IMPROVED CONTROLLER FOR A SUPERCONDUCTING MOTOR POWER SUPPLY CIRCUIT, SUPERCONDUCTING MOTOR POWER SUPPLY SYSTEM, AND AIRCRAFT COMPRISING SUCH A SYSTEM.
The power supply circuit controller effectively detects quench phenomena in superconducting power supply links by summing potential differences and inhibiting inverter control, addressing the challenge of inductive impedance interference and preventing Joule losses.
Patent Information
- Application Number
- FR2023010010
- Authority / Receiving Office
- FR · FR
- Patent Type
- Patents
- Current Assignee / Owner
- Filing Date
- 2023-09-21
- Publication Date
- 2025-10-17
- Estimated Expiration
- 2043-09-21
AI Technical Summary
Existing technologies struggle to reliably detect a quench phenomenon in superconducting power supply links carrying alternating current due to the inductive component of impedance, which disrupts conventional quench detection methods, leading to potential damage from excessive Joule losses.
A power supply circuit controller that processes signals from the three-phase power supply lines to detect a quench by summing potential differences and inhibiting the inverter control output if a predetermined threshold is exceeded, using low-pass filtering to eliminate noise and imperfections.
Rapid and reliable detection of quench phenomena in superconducting power supply links, limiting current delivery to prevent thermal dissipation and system damage.
Smart Images
Figure 00000014_0000 
Figure 00000014_0001 
Figure 00000015_0000
Abstract
Description
Title of the invention: IMPROVED CONTROLLER FOR THE ELECTRICAL POWER SUPPLY CIRCUIT OF A SUPERCONDUCTING MOTOR, SUPERCONDUCTING ELECTRICAL POWER SUPPLY SYSTEM FOR A MOTOR, AND AIRCRAFT COMPRISING SUCH A SYSTEM. Technical field
[0001] The present invention relates to a so-called superconducting motor, in particular of the type usable for the propulsion of an aircraft. The invention relates more particularly to the monitoring and control of a superconducting power supply link of an aircraft electric motor powered from an inverter, and to an aircraft. STATE OF THE PRIOR ART
[0002] The aeronautical industry is making profound changes in terms of aircraft design, with the aim of significantly reducing carbon dioxide and nitrogen oxide emissions, due to ecological and sustainable development constraints.
[0003] The use of liquid hydrogen as an energy source for an aircraft is a promising way to do this. Hydrogen can be used in a fuel cell, to generate electricity, or directly as fuel in a powertrain. In addition, work is specifically directed to optimize electric or hybrid propulsion systems for aircraft and hydrogen present in liquid form on board an aircraft can be used to increase the performance of electrical equipment by lowering its resistivity and consequently reducing Joule losses. It is also possible to use superconducting components. Superconducting conductors can be used for the distribution of alternating current in architectures comprising electric motors powered by controlled electronic power converters, such as inverters.In such motor power supply architectures, the superconducting link between the inverter and the motor must be protected against an unexpected transition from the superconducting state to the conventional state (a transition usually called quench in the field of superconductivity). It is therefore important to be able to detect early signs of such a transition in order to avoid excessive Joule losses that could damage the power supply circuits and their surrounding environment.
[0004] The detection of the quench of a superconducting conductor is generally carried out in monitoring the voltage across it and detecting the occurrence of an increase in the voltage level across its terminals. If detecting a quench on a connection carrying a direct current (so-called DC connection) is fairly easy since it involves discriminating between a zero and non-zero impedance (i.e. resistance) of the conductor, it is not the same for a connection carrying an alternating current (so-called AC connection). Indeed, in the latter case, the inductive component of the impedance is never zero and is even greater than the resistive component in most cases, which disrupts or even prevents quench detection using conventional means. There is therefore a need to obtain a fast, reliable, and low-cost solution that overcomes the inductive effect on the voltage across the terminals of the conductor(s) of the motor's electrical power supply link.
[0005] The situation can be improved. Statement of the invention
[0006] An object of the present invention is to propose a power supply circuit controller for an electric motor powered by alternating current, capable of rapidly and reliably detecting the appearance of a quench-type phenomenon in an electrical power supply link to a motor, so as to limit the current(s) delivered to the motor by an inverter when the power supply link between the inverter and the motor is about to pass from a nominal superconducting state to a state in which its resistivity increases.
