Abstract:
An electronic control unit for producing a sufficiently boosted voltage even when a supply voltage becomes high includes an inverter that drives switching elements and converts a DC voltage into an AC voltage, and booster means which boosts a DC voltage that is input into a voltage to lie within a voltage range in which a voltage necessary for driving the switching elements is a lower-limit value and a maximum boosted voltage is an upper-limit value, and outputs the boosted voltage to the inverter.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is based upon, claims the benefit of priority of, and incorporates by reference the contents of, Japanese Patent Application No. 2004-152411 filed on May 21, 2004.  
       FIELD OF THE INVENTION  
       [0002]     This invention relates to an electronic control unit and, particularly, to an electronic control unit which includes an electric power-assisted steering device and a transmission ratio-variable steering unit and controls an inverter.  
       BACKGROUND OF THE INVENTION  
       [0003]     An electronic control unit (hereinafter often referred to as ECU) requires various power source voltages depending upon the applications. Particularly, when a motor is to be driven by a known H-bridge or a three-phase bridge constituted by switching elements, there is usually used a method of boosting a power source voltage supplied to the ECU through a booster circuit as a gate drive source for the MOSFETs (metal oxide semiconductor field-effect transistors) which are the switching elements.  
         [0004]     The booster circuit boosts the supplied voltage to be roughly twice as great or more. There, however, exists an upper limit in the voltage (i.e., breakdown voltage) applied to the parts and elements constituting the booster circuit and the electric circuit such as the H-bridge or the three-phase bridge, and voltages in excess of the upper limit shall not be applied thereto. When a high voltage is supplied to the booster circuit, the voltage is further boosted and often exceeds the breakdown voltage causing the circuit to be broken down. So far, therefore, a monitoring function has been provided to monitor the supplied voltage, and the operation of the booster circuit is discontinued in case the supplied voltage exceeds a predetermined value.  
         [0005]     There have further been devised a semiconductor memory device which measures the time until the supplied voltage is boosted to a predetermined voltage and operates the booster circuit for the measured period of time only, and a circuit for forming the boosted voltage for the semiconductor memory device (see JP-A-2003-123495).  
         [0006]     In many cases, however, it has been demanded to obtain a predetermined boosted voltage despite an increase in the supplied voltage as a result of fluctuation. In an electric power-assisted steering device of a vehicle, for example, an alternator generates an overvoltage at the time of load dumping (in case a battery terminal is disconnected). Therefore, the power source voltage (voltage supplied from the battery) rises abruptly. In case the supplied voltage exceeds a predetermined value in the prior art, the operation of the booster circuit is discontinued to protect the circuit. Therefore, no voltage for driving is supplied to the switching elements of the inverter. When the supplied voltage becomes lower than the predetermined value, the booster circuit is driven again. While the booster circuit is not in operation, therefore, the motor is not driven by the inverter which is for assisting the steering force; i.e., a problem arouses in that the steer-assisting force abruptly decreases and the feeling of steering is deteriorated.  
         [0007]     The boosted voltage necessarily decreases when the booster circuit is no longer operated. However, the supplied voltage is an overvoltage generated by the alternator and does not decrease despite the booster circuit no longer being operated. Therefore, the ECU cannot monitor the supplied voltage to control the operation of the booster circuit so as not to interrupt the operation of the electric motor. This holds not only at the time of load dumping but also in the case of the ECU to which a high voltage is supplied at all times from the battery (e.g., when the battery voltage is 42 V).  
       SUMMARY OF THE INVENTION  
       [0008]     In view of the above problems, it is an object of the present invention to provide an electronic control unit which produces a sufficiently boosted voltage even when the supplied voltage becomes high, an electric power-assisted steering device, and a transmission ratio-variable steering unit.  
         [0009]     This invention provides an electronic control unit, an electric power-assisted steering device and a transmission ratio-variable steering unit for solving the above problems. That is, according to a first aspect of the invention, there is provided an electronic control unit comprising: 
        an inverter that drives the switching elements and converts a DC voltage into an AC voltage; and     booster means which boosts a DC voltage that is input into a voltage to lie within a voltage range in which a voltage necessary for driving the switching elements is a lower-limit value and a maximum boosted voltage is an upper-limit value, and outputs the boosted voltage to the inverter.        
 
         [0012]      FIG. 10  is a diagram illustrating a relationship between the supplied voltage and the boosted voltage. The boosted voltage (straight line  62 ) must be greater than a value (straight line  61 ) which is the sum of the battery voltage and a voltage Vgs (voltage across the gate and the source of MOSFET) for driving the switching elements  301  to  303  in a motor drive circuit shown in, for example,  FIG. 3 . Therefore, the lower-limit value of the supplied voltage necessary for the boosting becomes V 1 . In the prior art, the upper-limit value of the supplied voltage is V 2  with which the boosted voltage becomes the same as, or is slightly lower than, a breakdown voltage Vth 2  of the circuit or of the parts. Therefore, the boosting is not effected if the supplied voltage exceeds V 2 . In the present invention, on the other hand, in case the supplied voltage exceeds V 2 , the operation of the booster circuit is not readily stopped but, instead, booster means is driven and stopped at any time depending upon the state in which the voltage is boosted. This constitution makes it possible to produce the boosted voltage until the supplied voltage reaches V 3 . That is, the power source voltage can be selected over a wide range, and an electronic control unit can be shared among a plurality of systems having different power source voltages. Therefore, the electronic control unit can be produced at a decreased unit price owing to the effect of mass production. Further, since the boosted voltage does not exceed the breakdown voltage Vth 2 , the circuits or the parts are prevented from being broken down. Therefore, the circuits and the parts can be realized in small sizes to suppress the cost of production.  
