Abstract:
A brushless motor control apparatus detects a phase current flowing in a brushless motor, converts the phase current to a vector current including a d-axis current of a magnetic flux direction component and a q-axis current of a torque direction component orthogonal to the d-axis current, calculates a vector voltage instruction value based on differences between a vector current and a vector current instruction value, calculates a phase voltage instruction value and controls rotation of the brushless motor based on the phase voltage instruction value. The apparatus further calculates electric power produced in the brushless motor and protects the brushless motor based on the magnitude of electric power.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is based on and incorporates herein by reference Japanese Patent Application No. 2004-33206 filed on Feb. 10, 2004. 
   FIELD OF THE INVENTION 
   This invention relates to a brushless motor control apparatus, and more specifically, to a method for preventing a brushless motor from overheating. 
   BACKGROUND OF THE INVENTION 
   In a motor system, an excessive current higher than that under normal operating condition flows in motor windings due to dielectric breakdown, overload, forced constraint, or the like. If such an excessive current flows, temperatures of the windings rise and exceed a heat-withstanding temperature of insulating coating of the windings, dielectric breakdown occurs, the windings short out. This further leads to abnormal operations of an apparatus connected to the motor. 
   To prevent such overheating of the windings, temperature protective devices such as a thermistor and a thermal fuse are attached and the temperature rise is suppressed by limiting currents flowing in the windings in response to winding temperatures or cutting off currents when they exceed a predetermined temperature. However, such overheat protection needs addition of a thermistor, a thermal fuse, etc. and addition of a peripheral circuit for detection. This causes increase in cost. 
   Moreover, as overheat protection techniques of the direct current motor, winding temperatures are estimated from a motor current value squared and a motor resistance value, and the current is limited according to the estimated temperature because the winding temperature is proportional to a product of the motor current value squared by the motor resistance value. 
   The brushless motor uses three-phase alternating currents. Consequently its accurate state cannot be grasped, except that the sampling period and calculation period of current, voltage, etc. are done at high speed. In a worst case, it is likely that calculation results may be different from actual values depending on the sampling period (for example, only low current values of the alternating waveform are sampled), and hence dielectric breakdown may occur due to a failure of the overheat protection. 
   For example, supposing that the brushless motor with 16 poles is in rotation at 2000 rpm, a motor current waveform becomes an alternating waveform of a period of 3.75 ms. That is, if this alternating waveform is intended to be sampled accurately and subjected to calculation, it must be performed in a period of a few hundreds of micro seconds or less, which increases a calculation load of a microcomputer. To prevent the calculation load in the microcomputer from increasing, a microcomputer capable of high-speed processing is necessitated. This will result in an increase of cost. 
   It is also proposed in JP 2003-164185A to realize the overheat protection by using a q-axis current that is a torque component current of the brushless motor, or both the q-axis current and a d-axis current that is an exciting component current while preventing increase in cost and calculation load. 
   The brushless motor is driven by the three-phase alternating currents. Each of its voltage, current, magnetic flux, etc. is represented by a composite vector that is a vector sum of components generated by alternating components of each phase. When driving the brushless motor, its control is simplified by converting the three-phase alternating currents that necessitate handling these vectors to two-axis direct currents. 
   This two-axis direct current conversion means conversion whereby a motor having a fixed part and a rotating part is converted to one in an orthogonal coordinate system whose coordinates are both fixed, i.e., a rotating orthogonal coordinate system, and is called d-q conversion. The q-axis is in advance of the d-axis by a phase of π/2 and the d-axis is orientated in a direction of a magnetic flux formed by a field magnet. 
   In the direct current motor, a field magnet circuit is formed with permanent magnets or by flowing constant field-magnet currents in field-magnet windings. Independently from it, an armature current is supplied in a rotor conductor from the outside, whereby a torque proportional to the armature current can be generated. Therefore, the direct current motor is rotated with the torque proportional to the armature current. 
   On the other hand, in the brushless motor, the rotor is not electrically connected with the outside. That is, only a primary current flowing in the stator generates both a rotating magnetic field and an induction current equivalent to the armature current. Therefore, the primary current contains both a current (exciting component current) that generates a secondary interlinkage flux crossing a secondary-side rotor and a current (torque component current) that flows in a secondary-side conductor. 
