Apparatus for controlling steering force produced by power-steering system

There is disclosed an apparatus for controlling the ratio of the assisting force produced by a power-steering system installed in an automobile to the steering torque supplied by the driver. The apparatus primarily consists of a microprocessor, a ROM, and a RAM. The microprocessor controls the amount of current supplied to a solenoid valve according to the velocity of the automobile. The aforementioned ratio is varied according to the driver's condition and the road condition. Four graphs representing the relation of the electric current supplied to the solenoid valve to the angle through which the steering wheel is rotated are stored in the ROM. Each graph has two characteristic curves for two different values of the velosity of the automobile. The microprocessor calculates a driver's condition index from the output signal from a velocity sensor. Also, it calculates a road condition index from the output signal from a steering angle sensor. The value of the current supplied to the solenoid valve is selected from the data stored in the ROM according to these indices.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an apparatus which has an electronic 
control device to control the assisting force produced by a power-steering 
system, according to the conditions of a traveling automobile, for varying 
the ratio of the assisting force to the steering force supplied by the 
driver. 
2. Description of the Prior Art 
Usually, a power-steering system of this kind utilizes the velocity of the 
automobile to detect the conditions of the moving automobile. The 
assisting force is controlled according to the result of the detection in 
such a way that the assisting force is increased or decreased, depending 
on whether the velocity lies in a low-speed range or a high-speed range, 
respectively. In a known power-steering system as disclosed in Japanese 
Patent Laid-Open No. 59,574/1984, one characteristic curve is selected 
from several previously determined characteristic curves according to the 
road condition, i.e., whether the automobile goes down streets or mountain 
roads. Each of the characteristic curves determines the relation between 
the vehicle speed and the rotational frequency of the oil pump. This known 
system can vary the ratio of the assisting force to the steering effort 
supplied by the driver, depending on the vehicle speed and also on the 
road conditions. This ratio will be hereinafter referred to as the "power 
assistance ratio." 
However, the running conditions of the automobile varies not only depending 
on the aforementioned vehicle speed and road conditions but also depending 
on the driver's conditions, i.e., the temper and character of the driver. 
That is, the condition of the automobile changes, according to whether the 
driver drives boldly or moderately. Therefore, it has not been necessarily 
possible for the prior art techniques to provide appropriate power 
assistance ratio according to the driver's conditions. 
In order to solve this problem, the present inventors and others proposed a 
new technique in U.S. patent application Ser. No. 946,050 in 1986. 
According to this proposed technique, a first driving condition index 
K.sub..theta. corresponding to the road conditions and a second driving 
condition index K.sub.v corresponding to the driver's conditions are 
calculated. Then, the following arithmetic operation is performed. 
EQU I=I.sub..theta. +I.sub.v 
where I.sub..theta. is an electric current applied according to the first 
index K.sub..theta., and I.sub.v is an electric current applied according 
to the second index K.sub.v. The sum control current I is applied to a 
solenoid valve which varies the power assistance ratio. The value of the 
second term I.sub.v of the formula varies only according to the second 
condition index K.sub.v. Therefore, the characteristic curve of the 
control current I is obtained simply by shifting the characteristic curve 
of the applied current I.sub..theta. upwardly or downwardly according to 
the second index K.sub.v. For this reason, the control current I cannot 
vary sufficiently freely. Hence, the power assistance ratio changes merely 
within a limited range. 
SUMMARY OF THE INVENTION 
Accordingly, it is a main object of the present invention to provide an 
apparatus which controls a power-steering system in such a way that it 
permits the power-steering system to produce an appropriate assisting 
force, not only depending on the road conditions but also on the driver's 
conditions, and that it extends the range in which the power assistance 
ratio of the power-steering system can be varied. 
It is another object of the invention to provide an apparatus which 
controls a power-steering system, takes one control current value from 
each of at least four different graphs according to the velocity of the 
automobile, and then selects one control current value to be applied to a 
solenoid valve for controlling the power assistance ratio from the four 
selected values, depending on the road condition and also on the driver's 
condition. 