         [0013]     According to a second aspect of the invention, the electronic control unit of the invention includes voltage monitoring means for monitoring a voltage value that is boosted, and the booster means discontinues the boosting when the monitored voltage becomes greater than a predetermined threshold value included in the voltage range, and effects the boosting when the monitored voltage becomes smaller than the threshold voltage. In this constitution, the boosted voltage is monitored, and the boosting circuit is driven and stopped at any time depending upon the monitored result to control the boosted voltage. This makes it possible to obtain a sufficiently highly boosted voltage that could not be obtained so far even when the supplied voltage is high, yet preventing the circuitry from being broken down that results when the breakdown voltages are exceeded. Besides, the boosted voltage is stabilized at a value which is the same as, or slightly lower than, the breakdown voltage Vth 2  of the circuit or of the parts eliminating the need of halting the operation of the circuit or the actuator to which the boosted voltage is to be supplied. This improves the stability and reliability of the system as a whole inclusive of the electronic control unit.  
         [0014]     According to a third aspect of the invention, the electronic control unit of the invention is constituted as an electric power-assisted steering device in a vehicle in which a motor is energized and driven based on the steering operation by a driver to give a steer-assisting torque to a steering mechanism, wherein the motor is driven by the inverter. This constitution makes it possible to supply, at all times, a voltage (boosted voltage) necessary for driving the switching elements of the inverter in the electric power-assisted steering device, enabling the required steer-assisting torque to be produced at all times without halting the operation of the switching elements and without deteriorating the feeling of steering.  
         [0015]     According to a fourth aspect of the invention, the electronic control unit of the invention is constituted as a transmission ratio-variable steering unit comprising an input shaft connected to the steering side, an output shaft connected to the side of the wheels to be steered, and a transmission ratio-variable mechanical unit which varies the rotational angle of the output shaft relative to the rotational angle of the input shaft by energizing and driving a motor, wherein the motor is driven by the inverter. This constitution makes it possible to supply, at all times, a voltage (boosted voltage) necessary for driving the switching elements of the inverter in the transmission ratio-variable steering unit, enabling the transmission ratio-variable mechanical unit to be in operation at all times without halting the operation of the switching elements and without deteriorating the feeling of steering. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a diagram illustrating the whole constitution of a transmission ratio-variable control unit according to an embodiment of the invention;  
         [0017]      FIG. 2  is a diagram illustrating a transmission ratio control unit in detail;  
         [0018]      FIG. 3  is a diagram illustrating a motor drive circuit in detail;  
         [0019]      FIG. 4  is a flowchart illustrating the operation of a booster circuit of the invention (embodiments 1 to 3);  
         [0020]      FIGS. 5A-5B  are diagrams illustrating the booster circuit of the invention in detail (embodiment 1);  
         [0021]      FIG. 6  is a diagram illustrating the booster circuit of the invention in detail (embodiment 2);  
         [0022]      FIG. 7  is a diagram illustrating the booster circuit of the invention in detail (embodiment 3);  
         [0023]      FIG. 8  is a flowchart illustrating the operation of a booster circuit according to a prior art;  
         [0024]      FIGS. 9A-9B  are diagrams illustrating the booster circuit according to the prior art;  
         [0025]      FIG. 10  is a diagram illustrating an effective range of boosting the voltage;  
         [0026]      FIG. 11  is a diagram illustrating the whole constitution of an electric power-assisted steering device;  
         [0027]      FIGS. 12A-12C  are diagrams illustrating how to detect a decrease in the boosted voltage according to the prior art; and  
         [0028]      FIGS. 13A-13C  are diagrams illustrating how to detect a decrease in the boosted voltage according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]     The object of boosting the voltage to a degree that is required and sufficient even when the supplied voltage becomes high, is realized by using an electronic control unit that changes over the drive and stop of the booster circuit relying upon a value of the boosted voltage, by using an electric power-assisted steering device and by using a transmission ratio-variable steering unit.  
         [0030]     Embodiments of the electronic control unit, electric power-assisted steering device and transmission ratio-variable steering unit of the invention will now be described with reference to the drawings.  
         [0031]      FIG. 1  is a diagram of when the electronic control unit of the invention is applied to the transmission ratio-variable steering unit of a vehicle, and  FIG. 2  is a block diagram illustrating the circuit constitution of the whole transmission ratio-variable steering unit  1 . The electronic control unit of the invention can also be applied to those other than the transmission ratio-variable steering unit of a vehicle, and there is no limitation on the objects to which the invention can be applied.  
         [0032]     Referring, first, to  FIG. 1 , a steering wheel  10  of a vehicle is connected to an upper end of an input shaft  11 . The lower end of the input shaft  11  and the upper end of an output shaft  13  are connected together through a transmission ratio-variable unit  12 . Further, a pinion that is not shown is provided at the lower end of the output shaft  13 , and is in mesh with a rack  16  in a steering gear box  15 . In the steering gear box  15 , further, there is provided an electric power-assisted steering device that is not shown. To both ends of the rack  16 , there are connected rolling wheels  17  to be steered via tie rods and arms that are not shown.  
