# Patent

## Title
Torque sensor

## Abstract

A torque sensor (50) detects an input torque input into a torsion bar (51) using a magnetic force generating part (60), a rotating magnetic circuit (69), a fixed magnetic circuit (90), and a magnetic sensor (98). The rotating magnetic circuit (69) comprises a first soft magnetic member (70) and a second soft magnetic member (80), each of which comprises a magnetic ring (73, 83), a magnetic tip (71, 81) facing the magnetic force generating part (60), and a magnetic column (72, 82) connecting the tip (71, 81) and the ring (73, 83). By disposing the first soft magnetic member (70) and the second soft magnetic ring (80) so as to face each other, the soft magnetic rings (70, 80) are formed in an identical shape that can be manufactured by press-working a plate material, thereby realizing a compact and lightweight torque sensor (50).

## Background
FIELD OF THE INVENTION
This invention relates to a torque sensor for detecting a torque acting on a rotating body without contact.
BACKGROUND OF THE INVENTION
JP 2007-240496 A published by the Japan Patent Office in 2007 discloses a torque sensor for a steering system of a vehicle. The torque sensor detects a steering torque input into a torsion bar without contact.
The torque sensor comprises a magnetic force generating part which rotates together with a base of the torsion bar that has been operated to rotate, a rotating magnetic circuit which rotates together with an output shaft connected to a tip of the torsion bar, a fixed magnetic circuit fixed to a housing, and a magnetic sensor which detects a magnetic flux density in the fixed magnetic circuit.
As the torsion bar undergoes a torsional deformation due to an input torque, the magnetic force generating part rotates relative to the rotating magnetic circuit, and the magnetic flux density which the rotating magnetic circuit receives from the magnetic force generating part varies.
The rotating magnetic circuit and the fixed magnetic circuit are configured to transmit magnetic flux without contacting. By detecting the density of the magnetic flux in the fixed magnetic circuit using the magnetic sensor, it is possible to detect a torque acting on the torsion bar.
To transmit the magnetic flux from the rotating magnetic circuit to the fixed magnetic circuit, the rotating magnetic circuit comprises a pair of soft magnetic rings. The fixed magnetic circuit comprises a pair of magnetic collecting rings fixed to the inner peripheral surface of the housing.
One of the magnetic collecting rings is disposed to permanently face the outer peripheral part of one of the soft magnetic rings. The other magnetic collecting ring is disposed to permanently face the outer peripheral part of the other soft magnetic ring. According to this arrangement, when the magnetic collecting ring rotates relative to the corresponding soft magnetic ring, the magnetic flux density transmitted from the soft magnetic ring to the magnetic collecting ring does not vary. By providing such a magnetic flux transmission mechanism, the magnetic sensor can detect the torque acting on the torsion bar without contact.
SUMMARY OF THE INVENTION
In this torque sensor, however, it is difficult to manufacture the soft magnetic rings by press-working a plate material due to the complicated shape thereof. An expensive manufacturing process such as metal casting or sintering is required to form the soft magnetic rings. It is also inevitable that the torque sensor becomes large in size, especially in the axial direction, because of the complicated shape of the soft magnetic rings.
It is therefore an object of this invention to form the soft magnetic rings into a simple shape so that they can be manufactured by press-working a plate material, thereby realizing a compact and lightweight torque sensor.
To achieve the above object, this invention provides a torque sensor comprising a housing, a torsion bar inserted into the housing, a magnetic force generating part which rotates together with an end of the torsion bar while generating a magnetic flux in a direction of a rotation axis of the torsion bar, and a rotating magnetic circuit which rotates together with another end of the torsion bar.
The rotating magnetic circuit comprises a first soft magnetic member and a second soft magnetic member.
The first soft magnetic member comprises a first magnetic ring disposed around the torsion bar, first magnetic tips disposed in a peripheral direction at equal angular intervals so as to face the magnetic force generating part in the direction of the rotation axis, and first magnetic columns connecting the first magnetic tips magnetically to the first magnetic ring.
The second soft magnetic member comprises a second magnetic ring disposed around the torsion bar in an offset position from the first magnetic ring in the direction of the rotation axis, second magnetic tips disposed in a peripheral direction at equal angular intervals such that the second magnetic tips and the first magnetic tips are disposed alternately, and second magnetic columns connecting the second magnetic tips magnetically to the second magnetic ring.
The torque sensor also comprises a fixed magnetic circuit fixed to the housing to surround the rotating magnetic circuit so as to accept a magnetic flux from the rotating magnetic circuit without contact, and a magnetic sensor which detects a magnetic flux density in the fixed magnetic circuit.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a longitudinal sectional view of a power steering device comprising a torque sensor according to this invention.
FIG. 2 is an exploded perspective view of essential parts of the power steering device including the torque sensor.
FIGS. 3A and 3B are perspective views of a rotating magnetic circuit according to this invention in a broken-down state and a built-up state.
FIG. 4 is a plan view of essential parts of the torque sensor illustrating the positional relationship between a magnetic force generating part and the rotating magnetic circuit.



DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1 of the drawings, a power steering device 1 for a vehicle transmits an input torque that is input into an input shaft 10 from a steering wheel of the vehicle to an output shaft 20 via a torsion bar 51. The rotational torque of the output shaft 20 is then transmitted to a steered wheel via a rack-and-pinion mechanism.
The power steering device 1 comprises an assisting mechanism using an electric motor to assist a steering operation. The assisting mechanism comprises a worm wheel 9 fixed onto the outer periphery of the output shaft 20 and a worm 19 engaging with the worm wheel 9. The electric motor drives the worm 19 to rotate such that an auxiliary torque is input into the output shaft 20 in a direction of the steering operation.
To cause the assisting mechanism to generate the auxiliary torque corresponding to the input torque input into the input shaft 10 when the steering wheel is operated, the power steering device 1 comprises a torque sensor 50 which detects the input torque input into the input shaft 10. By controlling a power current driving the electric motor on the basis of a detected torque detected by the torque sensor 50, an auxiliary torque proportional to the input torque can be exerted on the output shaft.
The input shaft 10 is supported by the housing 30 via a ball bearing 7 so as to be free to rotate. The input shaft 10 is formed to have an axial hollow portion. The torsion bar 51 is accommodated in the axial hollow portion of the input shaft 10. A base of the torsion bar 51 is fixed to the input shaft 10 using a pin 5 while a tip of the torsion bar 51 engages with the output shaft 20 via a serration 4.
A dust seal 6 is interposed between the housing 30 and the input shaft 10. The output shaft 20 is supported by the housing 30 via a ball bearing 8 so as to be free to rotate. An outer periphery of a lower end of the input shaft 10 is supported by the output shaft 20 via slide bearing 3.
In the structure described above, the input shaft 10 and the output shaft 20 can rotate relatively about an identical rotation axis within a torsional deformation range of the torsion bar 51.
Referring to FIG. 2, the torque sensor 50 comprises a magnetic force generating part 60 which rotates together with the input shaft 10, a rotating magnetic circuit 69 which rotates together with the output shaft 20, a fixed magnetic circuit 90 fixed to the housing 30, and a magnetic sensor 98 which detects a magnetic flux density in the fixed magnetic circuit 90.
The magnetic force generating part 60 comprises a magnet ring 63 fixed onto the input shaft 10 via a back yoke 61.
Referring to FIG. 4, the magnet ring 63 is formed by six arc-shaped magnets made from a hard magnetic material. Each of the arc-shaped magnets has a tip magnetized as an N-pole and another tip magnetized as an S-pole. The arc-shaped magnets are arranged such that the N-pole of on magnet and the S-pole of an adjacent magnet abut each other. According to this construction, the magnet ring 63 has six N-poles and six S-poles which are disposed alternately along a circle at equal angular intervals.
Referring again to FIG. 2, the back yoke 61 is a cylindrical member formed from a soft magnetic material and press-fitted onto the outer periphery of the input shaft 10. The magnet ring 63 is fixed in advance onto the outer periphery of the lower end of the back yoke 61.
The back yoke 61 functions as a fixing member that fixes the magnet ring 63 onto the input shaft 10 as well as a yoke that transmits magnetic flux between an N-pole and an S-pole which abut on each other. By causing the back yoke 61 to contact the upper surface of the magnet ring 63, the magnetic flux of the magnet ring 63 is concentrated on the lower surface of the magnet ring 63.
It is also possible to provide a fixing member to fix the magnet ring 63 onto the input shaft 10 separately from the back yoke, whereby the back yoke is interposed between the fixing member and the magnet ring 63.
The rotating magnetic circuit 69 comprises a first soft magnetic member 70 and a second soft magnetic member 80 for receiving the magnetic flux generated by the magnet ring 63, a fixing member 77 fixed onto the output shaft 20, and a resin-molded body 87 which fixes the first soft magnetic member 70 and the second soft magnetic member 80 onto the fitting member 77.
