Current control for electric power steering apparatus motor

In an electric power steering apparatus using a direct drive electric motor for assisting steering force, the electric motor is operated with three-phase alternating current obtained from a direct-current power supply through an inverter, the three-phase alternating current being controlled by detecting the value of the direct current being supplied to the inverter. The difference between the detected direct current and a target current calculated on the basis of detected vehicle speed and steering torque is determined, based on which the three-phase alternating current is controlled.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an electric power steering apparatus which 
uses an electric motor to provide power to assist the steering force for a 
vehicle. 
2. Description of the Related Art 
An electric power steering apparatus has been developed in which the speed 
of the vehicle and the steering torque applied to the steering wheel are 
detected and, when the detected torque exceeds a prescribed dead zone, a 
driving current, whose magnitude is determined according to the detected 
torque and vehicle speed, is supplied to drive a steering assisting 
electric motor whose rotating force is used to assist the force required 
for the vehicle steering, thereby providing a comfortable steering feel to 
the driver. 
Such an electric power steering apparatus usually employs a direct drive 
motor as the steering assisting electric motor. The direct drive motor is 
suitable for use as a steering assisting electric motor since it does not 
require the use of a reduction gear and thus has the advantage of reducing 
the size of the motor-related hardware. In the electric power steering 
apparatus equipped with such a direct drive motor, a motor current 
controller sets a target current according to the detected torque and 
vehicle speed, and the direct drive motor is operated with three-phase 
alternating current supplied through an inverter according to the target 
current. However, the problem with this type of steering apparatus has 
been that the output torque of the electric motor is affected by a 
variation in the battery supply voltage as well as by the load change of 
the steering system, thereby causing an unnatural steering feel. 
One previous approach to resolving this problem has been by current 
feedback control wherein the value of the three-phase alternating current 
for operating the motor is detected and the result of the detection is fed 
back to the motor current controller to control the three-phase 
alternating current in such a manner as to correct any deviation between 
the detected three-phase alternating current value and the target current 
value. More specifically, a current detecting resistor is inserted in an 
electric circuit path between the inverter and the electric motor; the 
three-phase alternating current value is detected using this current 
detecting resistor, and the thus detected three-phase alternating current 
value is dq converted by software (coordinate transformation of rotor 
windings to the static coordinate system), the dq current obtained as a 
result of the conversion being fed back to the motor current controller. 
However, the above current feedback control of the prior art requires the 
use of a high-precision current sensor for detecting the three-phase 
alternating current value, but such a high-precision current sensor is 
large in size and not suitable for use in an electric power steering 
apparatus where miniaturization of hardware is a desired condition. 
Another problem with the prior art is that the complicated feedback 
control involving dq-converting the detected three-phase alternating 
current value increases the complexity of the software. This has created 
the further problem that the increased complexity of the software 
decreases the control response. 
OBJECTS OF THE INVENTION 
It is an object of the present invention to provide an electric power 
steering apparatus that can eliminate the unnatural steering feel caused 
by a variation in the supply voltage or by the load change of the steering 
system. 
It is another object of the invention to provide an electric power steering 
apparatus that makes it possible to reduce the size of the hardware for 
controlling the current supplied to the steering assisting motor. 
It is a further object of the invention to provide an electric power 
steering apparatus that makes it possible to simplify the software for 
controlling the current supplied to the steering assisting motor. 
It is yet another object of the invention to provide an electric steering 
power apparatus that makes it possible to increase the control response 
for controlling the current supplied to the steering assistance motor. 
According to the present invention, an electric power steering apparatus 
has means for detecting the direct current being supplied to an inverter 
and means for controlling three-phase alternating current on the basis of 
the detected direct current. 
