Power steering apparatus

A power steering apparatus including an electric motor for operating a hydraulic pump which supplies pressurized fluid, a directional control valve of center-closed type operable by a steering wheel for delivering the pressurized fluid supplied by the hydraulic pump selectively to a pair of cylinder chambers of a power cylinder through a supply passage, and motor controller for controlling the electric motor. While the steering wheel is at its neutral position, the controller drives the electric motor intermittently at such a low speed that does not deteriorate the efficiency of said electric motor so much so that differential pressure between a pressure at the supply passage and a higher one of pressures in the pair of cylinder chambers is returned to a predetermined first value.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a power steering apparatus for reducing a 
steering force which is required for a driver to steer a steering wheel of 
a vehicle. 
2. Discussion of the Prior Art 
The assignee of the present invention has proposed a power steering 
apparatus of the type wherein a hydraulic pump is driven by an electric 
motor for generating an oil pressure. In the power steering apparatus of 
this type, a rotary valve of center-closed type is used as a directional 
control valve. The directional control valve is operated upon relative 
rotation between an input shaft connected with a steering wheel and an 
output shaft connected with a road wheel. While the steering wheel is 
maintained at its neutral position, the hydraulic pump is shut off from a 
power cylinder which generates a force for assisting the steering 
operation. When the steering wheel is steered so that relative rotation is 
effected between the input and output shafts, the hydraulic pump 
communicates with one of two cylinder chambers of the power cylinder to 
supply pressurized oil thereto. The pressure in a passage connecting the 
hydraulic pump and the directional control valve is compared to a higher 
one of the pressures in the two cylinder chambers of the power cylinder. 
When the detected pressure difference becomes larger than a predetermined 
value, the electric motor is stopped. 
In such system, while the steering wheel is maintained at its neutral 
position so that the vehicle continues to travel straight, the electric 
motor is stopped. From the beginning of steering the steering wheel, oil 
pressure is immediately generated in one of the cylinder chambers of the 
power cylinder to assist the steering operation. Therefore, the energy 
consumed by the electric motor can be reduced and smooth steering 
operation can be realized. 
FIG. 1 is a graph showing the relationship between drive voltage Vm output 
to the electric motor and the pressure difference .DELTA.P in the 
aforementioned power steering apparatus. While the pressure difference 
.DELTA.P is smaller than a reference value Pa, in other words, in the 
state that sufficient power assist cannot be obtained, a maximum drive 
voltage Va is applied to the electric motor. When the pressure difference 
.DELTA.P is between the reference value Pa and a predetermined value Pe, 
the drive voltage Vm is decreased in proportion to increases in the 
pressure difference .DELTA.P. When the pressure difference .DELTA.P is 
equal to or larger than the predetermined value Pe, the drive voltage Vm 
is lowered to zero to stop the electric motor. 
In case where the pressure difference .DELTA.P does not sufficiently 
increase (in other words, it is fairly smaller than the predetermined 
value Pe) at the time when the steering wheel reaches the neutral 
position, the drive voltage Vm depending on that pressure difference 
.DELTA.P is applied to the electric motor. As a result, the pressure 
difference .DELTA.P increases, whereby the drive voltage Vm to the 
electric motor is decreased. In this manner, the pressure difference 
.DELTA.P reaches the predetermined value Pe, whereupon the electric motor 
is stopped. Namely, the drive voltage Vm is changed along the arrow 71 in 
FIG. 1. In the state that the steering wheel is maintained at the neutral 
position, the pressure difference .DELTA.P is maintained at the 
predetermined value Pe because the hydraulic pump is shut off from the 
power cylinder by the directional control valve of the normally 
center-closed type, so that the electric motor continues to be stopped. 
