Abstract:
An inverter circuit includes a bridge circuit constituted of a plurality of pairs of a high-side switching element and a low-side switching element, a command signal processing section, a pulse generating section for generating pulse signals to control the inverter bridge circuit according to the command signal to have a dead time to prevent short circuiting of the dc power source and a command signal compensation section. The compensation section modifies the command signal according to a current voltage level of the dc power source to control the dead zone, thereby preventing deformation of ac output power of the bridge circuit.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   The present application is based on and claims priority from Japanese Patent Application 2005-255380, filed Sep. 2, 2005, the contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an inverter circuit for converting dc electric power to ac electric power, which is to be supplied to a motor for a steering power assisting system. 
   2. Description of the Related Art 
   Generally, an inverter circuit includes an inverter constituted of a plurality of series-connected pairs of a high-side switching element and a low-side switching element and a PWM (pulse width modulation) control circuit. The inverter is controlled by the PWM control circuit to convert dc power into ac power as disclosed in JP-A 2004-201414. If each of the high-side and low-side switching elements is a MOSFET, the drain of the low-side switching element is grounded, the source of the positive side MOSFET is connected to a battery terminal, and the gate of each MOSFET is connected to the PWM control circuit. 
   In order to prevent short circuiting of the inverter from the battery terminal to the ground, it is necessary to provide a time lag or a dead time in the switching operation between the high-side switching elements and the low-side switching elements. Therefore, the inverter circuit can not provide its output current during the dead time even when the PWM control circuit sends a command voltage signal to the inverter circuit. As a result, the output ac power of the inverter circuit includes a waveform distortion, which may cause torque ripples when the output ac power is supplied to an ac motor. In JP-A 2004-201414, the waveform distortion is compensated or corrected by calculating a voltage deviation based on the command voltage signal, an output ac voltage of the inverter and an input dc voltage. However, the above way of compensation can not work very well if the dc power voltage changes. 
   The inventors conducted various tests and found that a dead zone of operation of an inverter, which is caused by the dead time, changes as the input dc power source voltage changes. 
   SUMMARY OF THE INVENTION 
   Therefore, an object of the invention is to provide an improved inverter circuit that can provide stable output ac power even if the voltage of an input power source fluctuates. For this purpose, the dead zone is controlled according to the voltage of the dc power source. 
   According to a feature of the invention, an inverter circuit includes an inverter constituted of a plurality of pairs of a high-side switching element and a low-side switching element connected in series with each other to form a pair of input terminals connected with a dc power source, a command signal processing section for providing a command signal to set a prescribed amount of the ac output power of the inverter, a pulse generating section for generating pulse signals for controlling the inverter according to the command signal to have a dead time to prevent short circuiting of the dc power source thereby forming a dead zone of operation, and a command signal compensation section for modifying the command signal according to a current voltage level of the dc power source to control the dead zone. 
   Because the command signal is modified according to the voltage level of the dc power source, the pulse signal is modified to control the inverter to provide the ac output power having a waveform that does not cause torque ripples of an ac motor to be connected to the inverter. 
   In the inverter circuit as featured above: the pulse generating section may include a booster circuit for boosting voltage of the dc power source to generate the pulse signal for controlling the inverter; the command signal compensation section may include a memory that stores data of the dead zone; the inverter is connected to an electric motor to drive the same; the inverter includes three pairs of a high-side switching element and a low-side switching element and the electric motor is a three-phase ac motor; the dc power source is a battery mounted in a vehicle. 
   Another object is to drive an electric motor that is free from torque ripples. 
   According to another feature of the invention, an inverter circuit includes a three-phase inverter, including a three-phase bridge circuit of switching elements, input terminals connected with a battery and output terminals connected with an electric motor, a command signal processing section for providing a command signal to control the electric motor; a pulse generating section for generating pulse signals for controlling the inverter according to the command signal to have a dead period to prevent the inverter from short circuiting of the battery, whereby the inverter has a dead zone of operation, and a command signal compensation section for modifying the command signal according to a voltage level of the battery to control the dead zone. 