   A technique of controlling these two currents independently is the d-q conversion described above. With this conversion, if the exciting component current (d-axis current) is controlled to be constant, the torque component current (q-axis current) will be proportional to the torque. Therefore, the use of this q-axis current enables the brushless motor to be vector-controlled as in the case of the direct current motor. 
   Here, the principle of vector control will be described using  FIG. 7 . The magnitude and phase of a current determines the torque of the brushless motor, the alternating current motor, or the like. In practice, the current is divided into a current component (magnetic flux current) that forms a magnetic flux in the direction of a main magnetic flux established inside the motor and a current component with a phase advanced by 90° that controls the torque directly (torque current). 
   The two components are controlled independently. The magnetic flux current and the torque current are defined as a current component that forms a magnetic field in the d-axis direction and a current component that forms a magnetic field in the q-axis direction, respectively. The fact that each of the current, voltage, and magnetic flux is controlled after being divided into a d-axis component and a q-axis component may account for a name of the vector control. These current components can be calculated by the well known three-phase to two-phase conversion based on a rotation angle θ of the main magnetic flux to the stator in the d-q axis coordinate system. 
   Moreover, in estimation calculation of the winding temperature (electric power), the use of the d-axis current in addition to the q-axis current enables estimation with higher accuracy than that of an estimation only with the q-axis current, although it adds a slight increase in the calculation load. 
   However, in the above conventional method, because the brushless motor uses the three-phase alternating currents, when the motor is in rotation or when the motor is not in rotation and yet predetermined currents are flowing through it so as to produce a torque (motor lock state), currents applied to phase windings might differ largely among them even if the q-axis current or the d-axis current is the same. The current also might differ depending on a motor locking position (electrical angle). 
   As a result, in the case where only the q-axis current or the d-axis current is used to estimate the winding temperatures and the overheat protection is performed, estimation errors may occur and excessive overheat protection may be executed, which impedes the production of sufficient torque by the motor. Otherwise, too little overheat protection may result in short circuit of the windings, which leads to abnormal operations of an apparatus connected to the motor. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of this invention to provide a brushless motor control apparatus capable of realizing overheat protection that avoids excessive or too little overheat protections with a minimum increase in cost and calculation load. 
   According to the present invention, a brushless motor control apparatus calculates electric power or energy produced in a brushless motor based on a q-axis current, and protects the brushless motor or a motor control unit based on the magnitude of the electric power. 
   Since the q-axis current of the brushless motor can be treated as in the case of a detected current value of the direct current motor, the use of the q-axis current for overheat protection makes it possible to employ a technique of overheat protection of the direct current motor. Beside, the use of the q-axis current, which hardly varies unlike the three-phase current varying periodically, enables the sampling period to be made longer. 
   This can prevent an increase in the calculation load of a microcomputer and checks an increase in the cost resulting from additional parts, such as a thermistor. In addition, since the electric power produced in the brushless motor and the winding temperature are in a proportional relationship, the overheat protection can be performed accurately by calculating the electric power produced in the brushless motor. 
   Preferably, the brushless motor control apparatus detects the electrical angle of the brushless motor, and calculates the electric power produced in the brushless motor when it makes rotation equivalent to one cycle period of the electrical angle. 
     FIG. 4A  shows schematically the windings of the brushless motor. Currents flowing in U-phase, V-phase and W-phase windings are represented by Iu, Iv and Iw. The peak value of theses phase currents are represented by Im, respectively. Here, Iu, Iv and Iw are expressed as follows.
   Iu=Im× sin ω t     Iv=Im× sin(ω t− 2π/3)   Iw=Im× sin(ω t− 4π/3) 
The electric power Wa, Wb, and Wc that are produced in the respective phase windings are expressed as follows.
   Wu=Iu   2   ×R=Im   2   ×R× sin 2  (ω t )   Wv=Iv   2   ×R=Im   2   ×R× sin 2  (ω t −2π/3)   Ww=Iw   2   ×R=Im   2   ×R× sin 2  (ω t− 4π/3), 
where ωt is the electrical angle of the motor.