In summary, the inventive apparatus for controlling the steering force 
produced by a power-steering system calculates a driver's condition index 
indicative of the driver's condition from the latest plural kinds of 
information furnished from a velocity sensor, the information indicating 
the velocity of the automobile. Also, the apparatus calculates a road 
condition index indicative of the road condition from the latest plural 
kinds of information delivered from an angle sensor that senses the angle 
through which the steering wheel of the automobile is rotated. Graphs 
containing characteristic curves corresponding to at least four 
conditions, including the combinations of first and second driver's 
conditions and first and second road conditions, are prepared. At least 
four control current values are taken from the graphs according to the 
information regarding the velocity of the automobile. One is selected from 
these at least four control current values according to the driver's 
condition index and the road condition index. An electric current having 
this selected value is supplied to the solenoid valve for controlling the 
power assistance ratio. Preferably, the first driver's condition is the 
state in which the driver drives quite moderately; the second driver's 
condition is the state in which the driver drives quite boldly; the first 
road condition is the state in which the automobile goes down streets 
having many straight roads and many right-angled corners; ad the second 
road condition is the state in which the automobile goes down mountain 
roads or winding roads. 
In this structure, the velocity of the automobile can assume values 
corresponding to the at least four conditions, including the two driver's 
conditions and the two road conditions. Those control current values which 
correspond to these values of the velocity are taken from their respective 
graphs. Then, one is selected from these control current values according 
to the driver's condition and the road condition. Consequently, the 
assisting force which is suited for any other driver's condition between 
the two driver's conditions and also for any other road condition between 
the two road conditions can be taken from the four graphs without 
requiring any graph containing intermediate characteristic curves which 
would otherwise be used for such intermdiate conditions. 
Other and further objects of this invention will become obvious upon an 
understanding of the illustrative embodiments about to be described or 
will be indicated in the appended claims, and various advantages not 
referred to herein will occur to one skilled in the art upon employment of 
the invention in practice.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIGS. 1 and 2, a rack-and-pinion power steering system is 
generally indicated by reference numeral 10. This power-steering system 10 
comprises a servo valve 11, a booster cylinder 12, and a reaction 
mechanism 13 incorporated in the valve 11. This servo valve 11 is 
connected to the steering wheel 46 of an automobile via a steering column 
47. The cylinder 12 has a piston rod 21 which is connected to the front 
wheels of the automobile via a link mechanism (not shown). 
A pump 15, such as a vane pump, is driven by the engine 41 of the 
automobile, and incorporates a bypass valve 18 to supply working fluid at 
a constant flow rate Q into a flow-dividing valve 14 through a discharge 
passage 17. The working fluid is distributed between a servo valve passage 
17a and another servo valve passage 17b at constant flow rates Q.sub.1 and 
Q.sub.2, respectively. The passage 17a is connected to the booster 
cylinder 12 via the servo valve 11. The reaction mechanism 13 and a 
solenoid valve 30 are connected to the passage 17b. 
The servo valve 11 is of the known rotary type and mounted between the 
booster cylinder 12 and the pump 15. When the steering wheel 46 is rotated 
by the driver, the steering effort is transmitted via the steering column 
47 to the input shaft 20 of the servo valve 11 to actuate the valve 11. 
Then, the valve 11 controls the flow of the working fluid into and out of 
both chambers of the cylinder 12 to produce an assisting force. The 
steering force augmented in this way is transmitted to the front wheels 
via the piston rod 21 of the cylinder 12. The used working fluid is 
returned to a reservorir 16 and drawn again into the pump 15. 