         [0033]     A steering angle sensor  6  is provided on the input shaft  11  to detect the steering angle of the steering wheel  10 , while an output angle sensor  14  constituted by a resolver is provided on the output shaft  13  to detect the steered angle of the rolling wheels  17  to be steered, The output angle sensor  14  may be provided in the transmission ratio-variable unit  12 . The steering angle of the input shaft  11  and the rotational angle of the output shaft  13  detected by the steering angle sensor  6  and the output angle sensor  14  are input to the transmission ratio control unit  3  which, further, receives a vehicle speed signal and an engine rotational speed signal from a car-mounted LAN (local area network)  7 . The transmission ratio control unit  3  produces a control signal for controlling the transmission ratio-variable unit  12 .  
         [0034]     The transmission ratio-variable unit  12  includes a motor  4  which is a known brushless motor and a reduction mechanism  5 , and varies the rotational angle of the output shaft  13  by turning the motor  4  which is the brushless motor relying upon the signals from the steering angle sensor  6  and from the car-mounted LAN  7 .  
         [0035]     In the steering mechanism described above, first, upon receiving a vehicle speed signal from the car-mounted LAN  7  and a steering angle detected by the steering angle sensor  6 , the transmission ratio control unit  3  operates a target rotational angle of the output shaft  13  based on these data. A motor control instruction based on a target rotational angle is sent as a PWM signal (pulse width modulation signal) from the transmission ratio control unit  3  to the transmission ratio-variable unit  12 . The motor  4  in the transmission ratio-variable unit  12  is driven by the motor control instruction, and the rolling wheels  17  to be steered are imparted with a steering angle corresponding to the rotational angle obtained by adding the target rotational angle of the output shaft  13  and the steering angle of the steering wheel  10  up together. The transmission ratio control unit  3  effects the operation by feedback to estimate the real steered angle of the rolling wheels  17  to be steered from the output angle sensor  14  and to impart, to the rolling wheels  17  to be steered, the steering angle which reliably corresponds to the target rotational angle.  
         [0036]     Upon detecting the abnormal condition in the transmission ratio-variable unit  12 , the transmission ratio control unit  3  sends an instruction to a solenoid drive circuit  32  to interrupt the supply of current to a solenoid coil  2 . Therefore, the input shaft  11  and the output shaft  13  are coupled together to execute the operation without the transmission ratio-variable unit  12 .  
         [0037]     Next, the constitution for controlling the transmission ratio-variable steering unit  1  will be described with reference to  FIG. 2 . The solenoid coil  2  is connected to the solenoid drive circuit  32  of the transmission ratio control unit  3  that will be described later, and generates an electromagnetic force relying upon a drive signal from the solenoid drive circuit  32  to couple the input shaft  11  and the output shaft  13  together or to disconnect them from each other.  
         [0038]     The transmission ratio control unit  3  controls the current that flows into the solenoid coil  2  and controls the drive of the motor  4  based on the steering angle data from the steering angle sensor  6  and the data such as the vehicle speed from the car-mounted LAN  7 . The transmission ratio control unit  3  is constituted by a microcomputer  31 , the solenoid drive circuit  32 , a relay  33 , a relay drive circuit  34 , a power supply circuit  35 , a voltage detection circuit  36 , a communication I/P (interface)  38 , a steering angle detection circuit  39 , a motor drive circuit  50 , a current detection circuit  51 , a motor terminal voltage detection circuit  52 , and an electric angle detection circuit  53 .  
         [0039]     The microcomputer  31  operates currents that flow into the solenoid coil  2  and the motor  4  based on the steering angle data from the steering angle sensor  6  and on the data such as vehicle speed from the car-mounted LAN  7 , and outputs a control signal based on the operated value, and is constituted by a CPU, a ROM, a RAM, an input/output interface and a bus line for connecting them, which have been known but which are not shown here. Various operation processings based on the signals input to the microcomputer  31  and control signal output processings of the microcomputer  31  for the peripheral circuits, are executed by control programs stored in the ROM that are not shown.  
         [0040]     The relay  33  is provided between the battery  8  and the power supply circuit  35  to make a connection or a break between the battery  8  and the power supply circuit  35  thereby to supply the electric power to, or interrupt the supply of electric power from, the transmission ratio-variable steering unit  1 . The relay  33  is turned on or off by the relay drive circuit  34  which is operated by a control signal from the microcomputer  31 .  
         [0041]     The power supply circuit  35  is connected to the battery  8  via an IG switch  9 , and a current is supplied from the battery  8  to the microcomputer  31 . The voltage detection circuit  36  detects the voltage of the battery  8 , and inputs the detected value to the microcomputer  31 .  
         [0042]     A booster circuit  22  is provided in the motor drive circuit  50 , boosts the voltage fed from the battery  8  to a predetermined voltage and applies it to a MOSFET gate drive circuit  314  (see  FIG. 3 ) included in the motor drive circuit  50 .  
         [0043]     The communication I/F  38  converts the vehicle speed signal from the car-mounted LAN  7  and the engine rotational speed signal into those signals that can be processed by the microcomputer  31 , and inputs the thus converted vehicle speed signal and the engine rotational speed to the microcomputer  31 . The communication I/F  38 , further, receives a signal for forcibly varying the above target rotational angle from the car-mounted LAN  7  to suppress unstable behavior of the vehicle, and inputs this signal to the microcomputer  31 . The signal for forcibly varying the target rotational angle is input to the communication I/F  38  in case, for example, the steering wheels  17  to be steered are skidded.  
