Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Bac~ground of the Invention
Field oE the Invention:
0 The invention is related to power steering systems
for automotive vehicles and in particular to electrically
assisted power steering systems.
Hydraulic power steering systems for automotive
vehicles have enjoyed wide acceptance by the using public
and are available on almost every type of commercially
available vehicle. With the down sizing of automobiles
and the use of smaller four cylinder engines for economy
reasons,~the power requirements of the hyraulic pump for
power`steering systems hàs become an appreciable portion
of the total available power. This is not only true
during parking or slow motion maneuvers when the engine
is idling but also at highway speed where the hydraulic
pump absorbs a portion of the available engine power in
parasitic losses such as just spinning the pump. To
prevent the engine from stalling due to the increased
load on the idling engine during low speed màneuvers, it
has been necessary to increase the engines idle speed.
Unfortunately the increased idle speed is only required
when the low speed steering maneuvers are being
executed. At all other times it is unnecessary.
There-Eore increasing the idle speed to accommodate the
use of the hydraulic power steering during low speed
09JRI0785 ~ ~j5;~ ~ 223--83--OO18/O435ITI
steering maneuvers ls co~lnterp~oductive to th~ econo~ical
advantages of the small engines.
Electrical power assisted steering systems, which
use the reserve power of the battery during low speed
maneuvers could overcome the disadvantages of the
hydraulic power steering systems in the smaller
vehicles. The concept of using electrical power to
assist steering is not new. Hepner in U.S. Patent
3,191,109 and Turible in U.S. Patent No. 3,351,152
disclose electrical power assisted steering systems
having a segmented steering column interconnected by an
electrically driven gearing arrangement. Turible also
discloses disabling the power assisted steering at speeds
above a predetermined low speed to give the operator a
normal feel of the road. The power steering arrangement
taught by Hepner and Turible are unsatisfactory because
the reverse torque generated by the electrically driven
gearing arrangement is transmitted back to the operator
through the steering wheel.
2~ This disadvantage was overcome by Goodacre et al in
U.S. Patent No. 3,534,623. In the assisted steering
system disclosed by Goodacre et al, the two sections of
the steering column are directly coupled by a gear
train. The gear train responds to a torque above a
predetermined level to energize an electric motor coupled
to the section of the steering column connected steering
gear box. Goodacre et al's system has the disadvantage
that it uses a high gear ratio between the steering
column and the electric motor and requires that the
steering wheel must be manually turned to return the
vehicle's wheels to their neutral position with the
wheels aligned along a straight path. Also the power
assist is either "on" or "off" giving the driver little
or no feel of the road at the higher speeds.
09JRI0785 1~55~ 223-83~0018/0435m
This problem was overcome by the ~se of torque
responsive manual clutches such as taught by Steinmann in
U.S. Paten~ No. 3,893,534, Bayle in U.S. Patent No~
3,983,953 and Adams in U.S. Patent No. 4,223,254.
Deininger et al in U.S. Pa-tent No. 4,241,804 teaches the
same basic concept but uses a counter-rotating clutch
mechanism with a non-reversible motor.
The concept of controlling the electrical power
applied to the electric motor as a function of the torque
applied to the steering wheel is also taught by Bayle,
cited above. Bayle discloses a torque sensor which
generates a torque signal proportional to the angular
displacement between the two sections of the steering
column and a pair of switches which generate direction
signals indicative of the direction of ~he applied
torque. A computer computes a current applied to the
electric motor which is a function of torque applied to
the steering wheel. In alternate embodiments taught by
Bayle, the computed electrical power applied to the motor
is further modified by an input indicative of the engine
speed to improve driveability at nominal road speeds
above a predetermined level. Further Bayle teaches the
use of an electric clutch to disengage the motor from the
steering wheel to eliminate the requirement for the
operator to manually return the wheels to their neutral
position.
The invention is an electrical power assisted
steering system which eliminates both the need for a
clutch to disengage the steering column from the electric
motor during the return of the wheels to their neutral
position and the need for an input indicative of the
engine speed for normal and hish speed operation.
