Magnetostrictive torque sensor system and electric power steering apparatus employing the same

A magnetostrictive torque sensor system stabilizes and detects steering torque applied to a steering shaft. The sensor system comprises magnetic-characteristic variation parts that are provided to the steering shaft, wherein magnetic characteristics of the magnetic-characteristic variation parts change in accordance with the applied torque; coils that are positioned around the magnetic-characteristic variation parts and that respond to changes in the magnetic characteristics; resistance elements that are serially connected to the coils; voltage-applying means for periodically applying a voltage to serial circuits that are formed from the coils and the resistance elements; terminals for retrieving changes in the terminal voltage of the coils; phase-shifting means for inverting the phase of the change in the terminal voltage of the coils; selecting means for alternatingly selecting and outputting the voltage change in the terminals and the voltage change in the output ends of the phase-shifting means; and smoothing means for smoothing the voltage signals output from the selecting means and for outputting a direct-current voltage.

FIELD OF THE INVENTION

The present invention relates to a magnetostrictive torque sensor system for detecting torque using changes in the inductance of a coil provided around a magnetostrictive film. The present invention also relates to an electric power-steering apparatus that uses the magnetostrictive torque sensor system.

BACKGROUND OF THE INVENTION

In electric power-steering apparatuses, an electric motor for producing a supplementary force is fixed to a mechanical steering apparatus, and the rotational torque provided by the motor is controlled using a control device, whereby the steering torque that must be applied by the driver is reduced. In conventional electric power-steering apparatuses, a steering-torque detecting part is provided to a steering shaft linked to the steering wheel. The steering-torque detecting part supplies detection signals to the control device in order to cause the motor to produce an appropriate supplementary steering torque.

Torsion-bar torque sensor systems that make use of the torsion of a torsion bar are the conventional steering-torque detecting parts that have been primarily used. Magnetostrictive torque sensor systems have also been proposed in recent years.

In magnetostrictive torque sensor systems, a magnetostrictive film formed of, e.g., a Ni—Fe plating is provided to two locations on the steering shaft. The magnetostrictive films in these two locations are both formed annularly in the circumferential direction on the surface of the steering shaft and are positioned vertically relative to one another in the axial direction. The magnetostrictive films in these two locations are also formed so as to have the necessary width in the axial direction and are made so as to be magnetically anisotropic in mutually opposing directions. When the driver applies a steering torque to the steering shaft, changes in the magnetostrictive characteristics that occur based on the magnetic anisotropy of the magnetostrictive films in these two locations are detected by a coil provided around the magnetostrictive films.

The magnetostrictive torque sensor systems described in JP-A 2001-133337 and JP-A 2002-168706 have a magnetizing coil and detection coil that are provided respectively to the two annular magnetostrictive films formed on the surface of the steering shaft. There are also magnetostrictive torque sensor systems that detect torque according to changes in the inductance of the detection coil, using only a detection coil without a magnetizing coil (see, e.g., JP-A 2002-71476 and JP-A 2005-321316).

The detection circuit of the magnetostrictive torque sensor system described in JP-A 2002-71476 has a coil around the magnetostrictive films formed on the surface of the steering shaft and also has a resistance element and a switching element that are serially connected to the coil. A power source that applies the necessary voltage is provided to the coil. A bottom-hold circuit for maintaining the minimum value of the output signal is also connected to the connecting part between the resistance element and the coil.

JP-A 2005-321316 discloses a magnetostrictive torque sensor system that improves on the magnetostrictive torque sensor system described in JP-A 2002-71476. This magnetostrictive torque sensor system also has a coil positioned around the magnetostrictive films, as well as a resistance element and a switching element that are serially connected to the coil.

