Abstract:
A brushless DC-motor driving controller is provided which can significantly reduce torque fluctuations in the brushless DC motor. The brushless DC-motor driving controller uses a position detector, generates rotation-angle signals representing rotor-rotation angles or a part thereof in high resolution, uses the rotation-angle signals, and performs rectangular-wave driving. By using the rotation-angle signals, the brushless DC-motor driving controller compensates for electrical angles corresponding to commutation-position skews in rectangular-wave driving which are caused by armature reaction in a brushless DC-motor.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a brushless DC-motor driving controller for driving and controlling a brushless DC-motor. Particularly, the present invention relates to a brushless DC-motor driving controller for suitably minimizing torque fluctuations in a brushless DC-motor. 
     2. Description of the Related Art 
     For conventional brushless DC-motors driven with rectangular waves, for example, a small number of Hall elements, as in the cases of three Hall elements for three phases and five Hall elements for five phases, is used to detect rotor positions to generate position-detecting signals. The generated position-detecting signals are used to detect the relative positional relation between the rotor and armature coils, and commutation positions are determined according to the positional relation. 
     According to requirements for cost reduction, miniaturization, and high output power, however, the torque constant and ampere-turns increase in conventional brushless DC-motors. Proportionally to the increase in ampere-turns, the armature-current magnetomotive force also increases, thereby increasing torque-form skew for each phase. 
     In the conventional brushless DC-motors, in which a small number of Hall elements is used to determine commutation positions and to perform rectangular-wave driving, phase-current commutation positions are fixed. In this case, addition of skewed torques in the individual phases, as shown in FIG. 1B, causes torque connection to be discontinuous, thereby causing torque fluctuations, as shown in FIG.  1 A. Torque fluctuations cause noise during operation of the brushless DC-motor. 
     With an electrical power steering device of vehicles which use the conventional brushless DC-motor causing torque fluctuations as its power source, when the steering wheel is slowly rotated, torque fluctuations effect steering sensation; and when the steering wheel is rotated quickly, the fluctuations cause noise. 
     SUMMARY OF THE INVENTION 
     The present invention has been developed under the above circumstances. A first object of the present invention is to provide a brushless DC-motor driving controller that significantly reduces torque fluctuations in brushless DC-motors. A second object is to provide a brushless DC-motor driving controller that can be used to improve steering sensation and to reduce noise occurrence in electrical power steering devices. 
     To achieve the described objects, the present invention provides a brushless DC-motor driving controller that uses a position detector, generates a rotation-angle signal representing rotor-rotation angle or a part thereof in high resolution, uses the rotor-rotation signal, and performs rectangular-wave driving. The rotor-rotation signal is used to compensate for an electrical angle corresponding to commutation-position skew in rectangular-wave driving which is caused by armature reaction in a brushless DC-motor. 
     In this case,,the electrical angle, which correspond to the commutation-position skew in rectangular-wave driving, which is caused by armature reaction in the brushless DC-motor, may be determined by using functions of values representing four operating conditions of normal-rotation driving, inverse-rotation driving, normal-rotation regenerative braking and inverse-rotation regenerative braking, and armature-reaction effects of the brushless DC-motor. 
     Also, the above-described brushless DC-motor driving controller, which compensates for the electrical angle corresponding to the commutation-position skew in rectangular-wave driving, which is caused by armature reaction in the brushless DC-motor, may be used to drive and control a brushless DC-motor in an electrical power steering device that uses rotation torque of the brushless DC-motor to support forces required for vehicle-wheel steering. 
     In this case also, the electrical angle corresponding to commutation-position skew in rectangular-wave driving which is caused by armature reaction in the brushless DC-motor may be determined by using functions of values representing four operating conditions of normal-rotation driving, inverse-rotation driving, normal-rotation regenerative braking and inverse-rotation regenerative braking, and armature-reaction effects of the brushless DC-motor. 
     According to the above, the objects of the present invention, described above, can be achieved, specifically, in the following manner. 
     The first object of the present invention can be achieved by the brushless DC-motor driving controller that uses a rotation-angle signal so as to compensate for an electrical angle corresponding, to commutation-position skew in rectangular-wave driving, which is caused by an armature reaction in the brushless DC-motor. This allows torque fluctuations in brushless DC-motors to be significantly reduced. The second object of the present invention can be achieved by the brushless DC-motor driving controller that uses the rotation-angle signal to drive and control a brushless DC-motor in an electrical power steering device that uses rotation torque of the brushless DC-motor to support forces required for vehicle-wheel steering. This improves steering sensation in the electrical power steering device, and in addition, minimizes noise occurrence. 
     Also, the first and second objects of the present invention can be achieved even more effectively because the electrical angle, which correspond to commutation-position skew in rectangular-wave driving which is caused by an armature reaction in the brushless DC-motor, is determined by using functions of values representing four operating conditions of normal-rotation driving, inverse-rotation driving, normal-rotation regenerative braking and inverse-rotation regenerative braking, and armature-reaction effects of the brushless DC-motor. 
