Method for operating electric machine of rotary drive

Abstract

The invention relates to a method for operating an electric machine (4) of a rotary drive having a transmission mechanism (2) with a transmission mechanism backlash, the electric machine (4) being adjusted as a function of a target value (nEMsoll) of an adjustment variable, the adjustment comprising a proportional controller, and the adjustment comprising a proportional controller. The proportional gain factor (KP) of the proportional controller is varied as a function of the variable which deduces the powerless state of the transmission (2).

Description

Technical Field

[0001] The invention relates to a method for operating an electric machine of a rotary drive, as well as to a computing unit and a computer program for carrying out the method. Background Art

[0002] In mechanical transmissions in rotary drives, transmission play or backlash may occur, that is, play may occur in the transmission between the driver and the driven member. Gear backlash can be described, for example, as an angular interval in which the driver can rotate without the driven member moving. Gear backlash occurs, for example, at each gear pair of the transmission. In the case of multi-stage transmissions, the gear backlash - angular interval multiplied by the corresponding transmission ratio - is added together. Gear backlash can occur, for example, in steering gears as steering play, in vehicle drives as play between thrust and drive force, in machine tools, for example at the spindle, or in rotary drives, such as the rotary mechanism of excavators or cranes. Summary of the invention

[0003] According to the invention, a method for operating an electric machine of a rotary drive, as well as a computing unit and a computer program for carrying out the method are proposed having the features of the independent claims. Advantageous embodiments are the subject matter of the dependent claims and the following description.

[0004] The invention uses the following measure, in which the electric motor of a rotary drive having a transmission with transmission backlash is regulated, and the gain of a P-regulator is varied as a function of a variable from which a force state, in particular a no-force state, is inferred. Oscillations and high mechanical loads which may occur with high control gains can be avoided, in particular, when the drive motor or electric machine accelerates strongly relative to the driven element in an angular range within the transmission backlash and the tooth flanks collide at high differential speeds. The transmission can have one or more transmission stages, each with a transmission backlash, which together give the overall transmission backlash.

[0005] It is assumed that the proportional gain factor and, if necessary, the integral gain factor (see further below) are positive.

[0006] In one embodiment, the method is used in a rotary drive having a transmission having a transmission backlash of at least 10°, at least 20° or at least 30° and / or a transmission ratio (between drive shaft and output shaft) of at least 20:1, at least 50:1 or at least 100:1. In this case, transmission backlash is to be understood as meaning the size of the (maximum) angular range within which a drive shaft of the transmission or an output shaft of an electric motor connected thereto can be rotated back and forth without a corresponding (corresponding to the transmission ratio) rotation of the output shaft of the transmission occurring.

[0007] In one embodiment, the variable from which the power-free state of the transmission is inferred is a control deviation of the control variable (e.g., the difference between a target value and an actual value of the control variable). In one embodiment, the proportional gain factor increases with increasing amplitude of the variable. The control deviation is already determined within the framework of the control, and no additional calculation is required accordingly. The control deviation indicates the extent to which the motor can follow the target value of the control variable and is accordingly a measure of how strongly the motor is loaded, wherein in the power-free state only a low load is present due to the moment of inertia of the motor, so that the control deviation is kept small.

[0008] In one embodiment, the manipulated variable is the rotational speed of the electric machine. Likewise, in one embodiment, the manipulated variable can be specified as the torque of the electric machine.

[0009] In one embodiment, the gradient of the proportional gain factor is limited upwards. The speed at which the proportional gain factor is increased or built up is then limited. Optionally, a downward limit can also be provided, that is to say a limit at which the proportional gain factor is reduced or lowered, but the limit at which the build-up is limited should be significantly smaller in magnitude than the limit at which the reduction is limited (e.g. by a factor of at least 10). It can be provided that the limit at which the gradient of the proportional gain factor is limited upwards depends on the proportional gain factor and in particular increases as the proportional gain factor increases.

[0010] In one embodiment, a torque request for controlling an inverter of an electric machine is determined (in the control), wherein the gradient of the magnitude of the torque request is limited asymmetrically, and wherein, in one embodiment, the magnitude of a lower limit for the gradient of the torque request is greater than an upper limit for the gradient of the torque request. Oscillations, for example due to delays in the system, can be avoided in particular by such asymmetrical limiting.