In the conventional magnetostrictive torque sensor systems disclosed in, e.g., JP-A 2005-321316, the frequency of the on/off operation of the switching element is, e.g., about 30 kHz. The frequency of the change in the voltage signal retrieved from the terminal of the coil that responds to changes in the magnetic characteristics of the magnetostrictive films is also about 30 kHz. As a result, the detection period of the peak-hold circuit used in the detection circuit can be calculated as the inverse of 30 kHz. When the detection period of the peak-hold circuit is in a frequency range of 30 kHz, then in terms of the frequency characteristics, the gain usually decays and phase lag increases. An increase in the phase lag in the output signal of the magnetostrictive torque sensor system results in reduced stability with which the electric power-steering apparatus is controlled, and greater loss in the uniformity of the supplementary force for reducing the steering torque that must be applied by the driver. Problems result in that the entire steering torque loses consistency, and the driver increasingly loses proper steering response.

A magnetostrictive torque sensor system has therefore been needed for stabilizing the steering torque applied to the steering shaft and performing detection, without affecting the frequency characteristics of the peak-hold circuit used in the detection circuit of conventional magnetostrictive torque sensor systems and without giving rise to gain decay or phase lag in the torque-detection signal. A demand has also arisen for an electric power-steering apparatus that can provide a good steering response using a magnetostrictive torque sensor system.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided a magnetostrictive torque sensor system comprising: at least one magnetic-characteristic variation part mounted on a rotating shaft and adapted so that a magnetic characteristic thereof varies in accordance with an applied torque; a coil that is positioned around the magnetic-characteristic variation part and responds to a change in the magnetic characteristic; a resistance element that is serially connected to the coil; voltage-applying means for periodically applying a voltage to a serial circuit that is formed from the coil and the resistance element; a terminal for retrieving a change in a terminal voltage of the coil; phase-shifting means for inverting a phase of the change in the terminal voltage of the coil; selecting means for alternatingly selecting and outputting a voltage change in the terminal and a voltage change in an output end of the phase-shifting means; and smoothing means for smoothing a voltage signal output from the selecting means and for outputting a direct-current voltage.

When the magnetic characteristics of the magnetostrictive films formed on the rotating shaft in this configuration change in accordance with a torque applied to the rotating shaft, the change in magnetic characteristics is retrieved as a change in the terminal voltage of the coil. The terminal voltage of the coil is retrieved as a periodic voltage signal by the voltage-applying part, which applies a periodic voltage. The waveform of this voltage signal starts to rise in accordance with an “on” operation, at which time the state of the waveform on startup varies in accordance with the torque applied to the rotating shaft. Finally, the periodic terminal-voltage signal of the coil and the periodic voltage signal resulting from the inversion of this signal are selected and synthesized in alternation, resulting in a voltage signal that is smoothed by the smoothing part. A direct-current voltage signal corresponding to the applied torque is accordingly obtained. The voltage signal output from the smoothing part corresponds to changes in the inductance of the coil that responds to changes in the magnetic characteristics of the magnetostrictive films.

Preferably, the magnetostrictive films are provided to two locations on the rotating shaft, and the coil, resistance element, voltage-applying part, terminal, phase-shifting part, selecting part, and smoothing parts are provided individually to the respective magnetostrictive films in these two locations. A calculating part is also provided for calculating a difference in the two voltage signals that are respectively output from each of the two smoothing parts. In this configuration, the difference in the voltage values output from each of the calculating parts of the two magnetostrictive films is determined, whereby a value is obtained for the voltage signal corresponding to the torque applied to the rotating shaft. The value of the voltage signal is input to a subsequent-stage sample-hold circuit. The sample-hold circuit is different from conventional bottom-hold circuits and can detect the stabilized applied torque without relying on the operational frequency characteristics of a switching element.

Desirably, the resistance element comprises a coil that is positioned around the rotating shaft, wherein a wrapping direction relative to the rotating shaft is the same for at least one of the coils responding to the change in the magnetic characteristic and for at least one of the coils used as the resistance element.

Preferably, the voltage-applying part comprises a constant voltage source and a switching element that is connected to the constant voltage source.

