BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a control apparatus of an electric power steering apparatus (EPS) for applying a steering assist force obtained by a motor of a steering system of an automobile or a vehicle.  
         [0003]     2. Description of the Related Art  
         [0004]     An electric power steering apparatus for assisting a steering apparatus of an automobile of a vehicle by rotation force of a motor applies a driving force of the motor to a steering shaft or a rack shaft by a transmitting mechanism such as gears and a belt through a speed reducer. Since such a conventional electric power steering apparatus precisely generates assist torque (steering assist force), a feedback control of a motor current is performed. In the feedback control, the motor applied voltage is adjusted such that a difference between a current command value and a motor current detection value becomes small. The motor applied voltage is generally adjusted by adjusting a duty ratio of PWM (pulse width modulation) control.  
         [0005]     Here, a general structure of the electric power steering apparatus will be explained with reference to  FIG. 1 . A column shaft  52  of a steering wheel  51  is connected to a tie rod  56  of wheels to be steered through speed reduction gears  53 , universal joints  54   a  and  54   b  and a pinion rack mechanism  55 . The column shaft  52  is provided with a torque sensor  57  for detecting a steering torque of the steering wheel  51 . A motor  58  for assisting a steering force of the steering wheel  51  is connected to the column shaft  52  through the speed reduction gears  53 . Electric power is supplied from a battery  60  to a control unit  59  which controls the electric power steering apparatus, and an ignition signal is inputted to the control unit  59  from an ignition key  61 . The control unit  59  calculates a steering assist command value I of assist command based on a steering torque T detected by the torque sensor  57  and a vehicle speed V detected by a vehicle speed sensor  63 , and current to be supplied to the motor  58  is controlled by the calculated steering assist command value I.  
         [0006]     As shown in Japanese Patent No. 3508702 B2, the electric power steering apparatus of the above kind includes an analog to digital converting means (refer to “ADC”, hereinafter) for converting magnitude of the motor current into a digital amount, and a current control means for controlling the motor current in accordance with the digital amount of the motor current converted by the ADC. The electric power steering apparatus detects a steering state, switches converting means used for controlling the motor current based on the steering state, and can selects the conversion digital amounts.  
         [0007]     According to the above conventional electric power steering apparatus, an output of a current detecting amplifier for detecting a current value flowing through the electric motor is inputted to a microcomputer as it is, and its signal is used for controlling the motor current as a current detection signal.  
         [0008]     As the output of the motor is increased, a measuring range of the current detection is increased. However, resolution of an ADC is relatively roughened. This is because that although ADCs having high resolution are available in recent market, but if cost up or using track record is taken into consideration, conventional products are still used because it is difficult to immediately change the ADC and meet a high resolution requirement.  
         [0009]     However, concerning the steering performance of the electric power steering apparatus, steering feeling in the small current region has become increasingly valued, and there is a problem that the steering feeling is deteriorated if the conventional resolution remains as it is.  
       SUMMARY OF THE INVENTION  
       [0010]     Hence, it is an object of the present invention to make the steering characteristics in a small current region excellent by correcting voltage in the small current region.  
         [0011]     The above object of the present invention is achieved by providing a control apparatus of an electric power steering apparatus for feedback controlling operation of a motor by a microcomputer (including a micro controller unit (MCU)) based on a current value of the motor detected by a current detector, wherein an arbitrary function generator of a current detecting characteristics is provided between a current detecting amplifier of the current detector and the microcomputer, and a motor current detection characteristics is corrected.  
         [0012]     The above object of the present invention is also achieved by providing a control apparatus of an electric power steering apparatus for feedback controlling operation of a motor by a microcomputer based on a current value of the motor detected by a current detector, wherein an arbitrary function generator of a current detecting characteristics is provided between a current detecting amplifier of the current detector and the microcomputer, both a signal whose motor current detecting characteristics is corrected and an output signal of the current detecting amplifier whose motor current detecting characteristics is not corrected are inputted to the microcomputer.  
