Method and device for determining the rotor position of an electric motor with several motor strings

[0001] The present invention relates to a method for determining the rotor position of an electromotor having a plurality of motor phase windings, in particular, a brushless DC motor. In addition, the present invention relates to a detection module for this purpose.

[0002] In many areas of technology, especially in the motor-vehicle area, so-called brushless DC motors have recently been used, which are also known as BLDC motors (BLDCBrushless Direct Current) having no brushes, which are subject to wear. Instead of a mechanical commutation, in BLDC motors an electronic commutation is provided, which is generally realized through a power electronics. BLDC motors are also called electronically commutated DC motors, or EC motors (ECelectronically commutated).

[0003] A BLDC motor is driven by a power-electronic actuator that functions as a commutator, for example, having a 6-pulse bridge converter, which, using pulse modulation, in general, pulse width modulation, produces from a battery or an intermediate-circuit DC-voltage a three-phase AC voltage system that is variable in frequency and voltage amplitude, so that for the BLDC motor current units are made available that are, for example, essentially rectangular. In this context, supplying current to the motor phase windings of the electromotor, i.e., supplying current to its windings, is carried out as a function of the specific rotor position. Usually, two phase windings are supplied with current simultaneously. In this context, the arms of the bridge converter, that are assigned to the phase windings, are active and supply current to a first phase winding in a positive charge and to a second motor phase winding in a negative charge. In this context, the switches of a third bridge converter arm are opened, and therefore the third arm is inactive.

[0004] As a result of the rotor of the electromotor, which has, e.g., a permanent magnet arrangement, a countervoltage is induced in the motor windings, i.e., in the specific motor phase windings. For a high degree of motor efficiency, the motor phase windings should be supplied with current such that the highest possible phase countervoltage, having the same polarity as the specific phase current, is induced in them.

[0005] In any case, in a BLDC motor, its instantaneous rotor positions must be known for determining the optimal commutation times. The rotor positions, i.e., the commutation times, can be determined, e.g., by a sensor arrangement or also without the use of sensors, for example, by evaluating the zero crossing points of the induced countervoltages in the phase windings that are not supplied with current. From the zero crossing points, it is possible to determine the rotor angle for the next commutation by extrapolation. It is true, this method is only suitable for electromotors that are run in continuous operation, e.g., in pumps or ventilators.

[0006] In motors having an automatic speed control that includes motor standstill, for example, in motors for positioning drives, more dynamic methods are required for determining the rotor position. In this context, certain difficulties undoubtedly arise:

[0007] Through the pulse width modulation (PWM) of the switches in the two active arms of the bridge converter, the current flowing in the active, current-supplied motor phase windings is adjusted and limited. Nevertheless, as a result of the pulse width modulation, interference pulses are generated, which are transmitted to the inactive phase winding, that is not supplied with current, inter alia, due to coupling inductances between the individual motor phase windings, so that the zero crossing point of the induced voltage cannot be measured in the inactive phase winding reliably and without distortion. The clock distortions for evaluation must be filtered out of the respective measuring signal.

[0008] However, in this context, in analog filters, phase shifts, among other things, occur, which generate the interfering measuring errors.

[0009] A digital filtering method is proposed in U.S. Pat. No. 5,859,520. In this method, a zero crossing point of the induced voltages is measured by clocking an upper switch of a bridge arm, so that a freewheeling current is generated via the lower switch of the same bridge arm. In freewheeling operation, the countervoltage that is induced in the motor phase winding that is assigned to the bridge arm in question is measured with respect to a ground potential of a measuring circuit. In this context, it is disadvantageous, on the one hand, that only the upper switches of the bridge arm can be clocked and, on the other hand, that by clocking the upper switches, so to speak, a forced freewheeling operation is generated in order to be able to carry out the measurements at all. However, from the forced freewheeling operation, the result is a reduced potential utilization of the motor.

[0010] In contrast, the method according to the present invention having the features of the main claim, as well as the detection module according to the present invention, that operates in accordance with the method, have the advantage that no forced freewheeling is required for determining the polarity of the countervoltages induced in the motor phase windings, and consequently an optimal utilization of the motor is possible.

