U.S. patent application number 10/336991 was filed with the patent office on 2004-04-15 for rotation sensor.
This patent application is currently assigned to The Furukawa Electric Co., Ltd.. Invention is credited to Morimoto, Hiroshi, Noguchi, Akira, Toratani, Tomoaki, Tsukamoto, Yukiaki, Yamamoto, Toshiro, Yamawaki, Kosuke, Yasujima, Toshiaki.
Application Number | 20040069074 10/336991 |
Document ID | / |
Family ID | 32025592 |
Filed Date | 2004-04-15 |
United States Patent
Application |
20040069074 |
Kind Code |
A1 |
Toratani, Tomoaki ; et
al. |
April 15, 2004 |
ROTATION SENSOR
Abstract
A contact type rotation sensor includes a contact brush having a
contact and an electrical resistor disposed for sliding motion
relative to the contact. The contact brush provided at a side face
of an arm portion with a projecting piece has asymmetric shape and
asymmetric weight distribution about its longitudinal axis, and is
large in moment of inertia in a direction of torsion. During
relative sliding motion, suppressed torsional vibration of the
contact occurs not only at natural frequencies but also at
dispersed frequencies other than natural frequencies, reducing the
sound pressure of sliding sound. A stable contact state is ensured
between the contact and the electrical resistor, resulting in
improved durability.
Inventors: |
Toratani, Tomoaki; (Tokyo,
JP) ; Yamamoto, Toshiro; (Tokyo, JP) ;
Morimoto, Hiroshi; (Tokyo, JP) ; Noguchi, Akira;
(Tokyo, JP) ; Yamawaki, Kosuke; (Tokyo, JP)
; Yasujima, Toshiaki; (Zama-shi, JP) ; Tsukamoto,
Yukiaki; (Zama-shi, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
The Furukawa Electric Co.,
Ltd.
Tokyo
JP
Tokyo Cosmos Electric Co., Ltd
Tokyo
JP
|
Family ID: |
32025592 |
Appl. No.: |
10/336991 |
Filed: |
January 6, 2003 |
Current U.S.
Class: |
73/862.22 |
Current CPC
Class: |
B62D 15/0215 20130101;
B62D 6/10 20130101; G01D 5/1655 20130101 |
Class at
Publication: |
073/862.22 |
International
Class: |
G01L 005/24 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 11, 2002 |
JP |
2002-299218 |
Claims
What is claimed is:
1. A rotation sensor comprising: a base plate that is rotatable; an
electrical resistor formed on a surface of said base plate so as to
extend in a rotating direction of said base plate; a holder
disposed near said base plate; and a contact member fixed at one
end portion thereof to said holder and provided at another end
portion with a contact, said contact member having an arm portion
thereof extending between these end portions of said contact
member, wherein rotation of said base plate causes relative sliding
motion between said contact and said electrical resistor, with said
contact being in electrical contact with said electrical resistor,
said arm portion has asymmetric widthwise weight distribution about
a longitudinal axis of said contact member, and information on the
rotation of said base plate, produced during the relative sliding
motion between said contact and said electrical resistor, is taken
out from said contact member in a form of electrical signal.
