Electricity: measuring and testing – Electrical speed measuring – Including speed-related frequency generator
Reexamination Certificate
1996-09-19
2001-05-08
Snow, Walter (Department: 2862)
Electricity: measuring and testing
Electrical speed measuring
Including speed-related frequency generator
C384S448000
Reexamination Certificate
active
06229298
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related to a rolling bearing unit with rotational speed detection device which is installed in an anti-lock braking system (ABS) or a traction control system (TCS) so that a vehicle wheel is rotatably supported with reference to a suspension apparatus while the rotational steed of the vehicle wheel is detected.
BACKGROUND OF THE INVENTION
Description of the Related Art
In an automotive vehicle, the wheels must be supported by a suspension apparatus so as to rotate freely. Also, in order to control an anti-lock braking system (ABS) or a traction control system (TCS), the rotational speed of the wheels must be detected. Rolling bearing units having a rotational speed detection device have heretofore been proposed, as disclosed for example in Japanese Utility Model First Publication KOKAI Nos. 3-128856, 5-4021 and in U.S. Pat. No. 5,063,345. A schematic diagram of the rolling bearing unit of U.S. Pat. No. 5,063,345 is shown in
FIGS. 1 and 2
.
The rolling bearing unit has a tone wheel or pulser ring
1
a part of which is illustrated in
FIG. 1 and a
stationery or fixed sensor
3
which is mounted on a suspension apparatus. The tone wheel
1
is generally made of a magnetic material and is ford with gear-shaped serrations
2
on an outer peripheral surface thereof. The tone wheel
1
is associated with a vehicle wheel through the bearing unit so as to rotate together with the vehicle wheel. The fixed sensor
3
comprises a permanent magnet
4
which produces an axially aligned (top-bottom direction in
FIG. 1
) magnetic field, a yoke
5
made of a magnetic material, end a coil
6
wound around the yoke
5
.
The yoke
5
has a base end
5
a
(upper end face in
FIG. 1
) and a tip end
5
b
(lower end face in FIG.
1
). The base end face
5
a
of the yoke
5
is abutted against one end face of the permanent magnet
4
(lower end face in
FIG. 1
), while the tip end face
5
b
of the yoke
5
faces towards the serrations
2
. As a result, the magnetic field is formed around the permanent magnet
4
and the yoke
5
as shown by the dotted lines with arrows in FIG.
1
.
The density of a magnetic flux producing the magnetic field becomes higher when a protrusion
2
a
of the serrations
2
is opposed to the tip end face
5
b
of the yoke
5
, and becomes lower when a recess
2
b
of the serrations
2
is opposed to the tip end face
5
b
of the yoke
5
. The induced electromotive force in the coil
6
thus varies as shown in
FIG. 2
, in accordance with the variation in magnetic flux density of the magnetic field formed around the coil
6
. Since the frequency of the induced electromotive force is proportional to the rotational speed of the vehicle wheel (not shown), then the ABS or the TCS can be appropriately controlled by inputting the induced electromotive force obtained as an output signal from the sensor
3
, to a controller
7
of the ABS or the TCS.
With the conventional rolling bearing unit fitted with the rotational speed detection device constructed and operated as mentioned above, there are the following problems.
Namely, in the support section for the rotating wheel, it is not always possible to maintain enough space inside for installation of the sensor
3
. It is thus necessary to miniaturize the sensor
3
. However mere miniaturization of the sensor
3
causes not only a drop off in output (the difference V in
FIG. 2
between the maximum and minimum voltages) of the sensor
3
, but also a loss in wheel speed detection reliability.
This is because, in order to miniaturize the sensor
3
, it is necessary to make the yoke
5
smaller in size. However, to sufficiently maintain the magnitude (voltage) of the output signal from the sensor
3
, a permanent magnet of high magnetic strength or high magnetic flux density must be used for the permanent magnet
4
. As a result, there is the likelihood of saturation of the magnetic flux inside the yoke
5
. If this occurs, the variation in magnetic flux density between the condition wherein the tip end face
5
b
of yoke
5
is opposed to a protrusion
2
a
of the serrations
2
and the condition wherein it is opposed to a recess
2
b
of the serrations
2
is reduced, so that the output becomes extremely small.
In this respect, a rolling bearing unit fitted with a rotational speed detection device incorporating a sensor which avoids saturation of the magnetia flux inside the yoke while still enabling installation in a limited space, and providing a sufficient output, is disclosed for example in Japanese Utility Model First Publication KOKAI No. 5-4021. A schematic diagram of this bearing unit is shown in
FIGS. 3 and 4
.
The construction disclosed in this publication has a tone wheel or pulser ring
1
and a permanent magnet
8
which has a magnetic field directed in a radial direction of the tone wheel
1
. A projection
9
is provided on an inner peripheral face of the permanent magnet
8
while a yoke
10
made of a magnetic material is provided with a central portion thereof in abutting contact with the outer peripheral face of the permanent magnet
8
and with both end portions
11
turned down.
The projection
9
together with the end portions
11
are arranged so as to be simultaneously faced to protrusions
2
a
of the serrations
2
on the outer periphery of a tone wheel
1
.
A coil
12
which is wound around the yoke
10
, comprises a first coil portion
12
a
and a second coil portion
12
b
wound in opposite directions to each other and connected together in series.
With such a construction, the magnetic flux from the outer side face of the permanent magnet
8
flows in two separate systems inside the yoke
10
as shown by arrows. Therefore, compared to the structure of
FIG. 1
, the magnetic flux is less likely to become saturated inside the yoke
10
. However there are still problems with obtaining a reliable effect. If the cross-sectional area of the yoke
5
of
FIG. 1
is assumed to be the same as that of the yoke
10
of
FIGS. 3 and 4
, then the structure of
FIGS. 3 and 4
can take up to approximately two times the magnet flux density of the structure of FIG.
1
. However if a permanent magnet
8
having a magnet flux density in excess of this level is used, the output of the sensor
13
will still drop considerably with saturation of the magnetic flux in the yoke
10
.
With the construction of
FIGS. 3 and 4
, all of the magnet flux utilized in detecting rotational speed passes through the space between the tip end face
9
of the permanent magnet
8
, and the tip of the protrusion
2
a
of the sarrations
2
which are opposed to the tip end face
9
of the permanent magnet
8
. However, in order to accurately detect rotational speed, it is necessary to make the pitch of the serrations
2
fine, so that the area of the tips of the protrusions
2
a
, and the cross sectional area of the space is reduced. Therefore when the magnetic flux density of the permanent magnet
8
is increased, the magnetic flux would become saturated in this space, so that there is still a considerable drop in output from the sensor
13
.
SUMMARY OF THE INVENTION
The rolling bearing unit with the rotational speed detection unit according to the present invention, is addressed at coping with the above mentioned problems.
An objective of the present invention is to provide a rotational speed detection device comprising a tone wheel and a sensor for use in a rolling unit comprising a rotating ring for supporting the tone wheel which is rotated with the rotating ring, and a fixed ring provided coaxial with the rotating ring for supporting the sensor to be faced to the one wheel, the tone wheel having a portion opposed to the sensor where first and second portions having different magnetic properties are alternatively positioned with a same pitch in a circumferential direction, and the sensor having a pair of permanent magnets, yoke of magnetic material disposed between the permanent magnets and having a mid portion, and a coil wound around the mid portion of the yoke, the yoke having opposi
Mashio Akihiro
Morita Kouichi
Okada Takamichi
Sakamoto Junshi
Evenson, McKeown, Edwards & Lenahan P.L.L.C.
NSK Ltd.
Snow Walter
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