 
   Since the currents flowing in the U-phase, V-phase and W-phase windings are alternating currents, the electric power produced in the brushless motor when it makes rotation corresponding to one cycle period of the electrical angle can be used as references. Since the q-axis current is used as a current whereby the electric power is calculated, this construction makes it possible to calculate the produced electric power accurately and prevent an increase in calculation load. 
   Further preferably, the brushless motor control apparatus uses a predetermined coefficient for setting a predetermined value for overheat protection. This predetermined coefficient takes different values in two states: when the brushless motor is energized but not rotated; and when the brushless motor is energized and rotated. 
   For example, when an ideal state where the phases of the brushless motor have no deviations at all is considered, the d-axis current i d  becomes zero and the q-axis current i q  becomes √(3/2)×Im. With the q-axis current i q  being constant, in the case where the motor is locked when any one of the U-phase, V-phase and W-phase current is at a peak current (being energized but not rotated), the phase electric power produced during one cycle period of the electrical angle (ωt=0−2π) will be 2 π×Im 2 ×R by the formula 1. This is equivalent to the area of a rectangular portion formed by a short side part defined by the current values of 0 and Im and a long side part defined by angles of 0° and 360°. 
   At the time of rotation, the phase electric power produced during one cycle period of the electrical angle (ωt=0−2π) will be π×Im 2 ×R by the formula 2. This is equivalent to the area of a shaded portion in  FIG. 4B . Representing the above value using the q-axis current i q  that is actually used for the calculation instead of Im, the phase electric power of any one of the U-phase, V-phase and W-phase windings when the motor is locked at the peak current will be 2π×2/3×i q   2 ×R, and the phase electric power at the time of rotation will be π×2/3×i q   2 ×R. 
   
     
       
         
           
             
               
                 
                   
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   This shows a relationship that the electric power per phase in the case of motor locking at the peak current double the electric power per phase at the time of rotation. Therefore, assuming that the coefficient when the motor is locked at the peak current is 1, the coefficient at the time of rotation may be set to one half. Because all that is necessary is to set a predetermined coefficient used to multiply the electric power in advance by virtue of this construction, a threshold for the overheat protection can be set simply and hence increase in calculation load can be avoided. 
   Furthermore, there is a case where a current applied to each phase winding may change also with the locking position (electrical angle). Representing the peak current of each phase current by Im as shown in  FIG. 5 , when the motor is locked with an electrical angle of, for example, 30° (point A), the V-phase current is represented by Im, whereas the U-phase current and the W-phase current are represented by 0.5×Im (point C), respectively. 
   Moreover, when the motor is locked with an electrical angle of 60° (point B), the W-phase current is zero, whereas the U-phase current and the V-phase current are represented by 0.866×Im, respectively. So, each phase current at the time of motor locking differs from phase to phase. 
   Here, there is a predetermined mutual relationship among the U-phase, V-phase and W-phase currents of the brushless motor. That is, to calculate the electric power (estimated temperature) of each phase winding using the q-axis current, all that is necessary is to multiply the electric power when the motor is locked at the peak current by a coefficient (sin ωt) that varies with the electrical angle (ωt) of each phase. 
   By these steps, it becomes possible to reduce estimation errors considerably and calculate the electric power (estimated temperature) of each phase winding, and hence the same effect as is given by estimation calculation of the winding temperatures of the brushless motor using the three-phase alternating currents can be obtained. Therefore, it becomes possible to perform appropriate overheat protection based on the maximum of the estimation temperature of each phase winding. 
   Therefore, in implementing the overheat protection of the brushless motor by estimating the motor winding temperatures (electric power) using the q-axis current, i.e., a torque component current of the brushless motor, the use of information on the rotation state of the motor and the position of the rotor enables to reduce estimation errors considerably and avoid excessive or too little overheat protections, and also enables to prevent both increase in the cost resulting from additional parts and increase in the calculation load resulting from the use of the phase current. 
   The brushless motor control apparatus can be applied to electronic power steering apparatus of vehicles whereby a brushless motor is driven on energization and assisting steering torque is given to a steering mechanism triggered by driver&#39;s steering operations. 
