Angular position sensor working to measure high linearity magnetic flux density

An angular position sensor is provided which is designed to an angular position of a rotary shaft. The angular position sensor has a magnet affixed to the rotary shaft. The magnet has an N-pole and an S-pole and is so geometrically shaped as to produce magnetic flux which is substantially uniform in amount within a range extending around each of centers of the N-pole and the S-pole. This improves the linearity of a change in sensor output upon rotation of the rotary shaft.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates generally to an angular position sensor working to measure an angular position of a rotary member, and more particularly to an improved structure of such an angular position sensor designed to sense a magnetic flux density that is higher in linearity.

2. Background Art

Typical angular position sensors working to measure an angular position of a rotary shaft are made up of a ring-shaped magnet with an N-pole and an S-pole arrayed in a circumferential direction thereof, a magnetic yoke disposed around the periphery of the magnet, and magnetic sensors. The magnetic yoke has formed therein radial grooves forming air gaps. The magnetic sensors are disposed within the air gaps and work to measure magnetic flux densities in the air gaps. For instance, U.S. Pat. No. 5,528,139 to Oudet et al., issued Jun. 18, 1996 (corresponding to Japanese Patent No. 2842482) teaches such a type of angular position sensor.

The N-pole and the S-pole of the magnet are disposed at an angular interval of 180° and create a magnetic flux density changing at a constant rate in the circumferential direction of the magnet. This causes the magnetic flux density as measured by the magnetic sensors to change in the form of a sine wave upon rotation of the rotary shaft. It is, thus, impossible for the magnetic sensors to measure the magnetic flux density that is higher in linearity. Determination of an absolute angular position of the rotary shaft requires large-scaled operations on trigonometric functions and/or using a map, thus posing the problem that the operation load on the system is undesirably high.

SUMMARY OF THE INVENTION

It is therefore a principal object of the invention to avoid the disadvantages of the prior art.

It is another object of the invention to provide an angular position sensor designed to measure a magnetic flux density that is higher in linearity.

According to one aspect of the invention, there is provided an angular position sensor which may be employed in electric power steering devices for automotive vehicles. The angular position sensor comprises: (a) a hard magnetic member connected to a rotary member, the hard magnetic member having a circumference and magnetized in a circumferential direction thereof to produce a magnetic field therearound; (b) a soft magnetic member disposed within the magnetic field produced by the hard magnetic member to form a magnetic circuit, rotation of the rotary member to change a relative position between the magnetic field and the hard magnetic member causes a magnetic flux density in the magnetic circuit to change; and (c) a magnetic flux density measuring sensor disposed at an interval away from the soft magnetic member. The magnetic flux density measuring sensor works to measure the magnetic flux density in the magnetic circuit to produce a signal as a function of the magnetic flux density as indicating an angular position of the rotary member. The hard magnetic member is so designed as to create magnetic flux that is substantially uniform in amount within a given angular range in a circumferential direction thereof, thereby causing the density of magnetic flux developed in the magnetic circuit to change substantially in proportion to a change in angular position of the rotary member, which enables the angular position sensor to provide a change in output that is higher in linearity upon rotation of the rotary member.

In the preferred mode of the invention, the hard magnetic member has a first magnetic pole and a second magnetic pole which are different in polarity from each other and joined at ends thereof together to define the circumference of the hard magnetic member. The first and second magnetic poles work to create the magnetic flux that is substantially uniform in amount within angular ranges defined around central portions of the first and second magnetic poles in the circumferential direction of the hard magnetic member. This structure provides a change in amount of the magnetic flux produced upon rotation of the rotary member in the form of substantially a rectangular wave within the angular range around the central portion of each of the first and second magnetic poles.

The central portions of the first and second magnetic poles may have a thickness defined in a direction perpendicular to a plane extending over the circumference of the hard magnetic member which is smaller than a thickness of portions of the first and second magnetic poles around interfaces between ends of the first and second magnetic poles. Specifically, the central portions of the first and second magnetic poles are thinner, in other words, peripheral areas of the first and second magnetic poles are smaller than those of the portions around the interfaces between the ends of the first and second magnetic poles, so that a total amount of magnetic flux produced from the central portions are decreased as compared with when the first and second magnetic poles have the thickness that is uniform over the circumference of the hard magnetic member, thereby resulting in the uniformity of the amount of magnetic flux within the angular ranges defined around the central portions of the first and second magnetic poles.