Referring to FIGS. 3A and 3B, the first soft magnetic member 70 comprises a first magnetic ring 73, six first magnetic columns 72 projecting respectively from the first magnetic ring 73 downward, and first magnetic tips 71 which are formed by bending a lower end portion of each first magnetic column 72 inward so as to face the lower end face of the magnet ring 63.
The second soft magnetic member 80 comprises a second magnetic ring 83, six second magnetic columns 82 projecting respectively from the second magnetic ring 83, and second magnetic tips 81 which are formed by bending an upper end portion of each second magnetic column 82 inward so as to face the lower end face of the magnet ring 63.
The first magnetic ring 73 and the second magnetic ring 83 are removed from each other in the direction of the rotation axis such that the first magnetic tips 71 and the second magnetic tips 81 are arranged alternately at equal angular intervals on an identical plain which is orthogonal to the rotation axis of the torsion bar 51. The first magnetic ring 73 and the second magnetic ring 83 are formed to have a predetermined width in the radial direction. The first magnetic ring 73 and the second magnetic ring 83 are formed into a continuous ring-shape. It is also possible to form the first magnetic ring 73 and the second magnetic ring 83 in a C-shaped ring having a slit.
Referring again to FIG. 4, the first magnetic tip 71 and the second magnetic tip 81 are disposed in a predetermined rotation position such that radial lines connecting the center of the torsion bar 51 and the center of the respective first magnetic tips 71 and radial lines connecting the center of the torsion bar 51 and the center of the respective second magnetic tips 81 correspond to the border between the N-pole and the S-pole of each of the six arc-shaped magnets forming the magnet ring 63 when the power steering device is in a neutral position in which no torque is exerted on the torsion bar 51.
Referring again to FIGS. 3A and 3B, the first magnetic columns 72 and the second magnetic columns 82 are disposed in parallel with the rotation axis of the torsion bar 51. Each of the first magnetic columns 72 and the second magnetic columns 82 is formed into a flat-plate-shape. When assembled into the rotating magnetic circuit 69, the first magnetic columns 72 and the second magnetic columns 82 form a planar shape of a dodecagon about the center of the torsion bar 51.
It is also possible to form each of the first magnetic columns 72 and the second magnetic columns 82 in a curved shape such that the first magnetic columns 72 and the second magnetic columns 82, when assembled into the rotating magnetic circuit 69, form a cylindrical shape which is coaxial with the torsion bar 51.
The first magnetic columns 72 extend downward from the first magnetic ring 73 and pass along the outer periphery of the magnet ring 63 to reach the first magnetic tips 71 located below the magnet ring 63. A predetermined space is kept between the first magnetic columns 72 and the magnet ring 63 to prevent a magnetic short-circuit therebetween. The second magnetic columns 82 extend upward from the second magnetic ring 83 to reach the second magnetic tips 81. The first magnetic columns 72 and the second magnetic columns 82 therefore extend in opposite directions to reach the first magnetic tips 71 and the second magnetic tips 81, which are located in the same plane.
According to the above construction, the size of the rotating magnetic circuit 69 can be made short in the direction of the rotation axis while preserving enough space between the first magnetic ring 73 and the second magnetic ring 83 to prevent a magnetic short-circuit therebetween.
The first magnetic ring 73, the first magnetic columns 72, and the first magnetic tips 71 are manufactured in advance by performing press-working on a soft magnetic plate material to form a one-piece first soft magnetic member 70. The second magnetic ring 83, the second magnetic columns 82, and the second magnetic tips 81 are also manufactured in advance by performing press-working on a soft magnetic plate material to form a one-piece second soft magnetic member 80. It is also possible to manufacture these members by means of metal casting or sintering.
When the first soft magnetic member 70 and the second soft magnetic member 80 are manufactured by means of metal casting or sintering, it is possible to form the first magnetic columns 72 and the second magnetic columns 82 into a block shape that is thicker than the press-formed magnetic columns in the radial direction with respect to the torsion bar 51.
