WO1987006732A1

WO1987006732A1 - Method of operating a system

Info

Publication number: WO1987006732A1
Application number: PCT/GB1987/000274
Authority: WO
Inventors: Savile Burdett
Original assignee: Savile Burdett
Priority date: 1986-04-25
Filing date: 1987-04-24
Publication date: 1987-11-05
Also published as: GB8728864D0; GB2198554A; GB8610170D0; AU7307487A

Abstract

A system for supplying compressed air and comprising several compressors (10, 14) is so controlled that the compressors are turned off in succession and the first of the compressors to be turned off is the first compressor to be turned on when there is a requirement to increase the number of compressors which are operating.

Description

Title: Method of operating a system.

This invention relates to a method of operating a system comprising a plurality of machines such as air compressors, vacuum generating machines, electrical supply machines, or other machines, each of which contributes, when operative, to a total capacity of the system. The invention will be described specifically in relation to a system comprising a plurality of air compressors, where the capacity of the system is the rate at which work can be done by the air supplied by the compressors , although the invention has many other varied and wide applications.

For example, where the machines are vacuum generating machines, the capacity of the system is related to the volume of and pressure in an evacuated space; where the machines comprise electrical supply machines, the capacity of the system is available electrical power.

Conventionally, the capacity of a system comprising a plurality of air compressors is sensed by one or more pressure sensors. As the pressure increases above a first, turn-off threshold value, a first of the air compressors is rendered inoperative. As the pressure rises still further above a second turn-off threshold value, a second of the compressors is rendered inoperative, and so on.

When the pressure falls below a first turn-on threshold value, for example as compressed air is drawn off for use, the second, or last air compressor to be rendered inoperative, is again rendered operative and is thus, brought on-line. If the pressure continues to fall to below a second turnon threshold value, the first or next to last air compressor to be rendered inoperative, is again rendered operative and is thus brought on-line. Thus conventionally, in air compressor systems, each of the plurality of air compressors is arranged to be rendered inoperative at a fixed turn-off threshold pressure, and rendered operative, at a fixed turn on threshold pressure which is typically lower than the turn-off pressure for that particular compressor. The fixed turn-off and turn-on threshold pressures for each compressor are usually different, so that one compressor is rendered inoperative at a threshold pressure which is higher than the threshold pressure at which the next compressor is rendered operative. This method of operating a system is known as cascading. When α system is operated in this way, the compressor which is arranged to be rendered inoperative at the highest pressure will be operative for a much longer time than the other compressors and will thus contribute more work to the capacity of the system. An object of the invention is to operate a system so that each machine contributes approximately equal work to the total capacity of the system.

According to a first aspect of the invention 1 provide a method of operating a system comprising a plurality of machines each of which, when operative, contributes to the total capacity of the system, a control means, and means to sense a parameter dependant upon the capacity of the system, the method comprising pre-setting into the control means a plurality of stepped turn-off threshold values of said parameter at each of which it is desired to render one of the machines which is operative, inoperative, presetting into the control means a plurality of stepped turn-on threshold values of said parameter at each of which it is desired to render operative, one of the machines which is inoperative, operating at least some of the machines to change said parameter in one direction until the next turn-off threshold is reached, rendering one of the machines inoperative and, when the value of said parameter changes in the opposite direction to reach the next turn-on threshold, rendering a different one of the machines operative in preferance to the last machine rendered inoperative.

Thus at least one machine will be switched "on" and "off" less frequently, and each machine will contribute more equally to the total capacity of the system. In one embodiment, the first one of a plurality of operative machines to be rendered inoperative is always the first of the thus inoperative machines to be rendered operative again when the parameter falls below the next turnon threshold value.

To achieve this, the control means may determine the order in which the machines are rendered inoperative and assign the machine which is rendered inoperative first, a position at a head of a turn-on queue. When subsequently one or more further machines is/are rendered inoperative as further turn-off thresholds are reached, the or each of these is assigned a subsequent position in the queue in the order in which they are rendered inoperative, the machine at the head of the turn-on queue being the first machine to be rendered operative again when the value of the parameter falls to the next highest turn-on threshold, the next machine in the turn-on queue thus becoming the machine at the head of the queue and accordingly being the next machine to be rendered operative again. Preferably, when α machine is rendered operative, the control means assigns the machine a position at the end of a turn-off queue, and when the parameter reaches the next lowest turn-off threshold value so that it is desired to render one of the machines inoperative, a machine at the head of the turn-off queue is rendered inoperative first, and any subsequently positioned machines in the turn-off queue, are rendered inoperative before the machine positioned at the end of the turn-off queue.