According to a second aspect of the present invention, there is provided an electric power-steering apparatus comprising: a motor for applying a torque to a steering shaft; a magnetostrictive torque sensor system for detecting a steering torque applied to the steering shaft; target-current calculating means for calculating a target electrical current of the motor in accordance with a signal from the sensor system; and driving means for driving the motor, wherein the magnetostrictive torque sensor system has a magnetic-characteristic variation part that is provided to the steering shaft, wherein a magnetic characteristic of the magnetic-characteristic variation part changes in accordance with the applied torque; a coil that is positioned around the magnetic-characteristic variation part and that responds to a change in the magnetic characteristic; a resistance element that is serially connected to the coil; voltage-applying means for periodically applying a voltage to a serial circuit that is formed from the coil and the resistance element; a terminal for retrieving a change in a terminal voltage of the coil; phase-shifting means for inverting a phase of the change in the terminal voltage of the coil; selecting means for alternatingly selecting and outputting a voltage change in the terminal and a voltage change in an output end of the phase-shifting means; and smoothing means for smoothing a voltage signal output from the selecting means and for outputting a direct-current voltage.

In this configuration, a voltage signal input to a hold circuit for holding a signal value corresponding to the applied torque becomes a direct-current voltage signal. The hold circuit is therefore not used in the region in which the frequency characteristics decay, and phase lag does not occur in the hold part or in the sensor output signal. The torque applied to the steering shaft can therefore be stabilized and detected.

Furthermore, a magnetostrictive torque sensor system having the aforementioned characteristics is used, whereby no phase lag is experienced in control, control is stabilized, and a smooth, satisfactory steering response can be obtained.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An electric power-steering apparatus10inFIG. 1is configured to provide a supplemental steering torque to a steering shaft (12a,12b) that is linked to a steering wheel11. The steering shaft is provided with an upper shaft portion12aand a lower shaft portion12b. The upper steering shaft12ais linked to the lower steering shaft12bvia a universal shaft coupling12c. The upper end of the steering shaft12ais linked to the steering wheel11. A pinion gear13is attached to the lower end of the steering shaft12b. A rack gear14aof a rack14is engaged with the pinion gear13. A rack-and-pinion mechanism15is formed from the pinion gear13and the rack gear14a. Tie rods16are provided to both ends of the rack14, and front wheels17are attached to the outside ends of the tie rods16.

An electric motor19is provided to the steering shaft12bvia a motive-force transmission mechanism18. The motive-force transmission mechanism18is formed from a worm gear18aand a worm wheel18b. The motor19outputs a rotational force (torque) that supplements the steering torque. This rotational force is provided to the steering shaft12bby way of the motive-force transmission mechanism18. A steering-torque detection part20is also provided to the steering shaft12b. The steering-torque detection part20detects steering torque applied to the steering shafts12a,12bwhen a driver operates the steering wheel11and thereby generates steering torque on the steering shafts12a,12b.

Reference number21designates a vehicle-speed detection part for detecting the speed of the vehicle, and reference number22designates a control device that is configured from a computer.

A steering torque signal (T) from the steering-torque detection part20and a vehicle speed signal (V) from the vehicle-speed detection part21are input to the control device22, which outputs a drive-control signal SG1for controlling the rotational operation of the motor19.

The rack-and-pinion mechanism15and the like are housed in a gear box that is not shown inFIG. 1. The gear box is designated by the reference number24inFIGS. 2 and 3.

In the electric power-steering apparatus10as described above, the steering-torque detection part20, the vehicle-speed detection part21, the control device22, the motor19, and the motive-force transmission mechanism18are added to a standard mechanical steering structure.

