CA 02427274 2006-08-O1
-1-
ELECTRONICALLY CONTROLLED POWER STEERING SYSTEM
FOR VEHICLE AND METHOD AND SYSTEM FOR MOTOR CONTROL
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to electrical motor drives and, in particular,
to
electric motors driven by switched converters which convert a do potential to
one or
more phases of pulsed current to drive the motor. The motor can be, for
example, a
brushless do motor having Hall sensors to control the commutation.
This invention further relates to a power steering device that generates
auxiliary steering power for driving to the steering mechanism of a vehicle by
means
of the oil pressure that it generated by a pump which is driven by electric
power.
2. Technology According~to Prior Art
Figure 1 shows a typical three phase motor drive from a do bus. The motor
may be a brushless DC motor having a permanent magnet rotor and a stator
comprising stator coils fed with switched pulsed phase drive signals. The do
bus
voltage is provided to an inverter 100 comprising three half bridges
comprising
transistors (e.g., MOSFETs, IGBTs, bipolar devices) gated by signals AH, AL,
BH,
BL and CH, CL. The high and low side devices are each connected in series
across
the bus and the output of each device comprises one of the three phases, U, V,
and W.
Each of the switching devices is controlled by a controller 200, which
receives Hall
signals controlling the commutation times from the electric motor 300. The
gate
drive signals AH, AL, BH, BL and CH, CL are provided to the respective
switches of
the inverter 100.
In a typical motor drive, shown, for example in Fig. 2, a Hall signal is
provided from the motor for each phase, one of which is shown. Only one of
each of
the gate drive high and low signals is shown. In a typical application, the
Hall
signals provide a signal for controlling the switching of the switches in the
converter
and thus the motor commutation. A typical motor drive is shown in Fig. 2
having a
120° conduction angle. As shown, the gate drives can be pulse width
modulated
(PWM) as shown by the low gate drive signal in Fig. 2. The gate drive signal
switch
events occur when the Hall transitions occur and any phase advance of the gate
drive
CA 02427274 2006-08-O1
-2-
signal switch is determined solely by the physical placement of the position
of the Hall
effect the sensors in motor. The conduction angle is forced to be 120°
or 180°. The
effective voltage at the outputs of the half bridges is controlled by varying
the duty
cycle of the PWM. The pulse width modulation may be done on the low side or
the
high side or on both the high side and the low side. In Fig 2, only one phase
is
shown. The other two phases are shifted by 120°.
Figure 3 shows another example of a typical motor drive having
180°
conduction angle. Similarly, the high or low side signals can be pulse width
modulated or both can be pulse width modulated.
In the past, if a phase advance of a gate drive signal was desired, this was
obtained solely by the physical placement of the position Hall effect sensors
in the
motor. That is, to obtain a phase advance, the position of the sensor in the
motor
would be moved forward by a certain number of degrees depending upon the
desired
phase advance. This phase advance is fixed and not electrically variable.
An object of the present invention is to provide a means for achieving a
variable phase advance and/or conduction angle requiring no mechanical changes
to
the motor to obtain phase advance and change the conduction angle, thereby
resulting in improved motor control.
It is a further object of the invention to provide an improved electric power
steering system for a vehicle.
A power steering device that assists the operation of the steering wheel of a
vehicle by supplying operating oil from the oil pump to the power cylinder
that is
joined to the steering mechanism has been known. The oil pump is driven by an
electric motor, with the auxiliary steering power which is in conformity with
the speed
of the motor rotation being generated by a power cylinder.
Into the steering shaft, a torsion bar that generates torsion which is in
conformity with the direction and size of the steering torque which has been
provided
by the steering wheel and an oil pressure control valve which changes its
opening
size in conformity with the direction and the size of the torsion of the
torsion bar are
incorporated. This oil pressure control valve is provided in the oil pressure
system
between the oil pump and the power cylinder and it causes an auxiliary
steering
power which is in conformity with the steering torque to be generated from the
power cylinder.
The drive control of the electric power motor is carded out on the basis of
the steering angular speed of the steering wheel. The steering angular speed
is
obtained on the basis of the output of the steering angle sensor that has been
provided
in connection with the steering wheel, and the target rotary seed of the
electric power
motor is set based on this steering angle rate. Voltage is supplied to the
electric
motor in such that this target rotary speed may be achieved.
As the electric motor, a triple-phase brushless motor is ordinarily used. The
triple-phase brushless motor comprises a stator which has field coils for the
U phase.
The V phase and the W phase, a rotor with a fixed permanent magnet that
receives the
CA 02427274 2006-08-O1
-3-
repulsive magnetic field from the field coils and Hall sensors for detecting
the rotation
position of this rotor. Three Hall sensors are provided at an interval of 120
degrees as
an electric angle in conformity with the U phase, the V phase and the W phase.
The triple-phase brushless motor is driven in accordance with the
conventional 120 degree power system in the ordinary case. This 120 degree
power
system is shown in Figure 13. The Hall signals that are outputted by the Hall
sensors
of the U phase, the V phase and the W phase deviate from each other by 120
degrees
in phase. The electrical power is passed during a period corresponding to an
electric
angle of 120 degrees to the U phase, the V phase and the W phase in turn so as
to
synchronize with the Hall signals of the U phase, the V phase and the W phase.
