METHOD OF CONTROLLING A PLURALITY OF MACHINES

Abstract: A method of operating a plurality of machines (11 to 14) such as air compressors, each of which contributes when operative, to a total capacity of a system, comprises sensing with a sensing means (20) the value of a variable parameter e.g. pressure, of the system, feeding a signal from the sensing means (20) to estimate from data stored in a memory (M) thereof, and the signal from the sensing means (20), the expected demand from the system in the immediate future and rendering and maintening operative selected machines (11 to 14) the selection being made by the control means (10) in response to the estimated expected demand.

Description: Title: "Method of controlling α plurality of machines" Description of Invention This invention relates to a method of controlling a plurality of machi each of which when operative, contributes a total capacity of a system. The invention has been devised particularly, but not exclusively, for with a plurality of machines comprising air compressors which may each of the same or different nominal rated capacity, and each of wh contributes to a total capacity of compressed air available for use. Conventional ly, a plurality of air compressors are operated in a casc mode in which, when the demand for compressed air increases so that available pressure falls below a first threshold pressure, an additio compressor is rendered operative, and when the demand decreases so that available pressure rises above a second threshold pressure, the additional compressor is rendered inoperative. Where the capacity can be genera matched to the demand, an air compressor, called a balance air compres would, when the air compressors are operated in a cascade mode, be cycl between operative and inoperative conditions to maintain the pressure wit a selected pressure range bounded by the first and second threshold pressur Devices are known which can estimate how long the balance compres is likely to remain inoperative, and when this time is less than predetermined time, instead of turning the air compressor to an off state, t air compressor is rendered inoperative by turning to an off load state which the compressor continues to run but does not contribute to the to capacity of the system. This protects the balance compressor from t frequent switching on and off. Obviously in such a system, the air compressors are not used equa resulting in uneven wear of the individual air compressors. Even when compressor is run in an off load state, the air compressor is subject to we and wastefυlly uses power. Thus it has been proposed alternatively to operate a plurality of compressors in a sequencing mode in which when it is desired to increase t capacity of the system as the pressure falls below a first threshold pressu the air compressor which has been rendered inoperative for the longest period of time is rendered operative, and when it is desired to decrease the capacity of the system as the pressure rises above a second threshold pressure the machine which has been operative for the longest period of time is rendered inoperative. This method is particularly useful where the air compressors are all of the same general rated capacity. All of the machines in the sequencing mode are used generally equally and usually, the air compressors are turned off rather than being run in an off load state. However, under certain load conditions it is possible for the cycle time between when any air compressor is rendered inoperative to when the same compressor is rendered operative again to be too small which would result in too frequent switching and this could result in damage to motors of the air compressor. It is an object of the present invention to provide a new or improved method of operating a plurality of machines. According to a first aspect of the invention I provide a method of operating a plurality of machines each of which contributes, when operative, to a total capacity of a system, the method comprising sensing with a sensinq means the value of a variable parameter of the system, feeding a signal from the sensing means to a control means, programming the control means to estimate from data stored in a memory thereof, and the signal from the sensing means, the expected demand from the system in the immediate future and rendering and maintaining operative selected machines, the selection being made by the control means in response to the estimated expected demand. It has been found that by estimating the expected demand and selecting machines to be maintained and rendered operative in accordance with the demand, the value of the variable parameter of the system can be kept below a second threshold value and above a first threshold value for at least a major portion of an immediate future time period until the expected demand is re- estimated. Further, the control means can select a machine or machines to be rendered or maintained operative or rendered inoperative to provide the most efficient use of the machines within given constraints which can b programmed into and stored in the memory of the control means. The method may be repeated at intervals so that the expected deman is frequently re-estimated. The intervals may be regular intervals o preferαbly the length of each interval is dependent upon a cycle time of th program of the control means and may be of regular or irregular length. The data stored in the memory of the control means may include dat derived from at least one signal from the sensing means indicating the valu of the variable parameter at a prior time. Thus where the method is repeate at intervals the expected demand can be re-estimated and revised