[0007] For this purpose, a method is proposed for controlling a power supply circuit of an electric motor, the power supply circuit comprising a power converter, called an "inverter", configured to deliver three alternating voltages from a direct voltage source and a superconducting three-phase power supply link comprising three electrical power supply lines configured to power the electric motor, the method being executed by a power supply control circuit comprising a power supply circuit controller, which controller comprises at least one control output configured to control the inverter, the method being such that it comprises: - i) obtaining a first signal representative of a potential difference between the ends of a first, among the three electrical supply lines of the three-phase supply link, - ii) obtain a second signal representative of a potential difference between the ends of a second, among the three power supply lines of the three-phase power link, - iii) obtain a third signal representative of a potential difference between the ends of a third, among the three, power supply lines of the three-phase power supply link, - iv) determining a sum signal representative of the sum of the first signal, the second signal and the third signal, and, - v) if the sum signal thus determined exceeds a predetermined threshold value, inhibit the control output of said controller, and otherwise, repeat steps i) to v).
[0008] Advantageously, it is thus possible to reliably and rapidly detect, and with limited means, the appearance of a quench phenomenon in a superconducting power supply link of an electric motor supplied with alternating current.
[0009] According to one embodiment, the method for controlling a power supply circuit further comprises, between steps iii) and iv), low-pass filtering of the determined sum signal.
[0010] The invention also relates to a control circuit for a power supply circuit of an electric motor, the power supply circuit comprising a power converter called an inverter configured to deliver three alternating voltages from a direct voltage source and a superconducting three-phase power supply link comprising three electrical power supply lines configured to power said electric motor, said control circuit comprising a power supply circuit controller comprising at least one control output configured to control said inverter, and the control circuit comprising electronic circuitry configured to: - i) obtain a first signal representative of a potential difference between the ends of a first, among the three electrical power supply lines of the three-phase power supply link, - ii) obtain a second signal representative of a potential difference between the ends of a second, among the three power supply lines of the three-phase power link, - iii) obtain a third signal representative of a potential difference between the ends of a third, among the three power supply lines of the three-phase power link, - iv) determining a sum signal representative of the sum of said first signal, said second signal and said third signal, and for, - v) if said sum signal exceeds a predetermined threshold value, inhibit said control output, and otherwise, repeat steps i) to v).
[0011] Advantageously, the control circuit of a power supply circuit further comprises a filtering circuitry (or circuit) configured to carry out low-pass filtering of the determined sum signal.
[0012] Another object of the invention is an electrical power supply system for an electric motor comprising a circuit for controlling a power supply circuit such as pre- previously described.
[0013] The invention also relates to an aircraft comprising at least one control circuit of a superconducting power supply circuit as previously described or an electrical power supply system as mentioned above.
[0014] Finally, the invention also relates to a computer program product comprising program code instructions for executing the steps of a method as previously described, when this program is executed by a processor of a control circuit of an electrical power supply circuit. Brief description of the drawings
[0015] The above-mentioned features of the invention, as well as others, will appear more clearly on reading the following description of an exemplary embodiment, said description being made in relation to the attached drawings:
[0016] [Fig.l] schematically illustrates a control circuit of an electrical power supply circuit of an electric motor operating in an electrical power supply system of an aircraft engine, according to one embodiment;
[0017] [Fig.2] is a flowchart illustrating a method for controlling an electrical power supply circuit of an electric motor, executed in an electrical power supply system of an aircraft engine, according to one embodiment;
[0018] [Fig.3] schematically illustrates an example of internal architecture of a power supply circuit control circuit already shown in [Fig.l]; and,
[0019] [Fig.4] illustrates an aircraft comprising an electrical power system comprising a circuit for controlling an electrical power circuit according to one embodiment.
[0020] DETAILED DESCRIPTION OF EMBODIMENTS
[0021] [Fig.l] is a schematic representation of an electrical power system 10 configured to electrically power an electric motor 100, according to one embodiment. According to the example described here, the electric motor 100 is an aircraft engine. To do this, the electrical power system 10 comprises a control circuit itself comprising a power circuit controller 106 for controlling an inverter 102 powering a superconducting electrical power link 104 connected to the electric motor 100. The inverter 102 operates power converter functions and provides a balanced three-phase electrical network of voltages to the electric motor 100, via the superconducting power link 104, from a DC voltage source 101.The balanced three-phase power network comprises a first power supply line 104a, a second power supply line 104b and a third power supply line 104c. These three power supply lines together compose the link. superconducting power supply 104.