However, the directional control valve of the aforementioned center-closed 
type cannot completely shut off the hydraulic pump from the power 
cylinder. Therefore, even when the steering wheel is maintained at the 
neutral position, with the electric motor being stopped, a small amount of 
the pressurized oil leaks to return to a reservoir. Such oil leak causes 
the pressure difference .DELTA.P to decrease, whereby the electric motor 
starts to be driven again. The electric motor is continuously rotated at 
such a super-low speed that the volume of the pressurized oil discharged 
from the hydraulic pump is equal to that of the oil leak. The rotation at 
the super-low speed of the electric motor makes the motor efficiency 
deteriorate notably. Consequently, the conventional system has a drawback 
that the electric motor continues to be used at a low efficiency, thereby 
resulting in the waste of battery power. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
improved power steering apparatus capable of reducing the energy consumed 
by the power steering apparatus. 
Another object of the present invention is to provide an improved power 
steering apparatus which can prevent an electric motor from being 
continuously driven at a super-low speed while a steering wheel is 
maintained at its neutral position. 
Briefly, a power steering apparatus according to the present invention 
comprises a hydraulic actuator having a pair of cylinder chambers and 
operable to generate assisting power, a hydraulic pump for supplying 
pressurized fluid, an electric motor for operating the hydraulic pump, a 
directional control valve mechanism of center-closed type operable by a 
steering wheel for delivering the pressurized fluid supplied by the 
hydraulic pump selectively to the pair of cylinder chambers of the 
hydraulic actuator through a supply passage, and motor control means for 
controlling the electric motor. The electric motor is driven 
intermittently at such a low speed that does not deteriorate the 
efficiency of said electric motor so much while the steering wheel is at 
its neutral position so that differential pressure between a pressure at 
the supply passage and a higher one of pressures in the pair of cylinder 
chambers is returned to a first predetermined first value. 
With this configuration, while the steering wheel is maintained at its 
neutral position, the electric motor is intermittently driven at a 
predetermined low speed. The predetermined low speed is such that the 
efficiency of the electric motor does not deteriorate so much: namely that 
the electric motor can operate at a fairly higher efficiency than it does 
at the aforementioned super-low speed. As a result, the electric motor can 
be prevented from being continuously rotated at a super-low speed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The preferred embodiment of the present invention will now be described in 
detail with reference to drawings. FIG. 2 is a block diagram showing the 
overall structure of a power steering apparatus according to the present 
invention. The power steering apparatus is mainly composed of a gear valve 
portion 10, a motor-driven pump portion 20 and a motor control portion 30 
for controlling the motor-driven pump portion 20. 
The gear valve portion 10 will be firstly described. Numeral 111 denotes an 
input shaft connected to a steering wheel 12 for rotation therewith, and 
numeral 112 denotes an output shaft which is coaxially connected to the 
input shaft 111 through a not-illustrated torsion bar for relative 
rotation. Between the input shaft 111 and the output shaft 112, an inner 
valve 131 and an outer valve 132 which are connected respectively to the 
input and Output shafts 111 and 112 are disposed to constitute a 
directional control valve 11, or preferably, a rotary servo valve. The 
inner valve 131 and the outer valve 132 are formed with a supply port 138, 
cylinder ports 133 and 134 and a drain port 135. This directional control 
valve 11 is of the center-closed type and operates upon relative rotation 
between the input shaft 111 and the output shafts 112 so that one of the 
two cylinder ports 133 and 134 communicates with the supply port 138, 
while the other thereof is shut off from the supply port 138. 
To describe in detail, in case where the steering wheel 12 is at the 
neutral position, both of the cylinder ports 133 and 134 are shut off from 
the supply port 138 by a first land portion 136 formed on the inner valve 
131 and communicate with the drain port 135 through second land portions 
137a and 137b formed on the inner valve 131. When the steering wheel 12 is 
steered in the clockwise direction to effect relative rotation between the 
inner and outer valves 131 and 132, the cylinder port 133 communicates 
with the supply port 138. To the contrary, when the steering wheel 12 is 
steered in the counterclockwise direction, the cylinder port 134 
communicates with the supply port 138. Preferably, the valve 11 is formed 
at each of several (e.g., three) circumferentially spaced portions thereof 
with a valve section comprising those shown by reference numerals 133-138. 