[0011] To determine the rotor position, the polarity of the phase voltage induced in the motor phase windings is ascertained. For this purpose, the specific phase voltages are compared to reference values, specifically to a real or simulated star-point voltage, that is applied at a star point, and in each case polarity values are calculated. To avoid distortions that falsify the polarity values, a synchronization to the starting times for supplying current to the motor phase windings is carried out, and a predetermined delay is allowed to elapse, in which the polarity values have achieved a steady-state, stable condition. Only then are the polarity values ascertained and, for example, supplied to a control unit for controlling the electromotor.

[0012] The method according to the present invention can be carried out together with the most various freewheeling methods. The necessary evaluation circuit is compact and can be integrated, for example, in a circuit for controlling the electromotor and/or for controlling a power electronics that supplies power to the electromotor. The method according to the present invention can be applied over a large speed range in a variable manner and without expensive adjustments. In any event, no measuring errors arise caused by phase shifts, such as, e.g., in the case of analog filters.

[0013] As a result of the measures indicated in the subclaims, advantageous refinements and improvements of the method according to the present invention and of the detection module according to the present invention are possible.

[0014] In order that polarity values are determined that are not affected by interferences, the values are preferably ascertained and read out only if no freewheeling of the electromotor has occurred during the delay.

[0015] Preferably, after the preestablished delay has elapsed, a sampling period is begun, in which for every motor phase winding not only a polarity value, but also, in accordance with the length of the sampling period, a plurality of polarity values can also be determined. The polarity values are each stored in a storage unit, it being provided that a polarity value that was calculated later replaces a polarity value that was calculated earlier. For example, only the most recent polarity value of a motor phase winding can be stored.

[0016] Advantageously, the most recently determined polarity value is then read out.

[0017] The sampling period can be terminated as a result of a plurality of events, e.g., as a result of a freewheeling of the electromotor or as a result of a subsequent switch-on time point for supplying current to the electromotor.

[0018] As was mentioned above, in conventional methods, the electromotor is supplied with current using pulse width modulation for setting and limiting the currents flowing in the individual motor phase windings. The switch-on times for supplying current to the motor phase windings of the electromotor, in this context, are preferably defined by a pulse-width-modulation basic timing signal. This pulse-width-modulation basic timing signal is advantageously used for synchronization in determining the polarity values.

[0019] It is also possible that the pulse-width-modulation basic timing signal is started anew, in each case, by a synchronization signal in response to a commutation of supplying the electromotor with current. In a commutation, the current supply generally alternates from one pair of phase windings to an adjacent pair of phase windings. In one advantageous variant of the present invention, the determination of the polarity values is synchronized anew, in each case, on the basis of the synchronization signal. Preferably, both the synchronization signal as well as the pulse-width-modulation basic timing signal help in synchronizing the ascertainment of the polarity values. It is very simple to realize this, for example, by combining the basic timing signal and the synchronization signal in a logical OR.

[0020] It is true that the phase voltages and the star voltage, in principle, can be picked off at the respective motor phase windings, or at the star point of the motor phase windings. However, the individual measuring points are often very inaccessible. Therefore, the phase voltages and/or the star point voltages are preferably simulated.

[0021] Exemplary embodiments of the present invention are depicted in the drawings and are discussed in greater detail in the description below. The following are the contents:

[0022] FIG. 1A depicts an arrangement of a bridge converter having connected phase windings of the electromotor, as well as a current characteristic in the current-supplied state,

[0023] FIG. 1B depicts the arrangement from FIG. 1A having a current characteristic during the freewheeling of the electromotor,

[0024] FIG. 2A depicts schematic and exemplary characteristics of currents when the electromotor from FIGS. 1A, 1B is supplied with current, as well as voltages that are induced in the motor phase windings in this context,

[0025] FIG. 2B depicts a pulsed current-supply of a phase winding from FIG. 2A,

[0026] FIG. 3 depicts the arrangement from FIGS. 1A, 1B having a motor control unit, which has a detection module according to the present invention,

[0027] FIG. 4 depicts a state-transition diagram of a first variant of the method according to the present invention, and

[0028] FIG. 5 depicts a state-transition diagram of a second variant of the method according to the present invention.