When the driver operates the steering wheel11and steers the automobile in the direction of travel, a rotational force is converted into linear motion in the axial direction of the rack14via the rack-and-pinion mechanism15and changes the direction of travel of the front wheels17via the tie rods16on the basis of the steering torque applied to the steering shaft (12a,12b). At the same time, the steering-torque detection part20provided to the steering shaft12bdetects the steering torque corresponding to the steering of the steering wheel11by the driver and converts the steering torque into the electrical steering torque signal T. The steering torque signal T is provided to the control device22. The vehicle-speed detection part21detects the speed of the vehicle and converts the speed into the vehicle speed signal V. The vehicle speed signal V is also output to the control device22. The control device22produces a motor current for driving the motor19on the basis of the steering torque signal T and the vehicle speed signal V. The motor19, which is operated by the motor current, provides a supplementary steering torque to the steering shaft (12a,12b) via the motive-force transmission mechanism18.

The steering force that must be applied by the driver on the steering wheel11is reduced via the drive from the electric motor19as above.

A cross section of a portion of the left and right ends of the rack14is shown inFIG. 2. The rack14is housed within a cylindrical housing31, which is positioned in the widthwise direction of the vehicle (the lateral direction inFIG. 2), so as to be able to slide in the axial direction. Ball joints32are screwed to both ends of the rack14protruding from the housing31. The left and right tie rods16are linked respectively to the left and right ball joints32. The housing31is provided with a bracket33and a stopper34. The purpose of the bracket33is to attach the housing31to the vehicle body (not shown).

InFIG. 2,35designates an ignition switch,36designates an onboard battery, and37designates an alternating-current generator (ACG) fixed to the vehicle engine. The alternating-current generator37starts to produce electricity when the vehicle engine is in operation. The necessary electrical power is supplied to the control device22from the battery36or the alternating-current generator37. The control device22is fixed to the motor19.

FIG. 3is a sectional view along the line A-A inFIG. 2.FIG. 3shows the specific structures of the steering-torque detection part20, the motive-force transmission mechanism18, the rack-and-pinion mechanism15, and the support structure of the steering shaft12b.

InFIG. 3, the steering shaft12bis rotatably supported by two bearing parts41,42in a housing24athat forms the gear box24. The rack-and-pinion mechanism15and the motive-force transmission mechanism18are housed within the housing24a, and the steering-torque detection part20is fixed to the upper part of the housing24a. The upper opening of the housing24ais covered by a lid43that is fixed by bolts. The pinion13provided to the lower end of the steering shaft12bis positioned between the bearing parts41,42. The rack14is guided by a rack guide45, powered by a compressed spring46, and pressed down towards the pinion13. The motive-force transmission mechanism18is formed from the worm gear18a, which is fixed to a transmission shaft48linked to the output shaft of the motor19, and the worm wheel18b, which is fixed to the steering shaft12b. The steering-torque detection part20is attached to the lid43.

The steering-torque detection part20as described above is provided to the steering gear box24. The steering-torque detection part20detects steering torque acting on the steering shaft12band inputs detection values to the control device22. The detection values of the steering-torque detection part20are used as reference signals for causing the motor19to produce an appropriate supplementary torque.

A magnetostrictive torque sensor system is used as the steering-torque detection part20shown inFIG. 3and will be referred to below as “magnetostrictive torque sensor system20.”

In the magnetostrictive torque sensor system20as shown inFIG. 3, two magnetostrictive films20b,20care formed annularly in the circumferential direction on the surface of the steering shaft12b. The magnetostrictive films20b,20care made of, e.g., Ni—Fe plating and are provided with magnetic anisotropy. The magnetostrictive films20b,20care formed in two locations, upper and lower, and are formed having prescribed widths in the axial direction of the steering shaft12b. The magnetostrictive films20b,20cin these two locations are formed so as to have magnetic anisotropy in mutually opposing directions.

When a steering torque is applied to the steering shaft12b, the opposite magnetostrictive characteristics generated in the magnetostrictive films20b,20care detected in the magnetostrictive torque sensor system20using the alternating-current resistance or other property of coils20d,20e, which are positioned around the magnetostrictive films20b,20c.