The reaction mechanism 13 mainly consists of plungers 13b and V-shaped 
inclined surfaces 13d. The output shaft 22 of the servo valve 11 has a 
pair of radially extending holes 13c in which the plungers 13b are fitted, 
respectively. The diametrically oppositely inclined surfaces 13d are 
formed on the input shaft 20 of the valve 11, and come into contact with 
the front ends of the plungers 13b. Working fluid is introduced into the 
rear portions of the plungers 13b of a port 13a formed in the reaction 
mechanism 13. The pressure of this fluid is varied by the solenoid valve 
30 to twist a torsion spring (not shown) to a larger or lesser extent. As 
a result, the characteristic of tee operation of the servo valve 11 with 
respect to the manual steering torque is varied. 
The structure and the operation of the servo valve 11, the booster cylinder 
12, and the reaction mechanism 13 are described in detail in U.S. patent 
application Ser. No. 865,337, filed 1986, assigned to the assignee of the 
present application. Accordingly, this patent application is herein 
incorporated by reference for a fuller understanding of the present 
invention. The solenoid valve 30 is similar in structure to tee solenoid 
valve disclosed in the aforementioned U.S. patent application Ser. No. 
865,337 (1986). As the electric current applied to the solenoid 36 of the 
valve 30 is increased, the valve narrows the opening of a variable 
restrictor to increase the pressure of fluid acting on the plungers 13b. 
The valve 30 is provided with a narrow fixed restrictor to allow fluid to 
pass through the valve at a given flow rate when the variable restrictor 
is fully closed. 
In the power-steering system constructed as described above, if the opening 
of the variable restrictor of the solenoid valve 30 is varied only 
according to the speed of the automobile, then the driver is required to 
supply a larger steering torque as the automobile speed increases, as 
indicated by the solid line in FIG. 3(A). Also, as the steering wheel is 
rotated further, the steering torque is required to be increased, as 
indicated by the solid line in FIG. 3(B). However, with this operation, 
the power assistance ratio, i.e., the ratio of the assisting torque to the 
manual steering torque, remains constant irrespective of whether the 
driver drives boldly or modelately or whether the road undulates or not. 
In the present example, the opening of the solenoid valve 30 is varied, 
depending not only on the vehicle speed but also on driver's condition 
index and road condition index (described later), by the use of an 
electronic control apparatus 50 shown in FIG. 1. Thus, the power 
assistance ratio is changed according to the driver's condition and the 
road condition. 
Referring again to FIG. 1, the electronic control apparatus 50 mainly 
consists of a microprocessor (CPU) 51, a read-only memory (ROM) 52, and a 
random access memory (RAM) 53. The CPU 51 is connected to the solenoid 36 
of the solenoid valve 30 via a solenoid energization circuit 54 to control 
the electric current applied to the solenoid 36. Also, the CPU 51 is 
connected to a vehicle speed sensor 40 via an interface (not shown). The 
sensor 40 comprises a tachometer connected to the output shaft 43 of the 
transmission, indicated by numeral 42, of the automobile. The driving 
force of the engine, indicated by numeral 41, is transmitted to the rear 
wheels 44 via the transmission 42. The vehicle speed v is determined from 
the frequency of pulses produced by the sensor 40. The CPU 51 is also 
connected to a steering angle sensor 45 via an interface (not shown). As 
an example, the angle sensor 45 comprises a rotating plate fixed to a 
steering column 47, two optical interrupters, and a phase discrimination 
circuit. The angle sensor 45 detects the angle .theta. through which the 
steering wheel 46 is rotated. 
When the driver drives moderately, the vehicle speed varies relatively 
infrequently, as shown in FIG. 5(A). When the driver drives boldly, the 
speed varies frequently, as shown in FIG. 5(B). When the driver drives 
moderately, the acceleration (v) curve has a small number of peaks and 
varies moderately, as shown in FIG. 5(C). When the driver drives boldly, 
the acceleration curve has more peaks and changes violently, as shown in 
FIG. 5(D). When the driver drives moderately, the curve of the absolute 
value of the acceleration, given by .vertline.v.vertline., has a small 
number of low peaks, as shown in FIG. 5(E). When the driver drives boldly, 
the curve of the absolute value has many high peaks, as shown in FIG. 
