WO2006051260A1

WO2006051260A1 - Vacuum pumping system

Info

Publication number: WO2006051260A1
Application number: PCT/GB2005/004128
Authority: WO
Inventors: Laurent Marc Philippe; Anthony John Wilder
Original assignee: The Boc Group Plc
Priority date: 2004-11-09
Filing date: 2005-10-25
Publication date: 2006-05-18
Also published as: GB0424734D0; TW200630544A

Abstract

A system for evacuating an enclosure comprises a plurality of vacuum pumps connected in parallel such that fluid is conveyed from an outlet of an enclosure to a respective inlet of each pump, and from the outlets of the pumps to a common exhaust. A respective variable speed motor drives each pump. A control system is provided for controlling the speeds of rotation of the pumps. In one embodiment, the control system sets a target speed for the pumps as a function of the speed of the slowest pump, and in another embodiment the control system sets the target speed as a function of the average speed of the pumps.

Description

VACUUM PUMPING SYSTEM

This invention relates to a vacuum pumping system comprising a plurality of variable speed pumps connected in parallel.

Vacuum processing is commonly used in the manufacture of semiconductor devices and flat panel displays to deposit thin films on to substrates. A processing chamber is evacuated using a vacuum pumping arrangement and feed gases are introduced to the evacuated chamber to cause the desired material to be deposited on one or more substrates located in the chamber. Upon completion of the deposition, the substrate is removed from the chamber and another substrate is inserted for repetition of the deposition process. Evacuated load lock chambers are used to transfer substrates to and from the processing chamber.

Pumping arrangements used to evacuate load lock chambers to the desired pressure generally comprise a booster pump connected in series with a backing pump. The inlet of the booster pump is connected to an outlet from the chamber, and the inlet of the backing pump is connected to the exhaust of the booster pump.

Booster pumps typically have oil-free pumping mechanisms, as any lubricants present in the pumping mechanism could cause contamination of the clean environment in which the vacuum processing is performed. Such "dry" vacuum pumps are commonly single or multi-stage positive displacement pumps employing inter-meshing rotors in the pumping mechanism. The rotors may have the same type of profile in each stage or the profile may change from stage to stage. The pumping mechanism is generally driven by a variable speed motor, which is in turn driven by an inverter. In order to vary the speed of rotation of the pumping mechanism, or "pump speed", a controller issues commands to the inverter to vary the frequency of the power supplied to the motor. By varying the pumping speed, the booster pump can maintain a constant system pressure even under conditions where the gas load may vary substantially.

When a booster pump becomes overloaded for any reason, the power consumption of the pump becomes equal to or greater than a predetermined value. The controller typically monitors the power consumption of the pump, for example by monitoring the current drawn by the motor or the torque required to rotate the pumping mechanism. In order to prevent overheating of the pumping mechanism, motor or drive when overload occurs, the controller instructs the inverter to reduce the frequency of the power supplied to the motor in order to decrease the pump speed and thereby reduce both the load on the pump and the power consumption of the pump. When the power consumption subsequently becomes lower than the predetermined value, or after a predetermined period of time, the controller instructs the inverter to increase the frequency of the power supply in order to return the pump speed to its pre-overload value.

Using such a pumping arrangement, significant vacuum pumping time is required to evacuate relatively large load lock chambers used, for example, to transfer flat- panel substrates to and from the processing chamber in the manufacture of flat panel displays. In order to increase the speed of evacuation of such chambers, rather than increase the size of the pumps it is cheaper to provide a pumping arrangement comprising a plurality of smaller booster pumps connected in parallel. In such pumping arrangements, an inlet manifold has an inlet connected to the outlet from the chamber, and a plurality of outlets each connected to an inlet of a respective booster pump. An exhaust manifold has a plurality of inlets each connected to an outlet of a respective booster pump, and an outlet connected to an inlet of a backing pump of suitable capacity for receiving the fluid exhaust from the booster pumps and exhausting the fluid at or around atmospheric pressure.