continuously. The data stored in the memory of the control means may alternativel or additionally include data input by an operator to cause the control mean to select at least one of the machines to be rendered or maintained operativ or rendered inoperative in preferance to a machine which would otherwise b selected by the control means. Thus for example, where a machine is foun to operate inefficiently, another more efficient machine preferably of th same nominal rated capacity, can be selected to be rendered operative i preferance to the inefficient machine, for the most efficient use of power. Particularly where each of the machines are of the same nominal rate capacity, the control means is preferably capable of operating the machine either in a cascade mode in which when it is necessary to increase th capacity of the system to match expected demand a further machine i rendered operative and when at a future time it is necessary to decrease th capacity of the system again to match expected demand the further machin is rendered inoperative again, and alternatively in a sequencing mode i which when it is necessary to increase the capacity of the system, the first machine of the inoperative machines to have been rendered inoperative is rendered operative when it is necessary to decrease the capacity of the system the first machine of the operative machines to have been rendered operative, is rendered inoperative. Thus the method may comprise calculating from the expected demand and data stored in the memory of the control means an estimated time from when a machine is rendered inoperative to when the machine is to be rendered operative again if the control means were operating the system in a sequencing mode, and where the estimated time is less than a preset time, the control means operating the system in a cascade mode and when the estimated time is greater than the preset time, the control means operating the system in a sequencing mode. Thus the problems of operating machines both in cascade mode and sequencing mode are overcome whilst the benefits of cascade mode and sequencing mode operation are adopted, when appropriate. When the control means operates the machines in a cascade mode and the further machine is to be rendered inoperative, the control means may calculate an expected period from the expected demand and the stored data, for which the machine is expected to remain inoperative before being rendered operative again, and where the expected period is greater than a predetermined time, the machine is rendered inoperative by turning the machine to an off state and where the expected period is less than the predetermined time, the machine is rendered inoperative by turning the machine to an off load state in which the machine continues to run but does not contribute to the capacity of the system. The method of the invention may also be used to control a plurality of machines at least some of which are of different capacities. The control means may select at least one of the machines to be rendered operative when it is necessary to increase the capacity of the system so that the capacity of the system is at least as great and close to the expected demand for the immediate future and the control means may select at least one of the machines to be rendered inoperative when it is necessary to decrease the capacity of the system so that the capacity of the system does not fall substantially below the expected demand in the immediate future. Again, when any machine is to be rendered inoperative the control means may calculate an expected period from the expected demand and the stored data for which the machine is expected to remain inoperative before being rendered operative aqain, and where the expected period is greater than a predetermined time, the machine is rendered inoperative by turninq the machine to an off state and where the expected period is less than the predetermined time, the machine is rendered inoperative by turning the machine to an off load state in which the machine continues to run but does not contribute to the capacity of the system. Conveniently, the expected demand at the time of estimation is estimated using the formula: fQ - C) t = R p. Where Q is the total capacity of the machines contributing to the system at the estimating time, C is the demand at the time of estimation t is the time since the last estimation. R is the storage size of the system and demand. Thus the value of C can be determined and may be used by the contr means as an estimation of expected demand in the immediate future. Preferably however, the control means is programmed to use not onl the value of C calculated at the estimating time, but at least one value o C calculated at a prior time, and the control means may apply a statistica connection to the value of C at the estimating time to more accuratel estimate the expected demand in the immediate future. Where the efficiencies of the various machines are known, the metho may calculate the total capacity Q of the system according to the formula: Q = (q l x e l ) + (q2 x e2) + (qn x en " ) Where q l to qn are the nominal output capacities of the machines, and e l to en are the actual efficiencies of the machines. The method of the invention is particularly applicable where th machines are each air compressors and the variable parameter is air pressur or air flow, the sensing means comprising at least one appropriate sensor. In a preferred arrangement, rather than sensing the air pressure in storage means or pressure or flow in an air delivery line near th compressors, at least one sensor is positioned at or adjacent an outlet fro the system to where the compressed air