[0011] In one embodiment, the output speed is determined, wherein the corresponding speed of the electric motor is determined from the output speed taking into account the transmission ratio of the transmission, and wherein the speed difference is determined as the difference between the actual speed of the electric motor and the corresponding speed of the electric motor. The output speed is the speed of a driven shaft of the transmission or of a load, which is driven via the transmission by a rotary drive. As long as the transmission is not in transmission backlash, there is a speed ratio between the driven shaft of the transmission and the drive shaft of the transmission (which corresponds to the output shaft of the electric motor) that corresponds to the transmission ratio of the transmission.

[0012] The speed difference is a measure or variable which indicates the extent to which the power-free state of the transmission exists (that is, whether the rotary drive is in the transmission backlash). Accordingly, in one embodiment, the variable from which the power-free state of the transmission is inferred is the speed difference, wherein in one embodiment, the proportional gain factor decreases as the magnitude of the variable increases.

[0013] In one embodiment, the target value of the manipulated variable is formed or filtered in such a way that the gradient of the target value of the manipulated variable is limited according to a limit that is dependent on the speed difference, wherein in one embodiment, the amplitude of the limit of the gradient of the target value of the manipulated variable decreases as the speed difference increases. This makes it possible to avoid high torque shocks on the tooth flanks in the gear mechanism, for example.

[0014] The computing unit according to the invention, for example a control unit of a rotary drive or of a machine having a rotary drive, is designed, in particular in terms of programming, to carry out the method according to the invention.

[0015] The implementation of the method according to the invention in the form of a computer program or computer program product with program code for executing all method steps is also advantageous, since this results in particularly low costs, in particular if the controller to be implemented also serves other purposes and is therefore already present. Suitable data carriers for providing the computer program are in particular magnetic, optical and electrical memories, such as, for example, hard disks, flash memories, EEPROMs, DVDs, etc. Downloading the program via a computer network (Internet, intranet, etc.) is also possible. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Further advantages and embodiments of the invention are apparent from the description and the drawings.

[0017] It goes without saying that the features mentioned above and those yet to be explained below can be used not only in the respectively specified combination but also in other combinations or alone, without departing from the scope of the present invention.

[0018] The present invention is schematically shown in the drawings based on exemplary embodiments and is described in detail below with reference to the drawings.

[0019] Figure 1 An exemplary topology of an electric rotary drive with a gear mechanism is shown.

[0020] Figure 2 A regulator structure according to one specific embodiment of the present invention is shown.

[0021] Figure 3 As an example of a machine in which the invention can be used, an excavator is shown which has an electrically driven rotary mechanism. DETAILED DESCRIPTION

[0022] Figure 1 An exemplary topology of an electric rotary drive with a transmission 2 is shown. The electric rotary drive has an electric motor 4, which drives a load 6, that is to say a driven element of a machine in which the electric rotary drive is used, via the transmission 2. The transmission 2 has, for example, a drive shaft, which is coupled to the output shaft of the electric motor 4, and a driven shaft, which is coupled to the load 6 or the driven element. The drive shaft and the driven shaft can be coupled to one another in the transmission via gears, toothed belts or the like. In this case, transmission backlash or play can occur, that is to say the drive shaft can rotate in an angular interval without a corresponding rotation of the driven shaft taking into account the transmission ratio of the transmission.

[0023] The electric machine 4 is controlled by an inverter 8, which typically has an inverter control, which is particularly designed to control the (semiconductor) switching elements of the inverter as a function of a control variable 14. The control variable 14 can be, in particular, a required torque (torque requirement) or a required speed (speed requirement). In the case of control according to the required speed or speed requirement, the inverter 8 or the inverter control can have a speed control. The inverter 8 is typically designed to determine the actual speed or actual speed of the electric machine 4. For example, with the aid of a resolver provided in the electric machine, wherein a corresponding signal of the resolver, that is to say a speed feedback of the resolver, is drawn as an arrow 9 in the figure.