5(E). The absolute value of the acceleration is integrated for a certain 
period of time, and the obtained value is used as a driver's condition 
index J.sub.1. This index assumes small values when the driver drives 
moderately and takes up large values when the driver drives boldly. Thus, 
it is possible to know whether the driver drives boldly or moderately. 
Streets have small number of curves but have many intersections. Therefore, 
when the automobile goes down streets, it often changes its direction by 
90.degree.. Hence, in this situation, the frequencies of moderate values 
of the angle .theta. through which the steering wheel is rotated are 
relatively low, as shown in FIG. 6(B). When the automobile goes down a 
mountain road, it turns many curves but its direction is rarely rotated 
through 90.degree.. Therefore, the frequencies of moderate values are 
relatively large, as shown in FIG. 6(A). The frequency distribution of the 
absolute value (.vertline..theta..vertline.) of the angle .theta. within a 
certain period of time is taken. The frequency of a moderate value of the 
angle .theta. corresponding to a movement of the automobile along a curve 
is found. This frequency is divided by the total frequency. The resulting 
quotient is used as a road condition index J.sub.2. The value of this 
index J.sub.2 is small when the automobile goes along streets, as shown in 
FIG. 6(B) and large when it goes down a mountain road, as shown in FIG. 
6(A). In this way, it is possible to know whether the automobile goes down 
streets or mountain roads. 
A control signal is applied to the solenoid 36 of the solenoid valve 30 to 
control the relation between the angle .theta. through which the steering 
wheel is rotated and the vehicle speed v. Four graphs containing 
characteristic curves of this control current are stored in the ROM 52. 
These graphs are A.sub.1, A.sub.2, A.sub.3, A.sub.4 shown in FIGS. 
4(A)-4(D). The graph A.sub.1 indicates characteristics of a first control 
current i.sub.1 applied to the solenoid 36 according to the varying angle 
.theta. and speed v when the driver's condition index J.sub.1 is equal to 
0, i.e., the driver drives quite moderately, and when the road condition 
index J.sub.2 is equal to 0, i.e., the automobile goes down streets and 
turns quite few curves. The first control current i.sub.1 increases at a 
given rate with increasing the angle .theta. and the speed v within 
certain ranges, i.e., .theta..sub.1 &lt;.theta.&lt;.theta..sub.2 and v.sub.1 
&lt;v&lt;v.sub.2, but it is held constant outside these ranges. The graph 
A.sub.4 indicates characteristics of a fourth control current i.sub.4 
applied to the solenoid 36 according to the varying angle .theta. and 
speed v when the driver's condition index J.sub.1 is equal to 1, i.e., the 
driver drives quite boldly, and when the road condition index J.sub.2 is 
equal to 1, i.e., the automobile goes along a mountain road having a very 
large number of curves. The value of the current i.sub.4 varies similarly 
to the current i.sub.1 with increasing the angle .theta. and the speed v, 
but it generally assumes values considerably larger than the values of the 
current i.sub.1. The graph A2 indicates characteristics of a second 
control current i.sub.2 applied when J.sub.1 =1 and J.sub.2 =0. The graph 
A.sub.3 indicates characteristics of a third control current i.sub.3 
applied when J.sub.1 =0 and J.sub.2 =1. The values of the currents i.sub.2 
and i.sub.3 vary similarly to the currents i.sub.1 and i.sub.4 with 
increasing the angle .theta. and the speed v. The currents i.sub.2 and 
i.sub.3 are set to values between the values of the currents i.sub.1 and 
i.sub.4. 