   In the above inverter circuit: the command signal processing section may provide the command signal according to signals of a steering angle sensor and a vehicle speed sensor; the command signal compensation section may include a memory that stores data of the dead zone relative to the voltage level of the battery. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and characteristics of the present invention as well as the functions of related parts of the present invention will become clear from a study of the following detailed description, the appended claims and the drawings. In the drawings: 
       FIG. 1  is a power steering system to which a power assisting motor according to a preferred embodiment of the invention is applied; 
       FIG. 2  is a block diagram of an inverter circuit for driving the motor shown in  FIG. 1 ; 
       FIG. 3  is a graph showing a relationship between output voltages of a booster circuit and battery voltages; and 
       FIG. 4  is a graph showing a dead zone voltage curve and a compensation curve relative to battery voltages. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A preferred embodiment of the present invention will be described with reference to the appended drawings. 
   As shown in  FIG. 1 , a power steering system  10  includes a steering wheel  12 , vehicle wheels  14  an input shaft  16 , an output shaft  18 , a transfer ratio control unit  30 , a rack-and-pinion steering gear box  40 , a steering angle sensor  42 , a vehicle speed sensor  46  and a rack shaft  48 . 
   The transfer ratio control unit  30  is constituted of a motor unit  20  and a speed reduction mechanism  32 , which is constituted of a planetary gear mechanism. 
   The power steering system assists a driver to steer vehicle wheels  14  by a steering wheel  12 , which is linked with the vehicle wheels  14  via the input shaft  16 , the speed reduction mechanism  32 , the output shaft  18 , the rack-and-pinion steering gear box  40  and the rack shaft  48 . The rack shaft  48  is connected with the vehicle wheels  14  via tie-rods (not shown). 
   The motor unit  20  includes an ac motor  22  that drives the speed reduction mechanism  32 , an inverter circuit  24  and a motor-rotation angle sensor  44 . The inverter circuit  24  controls rotation angle of the motor  22  so as to change the transfer ratio G of the output shaft  18  to the input shaft  16 . 
   The motor  22  is a permanent magnet type brushless motor that has a three-phase stator having three phase-coils. The motor  22  may have four or more phase-coils or may be a induction type motor. 
   The steering angle sensor  42  detects the rotation angle θ h  of the input shaft  16  or the steering wheel  12  and sends its output signal to the inverter circuit  24 . The motor-rotation angle sensor  44  detects the rotation angle (electric angle) θ m  of the motor  22  and sends the output signal thereof to the inverter circuit  24 . The vehicle speed sensor  46  detects the speed V of a vehicle and sends its output signal to the inverter circuit  24 . The inverter circuit  24  includes a microcomputer that is constituted of a CPU, a ROM, a RAM, etc, and calculates the output rotation angle θ p  of the output shaft  18  and steered angle of the wheels  14  based on the rotation angle θ m  of the motor  22  and the transfer ratio G of the output shaft  18 . 
   The inverter circuit  24  calculates the transfer ratio G of the transfer ratio control unit  30  based on the vehicle speed V. The inverter circuit  24  also calculates and the variation of the output rotation angle θ p  based on the transfer ratio G and the variation of the rotation angle θ h . A target motor rotation angle θ mm  is calculated based on a difference between an actual value of the motor-rotation angle θ m  and an actual value of the output rotation angle θ p  so that the output rotation angle θ p  can equal to a calculated value. A command voltage Vq* is calculated based on the target motor rotation angle θ mm . Then, an amount of motor current (Iu, Iv, Iw) of a sine wave is supplied to each phase coil of the three-phase stator winding. 
   When a vehicle stops or runs at a low speed, the power steering system  10  reduces steering work of a driver by controlling the motor  22  to change the transfer ratio G so as to increase the steered angle of the wheels  14  relative to the rotation angle θ h . On the other hand, the power steering system  10  increases the steering work by controlling the motor  22  to change the transfer ratio G so as to decrease the steered angle of the wheels  14  relative to the rotation angle θ h  when the vehicle is runs at a high speed. 
   As shown in  FIG. 2 , the inverter circuit  24  includes a command voltage processing section  50 , a compensation section  54 , a pulse generating section  56 , an inverter  60  and a memory  62 . 
   The pulse generating section  56  includes a pulse modulating circuit  57  and a driving circuit  58 . Incidentally, the driving circuit  58  includes a booster circuit B. The inverter  60  is a three-phase bridge circuit that is constituted of three high-side switching elements (e.g. MOSFET) T 1 , T 3 , T 5  and three low-side switching elements (e.g. MOSFET) T 2 , T 4 , T 6 . Each of the switching elements T 1 -T 6  has a gate connected to the driving circuit  58  to receive from it one of driving signals UH, VH, WH, UL, VL, WL. Incidentally, each of the switching elements may be an insulated gate bipolar transistor (IGBT). 