09JRI0785 l;~SS~3~; 223-83-ool~/0435m
Summary_of the Invention
The invention is an electrical power assisted
steering system designed specifically for todays down
sized fuel efficient automotive vehicles. The power
assisted steering sy~tem has a segmented sha~t joined by
a torsion element. One member of the shaft is adapted to
be connected to a conven~ional steering mechanism and the
other member connected to the vehicle's steering column~
A torque sensor responsive to torque applied between the
two members of the segmented shaft generates a signal
indicative of the direction and magnitude of the applied
torque. A low speed, high torque, reversible D.C.
electrical motor is connected to the member of the
segmented shaft connected to the steering mechanism
through a low gear ratio planetary gear system~ This
arrangement provides the desired power assist to the
vehicle's steering mechanism. The electrical motor is
energized by an antilog power amplifier in response to
the signal received from the torque sensor. The power
amplifier has a null output for a predetermined low input
signal range centered about the signal received from the
torque sensor indicative of zero torque being applied to
the torsion element. The input to output gear ratio of
the planetary gear system is between 1:1 and 14:1 and
permits the electric motor to be reverse driven through
the steering mechanism in the absence of electrical power
being applied to the motor.
One advantage of the power assisted steering system
is that no electrical or mechanical clutch is required to
disengage the electric motor from the driven member of
the shaft to permit the turned wheels to return to their
neutral position after the execution of a moving turn.
Another advantage of the power assisted steering system
is that the dead band and antilog gain characteristics of
O91RI0785 ~5~tj 223-~3-0018/0435m
the power ampliier eliminates the need for a vehicle
speed signal input yet affords the driver excellent feel
or the road at nominal and high speed operation where
power assistance is not required yet provides maximl~m
power assistance during parking and other low speed
maneuvers.
These and other advantages will become more apparent from
a reading of the specification in conjunction with the
drawings.
Brief Description of the Drawings
FIGURE 1 is a cross section of the electrical power
assisted steering system~
FIGURE 2 is an isolated view of the segmented
torque shaft rotated 90 from the position shown in
FIGURE 1.
FIGURE 3 is a cross section of the segmented torque
shaft.
FIGURE 4 is a partial cross section of the magnetic
induction type torque sensor.
FIGURE 5 is a partial cross section of the strain
gauge type torque sensor.
FIGURE 6 is a partial cross section of the strain
gauge type torque sensor taken normal to the view shown
in FIGU~E 5.
FIGURE 7 is a block diagram of the antilog power
amplifier.
FIGURE 8 is a graph showing the antilog and dead
band characteristics of the power amplifier.
Detailed Descri tion of the Invention
. ~
Referring to FIGURE 1, the electrical power
assisted steering system has a housing 10 riyidly mounted
to the steering mechanism of the vehicle and a segmented
O9JRI0785 12~ 223-83-0018/0435m
torque shaft 12 passing through the housing and supported
at its opposite ends by bearings 14 and 16. An
intermediate bearing 17 is disposed between the segmented
flexible shaft 12 and the rotor carrier 32 of an electric
motor 36.
A planetary gear assembly 18 driven by the electric
motor 36 has a stationary internal gear 20 fixedly
attached to housing 10, a cor.centric sun or center gear
22 fixedly attached to rotor carrier 32 of motor 36 and a
plurality of planet gears 24 disposed between internal
gear 20 and sun gear 22. The planet gears 24 are
connected to a planet carrier 26 circumscribing torque
shaft 12. The planet carrier 26 is keyed to the
segmented torque shaft 12 by means of a key pin 28 which
is received in a groove 30 formed in planet carrier 26.
The rotor carrier 32 of electric motor 36 is
attached to and rotatably supports the armature 34 of a
low speed, high torque reversible DC electric motor 36
circumscribing segmented torque shaft 12. A plurality of
permanent magnets 38 attached to the internal surface of
housing 10 comprise the stator of electric motor 36.Motor
36 receives electrical power from an antilog power
amplifi~r 40 as a function of the output from a torque
sensor 42 detecting the torque applied to the segmented
torque shaft 12 as shall be explained hereinafter. The
antilog power amplifier 40 receives electrical power from
the vehicles source of electrical power illustrated as
battery 41.
The torque sensor 42 may be a magnetic strain
sensor as illustrated in FIGURE 1 and 4, a resistive
bridge strain gauge as illustrated in FIGURES 5 and 6, or
any other type of torque sensor known in the art.
