WO2021219190A1

WO2021219190A1 - Method for determining a rotor position of an electric motor of a motor vehicle

Info

Publication number: WO2021219190A1
Application number: PCT/EP2020/061573
Authority: WO
Inventors: Martin ŠEBEST
Original assignee: Brose Fahrzeugteile Se & Co. Kommanditgesellschaft, Bamberg
Priority date: 2020-04-27
Filing date: 2020-04-27
Publication date: 2021-11-04

Abstract

The invention relates to a method (52) for determining a rotor position (46) of an electric motor (8) of a motor vehicle (2), in particular a commutator motor. The method (52) provides that a motor current (28) is detected, and that, on the basis of the motor current (28), a first motor signal (34) and a second motor signal (40) are produced, the first motor signal (34) being phase-offset with respect to the second motor signal (40). A deviation (42) between the first and second motor signals (34, 40) is determined, and, on the basis of the deviation (42), a deviation signal (62) is produced. If the deviation signal (62) exceeds a predetermined limit value (66), it is determined that the rotor position (46) has changed by a fixed angle. The invention also relates to an auxiliary unit (6) of a motor vehicle (2) and also to a computer program product (24).

Description

Description Method for determining a rotor position of an electric motor of a motor vehicle

The invention relates to a method for determining a rotor position of an electric motor of a motor vehicle. The invention also relates to an auxiliary unit of a motor vehicle, with an electric motor, and also to a computer program product for carrying out a method for determining a rotor position of an electric motor of a motor vehicle. The electric motor is preferably a commutator motor with brushes.

Motor vehicles have a multiplicity of auxiliary units which comprise an electric motor. Such an auxiliary unit is for example an electromotive window lifter. By means of this, a window pane is moved along an adjustment path, and is thereby opened and/or closed, when the electric motor is supplied with current. In order that mechanical loading of the individual component parts of the electromotive window lifter is reduced, in this case the window pane is not moved as far as an end stop by means of which the adjustment path is limited. Rather, the electric motor is already switched off when the window pane has approached the respective end stop to within a certain value. The value is in this case set such that for example in the closed state the window pane already lies in a seal, so that a sealing effect is achieved. In this case, the window pane is however kept at a distance from rigid component parts of the motor vehicle, such as the upper end stop. Consequently, there is no mechanical stress on the electromotive window lifter, which reduces wear.

It is consequently necessary to determine the position of the window pane, in order that the electric motor can be suitably switched off. The determination of the position of the window pane usually takes place on the basis of a determination of the rotor position of the electric motor. This exploits the fact that, on account of the fixed mechanical coupling, the window pane is always moved by the same amount when the electric motor has performed a complete revolution. It is consequently necessary to determine the rotational speed of the electric motor, and consequently the rotor position of the electric motor. Usually used for this are sensors, which for example comprise two Hall sensors. These are offset in relation to one another by 90° with respect to the axis of rotation, and by means of this the magnetic field provided by means of the rotor is detected. On account of the direct detection of the rotor position, accuracy is comparatively great. However, additional components are required, which increases production costs.

An alternative procedure envisages monitoring the motor current of the electric motor. If for example the electric motor is designed as a commutator motor, a characteristic signal, also referred to as ripple, is produced in the motor current when two adjacent commutator segments are electrically contacted by one of the brushes of the commutator motor. It is consequently possible by means of counting the ripples detected in the motor current to determine the number of commutator segments passed over, and consequently the rotor position. With the exception of the current sensor, no further sensors are required here, the current sensor usually already being present, and serving for example for the operation of antipinch protection. Consequently, production costs are not increased. It is however possible that such ripples occur in the motor current during the operation of the electric motor on account of electromagnetic disturbances. Furthermore, when the electric motor begins to be supplied with electric current, the ripples are only comparatively minor, so that in a run-up phase of the electric motor usually its rotor position is not correctly detected. Consequently, accuracy is reduced.

The invention is based on the object of providing a particularly suitable method for determining a rotor position of an electric motor of a motor vehicle and also a particularly suitable auxiliary unit of a motor vehicle, with an electric motor, and also a particularly suitable computer program product, production costs advantageously being reduced and/or accuracy increased.