is delivered for use. Thus the control means can account for the pressure fall along the ai delivery line and because the air pressure at the outlet is monitored, th control means can more easily maintain the pressure at the outlet within selected pressure range so that the selected pressure range can be smal le and the lower first threshold, lower than in known control methods. The maximum and minimum values of the variable parameter which ar allowed by the control means, may be changeable by the control means within an absolute value range where this would result in more efficient us of the machines. According to a second aspect of the invention 1 provide an ai compressor system comprising at least one air compressor, a storage mean for storing compressed air produced by the air compressor, a delivery lin from the storage means to a point of delivery to where the compressed air i delivered for use, a sensing means to sense the value of a variable paramete of the system and provide a signal to a control means which controls the ai compressor, characterised in that the sensor is positioned closer to the poin of delivery than the capacity generating machines. According to a third aspect of the invention I provide an apparatus comprising a system of machines, each of the machines when operativ — o— contributing to α total capacity of the system, sensing means to sense the value of a variable parameter of the system, control means to which a signal from the sensing means is fed, the control means comprising a memory containing stored data, the control means being programmed to estimate the expected demand from the system in the immediate future and and render and maintain operative selected machines, the control means being programmed to select machines, in response to the expected demand and the stored data. Conveniently, the control means comprises a microprocessor and the control means includes a user input means to enable the user to input into the memory of the control means at least some of the data to be stored. The machines may each comprise air compressors and the sensing means may be at least one sensor. The apparatus may include a storage means in which compressed air produced by the machines is stored ready for use. The apparatus may comprise any of the features of the apparatus referred to in the method of the first aspect of the invention. The invention will now be described with reference to the accompanying drawing which is an illustrative view of an apparatus comprising a plurality of air compressor machines which may be controlled by a method in accordance with the invention. The apparatus comprises a control means 10 which controls a plurality of air compressor machines I I -14, four in the present example, although If desired, the control means 10 may be arranged to control more or less than four machines as required. Each of the air compressors 1 1 -1 when operative, produces compressed air which is fed along a supply line 1 5 to a storage receiver 1 6 from where compressed air may be drawn off for use along a delivery line 17 to a delivery point 19. A valve 18 is provided at the point of delivery 19 so that the escape of compressed air from the delivery line 17 may be controlled. The air is used to power machines for example, in a factory environment. If desired, the delivery line 17 may deliver compressed air to more than one point of delivery. The control means 10 is a suitably programmed computer which may b dedicated to control the air compressors 1 1 - 14 or may be part of a large computer system. In each case, the control means 10 has a memory M, a tim clock c, and a control pane! P such as a keyboard, so that an operator ca provide input commands to the control means 10 to control the machines 1 1 14. The air pressure within the delivery line 17 at or adjacent the point o delivery 19, is sensed by an air pressure sensor 20 which provides a signal t the control means 10 along a line 2 1. Where more than one point of deliver is provided from the delivery line 17, a pressure sensor 20 may be provided a each point of delivery, or a single pressure sensor 20 may be provided at th point of delivery which demands the 'lowest air pressure as required. Alternatively, air flow could be sensed or even the position of movable part of valve 18. At intervals, the control means 10 responds to the signal of the pressur or other sensing means 20 and uses this reading, and information stored in the memory M, which includes one or more readings taken at prior times and other data, the control means 10 estimates the expected demand for compressed air at the or each point of delivery 1 9 for the immediate future. The demand may, for example, be estimated once during a program cycle of the control means so that the intervals may be regular or irregular. Where the intervals are irregular, the length of each interval needs to be timed. In one example, the control means may estimate the expected demand by applying the formula :- - (Q-C t = Rp. Where Q is the total capacity of the machines contributing to the system at the time of estimation, C is the demand at the time of estimation, t is the time since last estimation, R is the storage size of the system including the storage receiver 1 6 and all pipework, and, p is the actual change in pressure since the last estimation of demand. Hence, by knowing the demand C and preferably the demand at previous times and applying statistical correction, the control means 1 0 can determine which of the machines 1 1 - 14 to operate to ensure that the air pressure in the system, or at least at point of delivery 1 , remains within a selected pressure range in the immediate future until