GB2412186A - A method for controlling the rate of supply of a first substance to a fluid - Google Patents
A method for controlling the rate of supply of a first substance to a fluid Download PDFInfo
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- GB2412186A GB2412186A GB0406178A GB0406178A GB2412186A GB 2412186 A GB2412186 A GB 2412186A GB 0406178 A GB0406178 A GB 0406178A GB 0406178 A GB0406178 A GB 0406178A GB 2412186 A GB2412186 A GB 2412186A
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- 238000000034 method Methods 0.000 title claims abstract description 49
- 239000000126 substance Substances 0.000 title claims abstract description 44
- 239000012530 fluid Substances 0.000 title claims abstract description 28
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 claims abstract description 37
- RAHZWNYVWXNFOC-UHFFFAOYSA-N Sulphur dioxide Chemical compound O=S=O RAHZWNYVWXNFOC-UHFFFAOYSA-N 0.000 claims abstract description 26
- 239000000460 chlorine Substances 0.000 claims abstract description 20
- 229910052801 chlorine Inorganic materials 0.000 claims abstract description 20
- 235000010269 sulphur dioxide Nutrition 0.000 claims abstract description 13
- 239000004291 sulphur dioxide Substances 0.000 claims abstract description 13
- 230000008859 change Effects 0.000 claims description 14
- 101000642689 Entacmaea quadricolor Delta-actitoxin-Eqd1a Proteins 0.000 claims description 7
- 238000004590 computer program Methods 0.000 claims description 3
- 125000001309 chloro group Chemical group Cl* 0.000 claims 2
- 230000004044 response Effects 0.000 abstract description 31
- ZAMOUSCENKQFHK-UHFFFAOYSA-N Chlorine atom Chemical compound [Cl] ZAMOUSCENKQFHK-UHFFFAOYSA-N 0.000 abstract description 22
- 230000010355 oscillation Effects 0.000 abstract description 7
- 230000008569 process Effects 0.000 description 20
- 238000004659 sterilization and disinfection Methods 0.000 description 16
- 238000005070 sampling Methods 0.000 description 8
- 238000004422 calculation algorithm Methods 0.000 description 7
- 230000000694 effects Effects 0.000 description 6
- 239000004155 Chlorine dioxide Substances 0.000 description 4
- 235000019398 chlorine dioxide Nutrition 0.000 description 4
- OSVXSBDYLRYLIG-UHFFFAOYSA-N chlorine dioxide Inorganic materials O=Cl=O OSVXSBDYLRYLIG-UHFFFAOYSA-N 0.000 description 4
- 238000004519 manufacturing process Methods 0.000 description 4
- 230000006872 improvement Effects 0.000 description 3
- 238000004364 calculation method Methods 0.000 description 2
- 230000001419 dependent effect Effects 0.000 description 1
- 238000002474 experimental method Methods 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 230000009467 reduction Effects 0.000 description 1
Classifications
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
- F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
- F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
- F01N3/08—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous
- F01N3/10—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by thermal or catalytic conversion of noxious components of exhaust
- F01N3/18—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by thermal or catalytic conversion of noxious components of exhaust characterised by methods of operation; Control
- F01N3/20—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by thermal or catalytic conversion of noxious components of exhaust characterised by methods of operation; Control specially adapted for catalytic conversion ; Methods of operation or control of catalytic converters
- F01N3/2066—Selective catalytic reduction [SCR]
- F01N3/208—Control of selective catalytic reduction [SCR], e.g. dosing of reducing agent
-
- G—PHYSICS
- G05—CONTROLLING; REGULATING
- G05D—SYSTEMS FOR CONTROLLING OR REGULATING NON-ELECTRIC VARIABLES
- G05D11/00—Control of flow ratio
- G05D11/02—Controlling ratio of two or more flows of fluid or fluent material
- G05D11/13—Controlling ratio of two or more flows of fluid or fluent material characterised by the use of electric means
- G05D11/131—Controlling ratio of two or more flows of fluid or fluent material characterised by the use of electric means by measuring the values related to the quantity of the individual components
- G05D11/132—Controlling ratio of two or more flows of fluid or fluent material characterised by the use of electric means by measuring the values related to the quantity of the individual components by controlling the flow of the individual components