This object is achieved according to the invention with regard to the method by the features of Claim 1 , with regard to the auxiliary unit by the features of Claim 7 and with regard to the computer program product by the features of Claim 8. Advantageous developments and refinements are the subject of the respective subclaims. The method serves for determining a rotor position of an electric motor of a motor vehicle. Consequently, the electric motor is suitable, in particular intended and designed, for being installed in the motor vehicle, and consequently forming part thereof. The motor vehicle is preferably land-bound and suitably comprises a number of wheels, by means of which contact with a roadway or the like takes place. It is in this case expediently possible to position the motor vehicle on the roadway in any way desired, and the motor vehicle is consequently independent of rails or the like. The motor vehicle is for example a commercial vehicle, such as a truck or a bus. Particularly preferably, the motor vehicle is a passenger car. The electric motor is for example of a brushless design and is expediently a brushless DC motor (BLDC). Particularly preferably, however, the electric motor is provided with brushes and is for example a commutator motor with brushes, preferably a DC motor with brushes (DC motor). This has a commutator system with a commutator which comprises a number of commutator segments. The commutator segments are in particular structurally identical to one another and/or arranged at a constant distance from an axis of rotation of the electric motor and for example are produced from a copper. The commutator is fastened on a rotor of the electric motor that is mounted rotatably about the axis of rotation, and each of the commutator segments is electrically contacted by at least one end of an electrical coil of the electric motor that forms a component part of the rotor of the electric motor. The rotor has for example a laminated core, on which the electrical coils are attached. The electrical coils are expediently produced from an enamelled wire, for example enamelled copper wire. The commutator system also has at least two brushes, which are suitably produced from a compressed carbon powder. Preferably, the brushes are pressed against the commutator by means of a force, for example by means of a spring, so that the brushes are spring-loaded. The brushes are electrically contacted by a control unit, by means of which in particular an electrical DC voltage is provided, so that during operation the electrical DC voltage is applied to both brushes. As a result of this, there is a flow of electrical current between the brushes via the commutator segments respectively in contact with them, which are connected to one another via one of the electrical coils. Consequently, it is made possible by means of the commutator for the electrical coil of the rotor to be supplied with current, so that a magnetic field is produced. This expediently interacts with a magnetic field of a stator of the electric motor, which is preferably provided by means of one or more permanent magnets.

For example, the electric motor serves directly for the propulsion of the motor vehicle, and is consequently a component part of a main drive of the motor vehicle. For this, the electric motor is expediently mechanically coupled to at least one of the wheels. Particularly preferably, however, the electric motor is a component part of an auxiliary unit of the motor vehicle, and consequently does not serve directly for the propulsion of the motor vehicle. For this, the electric motor has in particular a corresponding size and/or performance class.

The auxiliary unit serves for example for providing comfort for a user of the motor vehicle. For example, the auxiliary unit is in this case an electromotive adjustment drive, which has an adjustment part that can be moved along an adjustment path by means of the electric motor. The electromotive adjustment drive is for example an electromotive seat adjustment, the adjustment part being for example the complete seat or part thereof, such as a backrest or a head restraint. In this case, by means of supplying current to the electric motor, the position of the respective adjustment part is set and it is for example moved transversally or tilted. In a further alternative, the adjustment part is an armrest, a central armrest or a sliding roof. In a further alternative, a convertible roof is used as the adjustment part. Particularly preferably, the adjustment part is a window lifter, so that the auxiliary unit is an electromotive window lifter. In a further alternative, the adjustment part is a door, so that the auxiliary unit is an electromotive door drive. In a further alternative, the auxiliary unit does not serve for increasing comfort but for example for operating the motor vehicle. In this case, the auxiliary unit is for example a pump, such as a lubricant pump, the lubricant being in particular a transmission oil or an engine oil. In a further alternative, the pump is a fuel pump or a water pump, a cooling circuit being suitably operated by means of the water pump. The pump always has in this case an impeller, which is adapted to the fluid that is respectively to be pumped. The impeller is expediently driven by means of the electric motor and is preferably fastened directly on a rotor of the electric motor. In a further alternative, the auxiliary unit is for example a brake booster, and in particular a pressure in a brake system is built up by means of the electric motor, or an actuation of an ABS system takes place by means of the electric motor. Alternatively, the electric motor is a component part of a fan, such as a cooler fan, or a heater blower, by means of which for example air is transported into an interior space of the motor vehicle.

The method provides that first a motor current is detected. The motor current is an electrical current, which is expediently caused when an electrical voltage is applied to the electric motor. The (electrical) motor current has fluctuations, which are caused in particular on account of operation of the electric motor. In this case, the motor current expediently has certain characteristics and/or signals (characteristic signals), which are caused by the rotation of the rotor. In particular, such a signal is a ripple that is produced on account of electrical contacting of two adjacent commutator segments by means of a single brush, if the electric motor is designed as a commutator motor. The detection of the motor current takes place for example by means of a current sensor, which particularly preferably has a shunt.

In this case, a variation over time of the motor current is detected in particular, so that a number of individual successive values of the motor current are obtained.

On the basis of the motor current, a first motor signal and a second motor signal are produced. Consequently, each of the motor signals corresponds to the motor current and is produced on the basis of the motor current. The two motor signals expediently have a variation over time. If the motor current changes, both the first motor signal and the second motor signal therefore also change. To sum up, there is a functional relationship between the first motor signal and the motor current and between the second motor signal and the motor current. The first motor signal is phase-offset, and consequently time-offset, with respect to the second motor signal. In this case, the two motor signals have in particular substantially the same form, differences occurring for example on account of fluctuations and/or measuring inaccuracies. Thus, for example, (local) maxima/minima of the first and second motor signals are offset in relation to one another, the amount of the offset being the same. Alternatively, the second motor signal corresponds to the time- offset first motor signal.

In a further working step, a deviation between the first motor signal and the second motor signal is determined. For determining the deviation, at each point in time the value of the first motor signal is subtracted from the value of the second motor signal at the same point in time, or vice versa. In other words, the difference between the individual values of the first and second motor signals is produced.

For example, the deviation has in this case a preceding sign, or is unsigned, so that only the amount of the difference between the first motor signal and the second motor signal is used.

