WO2018206843A1

WO2018206843A1 - A method and a control system for controlling a vacuum pump

Info

Publication number: WO2018206843A1
Application number: PCT/FI2018/050321
Authority: WO
Inventors: Lauri NYGREN; Manu HUTTUNEN; Teemu KINNARINEN; Tuomo Lindh; Jero Ahola; Antti HÄKKINEN; Vesa Karvonen
Original assignee: Lappeenrannan Teknillinen Yliopisto
Priority date: 2017-05-08
Filing date: 2018-05-03
Publication date: 2018-11-15
Also published as: FI20175411A

Abstract

A method for controlling a vacuum pump comprises: measuring (101) inlet pressure of the vacuum pump, determining (102) an estimate for theoretical suction speed based on data indicative of rotational speed of the vacuum pump and data indicative of the theoretical suction speed at a predetermined rotational speed, determining (103) an estimate for leak flow from the outlet of the vacuum pump back to the inlet of the vacuum pump based on the measured inlet pressure, determining (104) a difference between the estimates of the theoretical suction speed and the leak flow so as to obtain an estimate for suction speed of the vacuum pump, and controlling (105) the rotational speed of the vacuum pump based at least partly on the estimate of the suction speed. The rotational speed can be controlled for example so that the efficiency of a vacuum pump is maximized.

Description

A method and a control system for controlling a vacuum pump

Field of the disclosure

The disclosure relates generally to vacuum systems. More particularly, the disclosure relates to a method and to a control system for controlling a vacuum pump of a vacuum system. Furthermore, the disclosure relates to a computer program for controlling a vacuum pump of a vacuum system.

Background

Vacuum engineering deals with technological processes and equipment that use vacuum to achieve better results than those run under atmospheric pressure. Exemplifying application of vacuum technology are, among others, pyrolytic chromium carbide coating, manufacturing of non-reflecting glass, glass coloring, vacuum impregnation, vacuum filtering, vacuum dewatering and drying, and Vacuum coating. For example, vacuum coaters are capable of applying various types of coatings on metal, glass, plastic, and ceramic surfaces, providing high quality and uniform thickness and color. For another example, vacuum dryers can be used for delicate materials and save significant quantities of energy due to lower drying temperatures.

A typical vacuum system comprises a vacuum chamber, a vacuum pump for maintaining the vacuum inside the vacuum chamber, an electric drive for running the vacuum pump, and a control system for controlling the vacuum pump based on control and status information related to the vacuum system. The control system can be configured to optimize the operation of the vacuum system for example by adjusting the rotation speed of the vacuum pump so that the energy efficiency of the vacuum pump and the electric drive is maximized. Furthermore, the control system can be configured to carry out condition monitoring of the vacuum system. For example, the control system can be configured to monitor vacuum system leaks and vacuum evacuation. Vacuum systems of the kind described above are, however, not free from challenges. A challenge related to many vacuum systems is that instrumentation for producing the above-mentioned control and status information may comprise elements, such as e.g. a flow rate sensor, which are expensive and/or prone to failures.

Summary

The following presents a simplified summary in order to provide a basic understanding of some embodiments of the invention. The summary is not an extensive overview of the invention. It is neither intended to identify key or critical elements of the invention nor to delineate the scope of the invention. The following summary merely presents some concepts of the invention in a simplified form as a prelude to a more detailed description of exemplifying embodiments of the invention. In accordance with the invention, there is provided a new method for controlling a vacuum pump of a vacuum system. The vacuum pump can be for example a positive-displacement dry vacuum pump. A method according to the invention comprises:

- measuring inlet pressure of the vacuum pump, - determining an estimate for theoretical suction speed of the vacuum pump based on data indicative of rotational speed of the vacuum pump and data indicative of the theoretical suction speed at a predetermined rotational speed, the theoretical suction speed being determined by the compression volume of the vacuum pump and the rotational speed, - determining an estimate for leak flow from an outlet of the vacuum pump back to an inlet of the vacuum pump based on the measured inlet pressure,

- determining a difference between the estimate of the theoretical suction speed and the estimate of the leak flow so as to obtain an estimate for suction speed of the vacuum pump, and - controlling the rotational speed of the vacuum pump based at least partly on the estimate of the suction speed of the vacuum pump.