Thus the last machine to be rendered operative is the last machine to be rendered inoperative again and vice versa.

In another embodiment, any machine rendered inoperative subsequent to a first machine to be rendered inoperative, may be rendered operative in preferance to the first machine to be rendered inoperative, depending upon the cumulative time for which each machine has been operational.

To achieve this, the control means may determine the cumulative time for which each of the machines is operative, and when the capacity of the system reaches the next lowest threshold value at which it is desired to render one of the machines inoperative, the machine which has the longest cumulative operational time, is rendered inoperative first. Further, when the capacity of the system falls to the next highest turn-on threshold value, the machine which has the shortest cumulative operational time may be rendered operative first in preferance to the other machines.

In both embodiments, by operating the system in accordance with the invention, it has been found that each machine can be arranged to contribute substantially equal work to the system.

Preferably, the number of stepped turn-on and turn-off threshold values which are pre-set into the control means, are equal to the total number of machines of the system.

Of course, not all of the machines of the system may be available for use at any time, so the control means may be adapted to receive a signal from each machine and determine which of the machines are available for use. Where one of the machines due to be rendered operative is unavailable for use, the control means may render operative the next machine in the turn-on queue, or the machine with the next lowest accumulative operational time.

The control means may comprise a suitably programmed microprocessor or computer, and the pre-setting of the stepped threshold values may be achieved during programming, or may be input by a user as required by the program. According to α second aspect of the invention we provide a control means comprising means to receive a signal from each of a plurality of machines of a system, and means to provide a signal to each of a plurality of machines to render each of the plurality of machines of the system on or off, and means to receive a signal from at least one sensor, which senses the actual capacity of the system, the control means operating the system in accordance with the method according to the first aspect of the invention.

The method of the first aspect of the invention and the control means of the second aspect of the invention are particularly applicable where the system comprises a plurality of air compressor machines each of which contributes to a supply of compressed air, in which case the turn-on and turnoff stepped threshold values may comprise air pressures.

Each of the air compressors may feed compressed air to a reservoir from which compressed air may be drawn off for use. The sensor may comprise a pressure sensor to sense the pressure within the reservoir, or within a supply line from the reservoir to where the compressed air is to be used.

The invention will now be described with reference to the accompanying drawings in which:

FIGURE 1 is a diagrammatic representation of a system operating in accordance with the invention.

FIGURES 2a to 2c are flow diagrams illustrating the method of operation of the system of Figure 1.

Referring first to Figure 1, a system comprises a plurality of machines i.e. electrically operated air compressors, five In this example, indicated at 10-14. Each compressor feeds compressed air and thus contributes to the total pressure of air within a reservoir 15, from which compressed air may be drawn off for whatever purpose Is required, along a supply line 16.

A pressure sensor 17 senses the pressure within the reservoir 15 and feeds a signal to a control means 18, which in the present case comprises a suitably programmed microprocessor. If desired, the sensor 17 could be positioned to sense the pressure within supply line 16.

The air compressors 10 to 14 are all controlled by the control means 18. Each air compressor provides an input signal to the control means 18, along respective lines 10a to 14a, to indicate to the control means 18 the operational state or readiness of the respective compressors 10 to 14, that is whether each compressor 10 to 14 is available for use. It will be appreciated that air compressors can become too warm, and when the temperature of the compressor reaches a predetermined value, a thermostatic sensor which is conventionally provided, causes the compressor to be switched off until the temperature falls below a lower predetermined value. In the present example this may be achieved by feeding a signal from the thermostatic sensor of each air compressor to the control means 18, which switches off the respective compressor in preferance to any other desired control.

Preferably though, each compressor has Its own thermostatic circuit to switch off the compressor if It becomes too warm, and the signals provided to the control means 18 along lines 10a-14a merely indicate the readiness or otherwise of the air compressors.

Of course, an "unavailable" signal may be generated for other reasons, for example during maintenance of a compressor or in the event of any fault ocurring within a compressor.