2. The rotation sensor according to claim 1, wherein said arm
portion has a spring constant equal to or less than 0.1 N/mm, and
said contact produces a contact force applied to said electrical
resistor, the contact force falling within a range from 0.1 N to
0.24 N inclusive.
3. A rotation sensor comprising: first and second rotors
individually attached to first and second rotary shafts so as to be
rotatable in unison therewith, the first and second rotary shafts
being coupled through a torsion bar that allows a rotational
difference between the first and second rotary shafts; a base plate
rotatable in unison with said first rotor; first and second
electrical resistors individually formed on first and second faces
of said base plate so as to extend in a rotating direction of said
base plate; a first holder fixedly disposed on a side of the first
face of said base plate; a second holder disposed on a side of the
second face of said base plate so as to be rotatable in unison with
said second rotor; a first contact member fixed at its one end
portion to said first holder and provided at another end portion
with a contact, said first contact member having an arm portion
thereof extending between these end portions, rotation of said base
plate causing relative sliding motion between the contact and said
first electrical resistor, with the contact being in electrical
contact with said first electrical resistor; and a second contact
member fixed at its one end portion to said second holder, said
second contact member having another end portion thereof provided
with a contact and an arm portion thereof extending between the end
portions of said second contact member, rotation of said base plate
causing relative sliding motion between the contact and said second
electrical resistor, accompanied with a rotational difference
between the said base plate and said second holder, with the
contact being in electrical contact with said second electrical
resistor, wherein an angle of rotation of said first rotor, caused
by the relative sliding motion between the contact of said first
contact member and said first electrical resistor, is taken out
from said first contact member in a form of electrical signal, an
angle of rotation of said second rotor, caused by the relative
sliding motion between the contact of said second contact member
and said second electrical resistor, is taken out from said second
contact member in a form of electrical signal, and the arm portion
of at least one of said first and second contact members has
asymmetric widthwise weight distribution about a longitudinal axis
of the arm portion.
4. The rotation sensor according to claim 3, wherein the arm
portion having the asymmetric weight distribution has a spring
constant that is equal to or less than 0.1 N/mm, and the contact
that is provided in the contact member including the arm portion
having the asymmetric weight distribution is in contact with a
corresponding one of said first and second electrical resistors
with a contact force varying within a range from 0.04 N to 0.24 N
inclusive.
5. The rotation sensor according to claim 1 or 3, wherein the arm
portion having the asymmetric weight distribution is provided with
an asymmetrizing element.
6. The rotation sensor according to claim 5, wherein the
asymmetrizing element is comprised of a projecting piece formed
integrally with a distal end portion of said arm portion having the
asymmetric weight distribution.
7. The rotation sensor according to claim 5, wherein the
asymmetrizing element is comprised of a resin body provided at a
side edge of said arm portion having the asymmetric weight
distribution.
8. The rotation sensor according to claim 5, wherein the
asymmetrizing element is comprised of a notch formed at a side edge
of said arm portion having the asymmetric weight distribution.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Technical Field
[0002] The present invention relates to a rotation sensor, and more
particularly, to a contact type automotive rotation sensor.
[0003] 2. Related Art
[0004] In recent years, with increased attention to the global
environmental problem, reduction of automotive body weight have
been made as a measure to reduce automotive emission. By way of
example, a conventionally mainstream hydraulically-driven power
steering apparatus has been replaced by a power steering apparatus
of electrically-driven type which can realize automotive weight
reduction by eliminating a hydraulic pump.
[0005] However, as compared to a hydraulically-operated power
steering apparatus capable of generating a proper assist force
solely by use of a mechanical mechanism, an electrically-driven
power steering apparatus requires one or more rotation sensors for
detecting the steering angle and/or steering torque to enable a
computer to calculate an assist force based on steering angle and
steering torque.
[0006] Rotation sensors for an electrically-driven power steering
apparatus are classified into a non-contact type sensor that
detects the steering angle and/or torque based on a change in
magnetic field caused by steering operation and a contact type
sensor that makes detection on the basis of a voltage change caused
by steering operation and detected through a contact and an
electrical resistor which are disposed for relative sliding
motion.
[0007] A contact type rotation sensor includes a ring-shaped
electrical resistor formed on a surface of a disk-shaped base plate
that is rotatable with rotation of a steering handle, and a contact
disposed in sliding contact with the electrical resistor. In the
rotation sensor, a predetermined voltage is applied across two
reference points of the electrical resistor, and a voltage signal
is taken out from the contact.