To calculate the driver's condition index J.sub.1, the RAM 53 is equipped 
with a number of buffer registers D.sub.0 -D.sub.N-1. The CPU 51 
calculates the absolute value .vertline.v.vertline. of the acceleration v 
at regular intervals of T according to the formula 
EQU v=(v-ov)/T (1) 
where ov is the vehicle velocity stored in the registers D.sub.0 -D.sub.N-1 
last time. The absolute values calculated successively in this way are 
held in the registers D.sub.0 -D.sub.N-1. Each time an absolute value is 
stored in the last register D.sub.N-1, the CPU 51 then stores the next 
absolute value in the first register D.sub.0 and updates the contents of 
the register. The CPU 51 calculates the integral J.sub.1 of the values 
held in the registers D.sub.0 -D.sub.N-1 according to the formula 
##EQU1## 
where K.sub.1 is a constant. Where the driver drives quite boldly, the 
relation J.sub.1 .perspectiveto.1 is determined experimentally. The 
calculated value J.sub.1 is defined as the driver's condition index. These 
arithmetic operations are performed in accordance with a program as 
illustrated by the flowchart of FIG. 8. This program is stored in the ROM 
52. 
The RAM 53 is also equipped with a number of other buffer registers E.sub.0 
-E.sub.N-1 to calculate the road condition index J.sub.2.The CPU 51 
successively stores values of the angle .theta. in the registers E.sub.0 
-E.sub.N-1 at regular intervals of T. Whenever the contents of the last 
register E.sub.N-1 are updated, the CPU stores the next value in the first 
register E.sub.0. Thus, the contents of the registers are successively 
updated. The CPU 51 calculates the frequencies F from the contents of the 
registers E.sub.0 -E.sub.N-1 except for small and large values of the 
angle .theta., i.e., except where the steering wheel is rotated through 
small or large angles. The road condition index J.sub.2 is calculated 
according to the formula 
EQU J.sub.2 =K.sub.2 .times.F/N (3) 
where K.sub.2 is a constant. When the automobile goes down mountain roads 
having many curves, the relation J.sub.2 .perspectiveto.1 is 
experimentally determined. These arithmetic operations are performed 
according to a program as illustrated by the flowchart of FIG. 9. This 
program is stored in the ROM 52. 
The CPU 51 takes a first intermediate current value I.sub.1 to be applied 
to the solenoid 36 of the solenoid valve 30 either from the first graph 
A.sub.1 (FIG. 4(A)) or from the second graph A.sub.2 (FIG. 4(B)) according 
to the present vehicle speed v the angle .theta. through which the 
steering wheel is rotated, and the driver's condition index J.sub.1 when 
the automobile goes down streets. The CPU 51 takes a second intermediate 
current value I.sub.2 to be applied to the solenoid valve 30 either from 
the third graph A.sub.3 (FIG. 4(C)) or from the fourth graph A.sub.4 (FIG. 
4(D)) when the vehicle goes down a mountain road. Then, the CPU 51 
calculates the output control current value I to be applied to the 
solenoid 66 according to the road condition index J.sub.2, the first 
intermediate current value I.sub.1, and the second intermediate current 
value I.sub.2, employing the formula 
EQU I=(I.sub.2 -I.sub.1).times.J.sub.2 +I.sub.1 (4) 
The output current having this value I is supplied to the solenoid 36 of 
the solenoid valve 30. These arithmetic operations are carried out in 
accordance with a program as illustrated by the flowchart of FIG. 10. This 
program is stored in the ROM 52. 
As the values of the indices J.sub.1 and J.sub.2 increase, the value I of 
the output current applied to the solenoid 36 increases, reducing the 
opening of the solenoid valve 30. This increases the pressure introduced 
into the reaction mechanism 13. Then, as indicated by the broken lines in 
FIGS. 3(A) and 3(B), the steering torque to be supplied by the driver 
increases with increasing the vehicle velocity v and the angle .theta.. 
The value of the first term of the left side of formula (4) above varies 
according to the road condition index J.sub.2 and also according to the 
driver's condition index J.sub.1. Therefore, the characteristic curve of 
the control current I not only makes a parallel movement relative to the 
first intermediate current I.sub.1 but also generally or partially tilts. 
In this way, the control current I varies with a large degree of freedom. 
As a result, the characteristic curve also changes with a large degree of 
freedom. 
