JP2008191168A - Magnetostrictive torque sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetostrictive torque sensor capable of stably sensing a torque applied to a rotating shaft, without being affected by the frequency characteristics of a peak-and-holding circuit used in a conventional device, without generating attenuation of gain or delay in the phase of a torque sense signal. <P>SOLUTION: This magnetostrictive torque sensor 20 comprises the rotating shaft 12b, a plurality of magnetostrictive films 20b, 20c each of which the magnetic characteristics are changed, in response to the torque applied to the rotating shaft, and a plurality of coils 20d, 20e, 20f, 20g each being responsive to variations in the magnetostrictive characteristics of each of the plurality of the magnetostrictive films. The magnetostrictive torque sensor is adapted to sense the torque applied to the rotary shaft and is constituted, such that the two coils are coupled in series and the torque is sensed, based on the voltage of the midpoint between the two coils. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to a magnetostrictive truxene Sa for detecting the applied torque based on the change in value of the inductance of the coil provided around the magnetostrictive films.

  In an electric power steering device, an auxiliary force generation motor is attached to a mechanically configured steering device, and the rotational torque supplied from the motor is controlled using a control device, thereby reducing the driver's steering torque. is doing. In a conventional electric power steering apparatus, a steering torque detector (torque sensor) for detecting steering torque is provided on a steering shaft connected to a steering wheel. The detection value of the steering torque detection unit is supplied to the control device as a signal for causing the motor to generate an appropriate auxiliary steering torque.

  Conventionally, a torsion bar type torque sensor using torsion of a torsion bar is mainly used as the steering torque detection unit. In recent years, magnetostrictive torque sensors are also known. As an example of the magnetostrictive torque sensor, magnetostrictive films such as Ni—Fe plating are provided on the surface of the steering shaft connected to the steering wheel at two locations on the upper and lower sides. The two magnetostrictive films are provided with a predetermined axial width so as to have opposite magnetic anisotropy. When a steering torque is applied to the steering shaft, a change in magnetostrictive characteristics that occurs based on the magnetic anisotropy in the magnetostrictive film is detected by a coil disposed around the magnetostrictive film. Such a magnetostrictive torque sensor is disclosed in Patent Document 1 and Patent Document 2, for example.

  The magnetostrictive torque sensor disclosed in Patent Document 1 and Patent Document 2 has a configuration in which an excitation coil and a detection coil are provided on each of two magnetostrictive films formed on the surface of the steering shaft. In addition to the magnetostrictive torque sensor that requires an excitation coil, there is also a method that uses only a detection coil and detects torque based on a change in inductance of the detection coil (see, for example, Patent Document 3 and Patent Document 4).

  The detection circuit in the magnetostrictive torque sensor disclosed in Patent Document 3 includes a coil around a magnetostrictive film formed on the surface of the steering shaft, and a resistance element and a switching element connected in series with the coil. A power source for applying a required voltage is provided for the coil. Further, a bottom hold circuit that holds the minimum value of the output signal is connected to a connection portion between the resistance element and the coil.

Further, Patent Document 4 discloses a magnetostrictive torque sensor obtained by improving the above method. Also in this magnetostrictive torque sensor, a coil disposed around the magnetostrictive film and a resistance element and a switching element are connected in series with this coil.
JP 2001-133337 A JP 2002-168706 A JP 2002-71476 A JP-A-2005-321316

  In the conventional magnetostrictive torque sensor disclosed in Patent Document 4 or the like, the on / off frequency of the switching element is a high frequency of about 30 kHz, for example. For this reason, the frequency of change of the voltage signal taken out from the terminal of the coil that is sensitive to the change of the magnetic characteristics of the magnetostrictive film is also about 30 kHz. As a result, the detection period of the peak hold circuit is a short period of the reciprocal of about 30 kHz. However, normally, the frequency characteristics of the peak hold circuit used are such that the gain is attenuated and the phase delay is increased in the frequency region of 30 kHz. When the phase lag in the output signal of the magnetostrictive torque sensor is large, the control of the electric power steering device becomes unstable, and the assist output for assisting the steering becomes oscillating. As a result, there arises a problem that the steering torque becomes oscillating and the steering feeling becomes poor.