[0008] As the steering handle rotates, relative sliding motion
occurs between the electrical resistor and the contact, and the
voltage signal taken out from the contact varies depending on the
distance of relative sliding motion. Thus, the steering angle
and/or steering torque can be measured based on the voltage
signal.
[0009] However, the conventional contact-type rotational sensor has
a drawback that sliding sound is produced during the relative
sliding motion, giving discomfort to a person.
[0010] Furthermore, most components of the rotation sensor are
usually fabricated by resin molding at low costs, with an allowable
manufacturing error. Thus, the distance between the contact and the
base plate can vary normally about .+-.1 mm though a variation in
distance differs depending on the allowable manufacturing error. As
a result, there occurs a variation in contact force between the
contact and the electrical resistor formed on the base plate to
render a connection state therebetween unstable, resulting in poor
durability of the rotation sensor.
SUMMARY OF THE INVENTION
[0011] An object of the present invention is to provide a contact
type rotation sensor capable of suppressing sliding sound and
realizing improved durability by ensuring a stable contact state
between a contact and an electrical resistor.
[0012] The present inventors conducted measurements of sliding
sound generated during relative sliding motion between a contact
and an electrical resistor of a rotation sensor, and analyzed
measurement results with use of a computer, etc. to find the
following facts. Specifically, relative sliding motion between a
contact and an electrical resistor produces friction that serves as
energy source of torsional vibration of the contact. The torsional
vibration causes sliding sound. In a case where the contact is
formed into a shape particularly liable to make vibration,
torsional vibration promotes natural vibration of the contact,
producing extremely loud sound at natural frequencies, to thereby
give great discomfort to a person. Thus, the present inventors
recognized the necessity of finding a shape which makes a contact
hard to vibrate, and made concentrated experiences to find such a
shape.
[0013] Furthermore, the present inventors considered that a proper
contact force must be produced between a contact and an electrical
resistor in order to establish a stable contact state therebetween
to improve the durability of a rotation sensor, and made
concentrated experiences based on the recognition that a contact is
required to have a proper spring constant to produce a proper
contact force.
[0014] A rotation sensor according to the present invention,
created based on results of the experiences, comprises: a base
plate that is rotatable; an electrical resistor formed on a surface
of the base plate so as to extend in a rotating direction of the
base plate; a holder disposed near the base plate; and a contact
member fixed at its one end portion to the holder and provided at
another end portion with a contact, the contact member having an
arm portion thereof extending between these end portions. Rotation
of the base plate causes relative sliding motion between the
contact and the electrical resistor, with the contact being in
electrical contact with the electrical resistor. The arm portion
has asymmetric widthwise weight distribution about a longitudinal
axis of the contact member. Information on the rotation of the base
plate, produced during the relative sliding motion between the
contact and the electrical resistor, is taken out from the contact
member in a form of electrical signal.
[0015] According to the rotation sensor of the present invention,
the contact member is provided with the arm portion that has
asymmetric widthwise weight distribution about the longitudinal
axis of the contact member, and accordingly torsional vibration of
the contact member is suppressed that is generated during relative
sliding motion between the electrical resistor and the contact of
the contact member, and such torsional vibration may occur not only
at natural frequencies but also at dispersed frequencies other than
natural frequencies, whereby the sound pressure of sliding sound
caused by the relative sliding motion can be reduced.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a schematic view showing an automotive steering
system;
[0017] FIG. 2 is a schematic section view showing a rotation sensor
according to a first embodiment of the present invention in a state
accommodated in the steering system shown in FIG. 1;
[0018] FIG. 3 is a schematic plan view of a base plate of the
rotation sensor shown in FIG. 2;
[0019] FIG. 4 is a schematic perspective view showing a compliance
disk of the rotation sensor;
[0020] FIG. 5 is a schematic perspective view showing a contact
brush of the rotation sensor;
[0021] FIG. 6 is a graph showing a relationship between relative
sliding distance between an electrical resistor and a contact brush
and voltage signal taken out from the contact brush;
[0022] FIG. 7 is a graph showing sound pressure of sliding sound
generated in the rotation sensor shown in FIG. 5 in comparison with
sound pressure in a conventional rotation sensor;
[0023] FIG. 8 is a schematic perspective view showing a contact
brush of a rotation sensor according to a second embodiment of the
present invention; and
[0024] FIG. 9 is a schematic perspective view showing a contact
brush of a rotation sensor according to a third embodiment of the
present invention.