It
becomes possible to change the rotary speed of the brushless motor by the PWM
(pulse width modulation) control of the supply of the drive current to each
field coil
during the electricity-conducting period of 120 degrees.
Figure 14 shows the relationship between the rotary speed of the rotor and
the output torque in the triple-phase brushless motor. As is shown in Figure
14, it is
known that the output torque decreases along with an increase in the rotary
speed.
As can be understood from the formula relating to the motor as shown in (1)
below, if
the rotary speed of the motor (cu) increases, the electric current I that
flows to the
motor decreases along with an increase in the motor-generated induced voltage
kc~.
also known as the back emf, with a result that the output torque that is
proportional
to the electric current I becomes smaller.
V = IR. + L, di/dt + k w ( 1 )
where L = motor inductance di/dt - rate of change of current and V indicates
the
voltage impressed to the motor, I is the electric current that flows to the
motor. R is the
electric resistance of the motor, K is a constant and w indicates the speed of
rotation of
the motor.
SUMMARY OF THE INVENTION
The invention relates to a system and method for achieving a variable
phase advance and/or a variable conduction angle in a motor drive system.
It is a further object of the invention to provide a system and method that
uses any of variable phase advance, variable conduction angle and pulse width
modulation to suitably regulate the speed of an electric motor to obtain a
desired
torque characteristic.
The invention provides advantages in that increasing the phase advance
and/or conduction angle gives a higher achievable speed for any given torque.
That is,
the power is increased. Further, increasing the conduction angle reduces
torque ripple.
The above and other objects of the invention are achieved by a method for
controlling an electric motor having at least one sensor output for
determining a
switching instant for a switch of a switching inverter controlling a
conduction angle
CA 02427274 2006-08-O1
-4-
determining a conduction time during a revolution of the motor, the method
comprising; receiving the sensor output; and advancing a switching-on time of
a
switch of the switching converter connecting a d-c bus voltage to a motor
phase drive
input by a phase angle prior to the next sensor output determining the
switching
instant.
In recent years, there has been a demand for a greater rotary speed in the
medium low torque range of the triple-phase brushless motor. In order to meet
such
a demand, however, there will inevitably have to be a drastic rise in the cost
as it will
become necessary to review the control system of the triple phase brushless
motor and
re-evaluate the design of the triple-phase motor itself. Accordingly, a
purpose of this
invention lies in offering a power steering device which is capable of
obtaining a high
rotary speed in the medium low torque range of the electric motor and which
does not
bring about a drastic rise in manufacturing costs.
The invention for achieving the aforementioned objective is a power steering
device that generates an auxiliary steering power by oil pressure that is
generated by a
pump which is driven by an electric motor, the motor having a conduction angle
during which electrical power is provided to at least one motor phase, the
power
steering device comprising a rotary angle detector for detecting the rotary
angle of said
electric motor, a steering angular speed sensor for detecting a steering
angular speed
of a steering operating member, a drive target value rotational speed setting
device for
setting a drive target value rotational speed of said electric motor in
relation to an
output signal of the steering angular speed sensor, a drive signal generator
for
producing a drive signal for driving said electric motor and an angle setting
device for
determining a phase advance angle of the drive signal with respect to the
rotary angle
that is detected by said rotary angle detector on the basis of the drive
target value
rotational speed which is set by said drive target value rotational speed
setting device,
thereby changing the conduction angle.
According to the construction described above, the phase advance angle of the
drive signal is set in conformity with the drive target value rotational speed
of the
electric power motor (such as a brushless motor), with the conduction angle
being
changed accordingly.
If, for instance, the electric motor is a triple-phase brushless motor, with
said
triple-phase brushless motor being driven according to the 120 degree
conduction
angle method, the timing for the start of the electricity passing to the field
coils of the
U phase, the V phase and the W phase is variably set for die phase of the
output signal
of the rotary angle detector (such as a Hall sensor) corresponding to the U
phase, the V
phase and the W phase. As it becomes possible to increase the electric current
supplied (the electricity passing time or conduction angle) to the electric
motor by
setting a comparatively large phase advance angle for the drive target value
for the
high speed rotation range, the motor generating voltage (back emf) becomes
small,
thereby increasing the output torque.
CA 02427274 2006-08-O1
-5-
According to this invention, it becomes possible to increase the rotary speed
in
the medium low torque ranges without drastically changing the design of the
motor or
the design of the system as a whole. Accordingly, there will be no drastic
increase in
the cost.
Since it is possible to exercise control so as to set a suitable phase advance
angle (the minimum phase advance angle required) for the required motor rotary
speed, it becomes possible to control the major problems in the control of the
phase
advance angle (such as a reduction in permanent magnetism or lowering of
efficiency).
It is also conceivable to effect PWM control for passing electricity in a
period
of a certain phase advance angle by keeping the phase advance angle of the
drive
signal constant. In such a case, the heat loss in the switching means (such as
a field
effect transistor) for realizing the PWM control becomes a problem. According
to this
invention, it is not that PWM control is carried out during the period of the
phase
advance angle but that the period of power passing is varied by varying the
phase
advance angle, with a consequence that there is no need to consider an
increase in the
switching loss, and it becomes possible to control any possible increase in
the heat
loss.