-
- G—PHYSICS
- G05—CONTROLLING; REGULATING
- G05D—SYSTEMS FOR CONTROLLING OR REGULATING NON-ELECTRIC VARIABLES
- G05D11/00—Control of flow ratio
- G05D11/02—Controlling ratio of two or more flows of fluid or fluent material
- G05D11/13—Controlling ratio of two or more flows of fluid or fluent material characterised by the use of electric means
-
- G—PHYSICS
- G05—CONTROLLING; REGULATING
- G05D—SYSTEMS FOR CONTROLLING OR REGULATING NON-ELECTRIC VARIABLES
- G05D11/00—Control of flow ratio
- G05D11/02—Controlling ratio of two or more flows of fluid or fluent material
- G05D11/13—Controlling ratio of two or more flows of fluid or fluent material characterised by the use of electric means
- G05D11/135—Controlling ratio of two or more flows of fluid or fluent material characterised by the use of electric means by sensing at least one property of the mixture
- G05D11/138—Controlling ratio of two or more flows of fluid or fluent material characterised by the use of electric means by sensing at least one property of the mixture by sensing the concentration of the mixture, e.g. measuring pH value
-
- G—PHYSICS
- G05—CONTROLLING; REGULATING
- G05D—SYSTEMS FOR CONTROLLING OR REGULATING NON-ELECTRIC VARIABLES
- G05D21/00—Control of chemical or physico-chemical variables, e.g. pH value
- G05D21/02—Control of chemical or physico-chemical variables, e.g. pH value characterised by the use of electric means
Landscapes
- Engineering & Computer Science (AREA)
- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Automation & Control Theory (AREA)
- Chemical & Material Sciences (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Health & Medical Sciences (AREA)
- Toxicology (AREA)
- Combustion & Propulsion (AREA)
- Mechanical Engineering (AREA)
- General Engineering & Computer Science (AREA)
- Treatment Of Water By Oxidation Or Reduction (AREA)
Abstract
A method and apparatus for controlling the rate of supply of a first substance (for example sulphur dioxide or chlorine) to a fluid (for example water). The method comprises determining a supply rate parameter B from an input error parameter E indicative of the difference between a desired concentration R of a second substance in the fluid and a measured concentration C of the second substance in the fluid. The supply rate parameter is calculated by summing a proportional term updated at predetermined time interval T 1, an integral term updated at predetermined time interval T 2 and a correctional term CT at predetermined time interval T 3, T 2 is longer than TI, and also longer than T3. The correctional term is determined based upon the current value of E, the previous value of E, the desired concentration R and the measured concentration C. The correctional term increases the speed of response of the controller to changes in the desired concentration and reduces the amount of oscillation during changes in the supply rate parameter. The controller may be a Proportional - Integral (PI) controller.
Description
Supply Rate Control Method And Apparatus The present invention relates to
a method and apparatus for controlling the rate of supply of a first substance to a fluid. It is particularly suitable for, but not limited to, controlling the rate of supply of either chlorine or sulphur dioxide to water to disinfect the water.
In water treatment plants chlorine is commonly added to the water supply in order to disinfect it before it is suitable for domestic use. If too much chlorine has been added sulphur dioxide may be added to reduce the level of chlorine in the water.
The disinfection process is normally controlled by a controller having a single control output - a supply rate parameter. The supply rate parameter is passed to a dosing unit, which controls the addition of the chlorine or sulphur dioxide to the water. Typically, there may be two controllers, one controlling the addition of chlorine and one controlling the addition of sulphur dioxide.
Figure 1 illustrates the disinfection control process. The controller I has at least a first input 2, comprising a user defined desired concentration of chlorine in the water. Optionally, a flow meter 3 may be provided, providing a second input signal 4.
The second input allows the controller 1 to respond to changes in the rate of flow of the water 5 along a conduit 6 and change the supply rate parameter accordingly.