     Furthermore, according to the brushless DC-motor driving controller of the present invention, rotation-angle signals that represent close rotor-rotation angles are used to perform rectangular-wave driving. This allows phase-current commutation positions to be closely controlled. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
     FIGS. 1A to  1 C are characteristic graphs showing torque fluctuations due to armature-reaction effects in a conventional brushless DC-motor driving controller; 
     FIGS. 2A to  2 C are characteristic graphs showing magnetic-flux density distribution conditions on a rotor magnet surface and in an air gap of a brushless DC-motor of an embodiment when armature reaction is disregarded; 
     FIGS. 3A to  3 C are characteristic graphs showing waveforms of a synthesis torque, stationary-current torques in individual phases, and individual phase currents in the brushless DC-motor when armature reaction is disregarded; 
     FIGS. 4A to  4 C are characteristic graphs showing magnetic-flux density distribution conditions on the rotor magnet surface and in the air gap of the brushless DC-motor when armature reaction is considered in a first driving state; 
     FIGS. 5A to  5 C are characteristic graphs showing waveforms of a synthesis torque, stationary-current torques in individual phases, and individual phase currents in the brushless DC-motor when armature reaction is considered in the first driving state; 
     FIG. 6 is a schematic diagram showing a compensation method for armature reaction in the brushless DC-motor of the embodiment; 
     FIGS. 7A to  7 C are characteristic graphs showing waveforms of a synthesis torque, stationary current torques in the individual phases, and individual phase currents when armature reaction effects are compensated for in the brushless DC-motor of the embodiment in the first driving state; 
     FIGS. 8A to  8 C are characteristic graphs showing magnetic-flux density distribution conditions on the rotor magnet surface and the air gap of the brushless DC-motor when armature reaction is considered in a first regenerative braking state; 
     FIGS. 9A to  9 C are characteristic graphs showing waveforms of a synthesis torque, stationary current torques in the individual phases, and individual phase currents when armature reaction is considered in the brushless DC-motor in the first regenerative braking state; and 
     FIGS. 10A to  10 C are characteristic graphs showing waveforms of a synthesis torque, stationary current torques in the individual phases, and individual phase currents when armature reaction effects are compensated for in the brushless DC-motor of the embodiment in the first regenerative braking state. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Hereinbelow, a description will be given of an embodiment of the present invention with reference to the drawings. 
     A brushless DC-motor driving controller of the embodiment compensates for torque fluctuations caused by armature reaction. 
     (1) First Electrical Driving State (Normal-Rotation Driving) 
     In a first driving state, the rotor rotation direction is clockwise (CW), and the torque direction is also clockwise (CW). 
     FIGS. 2A to  2 C are characteristic graphs showing magnetic-flux density distribution conditions on a rotor magnet surface and in an air gap of a brushless DC-motor when armature reaction is disregarded. 
     When armature reaction in the brushless DC-motor is disregarded, the magnetic-flux density distribution condition on the magnet surface mainly determines the magnetic-flux density distribution condition in the air gap. FIG. 2C shows a magnetic-flux density distribution condition in the air gap; and FIG. 2A shows a magnetic-flux density distribution condition on the magnet surface. 
     FIGS. 3A to  3 C are characteristic graphs showing waveforms of a synthesis torque, stationary-current torques in individual phases, and individual phase currents in the brushless DC-motor when armature reaction is disregarded. 
     When the stationary current flows in individual phases of the brushless DC-motor, electromagnetic torque conditions in individual phases are as shown in FIG.  3 B. These conditions are substantially the same as magnetic-flux density distribution conditions in the air gap. For driving a five-phase brushless DC-motor by a four-phase exciting rectangular wave, currents in individual phases are commuted, as shown in FIG.  3 C. This provides an electromagnetic torque condition synthesized with the individual phase currents, as shown in FIG.  3 A. 
     FIGS. 4A to  4 C are characteristic graphs showing magnetic-flux density distribution conditions on the rotor magnet surface and in the air gap of the brushless DC-motor when armature reaction is considered in the first driving state. 
     When armature reaction is considered, the magnetic-flux density distribution condition in the air gap is determined by the synthesis waveform of magnetic-flux density distribution on the magnet surface and magnetic-flux density distribution caused by armature current in the air gap. As shown in FIG. 4C, the synthesis waveform representing the magnetic-flux density distribution. in the air gap is skewed. This skewed air-gap magnetic-flux density distribution causes electromagnetic torque conditions in the individual phases with the stationary current to be skewed, as shown in FIG.  4 B. 
     FIGS. 5A to  5 C are characteristic graphs showing waveforms of a synthesis torque, stationary-current torques in individual phases, and individual phase currents in the brushless DC-motor when armature reaction is considered in the first driving state. When the motor is excited using currents in the same rectangular waveforms as those in FIG. 3C, electromagnetic torques of two phases at current commutation positions are skewed because of armature reaction. Therefore, these waveforms are discontinuous, providing a synthesis electromagnetic torque condition of the motor shown in FIG.  5 A. 
     With current represented by I, an electrical angle (electric degree) of the commutation position which has been skewed because of armature reaction is expressed in a formula (1) below. 
     