In view of the above problems, the object of the present invention is not affected by the frequency characteristics of the peak hold circuit used in the conventional apparatus, and is applied to the rotating shaft without causing gain attenuation or phase delay in the torque detection signal. It is to provide a magnetostrictive truxene service can detect torque stably.

Magnetostrictive truxene service according to the present invention, in order to achieve the above object, configured as follows.

The first magnetostrictive torque sensor (corresponding to claim 1) includes a rotating shaft, a plurality of magnetic property changing portions whose magnetic properties change according to torque applied to the rotating shaft, and a plurality of magnetic property changing portions. In a magnetostrictive torque sensor that detects a torque applied to the rotating shaft, two coils are connected in series, and a middle point between the two coils is provided. Torque is detected based on the voltage.

In the second magnetostrictive torque sensor (corresponding to claim 2), preferably, the plurality of coils are four coils, the first circuit portion in which the two coils are connected in series, and the rest. A second circuit portion connected in series, and detecting a torque based on a voltage at a midpoint in the first circuit portion and a voltage at a midpoint in the second circuit portion. Features.
The third magnetostrictive torque sensor (corresponding to claim 3) is preferably characterized in that, in the above configuration, the winding directions of the plurality of coils are the same.

According to the magnetostrictive film torque sensor of the present invention , good detection sensitivity can be obtained with a simple configuration.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of the invention will be described with reference to the accompanying drawings.

  FIG. 1 shows an overall configuration of an electric power steering apparatus equipped with a magnetostrictive torque sensor according to the present invention. The electric power steering device 10 is configured to give an auxiliary steering force (steering torque) to a steering shaft 12 a and the like connected to the steering wheel 11. The steering shaft 12a is connected to the steering shaft 12b via a universal shaft joint 12c, the upper end of the steering shaft 12a is connected to the steering wheel 11, and the pinion gear 13 is attached to the lower end of the steering shaft 12b. A rack shaft 14 provided with a rack gear 14a meshing with the pinion gear 13 is disposed. A rack and pinion mechanism 15 is formed by the pinion gear 13 and the rack gear 14a. Tie rods 16 are provided at both ends of the rack shaft 14, and front wheels 17 are attached to the outer ends of the tie rods 16. A motor 19 is provided via a power transmission mechanism 18 for the steering shaft 12b. The power transmission mechanism 18 is formed by a worm gear 18a and a worm wheel 18b. The motor 19 outputs a rotational force (torque) that assists the steering torque, and applies this rotational force to the steering shafts 12 b and 12 a via the power transmission mechanism 18. A steering torque detector 20 is provided on the steering shaft 12b. The steering torque detector 20 detects the steering torque applied to the steering shafts 12a and 12b when the steering torque generated by the driver operating the steering wheel 11 is applied to the steering shafts 12a and 12b. Reference numeral 21 denotes a vehicle speed detection unit that detects the vehicle speed of the vehicle, and reference numeral 22 denotes a control device constituted by a computer. The control device 22 takes in the steering torque signal T output from the steering torque detection unit 20 and the vehicle speed signal V output from the vehicle speed detection unit 21, and uses the information related to the steering torque based on the information related to the vehicle speed. A drive control signal SG1 for controlling the rotation operation is output. The rack and pinion mechanism 15 and the like are housed in a gear box (not shown) in FIG.

  In the above, the electric power steering device 10 is configured by adding a steering torque detection unit 20, a vehicle speed detection unit 21, a control device 22, a motor 19, and a power transmission mechanism 18 to a normal steering system configuration. .

  In the above configuration, when the driver operates the steering wheel 11 to steer in the traveling direction during the traveling operation of the automobile, the rotational force based on the steering torque applied to the steering shafts 12a and 12b is applied to the rack and pinion mechanism 15. It is converted into a linear motion in the axial direction of the rack shaft 14 via the tie rod 16 and further changes the traveling direction of the front wheel 17 via the tie rod 16. At this time, at the same time, the steering torque detector 20 attached to the steering shaft 12b detects the steering torque corresponding to the steering by the driver at the steering wheel 11 and converts it into an electrical steering torque signal T. A steering torque signal T is output to the control device 22. In addition, the vehicle speed detection unit 21 detects the vehicle speed of the vehicle, converts it into a vehicle speed signal V, and outputs the vehicle speed signal V to the control device 22. The control device 22 generates a motor current for driving the motor 19 based on the steering torque signal T and the vehicle speed signal V. The motor 19 driven by the motor current causes an auxiliary steering force to act on the steering shafts 12 b and 12 a via the power transmission mechanism 18. By driving the motor 19 as described above, the steering force applied by the driver to the steering wheel 11 is reduced.