The apparatus constructed as described above operates in the manner 
described below. As shown in FIG. 7, the CPU 51 successively performs an 
arithmetic operation I for calculating the driver's condition index 
J.sub.1, an arithmetic operation II for calculating the road condition 
index J.sub.2, and an arithmetic operation III for calculating the output 
current value I. 
When the main switch (not shown) of the automobile is closed, the 
electronic control apparatus 50 sets various variables to 0 or other 
initial values. As the automobile moves, the vehicle speed v and the angle 
.theta. through which the steering wheel is rotated varies at every 
moment. The velocity v and the angle .theta. are detected by the speed 
sensor 40 and the angle sensor 45, respectively. Their present values are 
held in registers (not shown). A clock generator circuit 55 produces an 
interrupt signal at regular intervals of time, say 0.5 second, to the CPU 
51. The CPU 51 carries out the arithmetic operations according to the 
programs in synchronism with the interrupt signal. 
(I) Arithmetic Operation for Calculating the Driver's Condition Index 
J.sub.1 
Referring to FIG. 8, the CPU 51 first fetches the vehicle speed v from the 
register holding the present value (step 101). The speed v is 
differentiated to obtain the acceleration v according to the 
above-described formula (1)(step 102). Then, the CPU 51 compares the 
number n of the sampled values with the number of N of the buffer 
registers D.sub.0 -D.sub.N-1 (step 103). If the relation n.gtoreq.N does 
not hold, control directly proceeds to step 105. If the relation 
n.gtoreq.N holds, control goes to step 104, where the number n is reset to 
0. Then, control proceeds to step 105, where the absolute value 
.vertline.v.vertline. of the acceleration is held in the n-th buffer 
register D.sub.n. By these steps 103-105, the CPU 51 stores the absolute 
values .vertline.v.vertline. of acceleration detected at regular intervals 
of T in the N buffer registers D.sub.0 -D.sub.N-1 successively. Each time 
all the registers are stored with values, the CPU returns to the first 
register and updates successively the contents of this and subsequent 
registers. As a result, N absolute values .vertline.v.vertline. of the 
acceleration which were obtained during the latest period of T.times.N are 
stored in the register D.sub.0 -D.sub.N-1. 
The CPU 51 then successively reads out the contents of all the buffer 
registers D.sub.0 -D.sub.N-1 and calculates the driver's condition index 
J.sub.1, using the formula (2) (step 106). The CPU 51 then ascertains 
whether the index J.sub.1 is in excess of 1 (step 107). If this relation 
does not hold, then control directly proceeds to the next arithmetic 
operation II for calculating the road condition index J.sub.2. If this 
relation holds, then the index J.sub.1 is reset to 1 (step 108), followed 
by the execution of the operation II. 
(II) Arithmetic Operation for Calculating the Road Condition Index J.sub.2 
Referring next to FIG. 9, the CPU 51 reads the angle .theta. from the 
register holding the present value (step 111), and then it stores the 
absolute value .vertline..theta..vertline. of the angle in the n-th buffer 
register E.sub.n (step 112). Whenever the CPU 51 reads the angle .theta. 
from the register holding the present value, it successively changes the 
addresses of the registers E.sub.0 -E.sub.N-1 so that the N newest 
absolute values of the angle may be stored in the registers E.sub.0 
-E.sub.N-1. Subsequently, the CPU 51 resets the total count F obtained by 
a frequency counter at 0 (step 113). Thereafter, the CPU initializes the 
total count H obtained by a head-fetching counter at the number N of the 
buffer registers (step 114). 