The above-described method according to the invention is based on an assumption that the theoretical suction speed can be estimated on the basis of the rotational speed and that the theoretical suction speed is independent of the inlet and outlet pressures of the vacuum pump. Furthermore, the method is based on an assumption that above-mentioned leak flow can be estimated on the basis of the pressure difference between the outlet and the inlet of the vacuum pump. In typical cases the outlet pressure of the vacuum pump is the atmospheric pressure that can be assumed to be constant. If the outlet pressure of the vacuum pump can vary, a measured value of the outlet pressure of the vacuum pump or a measured value of the pressure difference over the vacuum pump can be used in addition to or instead of the measured value of the inlet pressure. It is worth noting that the estimated suction speed does not correspond to the actual amount of air or other gas, but to the volumetric flow rate of the gas in the conditions prevailing at the inlet of the vacuum pump. The estimated value of the suction speed can be converted into e.g. a volumetric flow rate value corresponding to given conditions by using known theoretic methods. A need for a flow rate sensor can be avoided in many cases because the suction speed is estimated based on the measured inlet pressure, the rotational speed of the vacuum pump, and properties of the vacuum pump. Therefore, the typical drawbacks of the flow rate measurement can be avoided.

The vacuum pump is advantageously driven with an electric motor supplied with a frequency converter that can be configured to produce an estimate for the rotational speed. Furthermore, the frequency converter can be configured to estimate the electric power received by the frequency converter from an electric power grid and/or the shaft power of the vacuum pump. In this exemplifying case, it is possible to estimate the isentropic efficiency of the vacuum pump, and/or the specific energy consumption of vacuum pump, and/or the total energy efficiency of the combination of the vacuum pump, the electric motor, and the frequency converter.

In accordance with the invention, there is provided also a new control system for controlling a vacuum pump of a vacuum system. A control system according to the invention comprises a processor configured to: - determine an estimate for theoretical suction speed of the vacuum pump based on data indicative of rotational speed of the vacuum pump and data indicative of the theoretical suction speed at a predetermined rotational speed, - determine an estimate for leak flow from an outlet of the vacuum pump back to an inlet of the vacuum pump based on measured inlet pressure of the vacuum pump,

- determine a difference between the estimate of the theoretical suction speed and the estimate of the leak flow so as to obtain an estimate for suction speed of the vacuum pump, and

- control the rotational speed of the vacuum pump based at least partly on the estimate of the suction speed of the vacuum pump.

In accordance with the invention, there is provided also a new vacuum system that comprises: - a vacuum chamber,

- a vacuum pump for maintaining vacuum inside the vacuum chamber, and

- a control system according to the invention for controlling the vacuum pump.

In accordance with the invention, there is provided also a new computer program for controlling a vacuum pump of a vacuum system. A computer program according to the invention comprises computer executable instructions for controlling a programmable processor to:

- determine an estimate for theoretical suction speed of the vacuum pump based on data indicative of rotational speed of the vacuum pump and data indicative of the theoretical suction speed at a predetermined rotational speed, - determine an estimate for leak flow from an outlet of the vacuum pump back to an inlet of the vacuum pump based on measured inlet pressure of the vacuum pump,

In accordance with the invention, there is provided also a new computer program product. The computer program product comprises a non-volatile computer readable medium, e.g. a compact disc "CD", encoded with a computer program according to the invention.

A number of exemplifying and non-limiting embodiments of the invention are described in accompanied dependent claims. Various exemplifying and non-limiting embodiments of the invention both as to constructions and to methods of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific exemplifying embodiments when read in connection with the accompanying drawings. The verbs "to comprise" and "to include" are used in this document as open limitations that neither exclude nor require the existence of also un-recited features. The features recited in the accompanied dependent claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of "a" or "an", i.e. a singular form, throughout this document does as such not exclude a plurality. Brief description of the figures

Exemplifying and non-limiting embodiments of the invention and their advantages are explained in greater details below in the sense of examples and with reference to the accompanying drawings, in which: figure 1 shows a flowchart of a method according to an exemplifying and non-limiting embodiment of the invention for controlling a vacuum pump of a vacuum system, figure 2 illustrates the principle utilized in methods according to exemplifying and non-limiting embodiments of the invention for controlling a vacuum pump of a vacuum system, figure 3 shows exemplifying actual and estimated suction speeds as functions of inlet pressure at several rotational speeds for a dry claw vacuum pump, and figure 4 illustrates a vacuum system according to an exemplifying and non-limiting embodiment of the invention.