       
         δ a   CWCW   =f   CWCW ( I )  (1) 
       
     
     In the above, the amount of skew in the electrical angle at the commutation position is represented by δa CWCW . 
     Now, an electrical angle at a conventional commutation position is assumed to be δb. With this, the commutation position δba in the brushless DC-motor driving controller of this embodiment is arranged to be a result of shifting from the electrical angle δb at the commutation position in the conventional brushless DC-motor to electrical angle δba for compensating for torque fluctuations due to skewed commutation position. The electrical angle δba is obtained from a formula (2) shown below. 
      δ ba=δb−δa   CWCW (δ a   CWCW   =f   CWCW ( I )≧0)  (2) 
     In the above, the relationship “δa CWCW =f CWCW (I)” between the current I and the skewed electrical angle of commutation is obtained by performing analyses of magnetic fields or experiments. 
     FIG. 6 is a schematic diagram showing a compensation method for armature reaction in the brushless DC-motor of the embodiment. As can be seen in FIG. 6, in the brushless DC-motor driving controller of the embodiment with the rotor position at the electrical angle δba for the current I, a drive pattern according to a rectangular wave references a Hall-element pattern table Tbls (δb, Sa e ) according to an electrical angle δb=δba+δa CCCW , and thereby generates a Hall-element signal Sa e . According to the Hall-element signal Sa e , the aforementioned drive pattern then references a drive pattern table Tblt (Sa e , mtdir, T 1 _ 10 ), and thereby determines an ON-OFF signal T 1 _ 10  for a switching element such as an FET. In this case, mtdir represents the direction of electromagnetic torque, which is clockwise (CW). Here, the table Tbls and the table Tblt may be integrated into a single table Tblst (δb, mtdir, T 1 _ 10 ) to obtain a direct drive table pattern T 1 _ 10  according to the electrical angle δb. 
     FIGS. 7A to  7 C are characteristic graphs showing waveforms of a synthesis torque, stationary current torques in the individual phases, stationary current torques in the individual phases, and currents in the individual phases when armature reaction effects are compensated for in the brushless DC-motor of the embodiment in the first driving state. As a result of shifting the commutation position from the electrical angle δb to the electrical angle δa, torque fluctuations at the commutation position are significantly reduced, providing a synthesis torque as shown in FIG.  7 A. 
     (2) Second Electrical Driving State (Reverse-Rotation Driving) 
     In a second driving state, the rotor rotation direction is counterclockwise (CCW), and the torque direction is also counter clockwise (CCW). 
     In the first driving state described above, because the rotor rotation direction is clockwise (CW), the commutation position must be advanced in the CW direction from the conventional commutation position so that the armature reactions caused by torque fluctuations are compensated for. In the second driving state, since the rotor rotation direction is counterclockwise (CCW), the commutation position must be advanced in the CCW direction from the conventional commutation position so that torque fluctuations caused by armature reaction are compensated for. Therefore, the current commutation position in the brushless DC-motor driving controller of the embodiment is shifted from the electrical angle δb, which is the commutation position in the conventional brushless DC-motor driving controller, to the electrical angle δba. The electrical angle δba can be obtained from a formula (3) shown below. 
     