         [0044]     The steering angle detection circuit  39  converts the signal from the steering angle sensor  6  into a form that can be recognized by the microcomputer  31 , and inputs the converted steering angle signal to the microcomputer  31 .  
         [0045]     The motor drive circuit  50  has six switching transistors  301  to  306  that constitute a known three-phase bridge circuit as shown in  FIG. 3 , and drives the motor  4  by controlling the six switching transistors by varying the PWM duty ratios based on the drive signals from the microcomputer  31 .  
         [0046]     The current detection circuit  51  detects the currents flowing into the U-phase, V-phase and W-phase of the motor  4 , and inputs the detected current values to the AD converter  31   a  in the microcomputer  31 . The motor terminal voltage detection circuit  52  detects terminal voltages of the U-phase, V-phase and W-phase of the motor  4 , and inputs the detected voltages to the AD converter  31   a  in the microcomputer  31 . In this embodiment, the AD converter  31   a  is incorporated in the microcomputer  31 , which, however, may be provided outside the microcomputer  31 .  
         [0047]     The electric angle detection circuit  53  converts the rotational angle (electric angle) detected by the output angle sensor  14  connected to the reduction mechanism  5  through the output shaft  13  into a form that can be processed by the microcomputer  31 , and inputs the thus converted rotational angle signal to the microcomputer  31 .  
         [0048]     As for the operation of the transmission ratio-variable steering unit  1 , the microcomputer  31  outputs a control signal to the solenoid drive circuit  32  which permits a current to flow into the solenoid  2  to disconnect the input shaft  11  and the output shaft  13  from each other. The motor  4  is driven based on the signals from the steering angle sensor  6  and the car-mounted LAN  7 , and the rotational force of the motor  4  is transmitted to the output shaft  13  through the reduction mechanism  5 . The torque sensor in the electric power-assisted steering device (not shown) detects the torque which is obtained by adding together the torque of the output shaft  13  to which the rotational force of the motor  4  is transmitted and the steering force exerted by the driver, and the steered angle of the rolling wheels  17  to be steered is varied by the electric motor of the electric power-assisted steering device.  
         [0000]     (Prior Technology)  
         [0049]     For easy comprehension of the constitution of the present invention, described below with reference to  FIGS. 8 and 9 A- 9 B is an example of controlling the booster circuit  22  according to a prior art.  FIG. 8  is a flowchart illustrating the flow of control, and  FIGS. 9A-9B  are diagrams illustrating the booster circuit and a timing chart at the time when the booster circuit operates.  
         [0050]     In  FIG. 9A , a comparator  22   a  is a known operational amplifier to which a resistor and a capacitor that are not shown are connected to compare the voltages. To one input terminal of the comparator  22   a , there is input a voltage Ve (see  FIG. 13B ) divided from the supply voltage VB of the battery  8  and to the other input terminal thereof, there is input a threshold value Vth for monitoring a high voltage of the supply voltage VB. The threshold value Vth may be formed in the booster circuit  22  based on the supply voltage VB, or may be formed by the power supply circuit  35  or by the microcomputer  31 .  
         [0051]     When the voltage Ve divided from the supply voltage VB of the battery  8  is smaller than the threshold value Vth, the comparator  22   a  outputs the H-level. When the voltage Ve divided from the supply voltage VB of the battery  8  is larger than the threshold value Vth, the comparator  22   a  outputs the L-level. The output from the comparator  22   a  is input to an AND circuit  22   b  constituted by a known logic circuit to AND with the clock signal and turn switching elements T 1  and T 2  on/off based on the results thereof. Therefore, the clock signal becomes effective only when the comparator  22   a  is producing the H-level, and the switching elements T 1  and T 2  are turned on/off. Reference numerals  22   c  and  22   d  denote buffer circuits that render the output from the AND circuit  22   b  to assume a voltage level necessary for turning the switching elements T 1  and T 2  on or off.  
         [0052]     When the clock signal is of the L-level, the switching element T 1  is turned off and the switching element T 2  is turned on to establish a passage of battery  8  (supplied voltage VB)—diode D 1  for preventing the reversal flow—capacitor C 1 —switching element T 2 —GND (ground), and an electric charge is accumulated in the capacitor C 1  depending upon the supply voltage VB of the battery  8 . When the clock signal is of the H-level, on the other hand, the switching element T 1  is turned on and the switching element T 2  is turned off to establish a passage of capacitor C 1 —diode D 2  for preventing the reversal flow—capacitor C 2 , and the electric charge accumulated in the capacitor C 1  is accumulated in the capacitor C 2 .  
         [0053]     The capacitor C 2  is applied with the supply voltage VB of the battery  8  at all times. Therefore, when the electric charge accumulated in the capacitor C 1  is accumulated in the capacitor C 2 , an electric charge corresponding to 2×VB (V) is finally accumulated in the capacitor C 2 . The voltage becomes 2×VB (V) across the terminals of the capacitor C 2 ; i.e., this voltage (boosted voltage) is applied to the MOSFET gate drive circuit  314 .  