Whilst the booster pumps in a parallel pumping arrangement will have the same rotor design and will be configured to operate at the same speed so that there is the same rate of gas flow through each pump, due to small manufacturing differences between the pumps it is possible that, during use, one of the booster pumps may reach an overload condition before the others. This can become increasingly likely where the booster pumps are normally operated at, or close to, full capacity in order to minimise "pump down" time, that is, the time required to evacuate the chamber to the required pressure. Under relatively high load conditions, as encountered during the evacuation of relatively large chambers used in the manufacture of flat panel displays, an inverter-driven booster pump tends to rapidly decelerate when an overload occurs in response to the reduction in the frequency of the power supplied to the motor by the inverter. Upon slowing, the power available to drive this booster pump reduces. However, as the power supplied to the other booster pumps remains high, these booster pumps are able to maintain a high pressure in the exhaust manifold. The relatively high pressure difference thus placed across a relatively slow, low powered booster pump can overload this pump, preventing it from subsequently re-accelerating and potentially causing the pump to stall, resulting in poor system performance.

Similar problems can be encountered during pump down where, due to slight manufacturing differences, one booster pump accelerates at a slower rate than the other booster pumps. This can also cause a relatively high pressure difference to be placed across a relatively slow booster pump, causing that pump to operate at a lower capacity than the others. As not all of the pumps are operating at full capacity, this has the effect of increasing the pump down time and reducing system performance.

Another example of how the booster speeds can become unbalanced is if the plurality of parallel booster pumps are operating with a time limited overload capability enabled in their drives, as described in earlier International patent application no. WO 2005/075827, the contents of which are incorporated herein by reference. In this case, if the overload condition is prolonged, then one of the booster pumps is very likely to reach the end of the overload period slightly before the others and will reduce its current limit, resulting in this booster pump slowing down. - A -

In at least its preferred embodiments, the present invention seeks to control parallel-connected pumps to operate at closely similar speeds during all operational conditions, thereby enhancing pump system performance.

In a first aspect, the present invention provides a vacuum pumping system comprising a plurality of vacuum pumps connected in parallel and each driven by a respective variable speed motor, and a control system for setting a target speed for the pumps as a function of at least the current speed of the slowest pump.

By controlling the speed of the pumps in this manner, one pump can be prevented from out-performing, or under-performing, another. This can prevent slower pumps from becoming overloaded by the faster pumps, and thereby inhibit pump stalling and improve pumping system performance. Controlling the pump speeds in this manner can also ensure that parallel pumps operating with overload enabled will always remain with a running speed within in a few Hz of each other and will not become unbalanced.

In the preferred embodiments, the control system comprises, for each pump, a pump controller for controlling the power supplied to the variable speed motor and thus the speed of rotation of the pump. The pump controller is preferably configured to change the frequency of the power supply to its respective motor to adjust pump speed, for example by transmitting an instruction to an inverter to change the frequency of the power supplied thereby to the motor. However, the controller may be configured to adjust another parameter of the power supply, such as the size (or amplitude) of the voltage or current of the power supply to the motor.

In one embodiment, the control system comprises a system controller for receiving from each pump controller a signal indicative of the current speed of its respective pump, determining the target speed from the current speeds of the pumps, and advising each pump controller of the target speed. The functionality for determining the target speed can thus be provided by software stored on a single system controller, with the individual pump controllers being responsive to the target speed received from the system controller to set its pump's speed. The system controller may be configured to set further parameters of the pumps' operation, such as the maximum speed of the pumps. This can enable the maximum speed of the pumps to be altered dynamically by the system controller in accordance with the operational state of the system. For example, the maximum speed can be adjusted depending on whether the pumping system is running at full speed or if low speed idling is required.

In an alternative embodiment, each pump controller has input means for receiving one or more signals indicative of the current speed of at least one of the other pumps, and is configured to use said signals to determine a target speed for its pump. Preferably, each pump controller has output means for outputting one or more signals indicative of the current speed of its pump, the system comprising means for connecting the output means of one pump controller to the input means of another pump controller. For rapid response, the input means and the output means from each pump controller preferably comprise analogue terminals.

