Magnetostrictive torque sensor

Details Magnetostrictive torque sensor Magnetostrictive torque sensor

1999-05-27

2002-07-02

Fuller, Benjamin R. (Department: 2855)

Measuring and testing

Dynamometers

Responsive to torque

C073S862193

Reexamination Certificate

active

06412356

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.
For example, Japanese Unexamined Patent Publication No. 5-118938 and Japanese Unexamined Patent Publication No. 59-77326 disclose such magnetostrictive torque sensors. A magnetostrictive torque sensor includes a shaft, a magnetostrictive member fitted about the shaft and detecting coil. Torque applied to the shaft deforms the magnetostrictive member, which changes the magnetic permeability of the magnetostrictive member. This varies the flux, or inductance, of the magnetostrictive member. Changes in the inductance are detected based on electromotive force induced in the detecting coil. The torque applied to the shaft is detected accordingly.
As illustrated in
FIG. 4
, a prior art magnetostrictive torque sensor includes a housing
30
, a shaft
32
rotatably supported by the housing
30
, a sleeve
33
fitted about the shaft
32
and a magnetostrictive member
34
fitted about the sleeve
33
. A cylindrical stator
35
is located about the magnetostrictive member
34
. The stator
35
is supported on the shaft
32
by bearings
38
. The bearings
38
allow the stator
35
rotate relative to the shaft
32
. The stator
35
includes a cylindrical yoke
36
and a pair of retainers
37
for supporting the yoke
36
. The yoke
36
has a pair of resin bobbins B. An exciting coil
39
and a detecting coil
40
are wound about each bobbin B. The exciting coils
39
and the detecting coils
40
surround and face the surface of the magnetostrictive member
34
. When an alternating current is supplied to the exciting coils
39
, the exciting coils
39
generate flux. The flux forms a magnetic circuit through the yoke
36
and the magnetostrictive member
34
.
The surface of the magnetostrictive member
34
is divided into two detection regions X and Y. Grooves
34
a
are formed in each region X, Y. The grooves
34
a
in the region X are inclined by forty-five degrees relative to the axis of the shaft
32
, and the grooves
34
a
in the region Y are inclined by minus forty-five degrees relative to the axis. When torque is applied to the shaft
32
, a compressive force acts on one of the detection regions X and Y and a tensile force acts on the other region depending on the rotational direction of the shaft
32
. A tensile force increases the magnetic permeability of the detection regions X, Y and a compression force decreases the magnetic permeability of the regions X, Y. Variations in the magnetic permeability of the magnetostrictive member
34
change the voltages induced in the detecting coils
40
. That is, the detecting coils
40
send varying voltage values to a processor (not shown). The processor computes the torque applied to the shaft
32
based on the voltage values from the detecting coils
40
.
The yoke
36
is located close to the magnetostrictive member
34
to improve the sensitivity of the torque sensor. To facilitate the installation of the bobbins B (the coils
39
,
40
) the yoke
36
includes two semi-cylindrical pieces
41
,
42
, as illustrated in FIG.
5
. When installing the coils
39
,
40
, the bobbins B, about which the coils
39
,
40
are wound, are attached to the inner wall of the yoke
36
. Thereafter, the pieces
41
,
42
are secured to each other along two joints
43
with non-conductive adhesive. Since the yoke
36
is divided into two pieces along a plane that includes the axis and since the non-conductive adhesive is located between the pieces
41
,
42
, eddy currents in the circumferential direction of the yoke
36
are blocked by the joints
43
. 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
43
is much greater than that of the other parts, the magnetic symmetry about the axis of the yoke
36
is impaired. The lowered magnetic symmetry of the yoke
36
causes the detection voltage of the torque sensor to change in accordance with the rotational position of the shaft
32
. Therefore, even if there is no torque acting on the shaft
32
, the torque sensor may erroneously detect that a torque is acting on the shaft
32
.
Torque deforms the magnetostrictive member
34
, and deformation of the member
34
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
34
and the coils
39
,
40
changes. When the distance between the magnetostrictive member
34
and the coils
39
,
40
changes, the torque sensor falsely detects torque even if there is no torque acting on the shaft
32
. Therefore, it is imperative that the distance between the member
34
and the coils
39
,
40
be constant to guarantee the precision of the torque sensor.
As described above, the stator
35
includes the yoke
36
and the two retainers
37
, shown in
FIG. 4
, and the yoke
36
includes two semi-cylindrical pieces
41
,
42
, shown in FIG.
5
. Due to dimensional errors created during assembly, the cross-section of the stator
35
is not completely round. That is, when assembling the pieces
41
and
42
to form the yoke
36
, their relative positions may vary. Also, the positions of the retainers
37
may vary relative to the yoke
36
. The variation of the relative positions of the parts prevents the inner surface of the retainer
35
from forming a perfect circle about the axis of the shaft
32
. Further, the edges of the pieces
41
,
42
are exposed on the inner surface of the yoke
36
, which disturbs the distribution of the flux.
The resin bobbins B are press fitted in the metal yoke
36
. The bobbins B have a greater coefficient of thermal expansion than that of the yoke
36
. Therefore, when the temperature of the sensor is relatively high, a stress F is generated between each bobbin B and the yoke
36
. The stress F displaces the positions of the exciting coils
39
and the detecting coils
40
. Therefore, even if there is no torque acting on the shaft
32
, the electromotive force induced in the detecting coils
40
is changed. In other words, the magnetic characteristics of the coils
39
,
40
are affected by temperature changes. This degrades the temperature characteristics of the torque sensor thereby lowering the detection accuracy of the torque sensor.
SUMMARY OF THE INVENTION
Accordingly, it is an objective of the present invention to provide a torque sensor that accurately detects torque despite temperature changes.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, a torque sensor for sensing torque applied to a shaft is provided. The sensor includes a magnetostrictive member, an exciting coil, a detecting coil, a bobbin and stator. The magnetostrictive member is fixed to the shaft is strained by the torque applied to the shaft. 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 exciting coil and the detecting coil are wound about the bobbin. The stator supports the bobbin such that the coils surround the magnetostrictive member. The stator is rotatably supported relative to the shaft and is formed with a plurality of stator pieces. The stator has an annular groove formed in its inner surface to accommodate w the bobbin. The annular groove has a first end wall, a second end wall and a circumferential wall. The bobbin further has a first end surface, a second end surface and a circumferential surface. The first end surface of the bobbin is fixed to the first end wall of the annular groove. A first annular clearance is formed between the circumferential surface of the bobbin and the circumferential wall of the annular groove. A second annular clearance is formed between the

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