On the basis of the deviation, a deviation signal is produced. There is therefore a functional relationship between the deviation signal and the deviation, and, when there is a changed deviation, consequently the deviation signal also changes. The deviation signal has in this case a certain variation over time. If the deviation signal satisfies a certain condition, it is determined that the rotor position has changed by a fixed angle. Used here as the condition is that the deviation signal exceeds a predetermined limit value. Consequently, the condition is in particular suitably formulated. As a consequence, the relationship of the deviation signal and the limit value is consequently monitored at temporally succeeding points in time. If the limit value is exceeded, that is to say if the deviation signal is greater at temporally succeeding considered points in time and subsequently smaller than the limit value, or vice versa, the change by the fixed angle is determined. In this case, both a reduction of the deviation signal, such that it is then less than the limit value, and an increase of the deviation signal, such that it is subsequently greater than the limit value, is regarded as exceeding the limit value.

Since the deviation between the two time-offset, that is to say phase-offset, motor signals is considered, it is possible to detect a trend within the motor signals that is caused by characteristic signals, such as a ripple. In particular, the phase offset/time offset is chosen appropriately. Consequently, accuracy in the determination of the change of the rotor position is improved. Since the two motor signals are produced, artefacts in the motor current, occurring for example on account of an electromagnetic coupling to further component parts of the motor vehicle, are present both in the first motor signal and in the second motor signal, so that they are equally taken into account when determining the change by the fixed angle. In other words, any possible artefact is consequently both in the first motor signal and in the second motor signal. As a consequence, an erroneous determination is avoided. Since no additional sensors are required, production costs are not increased. Furthermore, it is possible to carry out the method substantially completely by means of software routines, so that in addition there are also no other hardware requirements, for which reason production costs are reduced.

The motor current is suitably substantially sinusoidal during operation, in particular if the electric motor rotates at a substantially constant speed. The first and second motor signals are in this case expediently likewise sinusoidal and have for example substantially the same amplitude. The two motor signals are in this case offset in relation to one another by a certain phase, the phase expediently being less than 45°.

In particular, a speed of the electric motor is determined on the basis of the change of the rotor position. In this case, the time interval, that is to say the time span, that has elapsed between successive determinations of the change of the rotor position by the fixed angle, is determined. The fixed angle is specified in particular on the basis of the mechanical design of the electric motor, and is for example determined on the basis of the angular extent of the commutator segments. At least, however, the fixed angle is in particular the angle by which the electric motor must rotate between the occurrence of two characteristic signals in the motor current. If the motor current is substantially sinusoidal, the fixed angle is in particular equal to half the period, so that a complete sinusoidal signal, that is to say one period of the sinusoidal signal, corresponds to twice the fixed angle.

For example, the motor current is used in each case as the first motor signal and second motor signal, a timing element preferably being used for producing the second motor signal, so that the second motor signal corresponds to the time- shifted motor current, whereas the first motor signal corresponds exactly to the motor current. In a development, the first motor signal is also temporally shifted with respect to the motor current. Consequently, with the exception of the time offset, no complicated calculation is required for the determination of the motor signals, which increases robustness and processing speed.

Particularly preferably, however, the motor current is filtered with a first low-pass filter for producing the first motor signal. Preferably, in this case the filtered motor current is used directly as the first motor signal. The first low-pass filter has a first cut-off frequency. By means of the first low-pass filter, components of the motor current that have a greater frequency than the first cut-off frequency are removed, so that no components with a frequency greater than the first cut-off frequency are present in the first motor signal. At least, however, these components are reduced. As a consequence, noise that is introduced into the motor current, caused by an electromagnetic coupling or voltage fluctuations of an electrical system perhaps provided in the motor vehicle, by means of which the current is applied to the electric motor, is removed by means of the first low-pass filter, for which reason subsequent processing is made easier. False determination of the change by the fixed angle is also reduced.

To sum up, any artefacts are filtered out of the motor current by means of the first low-pass filter, and the first motor signal has a smoother variation than the unfiltered motor current. Also, the first low-pass filter causes a phase offset, so that the first motor signal is phase-offset with respect to the motor current. Consequently, no additional timing element is required, in particular if the motor current is used as the second motor signal.

The first cut-off frequency is expediently between 300 Hz and 500 Hz and for example equal to 400 Hz. Consequently, comparatively high-frequency components are filtered out of the motor current, whereas the components caused by the mechanical conditions during rotation of the electric motor, which form the characteristic signals, are substantially unchanged.

For example, the time-offset first motor signal is used as the second motor signal. Consequently, processing effort is reduced. Also, the second motor signal consequently has a comparatively smooth variation, and artefacts are removed. Particularly preferably, however, the motor current filtered by a second low-pass filter is used as the second motor signal. The second low-pass filter has in this case a second cut-off frequency, all of the components of the motor current that have a greater frequency than the second cut-off frequency being removed, or at least reduced, by means of the second low-pass filter. In this case, the second cut off frequency is chosen to be different from the first cut-off frequency. Consequently, on account of the different cut-off frequency, the phase offset between the two motor signals is obtained automatically, for which reason no additional processing is required. As a consequence, determination of the rotor position is made easier.