In the electric power steering apparatus, an unnatural steering feel is 
caused by a variation in the supply voltage or by the load change of the 
steering system. The variation in the supply voltage and the load change 
of the steering system both introduce a variation in the direct current 
being supplied to the inverter. In the electric power steering apparatus 
of the present invention, since the three-phase alternating current for 
operating the electric motor is controlled by detecting the direct current 
being supplied to the inverter, the direct current supplied to the 
inverter is prevented from varying due to a variation in the supply 
voltage or to a load change of the steering system, thus preventing an 
unnatural feel from being introduced into the steering feel. Furthermore, 
the current sensor used for controlling the motor current is one that 
detects direct current, which contributes to reducing the hardware size 
compared with the one used in the prior art. Moreover, the value of 
three-phase alternating current is controlled on the basis of the 
detection result of direct current. Since there is no need for complicated 
processing such as dq conversion required in the prior art apparatus, the 
control software can be simplified and thus, the control response is 
increased.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will now be described in detail by way of example and 
with reference to the accompanying drawing. 
Shown in FIG. 1 is a control system in the electric power steering 
apparatus of the present invention. In FIG. 1, the reference numeral 1 
designates a microcomputer for steering assisting control; within the 
microcomputer 1, the software elements are represented by functional 
blocks. A torque detection signal from a torque sensor 2 that detects the 
steering torque and a vehicle speed detection signal from a vehicle speed 
sensor 3 that detects the vehicle speed are analog-to-digital converted by 
an A/D converter 4 and fed to a target current calculation section 1a of 
the microcomputer 1. The target current calculation section 1a performs a 
prescribed operation on the basis of the supplied torque detection signal 
and vehicle speed detection signal, to determine a target current Ir(n) 
for operating a direct drive motor DM. The target current Ir(n) determined 
by the target current calculation section 1a is supplied to a current 
control section 1b. 
Both electrodes of a battery 9, which supplies power to the direct drive 
motor DM, are connected to an inverter 10 formed from power MOSFET 
circuits which are used for energization control of the direct drive motor 
DM. The negative electrode of the battery 9 is also grounded. Inserted in 
a direct-current circuit for the inverter 10 is a current detecting 
resistor 11 for detecting the supply current (the direct current supplied 
to the inverter 10), the current detecting resistor 11 being connected to 
a current detecting circuit 12. The current detecting circuit 12 detects 
the supply current value from the voltage detected across the current 
detecting resistor 11. The detected current value is analog-to-digital 
converted by an A/D converter 13 and supplied as a detected supply current 
If(n) to the current control section 1b of the microcomputer 1. 
The current control section 1b calculates the deviation of the detected 
supply current If(n) from the target current It(n), and the result of the 
calculation is supplied as a deviation current Id(n) to a three-phase 
voltage calculation section 1c. 
Mounted on the direct drive motor DM is a rotational position sensor 14, 
constructed from an absolute encoder, for detecting the absolute value of 
the rotational position of the direct drive motor DM. The rotational 
position data detected by the rotational position sensor 14 is supplied as 
a detected rotational position signal .theta. m to an offset circuit 15 
where the offset of the signal is adjusted. The signal with its offset 
adjusted is supplied as a rotational position signal .theta. to a 
sine-wave generating ROM 16. Of the three-phase sinusoidal waveforms 
having the phase angle given by the rotational position signal .theta., 
the sine-wave generating ROM 16 contains sinusoidal wave data for two 
phases [sin.theta., sin(.theta.+2/3.pi.)], the rotational position signal 
.theta. being used as the address. The sinusoidal wave data for two phases 
corresponding to the supplied rotational position signal .theta. are read 
into the three-phase voltage calculation section 1c. 
In the three-phase voltage calculation section 1c, the deviation current 
1d(n) is translated into a deviation voltage Vd(n) based on which 
operations of the following equations (1) to (3) are performed to 
determine three-phase voltages for supply to the direct drive motor DM. 
Equation (1) below is an operation expression for determining a sinusoidal 
voltage Vu(n) for the U phase of the direct drive motor DM; the sinusoidal 
voltage Vu(n) is calculated from the deviation voltage Vd(n) and the 
sinusoidal wave sin.theta. supplied from the sine-wave generating ROM 16. 