         [0054]     A flow of the drive control for the booster circuit  22  will be described with reference to the flowchart of  FIG. 8 . When the relay  33  is turned on to supply the voltage VB from the battery  8  (S 21 ), the booster circuit  22 , the battery voltage monitoring circuit included in the booster circuit  22 , the boosted voltage monitoring circuit included in the booster circuit  22 , the motor drive circuit  50  and the MOSFET gate drive circuit  314  are operated (S 22 ). Thereafter, as described earlier, an electric charge is accumulated in the capacitor C 1  and in the capacitor C 2 , and the voltage of the battery  8  is boosted. If the supply voltage VB from the battery  8  rises in this state (S 23 ), the boosted voltage (≈supply voltage VB of the battery  8 ×2) output from the booster circuit  22  is further increased (S 24 , corresponds to a region t 1  in  FIG. 9B ).  
         [0055]     The battery voltage monitoring circuit including the comparator  22   a  compares the voltage Ve divided from the battery voltage (supply voltage) VB with the threshold value Vth. When it is determined that the battery voltage is smaller than Vth 1  (S 25 : NO), the booster circuit  22  and the motor drive circuit  50  continue to operate (S 28 ). When it is determined that the battery voltage is greater than Vth 1  ( 525 : YES), on the other hand, the switching element T 1  is turned off and the switching element T 2  is turned on to discontinue the operation of the booster circuit  22 , and it is so determined that the supplied voltage VB is too large and an abnormal condition detection signal (too large supply voltage signal) is sent to the microcomputer  31 . The microcomputer  31  discontinues the operation of the motor drive circuit  50  (S 26 , corresponds to a region t 2  in  FIG. 9B ). It is so determined that the supply voltage is too large when the supply voltage VB is included in a region  68  of too large supply voltages.  
         [0056]     When it is so determined that the battery voltage becomes smaller than Vth 1  again in a state where the booster circuit  22  and the motor drive circuit  50  are not in operation, i.e., in a state where the battery voltage is greater than Vth 1  (S 27 ), the booster circuit  22  and the motor drive circuit  50  are operated again (S 27 →S 25 →S 28 , corresponds to a region t 3  in  FIG. 9B ).  
         [0057]     In  FIG. 13C , the comparator  22   g  is a known operational amplifier to which a resistor and a capacitor that are not shown are connected to compare the voltages. To one input terminal of the comparator  22   g  (voltage monitoring means of the invention), there is input a voltage Vc (see  FIG. 13A ) divided from the boosted voltage. To the other input terminal thereof, there is input a voltage Vd (see  FIG. 13B ) which is divided from the supply voltage, Upon comparing these values Vc and Vd, it can be examined if the voltage has been boosted relative to the supply voltage VB. When Vc is smaller than Vd, it is so determined that the voltage has not been properly boosted, and an abnormal condition detection signal (drop-of-boosted-voltage signal) is sent to the microcomputer  31 . The microcomputer  31  works to halt the operation of the motor drive circuit  50 .  
         [0058]     Referring to  FIG. 9B , a reference value (Vd) for comparison of the comparator  22   g  varies in proportion to the supply voltage VB and becomes as represented by a folded line  66 . Therefore, a region  67  where the boosted voltage decreases becomes a region where the values are smaller than the folded line  66 . Namely, when the boosted voltage lies in the region  67  where the boosted voltage decreases, it is so determined that the boosting operation has not been properly conducted.  
         [0059]     In  FIGS. 13A-13B , there is no particular limitation on the resistances R 1  to R 4  for dividing the boosted voltage, on the resistances R 5  to R 7  for dividing the supply voltage, or on the offset voltages v 1  and v 2  so far as the voltage Ve can be compared with the threshold value Vth by the comparator  22   a  and the voltage Vc can be compared with the voltage Vd by the comparator  22   g.    
       Embodiment 1 of the Invention  
       [0060]     A first embodiment of controlling the booster circuit  22  (booster means of the invention) by the method of the invention will be described with reference to  FIGS. 4, 5A  and  5 B.  FIG. 4  is a flowchart illustrating the flow of control operation, and  FIGS. 5A-5B  are diagrams illustrating the booster circuit and a timing chart of when the booster circuit operates. The diagram of  FIG. 5B  is a partial modification from the circuit diagram of  FIG. 9B , and is, hence, illustrated by using the same reference numerals. Further, the constitution same as that of the prior art is not described here again.  
         [0061]     In  FIG. 5A , the comparator  22   a  (voltage monitoring means of the invention) is a known operational amplifier to which a resistor and a capacitor that are not shown are connected to compare the voltages. To one input terminal of the comparator  22   a , there is input a voltage Va (see  FIG. 12A ) divided from the boosted voltage and to the other input terminal thereof, there is input a threshold value Vth for monitoring a high voltage of the boosted voltage. The threshold value Vth may be formed in the booster circuit  22  based on the supply voltage VB of the battery  8 , or may be formed by the power supply circuit  35  or by the microcomputer  31 .  
         [0062]     When the voltage Va divided from the boosted voltage is smaller than the threshold value Vth, the comparator  22   a  outputs the H-level. When the voltage Va divided from the boosted voltage is larger than the threshold value Vth, the comparator  22   a  outputs the L-level. The output from the comparator  22   a  is input to an AND circuit  22   b  constituted by a known logic circuit to find an AND with the clock signal to turn switching elements T 1  and T 2  on/off based on the results thereof. Therefore, the clock signal becomes effective only when the comparator  22   a  is producing the H-level, and the switching elements T 1  and T 2  are turned on/off. The operations of the switching elements T 1  and T 2 , and the flow of electric charge accumulated in the capacitors C 1  and C 2  (i.e., constitution of the boosting operation) are the same as those of the constitution of the prior art, and are not described here again.  