The output shaft 112 is formed at its bottom end with a pinion gear 113, 
with which a rack gear 141 formed on a rack shaft 14 is engaged. The rack 
shaft 14 is connected to a piston 16 of a power cylinder 15 which 
generates a force for assisting the steering operation. The interior of 
the power cylinder 15 is divided by the piston 16 into a left cylinder 
chamber 15a and a right cylinder chamber 15b which communicate with the 
cylinder ports 133 and 134, respectively. 
A shuttle valve 17 communicates with the left and right cylinder chambers 
15a and 15b and has a ball 171 in its interior passage. The ball 171 is 
moved left and right FIG. 2 in response to a pressure difference between 
the left and right cylinder chambers 15a and 15b. For example, when the 
pressure in the left cylinder chamber 15a is higher than that in the right 
cylinder chamber 15b, the ball 171 is moved right, and the pressure in the 
left cylinder chamber 15a is led from an output port 17a of the shuttle 
valve 17 to a differential pressure detector 18. On the contrary, when the 
pressure in the right cylinder chamber 15b is higher than that in the left 
cylinder chamber 15a, the ball 171 is moved left, and the pressure in the 
right cylinder chamber 15b is led from the output port 17a of the shuttle 
valve 17 to the differential pressure detector 18. When the pressures in 
the left and right cylinder chambers 15a and 15b are equal to each other, 
the equal pressure is led to the differential pressure detector 18. 
The differential pressure detector 18 detects a pressure difference between 
a pressure at a supply passage 26 described later and a pressure which is 
led from the shuttle valve 17, namely a pressure difference .DELTA.P 
between a pressure at the supply passage 26 and a higher one of the 
pressures in the left and right cylinder chambers 15a and 15b, and outputs 
an electric signal corresponding to the detected pressure difference 
.DELTA.P. 
The pump portion 20 will next be described. A hydraulic pump 21 is driven 
by an electric motor 22. An inlet port of the pump 21 is connected to a 
reservoir 25 in which fluid is stored, while an outlet port of the 
hydraulic pump 21 is connected to the supply port 138 of the directional 
control valve 11 via a check valve 23 and the supply passage 26. The drain 
port 135 of the directional control valve 11 is connected to the reservoir 
via a drain passage 25a. Connected to the supply passage 26 is an 
accumulator 24 for accumulating the pressurized oil discharged from the 
hydraulic pump 21. The pressurized oil accumulated in the accumulator 24 
is supplied to the supply port 138 of the directional control valve 11 and 
the differential pressure detector 18. At the beginning of the steering 
operation, the pressurized oil accumulated in the accumulator 24 is 
immediately supplied to the directional control valve 11. This avoids the 
assist delay in steering which would otherwise occurs when the delivery of 
the pressurized oil from the hydraulic pump 21 starts with a delay after 
the rotation of the electric motor 22 is started. 
The motor control portion 30 comprises an ECU (Electric Control Unit) 31 
and a drive circuit 32. An electric signal corresponding to the pressure 
difference .DELTA.P output from the differential pressure detector 18 is 
supplied to the ECU 31. A drive signal for driving the electric motor 22 
is output from the ECU 31 to the drive circuit 32. The drive circuit 32 
converts the drive signal to a drive voltage Vm and outputs it to the 
electric motor 22. 
FIG. 3 is a block diagram showing a minimum structure of the ECU 31 
required for carrying out the present invention. The ECU 31 is composed of 
a CPU (Central Processing Unit) 311, ROM 312, RAM 313, an input processing 
circuit 314 and an output processing circuit 315. The CPU 311 controls the 
whole of the ECU 31 according to a driving control program memorized in 
the ROM 312. A so-called "EPROM" or "EEPROM" is used as the ROM 312. A 
so-called "SRAM" or the like is used as the RAM 313 to store various kinds 
of data and input and output signals therein. An A-D converter is employed 
as the input processing circuit 314 for converting an analog signal input 
from an external apparatus (the differential pressure detector 18 in FIG. 