Further, according to the invention, the phase advance angle setting means
sets
a certain fixed phase advance angle irrespective of said drive target value at
the time
when the electricity passage to the electric motor is in an unsaturated state
but sets the
phase advance angle on the basis of the drive target value which is set by the
drive
target setting device at the time when power passing to the electric motor is
saturated.
According to this construction, a phase advance angle which is in conformity
with the drive target value can be set only after the 120 power passage has
been
saturated, (for example, it may be set at zero degrees) and, by carrying out
PWM
control within the power passing period of 120 degrees, for instance, both the
low
speed rotation control and the medium-speed rotational control of the power
motor can
be controlled. Once the 120 degree conduction angle period has been saturated
(100%
PWM duty cycle), further motor control is accomplished by varying the phase
advance
angle, with the motor being operated in the phase advance region in a
saturated state,
i.e., 100% PWM duty cycle.
Other features and advantages of the present invention will become apparent
from the following description of the invention which refers to the
accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS)
The invention will now be described in greater detail in the following
detailed
description with reference to the drawings in which:
Fig. 1 shows a generalized block diagram of a motor controller;
Fig. 2 shows a typical prior art motor drive control scheme;
CA 02427274 2006-08-O1
-6-
Fig. 3 shows another prior art motor drive control scheme;
Fig. 4 shows a motor drive control scheme in accordance with the invention
providing variable phase advance and/or conduction angle;
Fig. 5 shows several timing charts for motor drive signals for various casts
of
variable phase advance, fixed phase advance and conduction angle; and
Fig. 6 shows a speed controller in accordance with the invention that
selectively uses variable phase advancelconduction angle and pulse width
modulation.
Fig. 7 is a conceptual drawing showing the basic constitution of a power
steering device according to one example of this invention.
Fig. 8 is a block diagram showing the functional constitution of the electric
control unit in the above-described power steering device.
Fig, 9 is a characteristic chart showing the relationship between the steering
angle speed and the target rotary speed.
Fig. 10 is a chart shown for the purpose of explaining the power driving
method for operating the electric motor.
Fig. 11 is a figure showing the relationship between the phase advance angle
and the target rotary speed.
Fig. 12 is characteristics figure showing the relationship of the torque
versus
the rotary speed of the electric motor.
Fig. 13 is a time chart presented for the purpose of explaining the
conventional
120 degree conduction angle system.
Fig. 14 is a drawing showing the relationship between the rotary speed and the
output torque in the three-phase brushless motor.
DETAILED DESCRIPTION OF THE INVENTION
Turning now to Fig. 4, this figure shows gate drive high and gate drive low
signals for one motor phase, as well as the ideal and physical Hall signals
from the
motor. The ideal Hall signal is placed such that if 120° conduction
angle were used
with 0° phase advance, the switching instants would occur at the same
time as the Hall
signal transitions. This is shown in Fig. 4 by the dashed line x. If no phase
advance is
provided, the switching instants for the high drive signal would coincide with
the
rising edge of the ideal Hall signal. The physical Hall signal may be offset
(advanced)
from the ideal Hall signal by some amount, which can be 0°, or some
value greater
than 0°. An exemplary physical Hall signal is shown in Fig. 4. The
variable phase
advance (from the ideal Hall signal) is indicated in Fig. 4. Fig. 4 shows that
the gate
drive high signal is switched on some variable phase amount prior to the ideal
Hall
transition and some variable amount prior to the physical Hall signal
transition.
As shown in Figs. 4 and 5, the conduction angle may vary between
120° and
180°. The phase advance is variable. The phase advance and conduction
angle may
be independently adjustable although in practice a co-dependency is useful. In
CA 02427274 2006-08-O1
_7_
particular, a variable advance may be added to the conduction angle to provide
an
additional amount of conduction angle. Thus, the conduction angle equals
120° plus
the amount of variable advance a in the scheme shown. The total phase advance
p
equals a fixed amount of advance _k plus the variable advance a. Although the
phase
advance and conduction angle are shown as co-dependent in Fig. 4, they need
not be.
For example, a phase advance can be employed merely to shift the conduction
period,
but the conduction angle remains constant.
As shown in Fig. 4, the switching instants of the gate drive signals are not
constrained to coincide with the Hall transitions. A software algorithm can
place the
switching instants arbitrarily relative to the Hall sensor edges. As also
shown in Fig.
4, pulse width modulation may or may not be used depending upon the
application.
Adjusting the phase advance and/or conduction angle may be used to regulate
the
speed or current in certain situations, with or without PWM.
In order to provide the phase advance (which means the switching transition of
the gate signal is before the Hall signal transition) a software algorithm can
use the
prior Hall transition to cause the advance prior to the next corresponding
Hall signal
transition.
As described previously, increasing phase advance and conduction angle
provides a higher achievable speed for any given torque. That is, power is
increased.
The increase in conduction angle also reduces torque ripple.
The following data in Table 1 was recorded for a typical electric motor at
13.5
volts and 2.48 Nm torque.