Supply rate parameter 7 is passed to the dosing unit 8 where the chlorine or sulphur dioxide is added to the water 5 flowing along the conduit 6. Measuring device 9 measures the residual level of chlorine in the water. The measured concentration 10 is passed to the controller 1 completing a feedback loop. The difference between the desired concentration 2 and the measured concentration 10 is the input error parameter used by the controller to calculate the supply rate parameter.
Ignoring for now the optional flow meter 3 the disinfection control process may be considered to be a single input, single output system with the output being fed back to the controller. Typically, this is implemented as a discrete PI (proportional, integral) controller, including at least a proportional and an integral term. The input error parameter is determined periodically at the end of each sampling interval. For l water disinfection processes there may be a very long time lag from a change in the supply rate parameter leading to a consequential change in the measured concentration. This time lag makes it difficult to chose values for the sampling interval that provide a PI controller with a satisfactory response. If the time interval Ts is set to be shorter than the time lag, the supply rate parameter will be unstable. If Ts is set to be longer than the time lag, the response of the controller to changes in the desired concentration, and consequently the input error parameter, will be very slow.
One solution that has previously been used in water disinfection process controllers is to use two time intervals To and T2. At the end of each time interval To, the proportional term is updated. To is set to a very low value (typically less than one second), thereby providing a fast response to changes in the input error parameter. At the end of each time interval T2, the integral term is updated. T2 is set to be just greater than the time lag, minimising the delay in updating the integral term, and ensuring that the supply rate parameter remains stable. For changes in the desired concentration, the proportional response is very rapid, but the integral response will only occur after the time lag.
Using two sampling time intervals causes problems with the response of the controller to step changes in the desired concentration. The controller makes a change to the integral component of the control equation, and then waits for the time lag to expire and the integral component to be updated again. As the initial integral term is altered, the input error parameter will reduce. This reduction causes the proportional term to reduce, thereby reducing the control output. This can lead to oscillations as the supply rate parameter moves towards its new level, significantly reducing the speed of the response.
It is an aim of the embodiments of the present invention to provide an improvement to a water disinfection process controller leading to an improved speed of response to changes in the desired concentration. It is an aim of further embodiments of the present invention to reduce the amount of oscillation in the step response of a water disinfection process controller.
According to a first aspect, the present invention provides a method of controlling the rate of supply of a first substance to a fluid comprising: determining a supply rate parameter B indicative of a desired rate of supply of the first substance to the fluid, from an input error parameter E indicative of the difference between a desired concentration R of a second substance in the fluid and a measured concentration C of the second substance in the fluid; and the supply rate parameter being calculated by summing a proportional term updated with a current value of E as determined at predetermined time interval To an integral term updated with a current value of E as determined at predetermined time interval T2 where T2 is longer than To, and a correctional term CT updated with a current value of E as determined at predetermined time interval T3, where T2 is longer than T3; wherein the correctional term is determined based upon the current value of E, the previous value of E, the desired concentration R and the measured concentration C. By using such a correctional term, the speed of response to changes in the desired concentration is increased. Also, the amount of oscillation in the supply rate parameter is reduced when the supply rate parameter moves towards its new level in response to a step change in the desired concentration.
The proportional term may comprise Kp.En where En is the value of E as determined at the end of an nth time interval To, where n is an integer and Kp is a first predetermined gain factor constant. m
The integral term may comprise K, Em where Em is the value of E as m determined at the end of an mth time interval T2, where m is an integer, Em is the sum of the current and all previous values of Em and Kit is a second predetermined gain factor constant.
The supply rate parameter B may be determined by: B=Kp En +K, item + CT The first substance may be the same as the second substance and: if (Cr < Rr) and if (Kp.Er < Kp.Er I) then CT = +IKP Er - KP Er I |; where Cr is the value of C as determined at the end of an rth time interval T3, where r is an integer, Rr is the value of R as determined at the end of the rth time interval T3, Er is the value of E as determined at the end of the rth time interval T3 and Er is the value of E as determined at the end of the (r-l)th time interval T3.
Preferably, if (Cr > Rr) and if (Kp.Er > Kp Er I) then CT = -IKP Er - Kp Er I| . Preferably, if (Cr < Rr) and if (Kp Er > Kp.Er I), or if (C, > Rr) and if (Kp Er < Kp.Er I), then CT = 0.