Where the system comprises two pumps, the pumps can be cross-coupled such that the input terminals of one pump are connected to the output terminals of the other pump, and vice versa. Each pump can then determine a common target speed based on the current speed of its pump and the current speed of the other pump. Where the system comprises three or more pumps, the pump controllers can be daisy chained, such that each pump controller has its input terminals connected to the output terminals of one pump, and its output terminals connected to the input terminals of a different pump so that, in time, a common target speed will be set for the pumps.

The pump controller may be configured to continuously output its pump's current speed to the other pump controller, or alternatively may be configured to output its pump's current speed to the other pump controller only when its pump's speed has decreased, for example, when an overload occurs. In one preferred embodiment, the control system is configured to determine the target speed for each pump as a function of the speed of the slowest pump. The control system is preferably configured to set the target speed v according to the equation v = Av_s + B where v_s is the speed of the slowest pump, and A and B are constants, where A ≥ 1 and B > 0. By setting A >1 , the system will be continually increasing the target speed of the pumps up to the maximum speed. By setting B > 0, the target speed is moved away from zero. The constants A and B are preferably kept low so that any difference between the speeds of any two pumps is as small as possible.

In an alternative embodiment, the control system is configured to set the target speed as a function of the average speed of the pumps. For example, where a pump slows down, its speed becomes lower than the average, with the speed of the other pumps becoming greater than the average. With this target speed, the speed of the other pumps can be reduced towards that of the slower pump, and the speed of the slower pump can be increased to the average speed, thus enabling the pump speeds to be rapidly equalised. Preferably, the control system is configured to set the target speed v according to the equation v = Av _a + B where v_a is the average speed of the pumps, and A and B are constants, where A >1 and B > 0.

Thus, in a second aspect the present invention provides a system for evacuating an enclosure, the system comprising a plurality of vacuum pumps connected in parallel and each driven by a respective variable speed motor, and a control system for setting a target speed vfor the pumps according to the equation v - Au + B where u is one of the speed of the slowest pump and the average speed of the pumps, and A and B are constants. The control system may be configured to dynamically vary the values of A and B according to the required performance of the pumping system.

When the target speed is determined as a function of the average speed of the pumps, the system controller may be configured to receive from each pump controller a signal indicative of the current speed of its pump, determine, from the sum of the speeds of the pumps and the number of pumps, the average speed of the pumps, determine the target speed as a function of the average speed of the pumps, and advise the pump controllers of the average speed.

Alternatively, each pump controller may have input means for receiving a first signal indicative of the number of pumps and a second signal indicative of the sum of the speeds of the pumps, and be configured to determine the average speed of the pumps using the first and second signals. Both the first and second signals may be output from the system controller. Alternatively, the control system may comprise, for each pump, a current source, the control system being configured to adjust the size of the current produced by each current source depending on the speed of its respective pump, the current sources being connected in parallel across a load to generate, as the second signal, a voltage indicative of the sum of the speeds of the pumps. Each pump controller is preferably configured to control a respective current source. This arrangement can enable pump speeds to be rapidly adjusted in response to the variation in speed of one of the pumps, and so in a third aspect the present invention provides a system for controlling the speeds of rotation of a plurality of pumps connected in parallel, the system comprising, for each pump, a current source, means for adjusting the size of the current produced by each current source depending on the speed of its respective pump, the current sources being connected in parallel across a load to generate a voltage indicative of the sum of the speeds of the pumps, means for determining the average speed of the pumps from the generated voltage and the number of pumps, and means for setting a target speed for the pump as a function of the average speed of the pumps. In the preferred embodiments, the pumps are booster pumps, and so the system preferably comprises means for conveying fluid from an outlet of an enclosure to a respective inlet of each pump. Means are preferably provided for conveying fluid from the outlets of the pumps to a common exhaust, a backing pump having an inlet connected to the exhaust.

Depending on the required performance of the pumping system, the pumping system may include a set of additional pumps, for example booster or backing pumps, upstream or downstream from the aforementioned plurality of pumps, with these additional pumps similarly being connected in parallel and each driven by a respective variable speed motor. The control system may be additionally configured to set a target speed for the additional pumps as a function of at least the current speed of the slowest additional pump. Alternatively, a separate control system may be provided for these additional pumps. Further sets of additional pumps may be included as required, with the system controller preferably determining respective target speeds for each set of pumps, or with a dedicated system controller being provided for each set of pumps.