CA 02427274 2006-08-O1
U
~ '--~.--rM V1~
-~N O~.~O
~
N ~ 'Ol ~
a~ r r r r r
0
O
0oU
N .~~nd;~
U ~
r 000001
N v101v'1~
y ~ r -~N r
N N M M M
U
F."
00~ ~ O 00
~ ~ ~
r r
0
O
U
r O d:00W
N ~
U r r 0000p1
'~ O r ~t~nM
U
O~N r r v1
r O N ~'~O
v1 N M M M M
U
W ~U ~ r ~ ~
o o N ~t
W o O
h ~ ~ ~ h
O
O N O -i01'ct
d W vGooO N ~O
U r r r r 0000o0
O O ~ ~ 01.~N
0100~nM N
U
r o001O --~N M
N N N M M M M
U
' N d-r o 0oM o~oor .--
V N ~ M 4101~ ~ \001
.-;.~.-i.-~.-;O d100
~ ~cr r r r r r r ~ ~o
0
O
N ~ O ~ ~ O ~DooM m
O~ooO~a1O N M ~no0
U ~C\O~O~Or r r r r 00
b 00M O ~ d'~ V7~ 00In
U
OvN ~CO1M 00M 00~ O
'
~ ~n~n~wc vor r o0a,
v~ N N N N N N N N N N
U
bA
U
.O,
N
V
cd o 0 0 0 0 0 0 0 0 0 0 0 0 0 0
~
U ~ o ~ ~ c N c -~t~ ~ ~ ~ ~ o
~ n o
CA 02427274 2006-08-O1
-9-
In the Table 1, speed is in RPM, current is in amperes(A) and efficiency is in
percentage. The duty cycle is 100%, that is, there is 100% pulse width
modulation
(full on during conduction angle). The temperature was between 30 and
45°C. The
entries not filled in are considered not useable due to poor efficiency.
The data in Table 1 was recorded in order to develop a relationship between
phase advance and conduction angle that would result in useful motor
characteristics.
The data is useful for showing the trends in efficiency as phase advance and
conduction angle are varied. As shown in the table, for increasing conduction
angle, a
higher phase advance results in greater efficiency. For conduction angles of
160°, the
best efficiency occurs at phase advances of 40-60° (55° about
optimal) whereas at
180°, best efficiency occurs at phase advances of 60-80°
(75° about optimal). For
140° conduction angle, greatest efficiency occurs between 25 and
55°(50° about
optimal). At 120° conduction angle, maximum efficiency is between
5° and 50° (25°
about optimal).
Based upon Table 1, the following scheme can be chosen:
p = phase advance
c = conduction angle
k = fixed advance
a = variable advance (and additional conduction angle)
p=k+a,k<p<(k+60°)
c = 120° + a, 120° < c < 180°
0°_<a<60°
p a k = 15°
120° conduction: phase advance = k + 0° = 15°
140° conduction: phase advance = k + 20° = 35°
160° conduction: phase advance = k + 40° = 55°
180° conduction: phase advance = k + 60° = 75°
A fixed phase advance of k = 15° was chosen based on Table 1 with
the total
advance being equal to the fixed advance plus the variable advance a. In this
scheme,
the variable advance is also equal to the additional conduction angle. The
fixed
advance shifts the conduction angle period, while the variable advance
increases the
conduction angle.
Reviewing the data in Table 1, it is observable that; with this scheme and k =
15°, for both 160° and 180° conduction, the system is at
a maximum efficiency. At
120° and 140° conduction, the system is within one percent of
maximum efficiency
with k_ = 15°.
The above scheme has the advantages that it is simple, it results in higher
efficiency and it provides the possibility of placing the Hall sensor such
that a number
CA 02427274 2006-08-O1
- 10-
of switching instants will align with the Hall edges. This may improve the
accuracy
and simplicity of the software algorithm.
Figure 5 shows several examples of the control scheme according to the
present invention. In Fig. 5A, the variable advance equals 0°, the
total phase advance
equals the fixed phase advance k and the conduction angle equals 120°.
In Fig. 5B,
the variable phase advance is between 0 and 60°. The total phase
advance equals the
fixed advance _k plus the variable advance a and the conduction angle equals
120° plus
the variable advance a.
In Fig. SC, the variable advance equals 60°, the total phase advance
equals
fixed advance _k plus 60° and the conduction angle equals 180°.
The ideal and
possible physical Hall signals for a single phase are as shown at the top and
bottom of
Fig. 5, respectively.
By setting the fixed advance k, the result is that the turn off instants for
each corresponding switch (for each conduction angle) is at the same point
regardless
of the amount of variable advance. That is, the turn off instant for switch AH
is the
same for each of the three conduction angles. Similarly, the turn off instant
for the
switches AL for each scheme is at the same time, likewise for the switches BH,
BL,
CH and CL. This means that the Hall effect sensors can be positioned as shown
by the
possible physical Hall signal shown at the bottom of the plot, so that turn
off instants
always align with a Hall transition. The same would be true of the two other
phases.
This simplifies the software algorithm for controlling the switching of the
drive
transistors in each half bridge, thus simplifying the software for controlling
commutation.