   In the electronic power steering apparatus, there is a case where a vehicle is traveling with the steering maintained at a fixed angle. This case corresponds to the motor lock state described above. The rotation state of the motor varies rapidly with the operation state of the steering or a driving state. Even in such a case, the application of the brushless motor control apparatus can realize the overheat protection that avoids excessive or too little overheat protections. 
   In estimating the motor winding temperatures, the brushless motor control apparatus uses the q-axis current, which is a torque component current of the brushless motor and is conventionally proposed, or both the q-axis current and a d-axis current, which is an exciting component current. It additionally uses information on the rotation state of the motor and the position of the rotor (electrical angle). This enables estimation errors to be reduced considerably and the same effect as is given by a method of estimation calculation of the motor winding temperatures using the phase current to be obtained. 
   Consequently, it becomes possible to estimate a temperature (electric power) for each of the three-phase windings by virtue of this construction, select the highest temperature (largest electric power) among the three phases, and perform the overheat protection according to that temperature. This construction permits the overheat protection that avoids excessive or too little overheat protections while suppressing increase in cost and calculation load. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings: 
       FIG. 1  is a circuit diagram showing a three-phase brushless motor control apparatus of this invention; 
       FIG. 2  is a circuit diagram showing a drive circuit in  FIG. 1 . 
       FIG. 3  is a flowchart showing one embodiment of motor protection; 
       FIGS. 4A and 4B  are diagrams for explaining an electric power produced in the motor; 
       FIG. 5  is a diagram showing a relationship among phase currents; 
       FIG. 6  is a schematic diagram showing an electronic power steering control apparatus; and 
       FIG. 7  is a diagram showing the principle of vector control. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   [Embodiment] 
   A brushless motor control apparatus is shown in  FIG. 1 . Reference numeral  1  denotes a brushless motor, numeral  2  denotes a rotation angle sensor, such as a well known resolver for detecting electrical angle, numeral  3  denotes a controller, and numeral  4  denotes a current sensor. 
   The controller  3  comprises a drive circuit  31 , a PWM conversion unit  32 , a two-phase to three-phase conversion circuit  33 , a three-phase to two-phase conversion circuit  34 , a d-axis PI control unit  35  and a q-axis PI control unit  36  that perform well known PI (Proportional and Integral) control, subtraction circuits  37  and  38 , an instruction value arithmetic unit  39 , an electrical angle arithmetic unit  61 , an angular velocity arithmetic unit  64 , and an overheat protection arithmetic unit  65 , wherein a construction except for the overheat protection arithmetic unit  65  is the same as that of a known brushless motor control apparatus. The controller  3  includes a known microcomputer with an internal A/D (analog/digital) converter. The controller  3  may however be constructed with an exclusive hardware circuit. 
   As is well known, the drive circuit  31  includes a three-phase inverter circuit as shown in  FIG. 2 . Reference numerals  311 – 316  denote MOS power transistors, symbol D a flywheel diode, numeral  311  a U-phase upper arm element, numeral  312  a U-phase lower arm element, numeral  313  a V-phase upper arm element, numeral  314  a V-phase lower arm element, numeral  315  a W-phase upper arm element, and numeral  316  a W-phase lower arm element, wherein each of the elements  311 – 316  is connected to the flywheel diode D in reverse parallel individually. 
   Between a low electric potential power supply line LL and a high electric potential power supply line LH, a battery voltage is applied through an unillustrated smoothing circuit, and three-phase alternating voltages outputted from the drive circuit  31  (namely, a three-phase inverter) are applied to each end of a U-phase winding  11 , a V-phase winding  12  and a W-phase winding  13  of the motor  1 , individually. 
   The electrical angle arithmetic unit  61  generates a rotation angle signal θ from an analog rotation angle signal outputted from the rotation angle sensor, and outputs it to both the two-phase to three-phase conversion circuit  33  and the three-phase to two-phase conversion circuit  34 . The controller  3  performs PWM control of the three-phase brushless motor  1  based on the above phase current, the rotation angle signal and a torque instruction value externally applied. 
   The three-phase to two-phase conversion circuit  34  converts the applied three-phase current Iu, Iv and Iw into the q-axis current and the d-axis current based on the rotation angle signal ωt outputted from the rotation angle sensor  2 . The instruction value arithmetic unit  39  converts the torque instruction value externally applied into a q-axis current instruction value. 