[0024] The control device 10, for example a control unit, determines the control variable 14. For this purpose, the control device 10 determines the control variable 14, for example according to Figure 2To perform regulation in particular. On the one hand, the control device 10 uses a control signal 16 as an input variable, which indicates the desired speed of the motor or the load, and the control signal is generated, for example, by an operating element 12 (for example, a gear lever) that detects a user input. As an alternative or additional solution, the control signal 16 can also be generated at least partially by a (partially) autonomous machine control unit. On the other hand, the control device 10 uses actual parameters or actual parameters. The actual parameters include the actual speed n_EMist of the motor 4 and / or the actual torque M_EMist of the motor 4. Both of these can be determined, for example, by the inverter 8 and transmitted to the control device 10. Optionally, the actual speed n_Abtrieb of the driven member, that is, the load 6 or the driven shaft of the transmission mechanism 2 can be used as an input variable of the control device 10. If there is no sensor for detecting the actual speed of the output, an observer (not shown) can be used, which models the actual controlled object in order to reconstruct the actual speed n_Abtrieb of the output from the control variable and the actual speed of the electric machine 4 and / or the actual torque of the electric machine 4. The control variable 14 and the actual variable are transmitted, for example, via a signal bus (such as CAN: Controller Area Network). In this case, a delay time in the range of, for example, 0.5 to 5 ms may occur in each case.

[0025] In the embodiment shown, the control variable 14 is a target torque, which is transmitted to the inverter 8. According to other embodiments (not shown here), the control device 10 or the functionality implemented by it (for example by executing at least one corresponding computer program) can be at least partially part of the inverter 8, or the functionality of the control device can be at least partially implemented by the inverter 8 (for example in that at least one corresponding computer program is executed by the inverter control). In this case, for example, the prefilter 60 of the target speed (see Figure 2 ) also retains a part of the control device 10.

[0026] Figure 2 A controller structure according to one embodiment is shown, such as can be used, for example, in Figure 1 As in the control device 10 of the electric rotary drive shown in FIG.

[0027] Basically, the controller structure shown includes a proportional controller in which the speed difference Δn_EM of the electric machine between the target speed n_EMset and the actual speed n_EMist of the electric machine (Δn_EM=n_EMset−n_EMist) is multiplied by a proportional gain factor K_P (proportional element 24) in order to determine the torque request M_Anf for the electric machine. The speed difference Δn_EM in this example represents a control deviation. The speed of the electric machine is an example for a control variable, the target speed is an example for a target value of the control variable, and the actual speed is an example for an actual value of the control variable.

[0028] The proportional gain factor K_P is not constant, but can vary depending on the control deviation, that is, depending on the speed difference Δn_EM. The control deviation, that is, in the example shown, the speed difference Δn_EM is a variable that indicates whether the transmission is in a powerless state or in the transmission backlash, that is, in a state in which no torque is transmitted to the load. Since no torque is transmitted to the load, in the powerless state or in the transmission backlash, only the inertia moment of the motor itself (and the shaft and / or gear directly connected thereto) acts on the motor. However, if there is no powerless state or it is outside the transmission backlash, the inertia moment of the motor is much smaller (for example, by a factor of 100) than the inertia moment of the load acting on the motor, even when taking into account the transmission ratio of the transmission. In the transmission backlash, the control deviation (for example, the speed difference Δn_EM) can also be kept small with a relatively small proportional gain factor K_P, because the motor only has to apply a relatively small torque in order to overcome the inertia moment of the motor alone, and the control variable can therefore be adjusted quickly. Furthermore, by means of an optional prefiltering of the target speed (see below), the maximum speed gradient of the target variable (n_EMtarget) can be greatly limited, so that no large acceleration of the electric machine is required.

[0029] A stepwise proportional gain factor K_Pprog is determined (link 22), which varies or changes as a function of the speed difference Δn_EM (control deviation) between the minimum proportional gain factor and the maximum proportional gain factor. In one embodiment, it is provided that the stepwise determined proportional gain factor K_Pprog increases monotonically as the speed difference Δn_EM increases. This can be done in particular symmetrically, that is to say the stepwise determined proportional gain factor K_Pprog is a function of the amplitude of the speed difference Δn_EM (as shown by way of example). Correspondingly (in the case of monotonous increase), the stepwise determined proportional gain factor K_Pprog is equal to the minimum proportional gain factor when the control deviation is zero, and is equal to the maximum proportional gain factor when the control deviation is high. This embodiment prevents oscillations of the regulator even if the speed feedback is slightly delayed (delay time). A very high P-gain together with a low inertia of the machine can lead to oscillations (or jitters of the speed and torque) at a short delay time (e.g. 5 ms), which can be clearly perceived and can lead to high wear of the transmission. For example, there can be a ratio of at least 5 or at least 10 between the maximum proportional gain factor and the minimum proportional gain factor. The minimum proportional gain factor should not be equal to zero or should be selected to be so large that the free-running electric machine can largely follow the desired speed dynamics. However, the minimum proportional gain factor should be selected to be so small that speed fluctuations are avoided within the gear mechanism backlash.