The present invention also provides a method of controlling the speeds of rotation of a plurality of vacuum pumps connected in parallel and each driven by a respective variable speed motor, the method comprising the step of setting a target speed for the pumps as a function of at least the speed of the slowest pump.

The present invention further provides a method of controlling the speeds of rotation of a plurality of vacuum pumps connected in parallel and each driven by a respective variable speed motor, the method comprising the step of setting a target speed vfor the pumps according to the equation v = Au + B where u is one of the speed of the slowest pump and the average speed of the pumps, and A and B are constants. The present invention also provides a method of controlling the speeds of rotation of a plurality of pumps connected in parallel, the method comprising the steps of providing a current source for each pump, the current sources being connected in parallel across a load, adjusting the size of the current produced by each current source depending on the speed of its respective pump to generate a voltage indicative of the sum of the speeds of the pumps, determining the average speed of the pumps from the generated voltage and the number of pumps, and setting a target speed for the pumps as a function of the average speed of the pumps.

Features described above in relation to system aspects of the invention can equally be applied to the method aspects of the invention, and vice versa.

Preferred features of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 illustrates a vacuum pumping system comprising a plurality of booster pumps connected in parallel;

Figure 2 illustrates a pump controller for driving a motor, of a booster pump of Figure 1 ;

Figure 3 illustrates the connection of the pump controllers of the booster pumps of Figure 1 to a system controller in one embodiment of the invention;

Figure 4 is a flow chart illustrating one method of controlling the speeds of the booster pumps of Figure 1 ;

Figure 5a illustrates the connections between the pump controllers of two booster pumps in a second embodiment of the invention, and Figure 5b shows the connections between the pump controllers of three booster pumps in this second embodiment of the invention; Figure 6 is a flow chart illustrating a second method of controlling the speeds of the booster pumps of Figure 1 ;

Figure 7 illustrates the connection of the pump controllers of the booster pumps of Figure 1 to a system controller in a third embodiment of the invention;

Figure 8 illustrates in more detail the connection to the pump controller in the third embodiment; and

Figure 9 illustrates a circuit formed by the connection of the pump controllers to the system controller in the third embodiment.

Figure 1 illustrates a vacuum pumping system for evacuating an enclosure 10, such as a load lock chamber. The system comprises a plurality of similar booster pumps 12 connected in parallel, and a backing pump 14. Each booster pump 12 has an inlet connected to a respective outlet 16 from an inlet manifold 18, and an outlet connected to a respective inlet 20 of an exhaust manifold 22. The inlet 24 of the inlet manifold 18 is connected to an outlet from the enclosure 10, and the outlet 26 of the exhaust manifold 22 is connected to an inlet of the backing pump 14. Whilst in the illustrated pumping system there are three booster pumps connected in parallel, any number of booster pumps may be provided depending on the pumping requirements of the enclosure. Similarly, where a relatively high number of booster pumps are provided, two or more backing pumps may be provided in parallel.

With reference to Figure 2, each booster pump 12 comprises a pumping mechanism 30 driven by a variable speed motor 32. Booster pumps typically include an essentially dry (or oil free) pumping mechanism 30, but generally also include some components, such as bearings and transmission gears, for driving the pumping mechanism 30 that require lubrication in order to be effective.

Examples of dry pumps include Roots, Northey (or "claw") and screw pumps. Dry pumps incorporating Roots and/or Northey mechanisms are commonly multi-stage positive displacement pumps employing intermeshing rotors in each pumping chamber. The rotors are located on contra-rotating shafts, and may have the same type of profile in each chamber or the profile may change from chamber to chamber. The backing pump 14 may have either a similar pumping mechanism to the booster pumps 12, or a different pumping mechanism. For example, the backing pump 14 may be a rotary vane pump.

The motor 32 of the booster pump 12 may be any suitable motor for driving the pumping mechanism 30. In the preferred embodiment, the motor 32 comprises a three phase AC motor, although another technology could be used (for example, a single phase AC motor, a DC motor, permanent magnet brushless motor, or a switched reluctance motor).