The CPU 51 compares the value held in the H-th buffer register E.sub.H with 
two preset values B and C which are the upper limit and the lower limit, 
respectively, of the absolute value of intermediate values of the angle 
.theta. corresponding to mild curves, as shown in FIGS. 6(A) and 6(B) 
(step 115). If the relation B.ltoreq.E.sub.H .ltoreq.C holds, the CPU 51 
adds 1 to the total count F obtained by the frequency counter (step 116), 
and then control proceeds to step 117. If this relation does not hold, 
then control goes to step 117, where 1 is subtracted from the total count 
H obtained by the head-fetching counter. The CPU 51 compares the total 
count H with 0 and the execution of the steps 115-117 is repeated until 
the total count H becomes equal to or less than 0. The total count F 
obtained by the frequency counter is set to such a value that the 
relationship B.ltoreq.E.sub.n .ltoreq.C is satisfied. If the requirement 
H.ltoreq.0 is met, then control proceeds to step 119. 
The CPU 51 calculates the road condition index J.sub.2, using the formula 
(3) (step 119). Then, the CPU 51 makes a decision to see whether the index 
J.sub.2 is in excess of 1 or not (step 120). If this condition is not 
satisfied, control directly proceeds to processing III. If the condition 
is met, the index J.sub.1 is reset to 1 (step 121), followed by the 
execution of the processing III. 
(III) Arithmetic Operation for Calculating the Control Current I and 
Energization 
Referring to FIG. 10, the CPU 51 searches the first graph A.sub.1 stored in 
the ROM 52 for the first control current value i.sub.1 according to the 
present vehicle velocity v and the angle .theta. which were read into the 
CPU in steps 101 and 111 (step 131). The CPU similarly searches the second 
graph A.sub.2 for the second control current value i.sub.2 according to 
the velocity v and the angle .theta. (step 132). The CPU 51 substitutes 
the first current value i.sub.1, the second current value i.sub.2, and the 
driver's condition index J.sub.1 that was calculated by the aforementioned 
operation I into the following formula to calculate the first intermediate 
current value I.sub.1 by interpolation: 
EQU I.sub.1 =(i.sub.2 -i.sub.1).times.J.sub.1 +i.sub.1 
This current value I.sub.1 is appropriate as the current applied to the 
solenoid value 30, corresponding to the present vehicle speed, the angle 
.theta., and the driver's condition when the automobile goes down streets 
having a few curves. 
The CPU 51 searches the third graph A.sub.3 for the third control current 
value i.sub.3 according to the present speed v and the angle .theta. (step 
134). Then, the CPU searches the fourth graph A.sub.4 for the fourth 
control current value i.sub.4 according to the present speed v and the 
angle .theta. (step 135). The CPU substitutes the values i.sub.3, i.sub.4, 
and the driver's condition index J.sub.1 into the following formula to 
calculate the second intermediate current value I.sub.2 by interpolation 
(step 136): 
EQU I.sub.2 =(i.sub.4 -i.sub.3).times.J.sub.1 +i.sub.3 
This second intermediate current value I.sub.2 is suitable as a current 
value applied to the solenoid valve 30, according to the present vehicle 
speed, the angle, and the driver's conditions when the vehicle goes down a 
mountain road having many curves. 
The CPU 51 substitutes the intermediate current values I.sub.1, I.sub.2, 
and the road condition index J.sub.2 that was calculated by the operation 
II into the formula (4) to calculate the output current value I (step 
137). This value I is appropriate as a control current value applied to 
the solenoid value 30 according to the present speed, the angle, the 
driver's condition, and the road condition. The CPU supplies a control 
current having this value I to the solenoid 36 of the solenoid valve 30 to 
energize it (step 138), thus completing the execution of the program 
illustrated by the flowchart shown in FIGS. 8-10. 
Subsequently, the CPU 51 repeatedly runs the program in synchronism with 
the interrupt signal which is produced at regular close intervals of T to 
appropriately control the opening of the valve 30 according to the vehicle 
speed, the angle, the driver's condition, and the road condition. The 
manual steering torque that is best suited to the angle through which the 
steering wheel is rotated is determined. Thus, the assisting force 
produced by the power-steering system decreases as the driver's condition 
index J.sub.1 and the road condition index J.sub.2 increase. As indicated 
by the broken line in FIG. 3(A), the manual steering torque increases with 
increasing the speed v. As indicated by the broken line in FIG. 3(B), the 
manual steering torque increases with increasing the angle .theta.. The 
manual torque increases as the indices J.sub.1 and J.sub.2 increase. 