Description of exemplifying embodiments The specific examples provided in the description below should not be construed as limiting the scope and/or the applicability of the accompanied claims. Lists and groups of examples provided in the description are not exhaustive unless otherwise explicitly stated.

Figure 1 shows a flowchart of a method according to an exemplifying and non- limiting embodiment of the invention for controlling a vacuum pump of a vacuum system. The method comprises the following actions:

- action 101 : measuring inlet pressure p-_m of the vacuum pump,

- action 102: determining an estimate for theoretical suction speed Sth of the vacuum pump based on data indicative of rotational speed n of the vacuum pump and data indicative of the theoretical suction speed S_n at a predetermined rotational speed n_n, action 103: determining an estimate for leak flow Sieak from an outlet of the vacuum pump back to an inlet of the vacuum pump based on the measured inlet pressure, action 104: determining a difference Sth - eak between the estimate of the theoretical suction speed and the estimate of the leak flow so as to obtain an estimate for suction speed S of the vacuum pump, and action 105: controlling the rotational speed n of the vacuum pump based at least partly on the estimate of the suction speed of the vacuum pump.

The theoretical suction speed Sth is assumed to be substantially directly proportional to the rotational speed so that the theoretical suction speed Sth at the prevailing rotational speed n can be computed according to the following equation:

where n_est is the estimate of the prevailing rotational speed n, n_n is the above- mentioned predetermined rotational speed, and S_n is the above-mentioned theoretical suction speed at the predetermined rotational speed n_n. The predetermined rotational speed n_n can be for example the nominal rotation speed of the vacuum pump. Furthermore, the method is based on an assumption that the above-mentioned leak flow Sieak can be determined based on the pressure difference between the outlet and the inlet of the vacuum pump and that an estimate of the leak flow Sieak corresponding to a given pressure difference and a given rotational speed is a valid estimate for the leak flow also with different rotational speeds when the pressure difference is the above-mentioned given pressure difference, i.e. the pressure difference is unchanged.

Principles of methods according to different exemplifying embodiments of the invention are described below with reference to figure 2.

In a method according to an exemplifying and non-limiting embodiment of the invention, the leak flow is estimated based on a known suction curve of the vacuum pump. In figure 2, an exemplifying known suction curve is denoted with a reference 220. In this exemplifying case, the known suction curve 220 expresses the suction speed S as a function of the inlet pressure p_m at the above-mentioned predetermined rotational speed n_n. Figure 2 shows also a suction curve 221 which corresponds to the prevailing rotational speed n. The suction curve 221 is assumed to be unknown. In this exemplifying case, the estimate of the leak flow Sieak is a difference between the suction speed S_n at a first point 222 of the suction curve 220 where the inlet pressure iniet equals to the outlet pressure p_out of the vacuum pump and suction speed S_ni at a second point 223 of the suction curve 220 where the inlet pressure iniet is the measured inlet pressure p_n. Thus, the estimate of the leak flow eak is the following difference:

As the estimate of the leak flow eak is assumed to be a valid estimate also in cases where the rotational speed differs from predetermined rotational speed n_n and the inlet pressure is the measured inlet pressure pin, the difference S_n - S_ni is suitable for estimating the leak flow Seak also in a case where the rotational speed is the prevailing rotational speed n and the inlet pressure is the measured inlet pressure pin. Thus, the estimate for the suction speed S corresponding to the measured inlet pressure p,_n and the prevailing rotational speed n is:

S = Sth - Seak = SnHest/Hn - (^n - Sni ). (3) In a method according to another exemplifying and non-limiting embodiment of the invention, the leak flow Seak is estimated based on:

- a predetermined ultimate value p_uit of the inlet pressure at which the leak flow Seak equals the theoretical suction speed S_n at the predetermined rotational speed n_n, i.e. S_n = Seak, i.e. the suction speed S is zero, and - an assumption that the leak flow Seak is an isentropic nozzle flow from the outlet of the vacuum pump back to the inlet of the vacuum pump.