  FIG. 2 shows a specific configuration of the main part of the mechanical mechanism of the electric power steering apparatus 10 and the electric system. A part of the left end portion and the right end portion of the rack shaft 14 is shown in cross section. The rack shaft 14 is housed in a cylindrical housing 31 arranged in the vehicle width direction (left-right direction in FIG. 2) so as to be slidable in the axial direction. Ball joints 32 are screwed to both ends of the rack shaft 14 protruding from the housing 31, and left and right tie rods 16 are connected to these ball joints 32. The housing 31 includes a bracket 33 for attaching to a vehicle body (not shown), and includes stoppers 34 at both ends.

  In FIG. 2, 35 is an ignition switch, 36 is an in-vehicle battery, and 37 is an AC generator (ACG) attached to the vehicle engine. The AC generator 37 starts power generation by the operation of the vehicle engine. Necessary electric power is supplied from the battery 36 or the AC generator 37 to the control device 22. The control device 22 is attached to the motor 19. Reference numeral 38 denotes a rack end that contacts the stopper 34 when the rack shaft is moved. Reference numeral 39 denotes a dust seal boot for protecting the inside of the gear box from water, mud, dust, and the like.

  FIG. 3 is a cross-sectional view taken along line AA in FIG. In FIG. 3, the specific structure of the support structure of the steering shaft 12b, the steering torque detector 20, the power transmission mechanism 18, and the rack and pinion mechanism 15 is clearly shown.

  In FIG. 3, the steering shaft 12 b is rotatably supported by two bearing portions 41 and 42 in a housing 24 a that forms the gear box 24. A rack and pinion mechanism 15 and a power transmission mechanism 18 are housed inside the housing 24a, and a steering torque detector 20 is additionally provided at the top. The upper opening of the housing 24a is closed with a lid 43, and the lid 43 is fixed with bolts. The pinion 13 provided at the lower end portion of the steering shaft 12b is located between the bearing portions 41 and 42. The rack shaft 14 is guided by a rack guide 45 and is urged by a compressed spring 46 and pressed against the pinion 13 side. The power transmission mechanism 18 is formed by a worm gear 18a fixed to a transmission shaft 48 coupled to an output shaft of the motor 19 and a worm wheel 18b fixed to a steering shaft 12b. The steering torque detector 20 is attached to the lid 43.

  As described above, the steering torque detection unit 20 is provided in the steering gear box 24, detects the steering torque acting on the steering shaft 12b, and the detected value is input to the control device 22 to the motor 19. It is supplied as a reference signal for generating an appropriate auxiliary steering torque.

  The steering torque detection 20 used here is a magnetostrictive torque sensor. Hereinafter, it is referred to as “magnetostrictive torque sensor 20”. As shown in FIG. 3, two magnetostrictive films having magnetic anisotropy such as Ni—Fe plating are provided on the surface of the steering shaft 12b connected from the steering wheel 11 via the steering shaft 12a and the universal joint 12c. (20b and 20c) are provided with a predetermined axial width so as to have anisotropy in the reverse direction, and the reverse magnetostrictive characteristics generated when the steering torque is applied to the magnetostrictive films 20b and 20c are expressed by the magnetostrictive films 20b and 20c. The detection is performed by utilizing the AC resistance of the coils 20d and 20e disposed around the periphery. The coils 20f and 20g are coils used as resistance elements.

  Next, the magnetostrictive torque sensor 20 according to the embodiment of the present invention will be described in detail. As shown in FIG. 3, the magnetostrictive torque sensor 20 is provided with magnetostrictive films 20b and 20c at two locations on the peripheral surface of the steering shaft 12b made of a magnetic material, and the magnetization of the magnetostrictive film provided on the steering shaft 12b. Coils 20d, 20e and 20f, 20g for detecting changes are provided. A yoke portion 20h is provided on the outer periphery of the detection coils 20d and 20e.