The steps 107, 108, 120, and 121 are carried out to prevent the indices 
J.sub.1 and J.sub.2 from becoming too large when the driver drives quite 
boldly and the vehicle goes down a mountain road having quite many curves; 
otherwise an excessively large manual steering torque would be required. 
In the above example, the control current values i.sub.1 -i.sub.4 are taken 
from the graphs A.sub.1 -A.sub.4 shown in FIGS. 4(A)-4(D), depending on 
the vehicle velocity v and the angle .theta.. It is also possible to take 
the values i.sub.1 -i.sub.4 only according to the velocity v. In this 
case, the graphs shown in FIGS. 4(A)-4(D) should contain control current 
values which are set only depending on various values of the velocity, 
i.e., the current value curves would have a zero slope. 
FIG. 11 is a flowchart for illustrating a sequence of operations performed 
instead of the operations illustrated in FIG. 10. In this case, the CPU 
searches graph A.sub.1 shown in FIG. 4(A) for the control current value 
i.sub.1 (step 131). Then, the CPU searches the graph A.sub.3 shown in FIG. 
4(C) for the control current value i.sub.3 (step 132a). The CPU calculates 
the first intermediate current value I.sub.1 from the values i.sub.1, 
i.sub.3, and the road condition index J.sub.2 (step 133a). The CPU 
searches the graph A.sub.2 shown in FIG. 4(B) for the control current 
value i.sub.2 (step 134a). The CPU searches the graph A.sub.4 shown in 
FIG. 4(D) for the control current value i.sub.4 (step 135). Then, the CPU 
calculates the second intermediate current value I.sub.2 from the values 
i.sub.2, i.sub.4, and the index J.sub.2 (step 136a). Subsequently, the CPU 
calculates the output control current I from the first intermediate 
current value I.sub.1, the second intermediate current value I.sub.2, and 
the driver's condition index J.sub.1 (step 137a). Then, a control current 
having this value is produced (step 138). 
Instead of a signal indicating the vehicle speed as used in the above 
example, the output signal from an engine speed sensor 76 shown in FIG. 
12, the output signal from a throttle valve position sensor 81 shown in 
FIG. 13, or the output signal from an intake air flow sensor 91 shown in 
FIG. 14 may be used. The output signals from these sensors 76, 81, 91 vary 
with changes in the vehicle speed v. 
Referring particularly to FIG. 12, the engine speed sensor 76 receives the 
output signal from an igniter 70 which is driven by a signaling rotor 72 
via a pickup coil 11 to selectively ignite a number of spark plugs 75 via 
an ignition coil 73 and a distributor 74. The rotor 72 is mounted on a 
camshaft. 
Referring next to FIG. 13, the throttle valve position sensor 81 is 
mechanically coupled to a throttle valve 80 mounted in a fuel gas passage 
61 to detect the opening of the valve 80. This valve controls the flow of 
the fuel supplied to the engine 47. 
Referring to FIG. 14, the intake air flow sensor 91 is mechanically coupled 
to an airflow meter 90 mounted in an intake air passage 61 formed in an 
electronic fuel injection system. 
Also, in the above example, the four control current values i.sub.1 
-i.sub.4 are taken from the graphs A.sub.1 -A.sub.4 stored in the ROM 52. 
It should be understood, however, that the present invention is not 
limited to an apparatus using such graphs A.sub.1 -A.sub.4. For example, 
one or more calculational formulae, preferably four formulae corresponding 
to the graphs A.sub.1 -A.sub.4, may be stored in the ROM 52. The four 
control current values i.sub.1 -i.sub.4 may be calculated either from a 
signal indicating the present vehicle speed or from signals indicating the 
present speed and the angle through which the steering wheel is rotated, 
based on the calculational formulae. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the present 
invention may be practiced otherwise than as specifically described 
herein.