The estimate of the leak flow Seak can be computed according to the following equation based on the assumption that the leak flow is an isentropic nozzle flow:

where p_n is the gas density at the inlet of the vacuum pump, C is a flow coefficient, A is a nozzle throat area, p_out is the outlet pressure of the vacuum pump, out is the gas density at the outlet of the vacuum pump, k is the isentropic exponent that is about 1 .4 for air, and p_ratio is a pressure ratio that is the maximum of the following:

(1 + {k -†)l2†>^ - (5)

The estimates for the gas densities p_n and out at the inlet and the outlet of the vacuum pump, respectively, can be computed according to the following equation which is applied separately for the inlet of the vacuum pump and for the outlet of the vacuum pump:

P =

R.

(6) where p is the gas density at the inlet or the outlet of the vacuum pump, p is the inlet or outlet pressure, Rspedfic is the specific gas constant that is about 287.058 Jkg^_1K^" ¹ for air, and T is the gas temperature at the inlet or the outlet of the vacuum pump.

The temperatures at the inlet and the outlet of the vacuum pump can be measured with suitable temperature sensors. It is also possible that the temperature is measured only at the inlet the vacuum pump and the temperature at the outlet of the vacuum pump is estimated according to the following equation based on an assumption of isentropic compression:

where Tin is the temperature at the inlet of the vacuum pump and T_out is the temperature at the outlet of the vacuum pump, It is also possible that the temperature is measured only at the outlet the vacuum pump and the temperature at the inlet of the vacuum pump is solved from the above-presented equation (7).

The product CA in equation (4) can be computed with the aid of the following formula based on an assumption that the theoretical suction speed S_n at the predetermined rotational speed n_n equals the leak flow eak when the inlet pressure has the predetermined ultimate value

In conjunction with many vacuum pumps, the ultimate value p_uit of the inlet pressure, the theoretical suction speed S_n, and the predetermined rotational speed n_n are given as plate values of a vacuum pump or in a data sheet of the vacuum pump. As shown with above-presented equations, the estimate of the suction speed S can be obtained with the aid of the estimate of the prevailing rotational speed n, the measured inlet pressure p_m or a measured pressure difference over the vacuum pump, and the values Pult, Sn, and n_n. In cases where a suction curve, such as the suction curve 220, of a vacuum pump is known, the product CA in equation (4) can be solved also by applying a suitable fitting method, e.g. the least squares method, for finding a value of the product CA which minimizes the error between the known suction curve and a suction curve according to Sn ^— Sleak where eak corresponds to equation (4). A method according to an exemplifying and non-limiting embodiment of the invention comprises computing an estimate for the ideal isentropic power consumption Ps of the vacuum pump according to the following equation:

where S_est is the estimate of the suction speed. A method according to an exemplifying and non-limiting embodiment of the invention comprises varying the rotational speed of the vacuum pump so as to find a value of the rotational speed at which the energy efficiency of a combination of the vacuum pump and an electric drive running the vacuum pump is maximized, i.e. the following ratio is maximized:

where is electric power consumed for rotating the vacuum pump.

In a method according to an exemplifying and non-limiting embodiment of the invention, the vacuum pump is driven with an electric motor supplied with a frequency converter which produces the estimate n_est for the rotational speed n of the vacuum pump. It is also possible that the frequency converter produces an estimate for the shaft power of the vacuum pump. A method according to an exemplifying and non-limiting embodiment of the invention comprises computing the isentropic efficiency 77s of the vacuum pump:

(1 1 ) where P_est is the estimate of the shaft power of the vacuum pump.

A method according to an exemplifying and non-limiting embodiment of the invention comprises computing the specific energy consumption Es of the vacuum pump in accordance with the following equation:

ώ . (12)

A method according to an exemplifying and non-limiting embodiment of the invention is carried out with a processor configured to:

- determine the estimate for the theoretical suction speed Sth of the vacuum pump, - determine the estimate for the leak flow Sieak, - determine the estimate for the suction speed S of the vacuum pump by computing the difference Sth - Sieak between the estimate of the theoretical suction speed and the estimate of the leak flow, and control the rotational speed n of the vacuum pump based on the estimate of the suction speed of the vacuum pump.