In the magnetostrictive torque sensor system20, a yoke part20his provided around the coils20f,20g, which act as resistance elements, and the coils20d,20e, which detect changes in the magnetization (the opposite magnetostrictive characteristics) of the magnetostrictive films20b,20cthat are provided to the steering shaft12b.

The configuration of the electrical circuit of the magnetostrictive torque sensor system20will be described next with reference toFIG. 4. The magnetostrictive films20b,20cin the magnetostrictive torque sensor system20are formed in two locations on the steering shaft12b, and are formed so as to have mutually opposing magnetic anisotropy. When a torque is applied to the steering shaft12b, the magnetostrictive films20b,20cact as magnetic-characteristic variation parts in which the magnetic characteristics change in accordance with the torque. The coils20d,20e, which are provided around the magnetostrictive films20b,20c, respectively, detect the changes in magnetization as changes in inductance in response to changes in the magnetization state of the magnetostrictive films20b,20cwhen a torque is applied to the steering shaft12b. The coils20f,20gare serially connected to the coils20d,20e, respectively. The coils20f,20gact as resistance elements and will therefore also be referred to below as “resistance elements20f,20g.” A voltage-applying part52is also provided and is composed of a bridge circuit. The bridge circuit is composed of switching elements50a,50b,50c,50d, and a constant voltage source51, which supplies a voltage to the bridge circuit. The voltage-applying part52applies voltage of a prescribed period to a bridge circuit that is composed of the two serial circuits that are formed from the coils20d,20eand the resistance elements20f,20g.

In order to detect changes in the voltage (terminal voltage) of both ends of the coils20d,20ein the electrical circuit configuration shown inFIG. 4, a detection terminal53is provided to the connecting part between the resistance element20fand the coil20d, and a detection terminal54is provided to the connecting part between the resistance element20gand the coil20e. Phase-shifting parts55,56are also provided to invert (prompt a 180° shift in) the phase of the voltage change of both ends of the coils20d,20e. The voltage signals retrieved from the detection terminals53,54and the voltage signals retrieved from the output ends of the phase-shifting parts55,56are alternatingly selected and output by selecting parts57,58. The selecting part57is provided with a movable selector59that alternately selects the detection terminals53,55aon the fixed side. The selecting part58is also provided with a movable selector60that selects the detection terminals54,56a.

A filter61is provided in the stage subsequent to the selecting part57. The filter61cuts out noise included in the voltage signal output from the selecting part57and acts to smooth the changes in the voltage signal. The filter61is provided with a smoothing part63at the input stage for smoothing the changes in the voltage signal output from the selecting part57and for outputting a direct-current voltage. A filter62is also provided to the stage subsequent to the selecting part58. The filter62cuts out noise included in the voltage signal output from the selecting part58and smoothes the changes in the output voltage signal. The filter62is provided with a smoothing part64at the input stage for smoothing the changes in the voltage signal output from the selecting part58and for outputting a direct-current voltage. Amplifiers61a,62aare provided to the output stages of the filters61,62. A calculating part65is provided to the stage subsequent to the filters61,62in order to calculate the difference between the two direct-current voltages output from the amplifiers61a,62a.

The stage subsequent to the calculating part65is provided with an AD converting part66for converting the analog signal from the calculating part65into a digital signal; a sample-hold circuit67for holding the digital signal from the AD converting part66; and a voltage-torque converting part68for converting the signal output from the sample-hold circuit67to a torque value (T).

The coils20d,20e,20f,20gin the electrical circuit configuration above are wrapped around the steering shaft12bin the actual placement structure, as shown inFIG. 5. InFIG. 5, a wiring71from an electrical source terminal70is connected to one end of the coil20d, a wiring72from the other end of the coil20dis connected to one end of the coil20f, and a wiring73from the other end of the coil20fis connected to the other electrical source terminal74. A connection point75is also connected to a terminal VS1via a wiring76. Further, a wiring77from the electrical source terminal70is connected to one end of the coil20e, a wiring78from the other end of the coil20eis connected to one end of the coil20g, and a wiring79from the other end of the coil20gis connected to the other electrical source terminal74. A connection point80is also connected to a terminal VS2.