The first substance may different to the second substance and: if (Cr > Rr) and if (Kp.Er < Kp.Er I) then CT=+|Kp Er-Kp Er I|; where Cr is the value of C as determined at the end of an r th time interval T3, where r is an integer, Rr is the value of R as determined at the end of the rth time interval T3, Er is the value of E as determined at the end of the rth time interval T3 and Er is the value of E as determined at the end of the (r-1)th time interval T3 Preferably, if (Cr < Rr) and if (Kp.Er > Kp. Er) then CT = -IKP Er - KP Er | . Preferably, if (Cr > Rr) and if (Kp Er > Kp.Er I), or if (Cr < Rr) and if (Kp.Er < Kp.Er I), then CT = 0.
Preferably if ( rR r) is less than a predetermined deadband value then CT=0 Preferably T3 is equal to T' Preferably T2 is more than ten times longer than T; or T3.
There may be a time delay indicative of the time taken for a change in the supply rate parameter to lead to a consequential change in the measured concentration C after an update of the integral term, and T2 may be longer than said time delay.
According to a second aspect, the present invention provides an apparatus arranged to carry out the method as described above, wherein the method is implemented by a PI controller.
The controller may be a discrete PI controller.
The controller may be implemented in hardware.
The fluid may be flowing along a conduit.
The fluid may be water and the second substance may be chlorine.
Preferably, if the first substance is different to the second substance, then the first substance is sulphur dioxide.
According to the third aspect, the present invention provides a data carrier carrying computer program code means to cause a computer to carry out a method as described above.
According to the fourth aspect, the present invention provides a computer apparatus comprising a program memory containing processor readable instructions; and a processor for reading and executing the instructions contained in the program memory; wherein said processor readable instructions comprise instructions controlling the processor to carry out the method as described above.
According to the fifth aspect, the present invention provides a dosing device comprising an apparatus as described above; input means for setting the values of: the desired concentration R of the second substance in the fluid, predetermined time intervals To, T2 and T3, and predetermined gain factor constants Kp and K'; a sensor for measuring the concentration C of the second substance in the fluid and providing C to the apparatus; and dosing means for delivering a first substance into the fluid, depending on the value of a supply rate parameter B determined by the apparatus.
The fluid may be flowing along a conduit.
The fluid may be water, and the second substance may be chlorine.
Preferably, if the first substance is different to the second substance, then the first substance is sulphur dioxide.
Other objects and advantages of the present invention will become apparent
from the following description.
The present invention will now be described, by way of example only, with reference to the following drawings, in which: Figure 1 illustrates a schematic version of a known water disinfection process; Figure 2 illustrates a single input, single output controller, controlling a water disinfection process; Figure 3 illustrates a discrete PI controller; Figure 4 illustrates a modelled step response of the known discrete PI controller shown in Figure 1, having different sampling intervals for the proportional and integral terms; Figure 5 illustrates a modelled step response of a discrete PI controller having different sampling intervals for the proportional and integral terms implementing a control algorithm in accordance with an embodiment of the present invention; Figure 6 illustrates the step response of a controller in a water treatment works with the correctional term compensation turned off; and Figure 7 illustrates the step response of a controller in a water treatment works with the correctional term compensation turned on in accordance with an embodiment of the present invention.
Referring first to Figure 2. The controller I is provided with a desired input concentration of chlorine R from input 2, which is combined with the measured concentration C from input 10. The measured concentration C is subtracted from the desired concentration R by subtracter 11. The difference is an input error parameter E on the output 12 of the subtracter. This is passed to a controller 13, which determines the supply rate parameter B provided on output 7 via a control algorithm. The supply rate parameter B is input to the dosing unit 8. Dosing unit 8 varies the amount of chlorine or sulphur dioxide being added to the water based upon the value of B. The output 10 of the dosing unit 8 is the measured concentration C completing the feedback loop.
The control algorithm may conveniently be any suitable form of algorithm.
