Control device for power steering apparatus

A control device for a power steering apparatus includes a vehicle speed signal generator, a steering angle signal generating circuit, a first pattern memory, a second pattern memory, and a drive circuit. The vehicle speed signal generating circuit generates a vehicle speed signal corresponding to an actual vehicle speed. The steering angle signal generator detects a steering angle and generates a steering angle signal. The first pattern memory generates a power steering signal determined in correspondence with the vehicle speed signal. The second pattern memory generates a correction signal determined in correspondence with the vehicle speed signal and the steering angle signal representing a steering angle with respect to a neutral position of a steering wheel. The drive circuit outputs a correction result by correcting the power steering signal by the correction signal.

BACKGROUND OF THE INVENTION 
The present invention relates to a control device for a power steering 
apparatus. 
A steering force in a vehicle normally decreases when the vehicle speed 
increases. In a conventional power steering apparatus for giving an 
auxiliary power corresponding to the steering force, if a small steering 
force is set at a low vehicle speed, a steering force required at a high 
vehicle speed is undesirably too small to drive safely at the high speed. 
Various conventional devices have proposed to decrease a ratio of 
auxiliary power input to output in response to an increase in vehicle 
speed. These devices aim at requiring a sufficiently small steering force 
at the low vehicle speed but not a small steering force at the high 
vehicle speed. 
It is desirable to increase the steering force when the steering angle is 
increased with respect to the neutral position thereof. It is preferable 
to increase the steering force when the angle of the steering wheel turned 
by a driver is increased. This effect is typically preferred during 
high-speed driving. However, in a conventional power steering apparatus, 
an increase in steering force upon turning of the steering wheel is small. 
Strong demand has arisen for improving steering force control from the 
viewpoint of safety and steering feeling. In particular, the small 
increase in steering force during high-speed driving upon turning of the 
steering wheel leads to excess turning and hence a traffic accident. In 
order to avoid such danger, large steering forces are preset at both high 
and low vehicle speeds in some conventional power steering control 
devices. However, the large steering force leads to uncomfortable feeling 
during continuous steering with small steering angles. High-speed cruising 
therefore causes easy tiredness of drivers. 
SUMMARY OF THE INVENTION 
It is a principal object of the present invention to provide a control 
device for a power steering apparatus, wherein optimal steering forces 
corresponding to any driving conditions guarantee comfortable, safe 
driving. 
In order to achieve the above object of the present invention, there is 
provided a control device for a power steering apparatus, comprising: a 
vehicle speed signal generating circuit for generating a vehicle speed 
signal corresponding to an actual vehicle speed; a steering angle signal 
generator for detecting a steering angle and generating a steering angle 
signal; means for generating a power steering signal determined in 
correspondence with the vehicle speed signal; means for generating a 
correction signal determined in correspondence with the vehicle speed 
signal and the steering angle signal representing a steering angle with 
respect to a neutral position of a steering wheel; and means for 
outputting a correction result by correcting the power steering signal by 
the correction signal. 
According to the present invention, the optimal steering force can be 
obtained by generating a correction signal on the basis of an 
instantaneous vehicle speed and a steering angle and adding the correction 
signal to a vehicle speed response designation value.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows a control device for a power steering apparatus according to 
an embodiment of the present invention, and FIG. 2 shows a power steering 
apparatus employing the control device in FIG. 1. In order to best 
understand the present invention, the overall configuration of the power 
steering apparatus including the control device as the main feature of 
this invention will be first described with reference to FIG. 2. 
Referring to FIG. 2, the power steering apparatus includes a control device 
CONT in FIG. 1, a control valve CV, a vehicle speed sensor 1a, and a 
steering sensor 2a for generating a steering angle signal. In the control 
device CONT, a current i in FIG. 3 is supplied to the control valve CV on 
the basis of a steering angle signal generated by the steering sensor 2a 
and a vehicle speed (V) signal from the vehicle speed sensor 1a. The 
control valve CV controls a steering force in response to the current 
signal from the control device CONT. The larger the current becomes, the 
larger the steering force becomes. For this reason, if the vehicle speed 
is low, the corresponding steering force is small. In addition, the 
current change corresponding to the change in steering angle is small. The 
change in steering angle corresponding to the change in steering angle is 
small. However, when the vehicle speed is increased, the change in 
steering force corresponding to the change in steering angle is increased. 
A larger steering force is required at the high vehicle speed than that at 
the low vehicle speed. The steering force is abruptly increased when the 
steering angle is increased. Therefore, excess turning of the steering 
wheel during high-speed driving can be prevented, and the driver can 
recognize straight driving and obtain comfortable steering feeling. 
Control of the characteristics in FIG. 3 by the control device CONT in FIG. 