[0029] An electromotor 10 is connected to a six-pulse bridge converter 11 as a power-electronic power-supply unit of the electromotor. Electromotor 10 is a brushless, so-called electronically commutated DC electromotor (BLDC motor) of the type described above. Of electromotor 10, only motor phase windings 101, 201, 301 having motor windings 102, 202, 302 are depicted, which are disposed in the stator of electromotor 10 and which are interconnected in a star point 12. Running in the undepicted stator is a rotor 13 that is excited electrically, or by a current magnet, and that for reasons of simplicity is not depicted, the rotor, in response to its rotation, inducing voltages in motor phase windings 101, 201, 301.

[0030] A motor control unit 14, via a control line 22, controls bridge converter 11 and therefore electromotor 10. Bridge converter 11 has three bridge arms 103, 203, 303, each of which has an upper switchable valve 104, 204, 304 and a lower switchable valve 105, 205, 305. Valves 104, 105; 204, 205; 304, 305 are switched on and off by motor control unit 14, pulse width modulation (PWM) being utilized here.

[0031] On the input side, bridge converter 11 is connected to a battery or to a DC voltage intermediate circuit having a positive voltage potential U_B and a negative voltage potential U_B. On the output side, phase windings 101, 201, 301 are each connected to bridge arms 103, 203, 303 between valves 104, 105; 204, 205; 304, 305. Valves 104, 105; 204, 205; 304, 305 are, for example, power transistors, through which a current flow can be switched on and off in the direction from positive voltage potential U_B to negative voltage potential U_B or, from the pick-off of phase windings 101, 201, 301 to bridge arms 103, 203, 303, in the direction of negative voltage potential U_B. In the opposite direction, valves 104, 105; 204, 205; 304, 305, as protection here against induced overvoltages, each have an internal diode, indicated by an arrow. It is also possible that an external diode is connected to valves 104, 105; 204, 205; 304, 305. In any case, the diodes permit a freewheeling of the electromotor 10, e.g., the freewheeling state depicted in FIG. 1B.

[0032] Phase currents 106, 206, 306, flowing, in each case, across motor phase windings 101, 201, 301 are adjusted by motor control unit 14 by switching valves 104, 105; 204, 205; 304, 305 on and off. Typically, as depicted in the exemplary embodiment, in each case, two phase windings 101, 201, 301 are supplied with current simultaneously. In this context, one motor phase winding 101, 201, 301 is supplied with current in a positive charge and the second phase winding in a negative charge. An idealized commutation pattern for an electrical rotation of rotor 13 is shown in FIG. 2A, in which phase currents 106, 206, 306 flowing through motor phase windings 101, 201, 301 and phase voltages 107, 207, 307 induced in motor phase windings 101, 201, 301 by rotor 13, which is excited electrically or by a permanent magnet, are depicted by dotted lines.

[0033] It is apparent that between commutation time points 17, 18, which lie between the electrical angles 30 and 150 in the rotational direction of electromotor 10, phase winding 101 is supplied with current in a positive charge and motor phase winding 201 in a negative charge, and between commutation time points 19, 20 lying between electrical angles 210 and 330, phase winding 201 is supplied with current in a positive charge and motor phase winding 101 in a negative charge. For example, valves 104, 205, for adjusting a current-supply state, are switched on between commutation time points 17, 18, as depicted in FIG. 1A. In addition, a current 15 is shown that flows from positive voltage potential U_B via valve 104, motor windings 102, 202, and valve 205, to negative voltage potential U_B.

[0034] For a high motor efficiency of electromotor 10, a phase winding 101, 201, 301 is always supplied with current at the highest possible induced phase voltage 107, 207, 307 of the same polarity. In the circuit diagram depicted in FIG. 2A, motor phase windings 101, 201, 301 are each supplied with current between the specific commutation time points either continuously or not continuously. Electromotor 10, in this context, is under a full load.

[0035] As a result of the pulsed switching of valves 104, 105; 204, 205; 304, 305, here using pulse width modulation, phase currents 106, 206, 306, flowing in individual motor phase windings 101, 201, 301, are limited, electromotor 10 briefly moving into freewheeling operation. A freewheeling state of this type is shown in FIG. 1B. In this context, valve 205 is closed, so that a freewheeling current 16, flowing through valve 104, motor windings 102, 202, and the internal diode of valve 204, is set.