Preferably, the signal to the control means comprises a current when the compressor is available for use, which current is absent when the compressor becomes unavailable for use.

The control means 18 provides a signal to each and any of the air compressors 10 to 14 along respective lines 10b to 14b, to switch the compressors 10 to 14 on and off.

It will be appreciated that each of the compressors 10 to 14 when operative, contributes to the total capacity of the system i.e. the amount of compressed air available within reservoir 15.

As is conventional practice, on initial start-up, all of the compressors 10 to 14 which are available, are used until a first, lowest, stepped turn-off threshold pressure Is sensed in reservoir 15 by sensor 17. One of the compressors 10 to 14, for example compressor 10, is then switched off, and so compressors 1 1 to 14 only, continue to contribute to the total capacity of the system.

If the pressure within the reservoir 15 Increases further until a next lowest stepped turn-off threshold pressure is sensed, a second of the air compressors, for example compressor 1 1 is switched off, and so on until the capacity of the system reaches a maximum value when all of the compressors 10 to 14 would be switched off.

The stepped turn-off threshold pressures are pre-set into the control means 18. The steps may be equal, but preferably are unequal. If compressed air is drawn off from the reservoir 15 via the line 16 at any time, the pressure within the reservoir 15 may drop, if the rate at which the compressed air is drawn off exceeds the rate at which compressed air is fed to the reservoir by the air compressors 10 to 14, or any of them. Thus if for example, the compressors 13 and 14 only are operative, and compressed air is drawn off so that the pressure within the reservoir 15 falls below the next highest stepped turn-on threshold pressure, one or more of the inoperative compressors 10, 1 1 or 12 may be switched on at least until the pressure within the reservoir 15 again reaches the next lowest stepped turnoff threshold pressure. If the pressure still continues to drop to the next highest stepped turn-on pressure, another of the inoperative compressors may be switched on.

Again the stepped turn-on threshold pressures may be pre-set into the control means and preferably the steps are equal, but may be unequally spaced if required.

The method in accordance with the Invention provides where, for example compressor 10 is the last air compressor to be switched off as the pressure within the reservoir 15 rises, a different air compressor may be the first air compressor to be again switched on when the pressure within the reservoir 15 falls below the next highest turn-on threshold value.

Referring now to Figures 2a, 2b and 2c, the method of operation of the system, of Figure 1 will be described.

The microprocessor of the control means 18 is programmed to check which of the compressors 10 to 14 are available for use, and to continuously monitor the pressure within the reservoir 15 and to switch on or off one or more of the compressors 10 to 14 until a pressure within a desired range is obtained.

Referring first to Figure 2a a flow diagram of a check routine of a program of the microprocessor of control means 18 is illustrated. Upon startup, the control means 18 first checks which of the compressors 10 to 14 are available for use, as indicated at stage 20 in the flow diagram. If a signal is received that one or more of the compressors 10 to 14 are unavailable for any of the reasons mentioned above, the respective compressor/compressors is/are switched off where appropriate (stage 21), or merely ignored by the microprocessor until it/they become(s) available again.

The microprocessor then determines whether more compressors are required to increase the pressure within the reservoir 15 to within a desired range. First, the microprocessor reads the actual pressure as determined by sensor 17 (stage 22), and reads the next lowest turn-on step threshold (stage 23) from a look-up table 24. This information may be pre-set into the look-up table 24 during programming, or by a user of the system in accordance with the program.

The look-up table 24 preferably contains a plurality of stepped turn-on threshold pressures at each of which it is desired to switch on a, or another compressor, in order to maintain a total pressure within the reservoir 15 within a desired range. Preferably, the number of stepped turn-on threshold pressures pre-set into the look-up table 24 is equivalent to the number of machines of the system, in the present example five, even though all the machines 10 to 14 may not be available for use.

The microprocessor then decides whether the pressure sensed is less than the next lowest turn-on step threshold (see stage 25). If the pressure is less, another compressor is switched on by a turn-on routine (stage 26) which is described in detail below.

Of course, upon start-up the pressure sensed may be no greater than atmospheric pressure and all the available compressors 10-14 will be required. Thus on start-up, the next lowest turn-on step threshold pressure is of course the lowest step threshold pressure of the look-up table 24.