The wrapping directions of each of the coils20dthrough20gas described above are established so that the directions of magnetic fields H1, H2, H3, H4, which are produced in the longitudinal direction of the steering shaft12b, are all aligned. In other words, the wrapping directions of the coils20dthrough20gthat are wrapped around the steering shaft12bare established so as to be identical. An alternating current of a prescribed period is supplied to the electrical source terminals70,74by the voltage-applying part52.

According to the wiring and placement configuration of the coils shown inFIG. 5, the wrapping directions of the coils are established so that the directions (arrows A, B, C, D) of the magnetic fluxes (magnetic fields H1through H4), which are produced in the longitudinal direction of the steering shaft12bby the coils20dthrough20g, are all aligned when the switching terminals50athrough50dof the voltage-applying part52are turned “on” or “off,” as appropriate. The total magnetic field (arrow F) of the magnetic fields produced by the coils20dthrough20gcan therefore be applied over the entire steering shaft12b. A uniform and strong magnetic field can thereby be applied to the magnetostrictive films20b,20c, and hysteresis decreases.

According to the magnetostrictive torque sensor system20above, the hysteresis of the magnetostrictive torque sensor system20allows smooth and stable steering response to be obtained without any decline in steering assist or wheel (steering wheel11) return when the wheel is released by the driver and returns.

The operation of the magnetostrictive torque sensor system20will be described next with reference toFIGS. 6 and 7.FIG. 6shows an equivalent circuit that has been simplified from the electrical circuit ofFIG. 4.FIG. 7(a) through7(e) show the voltage waveforms at parts of the electrical circuit ofFIG. 6.

A switching element50A and a switching element50B in the electrical circuit shown inFIG. 6perform switching between “on” and “off” over a prescribed period. The switching element50A and the switching element50B are made from the four switching elements50athrough50dthat form the bridge circuit of the voltage-applying part52.

The detection circuit will be described for one of the magnetostrictive films, i.e., for the magnetostrictive film20c.

The waveform diagrams (a) through (e) inFIG. 7variously designate changes over time in the applied voltage when the switching element50A switches between “on” and “off” (a); changes over time in the output voltages from the terminal53, the terminal55a, and the output end of the selecting part57(b, c, d, respectively); and changes over time in the output voltage from the output end of the filter61(e). In the waveform diagrams (a) through (e) inFIG. 7, the horizontal axis designates time and the vertical axis designates voltage.

Electrical current flows to the serial circuit composed of the resistance element20gand the coil20ewhen the switching element50A switches “on” at times t1, t3, t5and “off” at times t2, t4, t6, as shown inFIG. 7(a). The voltage in the terminal53undergoes variations as shown by the waveform inFIG. 7(b). The value of the inductance (L) of the coil20eat this point is regarded to be L (μ1).

V(t) inFIG. 7(b) is the voltage of the terminal53at time t, and (e) is the electrical source voltage. The times t2, t4, t6when the switching element50A is switched to “off” are preferably set so that the maximum current that flows to the coil20ehas a value reaching the range in which the magnetization of the magnetostrictive film20cis saturated due to the magnetic flux from the coil20eproduced by the maximum voltage.

The voltage waveform from the output end55aof the phase-shifting part55is established so that the voltage waveform output is shifted by half a period (180°). This output voltage is shown inFIG. 7(e).

The selecting part57is set to switch at every half-period of the switching element50B. The waveform of the voltage signal output from the output end of the selecting part57at this point is shown inFIG. 7(d).

The voltage signal then passes through the noise reduction filter61, after which a direct-current voltage <V>1is output as shown inFIG. 7(e). A value that is proportional to the average value of the voltage output from the selecting part57is output in the smoothing part63of the filter61. The signal “<V>” in this instance refers to “the value proportional to the average voltage value.”