the demand is re-estimated. A controller of the apparatus can input into the control means 10 using the keyboard P, constraints under which the control means 10 must operate. For example, the controller may instruct the control means to operate one of the air compressors 1 1 - 14 in preferance to any other air compressor, wherever possible. It will be appreciated that there are many other methods by which the control means may be arranged to estimate the expected demand using pressure or flow readings. For example, usinq the formula above, a value of C may be determined taking account of the change of p only since last estimation although C could be averaged for a plurality of previous estimated and/or otherwise statistically adjusted as required and corrected to give an accurate estimation of demand. Any other suitable method for estimating demand may be used. If desired, in a simple arrangement e.g. where the delivery line 17 has only one point of delivery 19, the actual flow of the compressed air may be monitored from one time of estimation until the next. Where the delivery line 17 has a plurality of points of delivery 19, which may each require different amounts of compressed air or a plurality of delivery lines are provided from the storage means 16, the pattern of demand may be very complex and hence complex statistical methods may be required accurately to estimate the expected demand for the immediate future, but in each case, the sensed value of pressure, flow or whatever, alonq with data stored in the memory are used to estimate expected demand. The method of the invention may be used to control any group of machines, not only air compressors as described, provided that each of the machines contribute to the total capacity of the system. For example instead of air compressor machines 1 1 -14, the method may be used to control vacuum creating machines, or heat producing machines. In each case, a variable parameter, such as vacuum, temperature etc. is sensed and a signal is provided to the control means 10 which uses the signal to estimate expected future demand of vacuum or temperature. Two specific methods of operating the machines 1 1 -14 in accordance with the invention will now be described by way of example only. Method 1 For this method all of the air compressors 1 1 -14 are of the same nominal rated output capacity and the control means 10 is capable of operating the air compressors 1 1 -14 in either a cascade or a sequencing mode. The machines 1 1 -14 are al! three state machines, that is the machines may be operative in an on state during which the machines each contribute to the capacity of the system, or in an off inoperative state when the machines do not run; or in an inoperative off load state in which the machines continue to run but do not contribute to the total capacity of the system. Assuming for example that air compressors 1 1 , 12, are operative and air compressors 13 and 14 are both inoperative in an off state, at an estimating time, if the sensing means 20 senses that the pressure at the delivery point 19 has fallen sufficiently to indicate increased demand, so the control means determines that another of the air compressors 13 or 14 must be render operative so as to increase the capacity of the system to match the deman and to ensure that the value of the variable parameter i.e. pressure, does n fal l below a first threshold value in the immediate future. The control means 10 applies the formula given above and statistic methods as required, to estimate the expected demand of the system for t immediate future until the demand is re-estimated. The value of Q is known to be the total capacity of the machin currently operating i.e. air compressors I I and 12. This data may also contained in memory M. The value of t may be a selected value which may programmed into the memory M, using the keyboard P and is fixed or may a timed period which may vary. For example, the demand may be r estimated at intervals t which may be determined by a cycle time of t program of the control means. R is known from a previous determination, f example, as explained below. The value of p is calculated from the reading of the pressure sensor and a previous reading of the pressure sensor 20 when demand was la estimated, and is stored in the memory. Hence demand C is estimated, at t estimating time. As mentioned above, earlier calculated values of C may be used wi the latest calculated value to determine the expected demand in t immediate future. Now when the machines are operated in sequencing mode, the contr means may determine that air compressor 1 3 is the next machine to rendered operative according to a predetermined sequence i.e. 1 1 , then 1 then I 3, then 14, and so render compressor 13 operative. At the time of ne demand estimation, the pressure at the delivery point 19 is sensed again b sensor 20 and the demand is re-estimated. One of three conditions must ensue. First, demand has increased relative to capacity and the control mea predicts that the pressure sensed will fall below the first threshold pressur in the immediate future. The control means 10 responds by renderin operative the air compressor 14 which is the next in the sequence. Second, demand has changed relative to capacity but the control mea predicts that the pressure sensed will not rise above the second threshol pressure or fall below the first threshold pressure in the immediate futur The control means 10 responds by maintaining machines 1 1 -13 operative and machine 1 inoperative. Third, demand has fallen relative to capacity and the control means predicts that the pressure sensed will rise above the second threshold value which is greater