3) to a digital signal. A D-A converter is employed as the output 
processing circuit 315 for converting a digital signal sent from the CPU 
through a bus 316 to an analog one and for outputting it to an external 
apparatus (the drive circuit 32 in FIG. 3). 
The operation of the power steering apparatus as constructed above will be 
now described. When the hydraulic pump 21 is driven by the electric motor 
22, an oil is sucked from the reservoir 25 and is output from the 
hydraulic pump 21. The pressurized oil is then supplied to the accumulator 
24 and the supply passage 26. The pressurized oil supplied to the 
accumulator 24 is accumulated in the accumulator 24, while the pressurized 
oil supplied to the supply passage 26 is delivered to the directional 
control valve 11 and the differential pressure detector 18. 
In case where the steering wheel 12 is not steered so that the vehicle 
continues to travel straight, no relative rotation is effected between the 
input and output shafts 111 and 112. Since the directional control valve 
11 of the normally center-closed type is not operated in such state, the 
supply port 138 is shut off by the first land portion 136 and communicates 
with neither of the cylinder ports 133 and 134. The cylinder ports 133 and 
134 are in communication with the drain port 135, so that the pressures in 
the left and right cylinder chambers 15a and 15b of the power cylinder 15 
are equal to the pressure in the reservoir 25, namely to the atmospheric 
pressure. Therefore, the ball 171 of the shuttle valve 17 is maintained at 
its original position, and the atmospheric pressure is transmitted to the 
differential pressure detector 18 through the output port 17a. The 
differential pressure detector 18 outputs the pressure difference .DELTA.P 
between the pressure at the supply passage 26 and the atmospheric 
pressure. The electric motor 22 is stopped when the differential pressure 
.DELTA.P reaches a predetermined value, referred to later in detail. With 
this operation, a predetermined volume of the pressurized oil is 
accumulated in the accumulator 24. However, the differential pressure 
.DELTA.P becomes small thereafter due to an oil leak from between the 
supply port 138 and the first land portion 136 even when the steering 
wheel 12 is maintained at the neutral position. 
FIG. 4 is a flowchart illustrating processings executed by the CPU 311 for 
carrying out the present invention. The processings are performed when the 
drive control program stored in the ROM 312 is executed by the CPU 311. 
Firstly, at step S1, the pressure difference .DELTA.P output from the 
differential pressure detector 18 is read into the RAM 313. At step S2, a 
change amount of the pressure difference .DELTA.P is calculated based upon 
the pressure difference .DELTA.P read at step S1. To be concrete, the 
average of a plurality of the pressure differences .DELTA.P read within a 
predetermined time period is compared with the average of the pressure 
differences .DELTA.P read within the latest predetermined time period. At 
next step S3, it is Judged whether the change amount of the pressure 
difference .DELTA.P is equal to or smaller than zero. If the judgement is 
"YES", the processings proceeds to step S5, at which the drive voltage Vm 
corresponding to the mean pressure difference .DELTA.P is obtained based 
upon a .DELTA.P-Vm map shown in FIG. 5(b). If the judgement is "NO", the 
processing proceeds to step S4, at which the drive voltage Vm 
corresponding to the mean pressure difference .DELTA.P is obtained based 
upon another .DELTA.P-Vm map shown in FIG. 5(a). Then, at step S6, the 
drive voltage Vm obtained at step S4 or step S5 is output to the motor 
drive circuit 32. 
The .DELTA.P-Vm maps represented as graphs in FIGS. 5(a) and (b) define the 
drive voltages Vm which are selectively output to the electric motor 22 in 
dependence upon the pressure difference .DELTA.P between the pressure at 
the supply passage 26 and a higher one of the pressures in the left and 
right cylinder chambers 15a and 15b. The map of FIG. 5(a) is selected when 
the change amount of the pressure difference .DELTA.P is larger than zero, 
while the map of FIG. 5(b) is selected when the change amount of the 
pressure difference .DELTA.P is equal to or smaller than zero. The maps 
are stored in the ROM 312. 