If the microprocessor determines that the pressure sensed is not less than the next lowest turn-on step threshold pressure , the microprocessor then decides if the pressure is greater than the; next highest turn-off step threshold, which may be the maximum pressure within the desired operational range which it Is desired to obtain within the reservoir 15.

To achieve this, the microprocessor reads the next highest turn-off step threshold pressure from within the look-up table 24 (stage 26) which also contains a plurality of stepped turn-off threshold pressures within the operational range of the system, at each of which it is desired to switch off a, or another, compressor in order to maintain the total pressure within the reservoir 15 within the operational range. Preferably the number of stepped turn-off threshold pressures pre-set into the look-up table 24, is also equivalent to the number of machines of the system.

The microprocessor can then decide whether the sensed pressure is greater than the next highest turn-off threshold pressure (stage 27).

If the pressure sensed is not greater than the next highest turn-off step threshold, then a signal is sent to a "repeat" line 28 which causes the microprocessor again to check which of the compressors are available and to continuously monitor the pressure within the reservoir 15.

If however the pressure sensed is greater than the maximum next highest turn-off step threshold, a signal is sent to a turn-off routine (stage 29) which is described with reference to Figure 2c below, to switch off one or more of the compressors which are operating.

At the end of the turn-on routine 26 or the turn-off routine 29, a signal is sent to the "repeat" line 28, along lines 30 and 31 respectively, so that again, the microprocessor will determine which of the compressors 10-14 are available for use, and continuously monitor the pressure within the reservoir

131.

Referring now to Figure 2b the turn-on routine part of the program 26 is shown illustratively in flow diagram form.

The: check routine part of the program is illustrated for completeness at 32.

First, the turn-on routine determines whether or not the next compressor to be turned on is already specified (stage 23). If not, then the next compressor to be turned on needs to be specified.

The control means 28 may be operated manually In which case at stage 33 a compressor will already have been manually specified, as being the next compressor to be switched on. If however the control means is operated in an automatic mode of operation, the next compressor to be switched on needs to be determined.

In one embodiment of the invention, the next compressor to be switched on is the compressor which was switched off the longest ago by the turn-off routine described below.

To achieve this, when a compressor is switched off, for example compressor 10, the microprocessor positions the compressor 10 at the end of a "turn-on queue" and any others of the compressors 1 1 to 14 which are subsequently switched off are positioned in the queue behind compressor 10 in the order in which the compressors were switched off.

The next compressor to be specified for switching on at stage 34, of the turn-on routine, is the compressor at the head of the turn-on queue.

In another embodiment, the control means 18 includes a timer means which times the accumulative time since a datum time, which individual compressors 10 to 14 have been operative. When it is desired to switch on another compressor, the compressor with the shortest accumulative operational time accrued is specified at stage 34 of the turn-on routine. In both cases, when a compressor has been specified, the tum-on routine can continue.

When the next compressor is specified, the program then determines whether or not this compressor is available (stage 35). If so, then the compressor is switched on (stage 36) and a signal is sent on the repeat line 28 to the beginning of the check routine part of the program described above, to cause that program to again determine which of the compressors 10-14 are available and to continuously monitor the pressure within the reservoir 15.

If the specified compressor is not available, then another compressor is specified, If it has not already been manually specified. In the first embodiment mentioned above, this would be the next compressor in the turnon queue. In the second embodiment mentioned above, this would be the compressor with the next shortest accumulative operational time accrued. Stage 35 of the turn-on routine again decides when this next compressor is available, and if so, switches on this compressor as indicated at stage 36.

Referring now to Figure 2c the turn-off routine part of the program is illustratively described with reference to a flow diagram.

This routine is similar to the turn-on routine part of program of Figure

2b, and shows for completeness, the check routine at 32. The turn-off routine determines whether the next compressor to be switched off is already manually specified (stage 40), and if so, that compressor is turned off (stage

41). Otherwise, a compressor is specified.

In the first embodiment mentioned above, the compressor which has been operative for the longest time, i.e. the compressor which was first switched on, is specified for switching off.