A pump controller 34 drives the motor 32. In this embodiment, the pump controller 34 comprises an inverter 36 for varying the frequency of the power supplied to the AC motor 32. The frequency is varied by the inverter 36 in response to commands received from an inverter controller 38. By varying the frequency of the power supplied to the motor, the rotational speed of the pumping mechanism 30, hereafter referred to as the speed of the pump, or pump speed, can be varied. A power supply unit 40 supplies power to the inverter 36 and inverter controller 38. An interface 42 is also provided to enable the pump controller 34 to receive signals from an external source for use in controlling the pump 12, and to output signals relating to the current state of the pump 12, for example, the current pump speed, the power consumption of the pump, and the temperature of the pump.

In the embodiment shown in Figure 3, the pump controllers 34 of each of the booster pumps 12 are connected to a system controller 50. As illustrated, cables 52 may be provided for connecting the interfaces 42 of the pump controllers 34 to an interface of system controller 50. Alternatively, the pump controllers 34 may be connected to the system controller 50 over a local area network. As discussed earlier, in a pumping system such as that shown in Figure 1 it is important to ensure that the speeds of the booster pumps 12 are approximately the same at all times. In this first embodiment, the system controller 50 outputs signals to the pump controllers 34 to set a target speed for the pumps by controlling the frequency of the power supplied to the motors 32 of the booster pumps 12.

One method that may be used by the system controller 50 to set a target speed for the booster pumps 12 is illustrated in Figure 4. At step S1 , the system controller 50 issues a signal to each of the pump controllers 34 to set the maximum speed, V_MAX, of the booster pumps 12 to a specified value. The maximum speed of the booster pumps 12 may be set by setting a maximum value for the frequency of the power to be supplied to the motors 32 of the booster pumps 12, for example, v_MAχ = 100 Hz. Operation of the booster pumps 12 is subsequently started at step S2.

In steps S3 to S5, the system controller 50 determines a target speed vfor the booster pumps 12 that is a function of the speed of the slowest of the booster pump 12. In this embodiment, each pump controller 34 continually sends signals to the system controller 50.which advise the system controller 50 of the current speed of the booster pump 12. This may be conveniently provided in the form of the current frequency of the power supplied by the inverter 36 to the motor 32. At step S3, the system controller 50 reads the pump speeds supplied by each pump controller 34, and at step S4 determines which of the current speeds of the booster pumps 12 is the slowest. At step S5, the system controller 50 outputs a command to each of the pump controllers 34 to set a target speed ι/for the booster pumps 12 which is determined by the system controller 50 according to the equation: v = Av_s +B where v_s is the speed of the slowest pump, as determined at step S4, and A and B are constants. In this embodiment, as the current pump speed has been indicated by the current frequency of the power supplied to the motor 32, the target speed can be provided to the pump controllers 34 in the form of a target frequency for the inverters 36. In response to the command received from the system controller, each pump controllers 34 controls the frequency of the power supplied to its motor 32 according to the target frequency output from the system controller.

To enable the booster pumps 12 to increase their speed from zero when the booster pumps 12 are first started, the constant B is preferably set at a small positive value, for example 2 Hz. To enable the speed of the booster pumps 12 to be continually increased, the value of the constant A is preferably slightly greater than unity, for example, 1.04.

The system controller 50 then rapidly and continuously repeats steps S3 to S5 so that, as the frequency of the power supplied to the motors 32 increases in response to the commands output from the system controller 50, the target speed rapidly increases towards the maximum speed V_MAX-

Where the booster pumps are identical, the current speeds of the booster pumps will, at any given time, be the same. However, due to manufacturing tolerances it is likely that there may be.very small differences between the motors and/or the pumping mechanisms of the booster pumps. One consequence of this is that one of the booster pumps may reach an overload condition before any of the others, resulting in the pump controller of that pump automatically decreasing the frequency of the power supplied to the motor of that pump in order to remove the overload condition.

Whilst under normal conditions each pump controller will be able to adjust the frequency of the power supply to the target frequency, if an overload has just occurred a pump may have just reduced its frequency, and, while the overload persists, it will not be able to immediately adjust the frequency of the power supply to the target frequency. As a result of using this method of calculating the target frequency, the other pumps are prevented from "running away" from the slowest pump, keeping their speeds close to that of the slowest pump (typically within a few Hz, depending on the values of A and ^β) and so preventing that pump from stalling.