For controlling industrial processes, having a feedback loop and a calculated input error parameter E, PID controllers are extensively used. A P1D controller comprises at least three terms: a Proportional term, an Integral term and a Differential term. The proportional term is proportional to the input error parameter E and serves as a direct response to changes in E. The integral term reduces the steady state error (i.e. reducing the value of E towards zero if the input to the process is not changing). The differential term varies the response of the controller dependent upon the rate of change of E, improving the performance of the controller for fast changing processes.
For water disinfection processes, having a controller incorporating a differential term may lead to instability in the process. This is because the measured concentration C is typically noisy. Also the time lag between changes in the supply rate parameter B leading to consequential changes in the measured concentration C may be of the order of several minutes. Therefore, for water disinfection processes a PI controller is commonly used, comprising at least a Proportional term and an Integral term.
In the time domain the response of a PI controller may be expressed as: B(t)=Kp e(t)+ ' [e(t)dt 1 o PI controllers for water disinfection processes may be implemented in hardware, though are commonly implemented in software. In either case it is normal to implement the PI controller as a discrete PI controller operating on a sampled input error parameter. The analogue inputs are sampled at a predetermined sampling interval Ts. The output supply rate parameter B is now updated at the end of each sampling interval Ts.
Figure 3 illustrates a discrete PI controller 1, controlling a water disinfection process. Controller 1 has inputs a desired concentration of chlorine R on input 2, which is combined with the measured concentration C on input 10, to give the input error parameter E at the output of the subtracter 12 as described for Figure 2. At the end of each predetermined time interval Ts E is determined (shown schematically as a sampler 14 closing a switch 15). En is the value of E sampled at the end of an nth time interval Ts' where n is an integer. This is passed to the PI control algorithm 13, which determines the supply rate parameter B. The supply rate parameter B on output 7 is passed to the dosing unit 8 as before, and the resulting concentration is measured and the measured concentration C is fed back to the controller 1 completing the loop.
The response of the discrete PI controller may be expressed as B = proportional term + integral term. The proportional term may be expressed as Kp.En where En is the value of E sampled at the end of an nit time interval Ts' where n is an integer. Kp is a first predetermined gain factor constant. The integral term may be expressed as K' En, where En is the value of E sampled at the end of an nth time n interval Ts and n is an integer. En is the sum of the current and all previous values of En and Kit is a second predetermined gain factor constant. Therefore, the supply rate parameter B may be expressed as B = Kp En + K' nO En For water disinfection processes, the very long time lag between a change in the supply rate parameter B leading to a consequential change in the measured concentration C makes it difficult to choose values for Ts' Kp and K' which provide a PI controller with a satisfactory response. If the time interval Ts is set to be shorter than the time lag, the supply rate parameter B will be unstable. If Ts is set to be longer than the time lag, the response of the controller to changes in R will be very slow.
One solution that has previously been used in water disinfection process controllers is to use two time intervals of different duration (T' and T2) . At the end of each time interval of duration To the proportional term Kp.En is updated, where En is the value of E sampled at the end of an nth time interval To, where n is an integer. T' is set to a very low value (typically less than one second), thereby providing a fast response to changes in E. At the end of each time interval of duration T2 the integral m term is updated K, Em where Em is the value of E sampled at the end of an mth m=0 m time interval T2, m is an integer and Em is the sum of the current and all previous values of Em T2 is set to be just greater than the time lag, minimising the delay in updating the integral term, while ensuring that the supply rate parameter remains stable. For changes in the desired concentration R. the proportional response is very rapid, but the integral response will only occur after the time lag. The supply rate parameter B may be expressed as: m B = Kp.En + Kl To Em Figure 4 shows the step response of a model of a PI controller having different sampling intervals for the proportional and integral terms. For this model the value of parameter To is lOOmS, T2 is 150S, Kp is 0.8 and K, is 0.8. This clearly shows that a large amount of time taken to settle at the new value. During this delay in settling marked oscillation is observed.
The present inventor has realised that this undesirable step response is a direct consequence of using two time intervals to update the proportional and integral terms at different rates. This allows the proportional term of the PI control algorithm to reduce the effectiveness of the change in the integral term. To counteract this effect the present inventor has introduced an additional correctional term CT, which is summed with the proportional and integral terms, therefore: m B=Kp En +K' I0Em + CT To update the correctional term a third time interval, T3 is introduced. This may be the same as To, in which case the correctional term is updated at the same time as the proportional term. The correctional term is updated at the end of each time interval T3. T3 must be shorter than T2 to allow the correction to take effect during the relatively long intervals between updates of the integral term.