         [0063]     A flow of the drive control for the booster circuit  22  will be described with reference to the flowchart of  FIG. 4 . When the relay  33  is turned on to supply the voltage VB from the battery  8  (S 1 ), the booster circuit  22  the boosted voltage monitoring circuit included in the booster circuit  22 , the motor drive circuit  50  and the MOSFET gate drive circuit  314  are operated (S 2 ). Thereafter, as described above, an electric charge is accumulated in the capacitor C 1  and in the capacitor C 2 , and the supply voltage VB of the battery  8  is boosted. If the supply voltage VB from the battery  8  rises in this state (S 3 ), the boosted voltage (≈ supply voltage VB of the battery 8×2) output from the booster circuit  22  is further increased (S 4 , corresponds to a region t 1  in  FIG. 5B ).  
         [0064]     The boosted voltage monitoring circuit including the comparator  22   a  compares the voltage Va divided from the boosted voltage with the threshold value Vth. When it is determined that the boosted voltage is smaller than Vth 2  (S 5 : NO), the booster circuit  22  and the motor drive circuit  50  continue to operate (S 8 ). When it is determined that the boosted Voltage is greater than Vth 2  (S 5 : YES), on the other hand, the comparator  22   a  produces the L-level to invalidate the clock signal, whereby switching element T 1  is turned off and the switching element T 2  is turned on to discontinue the operation of the booster circuit  22 . However, the motor drive circuit  50  continues to operate (S 6 , corresponds to a region t 2  in  FIG. 5B ). When it is so determined that the boosted voltage becomes smaller than Vth 2  again in a state where the booster circuit  22  is not in operation (S 7 ), the comparator  22   a  produces the H-level to validate the clock signal whereby the switching elements T 1  and T 2  are turned on/off. The booster circuit  22  is operated again (S 7 →S 5 →S 8 , corresponds to a region t 3  in  FIG. 5B ). As shown in  FIG. 5B , the booster circuit  22  operates (corresponds to a region t 5  in  FIG. 5B ) and stops (corresponds to a region t 4  in  FIG. 5B ) repetitively to continuously produce the boosted voltage as a value close to Vth 2 .  
         [0065]     In  FIG. 12C , the comparator  22   g  and the comparator  22   h  (voltage monitoring means of the invention) are known operational amplifiers to which resistors and capacitors that are not shown are connected to compare the voltages. To one input terminal of the comparator  22   g , there is input a voltage Vc (see  FIG. 12A ) divided from the boosted voltage. To the other input terminal thereof, there is input a voltage Vd (see  FIG. 12B ) divided from the applied voltage VB. Further, to one input terminal of the comparator  22   h , there is input a voltage Vb (see  FIG. 12A ) divided from the boosted voltage. To the other input terminal thereof, there is input a threshold value Vth for monitoring a low voltage of the boosted voltage. The outputs of the comparators  22   g  and  22   h  are input to an AND circuit  22   i  including a known AND element.  
         [0066]     Upon comparing the above values, it can be examined if the voltage has been properly boosted. When Vb is smaller than the threshold value Vth and Vc is smaller than Vd, it is so determined that the voltage has not been properly boosted, and an abnormal condition detection signal (drop-of-boosted-voltage signal) is sent to the microcomputer  31 . The microcomputer  31  works to halt the operation of the motor drive circuit  50 .  
         [0067]     Referring to  FIG. 5B , a reference value (Vd) for comparison of the comparator  22   g  varies in proportion to the supply voltage VB and becomes as represented by a straight line  63 . On the other hand, the reference value (Vth) for comparison of the comparator  22   h  assumes a constant value and becomes as represented by a straight line  64 . Therefore, a region  65  where the boosted voltage decreases becomes a region where the boosted voltages are smaller than the values of the straight lines  63  and  64 . Namely, when the boosted voltage lies in the region  65  where the boosted voltage decreases, it is so determined that the boosting operation has not been properly conducted.  
         [0068]     In  FIGS. 12A-12C , there is no particular limitation on the resistances R 1  to R 4  for dividing the boosted voltage, on the resistances R 5  to R 7  for dividing the supply voltage VB, or on the offset voltages v 1  and v 2  so far as the voltage Va can be compared with the threshold value Vth by the comparator  22   a , the voltage Vc can be compared with the voltage Vd by the comparator  22   g , and the voltage Vb can be compared with the threshold value Vth by the comparator  22   h.    
       Embodiment 2 of the Invention  
       [0069]     A second embodiment of controlling the booster circuit  22  (booster means of the invention) by the method of the invention will be described with reference to  FIG. 6 .  FIG. 6  is a partial modification from the circuit diagram of  FIG. 5 , and is, hence, illustrated by using the same reference numerals. The portions overlapping those of the embodiment 1 of the invention are not described here again.  
         [0070]     In  FIG. 6 , the comparator  22   e  (voltage monitoring means of the invention) is a known operational amplifier to which a resistor and a capacitor that are not shown are connected to compare the voltages. To one input terminal of the comparator  22   e , there is input a voltage Va (see  FIG. 12A ) divided from the boosted voltage and to the other input terminal thereof, there is input a threshold value Vth for monitoring a high voltage of the boosted voltage. The threshold value Vth may be formed in the boosting circuit  22  based on the supply voltage VB of the battery  8 , or may be formed by the power supply circuit  35  or by the microcomputer  31 .  