than the first threshold value in the immediate future. The control means 10 responds by rendering inoperative one of the machines I I - 13 so as to decrease the capacity of the system, the machine 1 1 -13 to be rendered inoperative being selected depending on the expected demand and data stored in the memory. As the control means 10 is operating the machines in a sequencing mode, the next machine in the predetermined sequence to be rendered inoperative would be machine 1 1 i.e. machine 1 1 should be turned off and whenever it is desired to render operative one of the machines I I or 14, the control means 10 would ordinarily respond by rendering machine 14 operative because machine 14 had been rendered inoperative prior to machine I I . However, the control means 10 is arranged to predict how long it will be before machine I I is to be rendered operative again. If this time is greater than a preset off time, the control means 10 continues to operate the machines 1 1-14 in a sequencing mode as described, and so machine 1 I would be turned off. However if this predicted off time is less than the minimum preset off time, the control means 10 responds by switching operation of the machines 1 1 - 14 from a sequencing mode to operate the machines in a cascade mode, and machine 1 1 instead of being rendered inoperative by turning the machine off, is rendered inoperative by turning the machine to an off load inoperative state. Thus when it is desired to again increase the capacity of the system, machine I I is rendered operative again, rather than machine 14 beinq rendered operative. Thus at every estimation of expected demand, if the control means determines that the capacity of the system is to be decreased i.e. at least one of the machines 1 1 -14 is to be rendered inoperative, the expected demand is used with any data in memory M to decide which machine or machines is or are to be rendered inoperative and whether the machines I I - 14 are to be operated in a sequencing or cascade mode. Thus the net result is that the control means 10 select and renders operative until next estimation of expected demand, machines such that the pressure at delivery point 19 is kept between the first and second threshold pressures, at least for a major portion of the time period between estimations. Of course, the estimation of expected demand is unlikely ever to b very accurate, but it has been found that the estimation, particularly usin previous estimations, results in considerably more efficient use of th machines than hitherto possible, whilst protecting the machines from to frequent switching. The programming of the control means may ensure that the mos efficient machine or combination of machines are used where for example the required capacity can be obtained by operation of a different machine o different combination of machines. The operator can input into the control means 10 using the contro panel P conditions or constraints under which the control means mus operate. For example, if it is known that machine I I is particularly efficien in operation compared with machines 12- 1 , for example if the machine I I i newer, machine I I may be constrained to operate continuously whe appropriate and not be included in any sequencing consideration. The use may input into the control means the values of the first and second threshol pressure values, the value of the preset off time, and in some instances, th value of fixed time intervals t between readings when pressure is sensed an demand is estimated. The programming of the control means 10 may include safet considerations so that in the event that pressure sensed by sensor 20 (or further sensor provided in a safety circuit) rises above or fal ls below respective safe limit, the system is shut down or otherwise rendered safe o workable. Method 2 In this method al l of the machines 1 1 - 14 are al l of differen nominal rated output capacities, for example machine 12 is capable o producing twice as much compressed air as machine 1 1 , and machine 1 3 thre times as much as machine I I , and machine 14 four times as much as machin I I . If at the end of intervals, at estimating times, if the control means 1 determines that it is necessary to increase the capacity of the system i order to keep the pressure within a selected pressure range between an uppe second threshold pressure and a lower first threshold pressure in th immediate future, the control means 10 determines which of the available ai compressors I 1 -14 is to be rendered operative. For example, assuming that only air compressor 13 is operating and th control means estimates that in the immediate future, the demand will increase such that the air pressure at the delivery point 19 will fall below the first threshold pressure, the control means 10 may respond by rendering another of the machines 1 1 - 14 operative, for example machine 1 1 where the increase in demand is expected to be small, or where a great increase in demand is expected, machine 12 may be rendered operative instead or as well, or where a medium increase in demand is estimated, machine 13 may be rendered inoperative, and machine 1 rendered operative, or machines I I and 12 may be rendered operative whilst machine 13, 14, are inoperative, in each case, the machines I f -14 which are rendered operative are chosen to match the expected demand as close as possible, whilst the capacities of the machines which are operative are greater than the expected demand. Thus in the immediate future, or at least for a major portion of the time until expected demand is re-estimated, the available pressure at delivery point 1 9 preferably is kept within a range bounded by the first and second threshold pressure values. Again, constraints may be input to the control means ! 