In the embodiment shown in Figure 3, a system controller 50 calculates the target speed for the booster pumps, and advises the booster pumps of the target speed. In the second embodiment illustrated in Figure 5, the pump controllers 34 of the booster pumps 12 each determine a target speed for its pump based on a comparison between the current speed of its pump and the current speed of another pump.

With reference first to Figure 5a, the pumping system comprises two booster pumps connected in parallel. The input terminals 60a of the pump controller 34a of one of the booster pumps are connected to the output terminals 62b of the pump controller 34b of the other booster pumps, and the input terminals 60b of the pump controller 34b are connected to the output terminals 62a of the pump controller 34a. Where the input and output terminals are analogue terminals, connections preferably in the form of twisted wire pairs 64, 66 are used to cross couple the terminals of the pump controllers 34a, 34b.

In use, the target speed for the booster pumps is set in a similar manner to the method illustrated in Figure 4. Before the pumps are started, the maximum speed, v_MAχ, and the values of the constants A and B are set at each pump controller 34a, 34b. Operation of the booster pumps is subsequently started, and each pump controller continuously outputs to the other pump controller the current speed of its pump via the connections 64, 66. Each pump controller continuously compares the current speed of its pump with the current speed of the other pump and sets a target speed (/for its pump according to the equation v = Av_s +B where v_s is the speed of the slowest of the two pumps. As a result, at any given time the targets speeds of the two booster pumps are within a few Hertz. A similar method can be employed where the pumping system comprises three or more booster pumps. In the example shown in Figure 5b, for three booster pumps, the input terminals 60a of the pump controller 34a of a first one of the booster pumps are connected to the output terminals 62b of the pump controller 34b of a second one of the booster pumps by a first twisted wire pair 64. The input terminals 60b of the pump controller 34b are connected to the output terminals 62c of the pump controller 34c of a third one of the booster pumps by a second twisted wire pair 66. The input terminals 60c of the pump controller 34c are connected to the output terminals 62a of the pump controller 34a by a third twisted wire pair 68.

In use, before the pumps are started, the maximum speed, V_MAX, and the values of the constants A and B are set at each pump controller 34a, 34b, 34c. Operation of the booster pumps is subsequently started, and each pump controller continuously outputs to another pump controller the current speed of its pump. Each pump controller continuously compares the current speed of its pump with the current speed of that other pump and sets a target speed vfor its pump according to the equation v = Av_s +B where v_s is the speed of the slowest of the two pumps. As a result, if, say the first one of the booster pumps slows down, both the first and the third booster pumps will rapidly change their target speed based on current speed of the first booster pump. As the speed of the third booster pump decreases, due to the reduction in its target speed, the target speed of the second pump will start to decrease so that, in time, each of the three pumps will have the same target speed.

Figure 6 illustrates a second method that may be used by the system controller 50 of Figure 3 to set a target speed for the booster pumps 12. In contrast to the first method illustrated in Figure 4, instead of determining the target speed as a function of the slowest of the pumps, in this second method the target speed is determined as a function of the average speed of the pumps. At step S10, the system controller 50 issues a signal to each of the pump controllers 34 to set the maximum speed, V_MAX, of the booster pumps 12 to a specified value. As in the first method, the maximum speed of the booster pumps 12 may be set by setting a maximum value for the frequency of the power to be supplied to the motors 32 of the booster pumps 12, for example, VM_AX = 100 Hz. Operation of the booster pumps 12 is subsequently started at step S11.