[0022] In order to control the power supply system 10 of the electric motor 100, the controller 106 of the power supply circuit control circuit comprises at least:
[0023] - a current control CTRL output configured to control current inverter 102, and,
[0024] - an input, Vi, configured to receive a signal S representative of a sum of three signals A Va, A Vb and A Vc, themselves representative of voltages respectively measured at the terminals of each of the supply lines 104a, 104b and 104c of the superconducting supply link 104. The signal S is produced by an adder circuit 105 which comprises an output (therefore the one which delivers the signal S) and three inputs to which are respectively applied the signals A Va, A Vb and A Vc representative of voltages respectively measured at the terminals of each of the supply lines 104a, 104b and 104c.
[0025] Circuits 110a, 110b and 110c for determining a potential difference are used to each deliver a signal whose content or amplitude is representative of a difference in electrical potentials applied to these inputs.
[0026] Three-phase connections (or connection buses) 104u and 104d configured to carry out measurements of electrical potentials respectively connect the upstream and downstream ends of the superconducting electrical power supply link 104 to the potential difference determination modules 110a, 110b and 110c. Thus, the circuit 110a delivers the signal A Va representative of the potential difference between the ends of the electrical power supply link 104a; the circuit 110b delivers the signal A Vb representative of the potential difference between the ends of the electrical power supply link 104b and the circuit 110c delivers the signal A Vc representative of the potential difference between the ends of the electrical power supply link 104c. According to one embodiment, the signals A Va, A Vb and A Vc are analog signals and the adder circuit 105 is configured to deliver a signal S whose amplitude is the sum of these three analog signals.According to another embodiment, the signals AVa, AVb and AVc are digital signals (logic words) and the adder circuit 105 is configured to deliver a signal S in the form of a digital signal itself (a logic word) whose value is the sum of these three digital signals (these three logic words, for example signed integers 16 bits wide).
[0027] The control of the inverter 102 by the power supply circuit controller 106 uses a connection link 107. According to one embodiment, the connection link 107 carries a pulse width modulation (PWM) type signal generated by control circuits and modules operating a vector control type control of the inverter.
[0028] According to one embodiment, the three supply lines 104a, 104b and 104c of the superconducting power supply link 104 are arranged in the same cryogenic volume. According to a variant, each of the power supply lines 104a, 104b and 104c of the superconducting power supply link 104 is arranged in a cryogenic volume dedicated to it.
[0029] In any event, detection of an increase in the potential difference on at least one of the supply lines connected to one of the circuits 110a, 100b or 110c for determining the potential difference is sufficient to carry out quench detection and to define subsequent operations useful for preserving and securing the systems, since the sum of the voltages of the three-phase electrical supply link is in this case no longer zero or close to zero.
[0030] According to an alternative embodiment, a single potential difference determination module 110 comprises internal multiplexing circuits for carrying out a sequential scanning (monitoring) of the potential differences between the two ends of each of the electrical supply lines 104a, 104b and 104c, successively and iteratively, and the adder circuit 105, then digital, carries out an addition of three values respectively representative of the voltages at the terminals of the electrical supply lines 104a, 104b and 104c, received in a reduced time interval.
[0031] Cleverly and advantageously, the power supply circuit controller 106 performs processing and operations based on the sum S of the potential differences respectively measured at the terminals of the electrical power supply lines 104a, 104b and 104c, via the potential difference determination modules 110a, 110b and 110c and its input Vi.
[0032] According to one embodiment, its internal electronic circuitry is configured to inhibit the control output CTRL of the inverter 102 only if the sum S exceeds a predetermined threshold value for at least one predetermined threshold duration or for a duration exceeding this threshold duration.
[0033] The terms “inhibit the control output of the inverter” here designate any operation subsequent to the detection of the crossing of the threshold value (where applicable for a duration equal to or exceeding a threshold duration) and aimed at limiting the thermal dissipation by Joule effect in the superconducting power supply link 104, by limiting the current delivered by the inverter 102 in each of the electrical power supply lines 104a, 104b and 104c or by interrupting the operation of the inverter 102. This inhibition of the control output CTRL configured to control the inverter 102 will be called “safety of the inverter 102” here.