[0036] An alternating switching of valve 205, for alternating between a current-supplied and a freewheeling state for motor phase windings 101, 201, is depicted in FIG. 2B. Phase winding 301, in this context, is connected without current and is electrically separated from voltage potentials U_B, U_B. Valve 205 is switched on (current-supplied state), in each case, at switch-on time points t_e for a switch-on time t_ein, and is switched off (freewheeling state) at switch-on time points t_a for a switch-off time t_aus. In this context, motor control unit 14 initiates a sequence 21 from current units having positive phase current 106, between commutation time points 17, 18, and a sequence 22, of current units having a negative phase current 106, between commutation time points 19, 20.

[0037] It is evident that motor control unit 14 can also bring about other commutation patterns of phase currents 106, 206, 306 and/or pulse patterns of phase currents 106, 206, 306 by an appropriate switching of valves 104, 105; 204, 205; 304, 305.

[0038] Motor control unit 14 contains a detection module 23 for determining the rotor position of electromotor 10. Connected upstream of detection module 23 is a simulation module 24, which simulates phase voltages 107, 207, 307 as virtual phase voltages 108, 208, 308 and which simulates a virtual star point voltage 26 from virtual phase voltages 108, 208, 308. Virtual phase voltages 108, 208, 308 drop off at resistors 109, 209, 309, which are each connected, on the input side, to bridge arms 103, 203, 303, parallel to motor windings 102, 202, 302, and which, on the output side, are interconnected at a virtual star point 27, at which virtual star-point voltage 26 drops off. In any case, due to simulation module 24, it is not necessary that (real) star point 12 is contacted for detecting (real) star-point voltage 25.

[0039] Connected between simulation module 24 and detection module 23 is a comparator module 28 for determining the polarity of phase voltages 107, 207, 307. Comparator module 28 compares phase voltages 107, 207, 307 to virtual star-point voltage 26, which constitutes a reference value. Comparator module 28 contains operation amplifiers 110, 210, 310, that are connected as comparators, which, on the input side, compare the voltage differences applied at them between phase voltages 107, 207, 307, on the one hand, and virtual star-point voltage 26, on the other hand, and which, on the output side, read out the polarity, determined in this context, of the individual voltage different as polarity values 111, 211, 311 in the form of output voltages. Polarity values 111, 211, 311, in a polarity change of the individual voltage difference, fluctuate from a negative to a positive maximum value, and back again. Operation amplifiers 110, 210, 310 are connected in an undepicted manner to a power-supply voltage.

[0040] Phase voltages 107, 207, 307 are applied at voltage dividers 112, 212, 312, which are connected upstream of the plus-inputs of operation amplifiers 110, 210, 310, and which in each case have a resistor 29 connected to bridge arms 103, 203, 303 parallel to motor windings 102, 202, 302 and a resistor 30, that is connected to the latter and that is connected to ground. Virtual star-point voltage 26 is applied at a voltage divider 31, which has a resistor 32, which is connected between the virtual star point and the minus-inputs of operation amplifiers 110, 210, 310, and a resistor 33, which is connected between the minus-inputs and ground. In any case, phase voltages 107, 207, 307 as well as virtual star-point voltage 26, as a result of voltage dividers 112, 212, 312; 31, are dimensioned such that they can be processed by operation amplifiers 110, 210, 310. A suitable dimensioning of resistors 29, 30; 32, 33 is known to the worker skilled in the art and is a function, inter alia, of the properties of operation amplifiers 110, 210, 310.

[0041] Detection module 23, as a detection means, contains here a logical field that is programmed in accordance with the present invention, which can be, e.g., a so-called (Field) Programmable Logic Array ((F)PLA) or a so-called (Field) Programmable Gate Array ((F)PGA). In principle, the detection means can also be constituted as a discrete logic circuit or a suitable processor, which carries out a program module according to the present invention. The detection means of detection module 23, for determining polarity values 111, 211, 311, can be synchronized to switch-on time points, e.g., switch-on time points t_e, for supplying current to motor phase 101, 201, 301, specifically such that the means determine polarity values 111, 211, 311 after a preestablished delay t_wa that is subsequent to the specific switch-on time points. A delay t_wa of this kind, after current has begun to be supplied to motor phase winding 101, is drawn in, by way of example, in FIG. 2B.