In steps S12 to S14, the system controller 50 determines a target speed v for the booster pumps 12 that is a function of the average speed of the booster pump 12. In this embodiment, each pump controller 34 continually sends signals to the system controller 50 which advise the system controller 50 of the current speed of the booster pump 12. This may be conveniently provided in the form of the current frequency of the power supplied by the inverter 36 to the motor 32. At step S12, the system controller 50 reads the current pump speeds supplied by each pump controller 34, and at step S13 determines, from the sum of the current speeds of the booster pumps 12 and the number of booster pumps 12, the average speed of the pumps 12. At step S14, the system controller 50 outputs a command to each of the pump controllers 34 to set a target speed v for the booster pumps 12 which is determined by the system controller 50 according to the equation: v = Av₀ + B where v_a is the average speed of the pumps, as determined at step S13, and A and β are constants. In this embodiment, as the current pump speed has been indicated by the current frequency of the power supplied to the motor 32, the target speed can be provided to the pump controllers 34 in the form of a target frequency for the inverters 36. In response to the command received from the system controller, each pump controllers 34 controls the frequency of the power supplied to its motor 32 according to the target frequency output from the system controller.

Similar to the first method, to enable the booster pumps 12 to increase their speed from zero when the booster pumps 12 are first started, the constant B is preferably set at a small positive value, for example 2 Hz, and to enable the speed of the booster pumps 12 to be continually increased, the value of the constant A is preferably slightly greater than unity, for example, 1.04.

The system controller 50 then rapidly and continuously repeats steps S12 to S14 so that, as the frequency of the power supplied to the motors 32 increases in response to the commands output from the system controller 50, the target speed rapidly increases towards the maximum speed V_MAX-

Similar to the first method, by using this second method the system controller is able to keep the booster pumps at roughly the same speed. In the event that one of the pumps slows down, the average speed of the pumps will decrease, thereby causing the target speed to reduce and so cause the faster pumps to slow down. As these pumps slow down, the average speed of the pumps will continue to decrease and, in time, become roughly equal to the speed of the slowest pump. As a result, the second method can provide the same advantages and benefits as the first method.

This method of determining the target speed for the booster pumps is implemented in the third embodiment illustrated in Figures 7 to 9, in which the pump controllers 34 of the booster pumps 12, as opposed to the system controller 50, each determine the target speed for its pump.

With reference first to Figure 7, the booster pumps 12a, 12b, 12c are connected to the system controller 50 using multi-wire connectors 70, 72, 74, for example using the first and second wires of three RJ-45 connectors. As shown in Figure 8, a current source 76 is provided within each pump 12. Each current source 76 is configured to receive signals from the pump controller 34 via output 82, and to adjust the size of the current produced thereby in dependence on the signals. Each current source 76 is also connected between the first and second wires of the connectors. As shown in Figure 9, a load 78 is provided within the system controller 50 so as to form the circuit 80 illustrated in Figure 9 with the current sources 76a, 76b, 76c, of the respective booster pumps 12a, 12b, 12c.

Before the pumps have been started, the maximum speed, V_MAX. of the pumps, the number of booster pumps in the pumping system and the values of the constants A and B are set at each pump controller 34a, 34b, 34c. These values may be pre-programmed individually into the pump controllers, or may be sent to the pump controllers in respective signals output from the system controller 50. Once operation of the pumps has been started, each pump controller varies, in the same manner, the size of the current produced by the current source 76 in dependence on the current speed of its pump 12. As a result, the magnitude of the voltage V generated across the load 78 is proportional to the sum of the speeds of the pumps 12a, 12b, 12c. The voltage V is received at input terminal 84 of each pump controller 34a, 34b, 34c. From the magnitude of voltage V and the number of booster pumps in the pumping system, each pump controller determines the average speed, v_a, of the booster pumps, and, using the average speed, determines the target speed vfor the booster pumps according to the equation v = Av_α +B

As the speeds of the pumps increases, the current generated by the current sources 76a, 76b, 76c also increases, thereby increasing the magnitude of the voltage V across the load 78.

The invention has been described above with reference to a pumping system having a single plurality of booster pumps 12 connected in parallel. However, the pumping system may include one or more further pluralities of booster pumps connected in parallel and located either upstream or downstream from the booster pumps 12. The speed of each further plurality of booster pumps may be controlled in a similar manner to the speed of the booster pumps 12. The system controller 50 may be configured to set respective target speeds for each further plurality of pumps. Alternatively, a separate controller may be provided for each further plurality of pumps.