   The subtraction circuit  38  performs PI conversion of a difference Δi q  between the q-axis current i q  and the q-axis current instruction value i qc  in the q-axis PI control unit  36 , and outputs it to the two-phase to three-phase conversion circuit  33 . The subtraction circuit  37  performs PI conversion of a difference Δi d  between the d-axis current i d  and the d-axis current instruction value i dc  in the d-axis PI control unit  35 , and outputs it to the two-phase to three-phase conversion circuit  33 . 
   The two-phase to three-phase conversion circuit  33  converts PI control quantities of Δi q  and Δi d  applied from the d-axis PI control unit  35  and the q-axis PI control unit  36 , respectively, based on the rotation angle signal outputted from the rotation angle sensor  2  to target three-phase voltages Vu, Vv and Vw (phase voltage instruction value) through two-phase to three-phase conversion, and outputs them to the PWM conversion unit  32 . 
   The PWM conversion unit  32  outputs PWM signals, i.e., PWMU, PWMV and PWMW, each of which has a duty ratio corresponding to the applied target three-phase voltages Vu, Vv and Vw to the MOS power transistors  311 – 316  of the drive circuit  31 . The drive circuit  31  drives the motor  1  by outputting the three-phase alternating voltages Vu, Vv and Vw to the motor  1 . 
   Each of PWMU, PWMV and PWMW of the PWM signals includes two PWM signals having mutually opposite signs. To be more precise, the PWM signals include six signals (gate voltages) of a PWMU upper signal, a PWMU lower signal, a PWMV upper signal, a PWMV lower signal, a PWMW upper signal and a PWMW lower signal. 
   These signals are applied to the elements as follows: the PWMU upper signal to the U-phase upper arm element  311 , the PWMU lower signal to the U-phase lower arm element  312 , the PWMV upper signal to the V-phase upper arm element  313 , the PWMV lower signal to the V-phase lower arm element  314 , the PWMW upper signal to the W-phase upper arm element  315  and the PWMW lower signal to the W-phase lower arm element  316 . 
   Since a drive system itself of the three-phase brushless motor by the PWM control described above and its various variations are well known, further detailed explanation is omitted. 
   The motor overheat protection processing will be described using  FIG. 1  and the flowchart of  FIG. 3 . This processing is repeatedly performed in the controller  3 . First, each of the phase current of the U-phase, V-phase and W-phase windings of the motor  1  is detected by the current sensor  4  (S 1 ), and subsequently the electrical angle ωt is calculated in the electrical angle arithmetic unit  61  based on the rotation angle of the motor  1  detected by the rotation angle sensor  2  (S 2 ). Then, the kwon three-phase to two-phase conversion is performed using the formula 3 based on the phase current values detected in the three-phase to two-phase conversion unit  34  to obtain the q-axis current i q  (S 3 ). 
   
     
       
         
           
             
               
                 
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   The angular velocity is calculated in the angular velocity arithmetic unit  64  (S 4 ) based on the rotation angle of the motor  1  detected by the rotation angle sensor  2 . The angular velocity can be calculated from a change in the rotation angle per unit time. 
   The overheat protection arithmetic unit  65 , first, multiplies a squared value of the q-axis current i q  by a previously stored electric resistance r of each phase winding of the motor  1  to calculate the most recent basic electric power Po of the phase winding resulting from the q-axis current i q  (S 5 ). 
   Next, whether the motor  1  is in the lock state is determined. 
   This is done as follows: when the angular velocity obtained in the above is less than a predetermined value or when the angular velocity is less than a predetermined value and a value of the q-axis current exceeds a predetermined value, the motor  1  is determined to be in the lock state; and otherwise, the motor  1  is determined to be rotating (S 6 ). 
   When the motor  1  is in the lock state (S 6 : YES), a predetermined coefficient Ko multiplied by a sine component (sin ωt) of the electrical angle ωt of each phase is specified as a coefficient K (S 7 ). 
   When the motor  1  is determined to be rotating (S 6 : NO), a predetermined coefficient Ko (e.g., Ko=1) multiplied by ½ is specified as a coefficient K common to all phases: U-phase, V-phase and W-phase (S 8 ). 