Figure 6 shows a speed control utilizing the invention. At high loads,
losses due to switching in the power devices of the converter are significant.
Losses
occur when the transistors and the diodes switch. Thus, there are significant
losses
when pulse width modulating. Due to these losses, instead of pulse width
modulating,
when variable advance is greater than 0, a full duty cycle (100% PWM) may be
used.
The speed controller as shown in Fig. 6 can be provided that leaves the duty
cycle at
100% but varies variable advance a in order to regulate motor speed.
In Fig. 6, a gate drive comprising an inverter 100 is provided which
provides the three phases to the motor 300. The Hall signals axe provided to a
controller 200' which includes a commutator 200A and a pulse width modulator
200B.
The commutator 200A is provided with a signal comprising the variable amount
of
advance a, either 0 or some amount of advance for motor control. The pulse
width
modulator 200B is provided with a signal controlling the duty cycle, either an
amount
of duty cycle less than 100% or 100%. Depending on conditions, a switch 400
provides a variable advance a equal to 0 or a variable advance from a
controller 2 to
the commutator. Switch 400 also provides a duty cycle comprising either the
output
of a controller 1 comprising a variable duty cycle or 100% duty cycle to the
pulse
width modulator, as shown. Switch 400 may be controlled by a software
controller
and could comprise a transistor switching circuit. Controllers 1 and 2 are
provided
with a speed reference signal (Speed Ref.) which determines the desired speed.
A
CA 02427274 2006-08-O1
-11-
feedback signal 4000 is derived from the position sensors) and provided to the
controllers 1 and 2 as an indication of the actual motor speed.
Controller 1 is used when the desired speed is reached with 120°
conduction
angle and less than 100% duty cycle. If the current drawn by the motor is too
high
with 120° conduction and a 100% duty cycle, this scheme is also used
Thus, when
controller 1 is used to vary the duty cycle, variable advance a equals 0 as
shown in
Fig. 6.
Controller 2 is used if the desired speed cannot be reached with
120°
conduction angle and 100% duty cycle provided the current draw is not too
high.
Accordingly, when controller 2 is used, a variable advance a greater than 0 is
provided
to the commutator 200A with 100% pulse width modulation (full on during
conduction angle).
Controller 1 may include both speed and current control. Hysteresis may be
needed when switching between the two controllers.
The invention accordingly comprises a system for providing high efficiency
motor control and higher operating speeds at any given torque, thereby
increasing
power. Further, the increased conduction angle reduces the torque ripple. For
example, actual test results for a typical electric motor with 1Nm of torque,
show a
75% increase in current results in a 77% increase in motor speed. Table II
shows
some actual test results.
TABLE II
MOTOR SPEED (RPM)
Load Torque (Nm) 120 Conduction, 180 Conduction, 60
0 Phase Phase
Adv. Adv.
1.0 3360 5960
2.5 2530 3225
The forms of execution of the invention relating to a power steering system
will now be explained in detail by referring to Figs. 7-12.
Figure 7 is a conceptual figure indicating the basic constitution of a power
steering device according to an example of this invention. This steering
device is
arranged relative to the steering mechanism 1 of the vehicle, with an
auxiliary steering
power being provided given to this mechanism 1.
The steering mechanism 1 comprises for example, a steering wheel 2 which is
operated by the operator, a steering shaft 3 which is linked to this steering
wheel 2, a
pinion gear 4 coupled to the steering shaft 3, and a rack gear Sa which is
engaged with
the pinion gear 5, with a rack shaft 5 being extended in the right and left
directions. At
both ends of the rack axis 5, tie rods 6 are joined and the tie rods 6 are
linked to a
knuckle arm 7 that supports the wheels FL and FR at the right and at the left
as
steerable wheels The knuckle arm 7 is provided in such a fashion as to revolve
around
the king pin 8. The above arrangement is exemplary only. Other forms of
CA 02427274 2006-08-O1
-12-
steering gears and other components can be provided, as known to those of
skill in the
art.
In the above-described construction, when the steering wheel 2 is operated and
the steering shaft 3 is rotated, the rotation is converted into a linear
movement along
the right-left direction of the wheel by the pinion gear 4 and the rack shaft
5. This
straight-line movement is converted into a revolution amount around the king
pin of
the knuckle arm 7, with the result that the steering of the right and left
wheels FL and
FR is achieved.
Into the steering shaft 3, a torsion bar 9 that produces torsion in conformity
with the direction and the size of the steering torque that is added to the
steering wheel
2 and an oil pressure control valve 23 whose opening changes in conformity
with the
direction and the size of the torsion of the torsion bar 9 are incorporated.
The oil pressure control valve 23 is connected to a power cylinder 20 that
provides the auxiliary steering power to the steering mechanism 1. The power
cylinder 20 has a piston 21 that is integrally provided on the rack shaft 5
and a pair of
cylinder chambers 20a and 20b that have been divided by the piston 21. The
cylinder
chambers 20a and 20b are connected with the oil pressure control valve 23
through the
oil supply and return routes 22a and 22b respectively.
