Measuring and testing – Dynamometers – Responsive to torque
Reexamination Certificate
1999-02-24
2002-04-16
Fuller, Benjamin R. (Department: 2855)
Measuring and testing
Dynamometers
Responsive to torque
C073S862000, C073S862080, C073S862331
Reexamination Certificate
active
06370967
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a magnetostrictive torque sensor that detects torque acting on a shaft by sensing magnetostrictive changes in magnetostrictive property.
For example, Japanese Unexamined Patent Publication No. 5-118938 and Japanese Unexamined Patent Publication No. 59-77326 disclose such magnetostrictive torque sensors.
As illustrated in
FIG. 15
, a prior art magnetostrictive torque sensor includes a housing
81
, a shaft
83
, a cylindrical magnetostrictive member
84
and a stator
85
. The magnetostrictive member
84
is fitted about the shaft
83
, and the stator
85
is fixed to the inner wall of the housing
81
. The housing
81
is supported on the shaft
83
by a pair of bearings
82
to cover the shaft
83
. The shaft
83
rotates relative to the housing
81
and the stator
85
. The stator
85
is cylindrical and includes a pair of exciting coils
86
and a pair of detecting coils
87
. The coils
86
,
87
are located in the inner wall of the stator
85
. The exciting coils
86
and the detecting coils
87
face the surface of the magnetostrictive member
84
. When an alternating current is supplied to the exciting coils
86
, the exciting coils
86
generate flux. The flux forms a magnetic circuit through the stator
85
and the magnetostrictive member
84
.
The surface of the magnetostrictive member
84
includes two detection regions. Grooves
84
a
are formed in each region. The grooves
84
a
in one region are inclined by forty-five degrees relative to the axis, and the grooves
84
a
in the other region are inclined by minus forty-five degrees relative to the axis. When torque is applied to the shaft
83
, a compressive force acts on one of the detection regions and a tensile force acts on the other region depending on the rotational direction of the shaft
83
. A tensile force increases the magnetic permeability of the magnetostrictive member
84
and a compression force decreases the magnetic permeability of the magnetostrictive member
84
. Changes in the magnetic permeability of the magnetostrictive member
84
change the voltages induced by the detecting coils
87
. That is, the detecting coil
87
send varying voltage values to a processor
88
. The processor
88
calculates the difference between the voltage values from the detecting coils
87
. The processor
88
computes the torque applied to the shaft
83
based on the difference.
As illustrated in
FIG. 15
, the stator
85
is located close to the magnetostrictive member
84
to improve the sensitivity of the torque sensor. The coils
86
,
87
are embedded in the inner wall of the stator
85
. To facilitate the installation of the coils
86
,
87
, the stator
85
includes two semi-cylindrical pieces
85
a
. When installing the coils
86
,
87
, bobbins about which the coils
86
,
87
are wound are attached to the inner wall of the stator
85
. Thereafter, the pieces
85
a
are secured to each other with adhesive.
Since the stator
85
is divided into two pieces along a plane that includes the axis and since the non-conductive adhesive is located between the pieces
85
a
, eddy currents in the circumferential direction of the stator
85
are blocked by joints
85
b
.The eddy currents, which would otherwise adversely affect the sensitivity of the torque sensor, are reduced. This improves the sensitivity of the torque sensor. However, since the magnetic reluctance of the joints
85
b
is much greater than that of the other parts, the magnetic symmetry about the axis of the stator
85
is impaired. The lowered magnetic symmetry of the stator
85
causes the detection voltage of the torque sensor to change in accordance with the rotational position of the shaft
83
. Therefore, even if there is no torque acting on the shaft
83
, the torque sensor may erroneously detect that a torque is acting on the shaft
83
.
Torque deforms the magnetostrictive member
84
, and deformation of the member
84
changes the inductance of the coils. The torque sensor detects torque based on the changes of the inductance. However, the inductance also changes when the distance between the magnetostrictive member
84
and the coils
86
,
87
changes. When the distance between the magnetostrictive member
84
and the coils
86
,
87
changes, the torque sensor falsely detects torque even if there is no torque acting on the shaft
83
. Therefore, it is imperative that the distance between the member
84
and the coils
86
,
87
be constant to guarantee the precision of the torque sensor.
However, errors produced by assembling the stator
85
and the bearings
82
often displace the axis O
1
of the shaft
83
from the axis O of the stator
85
(
FIG. 17
illustrates the eccentricity in an exaggerated manner). The eccentricity causes the distance between the magnetostrictive member
84
and the coils
86
,
87
to fluctuate as the shaft
83
rotates.
Due to a dimensional error created during manufacture, the cross-section of the shaft
83
may not be completely round. If the cross-section of the shaft
83
is not completely round, the distance between the magnetostrictive member
84
and the stator
85
further fluctuates, which is a further source of error.
Further, when joining the semi-cylindrical pieces
85
a
, the relative positions of the pieces
85
a
can be radially displaced relative to each other. This forms steps in the inner wall of the stator
85
. The steps vary the distance between the stator
85
and the magnetostrictive member
84
, which may cause the torque sensor to falsely detect torque.
To prevent false detection of torque, a dead zone, in which changes of inductance are not judged to be the result of torque applied to the shaft, has been widened. However, in an apparatus that activates an actuator based on detection of torque, such as a power steering apparatus, a widened dead zone extends the time lag from when torque starts acting on a shaft to when the actuator is activated. This deteriorates the response of the actuator.
SUMMARY OF THE INVENTION
Accordingly, it is an objective of the present invention to provide a torque sensor having an improved detectivity and sensitivity.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, a torque sensor is provided. The torque sensor includes a magnetostrictive member and a stator facing the magnetostrictive member. The magnetostrictive member is strained by the torque applied to the shaft. An exciting coil and a detecting coil are accommodated in the inner surface of the stator. The exciting coil generates flux running through the magnetostrictive member. The flux varies in accordance with the strain of the magnetostrictive member. The detecting coil detects the flux variation. The cross-section of the stator's inner surface is substantially a round. The stator includes a plurality of stator pieces. The joint between the stator pieces substantially does not face the magnetostrictive member.
The features of the present invention that are believed to be novel are set forth with particularity in the appended claims. The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings.
REFERENCES:
patent: 4616512 (1986-10-01), Himmelstein et al.
patent: 5646356 (1997-07-01), Ling et al.
patent: 59-77326 (1984-05-01), None
patent: 5-118938 (1993-05-01), None
Fukasaku Hiroshi
Harasawa Takeshi
Kashiwagi Yoichiro
Kouketsu Yoshitaka
Odachi Yasuharu
Allen Andre
Fuller Benjamin R.
Kabushiki Kaisha Toyoda Jidoshokki Seisakusho
Morgan & Finnegan , LLP
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