DETAILED DESCRIPTION
[0025] With reference to the appended drawings, a rotation sensor
according to a first embodiment of the present invention will be
explained.
[0026] As shown in FIG. 1, an automobile to which the rotation
sensor of this embodiment is mounted has a steering handle 10
directly coupled to a column shaft 12. A steering shaft 16 is, on
one hand, coupled through a torsion bar 14 to the column shaft 12,
and on the other hand, is coupled to an electrically-operated power
steering mechanism 18 that accommodates therein a rack and pinion
for steering front wheels 19 and an electric motor for assisting a
steering operation.
[0027] The column shaft 12 and the steering shaft 16 are supported,
through bearings and the like, by support members (shown at 38 in
FIG. 2) on the side of automotive body, so as to smoothly rotate
with rotation of the steering handle 10. The front wheels 19 are
steered by the power steering mechanism 18 responsive to rotation
of the steering handle.
[0028] During the steering operation, the torsion bar 14 coupling
the column shaft 12 with the steering shaft 16 receives a reaction
force from a road surface, to be twisted. As a result, a rotational
deviation occurs between the column shaft 12 and the steering shaft
16.
[0029] The rotation sensor 20 of the first embodiment is designed
to determine, based on the rotational deviation, a reaction torque
applied to the steering shaft 16 and to the steering handle 10. As
shown in FIG. 1, the rotation sensor 20 is disposed to saddle
between the column shaft 12 and the steering shaft 16.
[0030] More specifically, as shown in FIG. 2, the rotation sensor
20 comprises a steering angle rotor 22 attached to a lower end of
the column shaft 12 and a torque rotor 24 attached to an upper end
of the steering shaft 16, so that the steering angle rotor 22 and
the torque rotor 24 rotate in unison with the column shaft 12 and
the steering shaft 16, respectively.
[0031] A disk-shaped base plate 26 is attached to an outer
periphery of the steering angle rotor 22, so as to rotate in unison
with the column shaft 12 and the steering handle 10.
[0032] An electrical resistor 28 for steering angle detection is
provided on an upper face of the base plate 26, which resistor 28
is formed by electrically conductive resin by using a technique for
printed circuit, for instance. As shown in FIG. 3, the electrical
resistor 28 is formed into an annular shape and has two reference
points A and B that are spaced at a circumferential distance of X
on the electrical resistor 28. A predetermined reference voltage is
applicable across these reference points A and B.
[0033] Referring to FIG. 2, an annular electrical resistor 30 for
reaction torque detection is formed on a rear face of the base
plate 26, which resistor 30 is similar to the electrical resistor
28 shown in FIG. 3.
[0034] Specifically, an annular brush holder 34 is disposed outside
the torque rotor 24, with a predetermined gap between the brush
holder 34 and the base plate 26, and is coupled to the torque rotor
24 through a compliance disk 32.
[0035] The compliance disk 32 has rigidity of connection that is
high in the circumferential direction of the brush holder 34 but
low in the radial direction thereof. The compliance disk 32 is
molded from metal such as stainless steel or plastic such as PBT
(poly-butylene telephthalate) to have a shape shown in FIG. 4.
[0036] More specifically, the compliance disk 32 is comprised of a
ring 32a, a pair of first elastic pieces 32b, and a pair of second
elastic pieces 32c. The elastic pieces 32b, 32c are formed
integrally with the ring 32a so as to project from an outer
peripheral edge of the ring 32a.