The details of the segmented flexible shaft 12 ~ill
now be explained with reference to FIGURES 1, 2 and 3.
09JRIo785 ~5~ 223-83-0018/0~35m
The shaft 1~ illustrated in FIGURE 2 i3 rotated 90
degrees from the position shown in FIGURE 1~ Segmented
flexible shaft 12 has an input member 44 having a splined
portion 46 at one end adapted to be connected to the
vehicle's steering wheel column (not shown) and an
opposite end 48. Intermediate the splined portion 46 and
the opposite end 48 is an external "poly spline" portion
50. An internal bore 52 formed in the opposite end 48 as
shown in FIGURE 1 and 3 is adapted to receive one end of
an output member 54. The other end of output member 54
is adapted to be connected to a conventional steering
mechanism (not shown).
The input member 44 and output member 54 are
rotatably connected by means of a pin 56 pressed into a
mating aperture formed in output member 54. The mating
aperture 58 in the end portion of input member 44 is
elongated as shown to limit the torque applied between
the input member 44 and output member 54 to a
predetermined value.
The input member 44 and output member 54 are
resiliently joined by a cylindrical torque tube 60 having
a female "poly spline" section at one end mating with the
external poly spline portion 50 of input member 44, The
opposite end of the torque tube 60 is fixedly attached to
a radial flange 62 formed at the end of output member 54
adjacent to the input member 44. The radial flange 62
may have a "polyspline" configuration corresponding to
"poly spline" portion 50 but preferably is circular
having a diameter corresponding to the internal diameter
of the torque tube 60. The torque tube 60 may be pinned
to radial flange 62 or fixedly attached using any other
means known in the art. The elongated aperture 58 in the
opposite end of input shaft 44 permits the torque tube 60
to be rotationally stressed to between 2.0 and 3.5 joules
O9JRI0785 ~5~ 223~3-0018/0435m
before pin 56 contacts the limits of elongated apertu~e
58. After contact, the torque applied to input member 4~
is communicated directly to output member 54 preventing
the torque tube from being over stressed. Pin 56 also
provides for a positive connection between the input
m~mber 44 and output member 54 allowing the operator to
retain steering control of the vehicle in the event of a
mechanical or electrical failure.
The details of the magnetic torque sensor 42 are
illustrated in Figures 1 and 4. The sensor comprises a
pair of identical but diametrically opposite sensor
assemblies 64 and 660 Since both sensor asse~blies are
identical only sensor asse~bly 64 will be explained in
detail. Sensor asse~bly 64 compri~es an arcuate
magnetically succeptable yoke 68 having three radial
poles ex~ending inwardly towards the torque tube 60 at
45 intervals. A primary coil 70 is wound around the
center poles and produces an alternating magnetic flux
flow through the torque tube 60 to the two outer poles.
Secondary or sensor coils 72 and 74 wound around outer
poles, generate induced electrical signals indicative of
the magnetic flux flow through its respective pole. With
no stress applied to torque tube 60, the signals induced
in the two sensor coils 72 and 74 are equal. However,
when a torque is applied in a first direction, torque
tube 60 is stressed which perturbates the flow of
magnetic flux therethrough. As a result, the electrical
signal induced in sensor coil 72 will differ from the
electrical signal induced in sensor coil 74 with the
difference being indicative of the torque applied to
torque tube 60. Reversing the direction of the applied
torque likewise changes the magnitude of the electrical
signals induced in the two sensor coils such that the
polarity of the differenc~ changes. Combining the
O9JRI0785 ~55~;3 223-83~00l8/0435m
signals induced in the sensor coils 72 and 74 produces an
output signal having an amplitude indicative of the
magnitude of the applied tcrque and a polarity indicative
of the direction of the applied torque.
In a like manner, sensor assembly 66 has a primary
coil 76 and sensor coils 78 and 80. The outputs of the
two sensor assemblies 64 and 66 are combined into a
single output signal having a in~reased signal to noise
ra~io. This type of torque sensor eliminates the need
for seperate direction switches as taught by the prior
art, in particular Adams U.S. Patent 4,223,254 ox Bayle
in U~S. Patent 3,983,9S3.