This is achieved because as a compressor is switched on by the turn-on routine already described with reference to figure 2b, the microprocessor positions that compressor at the head of a turn-off queue. Any subsequently switched-on compressors are positioned in the queue In the order in which they were switched on. Thus at any time, the compressor at the head of the queue has been operational for the longest time, and accordingly is the first to be specified for switching off. Thereafter, the next compressor in the queue becomes the head of the queue and is due for switching off next.

In the second embodiment mentioned above, when the Individual accumlative operational times of the compressors are measured by the timing means, the microprocessor switches the compressor off which has the longest accumulative operational time. In any case, when a compressor is switched off, at stage 41 , a signal is then sent again to the check routine part of the program 32 via repeat line 28, which checks which compressors are available for use and continuously monitors the pressure within the reservoir 15.

Thus In a method In accordance with the invention, instead of each compressor being switched on or off according to fixed switch on and switch off pressure threshold value, peculiar to each compressor, In the first embodiment the compressors are switched on or off in a sequence determined by the control means so that the compressor which was switched on first by the turn-on routine, is the compressor which is switched off first, and the compressor which is switched off first by the turn-off routine is the compressor which is switched on and brought on-line first, and in the second embodiment, the compressor which is operative and has the longest accumulative operational time is the first to be switched off, and the compressor which is inoperative and has the shortest accumulative operational time, is the first to be switched on.

This is in contrast to existing systems where the last compressor to be switched off Is the first compressor to be switched on, which results In uneven use of the compressors and frequent switching "on" and "off" of at least one of the compressors. In the method in accordance with the present invention, the compressors each contribute substantially more equal work to the system.

Although as described above, each of the compressors 10-14 may be switched off by the control means when a step threshold pressure is sensed, if desired, the compressors may continue to be switched on but otherwise become non operative. For example, the load may be relieved. This could be achieved by positioning valves in the supply lines from the compressors to the reservoir 15, which valves are closed by the control means to render the respective compressor Inoperative.

Although as described, the invention has been applied to a system comprising a plurality of air compressors, of course, the invention has many other wide and varied application, in which case the machines could be for example vacuum generating machines, electrical supply machines, or any other machines which contribute to a total capacity of a system. In the example described with reference to the drawing, the capacity of system is of course the amount of compressed air in the desired pressured range which is available for use. Where the machines comprise vacuum generating machines, the total capacity of the system may comprise the amount of vacuum available, and where the machines comprise electrical supply machines, the total capacity of the system may comprise the power available for use.

The features disclosed in the foregoing description, 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 system comprising a plurality of machines, each of which, when operative, contributes to the total capacity of the system, a control means and means for sensing a parameter dependant upon the capacity of the system, the method comprising pre-setting into the control means a plurality of stepped turn-off threshold values of said parameter at each of which values it is desired to render inoperative one of the machines which is operative, pre-setting into the control means a plurality of stepped turn-on threshold values of said parameter, at each of which values it is desired to render operative one of the machines which is inoperative, operating at least some of the machines to change said parameter in one direction until the next turn-off threshold value is reached, rendering one of the machines inoperative and, when the value of said parameter changes in the opposite direction to reach the next turn-on threshold value, rendering a different one of the machines operative in preference to the last machine rendered inoperative.

2. A method according to Claim 1 wherein a first one of a plurality of operative machines to be rendered inoperative is the first of the machines to be rendered operative again when said parameter falls below the-next turn-on threshold value.

3. A method according to Claim 2, wherein, when a machine is rendered operative, the control means assigns the machine a position at the end of a turn-off queue and, when the value of said parameter reaches the next lowest turn-off threshold value so that it is desired to render one of the machines inoperative, a machine at the head of the turn-off queue Is rendered inoperative first and any subsequently positioned machines in the turn-off queue are rendered inoperative before the machine positioned at the end of the turn-off queue.

4. A method according to Claim 1 wherein there is maintained a cumulative record of the time for which each machine has been operative and, when the value of said parameter reaches the next turn-off threshold value, the machine which has the greatest cumulative operational time is rendered inoperative first.

5. A control means comprising means for receiving a signal from each of a plurality of machines of a system, means for providing a signal to each of a plurality of machines to render each of the plurality of machines of the system on or off and means for receiving a signal from at least one sensor which senses a parameter dependant upon the actual capacity of the system, the control means operating the system in accordance with the method of claim 1.

6. Any novel feature or novel combination of features disclosed herein or in the accompanying drawing.