Magnetostrictive torque sensor

Details Magnetostrictive torque sensor Magnetostrictive torque sensor

1998-06-02

2001-05-29

Fuller, Benjamin R. (Department: 2855)

Measuring and testing

Dynamometers

Responsive to torque

Reexamination Certificate

active

06237428

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a torque sensor. More particularly, the present invention pertains to magnetostrictive torque sensors that are attached, for example, to a drive shaft or a steering shaft of vehicles to detect the magnitude of torque applied to the shaft and the rotational direction and rotational amount of the shaft.
FIG. 11
is a cross-sectional view illustrating a prior art magnetostrictive torque sensor
81
provided on a shaft
80
. The sensor
81
includes a sleeve
82
fitted about the shaft
80
. A magnetostrictive layer
83
is formed on the circumference of the sleeve
82
. The layer
83
is located at the axial center portion of the sleeve
82
. The layer
83
is divided into two magnetostrictive segments
83
A,
83
B at the axial center of the sleeve
82
by an imaginary plane perpendicular to the axis of the shaft
80
. Grooves are formed in the outer surfaces of the layer
83
. Half of the grooves are formed in the segment
83
A and the other half are formed in the segment
83
B. The grooves on one of the segments
83
A,
83
B are inclined by forty-five degrees relative to the axis of the sleeve
82
, whereas the grooves on the other segment are inclined by minus forty five degrees. The grooves on the segment
83
A and the grooves on the segment
83
B do not intersect with each other.
Bearings
84
are press fitted about the sleeve
82
. The bearings
84
support a cylindrical stator
85
to be rotatable relative to the sleeve
82
. Exciting coils
86
A,
86
B and detecting coils
87
A,
87
B are located in the inner wall of the stator
85
. The coils
86
A,
87
A face the segment
83
A and the coils
86
B,
87
B face the segment
83
B.
The segments
83
A,
83
B are strained in accordance with the magnitude and direction of torque acting on the shaft
80
. The strain of the segments
83
A,
83
B changes the permeability of the segments
83
A,
83
B. Changes in the permeability cause voltages induced by the detecting coils
87
A,
87
B to differ from each other. The value of the torque applied to the shaft
80
is represented by the magnitude of the voltage difference. The direction of the torque is represented by the sign of the subtraction result of the voltages. The magnetostrictive sensor detects the torque based on the difference between the output voltages of the coils
87
A,
87
B.
With no torque applied on the shaft
80
, the segments
83
A and
83
B have the same permeability. However, if the segments
83
A,
83
B are axially displaced relative to the corresponding coil pairs
86
A,
87
A and
86
B,
87
B, magnetic fluxes through the detecting coils
87
A,
87
B are changed. As a result, the voltage of the coils
87
A,
87
B become different even if there is no torque applied to the shaft
80
. This results in an erroneous detection of torque. In other words, the detection accuracy of the torque sensor
81
is degraded.
Accordingly, the torque sensor
81
has annular collars
88
for preventing the segments
83
A,
83
B from being displaced relative to the coil pairs
86
A,
87
A and
86
B,
87
B. The collar
88
are formed on the circumference of the sleeve
82
and prevent the bearings
84
, which are press fitted about the sleeve
82
, from moving axially. The bearings
84
hold the stator
85
thereby restricting the axial movement of the stator
85
. As a result, the positions of the coil pairs
86
A,
87
A and
86
B,
87
B are fixed relative to the segments
83
A,
83
B.
However, each element in the torque sensor
81
has a dimensional error that is created during manufacture process. Thus, the diameter of the sleeve
82
may be slightly smaller than the inner diameter of the bearings
84
. In this case, the bearings
84
are not firmly fitted about the sleeve
82
. This results in the bearings
84
being slightly inclined relative to the sleeve
82
. In other cases, the bearings
84
separate from the collars
88
during use of the sensor
81
. As a result, the coil pairs
86
A,
87
A and
86
B,
87
B are axially displaced from the predetermined positions in relation with the segments
83
A,
83
B. This degrades the detection accuracy of the sensor
81
.
In order to cope with the above drawbacks, a construction including snap rings fitted about the sleeve
82
is proposed. Specifically, a groove is formed in the sleeve
82
at a position adjacent to and axially outside of each bearing
84
. A snap ring is fitted to each groove. Each snap ring and the corresponding bearing
84
fix the position of the corresponding bearing
84
. However, the width of the grooves and thickness of the snap ring include dimensional errors. The errors may result in the bearings
84
separating from the collars
88
. As a result, the position of the stator
85
is displaced relative to the sleeve
82
.
SUMMARY OF THE INVENTION
Accordingly, it is an objective of the present invention to provide a magnetostrictive torque sensor that prevents a stator having exciting and detecting coils from moving axially and thus accurately maintains the positions of the coils relative to corresponding magnetostrictive layer thereby improving the torque detection accuracy.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.


REFERENCES:
patent: 4996890 (1991-03-01), Taniguchi et al.
patent: 5578767 (1996-11-01), Chikaraishi et al.
patent: 62-203027 (1987-09-01), None

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