The change amount of the pressure difference .DELTA.P will be concretely 
described hereinafter. When the steering wheel 12 is steered, for example, 
in the clockwise direction, the relative rotation is produced between the 
input and output shafts 111 and 112. Upon the relative rotation, the 
directional control valve 11 is operated, so that the supply port 138 is 
brought into communication with the cylinder port 133 while the cylinder 
port 134 is kept in communication with the drain port 135. In this state, 
the pressurized oil accumulated in the accumulator 24 is supplied to the 
left cylinder chamber 15a of the power cylinder 15, whereby the piston 16 
is moved rightward to steer the road wheels of the vehicle. During this 
operation, the oil in the right cylinder chamber 15b is drained to the 
reservoir 25 through the drain port 135 and the drain passage 25a. The 
pressure at the supply passage 26 thus decreases and the pressure in the 
left cylinder chamber 15a increases, whereby the pressure difference 
.DELTA.P decreases. Namely, the change amount of the pressure difference 
.DELTA.P becomes equal to or smaller than zero, so that the map of FIG. 
5(b) is selected. 
When the steering wheel 12 is being returned forward its neutral position, 
the relative rotation between the input and output shafts 111 and 112 
gradually decreases. The volume of the pressurized oil supplied to the 
left cylinder chamber 15a also decreases. Therefore, the pressure at the 
supply passage 26 increases and the pressure in the left cylinder chamber 
15a decreases, whereby the pressure difference .DELTA.P increases. Namely, 
the change of the pressure difference .DELTA.P becomes larger than zero, 
so that the map of FIG. 5(a) is selected. 
Referring to FIG. 5(a), while the pressure difference .DELTA.P is smaller 
than a reference value Pa, a maximum drive voltage Va is applied to the 
electric motor 22 to drive the hydraulic pump 21 at its maximum capacity. 
In a range between the reference value Pa and a second predetermined value 
Pb, the drive voltage Vm to the electric motor 22 is decreased in 
proportion to the increases in the pressure difference .DELTA.P. In 
another range between the second predetermined value Pb and a first 
predetermined value Pc, a constant low drive voltage Vb is applied to 
drive the electric motor 22 at a constant low speed. When the pressure 
difference .DELTA.P reaches the second predetermined value Pc, the drive 
voltage Vm is lowered to zero to stop the electric motor 22. 
The second predetermined value Pb represents a minimum pressure difference 
required for power assist operation. The drive voltage Vb is set to a 
limit value for driving the electric motor 22 without lowering the 
efficiency of the electric motor 22 so much. Since the drive voltage Vb is 
applied to the electric motor 22 in the range between the second 
predetermined value Pb and the first predetermined value Pc, an amount of 
pressurized oil more than the volume of the oil leak is discharged from 
the hydraulic pump 21. Because the electric motor 22 is not driven 
continuously by a smaller drive voltage than the drive voltage Vb, it can 
be avoided that the efficiency of the electric motor 22 notably 
deteriorates. 
Referring to FIG. 5(b), when the pressure difference .DELTA.P is between 
the first predetermined value Pc and the second predetermined value Pb, 
the drive voltage Vm is set to zero to stop the electric motor 22. As soon 
as the pressure difference .DELTA.P reaches the second predetermined value 
Pb, the drive voltage Vm is immediately increased to the voltage Vb and 
thereafter, is gradually increased between Vb and Va in proportion to the 
decreases of the pressure difference .DELTA.P. Then, after the pressure 
difference .DELTA.P decreases to the reference value Pa, the drive voltage 
Va is applied to the electric motor 22. 