0 under which the control means 10 is to operate. For example, an operator may decide that machine 12 is to be rendered operative whenever possible because for example, machine 12 is found to operate more efficiently than the other machines, where for example, machine 12 is a newer machine. Both of the two methods described above by way of example only, may, instead of operating individual machines 1 1 - 1 , be used to operate groups of machines or machines within designated groups. In each case, the expected demand is used by the control means to determine which of the machines is to be rendered operative and inoperative to achieve the most efficient use of the system. In both the methods above, and throughout this specification, by "immediate future" I mean the time until the expected demand is re- estimated which is preferably a short length of time of a few seconds. As explained above, this time may be fixed, or may be variable due to, for example, the cycle time of the program of the control means. To estimate the storage capacity R of the system, the formula given above may be applied when there is no demand from the system i.e. when C i zero. In this case:- Qt = Rp. Q is known provided that the control means can determine the total nominal rated capacity of each operative air compressor 1 1 -14 t is the predetermined time period and p is the increase in pressure during th time t. Of course, in practice, a system may have smal l leaks, but th approximation given for the total storage size of the system R will b adequate for determining the expected demand in future time periods. For improved accuracy, the estimation of R may be repeated iteratively during calibration. In all of the examples given above, the pressure sensor 20 is positioned at the point of delivery or at least closer to the point of delivery than the storage means. It has been found that by sensing the pressure at this point i.e. closer to the point of delivery than the storage means I 6, a more accurate determination of pressure can be achieved resulting in more efficient use of the machines. It is known that, particularly where the del ivery line 1 7 is long, there is a significant pressure fail along the delivery line 17 which increases as line flow increases. Where, as is conventional, pressure is sensed at or adjacent the storage means 1 , the value of at least the first threshold pressure has to be set high to ensure that sufficient pressure is always available at the point of delivery. In accordance with the invention, the lower threshold pressure can be set lower i.e. only slightly above the minimum acceptable pressure at the point of delivery. This contributes to energy saving. The features disclosed in the foregoing description, in the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, or a class or group of substances or compositions, as appropriate, may, separately or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims: CLAIMS 1. A method of operating a plurality of machines each of which contributes when operative, to a total capacity of a system, the method comprising sensing with a sensing means the value of a variable parameter of the system, feeding a signal from the sensing means to a control means, programming the control means to estimate from data stored in a memory thereof, and the signal from the sensing means, the expected demand from the system in the immediate future and rendering and maintaining operative selected machines, the selection being made by the control means in response to the estimated expected demand. 2. A method according to Claim I wherein the expected demand is re- estimated at intervals. 3. A method according to Claim 2 wherein the lengths of the intervals are dependent on a cycle time of the program of the control means. 4. A method according to any one of Claims I to 3 wherein the data stored in the memory of the control means includes data derived from at least one signal from the sensing means indicating the value of the variable parameter at a prior time. 5. A method according to any one of Claims I to 4 wherein the data stored in the memory of the control means includes data input by an operator to cause the control means to select at least one of the machines in preferance to a machine which would otherwise be selected by the control means. 6. A method according to any one of the preceding claims, wherein each of the machines are of the same nominal capacity. 7. A method according to Claim 6 wherein the control means is capable of operating the machines in a cascade mode in which when it is necessary to increase the capacity of the system to match expected demand, a further machine is rendered operative and when the at a future time it is necessary to decrease the capacity of the system to match demand the further machine is rendered inoperative again, and alternatively in a sequencing πrode in which when it is necessary to increase the capacity of the system, the first machine of the inoperative machines to have been rendered inoperative is rendered operative and when it is necessary to decrease the capacity of th system, the first machine of the operative machines to have been rendere operative is rendered inoperative, the method comprising calculating fro the expected demand and data stored in the memory of the control means a estimated time from when a machine is rendered inoperative to when th machine is to be rendered operative again and if the control means wer operating the system in a sequencing mode, and where the estimated time i less than a preset time, the control means operating the system in a cascad mode, and when the estimated time is greater than the preset time, th control means operating the system in a sequencing mode. 8. A method according to Claim 7 wherein when the control mean operates the machines in a cascade mode and the further machine is to b rendered inoperative, the control means calculates an expected period fro the expected demand and the stored data for which the machine is expecte to remain inoperative before being rendered operative again, and where th expected period is greater than a predetermined time, the machine i rendered inoperative by turning the machine to an off state and where th expected period is less than the predetermined time, the machine is rendere inoperative by turning the machine to an off load state in which the machin continues to run but does not contribute to the capacity of the system. 