   The driving signals UH, VH, WH applied to the gates of the high-side switching elements are boosted by the booster circuit according to the battery voltage, as shown in  FIG. 3 . On the other hand, the driving signals UL, VL, WL applied to the gates of the low-side switching elements have the same voltage as the battery. While the driver circuit respectively applies the driving signals to the gates of the switching elements, the switching elements turn on or off to form three-phase motor currents Iu, Iv, Iw, which are supplied to the three phase-coils of the motor  22 . 
   The command voltage processing section  50  includes a command voltage processing circuit  51  and a two-phase-to-three-phase converting circuit  52 . The command voltage processing circuit  51  calculates a command voltage Vq* to equalize the rotation angle θ m  of the motor  22  with the target motor rotation angle θ mm  based on the rotation angle θ h  of the input shaft  16 , the rotation angle θ m  and the vehicle speed V. The command voltage Vq* is outputted as a q-axis voltage to the two-phase-to-three-phase converting circuit  52 . 
   The two-phase-to-three-phase converting circuit  52  converts the command voltage Vq* to three phase command voltages Vu*, Vv*, Vw* based on the rotation angle θ m  of the motor  22 . The three phase command voltages Vu*, Vv*, Vw* are inputted to the compensation section  54 , which compensates the three-phase command voltages Vu*, Vv*, Vw* based on compensation data stored in the memory  62  to provide compensated command voltages Vu 1 , Vv 1 , Vw 1 , which are sent to the pulse modulating circuit  57  of the pulse generating section  56 . In the compensation section  54 , a compensation value is added to each of the three phase command voltages Vu*, Vv*, Vw*. 
   As shown in  FIG. 4 , the compensation value is selected from the compensation data (curve R) stored in the memory  62  based on the battery voltage (dc source voltage). The curve R is formed from a dead zone curve S, which is obtained from a test in which the dead time, the booster voltage, battery voltage, etc., are changed. The compensation value equalizes the level of the dead zone at a current battery voltage to the level of the dead zone when the battery voltage is 14 volts, which may be changed to 12 volts, 13 volts or other voltage according to a design policy or other circumstances. 
   For example: if the battery voltage is 14 volts, the compensation value is 0 (volt) to be added to the command voltages Vu*, Vv*, Vw*; and if the battery voltage is 11 volts, the compensation value is 0. 25 (volts) to be added to the command voltages Vu*, Vv*, Vw* so that the level of 0.5 volts on the dead zone curve S can be reduced by the level of 0.25 volts on the curve R of the compensation data to be the same level as the level of 0.25 volts on the dead zone S at 14 volts. Incidentally, the compensation has to be carried out so that the absolute value of the compensated command voltages Vu 1 , Vv 1 , Vw 1  can be larger than the command voltages Vu*, Vv*, Vw*. In other words, if the command voltages Vu*, Vv*, Vw* are negative, the compensation value to be added has to be negative. 
   The pulse modulating circuit  57  converts the compensated phase voltages Vu 1 , Vv 1 , Vw 1  into duty ratios (%). That is, the compensated phase voltage Vu 1  is converted into a U-PWM signal, the compensated phase voltage Vv 1  is converted into a V-PWM signal, and the compensated phase voltage Vw 1  is converted into a W-PWM signal, which are sent to the driving circuit  58 . 
   The driving circuit  58  provides driving signals UH, VH, WH, UL, VL, Wl to control the switching elements T 1 -T 6 . 
   Thus, the motor current Iu, Iv, Iw can be controlled to be free from the fluctuation of the battery voltage, so that the waveform distortion and the torque ripples can be minimized. 
   The above compensation can be carried out by utilizing the relationship between the battery voltage and the output voltage of the booster circuit shown in  FIG. 3 . In this case, the compensation value is set according to the output voltage of the booster circuit. 
   In the foregoing description of the present invention, the invention has been disclosed with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made to the specific embodiments of the present invention without departing from the scope of the invention as set forth in the appended claims. Accordingly, the description of the present invention is to be regarded in an illustrative, rather than a restrictive, sense.