Figure 3 shows exemplifying actual and estimated suction speeds as functions of the inlet pressure p_m at several rotational speeds for a dry claw vacuum pump. The actual suction speeds are depicted with solid curves and the estimated suction speeds are depicted with dashed curves. The estimated suction speeds are obtained with a method according to an exemplifying and non-limiting embodiment of the invention. As can be seen in figure 3, the estimated suction speeds have good accuracy at higher rotational speeds, but the accuracy decreases at lower rotational speeds, and especially at lower inlet pressures p-_m. The suction speeds correspond to volumetric flow rates, m³/h, of air or other gas in the conditions prevailing at the inlet of the vacuum pump. The actual amount of air or other gas can be described by mass flow rate, kg/h, or by standard volumetric flow rate. The standard volumetric flow rate describes the volumetric flow rate corresponding to standard conditions which are defined typically with the standard pressure p_std = 1 .01325 bar and the standard temperature T_std = 20 °C.

A method according to an exemplifying and non-limiting embodiment of the invention comprises converting the suction speed S into the standard volumetric flow rate Q in accordance with the following equation:

A computer program according to an exemplifying and non-limiting embodiment of the invention comprises computer executable instructions for controlling a programmable processing system to carry out actions related to a method according to any of the above-described exemplifying embodiments of the invention. A computer program according to an exemplifying and non-limiting embodiment of the invention comprises software modules for controlling a vacuum pump of a vacuum system. The software modules comprise computer executable instructions for controlling a programmable processor to: - determine an estimate for the theoretical suction speed Sth of the vacuum pump based on data indicative of the rotational speed n of the vacuum pump and data indicative of the theoretical suction speed S_n at a predetermined rotational speed n_n,

- determine an estimate for the leak flow Sieak from the outlet of the vacuum pump back to the inlet of the vacuum pump based on measured inlet pressure p-_m of the vacuum pump,

- determine the difference Sth - eak between the estimate of the theoretical suction speed and the estimate of the leak flow so as to obtain an estimate for the suction speed S of the vacuum pump, and - control the rotational speed n of the vacuum pump based at least partly on the estimate of the suction speed of the vacuum pump.

The above-mentioned software modules can be e.g. subroutines or functions implemented with a suitable programming language.

A computer program product according to an exemplifying and non-limiting embodiment of the invention comprises a computer readable medium, e.g. a compact disc "CD", encoded with a computer program according to an exemplifying embodiment of invention.

A signal according to an exemplifying and non-limiting embodiment of the invention is encoded to carry information defining a computer program according to an exemplifying embodiment of invention.

Figure 4 illustrates a vacuum system according to an exemplifying and non-limiting embodiment of the invention. The vacuum system comprises a vacuum chamber 409, a vacuum pump 406 for maintaining vacuum inside the vacuum chamber, a manometer 402 for measuring the inlet pressure p-_m of the vacuum pump, and a control system 401 according to an embodiment of the invention for controlling the vacuum pump 406. The control system 401 comprises a processor 403 configured to: - determine an estimate for the theoretical suction speed Sth of the vacuum pump 406 based on data indicative of the rotational speed n of the vacuum pump and data indicative of the theoretical suction speed S_n at a predetermined rotational speed n_n,

- determine an estimate for the leak flow Sieak from an outlet of the vacuum pump back to an inlet of the vacuum pump based on the measured inlet pressure p-_m,

- determine the difference Sth - Sieak between the estimate of the theoretical suction speed and the estimate of the leak flow so as to obtain an estimate for the suction speed S of the vacuum pump, and - control the rotational speed n of the vacuum pump based at least partly on the estimate of the suction speed of the vacuum pump.

In a control system according to an exemplifying and non-limiting embodiment of the invention, the processor 430 is configured to determine the estimate of the leak flow Sieak based on a suction curve expressing the suction speed as a function of the inlet pressure at a constant rotational speed, e.g. the above-mentioned predetermined rotational speed n_n. The estimate of the leak flow Sieak can be a difference between a first suction speed value at a first point of the suction curve where the inlet pressure equals to the outlet pressure p_out of the vacuum pump and a second suction speed value at a second point of the suction curve where the inlet pressure is the measured inlet pressure p-_m.

In a control system according to an exemplifying and non-limiting embodiment of the invention, the processor 403 is configured to determine the estimate of the leak flow >->leak based on: - a predetermined ultimate value p_uit of the inlet pressure at which the leak flow equals the theoretical suction speed at the predetermined rotational speed, and

- an assumption that the leak flow is an isentropic nozzle flow from the outlet of the vacuum pump back to the inlet of the vacuum pump.