For example, in this case the two low-pass filters are differently constructed or, particularly preferably, are identical to one another. Consequently, the two low- pass filters differ only on the basis of the choice of the respective cut-off frequency. The second cut-off frequency differs for example by between 30 Hz and 70 Hz, and for example substantially by 50 Hz, from the first cut-off frequency, or by 20%, 10% or 5%. Preferably, 300 Hz, 320 Hz, 350 Hz or 370 Hz is used here as the second cut-off frequency. Consequently, the phase offset between the two motor signals is comparatively small, so that the deviation is always substantially comparatively small. Consequently, the deviation signal likewise has only comparatively small fluctuations, for which reason the comparison with the limit value is facilitated. In a further alternative, the second cut-off frequency differs for example comparatively greatly from the first cut-off frequency, and is for example substantially half thereof. To reduce the phase offset obtained in this way, there is preferably a timing element, by means of which the phase offset is reduced. For example, in this case 220 Hz is used as the second cut-off frequency.

In a variant of the embodiment, the deviation is used directly as the deviation signal. Consequently, processing is made easier. In this case, zero (“0”) is expediently used as the limit value. Consequently, for the assessment whether the rotor position has changed by the fixed angle, it is determined whether the first motor signal is greater than the second motor signal, or vice versa. In other words, whenever a change of the preceding sign of the deviation takes place, it is determined that the rotor position has changed by the fixed angle. Consequently, comparatively few working steps are required for determining the change of the rotor position, which increases robustness.

In an alternative to this, for producing the currently applicable deviation signal, the currently applicable deviation is added to the temporally preceding deviation signal. In other words, the last value of the deviation signal is added to the currently applicable deviation, that is to say the currently applicable value of the deviation. Preferably, in this case the sum is used as the currently applicable value of the deviation signal. Consequently, the currently applicable deviation signal is based on the temporally preceding deviation signal, and the deviation signal is consequently produced in particular recursively.

For example, zero (“0”) is likewise used as the limit value. Particularly preferably, however, an upper and a lower bound are used, expediently differing from one another. Preferably, the bounds are equal in terms of the amount, but expediently only differ on the basis of the preceding sign. In this case, the upper and the lower bounds are alternately used as the limit value. As a consequence, as soon as the deviation signal exceeds the lower limit, it is subsequently monitored whether the deviation signal exceeds the upper bound. Each time this happens, a change by the fixed angle is determined. Consequently in turn, whenever subsequently the deviation signal exceeds the lower bound, it is determined that a renewed change by the fixed angle has taken place. In this case, the change by the fixed angle is expediently determined only when the deviation signal exceeds the upper bound from below, and is consequently thereafter greater than the upper bound. In the case of exceeding the lower bound, preferably only exceeding from above is considered, so that the change by the fixed angle is only determined when the deviation signal is reduced and becomes less than the lower bound. On account of the two bounds, there is a tolerance equalization, so that brief fluctuations are not falsely used for determination of the rotor position. As a consequence, robustness is increased.

For example, the currently applicable deviation signal is always determined in the same way, and is in particular always equal to the sum of the temporally preceding deviation signal and the currently applicable deviation. Particularly preferably, however, when the limit value is exceeded by the currently applicable deviation signal, that is to say the currently applicable bound, zero (“0”) is used as the preceding deviation signal in the temporally succeeding production of the currently applicable deviation signal. In other words, in this case the temporally succeeding deviation signal is equal to the currently applicable deviation. In other words again, when the limit value is exceeded, the deviation signal is subsequently set to zero (“0”), that is to say in particular whenever the limit value is changed to the other bound. To sum up, the last-produced deviation signal is used for the temporally succeeding production of the currently applicable deviation signal if it was between the lower and upper bounds. Otherwise, zero (“0”) is used for this. On the basis of such a procedure, excessive changing of the deviation signal is avoided, and it does not deviate excessively from the upper and lower bounds, so that the exceeding of the limit value can be detected in a comparatively short time. In other words, an excessive increase or decrease of the deviation signal is avoided, so that a change of the preceding sign of the deviation already leads in a comparatively short time to the determination of the change of the rotor position by the fixed angle. On account of the two bounds, any fluctuations however do not lead to the false determination of the change of the rotor position. Consequently, robustness is further increased. The auxiliary unit is a component part of a motor vehicle, and is consequently suitable, and preferably intended and designed, for being installed in a motor vehicle. The motor vehicle is for example a truck or a bus. As an alternative to this, the motor vehicle is a passenger car. The auxiliary unit serves in particular for performing various functions or at least one function, the function not corresponding to the direct propulsion of the motor vehicle. In other words, the motor vehicle is not driven by means of the auxiliary unit, but instead by means of a main drive. In this case, the auxiliary unit serves for example for the operation of the main drive, for example its cooling. Suitably, the auxiliary unit is designed as a pump, for example as a coolant pump. As an alternative to this, the auxiliary unit is a lubricant pump, for example an engine oil pump or a transmission oil pump. In a further alternative, the auxiliary unit is a fuel pump or a fan, such as an engine fan or some other fan or a blower. In a further alternative, the auxiliary unit is an electromotive air-conditioning compressor, an electromotive steering assistant or, particularly preferably, an adjustment drive, such as an electromotive window lifter (drive) or an electromotive door drive, the door being for example a side door or a tailgate. In a further alternative, the auxiliary unit is an electromotive seat adjustment.