Claims

1. A vacuum pumping system comprising a plurality of vacuum pumps connected in parallel and each driven by a respective variable speed motor, and a control system for setting a target speed for the pumps as a function of at least the speed of the slowest pump.

2. A system according to Claim 1 , wherein the control system comprises, for each pump, a pump controller for controlling the power supplied to the variable speed motor and thus the speed of rotation of the pump.

3. A system according to Claim 2, wherein each pump controller is configured to change the frequency of the power supply to its respective motor to adjust pump speed.

4. A system according to Claim 2 or Claim 3, wherein the control

.. system comprises a system controller for receiving from each pump controller a signal indicative of the current speed of its respective pump, determining a target speed for the pumps from the current speeds of the pumps, and advising each pump controller of the target speed.

5. A system according to Claim 4, wherein the system controller is further configured to control the maximum speed of the pumps.

6. A system according to Claim 2 or Claim 3, wherein each pump controller has input means for receiving one or more signals indicative of the speed of at least one of the other pumps, and is configured to use said signals to determine a target speed for its pump.

7. A system according to Claim 6, wherein each pump controller has output means for outputting a signal indicative of the speed of its pump, the system comprising means for connecting the output means of one pump controller to the input means of another pump controller.

8. A system according to Claim 7, wherein the input means and the output means of each pump controller comprise analogue terminals.

9. A system according to any preceding claim, wherein the control system is configured to set the target speed as a function of the speed of the slowest pump.

10. A system according to Claim 9, wherein the control system is configured to set the target speed in proportion to the speed of the slowest pump.

11. A system according to Claim 10, wherein the control system is configured to set the target speed v according to the equation v = Av_s +B where v_s is the speed of the slowest pump, and A and B are constants, where A >1 and B > 0.

12. A system according to any of Claims 1 to 8, wherein the control system is configured to set the target speed as a function of the average speed of the pumps.

13. A system according to Claim 12, wherein the control system is configured to set the target speed in proportion to the average speed of the pumps.

14. A system according to Claim 13, wherein the control system is configured to set the target speed v according to the equation v = Av_a +B where v_a is the average speed of the pumps, and A and β are constants, where A >1 and B > 0.

15. A system according to any of Claims 12 to 14, wherein each pump controller has input means for receiving a first signal indicative of the number of pumps and a second signal indicative of the sum of the speeds of the pumps, and is configured to determine the average speed of the pumps using the first and second signals.

16. A system according to Claim 15, wherein the control system comprises, for each pump, a current source, the control system being configured to adjust the size of the current produced by each current source depending on the speed of its respective pump, the current sources being connected in parallel across a load to generate a voltage indicative of the sum of the speeds of the pumps.

17. A system according to Claim 16, wherein each pump controller is configured to control a respective current source.

18. A system according to Claim 11 or Claim 14, wherein the control system is configured to dynamically vary the values of A and β.

19. A system according to any preceding claim, comprising means for conveying fluid from an outlet of an enclosure to a respective inlet of each pump.

20. A system according to any preceding claim, comprising means for conveying fluid from the outlets of the pumps to a common exhaust.

21. A system according to Claim 20, comprising an additional pump having an inlet connected to the exhaust.

22. A method of controlling the speeds of rotation of a plurality of vacuum pumps connected in parallel and each driven by a respective variable speed motor, the method comprising the step of setting a target speed for the pumps as a function of at least the speed of the slowest pump.

23. A method according to Claim 22, wherein the target speed is set as a function of the speed of the slowest pump.

24. A method according to Claim 23, wherein the target speed is set in proportion to the speed of the slowest pump.

25. A method according to Claim 24, wherein the target speed v is set according to the equation v = Av_^ +B where v_s is the speed of the slowest pump, and A and B are constants, where A >1 and B > 0.

26. A method according to Claim 22, wherein the target speed is set as a function of the average speed of the pumps.

27. A method according to Claim 26, wherein the target speed is set in proportion to the average speed of the pumps.

28. A method according to Claim 27, wherein the target speed v is set according to the equation v - Av_a +B where v_a is the average speed of the pumps, and A and B are constants, where A >1 and B > 0.

29. A method according to Claim 25 or Claim 28, wherein the values of A and B are dynamically varied.