         [0071]     When the voltage Va divided from the boosted voltage is smaller than the threshold value Vth, the comparator  22   e  outputs the H-level. When the voltage Va divided from the boosted voltage is larger than the threshold value Vth, the comparator  22   e  outputs the L-level. The output from the comparator  22   e  is input to a known semiconductor switch or to a switching circuit S 5  constituted by a relay circuit or the like. The switching circuit S 5  is turned on by the output of the H-level of the comparator  22   e  and is turned off by the output of the L-level.  
         [0072]     The constitution in which the electric charge is accumulated in the capacitor C 1  and in the capacitor C 2  by the turn on/off operation of the switching element T 1  and of the switching element T 2 , is the same as that of the embodiment 1 of the embodiment, and is not described here again.  
         [0073]     A flow of the drive control for the booster circuit  22  will be described with reference to the flowchart of  FIG. 4 . When the relay  33  is turned on to supply the voltage VB from the battery  8  (S 1 ), the boosted voltage monitoring circuit included in the booster circuit  22 , the booster circuit  22 , the motor drive circuit  50  and the MOSFET gate drive circuit  314  are operated (S 2 ). Here, the switching circuit S 5  is turned on. Thereafter, as described above, an electric charge is accumulated in the capacitor C 1  and in the capacitor C 2 , and the voltage of the battery  8  is boosted. If the supply voltage VB from the battery  8  is boosted in this state (S 3 ) the boosted voltage (≈supply voltage VB of the battery 8×2) output from the booster circuit  22  is further increased (S 4 , corresponds to a region t 1  in  FIG. 5B ).  
         [0074]     The boosted voltage monitoring circuit including the comparator  22   e  compares the voltage Va divided from the boosted voltage with the threshold value Vth. When it is determined that the boosted voltage is smaller than Vth 2  ( 55 : NO), the comparator  22   e  produces the H-level to turn the switching circuit S 5  on, and the booster circuit  22  and the motor drive circuit  50  continue to operate (S 8 ). When it is determined that the boosted voltage is greater than Vth 2  ( 55 : YES), on the other hand, the comparator  22   e  produces the L-level to turn the switching circuit S 5  off thereby to interrupt the supply of electric power from the battery  8 .  
         [0075]     Therefore, even when the switching element T 1  is turned off and the switching element T 2  is turned on, there is not established the passage of battery  8  (supply voltage VB)—diode D 1 —capacitor C 1 —switching element T 2 —GND (ground), and no electric charge is accumulated in the capacitor C 1 . There is not established, either, the passage of battery  8  (supply voltage VB)—switching element T 1 —capacitor C 1 —diode D 2 , and the electric charge accumulated in the capacitor C 1  is not accumulated in the capacitor C 2 . When the switching circuit S 5  is turned off, the booster circuit  22  ceases to operate. However, the motor drive circuit  50  continues to operate (S 6 , corresponds to a region t 2  in  FIG. 5B ).  
         [0076]     When it is so determined that the boosted voltage becomes smaller than Vth 2  again in a state where the booster circuit  22  is not in operation (S 7 ), the comparator  22   e  produces the H-level to turn the switching circuit S 5  on whereby the electric power is supplied again from the battery  8  and the booster circuit  22  is operated again (S 7 →S 5 →S 8 , corresponds to a region t 3  in  FIG. 5B ). As shown in  FIG. 5B , the electric power is supplied from the battery  8  (corresponds to a region t 5  in  FIG. 5B ) and is interrupted (corresponds to a region t 4  in  FIG. 5B ) repetitively to continuously produce the boosted voltage as a value close to Vth 2 .  
         [0077]     The method of determining whether the voltage has been boosted to a degree that is necessary and sufficient is the same as the one described in the embodiment 1 of the invention with reference to  FIGS. 5B and 12 , and is not described here again in detail.  
       Embodiment 3 of the Invention  
       [0078]     A third embodiment of controlling the booster circuit  22  (booster means of the invention) by the method of the invention will be described with reference to  FIG. 7 .  FIG. 7  is a partial modification from the circuit diagram of  FIGS. 5A-5B , and is, hence, illustrated by using the same reference numerals. The portions overlapping those of the embodiment 1 of the invention are not described here again.  
         [0079]     In  FIG. 7 , the comparator  22   f  (voltage monitoring means of the invention) is a known operational amplifier to which a resistor and a capacitor that are not shown are connected to compare the voltages. To one input terminal of the comparator  22   f , there is input a voltage Va (see  FIG. 12A ) divided from the boosted voltage and to the other input terminal thereof, there is input a threshold value Vth for monitoring a high voltage of the boosted voltage. The threshold value Vth may be formed in the booster circuit  22  based on the supply voltage VB of the battery  8 , or may be formed by the power supply circuit  35  or by the microcomputer  31 .  
         [0080]     When the voltage Va divided from the boosted voltage is smaller than the threshold value Vth, the comparator  22   f  outputs the H-level. When the voltage Va divided from the boosted voltage is larger than the threshold value Vth, the comparator  22   f  outputs the L-level. The output from the comparator  22   f  is input to known semiconductor switches or switching circuits S 1 , S 2 , S 3  and S 4  constituted by relay circuits or the like. These four switching circuits are turned on by the output of the H-level of the comparator  22   f  and are turned off by the output of the L-level.  
         [0081]     The constitution in which the electric charge is accumulated in the capacitor C 1  and in the capacitor C 2  by the turn on/off operation of the switching element T 1  and of the switching element T 2 , is the same as that of the embodiment 1 of the embodiment, and is not described here again.  