Alternatively the torque sensor may be a pair of
resistance strain gauges mounted on a flexible member
such as torsion bar as shown in Figure 50 In this
embodyment, torsion bar is a flat steel plate 82
interconnecting an input member 84 with an output member
86~ Like input member 44, input member 84 has a splined
portion 88 at one end adapted to be connected to the
ZO vehicle's steering column (not shown), a slot 90 at the
internal end for receiving one end of steel plate 82, and
an intermediate "poly spline" portion 92. The output
member 86 has one end adapted to be connected to the
vehicles steering gear box (not shown) and an internal
end having a slot 94 receiving the other end of steel
plate 820 The steel plate 82 is locked in slots 90 and
94 by means of pins as shown, or by any other means known
in the art.
A rigid cylindrical member 96 circumscribes steel
plate 82 and the internal ends of the input and output
members. Cylindrical member 96 has a female "poly
spline" which mates with the external poly spline portion
92 of input member 84 and slotted portion 98 which
circumscribes output member 86. A pin 99 locks
09JRI0785 ~ 223-83-0018/0435m
cylindrical member 96 to imput member 84. A pin lO0
pressed in outp~t member 86 has a diameter smaller that
width of the slotted portion 98 of cylindrical member 96
permitting a rotational displacement between member 96
and output member 86 and limiting the rotational torque
that may be applied to steel plate B2 to said
predetermined value.
A pair of resistance bridge strain gauges, 102 and
104, such as bridge shear torque gauges, part number
FAB-D12A-12SX, manufactured by BLH Electronics of Waltham
Massachusetts, are mounted in the central region of the
steel plate 82. the bridge strain gauges 102 and 104 are
bonded to the surface of steel plate 82, using any method
known in the art, such as with an epoxy cement. The
sensitivities of the two strain gauges are reversed, such
that for a torque applied in one direction, the output
signal of one of the two strain gauges increases while
the output signal of the other gauge decreases.
Reversing the direction of the applied torque will
therefore cause the output signal of the one strain gauge
to decrease and the output signal of the other strain
gauge to increase. By combining the output signals of
the two strain gauges 102 and 104 in a known way, the
combined output signal is a torque signal having a
magnitude indicative of the applied torque and a polarity
indicative of the direction of the applied torque.
The electrical power to and torque signal from the
two strain gauges 102 and 104 is provided by means of a
flexible flat cable 106 attached at one end to the steel
plate 82 through an aperture 108 in the cylindrical
member 96 and connected, at the other end to an
electrical terminal block 110 attached to the cover
housing 112. The electrical cable 114 connecting the
strain gauges to the power supply 40 is connected to the
12~
other end Oe terminal block 110 throucJh an insulator grommet
11~ inserted through an aperture in the cover housing 1~2
ad~acent to terminal block 110. The fle~ible cable 106 is
loosely spiraled abou-t the cylindrical member 96 as shown -~o
permit at least two complete revolutions of the illpUt member
84 in both directions from the neutral position. Neu-tral
position as used here and elsewhere means the position of the
wheels and or the position of the steerlng wheel when the
steered wheels are aligned along a straight path.
It is recognized that other types of torque sensors,
other the magnetic or resistive bridge strain gauge sensors
discussed above, may be used to detect the applied torque. For
instance, a fiber optic or other type of optical gauge may be
used to detect the relative displacement between the input and
output members of the segmentted shaft 12 to generate a signal
indicative of the magnitude and direction of the applied
torque. Alternatively, separate detectors may also be used to
generate separate signals of the magnitude of the applied
torque and the other indicative of the direction oE the applied
torque.