The first soft magnetic member 70 and the second soft magnetic member 80 are preferably made as identical members having an identical shape and size. By reversing the directions of assembling, the two identical members serve as the first soft magnetic member 70 and the second soft magnetic member 80.
It is still possible to form the soft magnetic member 70 and the second soft magnetic member 80 in different shapes. For example, the first soft magnetic member 70 may be formed in a flat shape in which the first magnetic columns 72 are omitted and the first magnetic tips 71 project inward directly from the first magnetic ring 73. In this case, only the second magnetic columns 82 of the second soft magnetic member 80 function to provide a space between the first magnetic ring 73 and the second magnetic ring 83 in the direction of the rotation axis of the torsion bar 51 so as to prevent a magnetic short-circuit between the first magnetic ring 73 and the second magnetic ring 83.
Similarly, the second soft magnetic member 80 may be formed in a flat shape in which the second magnetic columns 82 are omitted and the second magnetic tips 81 project inward directly from the second magnetic ring 83. In this case, only the first magnetic columns 72 provided in the first soft magnetic member 70 function to provide a space between the first magnetic ring 73 and the second magnetic ring 83 in the direction of the rotation axis of the torsion bar 51 so as to prevent a magnetic short-circuit between the first magnetic ring 73 and the second magnetic ring 83.
The fitting member 77 is made from ferrous metal and formed in a cylindrical shape. A resin-molded body 87 is used for integrating the fitting member 77, the first soft magnetic member 70, and the second soft magnetic member 80 into one-piece.
Referring again to FIG. 1, a plurality of holes 29 are formed on the outer periphery of the output shaft 20. The fitting member 77 has a lower end 76 which covers the holes 29 in a state where the rotating magnetic circuit 69 has been press-fitted onto the output shaft 20. Once the rotating magnetic circuit 69 has been press-fitted onto the output shaft 20, the fitting member 77 is secured onto the output shaft 20 so as not to displace in the axial and rotation directions by punching the corresponding parts of the lower end into the holes 29 using a staking jig.
By thus integrating the fitting member 77, the first soft magnetic member 70, and the second soft magnetic member 80 into one piece via the resin-molded body 87, no magnetic short-circuits occur among the fitting member 77, the first soft magnetic member 70, and the second soft magnetic member 80 even if the fitting member 77 is constructed from a ferrous metal.
It is possible to construct the fitting member 77 from a nonmagnetic material such as aluminum, but constructing the fitting member 77 from a ferrous metal helps to reduce the manufacturing cost of the torque sensor 50.
The resin-molded body 87 is formed as follows. The fitting member 77, the first soft magnetic member 70, and the second soft magnetic member 80 are first disposed in a predetermined die. A molten thermoplastic resin is then funneled into the die. The resin-molded body 87 is obtained by cooling the molten thermoplastic resin thus molded. The material for the resin-molded body 87 is not limited to molten thermoplastic resin, and thermosetting resin or reaction curing resin may be used instead.
Referring again to FIGS. 3A and 3B, the first magnetic ring 73 has a plurality of positioning holes 74 formed at regular angular intervals. The second magnetic ring 83 has a plurality of positioning holes 84 formed at regular angular intervals. When the resin-molded body 87 is molded, the magnetic ring 73 and the second magnetic ring 83 are positioned precisely in the die with respect to the rotation direction by inserting a positioning jig into the positioning holes 74 and 84.
An annular groove 78 is formed on the outer periphery of the fitting member 77 in advance. The fitting member 77 is provided with a plurality of rotation blocking holes 79 opening onto the annular groove 78. When forming the resin-molded body 87, the annular groove 78 and the rotation blocking holes 79 are filled up with the resin, thereby preventing the fitting member 77 from displacing in the axial and rotation directions with respect to the resin-molded body 87.
