signal S.sub.2 is adjusted to
the same level as the gas signal S.sub.g for a stoichiometric
ratio. This can be effected by a simple voltage divider in the air
flow sensor 32.
[0039] The error signal .epsilon. is fed to the non-inverting input
of the second comparator 104 and to the inverting input of the
third comparator 106, each of which compares the error signal
.epsilon. value with fixed positive and negative threshold values
generated by the positive and negative threshold value circuits
108, 110 respectively.
[0040] If the error signal value is greater than or equal to the
positive threshold value, then the comparator 104 applies an
actuation signal through the first operational amplifier 112 to the
first relay 116 which energises the air valve 28 to move in a first
direction, towards its fully closed position. Similarly, if the
error signal value is less than or equal to the negative threshold
value, the comparator 106 applies an actuation signal through the
second operational amplifier 114 to the second relay 116 which
energises the air valve 28 to move in the opposite direction
towards its fully open position.
[0041] If, however, the error signal value is less than the
positive threshold value and greater than the negative threshold
value, the second and third comparators are unaffected and the air
valve is not adjusted.
[0042] The threshold value circuits 108, 110 set an error signal
range within which the controller 100 takes no corrective action.
Thus, if the gas flow rate is changed in order to increase or
decrease the temperature of the burner 40 this will result in an
air/gas ratio which moves away from the desired value. This will
result in an error signal being generated by the comparator 102,
the error signal representing the difference between the actual
air/gas ratio and the desired air/gas ratio. It will therefore be
appreciated that if the change in the air flow rate which is
required to return the air/gas ratio to the desired level is less
than the change represented by the error signal range set by the
threshold value circuits 108, 110 then the error signal .epsilon.
will fall within this range and the air valve 28 will remain
unactuated. The error is in effect deemed to be zero and the air
valve is not adjusted. The threshold range set by the threshold
circuits 108, 110 is termed a "deadband". In practice, this reduces
the occurrence of limit cycling in the air flow and allows the
desired air/gas ratio to be maintained more closely.
[0043] The value of the deadband affects the performance of the
air/gas ratio controller 100 which in turn affects the efficiency
of the combustion plant. Selection of the correct value for the
deadband is therefore important. By making the deadband value high,
limit cycle oscillations are reduced, but the accuracy of control
of the air valve to provide the desired air/gas ratio is reduced.
Conversely, a low threshold value gives good accuracy but increases
the occurrence of limit cycling. It is preferable, therefore to
make the deadband as small as possible, without causing limit
cycling.
[0044] It is apparent from the above description that if the
deadband value represents a change in air flow rate which is
slightly larger than the movement of the air valve (change in air
flow rate) in a single sample interval, then adjustment of the
valve can be made without limit cycling occurring. A constant
deadband value can therefore be used for valves with linear
characteristics. However, for non-linear valves having an error
envelope such as that shown in FIG. 8, the use of a constant
deadband value is ineffective since limit cycling may occur in some
parts of the operating range of the valve even though a deadband is
used.
[0045] A solution is to vary the value of the deadband according to
the valve characteristic over the valve's operating range. It is
found that the optimum deadband value for a given valve position is
equal to twice the value of the differential of the valve
characteristic at that position Since the deadband is centred
around a zero value the upper and lower threshold levels of the
deadband (set by the positive and negative threshold circuits 108,
110) correspond to the positive and negative derivative curves of
the valve. Thus, the deadband is chosen to map exactly the error
envelope of the valve. Thus, the controller will adjust the air
valve in the instance where: .di-elect cons. ( T .times. .times.
.delta. , u ) .gtoreq. D .function. ( u ) 2 ##EQU1## where
D(u)=.delta.(u) and represents the deadband value defined by the
error envelope at a given valve position (u) which is the region
within which limit cycling does not occur even in the absence of a
deadband since the value of an error within which region is greater
than or equal to the change in flow caused by adjustment of the
valve during one sample interval. Conversely, the controller will
not adjust the air valve in the instance where: .di-elect cons. ( T
.times. .times. .delta. , u ) < D .function. ( u ) 2 ##EQU2## In
this case, the error lies within the deadband which is the region
in which limit cycling would occur if the air valve were adjusted
and the deadband were not present
[0046] A solution is to vary the value of the deadband in
dependence on the valve characteristic over the operating range of
the valve.
[0047] FIG. 4 shows a second embodiment of air/gas ratio controller
200 as part of a control system 190. In FIG., 3, 4 and 5 like
reference numerals indicate like parts. As can be seen, the
controller 200 is similar in form to the controller 100 of FIG. 3
but with the fixed threshold value circuits replaced by variable
threshold value circuits 208, 210 each of which comprises a look-up
table. The variable threshold value circuits 208, 210 are connected
to receive a signal from an air valve position sensor 222 via an
operational amplifier 220. The valve position sensor 222 can be of
the form which simply monitors the voltage applied to the valve to
drive the valve between its open and closed positions.