The auxiliary unit has an electric motor, which is for example a brushless DC motor. Particularly preferably, however, the electric motor has brushes and is expediently a commutator motor with brushes. The auxiliary unit also has a control unit, by means of which in particular operation of the electric motor takes place. The control unit expediently has an application-specific circuit (ASIC) and/or a microprocessor, which is preferably programmable. Preferably, during operation an electrical voltage, preferably an electrical DC voltage, is applied to the electric motor by means of the control unit. On account of the electrical voltage, the electric motor is in this case set in a rotational movement. In particular, on account of the electrical voltage, a motor current is induced, which leads to the formation of a magnetic field. For this, the electric motor expediently has an electromagnet, which is supplied with current by means of the motor current. This magnetic field preferably interacts with a permanent magnet of the electric motor, so that the electric motor, in particular a rotor of the electric motor that suitably has the electromagnet, is set in the rotational movement. Suitably, it is connected to a commutator motor of the electric motor, if it is designed as a commutator motor. In the case of the auxiliary unit, the rotor position of the electric motor is determined by means of a method in which the motor current is detected. On the basis of the motor current, a first motor signal and a second motor signal are produced, the first motor signal being phase-offset with respect to the second motor signal. A deviation between the first motor signal and the second motor signal is determined, and on the basis of the deviation a deviation signal is produced. If the deviation signal exceeds a limit value, it is determined that the rotor position has changed by a fixed angle.

The fixed angle is in particular stored in the control unit and is specified on the basis of the mechanical design of the electric motor. Expediently, the auxiliary unit has a current sensor, which is for example designed as a shunt. Suitably, the control unit comprises an analogue-to-digital converter (ADC), by means of which the analogue value of the motor current is transformed into a digital value. It is consequently possible to carry out the method substantially completely by means of software routines, in particular by means of a computer program product. Preferably, the auxiliary unit, suitably the control unit, has for this a suitable storage medium and/or a computer, like the microprocessor. In an alternative to this, the method is at least partially or completely carried out by means of analogue components.

The computer program product comprises a number of commands which, during the execution of the program by a computer, cause the latter to carry out a method for determining a rotor position of an electric motor of a motor vehicle. In the method, a motor current is detected, and on the basis of the motor current a first motor signal and a second motor signal are produced, the first motor signal being phase-offset with respect to the second motor signal. A deviation between the first motor signal and the second motor signal is determined, and on the basis of the deviation a deviation signal is produced. If the deviation signal exceeds a predetermined limit value, it is determined that the rotor position has changed by a fixed angle. On the basis of the change by the fixed angle, in particular a rotational speed of the electric motor is determined. For this, the variation over time of the change of the rotor position is detected and from this preferably the speed is derived. The computer is expediently a component part of a control unit and for example is formed by means thereof. The computer preferably comprises a microprocessor or is formed by means thereof. The computer program product is for example a file or a data carrier which contains an executable program that automatically performs the method when it is installed on a computer.

The invention also relates to a storage medium on which the computer program is stored. Such a storage medium is for example a CD-ROM, a DVD or a Blu-ray disc. As an alternative to this, the storage medium is a USB stick or some other store, which is for example rewritable or can only be written to once. Such a store is for example a flash memory, a RAM or a ROM.

The developments and advantages cited in connection with the method can also be transferred analogously to the auxiliary unit and/or the computer program product and also to one another, and vice versa.

Exemplary embodiments of the invention are explained in more detail below on the basis of a drawing, in which:

Fig. 1 schematically shows a motor vehicle with an auxiliary unit having an electric motor and a control unit,

Fig. 2 schematically shows a first embodiment of part of the control unit, Fig. 3 shows according to Fig. 2 a second embodiment of the part of the control unit,

Fig. 4 shows a method for determining a rotor position of the electric motor, Fig. 5 shows the variation over time of a motor current, a first motor signal and a second motor signal and also a deviation signal when the electric motor has a constant rotational speed, Fig. 6 shows in two graphs the variation over time of the motor current, the first motor signal and the second motor signal and also an alternative deviation signal when the electric motor has a constant rotational speed, and Fig. 7 shows the variations over time according to Fig. 6 in a run-up phase of the electric motor.

Parts corresponding to one another are provided with the same reference signs in all of the figures.

In Figure 1 a motor vehicle 2 is represented in the form of a passenger car. The motor vehicle 2 has a number of wheels 4, by means of which contact with a road that is not represented any more specifically takes place. At least two of the wheels 4 are driven by means of a main drive that is not represented any more specifically. The motor vehicle 2 also has an auxiliary unit 6, which is not a component part of the main drive, and which consequently does not serve for the direct propulsion of the motor vehicle 2. The auxiliary unit 6 serves in a variant for supplying the main drive and is for example an oil pump. In a further alternative, the auxiliary unit 6 is a component part of an electromotive steering device or an electromotive adjustment drive, such as an electromotive window lifter.

The auxiliary unit 6 has an electric motor 8, which is designed as a commutator motor. The electric motor 8 has a rotor 10, which is mounted rotatably about an axis of rotation. The rotor 10 comprises a laminated core, onto which a number of electrical coils that are not represented any more specifically are wound. These are electrically contacted by a commutator 12 that is fastened on the rotor 10 for rotation therewith, to be specific the commutator segments thereof that are not represented any more specifically. The commutator 12 is a component part of a commutator system 14, which also has two brushes 16, which are produced from sintered or compressed carbon.