Referring again to FIG. 2, the die is formed in advance in such a shape that twelve inward depressed portions 88 are formed on the inner periphery of the resin-molded body 8 to accommodate the first magnetic columns 72 and the second magnetic columns 82.
By forming the die in this way, the resin funneled into the die to mold the resin-molded body 87 is prevented by the die from contacting the first magnetic columns 72 and the second magnetic columns 82. Such an arrangement of the die is preferable in preventing an internal stress from being generated in the first magnetic columns 72 and the second magnetic columns 82 due to a forming pressure of the resin during the process of solidification and shrinking, or due to thermal expansion or thermal shrinkage of the resin after the resin-molded body 87 is molded.
Internal stress causes the magnetic flux transmitting performance of the first magnetic columns 72 and the second magnetic columns 82 to deteriorate. By forming the die to provide the depressed portions 88, the first magnetic columns 72 and the second magnetic columns 82 can be positioned easily with respect to the die.
The die is formed such that the depressed portions 88 are provided on the outside of the first magnetic columns 72 and the second magnetic columns 82. It is however possible to form the die such that the depressed portion 88 is formed on the inside of the first magnetic columns 72 and the second magnetic columns 82.
The fixed magnetic circuit 90 comprises a first magnetic collecting ring 91 facing the outer periphery of the first magnetic ring 73, a second magnetic collecting ring 92 facing the outer periphery of the second magnetic ring 83, a first magnetic collecting yoke 93 connected to the first magnetic collecting ring 91, and a second magnetic collecting yoke 94 connected to the second magnetic collecting ring 92.
The first magnetic collecting ring 91 is constructed from a soft magnetic material into a C-shaped ring which has a slit 91A. The second magnetic collecting ring 92 is constructed from a soft magnetic material into a C-shaped ring which has a slit 92A. The first magnetic collecting ring 91 and the second magnetic collecting ring 92 are formed in an identical shape. The first magnetic collecting ring 91 and the second magnetic collecting ring 92 are fixed on the inner periphery of the housing 30.
Referring again to FIG. 1, a first annular groove 31 into which the first magnetic collecting ring 91 is fitted and a second annular groove 32 into which the first annular groove 31 is fitted are formed on the inner periphery of the housing 30. The first magnetic collecting ring 91 narrows the width of the slit 91A when it is fitted into the first annular groove 31. The second magnetic collecting ring 92 narrows the width of the slit 92A when it is fitted into the second annular groove 32. When the widths of the slits 91A and 92A are narrowed, the effect of the magnetic gap formed by the slits 91A and 92A also decreases. The housing 30 is formed from a nonmagnetic material such as aluminum.
The depth of the first annular groove 31 is set to be smaller than the thickness of the first magnetic collecting ring 91 in the radial direction. The depth of the second annular groove 32 is set to be smaller than the thickness of the first magnetic collecting ring 92 in the radial direction. As a result, the first magnetic collecting ring 91 projects inward from an inner peripheral surface 38 of the housing 30 in a state where it is fitted into the first annular groove 31. The second magnetic collecting ring 92 projects inward from the inner peripheral surface 38 of the housing 30 in a state where it is fitted into the second annular groove 32.
The first magnetic collecting ring 91 has a central portion with respect to the axial direction which faces the outer periphery of the first magnetic ring 73 at a predetermined gap. The second magnetic collecting ring 92 has a central portion with respect to the axial direction which faces the outer periphery of the second magnetic ring 83.
When the fixed magnetic circuit 90 is fitted into the housing 30, staking parts 33 are formed on the inner periphery of the housing 30 along the first annular groove 31 by punching the inner peripheral surface 38 of the housing 30 using a jig to deform the portions on both sides of the first annular groove 31, thereby securing the first magnetic collecting ring 91 within the first annular groove 31.