The oil pressure control valve 23 is further provided on an oil circulation
route
24 that passes through a reserve tank 25 and an oil pump 26. The oil pump 26
is
driven by a motor M(27) of the electromotive type; it draws the operating oil
which is
stored in the reservoir tank 25 to supply same to the oil pressure control
valve 23. The
excess operating oil is returned to the reservoir tank 25 from the oil
pressure control
valve 23 through the oil circulation route 24.
The oil pressure control valve 23 supplies the operating oil to either the
cylinder chamber 20a or cylinder chamber 20b of the power cylinder 20 through
either
the oil supply or return route 22a and 22b in the case where torsion is
impressed to the
torsion bar 9 in one direction. In the event that torsion is impressed to the
torsion bar
9 in the other direction, further, it supplies the operating oil to the other
of the cylinder
chambers 20a and 20b through the other of the oil supply or return routes 22a
and 22b.
In the case where no torsion or torsion is scarcely impressed to the torsion
bar
9, the oil pressure control valve 23 will be in the so-called equilibrium
state and the
operating oil circulates in the oil circulation route 24 without being
supplied to the
power cylinder.
When the operating oil is supplied to either one of the cylinder chambers of
the
power cylinder 20, the piston 21 moves in the direction of the width of the
steerable
wheels. As a result, auxiliary steering power is impressed to the rack shaft
5.
Examples of the Construction of the oil pressure control valve 23 are
disclosed
in detail in U.S. 4,624,283 to cite an example.
The electric motor 27 consists, for example, of a triple phase brushless motor
and it is controlled by an electronic control unit 30 through a drive circuit
28. The
drive circuit 28 comprises, for instance, a power transistor bridge circuit.
It supplies
CA 02427274 2006-08-O1
-13-
electric power from a battery 40 as an electric power source to the electric
motor 27 in
accordance with the control signal that is provided by an electronic control
unit 30.
The electronic control unit 30 includes a micro-computer which is
activated upon receiving a power supply from the battery 40. This micro-
computer
comprises a CPU 31, a RAM 32 that provides the work area for the CPU 31, a ROM
33 that has memorized the data for control as well as the action program' of
the CPU
31, and a bus 34 for the mutual connection of the CPU 31, RAM 32 and ROM 33.
To the electronic control unit 30, steering angle data as outputted from the
steering angle sensor 11 is provided. The steering angle sensor 11 is provided
in
relation to the steering wheel 2. By setting the steering angle of the
steering wheel 2
at the time when the ignition switch is activated and the engine has started
at the initial
value "0", a steering angle data of the sign in conformity wills the steering
direction is
outputted. On the basis of this steering data, the CPU 31 calculates the
steering speed
that corresponds to its tune differential value.
An electric current detection signal from an electric current sensor 12 that
detects the electric current that flows to the electric motor 27 and a
detection signal
from the Hall sensor 15 as a rotor position sensor for the detection of the
rotor position
of the electric power motor 27 are provided to the electronic control unit 30.
Moreover, a wheel speed signal that is outputted from the wheel speed
sensor 13 is given to the electronic control unit 30. The wheel speed sensor
13 may be
a sensor that directly detects the wheel speed (proportional to vehicle speed)
or the
wheel speed may be obtained by calculation on the basis of the output pulse of
the
wheel speed sensor that has been provided in relation to the wheel.
The electronic control unit 30 controls the electric power motor 27 on the
basis of the steering angle data, the current data and the wheel spiced data
that are
given from the steering angle sensor 11, the current sensor 12 and the wheel
speed
sensor 13 respectively.
Figure 8 is a block diagram showing the construction of the electronic
control unit as viewed from its functional standpoint. The electronic control
unit 30
substantially possesses a plurality of functional means that are realized
through the
execution of a program stored in ROM 33 by the CPU 31. The electronic control
unit
30 thus comprises steering angular speed operating part 41 for the calculation
of the
steering angular speed on the basis of the output signal of the steering angle
sensor 11
and a target rotary speed setting part 42 that sets the target rotary speed R
of the
electric motor 27 on the basis of the wheel speed as detected by the wheel
speed
sensor 13 as well as the steering angular speed calculated by the steering
angular
speed operating part 41.
In addition, the electronic control unit 30 is provided with a motor driving
control part 45 that controls and drives the electric power motor 27 so as to
achieve
the target rotary speed R as set by the target rotary speed setting part 42.
The motor
drive control part 45 generates a drive signal for achieving the target rotary
speed R on
the basis of the motor electric current that is detected by the electric
current sensor 12
and provides this drive signal to the drive circuit.
CA 02427274 2006-08-O1
-14-
The electric motor 27 is provided with a stator that has a U -phase field coil
27U, a V-phase field coil 27V and a W-phase field coil 27W and a rotor with a
fixed
permanent magnet that receives a repulsion field from these field coils 27U,
27V and
27W, with the rotary angle of this rotor detected by the Hall sensor 15. The
Hall
sensor 15 comprises the Hall sensors 15U, 15V and 15W that have been provided
in
conformity with the U phase, the V phase and the W phase.