[0037] The first elastic pieces 32b are spaced apart from each
other so as to diametrically opposite each other. Each of the first
elastic pieces 32b includes a rising portion extending upwardly
from the ring 32a, a spring portion formed into an arcuate shape to
extend radially outward and vertically downward from an upper end
of the rising portion, and a mounting portion extending
horizontally and radially outward from a lower end of the spring
portion. A pair of through holes 32d are formed in the mounting
portion.
[0038] The mounting portion of each of the first elastic pieces 32b
is disposed on and then fixed to an upper face of the brush holder
34 by means of mounting screws (not shown) that are inserted
through the through holes 32d and screwed into the brush holder
34.
[0039] The second elastic pieces 32c are spaced apart from each
other so as to diametrically opposite each other, such that a line
connecting the second elastic pieces 32c extends perpendicularly to
a line connecting the first elastic pieces 32b. Thus, the first and
second elastic pieces 32b and 32c are alternately disposed at equal
intervals of 90 deg in the circumferential direction of the ring
32a.
[0040] Each of the second elastic pieces 32c includes a rising
portion extending upwardly from the ring 32a, a spring portion
formed into an arcuate shape so as to extend radially inward and
vertically downward from an upper end of the rising portion, and a
mounting portion extending horizontally and radially inward from a
lower end of the spring portion and formed with a pair of through
holes 32e.
[0041] The mounting portion of each of the second elastic pieces
32c is disposed on and connected to an upper face of the torque
rotor 24 by means of mounting screws (not shown) passing through
the through holes 32e and screwed into the torque rotor 24.
[0042] The compliance disk 32 has high rigidity, due to the
presence of the ring 32a, in the rotating direction of the brush
holder 34 and the torque rotor 24, whereas it has low rigidity in
the radial direction of the brush holder and the torque rotor due
to the presence of the spring portions of the first and second
elastic pieces 32b, 32c. Thus, the compliance disk 32 can absorb
concentric misalignment of the axes of the column shaft 12 and the
steering shaft 16, while positively transmitting the rotation of
the steering shaft 16, i.e., the rotation of the torque rotor 24,
to the brush holder 34.
[0043] Accordingly, as viewed in the radial direction of the
compliance disk 32, the compliance disk 32 serves as an elastic
coupling member that elastically couples the torque rotor 24 and
the brush holder 34, whereby misalignment of the axes of the column
shaft 12 and the steering shaft 16 is prevented from affecting on
steering angle detection. Meanwhile, the compliance disk 32 is not
essentially required for the rotation sensor 20.
[0044] The base plate 26, the brush holder 34, and the like are
accommodated in a casing 36 fixed to the support member 38 on the
automotive body side.
[0045] The casing 36 has a ceiling wall 36a spaced from a surface
of the base plate 26 at a given distance. The ceiling wall 36a is
mounted with a pair of contact brushes 40 for steering angle
detection, which are directed downward. Only one of the contact
brushes 40 is shown in FIG. 2. Each of the contact brushes 40 has
one end thereof fixed to the ceiling wall 36a of the casing 36, and
the other end thereof disposed in contact with the electrical
resistor 28. Thus, the ceiling wall 36a of the casing 36 serves as
a brush holder for the contact brushes 40.
[0046] A pair of contact brushes 42 for reaction torque detection
are mounted on an upper face of the brush holder 34 so as to be
directed upward. These contact brushes 42 have their one ends fixed
to the brush holder 34 and the other ends thereof disposed in
contact with the electrical resistor 30. An annular projection 36b
for guiding the rotating brush holder 34 is formed in a bottom wall
of the casing 36.
[0047] The contact brushes 40, 42 have the same construction, and
therefore, an explanation on the contact brushes 40 will be given
with reference to FIG. 5, whereas an explanation on the contact
brushes 42 will be omitted.