[0034] According to an alternative embodiment, the circuits 110a, 110b and 110c for determining a potential difference and / or the adder circuit 105 are integrated into the controller 106 of the power supply link 104, and the power lines of the connection buses 104u and 104d are then directly connected to inputs of the controller 106 of the power supply link 104.
[0035] According to an exemplary embodiment, the inhibition of the CTRL output defines a degraded operating mode of the inverter 102, at reduced power. According to another exemplary embodiment, the inhibition of the CTRL output isolates the DC voltage source 101 from the inverter 102. These exemplary embodiments are not limiting and other modes of controlling the power dissipated in the electrical power supply link 104, via the CTRL control output of the controller 102 can of course be implemented.
[0036] [Fig. 2] is a flowchart diagram which illustrates steps of a method for controlling the power supply link 104 of the electrical power supply system 10 of the electric motor 100, executed in whole or in part by the power supply circuit controller 106 (and therefore of the power supply link 104), according to one embodiment.
[0037] A step S0 comprises operations of initialization and configuration of the set of systems present aiming to obtain a nominal state defined as a normally operational configuration for use of the electric motor 100 and its electrical power supply circuit 10.
[0038] A step S11 comprises obtaining the signals AVa, AVb and AVc at the output respectively of the circuits 100a, 100b and 100c for determining a potential difference between the ends of the power supply link 104, for each of the power supply lines 104a, 104b and 104c. According to the example described, this step S11 is broken down into three steps S10, S11 and S12 operated in parallel (i.e. simultaneously and independently of one another). During step S10, the circuit 110a determines the signal AVa to deliver it to a first input of the adder circuit 105; during step S11, the circuit 110b determines the signal AVb to deliver it to a second input of the adder circuit 105; and, during step S12, the circuit 110c determines the signal AVa to deliver it to a third input of the adder circuit 105.
[0039] During a step S2, the adder circuit 105 adds the three signals AVa, AVb and AVc and delivers the sum signal S = AVa + AVb + AVc at its output. This signal S must be substantially close to zero in the absence of a quench phenomenon, since the power supply link 104 is a balanced three-phase type link for which, in theory, the sum of the voltages is always zero. The signal S thus determined is applied to the input Vi of the power supply circuit controller 106 via an electrical link 109.
[0040] According to one embodiment, the three signals AVa, AVb and AVc are corrected, respectively during optional steps S10', SU' and S12', before being summed during step S2 by the adder circuit 105. For example, each of the three signals AVa, AVb and AVc can be filtered, or a gain can be applied to each of these AVa, AVb and AVc signals, or a time shift (an "offset" in English) can be applied to each of these AVa, AVb and AVc signals.
[0041] A filtering step S3, for example low-pass filtering, is then carried out by the power supply circuit controller 106 to filter the result of the addition, namely the previously determined sum sign S. This filtering makes it possible to eliminate parasitic noise, but also to attenuate any imperfections in the system, such as an imbalance between the phases. The signal resulting from the filtering carried out is then compared to a predetermined threshold value during a step S4. Indeed, if the sum S of the three signals AVa, AVb and AVc is supposed to be zero in theory, the uncertainties and disparities inherent in the hardware implementation of the three power supply lines 104a, 104b and 104c, as well as in the output stages of the inverter 102 and in the electric motor 100 are such that in practice, this sum, although close to zero in the absence of a quench phenomenon, is not quite zero.Therefore, it should not be compared to zero, but to a reduced threshold value determined by analyses and / or laboratory experiments in the systems concerned.
[0042] In the case where the value S compared with the threshold value exceeds the predetermined threshold value, and therefore in the presence of a quench phenomenon, the control output CTRL of the inverter 102 is inhibited during a step S5 of securing the electrical power supply system 10 so as to preserve the integrity of the power supply systems of the electric motor 100 present. This state is transmitted to the inverter via a control line 107 connecting the controller 106 to the inverter 102. In the opposite case, and therefore in the absence of a quench phenomenon detected in the superconducting electrical power supply link 104, the method implemented by the control circuit comprising the power supply circuit controller 106 loops back to step S1 to carry out a new iteration of the steps of the method described above.Optionally, during step S4, and after detection of an exceeding of the predetermined threshold value, it is verified that the exceeding of the predetermined threshold value is detected for at least a predetermined duration to authorize the inhibition operations of step S5, failing which, if the exceeding of the predetermined threshold value only occurs for a time less than the predetermined threshold duration and is then resolved, the method continues in sequence by looping back from step SL According to one embodiment, such a one-off detection results in a configuration of supervision and control systems of the electrical power supply circuit 10 of the motor 100.