The current sensor 12 whose purpose it is to detect the electric current that
flows to the electric motor 27 is equipped with electric current sensors 12U,
12V and
12W that detect the electric currents that flow to the U phase, the V phase
and the W
phase respectively. The output signals of the electric current sensors 12U,
12V and
12W and the Hall sensors 15U, 15V and 15W are suitably amplified and provided
to
the motor drive control part 45. Alternately, the current sensor 12 can be
implemented
as a single current sensor coupled to the DC bus.
The drive circuit 28 comprises a series circuit of a pair of field effect
transistors UH and UL that correspond to the U phase, a pair of field effect
transistors
VH and VL that correspond to the V phase and a pair of field effect
transistors WH
and WL that correspond to the W phase coupled in parallel across the battery
40.
The U phase field coil 27U of the electric motor 27 is connected to a
connecting point between the field effect transistor UH and UL, the V phase
field coil
27 V is connected to a connecting point between the field effect transistors
VH and VL
and the W phase field coil 27W is connected to a connective point between the
field
effect transistors WH and WL.
The motor drive control part 45 brings the field effect transistors UH, VH
and WH into the ON state in this order during a certain period of electric
angle and, at
the same time, controls the rotation of the electric motor 27 by providing a
drive signal
consisting of the PWM pulses for the electric field effect transistors UL, VL
and WL.
In particular, the motor drive control part 45 comprises a PWM duty cycle
setting part 46 for setting the PWM duty cycle corresponding to the target
rotary speed
R that is set up by the target rotary speed setting part 42, a phase advance
angle setting
part 47 for setting the phase advance angle D 8 which correspond to the target
rotary
speed that is set likewise by the target rotary speed setting part 42 and a
drive signal
producing part 48 that produces the drive signals to be given to the field
effect
transistors UH, UL, VH, VI, WH and WL of the drive circuit 28 on the basis of
the
phase advance angle 0 A that is set by the phase advance angle setting part 47
as well
as the PWM duty cycles that are set by the PWM duty setting park 46.
Figure 9 is a figure showing the relation between the steering angular speed
and the target rotary speed as set by the target rotary speed setting part 42.
The target
rotary speed R is set between the lower limit Rl and the user limit R2 so that
it will
monotonously increase (the increase being linear in this form of execution) in
the
range of zero being no larger than V(8), which is no larger than VT (VT being
a
threshold value) regarding the steering angular speed V(8).
The target rotary speed setting apart 42 variously sets the incline of the
target rotary speed R as compared with the steering angle speed B(8) on the
basis of the
CA 02427274 2006-08-O1
-15-
wheel speed as is shown in Figure 3. In other words, the threshold value VT is
variously set in accordance with the wheel speed range. To be more specific,
the
threshold value is set higher when the wheel speed becomes higher, i.e., when
the
vehicle is moving faster. Accordingly, the target rotary speed R will be set
lower as
the wheel speed becomes higher, with a consequence that the auxiliary steering
power
becomes smaller. In this manner, wheel-speed responsive control is carried out
for
generating a suitable steering auxiliary power in conformity with the speed of
the
vehicle.
Figure 10 is a time chart presented for the purpose of explaining the method
of
passing the electric current for driving the electric motor 27. Figure 10(a)
shows the
U-phase Hall signal that is outputted by the Hall sensor 15U and Figure 10(b)
shows
the V-phase Hall signal that the Hall sensor 1 SV outputs. In addition, Figure
10(c)
shows the W-phase Hall signal that the Hall sensor 15 W outputs.
Moreover, Figure 10(d) shows the drive signal wave-form that is provided to
the field effect transistor UH, Figure 10(e) shows the waveform of the drive
signal that
is provided to the field effect transistor VH and Figure 10(f) shows the drive
signal
waveform that is provided to the electric field effect transistor WH.
Along with the rotation of the electric motor 27, the U phase Hall signal, the
V
phase Hall signal and the W phase Hall signal assume the waveforms phase-
delayed
by an electric angle of 120 degrees each.
The drive signal producing part 48 produces the drive signals that basically
follow the 120 degree power passing system. In other words, the drive signal
that is
provided to the field effect transistor UH rises in advance of the U-phase
Hall signal
and, after being held in an ON state only during the period of an electric
angle
obtained by adding the phase advance angle ~ 8 to 120 degrees, it is turned
back to
the OFF state in synchronization with a Hall signal. Likewise, the drive
signal that is
provided to the field effect transistor VH rises in advance of the rising edge
of the V-
phase Hall signal and, after being held in the ON state only during the period
of the
electric angle obtained by adding the phase advance angle 0 8 to 120 degrees,
it is
turned to the OFF state in synchronization with a Hall signal.
The same can be stated about the drive signal of the field effect transistor
WH
and it rises to the ON state in advance of the leading edge of the W-phase
Hall signal
and, at the same time, it is kept in the ON state only during the period of
the electric
angle obtained by adding the phase advance angle D 8 to 1120 degrees, followed
by
turn-back to the OFF state in synchronization with a Hall Signal.
While these controls are being carried out, the pulse width control signal for
the duty ratios set at the PWM duty setting part 46 is provided to the field
effect
transistors UL, VL and WL.