Similarly, staking parts 34 are formed on the inner periphery of the housing 30 along the second annular groove 32 by punching on the inner peripheral surface 38 of the housing 30 using a jig to deform the portions on both sides of the second annular groove 32, thereby securing the second magnetic collecting ring 92 within the second annular groove 32.
According to this process, looseness is eliminated in the first magnetic collecting ring 91 and the second magnetic collecting ring 92 and the rotation of the first magnetic collecting ring 91 and the second magnetic collecting ring 92 with respect to the housing 30 is prevented.
It is possible to form the housing 30 from a resin. In this case, the first magnetic collecting ring 91 and the second magnetic collecting ring 92 may be fixed to the housing 30 by heat staking.
The first magnetic collecting yoke 93 and the second magnetic collecting yoke 94 are fixed in advance to the interior of a sensor housing 39. By press-fitting the sensor housing 39 into a lateral opening of the housing 30, the first magnetic collecting yoke 93 comes into contact with a back portion of the first magnetic collecting ring 91 on the opposite side of the slit 91A and the second magnetic collecting yoke 94 comes into contact with a back portion of the second magnetic collecting ring 92 on the opposite side of the slit 92A.
Referring again to FIG. 2, the first magnetic collecting yoke 93 is formed into a block shape and has a pair of magnetic collecting projections 93a projecting downward. The second magnetic collecting yoke 94 is formed into a block shape and has a pair of magnetic collecting projections 94a projecting upward. The pair of magnetic collecting projections 93a and the pair of magnetic collecting projections 94a have end faces facing each other on both sides of a pair of magnetic gaps 96.
Referring again to FIG. 1, a magnetic sensor 98 using a hall element is inserted into the pair of magnetic gaps 96. The magnetic sensor 98 outputs signals representing a magnitude and direction of a magnetic field formed in the magnetic gaps 96 via a signal cable 97. The magnetic sensor 98 may further comprise an amplifying circuit which amplifies the signals generated by the hall element, a circuit for temperature compensation, or a circuit for noise filtering.
When the power steering device is in the neutral position in which no torque is exerted on the torsion bar 51, the first magnetic tips 71 of the first soft magnetic member 70 and the second magnetic tips 81 of the second soft magnetic member 80 face the N-poles and the S-poles of the magnet ring 63 evenly, thereby causing a magnetic short circuit between an N-pole and an adjacent S-pole. In this state, the magnetic flux of the magnet ring 63 is not transmitted to the rotating magnetic circuit 69 and the fixed magnetic circuit 90.
When a driver of a vehicle operates the steering wheel, a torque in one direction is input into the torsion bar 51 and the torsion bar 51 undergoes torsional deformation according to the direction of the input torque.
Providing that the rotating magnetic circuit 69 has rotated clockwise in FIG. 4 with respect to the magnet ring 63 as a result of the torsional deformation of the torsion bar 51, the total area of the first magnetic tips 71 facing the N-poles increases and the total area of the second magnetic tips 81 facing the S-poles increases.
As a result, the magnetic flux generated by the magnet ring 63 is transmitted to the field magnetic circuit 90 via the rotating magnetic circuit 69, and the magnetic sensor 98 outputs signals in response to the magnitude and the direction of the magnetic field formed in the magnetic gaps 96.
The magnetic path formed through the rotating magnetic circuit 69 and the fixed magnetic circuit 90 in this state starts from the N-poles of the magnet ring 63, and then passes through the first magnetic tips 71, the first magnetic columns 72, the first magnetic ring 73, the first magnetic collecting ring 91, the first magnetic collecting yoke 93, the second magnetic collecting yoke 94, the second magnetic collecting ring 92, the second magnetic ring 83, the second magnetic columns 82, and the second magnetic tips 81 to reach the S-poles of the magnet ring 63.