         [0013]     Further, the above object is effectively achieved if the corrected current detecting signal is used for controlling a current of a small current region, and the non-corrected current detecting signal is used for controlling a current from an intermediate current region to a large current region. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     In the accompanying drawings:  
         [0015]      FIG. 1  is a diagram showing an example of a structure of a general electric power steering apparatus;  
         [0016]      FIG. 2  is a block diagram showing an outline structure of a control apparatus of an electric power steering apparatus according to the present invention;  
         [0017]      FIG. 3  is a diagram for schematically explaining a motor current detecting characteristic;  
         [0018]      FIG. 4  is a diagram showing a relation of a detection voltage with respect to a motor current detected by a current detecting amplifier;  
         [0019]      FIG. 5  is a diagram showing a current detecting characteristic for explaining a relation of the detection voltage with respect to the motor current with linear indication when the detection data is corrected by the arbitrary function generator;  
         [0020]      FIG. 6  is a diagram of a current detecting characteristic for explaining a relation of detection voltage with respect to the motor current with logarithm indication when detection data is corrected by an arbitrary function generator;  
         [0021]      FIG. 7  is a diagram showing that a current detection characteristic of the arbitrary function generator is made variable, and an output signal of the current detecting amplifier is sent to the microcomputer; and  
         [0022]      FIG. 8  is a diagram showing a relation of detection voltage with respect to the motor current in a state where the current detecting characteristic of the arbitrary function generator are made variable.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]     An embodiment of the present invention will be explained with reference to the drawings.  
         [0024]      FIG. 2  is a block diagram showing an outline structure when a control apparatus of an electric power steering apparatus according to the present invention is applied to a vehicle. A control section  10  includes a microcomputer (or MCU)  11 , a motor drive circuit  12  comprising a plurality of switching elements, and a current detecting circuit  13  for detecting a current flowing through a motor  30 .  
         [0025]     The microcomputer  11  includes a CPU  21 , an ADC  22  for converting an analog signal into a digital signal, an input interface  23 , an output interface  24  and a memory  25  comprising a ROM and a RAM. The CPU  21  controls the motor drive circuit  12  through the output interface  24  based on an input signal from the input interface  23 .  
         [0026]     As shown in  FIG. 3 , the current detecting circuit  13  is connected to the motor drive circuit  12 . The current detecting circuit  13  includes a register (shunt register)  31 , and a current detecting amplifier  32  which is connected to both ends of the register  31  and which detects motor current. The current detecting amplifier  32  detects voltages of both ends of the shunt register  31 , amplifies the voltages with predetermined amplification factor, and inputs the same to the microcomputer  11 .  
         [0027]     An arbitrary function generator  35  which generates a current detecting characteristic as an arbitrary function is provided between the microcomputer  11  and an output side of the current detecting amplifier  32 . An output signal of the current detecting amplifier  32  is inputted to the microcomputer  11 , and an arbitrary signal from the arbitrary function generator  35  is also inputted to the microcomputer. The arbitrary function generator  35  optimally corrects the motor current detecting characteristic.  
         [0028]     The arbitrary function generator  35  comprises a logarithmic amplifier and so on. The arbitrary function generator  35  has a logarithmic characteristic as the current detecting characteristic and can correct voltage from a small current region to a large current region of the motor current. That is, the logarithmic amplifier amplifies logarithm in such a manner that a small signal is amplified largely and a large signal is amplified small. Therefore, a current detecting signal obtained by the current detecting amplifier  32  can be corrected more finely than in a region from an intermediate current to a logarithmic characteristic in a small current region of the motor current.  
         [0029]     For example,  FIG. 4  shows a motor current detecting characteristic of the current detecting circuit  13 , and shows a relation of a detected voltage with respect to the motor current. In this case, it can be found that if the output of the current detecting circuit  13  is corrected by the arbitrary function generator  35 , since the arbitrary function generator  35  such as the logarithmic amplifier has a logarithmic characteristic, the output is largely amplified in the small current region and the output is amplified only little in the large current region as shown in  FIG. 5  (linear indication) and  FIG. 6  (semi-logarithm indication).  