At the end of each time interval T3, CT is updated with the current value of E. The value of CT is determined based upon the current value of E, the previous value of E at the time of the last update of CT, the desired concentration R and the measured concentration C. For the addition of chlorine, if the measured concentration C is less than the desired concentration R. and the current value of the proportional term is lower than the previous value, then the correctional term is the modulus of the difference between the current and previous values of the proportional term, i.e.: if (Cr < Rr) and if (Kp.Er < Kp.Er I) then CT = +IKP Er-KP Er | Cr is the value of C sampled at the end on an rth time interval T3, where n is an integer. Rr is the value of R sampled at the end of the rth time interval T3. Er is the value at E sampled at the end of the rth time interval T3, and Er is the previous value of E. The effect of this is to stop the supply rate parameter B reducing if the desired concentration C has not been reached by the measured concentration R. Conversely, if the desired concentration R has been exceeded, and the current value of the proportional term is larger than the previous value then the correctional term CT is minus the modulus of the difference between the current and previous values of the proportional term, i.e.: if (Cr > Rr) and if (Kp. Er > Kp.Er) then CT = - |KP Er-KP Er | . If neither of these conditions is met then the correctional term is set to zero and has no effect on the supply rate parameter B. i.e.: if (Cr < Rr) and if (Kp.Er > Kp.Er '), or if (Cr > Rr) and if (Kp.Er < Kp.Er '), then CT=O.
For the addition of sulphur dioxide to counteract an inadvertent excess of chlorine in the water then again, the supply rate parameter is determined based upon the current value of E, the previous value of E, the desired concentration of chlorine R and the measured concentration of chlorine C. This time if the measured concentration of chlorine C exceeds the desired concentration R. and the current value of the proportional term is lower than the previous value, then the correctional term CT is the modulus of the difference between the current and previous values of the proportional term, i.e.: if (Cr > Rr) and if (Kp.Er < Kp.Er I) then CT = +IKP Er - Kp Er | . This increases the amount of sulphur dioxide being added to the water to counteract the excess chlorine.
Conversely, if the measured concentration C is less than the desired concentration R. but the proportional term is still increasing then the correctional term is minus the modulus of the difference between the current and previous values of the proportional term, i.e.: if (Cr < Rr) and if (Kp.Er > Kp.Er l) then CT =-IKP Er - KP Er I | If neither of these conditions is met then the correctional term is set to zero and has no effect on the supply rate parameter B. i.e.: if (Cr > Rr) and if (Kp.Er > Kp.Er I), or if (Cr < Rr) and if (Kp.Er < Kp.Er i), then CT=O.
Figure 5 shows the step response of a model of a PI controller, incorporating the correctional term in accordance with the present invention. In comparison with Figure 4, it is clear that there is significantly less oscillation in the signal. For this model To is lOOmS, T2 is 150S, Kp is 0.6 and Kit is 0.5.
In order to prevent residual oscillation and instability due to signal noise, the calculation of the correctional term can be further improved by placing a predetermined percentage dead band around the desired concentration R. In other words, if the measured concentration C is within the predetermined percentage of R then CT=O, e.g. if the measured concentration is within 5% of R then CT=O.
However, this deadband percentage can be any predetermined value or percentage e.g. 15%, 10%, 5%, 2% or 1%.
The improved PI control algorithm of the present invention has been implemented in a PI controller in a water treatment works, treating over 1001itres per second. Figure 6 shows the step response with the correctional term permanently set to zero, (i.e. a PI controller of the prior art). Figure 7 shows the step response with the correctional term being calculated as outlined above in accordance with the present invention. In order to enable direct comparison in both cases a step change in the desired concentration R has been introduced from 1.5mg/1 to 2mg/1. The graphs are plotted on the same scale. For this water treatment plant, an improvement in the step response from 19 minutes to 3 minutes was observed.