[0048] Before the control system is put into operation the
characteristic of the air valve is measured and the differential
curve shown in FIG. 7 determined for the valve in order to provide
the error envelope shown in FIG. 8. A number of different threshold
values or levels are then taken from the envelope of FIG. 8, a
positive and a negative value for selected valve positions. The
positive values are stored in the look-up table of the threshold
value circuit 208 and the negative values are stored in the look-up
table of the threshold value circuit 210.
[0049] During operation, as the valve position changes, the
threshold value in the look-up table which is compared with the
error signal is selected according to the position signal from the
air valve position sensor.
[0050] The value generated by each variable threshold value circuit
208, 210 is thus a function of the position of the air valve 28 and
thus of the air flow rate. As the position of the air valve varies,
the change in the air flow rate which occurs during each sample
interval also varies. The air valve characteristics are effectively
stored in the look-up table in each threshold circuit 208, 210. The
look-up table therefore gives the characterstic at a given valve
position and thus determines the deadband value for that position.
The deadband is thus varied according to the instantaneous position
of the air valve 28.
[0051] As in the previous embodiment, if the error signal E
calculated by comparator 202, lies within the range defied by the
instantaneous positive and negative threshold values generated by
the threshold circuits 208, 210, then the error is deemed to be
zero and no corrective action is made to air valve 28.
[0052] If, however, the error value lies on or outside the error
envelope, the air valve 28 is adjusted as described previously.
[0053] Since the deadband value is always greater than the change
in air flow effected by movement of the air valve during one sample
interval, the occurrence of limit cycling is used. In addition, the
accuracy of the air/gas ratio controller 200 is increased. This
results in a significant improvement in combustion efficiency of
the gas combustion plant since the air/gas ratio is maintained at
an optimum.
[0054] It will be apparent that various modifications and
improvements can be made to the present invention.
[0055] The present invention may be modified such that the movement
of the air valve 28 is continuously monitored to determine whether
the characteristics of the valve have changed owing to wear, for
example. If the valve characters tics have changed, this
information can be fed to the variable threshold value circuits to
modify the deadband value for each position of the valve. An
example of such a modification to the present invention is shown in
FIG. 5 in which like reference numerals indicate like parts.
[0056] In FIG. 5 one of the relays, in this case relay 116 which
actuates the valve towards its fully closed position, is connected
to an input of a multiplexer 300. An output of the air flow sensor
32 and the valve position sensor 222 are also connected to the
multiplexer 300.
[0057] The output from the multiplexer 300 is connected to a
parameter estimator 302 whose output in turn is connected to the
variable threshold value circuits 208, 210.
[0058] The parameter estimator 302 may be a microprocessor running,
for example, MATLAB.
[0059] Before the control system is put into operation the
characteristic of the air valve is measured and the response curve
shown in FIG. 6 is stored in a store in the parameter estimator
302.
[0060] This can be effected by moving the valve from one of a fully
open and closed position to the other and monitoring the signals
from the valve position sensor 222 and the air flow rate sensor 32
which are then stored in the parameter estimator 302 as
continuously variable values or discrete values.
[0061] When relay 116 is actuated to move the air flow valve 28
towards its fully closed position the parameter estimator 302 is
also enabled During closing of the valve 29 the parameter estimator
302 processes the outputs from the air flow rate and position
sensors 32, 222 and compares the monitored flow rate with the
previously stored flow rate. If there is a deviation between the
monitored flow rate with the previously stored flow rate this would
suggest, for example, wear in the valve mechanism. The parameter
estimator 302 then adjusts the threshold values in the look up
tables in the threshold value circuits 208, 210 which relate to the
monitored valve position to ale account of changes in the valve
characteristics which have occurred. It will be appreciated that
equally the movement of the valve towards its fully open position
maybe used to update the look up tables to take account of wear, in
which case the estimator 302 would be enabled with relay 118.
[0062] Whilst the above description is made with reference to a
lag-lead control system wherein the lead parameter is the gas flow
rate and the lag parameter is the air flow rate, it will be
appreciated that the invention is equally applicable to systems
wherein the lead parameter is the air flow rate and the lag
parameter is the gas flow rate, or any other lag-lead system.
[0063] It Will also be appreciated that,whilst the preferred form
of the invention has been described with reference to an air/gas
combustion plant or furnace, the invention is equally applicable to
lag-lead control systems for controlling the ratio of two fluids
where the fluids may be in gas or liquid form.
* * * * *