[0030] The proportional gain factor K_Pprog determined step by step can be used directly as the proportional gain factor K_P. Optionally, (instead of direct use) a gradient limitation of the proportional gain factor K_P can be set (block 30). The illustrated implementation of this gradient limitation is intended to be exemplary; it goes without saying that other implementations are also conceivable.

[0031] In the case of (exemplary) gradient limiting, the gradient (rate of change) of the proportional gain factor is limited upwards. The corresponding upper limit depends on the proportional gain factor K_Pprog determined step by step, for example, corresponding to a gradient limiting function (link 32), which increases monotonically, especially strictly monotonically, with the proportional gain factor K_Pprog determined step by step (especially not constant over the entire value range). The upper limit is positive, corresponding to an increasing proportional gain factor, that is to say a positive gradient. In the minimum link 34, the smaller of the (limited) value obtained by means of the gradient limiting function on the one hand and the deviation ΔK on the other hand is selected to achieve an upward limitation, the deviation being determined as the difference between the desired step-by-step determined proportional gain factor K_Pprog and the actually used value of the proportional gain factor K_P (ΔK=K_Pprog-K_P). As shown, the deviation ΔK can also be scaled (link 36) or normalized. The scaling in link 36 can be regarded as a control gain of the feedback loop formed in block 30. This can also be achieved by suitable parameter selection in other links of the gradient limiting. The dynamics of the gradient limitation (block 30 ) can be set by appropriately choosing the values ​​of the parameters of the link 32 , the link 36 and the limit 38 .

[0032] In addition, in the case of gradient limiting, the gradient of the proportional gain factor can be limited downwards as shown. For this purpose, a lower (negative) limit 38 is provided, the magnitude of which is (significantly) greater than any value assumed by the gradient limiting function (in the possible value range of the proportional gain factor K_Pprog determined step by step, link 32). The downward gradient limiting is then weaker than the upward gradient limiting. Here, a maximum value link 40 is used to achieve the downward limiting.

[0033] Overall, a limited upward and, if appropriate, downward gradient ΔK_lim of the proportional gain factor is obtained, which is integrated in the integration element 42 in order to determine the proportional gain factor K_P.

[0034] In addition, an integral regulator 50 can be optionally provided, which has a variable integral gain coefficient K_I and determines the additive integral component for the torque request M_Anf. As shown, the integral gain coefficient K_I is particularly dependent on the amplitude of the actual speed n_EMist of the motor formed in the amplitude link 52. The integral gain coefficient K_I is formed as a function of the amplitude of the actual speed (link 54), wherein the integral gain coefficient K_I decreases monotonically as the amplitude of the actual speed increases. This function can contain a constant section (as here at a small amplitude), but should not be constant. The speed difference Δn_EM is integrated in the integral link 56 according to the integral gain coefficient K_I (for example, multiplied by the integral gain coefficient).

[0035] The integral regulator 50 can optionally have a maximum value (Max) and a minimum value (Min) as shown, that is to say that the integral is limited to a value range from a minimum value to a maximum value. The maximum value and the minimum value can vary. Here, the maximum value and the minimum value can depend on the amplitude of the actual speed n_EMist of the motor similarly to the integral gain coefficient K_I, wherein the amplitude of the maximum value and the minimum value especially decreases monotonically as the amplitude of the actual speed increases. For example, a variable integral gain coefficient K_I that already has a corresponding functional correlation can be used as a starting point. The variable integral gain coefficient K_I can be multiplied by an integral limiting coefficient (link 57). The obtained value can be directly used as a maximum value and additionally multiplied by -1 to obtain a minimum value.

[0036] Likewise, a differential controller, in particular with a variable gain factor (variable differential component), can optionally be provided (in addition to or as an alternative to an integrating controller; not shown).