         [0082]     A flow of the drive control for the booster circuit  22  will be described with reference to the flowchart of  FIG. 4 . When the relay  33  is turned on to supply the voltage VB from the battery  8  (S 1 ), the booster circuit  22 , the boosted voltage monitoring circuit included in the booster circuit  22 , the motor drive circuit  50  and the MOSFET gate drive circuit  314  are operated (S 2 ). Here, the switching circuits S 1 , S 2 , S 3  and  54  are turned on. Thereafter, as described above, an electric charge is accumulated in the capacitor C 1  and in the capacitor C 2 , and the voltage of the battery  8  is boosted. If the supply voltage VB from the battery  8  is boosted in this state (S 3 ), the boosted voltage (≈supply voltage VB of the battery 8×2) output from the booster circuit  22  is further increased (S 4 , corresponds to a region t 1  in  FIG. 5B ).  
         [0083]     The boosted voltage monitoring circuit including the comparator  22   f  compares the voltage Va divided from the boosted voltage with the threshold value Vth. When it is determined, that the boosted voltage is smaller than Vth 2  (S 5 : NO), the booster circuit  22  and the motor drive circuit  50  continue to operate (S 8 ). When it is determined that the boosted voltage is greater than Vth 2  (S 5 : YES), on the other hand, the comparator  22   f  produces the L-level to turn the switching circuits S 1 , S 2 , S 3  and S 4  off. Therefore, no passage is formed for accumulating the electric charge in the capacitor C 1 , and no electric charge is accumulated in the capacitor C 1 . There is not established, either, the passage from the capacitor C 1  to the capacitor C 2 , and the voltage is not boosted. However, the motor drive circuit  50  continues to operate (S 6 , corresponds to a region t 2  in  FIG. 5B ).  
         [0084]     When it is so determined that the boosted voltage becomes smaller than Vth 2  again in a state where the booster circuit  22  is not in operation (S 7 ), the comparator  22   f  produces the H-level to turn the switching circuits S 1 ,  52 , S 3  and S 4  on whereby the booster circuit  22  is operated again (S 7 →S 5 →S 8 , corresponds to a region t 3  in  FIG. 5B ). As shown in  FIG. 5B , the electric power is supplied from the battery  8  (corresponds to a region t 5  in  FIG. 5B ) and is interrupted (corresponds to a region t 4  in  FIG. 5B ) repetitively to continuously produce the boosted voltage as a value close to Vth 2 .  
         [0085]     In the circuit diagram of  FIG. 7 , it is allowable to employ any one of the constitution having switching circuits S 1  and S 2  only, the constitution having switching circuits S 3  and S 4  only, or the constitution having any one of S 1  to S 4 .  
         [0086]     Further, the method of determining whether the voltage has been boosted to a degree that is necessary and sufficient is the same as the one described in the embodiment 1 of the invention with reference to  FIGS. 5B and 12 , and is not described here again in detail.  
         [0000]     (Application to the Electric Power-Assisted Steering Device)  
         [0087]     The rotation detecting device of the invention can be preferably applied to the electric power-assisted steering (EPS) device for a vehicle.  FIG. 11  is a diagram schematically illustrating the constitution of an electric power-assisted steering device  101 . A steering wheel  110  is connected to a steering shaft  112   a  which is connected at its lower end to a torque sensor  111  that detects the motion of the steering wheel  110 . An upper end of a pinion shaft  112   b  is connected to the torque sensor  111 . Further, a pinion (not shown) is provided at the lower end of the pinion shaft  112   b , and is brought into mesh with a rack bar  118  in a steering gear box  116 . Further, the ends on one side of tie rods  120  are connected to both ends of the rack bar  118 , and rolling wheels  124  to be steered are connected to the ends on the other side of the tie rods  120  via knuckle arms  122 . Further, a motor  115  is attached to the pinion shaft  112   b  via a gear (not shown). The motor  115  may be mounted in concentric with the rack bar  118 .  
         [0088]     A steering control unit  130  includes a CPU  131 , a RAM  132 , a ROM  133 , an I/O  134  which is an input/output interface, and a bus line  135  for connecting them, which have been known. The CPU  131  executes the control operation relying upon the program and data stored in the ROM  133  and the RAM  132 . The ROM  133  includes a program storage region  133   a  and a data memory region  133   b . The program storage region  133   a  stores an EPS control program  133   p . The data storing region  133   b  stores the data necessary for operating the EPS control program  133   p.    
         [0089]     In the steering control unit  130 , the CPU  131  executes the EPS control program stored in the ROM  133  to calculate a drive torque to be produced by the motor  11 . 5 , that corresponds to a torque detected by the torque sensor  111  and to a steering angle detected by the steering angle sensor  113 . A voltage is applied to the motor  115  via the motor drive circuit  114  to produce the drive torque as calculated.  
         [0090]     The steering control unit  130  and the motor drive circuit  114  in the electric power-assisted steering device  101  is constituted in nearly the same manner as those of the transmission ratio-variable control unit  1  of  FIG. 1 . Therefore, the electronic control unit of the invention can be applied thereto.  
         [0091]     In the foregoing were described the embodiments of the invention, which, however, are merely examples and to which only the invention is in no way limited. The invention can be further modified in a variety of ways based on the knowledge of a person skilled in the art without departing from the spirit and scope of the invention.