One of the key features of the invention is the
combination of the low speed, high torque reversible DC
electrical motor 36 in combination with the low gear ratio
planetary gear drive system 18 which permits the mo-tor 36 to
be reverse driven from the output member of the seymentecl
torque shaft 12. It has been Eound that with this combination,
the forces generated by a moving vehicle, tending to return the
wheels to their neutral position after the execution of a turn,
is sufficient to drive the electric motor 36 and the vehicles
steering column in the reverse direction to t:he neutral
position. This factor eliminates the need for mechanical or
MLS/lcm
O9J~I0785 223-83-0018/0435m
electrical clutches to decouple the electric motor 36
from the output member of the segmented shaft during the
return to neutral maneuv~r or the need for the vehicle's
operator to physically return the wheels to their neutral
S position. In the prototype model of the electrical power
assisted steering system, the electric motor 36 produced
an output torque of approximately 6.1 joules (4.5 ft-lbs)
at 400 RPM's with an applied electrical power of 400
watts (~0 amperes at 10 volts). The input to output gear
ratio of the planetary gear mechanism 18 was 4:1
resulting in a maximum torque of approximately 24 joules
(18 ft-lbs) capable of being applied to the output member
of the segmented shaft 12 by the electric motor assisting
the driver in turning the wheels of the vehicle~ With
this combination, the effort required to turn the wheels
of a stationary down sized front wheel drive vehicle was
found to be significantly reduced from 20 joules to 2.3
joules (200 inch-lbs) which is well within the
- capabilities of an operator having limited physlcal
capabilities such as a female operator~ The trade offs
between the output torque of the DC motor 36 and the gear
ratio of the planetary gear mechanism 18 are obvious
however, in order to preserve the reverse drive
capabilities of the electric motor, and the operators'
feel of the road at nominal and higher speeds, the gear
ratio of the planetary gear mechanism 18 is preferably
limited to the range between 1:1 and 14:1 with the torque
output of the electric motor adjusted accordingly.
In order to preserve the operators feel of the road
and reduce the tendency of the electrically assisted
power steering system to result in oversteering at
nominal and higher vehicle speeds the prior art teaches
either disabling the electric motor above a predetermined
vehicle speed or reducing the authority of the motor as a
O9JRI0785 22~-83-0018/04351n
13
function of vehicle speed. This vehicle ~peed dependancy
is eliminated by the use of power amplifier 40 having an
antilog power output characteristic. U.S. patent
4,471,280 "Anti-Log Power ~mplifier" by Tho~as E. Stack,
gives the details o the antilog power amplifier 40.
Briefly, referring to Figure 7, antilog power amplifier
40, receives the torque signal output from the torque
sensor 42 having a magnitude indicative of the applied
torque and a polarity indicative of the direction of the
applied torque. The torque signal is amplified in a
preamplifier 120. The output of the preamplifier 120 has
its output coupled directly to an active filter 122 which
attenuates any mechanically produced resonances. The
output of active filter 122 is amplified in antilog
amplifier 124 which exponentially modifies the filtered
signal in both negative or positive directions from a
predetermined reference voltage depending upon the
magnitude and the polarity of the signal received from
the torque sensor 42. The antilog amplifier 124 uses the
forward "turn on" characteristics of a silicon PN
juctions to produce the antilog characteristics of
amplifier 40. The output of the antilog amplifier 124 is
received by a pulse width mocluator 126 which generates a
pair of pulse trains having pulse widths proportional to
the input voltage. These two pulse trains are 180 out
of phase and control the switching operation of the
bridged power amplifiers 128 and 1300 The outputs of
power amplifiers 128 and 130 provide direct current
inputs to the reversible DC electric motor 36. The
operation of the circuit is such that the outputs of
power amplifiers 128 and 130 are equal in response to a
torque signal indicative of zero torque or a null. A
torque signal having a first polarity will increase the
integrated output current of amplifier 128 and decrease
09JRI0785 1~5~ 223-83-0018/0435m
14
~he integrated output current o~ amplifier 130 producing
a eurrent differential causing motor 36 to rotate in a
first direction. A torque signal having the opposite
polarity will decrease the integrated output current of
amplifier 128 and increases the integrated output current
of amplifier 130 producing a current differential causing
motor 36 to rotate in the reverse direction. FIGURE 8 is
a graph showing the output characteristics of the antilog
power amplifier 40 as a function of the magnitude and
polarity of the torque applied to the vehicle's steering
wheel. ~he antilog gain characteristics of power
amplifier 40 are adjusted to produce a dead band
indicative of a torque ranging from 0.4 to 0.8 joules
about a torque signal indicative of zero torqueO This
dead band has been found to improve the road feel at
nominal and above vehicle speeds while having an almost
insignificant effect during low speed maneuvers. The
dead band may be adjusted to accomodate different types
of vehicles having steering gear trains of varying
stiffness.
It is recognized that those skilled in the art may
modify the disclosed electrically assisted power steering
system by using different types of torsion members,
torque sensors and gearing arrangements without departing
form the spirit of the invention as described above and
set forth in the appended claims.