If an electrical DC voltage is applied to the brushes 16, an electrical current flows via the commutator segments of the commutator 12 contacted by the brushes 16, and the electrical coil connected thereto, so that a magnetic field is formed. This interacts with a magnetic field that is provided by means of a stator 18, which for this has a number of permanent magnets, and surrounds the rotor 10. As a consequence of this, the rotor 10 is rotated, and the brushes 16 come into electrical contact with adjacent commutator segments of the commutator 12, so that current is supplied to a further one of the electrical coils.

The brushes 16 are electrically contacted by a control unit 20 of the auxiliary unit 6 and are consequently supplied with current by means thereof. For this, the control unit 20 is electrically contacted by an electrical system of the motor vehicle 2 that is not represented any more specifically. Also, the control unit 20 is coupled in terms of signalling to a bus system of the motor vehicle 2, so that by means of an on-board computer of the motor vehicle 2 a functioning mode of the auxiliary unit 6 can be changed. The control unit 20 also has a computer 22 in the form of a microprocessor and also a computer program product 24.

In Figure 2, a first embodiment of part of the control unit 20 is schematically represented in a simplified form, individual component parts thereof being provided by means of discrete components or by means of software routines, which are defined by means of the computer program product 24. The control unit 20 has an A/D converter 26 (analogue-to-digital converter), the input of which is connected to a current sensor, which has a shunt. By means of the current sensor and the A/D converter 26 connected to it, the motor current 28 flowing via the electric motor 8 is detected.

The A/D converter 26 is connected in signalling terms to a first low-pass filter 30, which has a first cut-off frequency 32, which is 400 Flz. By means of the first low- pass filter 30, a first motor signal 34 is produced, corresponding to the motor current 28 filtered by means of the first low-pass filter 30. A subtractor 36 is acted on by the first motor signal 34, and is consequently connected in signalling terms to the first low-pass filter 30. A timing element 38 is also connected in signalling terms to the first low-pass filter 30, so that the timing element 38 is acted on by the first motor signal 34. By means of the timing element 38, the first motor signal 34 is in this case time-offset, and therefore phase-offset, so that a second motor signal 40 is produced, corresponding to the time-offset first motor signal 34. The subtractor 36 is likewise acted on by the second motor signal 40, and by means of the subtractor 36 the deviation 42 between the two motor signals 34, 40 is produced.

The subtractor 36 is connected in signalling terms to an evaluation unit 44, which is consequently acted on by the deviation 42. By means of the evaluation unit 44, a rotor position 46 of the electric motor 8 is output, that is to say an angular position of the rotor 10 with respect to the stator 18. On the basis of the rotor position 46, to be specific the change thereof, a rotational speed of the electric motor 8 is derived by means of units not represented any more specifically, the supplying of current to the electric motor 8 being adapted in dependence on the rotational speed determined, so that the electric motor 8 rotates at a certain rotational speed. Also, a switching off of the electric motor 8 takes place in dependence on the rotor position 46 determined, so that for example an adjustment part connected thereto, such as the window pane, can be moved to a predetermined location. In Figure 3, an alternative embodiment of the control unit 20 is represented. Present again here, too, is the A/D converter 26, which is acted on by the motor current 28. In turn, the first low-pass filter 30, which has the first cut-off frequency 32, is connected in signalling terms between said converter and the subtractor 36. The first cut-off frequency 32 is in turn equal to 400 Hz. There is however additionally a second low-pass filter 48, which has a second cut-off frequency 50, which deviates from the first cut-off frequency 32. The second cut-off frequency 50 is in this case equal to 350 Hz. Consequently, the first low-pass filter 30 represents a fast low-pass filter and the second low-pass filter 48 represents a comparatively slow low-pass filter.

The output of the second low-pass filter 48 is connected in signalling terms by way of the timing element 38 to the subtractor 36, the second motor signal 40 being produced by means of the timing element 38, and consequently corresponding to the motor current 28 filtered by the second low-pass filter 48 and shifted by means of the timing element 38. By means of the subtractor 36, in turn the deviation 42 is produced and is passed to the evaluation unit 44, so that there in turn the rotor position 46 can be determined. In a variant that is not represented any more specifically, the timing element 38 is not present, and the second motor signal 40 corresponds to the motor current 28 filtered by means of the second low-pass filter 48.

Represented in Figure 4 is a method 52 for determining the rotor position 48, which is carried out at least partially by means of the control unit 20. For this, the computer program product 24 has corresponding commands, which, during execution by the computer 22, cause the latter to carry out the method 52. In an alternative, the method 52 is carried out by means of corresponding analogue or at least discrete components.

In a first working step 54, the method 52 is started. This takes place for example whenever the request to carry out a certain function for which it is necessary to supply the electric motor 8 with current is received by the auxiliary unit 6 via the bus system represented more specifically. For example, the window pane is to be adjusted. As a consequence of this, an electrical DC voltage is applied by means of the control unit 20 to the electric motor 8, to be specific to the two brushes 16, so that the motor current 28 flows via the electric motor 18 and, as a consequence of this, the rotor 10 rotates with respect to the stator 18.

In a following second working step 56, the motor current 28 is detected. For this, the signals provided by means of the current sensor are converted by means of the A/D converter 26 into a digital value.