It will be readily obvious to the appropriately skilled person that although the calculations outlined above suggest a software implementation, they may equally be implemented in hardware or software, without effect upon the operation of the invention. Further, although the experiment has been outlined in terms of the treatment of water supplies by controlling the levels of chlorine in the water, this improvement may be applied to any water disinfection process. More generally, the invention has utility in any industrial process controlled by a Pl or PID controller where the proportional and integral terms are updated at differing rates. Such other industrial processes may include any other form of dosing process, wherein a substance is added to a fluid, or any other form of industrial process.
Further modifications and applications of the present invention will be readily apparent to the appropriately skilled person, without departing from the scope of the appended claims.
Claims (31)
1. A method of controlling the rate of supply of a first substance to a fluid comprising: determining a supply rate parameter B indicative of a desired rate of supply of the first substance to the fluid, from an input error parameter E indicative of the difference between a desired concentration R of a second substance in the fluid and a measured concentration C of the second substance in the fluid; and the supply rate parameter being calculated by summing a proportional term updated with a current value of E as determined at predetermined time interval To, an integral term updated with a current value of E as determined at predetermined time interval T2, where T2 is longer than To, and a correctional term CT updated with a current value of E as determined at predetermined time interval T3, where T2 is longer than T3; wherein the correctional term is determined based upon the current value of E, the previous value of E, the desired concentration R and the measured concentration C.
2 A method according to claim 1, wherein the proportional term comprises Kp.En where En is the value of E as determined at the end of an nth time interval To, where n is an integer and Kp is a first predetermined gain factor constant.
3. A method according to claim 1 or claim 2, wherein the integral term comprises m K' Em where En, is the value of E as determined at the end of an mth time interval m T2, where m is an integer, Em is the sum of the current and all previous values of Em and K, is a second predetermined gain factor constant.
4. A method according to claim 3, wherein the supply rate parameter B is determined by: m B = Kp.En + Kit I0 Em + CT
5. A method according to claim 4, wherein the first substance is the same as the second substance and: if (Cr < Rr) and if (Kp.Er < Kp.Er I) then CT = +IKP Er-KP Er-l |; where Cr is the value of C as determined at the end of an rth time interval T3, where r is an integer, Rr is the value of R as determined at the end of the r0' time interval T3, Er is the value of E as determined at the end of the rib time interval T3 and Er is the value of E as determined at the end of the (r- 1)th time interval T3.
6. A method according to claim 5, wherein: if (Cr > Rr) and if (Kp.Er > Kp.Er I) then CT = - |KP Er-KP Er I| .
7. A method according to claim 5 or claim 6, wherein: if (Cr < Rr) and if (Kp.Er > Kp.Er I), or if (Cr > Rr) and if (Kp.Er < Kp.Er I), then CT = 0.
8. A method according to claim 4, wherein the first substance is different to the second substance and: if (Cr > Rr) and if (Kp.Er < Kp.Er I) then CT = +IKP Er-KP Er I |; where Cr is the value of C as determined at the end of an r th time interval T3, where r is an integer, Rr is the value of R as determined at the end of the rth time interval T3, Er is the value of E as determined at the end of the rth time interval T3 and Er is the value of E as determined at the end ofthe (r-l)th time interval T3.
9. A method according to claim 8, wherein: if (Cr < R.) and if (Kp.Er > Kp.Er I) then CT = - |KP Er-KP Er | .
10. A method according to claim 8 or claim 9, wherein: if (C, > Rr) and if (Kp.Er Kp.Er i), or if (Cr < Rr) and if (Kp.Er < Kp.Er i), then CT=o.
11. A method according to any one of claims 5 to 10, wherein if ( r r) is less than a predetermined Headband value then CT = 0. R.
12. A method according to any one of claims 5 to 10, wherein T3 is equal to To
13. A method according to any one of the preceding claims, wherein T2 is more than ten times longer than To or T3.
14. A method according to any one of the preceding claims, wherein there is a time delay indicative of the time taken for a change in the supply rate parameter to lead to a consequential change in the measured concentration C after an update of the integral term, and T2 is longer than said time delay.