1 will be described below. Referring to FIG. 1, a vehicle speed pulse 
generator 1 includes a reed switch (i.e., a vehicle speed sensor) 1a, a 
resistor 1b, and a waveshaper 1c. The reed switch 1a repeats on/off 
operation upon rotation of a magnet (not shown) mounted on the 
transmission output shaft. The waveshaper 1c generates short vehicle speed 
pulses at the off timings of the reed switch 1a. A steering angle signal 
generator 2 includes a potentiometer 2a as a steering sensor, and an A/D 
converter for converting an analog signal from the potentiometer 2a to a 
digital signal representing one of the values (e.g., 0 to 255). The 
potentiometer 2a includes a main body and a slider C. The main body 
consists of a slip ring SL and a resistor R which are mounted on the 
steering column. The slider C is rotated together with the steering shaft. 
An instantaneous vehicle speed signal generator 3 includes a counter 3a 
and a register 3b. The counter 3a counts 1-ms clock pulses CL1 from a 
clock pulse generator 7. The count of the counter 3a is stored in the 
register 3b in response to the vehicle speed pulse from the vehicle speed 
pulse generator 1. The counter 3a is then reset. Therefore, if the vehicle 
speed is low, the instantaneous vehicle speed signal generator 3 generates 
a large count and the count data is updated in response to each vehicle 
speed pulse. 
A pattern memory 4 stores a vehicle speed response designation value (i.e., 
a signal for determining a power steering angle at the start of steering) 
for the instantaneous vehicle speed. A pattern memory 5 stores correction 
signals for correcting the vehicle speed response designation values, as 
shown in FIG. 6. Each correction signal is determined by the corresponding 
instantaneous vehicle speed and the corresponding steering angle. In this 
case, the steering angle is used as a variable and the instantaneous 
vehicle speed is used as a parameter. A characteristic curve a in FIG. 6 
represents correction values at the maximum vehicle speed; e, correction 
values at the minimum vehicle speed; and b to d, correction values at the 
intermediate vehicle speeds. 
A drive circuit 6 includes adders 6a and 6c, a register 6b, a timer 6d, a 
transistor 6e, an A/D converter 6f, a comparator 6g, an up/down counter 
6i, and a reference current memory 6h. The timer 6d generates an ON signal 
having a duration corresponding to that of an output from the adder 6c for 
every 20 ms of a clock pulse CL3 from the clock pulse generator 7. If the 
output from the adder 6c is disabled, the transistor 6e is not turned on. 
It should be noted that the clock pulse generator 7 generates the 1-, 10-, 
and 20-ms clock pulses CL1, CL2, and CL3. 
The operation of the control device CONT having the above arrangement will 
now be described. When a vehicle starts to travel, the vehicle speed pulse 
generator 1 generates a vehicle speed pulse corresponding to the actual 
vehicle speed. This pulse is processed by the instantaneous vehicle speed 
signal generator 3 and converted thereby to the instantaneous vehicle 
speed signal. This signal is supplied to the pattern memory 4. A vehicle 
speed response designation value corresponding to the input instantaneous 
vehicle speed signal is read out from the pattern memory 4 and supplied to 
the adder 6a. 
When the driver turns the steering wheel during driving, the steering angle 
signal generator 2 generates a steering angle signal. The steering angle 
signal is supplied to the pattern memory 5. Since the pattern memory 5 has 
received the instantaneous vehicle speed signal from the instantaneous 
vehicle speed signal generator 3, one of the characteristics represented 
by the characteristic curves a to e in FIG. 6 is selected. The selected 
characteristic curve and the steering angle signal determine a correction 
signal. The correction signal is supplied to the adder 6a. 
The selected vehicle speed response designation value from the pattern 
memory 4 and the selected correction signal from the pattern memory 5 are 
added by the adder 6a. The sum signal from the adder 6a drives the timer 
6d through the register 6b and the adder 6c. The timer 6d generates a 
signal having a duty ratio corresponding to the input signal. This signal 
is supplied to the control valve CV through the transistor 6e, thereby 
performing power steering control. 
A current supplied to the control valve CV cannot be constant due to 
variations in power source voltage and heat from the solenoid. For this 
reason, the current supplied to the control valve CV is compared by a 
comparator 6g with a reference value read out from the reference current 
memory 6h. The comparison result is supplied to the adder 6c through the 
up/down counter 6i, thereby controlling the current of the control valve 
CV to be constant. 
If the instantaneous vehicle speed is small, the change in steering force 
corresponding to the change in steering angle is not large, as indicated 
by a characteristic curve a in FIG. 7. However, when the instantaneous 
vehicle speed is increased, the steering forces are larger, as indicated 
by characteristic curves b and c. As shown in FIG. 3, the characteristics 
representing the relationship between the vehicle speed V, the steering 
angle, and the steering force (current) are obtained. The "current" here 
indicates the current supplied to the solenoid for driving the control 
valve CV. A large current corresponds to a large steering force. 
Since the power steering apparatus has the characteristics in FIG. 3, the 
steering force is decreased near the zero steering angle, thus easily 
identifying the straight steering position. In addition, when the driver 
turns the steering wheel at the high vehicle speed, the steering force is 
abruptly increased, thereby eliminating a dangerous driving state. 
According to the present invention as described above, the optimal steering 
force is determined according to the vehicle speed and the steering angle. 
Therefore, comfortable, safe steering can be achieved.