[0042] Detection module 23 scans polarity values 111, 211, 311 in accordance with a method according to the present invention that is discussed below, by way of example, on the basis of two variants, and it filters out the interferences containing any so-called raw polarity values 111, 211, 311. Subsequently, detection module 23, from raw polarity values 111, 211, 311, reads out the calculated, so-called digitally filtered polarity values 114, 214, 314 to a control module 34 of motor control unit 14, which controls bridge converter 11.

[0043] Control module 34 then determines the specific electrical reference angle of rotor 13 on the basis of signal changes in polarity values 114, 214, 314 and, therefore, the assigned commutation time points. However, this function could also be satisfied by detection module 23. Intermediate values of the reference angles, which cannot be picked off directly from polarity values 114, 214, 314, can be determined by control module 34, or detection module 23, for example, using extrapolation. In addition, it is possible that detection module 23 and/or control module 34 only reads out the one polarity value from 114, 214, 314 that is assigned to an instantaneously not current-supplied phase winding 101, 201, 301, e.g., between commutation time points 17, 18, first polarity value 311 of phase winding 301 and then polarity value 211 of phase winding 201.

[0044] Control module 34 is depicted only in a schematic fashion and, by way of example, in the form of a control means 35 and storage means 36. Control means 35 is a processor or a group of processors, for example, digital signal processors, which can execute the program code from program modules that are stored in storage means 36. Control module 34, on the one hand, controls bridge converter 11 and, in this context, provides the circuit pattern for valves 104, 105; 204, 205; 304, 305, for setting the current-supplied and freewheeling states. On the other hand, control module 34 supplies to detection module 23 as synchronization signals a Freewheeling-ON signal 40, a PWM-pulse-start signal 41, and a PWM-pulse signal 42.

[0045] PWM-pulse signal 42 is, so to speak, the PWM basic timing signal and is read out, in each case, at switch-on time points t_e, for example, as logic 1. It represents, in each case, the beginning of a current pulse t_ein on one of motor phase windings 101, 201, 301, i.e., switch-on time points t_e of the current-supply of motor phase windings 101, 201, 301.

[0046] In the present example, the PWM basic timing signal, i.e., also PWM-pulse signal 42, is in each case started anew in response to a commutation, thus, e.g., at commutation time points 17, 18, 19, 20. PWM-pulse-start signal 41, in this context, is read out as logical 1. Therefore, motor phase windings 101, 201, 301, to the extent that they had previously not been supplied with current, and a current supply for specific motor phase winding 101, 201, 301 is provided, are, in each case, directly supplied with current using a current pulse, immediately after a commutation. The beginning of a current-supply of this kind is determined, for example, by commutation time points 17, 19 for motor phase winding 101 in FIG. 2B.

[0047] Freewheeling-ON signal 40, in each case, is read out at the beginning of a freewheeling state on one of motor phase windings 101, 201, 301, i.e., at time points t_a, for example, as logical 1.

[0048] A first variant for determining polarity values 114, 214, 314 is depicted in FIG. 4. The starting state there is a state Waiting for current supply 401. If detection module 23, in this state, receives one of signals PWM-pulse-start signal 41 or PWM-pulse signal 42 having logical 1, i.e., a switch-on signal is given by control module 34 for supplying current to motor phase windings 101, 201, 301, then detection module 23 moves into a state Current-supply switched on 402, which is indicated by a transition 412. Detection module 23 is therefore synchronized to a switch-on time point for supplying current to at least one of motor phase windings 101, 201, 301.

[0049] In state Current-supply switched on 402, detection module 23 starts a timer, to wait to the end of a preestablished delay t_wa until the scanning of polarity values 111, 211, 311. Delay t_wa is, in particular, provided so that operation amplifiers 110, 210, 310 achieve a steady-state condition and read out stable polarity values 111, 211, 311, and the delay can be determined and set accordingly by a worker skilled in the art.