[0043] According to one embodiment, the aforementioned predetermined threshold duration value is determined by calculation or during operational tests in a research and development laboratory. development, or during validation and / or approval tests. According to one embodiment, these two threshold values are programmable, preferably remotely via a remote control interface of the control circuit and the power supply circuit controller 106.
[0044] The steps of the method described above can be implemented indifferently by hardware circuitry, by software functions or by a combination of these two forms of implementation.
[0045] According to an alternative embodiment, steps S10, S11 and S12 are operated sequentially by a single potential difference determination circuit 100 which uses an input multiplexer to connect successively to the supply lines 104a, 104b then 104c and an output demultiplexer to supply the signals A Va, AVb and AVc to the adder circuit 105.
[0046] [Fig. 3] is a diagram illustrating an example of internal architecture of the power supply circuit control circuit or of the power supply circuit controller 106, according to one embodiment. It is noted that [Fig. 3] could also schematically illustrate an example of hardware architecture of a processing module included in the power supply circuit controller 106 or comprising the power supply circuit controller 106, in addition to other modules configured to operate other functions related to the implementation of the method.
[0047] According to the example of hardware architecture shown in [Fig. 3], the control circuit or where appropriate the power supply circuit controller 106 then comprises, connected by a communication bus 1060: a processor or CPU (“Central Processing Unit” in English) 1061; a RAM (“Random Access Memory” in English) 1062; a ROM (“Read Only Memory” in English) 1063; a storage unit such as a hard disk (or a storage media reader, such as an SD (“Secure Digital” in English) card reader) 1064; at least one interface module 1065 allowing the power supply system controller 106 to communicate with devices present in the power supply system 10, such as for example, the inverter 102, the modules for determining a potential difference 110a, 110b or 110c and the electric motor 100.Advantageously, the INTER 1065 interface module comprises in particular input-output ports, inputs of digital / analog converters and analog / digital converters, outputs controlled by pulse width modulation, and more generally all types of interfaces useful for managing a system for supplying a motor by a three-phase current source of the inverter type operating from a continuous energy source. In particular, the INTER 1065 interface module of the controller 106 is configured to operate in particular vector control functions of an inverter such as the inverter. 102.
[0048] The processor 1061 is capable of executing instructions loaded into the RAM 1062 from the ROM 1063, an external memory (not shown), a storage medium (such as an SD card), or a communications network. When the power circuit controller 106 is powered on, the processor 1061 is capable of reading program code instructions from the RAM 1062 and executing them. These instructions form a computer program causing the processor 1061 to implement all or part of a method described in relation to [Fig. 2], or all or part of the described variations of this method.
[0049] All or part of the method described in relation to [Fig. 2], or its described variants may be implemented in software form by executing a set of instructions by a programmable machine, for example a DSP (Digital Signal Processor) or a microcontroller, or be implemented in hardware form by a machine or a dedicated component, for example an FPGA (Field-Programmable Gate Array) or an ASIC (Application-Specific Integrated Circuit). In general, the power supply circuit controller 106 comprises electronic circuitry configured to implement the methods described in relation to the control circuit or the power supply circuit controller 106.Obviously, the power supply circuit controller 106 further comprises all the elements usually present in a system comprising a control unit and its peripherals, such as a power supply circuit, a power supply supervision circuit, one or more clock circuits, a reset circuit, related input-output ports, interrupt inputs, bus drivers, this list being non-exhaustive.
[0050] [Fig. 4] illustrates an aircraft 1 comprising the electrical power supply system 10 previously described, which system comprises a control circuit itself comprising the power supply circuit controller 106. The use of such a system on board an aircraft powered using at least one electric motor such as the electric motor 100 makes it possible to offer an increased level of safety in the event of the appearance of a quench phenomenon on an electrical power supply link of a motor.