The torsion bar 51 undergoes a torsional deformation in response to the input torque. As the difference in the N-pole facing area and the S-pole facing area of the first magnetic tips 71 and the difference in the S-pole facing area and the N-pole facing area of the second magnetic tips 81 increases, the magnitude of the magnetic field formed in the magnetic gap 96 increases and the output signals from the magnetic sensor 98 vary accordingly.
It should be noted that the number of the poles of the magnet ring 63 can be set arbitrarily as long as it is equal to or greater than two. Providing that the area of the first soft magnetic member 70 facing the magnet ring 63 is identical to the area of the second soft magnetic member 80 facing the magnet ring 63, the magnetic flux density transmitted to the magnetic sensor 98 is increased by increasing the number of poles of the magnet ring 63.
The first magnetic ring 73 and the second magnetic ring 83 must be removed from each other in the direction of the rotation axis so as prevent a magnetic short-circuit therebetween.
In the torque sensor according to the aforesaid prior art, both the first magnetic ring and the second magnetic ring are located under the magnet and both the first magnetic columns and the second magnetic columns are formed upward toward the first magnetic tips and the second magnetic tips, respectively. As a result, the first magnetic columns can be made short, but the second magnetic columns inevitably become long. Therefore it is difficult to manufacture the second soft magnetic member comprising the second magnetic columns by means of press-working.
In the torque sensor 50 according to this invention, the magnet ring 63 is disposed between the first magnetic ring 73 and the second magnetic ring 83 with respect to the direction of the rotation axis and the first magnetic columns 72 and the second magnetic columns 82 are arranged to project in opposite directions.
More specifically, the first magnetic column 72 projects downward from the first magnetic ring 73, thereby passing the outside of the magnetic force generating part 60 in the direction of the rotation axis to reach the first magnetic tips 71. The second magnetic columns 82 project upward from the second magnetic ring 83 so as to approach the magnetic force generating part 60 in the direction of the rotation axis to reach the second magnetic tips 81.
According to the above construction of the rotating magnetic circuit 69, the length of the first magnetic columns 72 and the length of the second magnetic columns 82 in the direction of the rotation axis can be made shorter with respect to the distance between the first magnetic ring 73 and the second magnetic ring 83. By shortening the length of the first magnetic columns 72 and the length of the second magnetic columns 82 in this way, the first soft magnetic member 70 and the second soft magnetic member 80 can be manufactured by means of press-working.
Since the press-working requires less material than metal casting or sintering, manufacturing the first soft magnetic member 70 and the second soft magnetic member 80 by means of press-working lowers the manufacturing cost of the torque sensor 50.
Since press-working manufactures a member which is thinner than a member manufactured by metal casting or sintering, the cross-sectional area of the magnetic path which the member can provide is inevitably smaller. However, by constructing the first soft magnetic member 70 and the second soft magnetic member 80 using a high-density material, magnetic hysteresis characteristics of the torque sensor 50 can be improved.
Further, since the first magnetic columns 72 are disposed to surround the magnet ring 63, the space required in the direction of the rotation axis for disposing the first soft magnetic member 70 becomes small, thereby realizing a compact and lightweight torque sensor.
The contents of Tokugan 2008-093636, with a filing date of Mar. 31, 2008 in Japan, are hereby incorporated by reference.
Although the invention has been described above with reference to certain embodiments, the invention is not limited to the embodiments described above. Modifications and variations of the embodiments described above will occur to those skilled in the art, within the scope of the claims.
For example, application of the torque sensor according to this invention is not limited to a power steering device for a vehicle. It is widely applicable for the detection of a torque acting between relatively rotating objects connected by a torsion bar.
The embodiments of this invention in which an exclusive property or privilege is claimed are defined as follows:

## Claims

1. A torque sensor comprising:
a housing;
a torsion bar inserted into the housing, the torsion bar having a rotation axis, the torsion bar having opposite first and second ends;
a magnetic force generating part which rotates together with the first end of the torsion bar while generating a magnetic flux in a direction of the rotation axis;
a rotating magnetic circuit which rotates together with the second end of the torsion bar, the rotating magnetic circuit comprising a first soft magnetic member and a second soft magnetic member, the first soft magnetic member comprising a first magnetic ring disposed around the torsion bar, a first magnetic tip disposed to face the magnetic force generating part in the direction of the rotation axis, and a first magnetic column connecting the first magnetic tip magnetically to the first magnetic ring, and the second soft magnetic member comprising a second magnetic ring disposed around the torsion bar in a position offset from a position of the first magnetic ring in the direction of the rotation axis, a second magnetic tip disposed such that the second magnetic tip and the first magnetic tip are disposed on an identical plane orthogonal to the rotation axis, and a second magnetic column connecting the second magnetic tip magnetically to the second magnetic ring, the second magnetic column and the first magnetic column being oriented oppositely;
a fixed magnetic circuit fixed to the housing to surround the rotating magnetic circuit so as to accept without contact a magnetic flux from the rotating magnetic circuit; and
a magnetic sensor which detects a magnetic flux density in the fixed magnetic circuit.

2. The torque sensor as defined in claim 1, wherein the magnetic force generating part comprises a magnet ring which is disposed coaxially with the torsion bar on the outside thereof and has an N-pole and an S-pole formed alternately in a peripheral direction at equal angular intervals, and the first magnetic tip has an N-pole facing area and an S-pole facing area that are equal to each other, while the second magnetic tip has an N-pole facing area and an S-pole facing area that are equal to each other, in a neutral position where the torsion bar does not undergo torsional deformation.

3. The torque sensor as defined in claim 1, wherein the first magnetic ring and the first magnetic tip are offset from the magnetic force generating part in different directions along the rotation axis, the first magnetic column is arranged to pass the outside of the magnetic force generating part so as to connect the first magnetic ring and the first magnetic tip.

4. The torque sensor as defined in claim 3, wherein the first soft magnetic member and the second soft magnetic member are formed into an identical shape and disposed in opposite directions with respect to the direction of the rotation axis.

5. The torque sensor as defined in claim 1, wherein the first soft magnetic member and the second soft magnetic member are formed by press-working a soft magnetic plate material.

6. The torque sensor as defined in claim 1, wherein the rotating magnetic circuit further comprises a resin-molded body which fixes the relative rotation position between the first soft magnetic member and the second soft magnetic member, and a fixing member which fixes the first soft magnetic member and the second soft magnetic member to the second end of the torsion bar via the resin-molded body.

7. The torque sensor as defined in claim 6, wherein the resin-molded body comprises depressed portions which accommodate the first magnetic column and the second magnetic column.

8. The torque sensor as defined in claim 7, wherein a minute gap is formed between the depressed portion and the first magnetic column as well as between the depressed portion and the second magnetic column.

9. The torque sensor as defined in claim 1, wherein the fixed magnetic circuit comprises a first magnetic collecting ring which surrounds the first magnetic ring and a second magnetic collecting ring which surrounds the second magnetic ring, a first magnetic collecting yoke contacting the first magnetic collecting ring, and a second magnetic collecting yoke contacting the second magnetic collecting ring, and the magnetic sensor is interposed between the first magnetic collecting yoke and the second magnetic collecting yoke.