[0050] After the elapsing of delay t_wa, detection module 23 moves into a state Sampling and reading out the polarity values (transition 423). In this state, detection module 23 scans polarity values 111, 211, 311, and reads them out to control module 34 as polarity values 114, 214, 314. Subsequently, detection module 23, in a transition 431, moves once again to state Waiting for current-supply 401.

[0051] If, in state Current-supply switched on 402, control module 34 connects bridge converter 11 for initiating a freewheeling operation, and detection module 23 receives Freewheeling-ON signal 40, then detection module 23 also moves into state Waiting for current-supply 401.

[0052] If detection module 23 in state Current-supply switched on 402 once again receives a signal Current-supply switched on, in particular PWM-pulse-start signal 41 produced on the basis of a commutation, possibly also PWM-pulse signal 42, then detection module 23 starts a timer once again for determining preestablished delay t_wa, which is depicted by transition 422.

[0053] A second variant for determining polarity values 114, 214, 314 is depicted in FIG. 5. The point of departure is a state Current-supply switched on 501, which is adopted on the basis of one of signals PWM-pulse-start signal 41 or PWM-pulse signal 42. In state 501, detection module 23 starts a timer, to wait to the end of a preestablished delay t_wa until the sampling of polarity values 111, 211, 311.

[0054] If detection module 23 in state Current-supply switched on 501 once again receives a signal Current-supply switched on (PWM-pulse-start signal 41 or PWM-pulse signal 42), then it starts a timer once again for determining preestablished delay t_wa, as is depicted by a transition 511.

[0055] If detection module 23, in state Current-supply switched on 501, receives the Freewheeling-ON signal 40, then detection module 23 moves into state Freewheeling before end of delay 502 (transition 512), in which no sampling of polarity values 111, 211, 311 takes place. If detection module 23, in state Free running before termination of delay 502, receives a signal Current supply switched on (PWM-pulse-start signal 41 or PWM-pulse signal 42), then it once again moves into state Current supply switched on 501 (transition 521).

[0056] If, in state Current-supply switched on 501, delay t_wa has elapsed, without one of synchronization signals 40, 41, 42 having been received, then detection module 23 moves into a state Current-supply active after delay 503 (transition 513). In this state, detection module 23 scans polarity values 111, 211, 311 once or, during a sampling period t_ab, repeatedly, e.g., using a preestablished sampling frequency, and stores them in storage unit 43 that is configured, e.g., as a shift register.

[0057] If, during state Current supply active after delay 503, Freewheeling-ON signal 40 is received, and electromotor 10 therefore moves into freewheeling operation, then detection module 23, in a transition 534, moves into a state Read out in freewheeling 504 and reads out the most recently scanned polarity values 111, 211, 311 as polarity values 114, 214, 314. In response to a signal Current-supply switched on (PWM-pulse-start signal 41 or PWM-pulse signal 42), detection module 23 once again moves into state Current-supply switched on 501 (transition 541).

[0058] If, during state Current-supply active after delay 503, a signal Current-supply switched on is received (PWM-pulse-start signal 41 or PWM-pulse signal 42), detection module 23 moves into a state Read out without freewheeling 505 (transition 535), in which it reads out most recently scanned polarity values 111, 211, 311 as polarity values 114, 214, 314. Immediately thereafter, detection module 23, in a transition 551, once again goes into state Current-supply switched on 501.

[0059] Further variants of the present invention are easily possible:

[0060] Detection module 23 can also be configured as a software module, which contains a program code that can be executed by a control means, e.g., a processor. For example, processor 35 of motor control unit 14 could execute a software module of this type. Detection module 23, from a functional standpoint, would then be integrated into control module 34.

[0061] It is, in principle, also possible that comparator module 28 compares virtual phase voltages 108, 208, 308 to actual star-point voltage 25 or compares virtual phase voltages 108, 208, 308 to virtual star-point voltage 26.

[0062] Simulation module 24 could be a component part of detection module 23 and/or motor control unit 14.

[0063] In addition, comparator module 28 could be integrated in detection module 23 and/or motor control unit 14. For example, comparator module 28 and/or detection module 23 could be realized using an ASIC component (Application Specific Integrated Circuit).

[0064] In principle, PWM-pulse signal 42 would also be sufficient for synchronizing detection module 23 to the specific switch-on time points for supplying current to motor winding phases 101, 201, 301.