As described above, upon starting of returning the steering wheel 12 toward 
the neutral position, the pressure difference .DELTA.P increases. In such 
state, the processings are executed in order of steps S1, S2, S3, S4 and 
S6 of FIG. 4, during which time the drive voltage Vm obtained at step S4 
in dependence upon the detected mean pressure difference .DELTA.P 
calculated at step S4 is output to the electric motor 22. The drive 
voltage Vm is therefore changed in the order indicated by the arrows 41, 
42, 43 and 44 in FIG. 5(a). When the pressure difference .DELTA.P reaches 
the first predetermined value Pc, the electric motor 22 is stopped. To the 
contrary, as the steering wheel 12 is steered from the neutral position in 
either direction, the pressure difference .DELTA.P decreases. In such 
state, the processings are executed in the order of steps S1, S2, S3, S5 
and S6 of FIG. 4, during which time the drive voltage Vm obtained at step 
S5 in dependence upon the detected mean pressure difference .DELTA.P is 
output to the electric motor 22. The drive voltage Vm is therefore changed 
in the order indicated by the arrows 45, 46, 47 and 48 in FIG. 5(b). 
When the steering wheel 12 is maintained at the neutral position, the 
pressure difference .DELTA.P slowly decreases due to the oil leak. This 
situation is represented by the arrow 45 in FIG. 5(b). When the pressure 
difference .DELTA.P decreases to the second predetermined value Pb, the 
drive voltage Vb is output to drive the electric motor 22 at the constant 
low speed. This situation is represented by the arrow 46 in FIG. 5(b) and 
the arrow 43 in FIG. 5(a). Then, when the pressure difference .DELTA.P 
again increases to the first predetermined value Pc, the drive voltage Vm 
is lowered to zero to stop the electric motor 22. This situation is 
represented by the arrow 44 in FIG. 5(a). 
Therefore, while the steering wheel 12 is maintained at the neutral 
position, a series of processings of steps S1, S2, S3, S4 and S6 and 
another series of processings of steps S1, S2, S3, S5 and S6 of FIG. 4 are 
alternately executed. Namely, when the electric motor 22 is driven to make 
the pressure difference .DELTA.P become larger than the second 
predetermined value Pb, the series of the processings of steps S1, S2, S3, 
S4 and S6 are selected and repeatedly executed until the pressure 
difference .DELTA.P reaches the first predetermined value Pc. The electric 
motor 22 is stopped at the first predetermined value Pc of the pressure 
difference .DELTA.P. Subsequently, the series of the processings of steps 
S1, S2, S3, S5 and S6 are selected and repeatedly executed until the 
pressure difference .DELTA.P decreases to the second predetermined value 
Pb. Such control is repeated to drive the electric motor 22 
intermittently. 
FIG. 6 is a graph showing time-dependent changes of the drive voltage and 
the pressure difference while the steering wheel 12 is maintained at its 
neutral position. In the graph, the drive voltage Vm and the pressure 
difference .DELTA.P are taken in the ordinate, and the time t is taken in 
the abscissa. 
Between time t11 and time t12, steps S1-S4 and S6 are executed, and the 
drive voltage to the motor 22 is kept to be the value Vb, as indicated by 
the curve fd, whereby the pressure difference .DELTA.P increases from the 
second predetermined value Pb to the first predetermined value Pc, as 
indicated by the curve fc. Between time t12 and time t21, steps S1-S3 and 
S5-S6 are executed, and the drive voltage Vm to the motor 22 is kept zero, 
whereby the pressure difference .DELTA.P gradually decreases and finally 
reaches the second predetermined value Pb. The increase and decrease of 
the pressure difference .DELTA.P between time t11 and t21 is repeated 
thereafter, that is between time t21 and time t31, and after time t31. 
Consequently, the pressure difference .DELTA.P (second predetermined value 
Pb in this embodiment) which is required for avoiding the assist delay can 
always be maintained at the very beginning of steering operation. Further, 
since the electric motor 22 is intermittently driven at a low speed (drive 
voltage Vb in this embodiment) within a range that the efficiency of the 
electric motor 22 does not deteriorate so much, the consumption of 
electric power can be minimized. 
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.