In a control system according to an exemplifying and non-limiting embodiment of the invention, the processor 403 is configured to compute the estimate of the leak flow Sieak according to the following equation based on the assumption that the leak flow is an isentropic nozzle flow:

where _n is gas density at the inlet of the vacuum pump, C is a flow coefficient, A is a nozzle throat area, p_out is outlet pressure of the vacuum pump, out is gas density at the outlet of the vacuum pump, k is an isentropic exponent, and p_ratio is a pressure ratio being a maximum of the following: p pout and (1 + (k - 1 )l2f^ ^{~ k)}.

In a control system according to an exemplifying and non-limiting embodiment of the invention, the processor 403 is configured to compute estimates for the gas densities at the inlet and the outlet of the vacuum pump 406 according to the following equation applied separately for the inlet of the vacuum pump and for the outlet of the vacuum pump:

D—

where p is the gas density, p is the pressure, Rspedfic is the specific gas constant, and T is the gas temperature.

A control system according to an exemplifying and non-limiting embodiment of the invention comprises a temperature sensor 410 for measuring the temperature Tin at the inlet of the vacuum pump 406, and the processor 403 is configured to compute an estimate for the temperature Tout at the outlet of the vacuum pump 406 accord to the following equation based on an assumption of isentropic compression:

In a control system according to an exemplifying and non-limiting embodiment of the invention, the processor 403 is configured to compute an estimate for the product CA according to the following formula based on an assumption that the theoretical suction speed S_n at the predetermined rotational speed n_n equals the leak flow Sieak when the inlet pressure has the predetermined ultimate value uit:

In a control system according to an exemplifying and non-limiting embodiment of the invention, the processor 403 is configured to compute an estimate for ideal isentropic power consumption Ps according to the following equation:

where S_est is the estimate for suction speed. In a control system according to an exemplifying and non-limiting embodiment of the invention, the processor 403 is configured to vary the rotational speed of the vacuum pump 406 so as to find a value of the rotational speed at which the following ratio is maximized:

where P_ei is electric power consumed for rotating the vacuum pump. A control system according to an exemplifying and non-limiting embodiment of the invention comprises an electric motor 407 for driving the vacuum pump 406 and a frequency converter 408 for supplying voltage to the electric motor in accordance with a control signal produced by the processor 403. It is however also possible that a control system according to another exemplifying embodiment of the invention is a separate device with respect to means, such as the electric motor 407 and the frequency converter 408, for driving the vacuum pump.

The frequency converter 408 is advantageously configured to produce an estimate nest for the rotational speed n of the vacuum pump 406. It is also possible that the control system comprises a speed sensor connected to the shaft of the vacuum pump 406. Furthermore, the frequency converter 408 can be configured to produce an estimate for electric power received by the frequency converter 408 from an electric power grid, an estimate f_est for torque directed to the vacuum pump 406 and/or an estimate for mechanical power supplied to the vacuum pump. The processor 403 can be configured to compute an estimate of the isentropic efficiency of the vacuum pump 406, and/or an estimate of the specific energy consumption of vacuum pump, and/or an estimate of the energy efficiency of the combination of the vacuum pump 406, the electric motor 407, and the frequency converter 408.

The processor 403 can be implemented with one or more processor circuits each of which can be a programmable processor circuit provided with appropriate software, a dedicated hardware processor such as for example an application specific integrated circuit "ASIC", or a configurable hardware processor such as for example a field programmable gate array "FPGA". The processor 406 can be communicatively connected to a memory 405 that can be implemented with one or more memory circuits each of which can be a random access memory "RAM" circuit.

In a control system according to an exemplifying and non-limiting embodiment of the invention, the processor 403 and/or the memory 405 are part/parts of the frequency converter 408. In a control system according to another exemplifying and non-limiting embodiment of the invention, the processor 403 and/or the memory 405 are part/parts of a programmable logic circuit "PLC" which is a separate device with respect to the frequency converter 408. The specific examples provided in the description given above should not be construed as limiting. Therefore, the invention is not limited merely to the exemplifying and non-limiting embodiments described above. Lists and groups of examples provided in the description are not exhaustive unless otherwise explicitly stated.