  FIG. 4 shows a circuit configuration of the magnetostrictive torque sensor 20. In the magnetostrictive torque sensor 20, magnetostrictive films 20b and 20c are formed so as to have opposite magnetic anisotropies at two locations on the steering shaft 12b. These magnetostrictive films 20b and 20c act as a magnetic characteristic changing portion in which the magnetic characteristics change according to the torque when a torque is applied to the steering shaft 12b. In this circuit, coils 20d and 20e are provided around the magnetostrictive film in response to a change in magnetization of the magnetostrictive films 20b and 20c when torque is applied to the steering shaft 12b, and detecting the change in magnetization as a change in inductance. ing. In this circuit, coils 20f and 20g (hereinafter also referred to as “resistance elements 20f and 20g”) connected in series to the coils 20d and 20e, respectively, are provided. Further, switching elements 50a, 50b, 50c, 50d for applying a voltage at a predetermined period to a bridge circuit composed of two series circuits formed by the coils 20d, 20e and the resistance elements 20f, 20g are connected to these. A voltage application unit 52 including a constant voltage source 51 is provided.

  Further, in the circuit configuration shown in FIG. 4, in order to detect a change in voltage (terminal voltage) at both ends of each of the coils 20d and 20e, the detection terminal 53 is connected to each connection portion of the resistance elements 20f and 20g and the coils 20d and 20e. , 54 are provided. Furthermore, phase shift sections 55 and 56 are provided that invert (180 ° shift) the phase of the voltage change at both ends of the coils 20d and 20e. Selection units 57 and 58 for alternately selecting and outputting the voltage signal extracted from the detection terminals 53 and 54 and the voltage signal extracted from the output terminals of the phase shift units 55 and 56 are provided. The selection unit 57 includes a selector 59 (for example, a multiplexer) that alternately selects the detection terminals 53 and 55a on the fixed side. Similarly, the selection unit 58 includes a selector 60 that selects the detection terminals 54 and 56a.

  A filter 61 is provided at the next stage of the selection unit 57. The filter 61 functions to cut noise included in the voltage signal output from the selection unit 57 and smooth the change in the voltage signal. The filter 61 includes, at its input stage, a smoothing unit 63 that smoothes changes in the voltage signal output from the selection unit 57 and outputs a DC voltage. Similarly, a filter 61 that cuts noise included in the output voltage signal and smoothes the change in the output voltage signal is provided at the next stage of the selection unit 58. The input stage of the filter 61 includes a smoothing unit 64 that smoothes changes in the output voltage signal from the selection unit 58 and outputs a DC voltage. The next stage of the filters 61 and 62 includes an arithmetic unit 65 that calculates the difference between the two DC voltages output from the amplifiers 61 a and 62 a of the filters 61 and 62.

  At the subsequent stage of the calculation unit 65, an AD conversion unit 66 that converts an analog signal from the calculation unit 65 into a digital signal, a sample hold circuit 67 that holds the digital signal from the AD conversion unit 66, and an output from the sample hold circuit 67 A voltage-torque converter that converts the signal to be converted into a torque value is provided.

  The coils 20d, 20e, 20f, and 20g (the coils 20f and 20g also function as resistance elements) in the above-described electric circuit configuration are wound around the actual steering shaft 12b as shown in FIG. ing. As shown in FIG. 5, the wiring 71 from the power supply terminal 70 is connected to one end of the coil 20d, the wiring 72 from the other end of the coil 20d is connected to one end of the coil 20f, and the wiring from the other end of the coil 20f. 73 is connected to another power supply terminal 74. The connection point 75 is connected to the terminal of VS1 by a wiring 76. Further, the wiring 77 from the power terminal 70 is connected to one end of the coil 20e, the wiring 78 from the other end of the coil 20e is connected to one end of the coil 20g, and the wiring 79 from the other end of the coil 20g is connected to the power terminal 74. It is connected. The connection point 80 is connected to the terminal of VS2. As described above, the winding directions of the coils 20d to 20g are set so that the directions of the magnetic fields H1, H2, H3, and H4 generated in the longitudinal direction of the steering shaft 12b all coincide. The winding directions of the coils 20d to 20g wound around the steering shaft 12b are set to be the same. The power supply terminals 70 and 74 are supplied with an AC voltage having a predetermined cycle by the voltage application unit 52 described above.