[0048] As shown in FIG. 5, the contact brush 40 is provided with a
spring arm 44 formed by a thin plate. The spring arm 44 is
comprised of a fixture end portion 44a used to mount the spring arm
to the brush holder, and a rectangular arm portion 44b having 2.5
mm width and 4 mm length that integrally extends from the fixture
end portion 44a.
[0049] A projecting piece 44c is formed integrally with a distal
end portion of the arm portion 44b, so as to project from a side
edge of the arm portion 44b. Further, a brush portion 46 is
attached to the distal end portion of the arm portion 44b. The
brush portion 46 is comprised of wires 46a and a plate-like wire
holder 46b that is fixed by welding to the distal end portion of
the arm portion 44b. The wires 46a are supported at their roots by
the wire holder 46b.
[0050] Distal end portions of the wires 46a are curved into a hook
shape, thereby forming curved portions that constitute an
electrical contact 48, which is in contact with the electrical
resistor 28 and serves as a contact end of the contact brush
40.
[0051] The spring arm 44 having the projecting piece 44c is
fabricated by e.g., stamping a sheet of beryllium copper having
0.06 mm thick and by bending the stamped sheet so as to define the
fixture end portion 44a and the arm portion 44b. The wires 46a of
the brush portion 46 are constituted by a septinary alloy.
[0052] The spring arm 44 and spring arm portion 44b of the contact
brush 40 have a spring constant preferably equal to or less than
0.1 N/mm. A contact force of the contact brush 40 applied to the
electrical resistor 28 is preferably within a range from 0.04 N to
0.24 N inclusive. In the present embodiment, the spring constant
and the contact force are set to 0.056 N/mm and 0.14 N,
respectively.
[0053] In FIG. 5, the longitudinal axis L of the spring arm 44 is
shown by one-dotted chain line, which passes through the center of
the brush portion 46 to pass through the center of the electrical
contact 48. The arm portion 44b is asymmetric in shape about the
longitudinal axis L since it has the projecting piece 44. Thus, the
arm portion 44b is also asymmetric in weight distribution as viewed
in the widthwise direction.
[0054] The following is an explanation of the operation of the
rotation sensor 20 according to the first embodiment.
[0055] When the steering handle 10 is rotated, the rotation of the
steering handle is transmitted, on one hand, to the base plate 26
through the column shaft 12 and the steering angle rotor 22, thus
rotating the base plate 26. The rotation of the steering handle 10
is transmitted, on the other hand, to the power steering mechanism
18 and the torque rotor 24 through the column shaft 12, the torsion
bar 14 and the steering shaft 16. Thus, the front wheels 19 are
steered by the power steering mechanism 18, and the brush holder 34
rotates in unison with the torque rotor 24 coupled thereto through
the compliance disk 32.
[0056] During the front wheels 19 being steered, the torsion bar 14
receives a reaction force from a road surface and is twisted,
causing a rotational deviation to occur between the column shaft 12
and the steering shaft 16. As a result, a difference is caused
between the rotation angle of the base plate 26, fixed to the
column shaft 12 through the steering angle rotor 22, and the
rotation angle of the brush holder 34 that is coupled to the
steering shaft 16 through the compliance disk 32 and the torque
rotor 24. The rotation angle difference is within a range from +12
deg to -12 deg.
[0057] With the rotation of the base plate 26, there occurs
relative sliding motion between the electrical contacts 48 of the
contact brushes 40 fixed to the casing 36 and the annular
electrical resistor 28 formed on the base plate 26, under condition
that voltages of zero and Vcc volts are applied individually to two
reference points A, B (refer to FIG. 3) of the electrical resistor
28. Relative sliding motion of the electrical contact 48 relative
to the electrical resistor 28 from the reference point A toward the
reference point B in the circumferential direction of the
electrical resistor 28 causes the voltage signal taken out from the
contact brush 40 to increase from zero volts toward Vcc volts in
proportion to the distance of sliding motion, as shown in FIG. 6,
as understood from the principle of potentiometer. On the other
hand, relative sliding motion of the electrical contact 48 of the
contact brush 40 from the reference point B toward the reference
point A causes the output voltage of the contact brush 40 to
decrease from Vcc volts toward zero volts.