The phase advance angle setting part 47 is for setting the advance angle of
the
phase of the drive signal as compared with the Hall signal on the basis of the
target
rotary speed R. The phase advance angle setting part 47 sets the phase advance
angle
0 O at zero insofar as the PWM duty setting part 46 sets a PWM duty of less
than 100
CA 02427274 2006-08-O1
- 16-
percent. At this time, the drive signal producing part 48 produces a drive
signal that
follows the ordinary 120 degree conduction angle system.
When the PWM duty setting part 46 sets a 100 percent PWM duty and,
accordingly, in the state where the electric passage due to the PWM control is
saturated, the phase advance angle setting part 47 variously sets the phase
advance
angle 0 8 in accordance with the target rotary speed R. At this time the drive
signal
producing part 48 brings the field effect transistor UH, VH and WH into the ON
state
at the timing where the phase has been advanced by the phase advance angle 0 8
as
compared with the Hall signal. As a consequence, the power passing (conduction
angle) time will become the time that corresponds to 120 degrees plus D 8 ,
with the
power passing time becoming longer by the time corresponding to the phase
advance
angle 0 8.
In order to bring the drive signals of the U phase, the V phase and the W
phase
into the ON state at the timing which is ahead by the phase advance angle 0 8
as
compared with the Hall signal, it is only necessary to set the ON timing of
the drive
signal of the W phase, the U phase and the V phase by using the rolling signal
one
cycle before.
Figure 11 shows the relationship between the phase advance angle 0 8 that is
set by the phase advance angle setting part 47 and the target rotary speed R
that is set
by the target rotary speed setting part 42. Let us assume an example where the
PWM
duty setting part 46 sets a 100 percent PWM duty at the target rotary speed of
4,000
rpm, with the highest rotary speed of the electric power motor 27 required
being 5,000
rpm. In this case, the phase advance angle setting part 47 sets the phase
advance angle
d 8 in such a way as to monotonously increase from zero to 60 degrees in the
target
rotary speed R region between 4,000 rpm and 5,000 rpm.
The phase advance angle D 8 may be set in such a fashion as will increase
linearly along with an increase in the target rotary speed R or the change in
the phase
advance angle 0 8 as compared with the target rotary speed R may become a non-
linear change. It is desirable that the upper limit of the phase advance angle
D 8 be
set at 60 degrees. If a phase advance angle O 8 that exceeds 60 degrees is
set, the
field effect transistors UH, UL, VH, VL, WH and WL are set on simultaneously,
thereby damaging the power element of the drive circuit 28 (field effect
transistors
UH, UL, VH, VL, WH and WL).
Figure 12 is a characteristic figure showing the relation of the torque
against
the rotary speed of the electric motor 27. As has been shorn in Formula ( 1 )
above,
when the rotary speed w increases, the motor electric current I is reduced due
to the
motor generated induced voltage kw that is produced thereby, with a result
that the
torque that is proportional to this motor currant decreases.
In this form of execution, while the rotation of the electric power motor 27
is
controlled by the PWM control in the low and medium speed rotary ranges up to
4,000
rpm, the PWM duty is at 100 percent in the medium high rotation range higher
than
4,000 rpm, with the rotation of the electric power motor 27 being controlled
by the
phase advance angle control. As a result, the power passing time becomes
longer by
CA 02427274 2006-08-O1
17-
the portion of the phase advance angle D 8 in the medium high speed range
where
phase advance angle control is carried out, with a result that the actual
magnetic flux
density decreases and the motor generation induced voltage at the high speed
rotation
becomes small. Thus, it becomes possible to obtain a high rotational speed in
the
medium low torque range as is shown in Figure 6.
According to this form of execution which is shown above, it becomes possible
to increase the rotational speed in the medium low torque range by means of
well-
contrived control without changing the design or specifications of the
electric motor
27. Accordingly, it becomes possible to obtain auxiliary steering power
without
bringing about a drastic increase in the manufacturing costs.
In view of the fact that a phase advance angle D 8 which is satisfactory in
conformity with the target rotary speed R is set without setting the phase
advance
angle D 8 at a fixed value, it becomes possible to minimize the problems that
may
arise in the case where excessive phase advance angle control has been carried
out (the
problem involving a decline in magnetism and efficiency of the motor in the
case
where the phase advance angle control volume has been increased).
As compared with the case where, while the phase advance angle (D) (8) is
fixed at a certain value, PWM control is carried out during the period where
the phase
advance angle 0 8 constant, heat loss can be prevented and also the heat
design of the
drive circuit becomes easier to carry out, as it will not be necessary to take
the
switching loss of the field effect transistors into consideration.
A form of the execution of this invention has been explained above. However,
the invention can be implemented in other forms as well. Even though, in the
above-
described form of execution, PWM control was conducted in the low medium speed
rotary range, with the phase advance angle control being conducted in the
medium
high speed rotary range, it is also possible to carry out the phase advance
angle control
only in the high speed rotation range.
Moreover, various design modifications can be made within the range of the
items that have been described above.
Although the present invention has been described in relation to particular
embodiments thereof, many other variations and modifications and other uses
will
become apparent to those skilled in the art. Therefore, the present invention
should be
limited not by the specific disclosure herein, but only by the appended
claims.