Represented in Figure 5 is the variation over time of the motor current 28 that is obtained when the electric motor 8 is operated at a constant (rotational) speed.

The motor current 28 has a substantially sinusoidal variation, which is obtained on account of the changing electrical contacting of the commutator segments by the two brushes 16. On account of electromagnetic couplings to further component parts and/or on account of fluctuations in the voltage of the electrical system in the motor vehicle 2, the motor current 28 may have brief fluctuations.

In a following third working step 58, both the first motor signal 34 and the second motor signal 40 are produced. For this, the motor current 28 is filtered. Depending on the variant of the control unit 20 that is used, only the first low-pass filter 30 and the timing element 38 or both the first low-pass filter 30 and the second low-pass filter 48 and the timing element 38 or only the first low-pass filter 30 and the second low-pass filter 48 are used for this. In other words, the motor current 28 filtered by the first low-pass filter 30 is used as the first motor signal 34. Depending on the variant that is used, the first motor signal 34 time-offset by means of the timing element 38 or the motor current 28 filtered by the second low-pass filter 48 is used as the second motor signal 40, in a variant an additional offset taking place by means of the timing element 38. In this case, the second cut-off frequency 50 is different from the first cut-off frequency 32 of the first low-pass filter 30. To sum up, the first motor signal 34 and the second motor signal 40 are produced on the basis of the motor current 28, the first motor signal 34 being phase-offset with respect to the second motor signal 40.

As a consequence, the variation over time of the first motor signal 34 and the second motor signal 40 that is represented in Figure 5 is obtained. The two motor signals 34, 40 are phase-offset, and consequently temporally shifted, with respect to the motor current 28. The second motor signal 40 is also temporally shifted with respect to the first motor signal 34. On account of the filtering by the first low-pass filter 30 or the second low-pass filter 48, in this case the first motor signal 34 and the second motor signal 40 no longer have the brief fluctuations that may occur in the motor current 20. Therefore, the variation of the two motor signals 34, 40 is smooth.

In a following fourth working step 60, the deviation 42, that is to say the difference between the first motor signal 34 and the second motor signal 40, is produced by means of the subtractor 36. In other words, the deviation 42 between the first motor signal 34 and the second motor signal 40 is determined. On the basis of the deviation 42, a deviation signal 62, the variation overtime of which is represented in Figure 5, is produced by means of the evaluation unit 44. In the case of the variant represented in Figure 5, the deviation 42 itself is used as the deviation signal 62. If, consequently, the second motor signal 40 is greater than the first motor signal 34, the deviation signal 62 is negative.

In a following fifth working step 64, the deviation signal 62 is compared with a predetermined limit value 66, which is also referred to just as a limit value. In the case of the variant represented in Figure 5, zero (“0”) is used as the limit value 66. Each time the deviation signal 62 exceeds the limit value 66, the value of a position signal 70, which can only assume two different values, is changed in a following sixth working step 68. Consequently, each time the deviation signal 62 reaches the limit value 66, to be specific zero (“0”), the value of the position signal 70 is changed from the one admissible value to the other admissible value.

Each time the position signal 70 changes the value, it is determined that the rotor position 46 has changed by a fixed angle. The fixed angle is specified on the basis of the mechanical specifications of the electric motor 8. In the example represented in Figure 5, consequently, a sixfold change of the position signal 70 by the fixed angle has taken place, so that in this case the rotor position 46 output by means of the evaluation unit 44 has been increased by six times the fixed angle.

If the currently output rotor position 46 still does not correspond to one specified by means of the request transmitted in the first working step 54, the electric motor 8 continues to be supplied with current and the second to sixth working steps 56, 68 continue to be carried out. If the rotor position 46 corresponds to the transmitted position, a seventh working step 72 is carried out and the supply of current to the electric motor 8 is ended. The then currently applicable rotor position 46 is stored in a storage unit of the control unit 20, so that, when the method 52 is carried out once again, a change by the fixed angle takes place on the basis of this stored rotor position 46 in accordance with the position signal 70. In Figure 6, two graphs are represented, one of the graphs showing the variation over time of the motor current 28 and of the first and second motor signals 34, 40. In this case, the electric motor 8 in turn rotates at a constant speed, and it is possible to use both the variant represented in Figure 2 and the variant represented in Figure 3 of the control unit 20, and the associated way of determining the motor signals 34, 40.

The lower of the two graphs shows the in this respect synchronous variation over time of the deviation 42. In comparison with the variant shown in Figure 5, the deviation signal 62 has changed. Instead of using the deviation 42, the temporally preceding deviation signal 62, that is to say the directly temporally preceding value of the deviation signal 62, is used for producing the currently applicable deviation signal 62, that is to say the currently applicable value of the deviation signal 62, and the currently applicable deviation 42, that is to say the currently applicable value of the deviation 42, is added thereto. Consequently, the deviation signal 62 is recursively defined.