Claims

What is claimed is:

1 . A method for controlling a vacuum pump of a vacuum system, characterized in that the method comprises:

- measuring (101 ) inlet pressure (p,_n) of the vacuum pump,

- determining (102) an estimate for theoretical suction speed (Sth) of the vacuum pump based on data indicative of rotational speed (n) of the vacuum pump and data indicative of the theoretical suction speed (S_n) at a predetermined rotational speed (n_n),

- determining (103) an estimate for leak flow (Sieak) from an outlet of the vacuum pump back to an inlet of the vacuum pump based on the measured inlet pressure,

- determining (104) a difference (Sth - Sieak) between the estimate of the theoretical suction speed and the estimate of the leak flow so as to obtain an estimate for suction speed (S) of the vacuum pump, and

- controlling (105) the rotational speed (n) of the vacuum pump based at least partly on the estimate of the suction speed of the vacuum pump.

2. A method according to claim 1 , wherein the leak flow is estimated based on a suction curve (220) expressing the suction speed as a function of the inlet pressure at a constant rotational speed, the estimate of the leak flow being a difference between a first suction speed value at a first point (222) of the suction curve where the inlet pressure equals to outlet pressure (p_out) of the vacuum pump and a second suction speed value at a second point (223) of the suction curve where the inlet pressure is the measured inlet pressure.

3. A method according to claim 1 , wherein the leak flow is estimated based on:

- a predetermined ultimate value (p_uit) of the inlet pressure at which the leak flow equals the theoretical suction speed at the predetermined rotational speed, and - an assumption that the leak flow is an isentropic nozzle flow from the outlet of the vacuum pump back to the inlet of the vacuum pump.

4. A method according to claim 3, wherein the method comprises computing the estimate of the leak flow Sieak according to the following equation based on the assumption that the leak flow is the isentropic nozzle flow:

where p_n is gas density at the inlet of the vacuum pump, C is a flow coefficient, A is a nozzle throat area, p_out is outlet pressure of the vacuum pump, out is gas density at the outlet of the vacuum pump, k is an isentropic exponent, and p_ratio is a pressure ratio being a maximum of the following: p pout and (1 + (k - 1 )l2f^ ^{~ k)}, where p-_m is the inlet pressure of the vacuum pump.

5. A method according to claim 4, wherein the method comprises computing estimates for the gas densities at the inlet and the outlet of the vacuum pump according to the following equation applied separately for the inlet of the vacuum pump and for the outlet of the vacuum pump:

P

p ⁼— ^:—^— where p is the gas density, p is pressure, Rspedfic is a specific gas constant, and T is gas temperature.

6. A method according to claim 5, wherein the method comprises measuring a temperature Tin at the inlet of the vacuum pump and computing an estimate for temperature Tout at the outlet of the vacuum pump according to the following equation based on an assumption of isentropic compression:

7. A method according to claim 6, wherein the method comprises computing an estimate for the product CA of the flow coefficient and the nozzle throat area according to the following formula based on an assumption that the theoretical suction speed at the predetermined rotational speed equals the leak flow when the inlet pressure has the predetermined ultimate value

8. A method according to any of claims 1 -7, wherein the method comprises computing an estimate for ideal isentropic power consumption Ps according to the following equation:

where k is an isentropic exponent, S_est is the estimate for suction speed, p,_n is the inlet pressure of the vacuum pump, and p_out is outlet pressure of the vacuum pump.

9. A method according to claim 8, wherein the method comprises varying the rotational speed of the vacuum pump so as to find a value of the rotational speed at which the following ratio is maximized:

where Ps is the estimate for the ideal isentropic power consumption and P_ei is electrical power consumed for rotating the vacuum pump.

10. A method according to any of claims 1 -9, wherein the vacuum pump is driven with an electric motor supplied with a frequency converter which produces an estimate for the rotational speed (n) of the vacuum pump.

1 1 . A method according to any of claims 1 -10, wherein the method is carried out by a processor configured to: - determine the estimate for the theoretical suction speed (Sth) of the vacuum pump,

- determine the estimate for the leak flow (Seak),

- determine the estimate for the suction speed (S) of the vacuum pump by computing the difference (Sth - Seak) between the estimate of the theoretical suction speed and the estimate of the leak flow, and

- control the rotational speed (n) of the vacuum pump based on the estimate of the suction speed of the vacuum pump.