The waveforms (a) through (e) inFIG. 8variously designate the changes over time in the applied voltage when the switching element50A switches “on” or “off” in the case where the inductance value (L (μ2)) of the coil20eis larger than the aforedescribed inductance value (L (μ1)) (a); changes over time in the output voltages from the terminal53, the terminal55a, and the output end of the selecting part57(b, d, d, respectively); and changes over time in the output voltage from the output end of the filter61(e).

Electrical current flows to the serial circuit composed of the resistance element20gand the coil20ewhen the switching element50A switches “on” at times t1, t3, t5and “off” at times t2, t4, t6, as shown inFIG. 8(a). The voltage in the coil20eundergoes variations as shown by the waveform inFIG. 8(b). The value of the inductance (L) of the coil20eat this point is regarded to be L (μ2).

The voltage signal output from the output end55aof the phase-shifting part55is established so that the voltage waveform output is shifted by half a period, as described above. The waveform of this output voltage is shown inFIG. 8(c).

The selecting part57is set to switch at every half-period of the switching element as described above. The waveform of the voltage signal output from the selecting part57at this point is shownFIG. 8(d).

The voltage signal then passes through the noise reduction filter61, after which a direct-current voltage <V>2is output from the output part of the filter61as shownFIG. 8(e). A value output from the output end of the filter61is proportional to the average value of the voltage output from the selecting part57.

As shown inFIGS. 7 and 8, the direct-current voltage <V>1and the direct-current voltage <V>2designate values that differ depending on changes in the inductance value L (μ). The inductance value L (μ) depends on the magnetic permeability of the magnetostrictive films20b,20c. The magnetic permeability μ changes depending on the action of the torque of the steering shaft12bon the magnetostrictive films20b,20c, and therefore the steering torque applied to the steering shaft (12a,12b) can be detected by measuring the aforedescribed direct-current voltage.

The configuration and operation of the detection electrical circuit of the other magnetostrictive film, i.e., the magnetostrictive film20b, are identical to those in the aforedescribed detection electrical circuit of the magnetostrictive film20c. The direct-current voltage output from the output end of the filter62reflects the steering torque applied to the steering shaft (12a,12b).

FIG. 9is a graph that shows the relationship between the direct-current voltage and the torque applied to the steering shaft (12a,12b), i.e., the steering torque. The direct-current voltages detected by way of the detection electrical circuits of the two magnetostrictive films20b,20care line L10and line L11, respectively. The magnetostrictive films20b,20care formed in two locations, upper and lower, so as to have magnetic anisotropy in mutually opposing directions. The result of these magnetic anisotropies is reflected in the symmetry about the vertical axis. The line L12designates a value resulting from subtracting the characteristic line L11from the characteristic line L10, which were detected by the two coils20e,20f. The value of line12, which is zero when the steering torque is zero, displays a linear change with the change in the steering torque. The steering torque can be detected from the values of the respective detection electrical circuits, which include the two detection coils20e,20f, by using the characteristics of the line12.

The two direct-current voltages from the amplifiers61a,62aare calculated in the calculating part65shown inFIG. 6. The output voltage of the calculating part65is converted from analog to digital in the AD converting part66and input to the sample-hold circuit67. Downstream from the sample-hold circuit67, the voltage value is converted into the torque value (T) in the voltage-torque converting part68and is then output.

The torque (T) can thus be detected using the voltage-torque converting part68, which includes a table of previously established relationships between torque and voltage, and the voltage output by the calculating part65.

The voltage input to the sample-hold circuit67as above is a direct-current voltage. The sample-hold circuit67is therefore not used in the range in which the frequency characteristics decay, and phase lag does not occur in the sample-hold circuit or in the sensor output. As a result, phase lag is not experienced in the control of the electric power-steering apparatus, and control is stabilized, allowing smooth steering sensitivity to be obtained.