Whenever the sum produced in this way exceeds an upper bound 74, that is to say increases by more than the upper bound 74, in the temporally succeeding determination of the deviation signal 62 the sum is determined on the basis of zero (“0”), so that then the currently applicable value of the deviation 42 is used as the value of the deviation signal 62. Similarly, whenever the deviation signal 62 exceeds a lower bound 76, that is to say decreases by more than the lower bound 76, in the succeeding determination of the value of the deviation signal 62 only the then currently applicable value of the deviation 42 is used. In other words, consequently, whenever the deviation signal 62 falls below the lower bound 76, the deviation signal 62 is set to zero in the temporally succeeding determination. Consequently, the deviation signal 62 is always between the lower bound 76 and the upper bound 74. The amount of the upper bound 74 and the lower bound 76 is equal, and they only differ in their preceding sign. In other words, the upper bound 74 is positive, whereas the lower bound 76 is negative. The two bounds 74, 76 are used as the limit value 66, the lower bound 76 being used as the limit value 66 after exceeding of the upper bound 74, and vice versa. Here, too, each time the respective limit value 66 is exceeded, the value of the position signal 70, which in turn can only assume the two different values, is changed. As a consequence, when the upper bound 74 is exceeded twice without the lower bound 76 having been exceeded in the meantime, the value of the position signal 70 is unchanged.

In Figure 7, two graphs are represented in accordance with Figure 6. In one, the variation over time of the motor current 28 and of the first and second motor signals 34, 40 during a run-up phase of the electric motor 8 is represented. On the basis of a value of 0 A, the motor current 28 increases comparatively strongly in terms of the amount to a value of substantially -15 A. Consequently, a comparatively great torque is built up, so that static friction is overcome Subsequently, on account of the reduced friction, the motor current 28 decreases over an extended period of time. On account of the commutator segments, in this case the sinusoidal oscillation is present in the motor current 28, the period being shortened on account of the increasing rotational speed of the electric motor 8. Represented in the lower graph of Figure 7 are the resultant deviation 42 and also the deviation signal 62, determined in accordance with the variant represented in Figure 6. In the method 52, a determination of the rotor position 46 during the run up phase is also possible, since the different effects that occur in the motor current 28 during the run-up phase are reflected both in the first and in the second motor signal 34, 40. Thus, each of the motor signals 34, 40 correspondingly increases or decreases, and each has the superposed sinusoidal oscillation. Since the number of revolutions of the electric motor 8, that is to say its rotational speed, increases, the time interval between the points in time of the change of the values of the position signal 70 decreases. This results in an accelerated change of the rotor position 46 by in each case the constant fixed angle, which corresponds to the increase in the number of revolutions during the run-up phase. The invention is not restricted to the exemplary embodiments described above. Rather, other variants of the invention may also be derived therefrom by a person skilled in the art without departing from the subject matter of the invention. In particular, furthermore, all of the individual features described in connection with the individual exemplary embodiments can also be combined with one another in some other way without departing from the subject matter of the invention.

List of reference signs

2 Motor vehicle

4 Wheel

5 6 Auxiliary unit

8 Electric motor

10 Rotor

12 Commutator

14 Commutator system o 16 Brush

18 Stator

20 Control unit

22 Computer

24 Computer program products 26 A/D converter

28 Motor current

30 First low-pass filter

32 First cut-off frequency

34 First motor signal 36 Subtractor

38 Timing element

40 Second motor signal

42 Deviation

44 Evaluation unit as 46 Rotor position

48 Second low-pass filter

50 Second cut-off frequency 52 Method

54 First working step so 56 Second working step

58 Third working step

60 Fourth working step

62 Deviation signal 64 Fifth working step 66 Limit value

68 Sixth working step 70 Position signal 72 Seventh working step

74 Upper bound

76 Lower bound

Claims

1. Method (52) for determining a rotor position (46) of an electric motor (8) of a motor vehicle (2), in particular a commutator motor, in which - a motor current (28) is detected,

- on the basis of the motor current (28), a first motor signal (34) and a second motor signal (40) are produced, the first motor signal (34) being phase-offset with respect to the second motor signal (40),

- a deviation (42) between the first and second motor signals (34, 40) is determined,

- on the basis of the deviation (42), a deviation signal (62) is produced, and

- if the deviation signal (62) exceeds a predetermined limit value (66), it is determined that the rotor position (46) has changed by a fixed angle.

2. Method (52) according to Claim 1 , characterized in that the motor current (28) filtered by a first low-pass filter (30), which has a first cut-off frequency (32), is used as the first motor signal (34).

3. Method (52) according to Claim 2, characterized in that the motor current (28) filtered by a second low-pass filter (48), which has a second cut-off frequency (50), is used as the second motor signal (40), the second cut-off frequency (50) being chosen to be different from the first cut-off frequency (32).

4. Method (52) according to one of Claims 1 to 3, characterized in that the deviation (42) is used as the deviation signal (62) and zero is used as the limit value (66).

5. Method (52) according to one of Claims 1 to 3, characterized in that, for producing the currently applicable deviation signal (62), the currently applicable deviation (42) is added to the temporally preceding deviation signal (62), an upper and a lower bound (74, 76) being alternately used as the limit value (66).

6. Method (52) according to Claim 5, characterized in that, when the limit value (66) is exceeded by the currently applicable deviation signal (62), zero is used as the preceding deviation signal (62) for the temporally succeeding production of the currently applicable deviation signal (62).

7. Auxiliary unit (6) of a motor vehicle (2), with an electric motor (8), the rotor position (46) of which is determined according to one of Claims 1 to 6.

8. Computer program product (24), comprising commands which, during the execution of the program by a computer (22), cause the latter to carry out the method (52) according to one of Claims 1 to 6.