[0051] Furthermore, the clever use of a low-pass filter circuit, possibly with a cut-off frequency controlled as a function of the rotation frequency of the motor, makes it possible to obtain a compromise between the bandwidth of the signals for which protection is implemented and the rejection of higher-frequency parasitic signals.
[0052] Finally, an implementation according to the embodiments described advantageously makes it possible to significantly increase the level of security while being simple and with a very limited additional use of hardware resources.
Claims
Claims
1. Method for controlling a power supply circuit (102, 104) of an electric motor (100), the power supply circuit (102, 104) comprising a power converter (102) called an inverter (102) configured to deliver three alternating voltages from a direct voltage source (101) and a superconducting three-phase power supply link (104) comprising three electrical power supply lines (104a, 104b, 104c) configured to power said electric motor (100), said method being executed by a power supply control circuit comprising a power supply circuit controller (106), which controller comprises at least one control output (CTRL) configured to control said inverter (102), the method being characterized in that it comprises: - i) obtaining a first signal (AVa) representative of a potential difference between the ends of a first (104a), among the three lines power supply (104a, 104b,104c) of the three-phase power supply link (104), - ii) obtaining a second signal (AVb) representative of a potential difference between the ends of a second (104b), among the three power supply lines (104a, 104b, 104c) of the three-phase power supply link (104), - iii) obtaining a third signal (AVc) representative of a potential difference between the ends of a third (104c), among the three power supply lines (104a, 104b, 104c) of the three-phase power supply link (104), - iv) determining a sum signal (S) representative of the sum of said first signal (AVa), said second signal (AVb) and said third signal (AVc), and, - v) if said sum signal exceeds a predetermined threshold value, inhibiting said control output (CTRL), and otherwise, repeating steps i) to v).,
2. A method of controlling a power supply circuit (102, 104) according to claim 1, further comprising, between steps iii) and iv), low-pass filtering of the sum signal (S).
3. Control circuit of a power supply circuit (102, 104) of an electric motor (100), the power supply circuit (102, 104) comprising a power converter (102) called inverter (102) configured to deliver three alternating voltages from a direct voltage source (101) and a superconducting three-phase power link (104) comprising three electrical power supply lines (104a, 104b, 104c) configured to power said electric motor (100), said control circuit comprising a power circuit controller (106) comprising at least one control output (CTRL) configured to control said inverter (102), and the control circuit being characterized in that it comprises electronic circuitry configured to: - i) obtain a first signal (AVa) representative of a potential difference between the ends of a first (104a), among the three electrical power supply lines (104a, 104b, 104c) of the three-phase power link (104), - ii) obtain a second signal (AVb) representative of a potential difference between the ends of a first (104a), among the three electrical power supply lines (104a, 104b, 104c) of the three-phase power link (104), - iii) obtain a second signal (AVb) representative of a potential difference between the ends of a first (104a), among the three electrical power supply lines (104a, 104b, 104c) of the three-phase power link (104), - iv) obtain a second signal (AVb) representative of a potential difference between the ends of a first (104a), among the three electrical power supply lines (104a, 104b, 104c) of the three-phase power link (104), - v ... potential between the ends of a second (104b),among the three power supply lines (104a, 104b, 104c) of the three-phase power supply link (104), - iii) obtaining a third signal (AVc) representative of a potential difference between the ends of a third (104c), among the three power supply lines (104a, 104b, 104c) of the three-phase power supply link (104), - iv) determining a sum signal (S) representative of the sum of said first signal (AVa), said second signal (AVb) and said third signal (AVc), and for, - v) if said sum signal (S) exceeds a predetermined threshold value, inhibiting said control output (CTRL), and otherwise, repeating steps i) to v).,
4. Control circuit of a power supply circuit (102, 104) according to claim 3, further comprising filtering circuitry configured to operate a low-pass filtering of said determined sum signal (S).
5. Electrical power supply system (10) for an electric motor (100) comprising a control circuit of a power supply circuit (102, 104) according to one of claims 3 and 4.
6. Aircraft (1) comprising at least one control circuit of a superconducting power supply circuit (102, 104) according to one of claims 3 and 4 or an electrical power supply system (10) according to the re- claim 5.
7. Computer program product comprising program code instructions for executing the steps of the method according to one of claims 1 and 2, when said program is executed by a processor of a control circuit of an electrical power supply circuit (102, 104).
8. A storage medium comprising a computer program product according to claim 7.