The central portions of the first and second magnetic poles may alternatively have a width defined in a direction oriented parallel to a plane extending cover the circumference of the hard magnetic member which is smaller than a width of the portions of the first and second magnetic poles around the interfaces between ends of the first and second magnetic poles. Specifically, the peripheral areas of the first and second magnetic poles are smaller than those of the portions around the interfaces between the ends of the first and second magnetic poles, so that a total amount of magnetic flux produced from the central portions are decreased as compared with when the first and second magnetic poles have the width that is uniform over the circumference of the hard magnetic member, thereby resulting in the uniformity of the amount of magnetic flux within the angular ranges defined around the central portions of the first and second magnetic poles.

The hard magnetic member may alternatively have sub-soft magnetic members which work to convert magnetic flux generated from the first and second magnetic poles into the magnetic flux that is substantially uniform in amount within the given angular range. The sub-soft magnetic members are disposed on outer peripheries of the central portions of the first and second magnetic poles.

In the structure wherein the hard magnetic member has the thickness defined in the direction perpendicular to a plane extending over the circumference thereof which is smaller than that of the soft magnetic member, the hard magnetic member and the soft magnetic member may be so disposed that a plane defined on a circumferential center line of the hard magnetic member in a thickness-wise direction thereof coincides with a plane defined on a circumferential center line of the soft magnetic member in a thickness-wise direction thereof. This structure serves to keep the hard magnetic member inside the soft magnetic member in a thickness-wise direction of the hard magnetic member even if a slight shift between the hard magnetic member and the soft magnetic member occurs in the thickness-wise direction of the hard magnetic member, thereby minimizing a change in magnetic flux density to be measured by the magnetic flux density measuring sensor.

The angular position sensor may further comprise a magnetic shield which surrounds the soft magnetic member to minimize an error of a sensor output arising from external magnetic disturbances.

The soft magnetic member may have a circumference and be disposed outside the circumference of the hard magnetic member. The soft magnetic member may have a first, a second, a third, and a fourth gap formed therein at an interval of approximately 90° in a circumferential direction of the soft magnetic member. A distance between an outer periphery of the soft magnetic member and the magnetic shield is greater than a length of each of the first to fourth gaps in the circumferential direction of the soft magnetic member, thereby avoiding leakage of the magnetic flux from the soft magnetic member to the magnetic shield.

The widths of the first and second magnetic poles of the hard magnetic member defined in the direction perpendicular to the plane extending over the circumferential direction of the hard magnetic member may decrease toward circumferential centers of the first and second magnetic poles. Specifically, the peripheral areas around the circumferential centers of the first and second magnetic poles are smaller than those of the portions around the interfaces between the ends of the first and second magnetic poles, so that a total amount of magnetic flux produced from the central portions are decreased as compared with when the first and second magnetic poles have the width that is uniform over the circumference of the hard magnetic member, thereby resulting in the uniformity of the amount of magnetic flux within the angular ranges defined around the central portions of the first and second magnetic poles.

Each of the hard magnetic member and the soft magnetic member may have a circular inner circumference. The hard magnetic member may have substantially circular outer circumference defined by geometry that widths of the circumferential centers of the first and second magnetic poles in the direction perpendicular to the circumferential direction of the hard magnetic member are smaller than widths of interfaces between the first and second magnetic poles.

In the structure wherein the thickness of the hard magnetic member in the direction perpendicular to the plane extending over the circumference of the hard magnetic member is greater than that of the soft magnetic member, ends of the hard magnetic member opposed in the direction perpendicular to the plane extending over the circumference of the hard magnetic member may project outside ends of the soft magnetic member in the direction perpendicular to the circumference of the hard magnetic member. This causes the magnetic flux to flow from corners of the hard magnetic member out of the soft magnetic member which serves to attract incoming iron powders to avoid sticking thereof to the inner periphery of the soft magnetic member and an opposed portion of the outer periphery of the hard magnetic member, thus ensuring the stability of flow of magnetic flux from the hard magnetic member to the inner periphery of the soft magnetic member for an extended period of time.

According to the second aspect of the invention, there is provided an angular position determining apparatus which comprises: (A) an angular position sensor including (a) a hard magnetic member connected to a rotary member, the hard magnetic member having a circumference and magnetized in a circumferential direction thereof to produce a magnetic field therearound and working to produce magnetic flux that is substantially uniform in amount within a given angular range in a circumferential direction thereof, the hard magnetic member having a first magnetic pole and a second magnetic pole different in polarity from the first magnetic pole, the first and second magnetic poles being jointed at ends thereof at locations 180° apart from each other in a circumferential direction of the hard magnetic member, (b) a soft magnetic member disposed outside the circumference of the hard magnetic member within the magnetic field produced by the hard magnetic member to form a magnetic circuit, rotation of the rotary member to change a relative position between the magnetic field and the hard magnetic member causes a magnetic flux density in the magnetic circuit to change, the soft magnetic member having gaps formed therein at an interval of approximately 90°, and (c) a magnetic flux density measuring sensor including a first and a second sensor element respectively disposed in two of the gaps adjacent in the circumferential direction of the soft magnetic member, the first and second sensor elements working to measure magnetic flux densities within the two gaps and produce electric signals indicative thereof; and (B) an angular position computing circuit working to computing an angular position of the rotary member based on the electric signals produced by the magnetic flux density measuring sensor. Specifically, the densities of magnetic flux produced in the gaps are 90° out of phase with each other, so that outputs of the first and second sensor elements will also be 90° out of phase with each other, thereby enabling the angular position computing circuit to determine the angular position of the rotary member in a full angular range.