       
         δ ba=δb+δa   CCWCCW (δ a   CCWCCW   =f   CCWCCW ( I )≧0)  (3) 
       
     
     In the above, the electrical angle δa CCWCCW , which is the amount of skew of the commutation position caused by armature reaction, is represented by the function f CCWCCW (I) 
     (3) First Regenerative Braking State (Normal-Rotation Regenerative Braking) 
     In a first regenerative braking state, the rotor rotation direction is clockwise (CW), and the torque direction is counterclockwise (CCW). 
     FIGS. 8A to  8 C are characteristic graphs showing magnetic-flux density distribution conditions on the rotor magnet surface and in the air gap of the brushless DC-motor when armature reaction is considered in the first regenerative braking state. 
     In the first regenerative braking state, because the current direction is opposite to that in the first driving state, the magnetic-flux density distribution condition caused by armature current in the air gap is as shown in FIG.  8 C. In this case, armature-and-magnet synthesis magnet flux distribution is skewed, as shown in FIG.  8 C. 
     Because of the skewed synthesis magnetic flux distribution, when commutation occurs at the commutation position in the conventional brushless DC-motor driving controller, electromagnetic torque conditions with stationary currents in the individual phases are skewed, as shown in FIG.  7 B. FIGS. 7A to  7 C are characteristic graphs showing waveforms of a synthesis torque, stationary current torques in the individual phases, and currents in the individual phases when armature reaction is considered in the brushless DC motor in the regenerative braking state. 
     For compensation for these torque fluctuations caused by armature reaction, the commutation position must be shifted back in the CW direction from the commutation position in the conventional brushless DC-motor driving controller. For this reason, to compensate for the torque fluctuations due to armature reaction, the commutation position in the brushless DC-motor driving controller of the embodiment is arranged to be a result of shifting from the electrical angle δb at the commutation position in the conventional brushless DC-motor to electrical angle δba for compensating for torque fluctuations due to skewed commutation position. The electrical angle δba is obtained from formula (4) shown below. 
     
       
         δ ba=δb+δa   CWCCW (δ a   CWCCW   =f   CWCCW (  I )≧0)  (4) 
       
     
     In the above, the electrical angle δa CWCCW  for the amount of skew caused by armature reaction commutation position is represented by the function f CWCCW (I). 
     FIGS. 10A to  10 C are characteristic graphs showing waveforms of a synthesis torque, stationary current torques in the individual phases, and currents in the individual phases when armature reaction effects are compensated for in the brushless DC-motor of the embodiment in the first regenerative braking state. As a result of shifting of the commutation position from the electrical angle δb to the electrical angle δa, torque fluctuations at the commutation position are significantly reduced, providing a synthesis torque as shown in FIG.  10 A. 
     (4) Second Regenerative Braking State (Inverse-Rotation Regenerative Braking) 
     In a second regenerative braking state, the rotor rotation direction is clockwise (CW), and the torque direction is counterclockwise (CCW). 
     In the first regenerative braking state described above, the rotor direction is clockwise (CW), and for compensating for torque fluctuations caused by armature reaction, the commutation position must be shifted back in the CW direction from the commutation position in the conventional brushless DC-motor driving controller. In the second regenerative braking state, however, the rotor direction is counterclockwise(CCW). Therefore, for compensating for the torque fluctuations caused by armature reaction, the commutation position must be shifted back in the CCW direction from the commutation position in the conventional brushless DC-motor driving controller. For this reason, to compensate for the torque fluctuations due to armature reaction, the commutation position in the brushless DC-motor driving controller of the embodiment must be a result of shifting from the electrical angle δb at the commutation position in the conventional brushless DC-motor to electrical angle δba. The electrical angle δba can be obtained from a formula (5) shown below. 
     
       
         δ ba=δb−δa   CCWCW (δ a   CCWCW   =f   CCWCW ( I )≧0)  (5) 
       
     
     In the above, the electrical angle δa CCWCW  for the amount of skew of commutation position caused by armature reaction is represented by the function f CCWCW (I). 
     In these ways, for rectangular-wave driving by using close positioning signals, the brushless DC-motor driving controller of the embodiment can compensate for skew of the current commutation position caused by armature reaction, thereby significantly reducing torque fluctuations during commutation, which are caused by effects of armature reaction. 
     As above, the present invention has been described with reference to the preferred embodiment. However, the invention is not restricted to the embodiment, and it may be modified in various ways within the scope of the invention.