[0058] The voltage signals from the contact brushes 40 are supplied
to an external arithmetic processing device (not shown) through
signal cables (not shown), and the rotation angle of the steering
angle rotor 22 or the steering angle of the steering handle 10 is
calculated by the arithmetic processing device. The voltage signals
from the contact brushes 40 have a predetermined phase difference
therebetween, from which difference the arithmetic processing
device detects the rotating direction of the steering handle
10.
[0059] With the rotation of the steering handle 10, the brush
holder 34 rotates, accompanied with a rotational angle difference
relative to the base plate 26. In other words, electrical contacts
of contact brushes 42 held by the brush holder 34 make sliding
motions relative to an annular electrical resistor 30 formed on the
base plate 26. The voltage signals taken out from the contact
brushes 42, which vary depending on the distance of sliding motion
or the rotation angle of the brush holder 34, are supplied to the
arithmetic processing device. In the processing device, a
rotational deviation between the steering angle rotor 22 and the
torque rotor 24, i.e., reaction torque, is calculated based on the
two output signals of the contact brushes 42.
[0060] In FIG. 7, the solid line indicates results of measurement
showing a relation between sound pressure and vibration frequency
caused by vibration of the contact brush 40 or 42 of the rotation
sensor, which vibration was caused during relative sliding motion
of the contact brush relative to an electrical resistor. The dotted
line shown in FIG. 7 indicates results of similar measurement for a
rotation provided with conventional contact brushes which have the
same construction as the contact brushes 40, 42 except that they
are provided with no projection pieces 44c.
[0061] As apparent from FIG. 7, the sound pressure of sliding sound
observed in an audible sound frequency range from 300 Hz to 1600 Hz
is lower in the rotation sensor 20 having contact brushes 40 or 42
than in the rotation sensor having conventional contact
brushes.
[0062] Specifically, the contact brush receives friction energy
during the relative sliding motion. As for the conventional contact
brush having a spring arm whose arm portion is symmetric in shape
about its longitudinal axis, it is considered that friction energy
tends to cause torsional vibration T in the arm portion around the
longitudinal axis thereof, as shown by the arrow in FIG. 5. The
torsional vibration T has a frequency such as to promote principal
vibration of the arm portion in an audible sound frequency range,
thus extremely increasing the sound pressure at the natural
vibration frequency.
[0063] On the other hand, the contact brush 40 or 42 shown in FIG.
5 is provided at the arm portion 44b with the projecting piece 44c,
so that the arm portion 44b has an asymmetric shape about its
longitudinal axis, resulting in having asymmetric weight
distribution in the widthwise direction. This increases the moment
of inertia of the arm portion 44b in a direction of torsion around
the longitudinal axis L, thus suppressing the torsional vibration T
of the arm portion 44b.
[0064] Even when the torsional vibration T causes principal
vibrations in the arm portion 44b, the projecting piece 44c makes
it possible to cause such principal vibrations of the arm portion
44b to have frequencies falling outside the audible sound frequency
range. In addition, it is possible for torsional vibration T of the
arm portion 44b to occur not only at natural frequencies but also
at dispersed vibration frequencies other than natural frequencies.
As a consequence, the contact brush 40 or 42 makes it possible to
reduce the sound pressure of sliding sound, as a whole, in the
audible sound frequency range, and to reduce peak values of sound
pressure at natural frequencies.