The angular position computing circuit is designed to combine the electric signals to determine the angular position of the rotary member within the full angular range.

The angular position computing circuit is designed to perform at least one of addition, subtraction, multiplication, and division operations on the electric signals provided by the magnetic flux density measuring sensor, thus resulting in a decrease in operation load on the angular position computing circuit.

The rotary member may be a steering shaft connected to a steering wheel of an automotive vehicle.

According to the third aspect of the invention, there is provided an angular position determining apparatus which comprises: (a) a ring-shaped hard magnetic member connected to a rotary member, the hard magnetic member including a first magnetic pole and a second magnetic pole different in polarity from the first magnetic pole, the first and second magnetic poles being jointed at ends thereof at locations 180° apart from each other in a circumferential direction of the hard magnetic member, the hard magnetic member being so designed as to create magnetic flux that is substantially uniform in amount within a given angular range in a circumferential direction thereof; (b) a soft magnetic member disposed outside a circumference of the hard magnetic member having formed therein gaps arrayed at an interval of approximately 90°, rotation of the rotary member to change a relative position between the hard magnetic member and the soft magnetic member causes a magnetic flux density in the gaps to change; (c) magnetic flux density measuring sensors disposed one in each of two of the gaps adjacent in a direction of array of the gaps, the magnetic flux density measuring sensors working to magnetic flux densities within the two gaps and produce electric signals indicative thereof which exhibit substantially triangular waves shifted 90° apart in phase from each other and each of which has a straight portion; and (d) an angular position computing circuit working to combine and correct the straight portions of the triangular waves to form a substantially single straight line. The angular position computing circuit computes an angular position of the rotary member using the straight line. This structure serves to provide outputs of the magnetic density measuring sensors which are higher in linearity as a function of a change in angular position of the rotary member. The above combination and correction minimizes an error in determining the angular position of the rotary member.

The electric signals produced by the magnetic flux density measuring sensors are voltage signals whose level change as a function of the angular position of the rotary member. The correction of the straight portions of the triangular waves is achieved in the angular position computing circuit by extracting segments from the straight portions each of which extends over one of preselected angular ranges of rotation of the rotary member, bringing signs of inclinations of the segments into agreement with each other, moving the segments in parallel to bring a voltage level of an end of each of the segments into agreement with that of an end of an adjacent one of the segments, and joining the moved segments to produce a single voltage-to-angle line, defining a straight voltage-to-angle line extending between a maximum voltage level and a minimum voltage level indicated by the single voltage-to-angle line, determining a middle voltage level intermediate between the straight voltage-to-angle line, determining a voltage correction value required to bring the middle voltage level into agreement with an ideal one, and determining an inclination correction value required to bring an inclination of the straight voltage-to-angle line into agreement with an ideal one.

Each of the magnetic flux density measuring sensors may be designed to correct the electric signal so as to compensate for an error arising from an ambient temperature.

Each of the magnetic flux density measuring sensors may be equipped with a temperature-to-correction value map. Each of the magnetic flux density measuring sensors works to pick up a correction value from the temperature-to-correction value map which corresponds to the ambient temperature and correct the electric signal using the correction value.

The angular position computing circuit may store therein an ideal maximum voltage level and an ideal minimum voltage level of the electric signals, determine a first difference between an actual maximum voltage level of the electric signals and the ideal maximum voltage level and a second difference between an actual minimum voltage level and the ideal minimum voltage level, and correct the actual maximum and minimum voltage levels using the first and second differences. Usually, the amount of magnetic flux produced by the hard magnetic, member decreases gradually with a rise in ambient temperature, thus resulting in a decrease in magnetic flux density to be measured by the magnetic flux density measuring sensors. This will cause the voltage levels of outputs of the magnetic flux density measuring sensors to drop. In order to eliminate this problem, the angular position computing circuit is designed to compensate for the drops in voltage levels of the outputs from the magnetic flux density measuring sensors in the above manner.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, wherein like reference numbers refer to like parts in several views, particularly toFIGS. 1(a) and1(b), there is shown an angular position detector1according to the first embodiment of the invention.