15. An apparatus arranged to carry out the method of any one of the preceding claims, wherein the method is implemented by a PI controller.
16. An apparatus according to claim 15, wherein the controller is a discrete PI controller.
17. An apparatus according to claim 15 or claim 16, wherein the controller is implemented in hardware.
18. An apparatus according to any one of claims 15 to 17, wherein the fluid is flowing along a conduit.
19. An apparatus according to any one of claims 15 to 18, wherein the fluid is water and the second substance is chlorine.
20. An apparatus according to any one of claims 15 to 19, wherein if the first substance is different to the second substance, then the first substance is sulphur dioxide.
21. A data carrier carrying computer program code means to cause a computer to carry out a method according to any one of claims 1 to 17.
22. A computer apparatus comprising: a program memory containing processor readable instructions; and a processor for reading and executing the instructions contained in the program memory; wherein said processor readable instructions comprise instructions controlling the processor to carry out the method of any one of claims 1 to 17.
23. A dosing device comprising: an apparatus according to any one of claims 18 to 20; input means for setting the values of: the desired concentration R of the second substance in the fluid, predetermined time intervals To, T2 and T3, and predetermined gain factor constants Kp and K, ; a sensor for measuring the concentration C of the second substance in the fluid and providing C to the apparatus; and dosing means for delivering a first substance into the fluid, depending on the value of a supply rate parameter B determined by the apparatus.
24. A dosing device according to claim 23, wherein the fluid is flowing along a conduit.
25. A dosing device according to claim 23 or claim 24, wherein the fluid is water, and the second substance is chlorine.
26. A dosing device according to any one of claims 23 to 25, wherein if the first substance is different to the second substance, then the first substance is sulphur dioxide.
27. A method, substantially as hereinbefore described with reference to the accompanying Figures 2 to 7.
28. An apparatus, substantially as hereinbefore described with reference to the accompanying Figures 2 to 7.
29. A data carrier carrying computer program code means, substantially as hereinbefore described with reference to the accompanying Figures 2 to 7.
30. A computer apparatus, substantially as hereinbefore described with reference to the accompanying Figures 2 to 7.
31. A dosing device, substantially as hereinbefore described with reference to the accompanying Figures 2 to 7.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB0406178A GB2412186B (en) | 2004-03-19 | 2004-03-19 | Supply rate control method and apparatus |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB0406178A GB2412186B (en) | 2004-03-19 | 2004-03-19 | Supply rate control method and apparatus |
Publications (3)
| Publication Number | Publication Date |
|---|---|
| GB0406178D0 GB0406178D0 (en) | 2004-04-21 |
| GB2412186A true GB2412186A (en) | 2005-09-21 |
| GB2412186B GB2412186B (en) | 2008-06-04 |
Family
ID=32117999
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB0406178A Expired - Fee Related GB2412186B (en) | 2004-03-19 | 2004-03-19 | Supply rate control method and apparatus |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2412186B (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN104156012A (en) * | 2014-08-01 | 2014-11-19 | 江苏大学 | Real-time control method for improving steady precision of mixed pesticide concentration of pesticide mixing device |
Citations (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0420146A1 (en) * | 1989-09-29 | 1991-04-03 | Tokheim Corporation | Variable blending dispenser |
| US6445980B1 (en) * | 1999-07-10 | 2002-09-03 | Mykrolis Corporation | System and method for a variable gain proportional-integral (PI) controller |
-
2004
- 2004-03-19 GB GB0406178A patent/GB2412186B/en not_active Expired - Fee Related
Patent Citations (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0420146A1 (en) * | 1989-09-29 | 1991-04-03 | Tokheim Corporation | Variable blending dispenser |
| US6445980B1 (en) * | 1999-07-10 | 2002-09-03 | Mykrolis Corporation | System and method for a variable gain proportional-integral (PI) controller |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN104156012A (en) * | 2014-08-01 | 2014-11-19 | 江苏大学 | Real-time control method for improving steady precision of mixed pesticide concentration of pesticide mixing device |
Also Published As
| Publication number | Publication date |
|---|---|
| GB0406178D0 (en) | 2004-04-21 |
| GB2412186B (en) | 2008-06-04 |
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