[0037] The target value of the control parameter, that is, Figure 2 The target speed n_EMset in the electric machine can be obtained from the control signal 16, which can be the desired speed n_w of the electric machine (for example, the speed desired by a user of the machine in which the rotary drive is used). It can be provided that the desired speed n_w or the control signal 16 (corresponding to the deflection of the gear lever, for example) is used directly as the target speed n_EMset. Alternatively, a preprocessing of the desired speed n_w or the control signal 16 can be optionally provided, for example, a step-by-step mapping of the deflection of the gear lever onto the target speed n_EMset, as well as a first prefiltering and gradient limiting of the gear lever signal, in order to enable sensitive operation.

[0038] According to an optional embodiment, a (second) pre-filter 60 for the target speed n_EMset is provided as pre-processing, wherein the gradient (or rate of change) of the target speed n_EMset is limited as a function of the magnitude of a differential speed n_diff between the actual speed n_EMist of the electric machine and an (expected) speed i·n_Abtrieb of the electric machine that is correlated with the actual speed n_Abtrieb of the driven element, wherein i is the transmission ratio of the gear mechanism (n_diff=n_EMist-i·n_Abtrieb). The correlated (expected) speed i·n_Abtrieb is the speed at which the electric machine must rotate at a given actual speed n_Abtrieb of the driven element when the gear mechanism is outside the gear mechanism backlash or when there is a frictional fit in the gear mechanism. If the differential speed n_diff is not equal to zero, it is assumed that the gear mechanism is in the gear mechanism backlash. The differential speed n_diff is correspondingly also a variable which indicates whether the transmission is in a power-free state or in transmission backlash. In principle, the differential speed n_diff can be used as an alternative or in addition to the control deviation when determining the step-by-step proportional gain factor.

[0039] In the prefilter 60, it is provided that the target speed n_EMset is determined from the set speed n_w in such a way that the gradient of the target speed n_EMset is limited as a function of the magnitude of the differential speed n_diff, wherein in particular the magnitude of the gradient of the target speed n_EMset is smaller than a limit that is dependent on the magnitude of the differential speed n_diff. In one embodiment, the limit decreases (in particular strictly monotonically) to a minimum value that should not be equal to zero (the limit should not be constant over the entire range).

[0040] By limiting the gradient of the target speed, excessive acceleration of the motor in the transmission backlash is prevented, thereby avoiding high torque impact in the transmission. The specific implementation of the pre-filter 60 can be implemented similarly to the gradient limitation of the proportional gain coefficient K_P (block 30).

[0041] The torque request M_Anf can be used directly to control the electric machine or the inverter (ie, used as actuated variable 14 ). Optionally, a torque limitation can be provided (block 70 ), in which a limited torque request M_Anflim is determined from the torque request M_Anf (which is used as actuated variable 14 ).

[0042] In the torque limitation (block 70), the gradient (rate of change) of the amplitude of the torque request is limited (if a distinction is made between positive and negative torques depending on the direction of rotation), that is, the limitation is carried out in a range of the torque request M_Anf in which the gradient exceeds an upper limit or falls below a lower limit, according to which the torque request is adapted in order to determine the limited torque request M_Anflim. In one embodiment, the amplitude of the lower limit is greater than the upper limit, for example by at least a factor of 5 or at least a factor of 10. That is, the gradient of the amplitude of the torque request is limited asymmetrically. In this way, oscillations can be avoided in particular.

[0043] exist Figure 2 In the example, the rotational speed of the electric machine is used as the controlled variable. As an alternative or in addition, other controlled variables, such as the rotation angle of the electric machine or possibly the torque of the electric machine, can also be used.