         [0030]     With this, in the motor control of the electric power steering apparatus, if the current detecting characteristic is switched to the logarithmic characteristic, it is possible to precisely control the operation of the motor  30  in a small current region such as near the neutral point of a steering wheel where the motor current becomes small. That is, if the output from the current detecting amplifier  32  is sent to the microcomputer  11  in such a manner that the current detecting characteristic is switched in accordance with the magnitude of the motor current by the arbitrary function generator  35  in a small current region and that the arbitrary function generator  35  is bypassed in a region from the intermediate current region to the large current region, it is possible to precisely control the operation of the motor  30  in the small current region where the motor current becomes small, and an excellent steering feeling can be obtained in the small current region.  
         [0031]     Therefore, the arbitrary function generator  35  is provided between the current detecting amplifier  32  and the microcomputer  11 , the current detecting characteristic of the arbitrary function generator  35  has a logarithmic characteristic. Accordingly, it is possible to finely correct a current detecting signal of the current detecting amplifier  32  in the small current region where the motor current is small. That is, a control can be performed such that in the region from the intermediate current region to the large current region of the motor current, the output from the current detecting amplifier  32  is inputted to the microcomputer  11  as it is, and the current detecting characteristic is switched to the logarithmic characteristic in the small current region where the motor current is small such as a region in the vicinity of the neutral point of the steering wheel. Thus, the steering feeling becomes excellent in a region near the neutral point of the steering wheel and when the motor current is small and the motor current must be controlled finely.  
         [0032]      FIG. 7  shows a modification in which the current detecting characteristic of the arbitrary function generator  35  is made variable in accordance with magnitude of the motor current. In this modification, the arbitrary function generator  35  is provided between the current detecting amplifier  32  and the microcomputer  11 , and both a signal whose motor current detecting characteristic is corrected and an output signal of the current detecting amplifier  32  whose motor current detecting characteristic is not corrected are inputted to the microcomputer  11 . As shown in  FIG. 8 , for example, the current detecting characteristic of the arbitrary function generator  35  can be made variable, and the current detecting characteristic in the small current region can be set different from those in other regions. In this example, a relation of a detected voltage with respect to the motor current is set such that the small current region (Y=2X) is different from a region from the intermediate current region to the large current region (Y=X) with respect to the point “t”. Thus, the bypass path of the arbitrary function generator  35  can be omitted, and the same effect as that of the above embodiment can be obtained.  
         [0033]     The characteristic of the arbitrary function generator  35  can arbitrarily be varied based on commands from the CPU  21 , the characteristic can be selected from small, large various kinds of current regions in accordance with a using purpose, and the steering feeling becomes more excellent.  
         [0034]     According to the control apparatus of the electric power steering apparatus of the present invention as described above, the arbitrary function generator such as a logarithmic amplifier is provided between the current detecting amplifier and the microcomputer, and the output signal from the current detecting amplifier is corrected as a logarithmic characteristic. With this, if the motor current detecting characteristic is switched to the logarithmic characteristic, an output voltage of the current detecting amplifier can finely corrected. Especially, in the vicinity of a neutral point of a steering wheel, such as in a small current region where a vehicle is in a driving state where the motor current must be controlled finely, an excellent steering feeling can be obtained. If the current detecting characteristic is made variable in accordance with the magnitude of the motor current with respect to the arbitrary function generator, a bypass path of the arbitrary function generator can be omitted.
It is an object of the present invention to make steering characteristics excellent in a small current region by correcting voltage in the small current region. A control apparatus of an electric power steering apparatus for feedback controlling operation of a motor by a microcomputer based on a current value of the motor detected by a current detector, wherein an arbitrary function detector of current detection characteristics is provided between a current detection amplifier of the current detector and the microcomputer, and motor current detection characteristics are corrected.