  In the arrangement and wiring of the coils shown in FIG. 5, the direction (arrows A, B, C, D) of magnetic fluxes (magnetic fields H1 to H4) generated in the longitudinal direction of the steering shaft 12b when each switching element is turned on. Since the coil winding direction is set so that all of the coils are matched, it is possible to apply the combined magnetic field (arrow F) of the magnetic field generated by each coil to the entire steering shaft 12b. As a result, a uniform strong magnetic field can be applied to the magnetostrictive films 20b and 20c, the hysteresis is reduced, and the assist does not decrease due to the hysteresis of the magnetostrictive torque sensor when the handle is turned back. Thus, the steering wheel return does not deteriorate, and a smooth steering feeling can be stably obtained.

  Next, the operation of the magnetostrictive torque sensor 20 configured as described above will be described with reference to FIGS. FIG. 6 shows an equivalent circuit in which FIG. 5 is simplified, and FIG. 7 shows voltage changes at various points in the circuit of FIG. In the electric circuit shown in FIG. 6, the switching element 50A and the switching element 50B are turned on and off at a predetermined cycle. The switching element 50A and the switching element 50B are formed by four switching elements 50a, 50b, 50c, and 50d that form a bridge circuit in the constant voltage source 51.

  A detection circuit for one magnetostrictive film 20c will be described. 7A to 7E respectively show changes in applied voltage with time when the switching element 50A is turned on / off, and output voltage times from the terminal 53, the terminal 55a, and the output terminal of the selector 57, respectively. The change and the time change of the output voltage from the output terminal of the smoothing part 63 are shown. In FIG. 7, the horizontal axis indicates time, and the vertical axis indicates voltage. First, as shown in FIG. 7A, when the switching element 50A is turned on at time t1, t3, t5 and turned off at t2, t4, t6, a current flows through the circuit including the resistance element 20g and the coil 20e. The voltage of the terminal 53 changes with the time change shown in FIG. It is assumed that the inductance (L) of the coil 20e at this time is L (μ1).

  In FIG. 7B, V (t) is the voltage at time t of the terminal 53, and E is the voltage of the power supply. T is time. The time t2, t4, t6 when the switching element 50A is turned off is preferably until the maximum current flowing through the coil 20e reaches a region where the magnetization of the magnetostrictive film 20c is saturated by the magnetic flux from the coil 20e generated by the maximum current. To be a value.

  The voltage waveform from the output terminal of the phase shift unit 55 is set so that a voltage waveform shifted by a half cycle (180 °) is output. The output voltage is shown in FIG.

  The selector 57 is set to switch every half cycle of the switching element 50B. The waveform of the voltage signal output from the output terminal of the selection unit 57 at that time is shown in FIG.

Thereafter, after passing through the noise cut filter 61, the smoothing unit 63 outputs a DC voltage <V> _{1 as} shown in FIG. The smoothing unit 63 outputs a value proportional to the average value of the voltage waveform output from the selection unit 57. The symbol “<V>” means “a value proportional to the average value of the voltage waveform”.

  8A to 8E, the switching element 50A is turned on / off when the inductance value (L (μ2)) of the coil 20e is larger than the inductance value (L (μ1)). The time change of the applied voltage at the time, the time change of the output voltage from each of the output terminals of the terminal 53, the terminal 55a and the selection unit 57, and the time change of the output voltage from the output terminal of the smoothing unit 63 are shown. First, as shown in FIG. 8A, when the switching element 50A is turned on at times t1, t3, and t5 and turned off at times t2, t4, and t6, a current is supplied to the circuit including the resistance element 20g and the coil 20e. Flowing. The voltage of the coil 20e changes with time as shown in FIG. It is assumed that the inductance (L) of the coil 20e at this time is L (μ2).

  Further, the voltage signal output from the output terminal of the phase shift unit 55 is set so that a voltage waveform shifted by a half period as described above is output. The output voltage is shown in FIG.

  The selector 57 is set so as to switch every half cycle of the switching element as described above, and the waveform of the voltage signal output from the selector 57 at that time is shown in FIG.

After passing through the noise cut filter 61, the smoothing unit 63 outputs a DC voltage <V> _{2 as} shown in FIG. A value proportional to the average value of the waveform output from the selection unit 57 is output from the output end of the smoothing unit 63.

As shown in FIGS. 7 and 8, the DC voltage <V> ₁ and the DC voltage <V> ₂ show different values depending on the difference in inductance value L (μ). The inductance L (μ) depends on the magnetic permeability μ of the magnetostrictive film, and the magnetic permeability μ changes when the torque acts on the magnetostrictive film. Therefore, the steering torque can be detected by measuring this voltage. it can.