[0044] As an example for a machine in which the present invention can be used, Figure 3 An excavator 130 is shown in which an electric drive, in particular an electric rotary drive, which can be adjusted according to the invention can be provided. The excavator 130 comprises a chassis 132 and a body 134 which is rotatably mounted thereon. The wheels 136 mounted on the chassis can be driven by an electric travel machine, not shown in detail, which is operated according to the invention, wherein at least one axle on which at least one of the wheels is fastened is coupled to the electric travel machine directly or via a transmission. The electric travel machine, the transmission if necessary and the wheels together form a travel drive which enables the travel movement of the excavator. The rotation of the body relative to the chassis can be achieved by a rotary mechanism 138 which can be regarded as a transmission or as part of a transmission and is driven by an electric motor operated according to the invention. The rotary mechanism 138 or its drive is shown enlarged in the figure, wherein the output shaft of the electric motor 4 is coupled to a gear 137 which cooperates with a ring gear 139. The gear 137 is coupled to the motor 4 or its output shaft, for example, via a planetary gear 149 having one or more planetary gear sets (not shown in detail). The gear backlash occurs here as a gap between the movement of the motor 4 or its output shaft and the movement of the vehicle body. The gear backlash includes in particular a component from the gap of the planetary gear 149 and a component from the gap between the gear 137 and the ring gear 139. A boom or excavator arm 140 is fastened to the vehicle body 134, at the end of which a bucket 142 is located. The boom, arm and bucket are moved here, for example, by an electrohydraulic device 144 with the aid of a hydraulic cylinder 146. A battery 148 supplies electrical energy to the travel motor, the electric rotary device-machine and / or the electrohydraulic device.

Claims

1. A method for operating an electric motor (4) of a rotary drive having a transmission (2) with a transmission backlash, in, regulating the electric motor (4) according to a target value (n_EMset) of a control variable, wherein the regulation includes a proportional controller; In this case, the proportional gain factor (K_P) of the proportional controller is changed according to a variable from which the power-free state of the transmission (2) is inferred.

2. The method according to claim 1, wherein: The variable from which the force-free state of the transmission (2) is inferred is the control deviation (Δn_EM) of the control variable.

3. The method according to claim 2, wherein: The proportional gain coefficient (K_P) increases as the amplitude of the parameter increases.

4. A method according to any one of the preceding claims, wherein: The manipulated variable is the rotational speed of the electric motor.

5. A method according to any one of the preceding claims, wherein: The gradient of the proportional gain coefficient (K_P) is limited upward.

6. The method according to claim 5, wherein: The limit with which the gradient of the proportional gain factor is limited upwards depends on the proportional gain factor, in particular increases as the proportional gain factor increases.

7. A method according to any one of the preceding claims, wherein: A torque requirement (M_Anf, M_Anflim) for controlling an inverter of the electric machine is determined; wherein a gradient of the magnitude of the torque requirement is limited asymmetrically; wherein in particular a lower limit of the gradient for the torque requirement has a greater magnitude than an upper limit of the gradient for the torque requirement.

8. A method according to any one of the preceding claims, wherein: The regulation comprises an integral regulator (50); wherein an integral gain factor (K_I) of the integral regulator is changed according to the amplitude of the actual value (n_EMist) of the regulation variable; wherein the integral gain factor (K_I) decreases monotonically in particular as the amplitude of the regulation variable increases.

9. The method according to claim 8, wherein: The integral regulator (50) has a maximum value and a minimum value; wherein the amplitude of the maximum value and the amplitude of the minimum value of the integral regulator (50) are changed according to the amplitude of the actual value (n_EMist) of the control variable; wherein the amplitude of the maximum value and the amplitude of the minimum value of the integral regulator decrease monotonically in particular as the amplitude of the control variable increases.

10. A method according to any one of the preceding claims, wherein: The regulation includes a differential regulator.

11. A method according to any one of the preceding claims, wherein: An output speed (n_Abtrieb) is determined; wherein a corresponding speed of the electric motor is determined from the output speed while taking into account a transmission ratio of the transmission; and wherein a speed difference is determined as a difference between an actual speed of the electric motor (n_EMist) and the corresponding speed of the electric motor; wherein the output speed (n_Abtrieb) is determined in particular by an observer.

12. The method according to claim 11, wherein: The manipulated variable target value is formed or filtered in such a way that the gradient of the target value of the manipulated variable is limited according to a limit that depends on the speed difference, wherein in particular the amplitude of the limit of the gradient of the target value of the manipulated variable (n_EMtarget) decreases with increasing speed difference.

13. The method according to claim 11 or 12, wherein: The variable from which the force-free state of the transmission is inferred is the rotational speed difference; in particular, the proportional gain factor decreases with increasing amplitude of the variable. 14 . A computing unit comprising a processor, which is configured in such a way that it carries out the method according to claim 1 .

15. Computer program comprising instructions which, when said program is executed by a computer, cause the computer to carry out the method according to claims 1 to 13.

16. A computer-readable data carrier having stored thereon a computer program according to claim 15.

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