[0065] As for the contact brushes 40, 42 whose arm portions 44b
have spring constant of 0.056 N/mm to produce a contact force of
0.14 N (central value) applied to the electrical resistors 28, 30,
a stable contact state is maintained between the contact brushes
40, 42 and the electrical resistors 28, 30 to thereby realize
excellent durability, even when there is variation (normally, about
.+-.1 mm) in distances between a ceiling wall 36a of a casing 36
and a surface of a base plate 26 and between a rear face of the
base plate 26 and an upper face of the brush holder 34.
[0066] Next, a rotation sensor according to a second embodiment of
this invention will be explained.
[0067] The rotation sensor of this embodiment is the same in
construction as that shown in FIG. 2 except for contact brushes.
Like numerals are used to denote like elements similar to those of
FIG. 2, and explanations of these elements are omitted.
[0068] As shown in FIG. 8, a contact brush 50 in this embodiment is
provided at a side edge of an arm portion 44b with an epoxy resin
member 52 serving as an asymmetrizing element, instead of the
projecting piece 44c shown in FIG. 5. Except for such a difference,
the contact brush 50 can be fabricated in the same manner as the
contact brushes 40, 42 shown in FIG. 5.
[0069] Unlike the first embodiment, a spring arm 44 of the contact
brush 50 is fabricated by stamping and bending a sheet of phosphor
bronze of 1 mm thick, and wires 46a of a brush portion 46 are each
constituted by a hexinary alloy.
[0070] The rotation sensor of the second embodiment can achieve
similar advantages as those attained by the first embodiment, since
the arm portion 44b of the contact brush 50 has an asymmetric shape
about its longitudinal axis L as viewed in the widthwise direction
because of the presence of the epoxy resin member 52, thus having a
weight distribution that is asymmetric in the widthwise
direction.
[0071] Next, a rotation sensor according to a third embodiment of
this invention will be explained.
[0072] The rotation sensor of this embodiment has the same
construction as that shown in FIG. 2 except for contact
brushes.
[0073] As shown in FIG. 9, a contact brush 54 of the rotation
sensor of the third embodiment is provided at a side edge of an arm
portion 44b of a spring arm 44 with a notched portion serving as an
asymmetrizing element. A fixture end portion 44a of the spring arm
44 has a width that is wider than that of the arm portion 44b.
[0074] The arm portion 44b is formed at its distal end with a brush
portion 44e integral therewith. The brush portion 44e is formed
into a comb-like shape having bent teeth thereof constituting an
electrical contact 48.
[0075] The spring arm 44 comprised of an fixture end portion 40a,
arm portion 40b and brush portion 44e can be fabricated from a
rectangular sheet of beryllium copper having 0.08 mm thick by
stamping and bending such a sheet.
[0076] The rotation sensor of the third embodiment can achieve
advantages similar to those achieved by the first embodiment since
the arm portion 44b of the contact brush 54 is formed into an
asymmetric shape about the longitudinal axis thereof as viewed in
the widthwise direction to realize asymmetric widthwise weight
distribution.
[0077] In addition, the contact brush 54 having the brush portion
44e that is formed integrally with the arm portion 44b can be
fabricated with ease at low costs, as compared to the contact
brushes 40, 42 and 50 each having a brush portion 46 that is
constituted by wires 46a.
[0078] The rotation sensor of this invention is not limited to the
first to third embodiments and may be modified variously.
[0079] For instance, the projecting piece 44c, epoxy resin member
52 or notched portion 44d is provided in each of the first to third
embodiments in order to form the arm portion 44b into an asymmetric
shape, but other means may be used to obtain an asymmetrically
shaped arm portion 44b.
[0080] Alternatively, respective portions of the arm portion 44b
may be formed by different materials to have asymmetric weight
distribution.
[0081] Although cases where the rotation sensor for
electrically-driven automotive power steering apparatus have been
explained in the first to third embodiments, the present invention
is applicable to a variety of rotation sensors, such as for
example, a rotation sensor for a robot arm, for detecting a
rotation angle of a rotary shaft, torque between two shafts
arranged for relative rotation, or the like.
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