EQU Vu(n)=Vd(n).multidot.sin.theta. (1) 
Equation (2) below is an operation expression for determining a sinusoidal 
voltage Vv(n) for the V phase of the direct drive motor DM; the sinusoidal 
voltage Vv(n) is calculated from the deviation voltage Vd(n) and the 
sinusoidal wave sin(.theta.+2/3.pi.) supplied from the sine-wave 
generating ROM 16. 
EQU Vv(n)=Vd(n).multidot.sin(.theta.+2/3.pi.) (2) 
Equation (3) below is an operation expression for determining a sinusoidal 
voltage Vw(n)for the W phase of the direct drive motor DM; the sinusoidal 
voltage Vw(n) is calculated from the sinusoidal voltage Vu(n) obtained 
from equation (1) and the sinusoidal voltage Vv(n) obtained from equation 
(2). 
EQU Vw=-[Vu(n)+Vv(n)] (3) 
The three-phase sinusoidal voltages Vu(n), Vv(n), and Vw(n) obtained from 
the above operations by the three-phase voltage calculation section 1c are 
digital-to-analog converted by a D/A converter 5 and supplied to a 
sine-wave PWM circuit 6. Also supplied to the sine-wave PWM circuit 6 is a 
trigonometric wave signal generated by a trigonometric wave generator 7. 
The sine-wave PWM circuit 6 compares the three-phase sinusoidal voltages 
with the trigonometric wave signal and, based on the results of 
comparison, PWM signals for controlling the direct drive motor DM are 
supplied to a power MOSFET driver circuit 8. Based on the PWM signals, the 
power MOSFET driver circuit 8 drives power MOSFETs (not shown) in the 
inverter 10. The inverter 10 is formed from three series circuits of power 
MOSFETs, the three circuits being connected in parallel. The power MOSFET 
driver circuit 8 commutatively controls these power MOSFETs to supply the 
three-phase sinusoidal current to the direct drive motor DM, thus driving 
the direct drive motor DM. 
In the electric power steering apparatus having the above configuration, 
when there occurs a variation in the supply voltage from the battery 9 or 
when the load change of steering system is caused, the supply current 
exhibits a variation. The variation of the supply current is detected by 
the current detecting circuit 12, in accordance with which the detected 
supply current If(n) varies. The current control section 1b calculates the 
deviation current Id(n) to compensate for the variation of the detected 
supply current If(n), and supplies the deviation current Id(n) to the 
three-phase voltage calculation section 1c. The three-phase voltage 
calculation section 1c transforms the supplied deviation current Id(n) 
into the three-phase sinusoidal voltages which are then supplied to the 
sine-wave PWM circuit 6 through the D/A converter 5. The three-phase 
sinusoidal voltages ape thus corrected to compensate for the variation of 
the supply current, thus correcting for the variation of the supply 
current caused by a variation in the supply voltage from the battery 9 or 
by the load change of steering system. That is, in the above control 
system, feedback control is performed with respect to the supply current. 
In the above supply current feedback control, when there occurs a variation 
in the supply voltage from the battery 9 or when the load change of 
steering system is caused, the resulting supply current variation is 
corrected as described above. This contributes to preventing an unnatural 
feel from being introduced in the steering feel by a variation in the 
supply voltage from the battery 9 or by the load change of steering 
system. 
As described above, according to the present invention, the direct current 
being supplied to the inverter is detected in order to control the 
three-phase alternating-current voltages. Since the detection of direct 
current can be accomplished by a small-size current sensor, the hardware 
can be reduced in size, and since the control of three-phase alternating 
current is performed on the basis of the direct current being supplied to 
the inverter, i.e., the detection result of the direct current, the above 
configuration serves to eliminate the need for complicated processing, 
such as dq conversion in the prior art, and offers excellent advantages 
such as simplified software and hence, increased response of control. 
As this invention may be embodied in several forms without departing from 
the spirit of essential characteristics thereof, the present embodiment is 
therefore illustrative and not restrictive, since the scope of the 
invention is defined by the appended claims rather than by the description 
preceding them, and all changes that fall within metes and bounds of the 
claims, or equivalence of such metes and bounds thereof are therefore 
intended to be embraced by this claims.