  The circuit for the other magnetostrictive film 20b is also operated in the same manner as described above, and a DC voltage reflecting torque is output from the smoothing unit 64.

  FIG. 9 is a graph showing the relationship between applied torque (steering torque) and DC voltage. The DC voltages detected via the detection circuits of the two magnetostrictive films 20b and 20c are a curve L10 and a straight line L11. This is because the magnetostrictive films 20b and 20c are deposited so as to have magnetic anisotropies in opposite directions at two upper and lower positions, and as a result of reflecting the magnetic anisotropy, the magnetostrictive films 20b and 20c are symmetrical with respect to the vertical axis. It has become. The straight line L12 indicates a value obtained by subtracting the characteristic curve L11 from the characteristic curve L10 detected by the two coils 20e and 20f. When the steering torque is zero, the value becomes zero, and the change in the steering torque is almost linear. That change By using the characteristic of the straight line L12, the steering torque can be detected from the values of the respective detection circuits including the two detection coils 20e and 20f.

  The arithmetic unit 65 performs an operation of subtracting two DC voltages, AD-converts by the AD converter 66, and inputs to the sample hold circuit 67. The output from the sample and hold circuit 67 is converted into a torque value by the voltage-torque converter 68 and output.

  Thus, the torque (T) can be detected by the voltage-torque conversion unit 68 having a table in which the voltage output from the calculation unit 65 and the relationship between the torque and the voltage are obtained in advance.

  As described above, the voltage input to the sample and hold circuit 67 is a DC voltage. For this reason, the hold circuit is not used in the attenuation range of the frequency characteristic, the phase delay of the hold circuit is eliminated, and the phase delay of the sensor output is eliminated. Thereby, there is no phase delay in the control of the electric power steering apparatus, the control is stabilized, and a smooth steering feeling can be obtained.

  The present invention is used to realize a magnetostrictive torque sensor that can stably detect torque applied to a steering shaft, and is used to realize an electric power steering apparatus having good steering feeling characteristics.

1 is an overall configuration diagram of an electric power steering apparatus to which a magnetostrictive torque sensor of the present invention is applied. It is a figure which shows the specific structure of the principal part of the mechanical mechanism of an electric power steering apparatus, and an electric system. It is the sectional view on the AA line in FIG. It is an electric circuit diagram for demonstrating embodiment of the magnetostrictive torque sensor which concerns on this invention. It is a figure which shows concretely the arrangement | positioning and connection of a coil. It is an electric circuit diagram which shows the principal part structure of the magnetostrictive torque sensor which concerns on this invention. It is a wave form diagram which shows the voltage waveform in each location in the electric circuit shown in FIG. 6 in the 1st inductance value. It is a wave form diagram which shows the voltage waveform in each location in the electric circuit shown in FIG. 6 in the 2nd inductance value. It is a graph which shows the relationship between steering torque and a detection voltage.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Electric power steering apparatus 11 Steering wheel 18 Power transmission mechanism 19 Motor 20 Steering torque detection part (magnetostrictive torque sensor)
20b, 20c Magnetostrictive film 20d, 20e Coil 20f, 20g Coil (resistance element)
22 control device 51 constant voltage source 52 voltage application unit 53, 54 detection terminal 55, 56 phase shift unit 57, 58 selection unit 63, 64 smoothing unit 65 calculation unit

Claims (3)

A rotating shaft; a plurality of magnetic characteristic changing portions whose magnetic characteristics change according to a torque applied to the rotating shaft; and a plurality of coils sensitive to changes in the magnetic characteristics of each of the plurality of magnetic characteristic changing portions. In a magnetostrictive torque sensor that detects the torque applied to the rotating shaft,
A magnetostrictive torque sensor, wherein the two coils are connected in series, and the torque is detected based on a midpoint voltage between the two coils. The plurality of coils are four coils,
  A first circuit portion in which the two coils are connected in series and a second circuit portion in which the remaining two coils are connected in series;
2. The magnetostrictive torque sensor according to claim 1, wherein the torque is detected based on a midpoint voltage in the first circuit portion and a midpoint voltage in the second circuit portion. 3. The magnetostrictive torque sensor according to claim 1, wherein the winding directions of the plurality of coils are the same.

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