9. A method according to any one of Claims I to 5, wherein at least som of the machines are of different capacities, the control means selecting a least one machine to be rendered operative when it is necessary to increas the capacity of the system so that the capacity of the system is at least a great and close to the expected demand for the immediate future and th control means selecting at least one machine to be rendered inoperative whe it is necessary to decrease the capacity of the system so that the capacity o the system does not fall substantially below the expected demand in th immediate future. 10. A method according to Claim 9 wherein when any machine is to b rendered inoperative the control means calculates an expected period fro the expected demand and the stored data for which the machine is expecte to remain inoperative before being rendered operative again, and where th expected period is greater than a predetermined time the machine i rendered inoperative by turning the machine to an off state and when th expected period is less than the predetermined time the machine is rendere inoperative by turning the machine to an off load state in which the machin continues to run but does not contribute to the capacity of the system. 1 1. A method according to any one of claims 4 to 10 where appendent t claim 4 wherein the expected demand at the time of estimation is estimate using the formula: (Q - C) t = R p Where Q is the total capacity of the machines contibuting to the syste at the estimated time, C is the demand at the time of estimation, t is the time since the last estimation, R is the storage size of the system, and p is the actual change in variable parameter since the last estimation o demand. 12. A method according to Claim I I wherein the control means i programmed to use not only the value of C calculated at the estimating time but at least one value of C calculated at a prior time, the control mean applying a statistical correction to the value of C at the estimating time. 13. A method according to Claim I I or Claim 12 wherein the total capacit Q of the system is given by the formula: Q = (ql x el) + ( q2 x e2) + ( qn x en) Where ql to qn are the nominal rated output capacities of the machines, and el to en are the actual efficiencies of the machines. 14. A method according to any one of the preceding claims wherein th machines are each air compressors and the variable parameter is one of ai pressure and air flow, the sensing means comprising at least one appropriat sensor. 15. A method according to Claim 14 wherein the at least one sensor i positioned at an outlet from the system to where the compressed air i delivered for use. 1 6. A method according to any one of the preceding claims wherein th maximum and minimum values of the variable parameter which are allowe by the control means are changeable by the control means within an absolut value range. 17. An air pressure system comprising at least one air compressor, storage means for storing compressed air produced by the air compressor, delivery line from the storage means to a point of delivery to where th compressed air is delivered for use, a sensing means to sense the value of variable parameter of the system and provide a signal to a control mean which controls the air compressor, characterised in that the sensor i positioned closer to the point of delivery than the capacity generatin machines. 18. An apparatus comprising a system of machines, each of the machine when operative contributing to a total capacity of the system, sensing mean to sense the value of a variable parameter of the system, control means t which a signal from the sensing means is fed, the control means comprising memory containing stored data, the control means being programmed t estimate the expected demand from the system in the immediate future an render and maintain operative selected machines, the control means bein programmed to select machines, in response to the expected demand and th stored data. 19. An apparatus according to Claim 18 wherein the control mean comprises a microprocessor and the control means includes a user inpu means to enable a user to input into the memory of the control means a least some of the data to be stored. 20. An apparatus according to Claim 18 or Claim 1 9 wherein each of th machines comprise air compressors and the sensing means comprising at least one pressure sensor. 2 1. An apparatus according to Claim 20 which includes a storage means in which compressed air produced by the machine is stored ready for use. 22. An apparatus according to any one of Claims 10 to 21 having any of the features of the apparatus specified in claims I to 17.

Inventors: BURDETT SAVILE, BURDETT, SAVILE
Applicants: BURDETT SAVILE, BURDETT, SAVILE
Application year: 1988
Main classification: G05D16
IPC codes: G05D16