   A product obtained by multiplying the most recent basic electric power Po calculated previously by the coefficient obtained in the above is specified as the most recent electric power P (S 9 ). 
   The most recent value P is added to stored past history data of electric power and this history data is substituted into a previously stored calculation formula to calculate the temperature T of the phase winding (S 10 ). 
   Then, when any one of the calculated temperatures T of the phase windings exceeds a predetermined threshold temperature Tth stored in advance (S 11 : YES), the overheat protection arithmetic unit  65  sends a predetermined signal to the instruction value arithmetic unit  39 . The instruction value arithmetic unit  39  receives the signal and sends a value for reducing the current flowing in the motor  1 , i.e., a value that is smaller than the q-axis current instruction value and calculated according to the torque instruction value, to the subtraction circuit  38  as a q-axis current instruction value (S 12 ). As a result, the current flowing in the motor  1  decreases, the electric power of each phase winding decreases, and the calorific value reduces as well. 
   Example of Application to Electronic Power Steering Apparatus 
   The brushless motor control apparatus  3  is also suitable for an electronic power steering (EPS) apparatus of vehicles.  FIG. 6  is an outline block diagram of an electronic power steering apparatus  101  to which the brushless motor control apparatus  3  is applied. 
   A steering wheel  110  is coupled to a steering shaft  112   a  and the lower end of this steering shaft  112   a  is coupled to a torque sensor  111  for detecting a movement of the steering wheel  110  by a driver. The upper end of a pinion shaft  112   b  is coupled to the torque sensor  111 . A pinion (not illustrated) is provided on the lower end of the pinion shaft  112   b , and this pinion is engaged with a rack bar  118  in a steering gear box  116 . 
   Moreover, one end of each tie rod  120  is coupled to each end of the rack bar  118 , respectively. The other end of the each tie rod  120  is coupled to a steering wheel  124  through a knuckle arm  122 . Furthermore, an electric motor  115  that is the three-phase brushless motor is attached to the pinion shaft  112   b  through gears (not illustrated). A method of attaching the electric motor  115  to the rack bar  118  coaxially may be adopted. 
   A steering control unit  130  is equipped with a known CPU  131 , RAM  132 , ROM  133 , an I/O  134  serving as an input and output interface, and a bus line  135  connecting these components. The CPU  131  performs control with a program and data stored in the ROM  133  and the RAM  132 . The ROM  133  has a program storage area  133   a  and a data storage area  133   b . The program storage area  133   a  stores an EPS control program  133   p . The data storage area  133   b  stores data necessary for operations of the EPS control program  133   p.    
   Through execution of the EPS control program stored in ROM  133  by the CPU  131 , the steering control unit  130  calculates a drive torque that corresponds to the torque detected by the torque sensor  111  and should be produced by the electric motor  115 , and makes a motor driver  114  apply a voltage for producing the calculated drive torque to the electric motor  115 . 
   At this time, a resolver  109  detects a rotation angle of the electric motor  115  and a current sensor  108  detects a current flowing in the electric motor  115 , whereby the apparatus checks whether the motor is rotating that corresponds to the drive torque and calculates a voltage to be applied to the electric motor  115  according to the result. 
   That is, this electronic power steering apparatus detects a current flowing in the electric motor  115  with the current sensor  108 , subsequently calculates a voltage to be applied to the electric motor  115  and performs control for driving the electric motor  115  in the construction shown in  FIG. 1  and  FIG. 2 . 
   In this electronic power steering apparatus  101 , the vehicle may travel with the steering wheel  110  maintained at a fixed angle. This corresponds to the motor lock state described above. The rotation state of the electric motor  115  changes quickly depending on operation states of the steering wheel  110  or driving conditions. That is, by applying the brushless motor control apparatus, appropriate overheat protection that is responsive to operation states of the steering wheel  110  can be realized with excessive or too little overheat protections being avoided. 
   It is to be understood that these embodiments are only illustrative of the application of this invention and this invention is not limited to them, but that changes and modifications may be made based on knowledge of the those skilled in the art without departing from the spirit and scope of the invention as defined in the appended claims.