12. A control system (401 ) for controlling a vacuum pump of a vacuum system, characterized in that the system further comprises a processor (403) configured to:

- determine an estimate for theoretical suction speed (Sth) of the vacuum pump based on data indicative of rotational speed (n) of the vacuum pump and data indicative of the theoretical suction speed (S_n) at a predetermined rotational speed (n_n), - determine an estimate for leak flow (Sieak) from an outlet of the vacuum pump back to an inlet of the vacuum pump based on measured inlet pressure (p,_n) of the vacuum pump,

- determine a difference (Sth - Sieak) between the estimate of the theoretical suction speed and the estimate of the leak flow so as to obtain an estimate for suction speed (S) of the vacuum pump, and

- control the rotational speed (n) of the vacuum pump based at least partly on the estimate of the suction speed of the vacuum pump.

13. A control system according to claim 12, wherein the processor is configured to determine the estimate of the leak flow based on a suction curve (220) expressing the suction speed as a function of the inlet pressure at a constant rotational speed, the estimate of the leak flow being a difference between a first suction speed value at a first point (222) of the suction curve where the inlet pressure equals to outlet pressure (p_out) of the vacuum pump and a second suction speed value at a second point (223) of the suction curve where the inlet pressure is the measured inlet pressure.

14. A control system according to claim 13, wherein the processor is configured to determine the estimate of the leak flow based on: - a predetermined ultimate value (p_uit) of the inlet pressure at which the leak flow equals the theoretical suction speed at the predetermined rotational speed, and

15. A control system according to claim 14, wherein the processor is configured to compute the estimate of the leak flow Sieak according to the following equation based on the assumption that the leak flow is the isentropic nozzle flow:

where _n is gas density at the inlet of the vacuum pump, C is a flow coefficient, A is a nozzle throat area, p_out is outlet pressure of the vacuum pump, out is gas density at the outlet of the vacuum pump, k is an isentropic exponent, and p_ratio is a pressure ratio being a maximum of the following: p p₀ui and (1 + (k - 1 )l2f^ ^{" k)}, where p,_n is the inlet pressure of the vacuum pump.

16. A control system according to claim 15, wherein the processor is configured to compute estimates for the gas densities at the inlet and the outlet of the vacuum pump according to the following equation applied separately for the inlet of the vacuum pump and for the outlet of the vacuum pump: where p is the gas density, p is pressure, Rspedfic is a specific gas constant, and T is gas temperature.

17. A control system according to claim 16, wherein the control system comprises a temperature sensor (410) for measuring a temperature Tin at the inlet of the vacuum pump and the processor is configured to compute an estimate for temperature Tout at the outlet of the vacuum pump according to the following equation based on an assumption of isentropic compression:

18. A control system according to claim 17, wherein the processor is configured to compute an estimate for the product CA of the flow coefficient and the nozzle throat area according to the following formula based on an assumption that the theoretical suction speed at the predetermined rotational speed equals the leak flow when the inlet pressure has the predetermined ultimate value p_uit:

19. A control system according to any of claims 12-18, wherein the processor is configured to compute an estimate for ideal isentropic power consumption Ps according to the following equation:

20. A control system according to claim 19, wherein the processor is configured to vary the rotational speed of the vacuum pump so as to find a value of the rotational speed at which the following ratio is maximized:

21 . A control system according to any of claims 12-20, wherein the control system comprises an electric motor (407) for driving the vacuum pump and a frequency converter (408) for supplying voltage to the electric motor in accordance with a control signal produced by the processor, the frequency converter being configured to produce an estimate for the rotational speed (n) of the vacuum pump.

22. A vacuum system comprising:

- a vacuum chamber (409),

- a vacuum pump (406) for maintaining vacuum inside the vacuum chamber, and

- a control system (401 ) according to any of claims 12-21 for controlling the vacuum pump.

23. A computer program for controlling a vacuum pump of a vacuum system, characterized in that the computer program comprises computer executable instructions for controlling a programmable processor to:

- determine an estimate for theoretical suction speed (Sth) of the vacuum pump based on data indicative of rotational speed (n) of the vacuum pump and data indicative of the theoretical suction speed (S_n) at a predetermined rotational speed (n_n), - determine an estimate for leak flow (Sieak) from an outlet of the vacuum pump back to an inlet of the vacuum pump based on measured inlet pressure (p,_n) of the vacuum pump, - determine a difference (Sth - Sieak) between the estimate of the theoretical suction speed and the estimate of the leak flow so as to obtain an estimate for suction speed (S) of the vacuum pump, and

24. A computer program product for controlling a vacuum pump of a vacuum system, the computer program product comprising a non-volatile computer readable medium encoded with a computer program according to claim 23.