The angular position detector1consists essentially of an angular position sensor installed on an outer periphery of a rotary shaft2and an angular position computing circuit6. The angular position computing circuit6is designed to determine an angular position of the rotary shaft2using an output of the angular position sensor.

The angular position sensor includes a magnet3made of a hard magnetic material, a yoke4made of a soft magnetic material, and a magnetic sensor5working to measure the density of magnetic flux.

The magnet3is of a ring-shape and affixed to the outer periphery of the rotary shaft2. The magnet3is made up of two semicircular parts: one having an N-pole3a, and the other having an S-pole3b. The N-pole3aand the S-pole3bare joined integrally at ends thereof at locations 180° far away from each other. The magnet3has a thickness h, as shown inFIG. 2, which decreases gradually from interfaces3cbetween the N-pole3aand the S-pole3bto circumferential centers of the N-pole3aand the S-pole3b.

The yoke4is of an annular shape and made up of four segments4ato4b(will also be referred to as a first, a second, a third, and a fourth yoke segment below) which are arrayed in a circle around the periphery of the magnet3through air gaps41located at approximately 90° away from each other. The yoke4has a thickness, as shown inFIG. 1(a), greater than that of the magnet3. The circumferential center line of the yoke4(i.e., a line extending through the middle of the thickness of the yoke4) coincides with that of the magnet3over the entire circumference thereof. In other words, the magnet3and the yoke4are so disposed that a plane defined on the circumferential center line of the magnet3in a thickness-wise direction thereof coincides with that defined on the circumferential center line of the yoke4in a thickness-wise direction thereof.

The magnetic sensor5is made up of a first sensor element5aand a second sensor element5b. The first sensor element5ais disposed within the gap41between the first and fourth yoke segments4aand4b. The second sensor element5bis disposed within the gap41between the first and second yoke segments4aand4b. The first and second elements5aand5bwork to measure magnetic flux developed in the gaps41as indicating the density of magnetic flux, respectively. The first and second sensor elements5aand5bare separate from the yoke4and each implemented by, for example, a Hall sensor, a Hall IC, or a magneto-resistive device which works to output an electric signal (e.g., a voltage signal) as a function of the density of magnetic flux within the gap41to the angular position computing circuit6.

The angular position computing circuit6works to determine an angular position (i.e., an absolute angle) of the rotary shaft2using the electric signals outputted from the first and second sensor elements5aand5b. Specifically, the angular position computing circuit6combines or links the outputs of the first and second sensor elements5aand5btogether to determine the angular position of the rotary shaft2over 90°.

The density of magnetic flux generated by the magnet3will be described below.

The thickness h of the magnet3, as described above, decreases from the interfaces3cbetween the ends of the N-pole3aand the ends of the S-pole3bto the circumferential centers thereof, so that the thickness of the circumferential centers of the N-pole3aand the S-pole3bis smaller than that of the interfaces3c. Specifically, an area of a peripheral surface around the circumferential centers of the N-pole3aand the S-pole3bof the magnet3is smaller than that when the thickness h is constant over the entire circumference of the magnet. In other words, the amount of magnetic flux produced in the radius direction of the magnet3from the circumferential centers of the N-pole3aand the S-pole3bwhich are the greatest in magnetic flux density is decreased. This causes a total amount of magnetic flux to be almost uniform around the circumferential centers of the N-pole3aand the S-pole3bof the magnet3. Rotation of the magnet3(i.e., the rotary shaft2) will cause the amount of magnetic flux flowing through each of the sensor elements5aand5bof the magnetic sensor5to change cyclically in the form of a wave, as shown inFIG. 3(b). The amount of magnetic flux within a range X (i.e., around the circumferential center of the N-pole3a) is substantially identical with that within a range Y (around the circumferential center of the N-pole3b).

A decrease in thickness h of the magnet3from the interfaces3cbetween the N-pole3aand the S-pole3bis so selected that the amount of magnetic flux created from around each of the circumferential centers of the N-pole3aand the S-pole3bis substantially constant.

A change in magnetic flux density as measured by the magnetic sensor5when the rotary shaft2rotates in a circumferential direction thereof will be described below with reference toFIGS. 4(a) to4(d).

When the rotary shaft2is, as shown inFIG. 4(a), at an angular position I of zero (0°), no magnetic flux flows through the gap41between the first and fourth yoke segments4aand4b, so that the magnetic flux density shows zero (0), while a maximum magnetic flux density of a negative polarity is developed in the gap41between the first and second yoke segments4aand4b. The first and second sensor elements5aand5boutput voltage signals having levels on a broken line I, as illustrated inFIG. 4(d).

When the rotary shaft2rotates 90° in a clockwise direction from the angular position I to an angular position II, as shown inFIG. 4(b), it causes a maximum magnetic flux density of a positive polarity to be developed in the gap41between the first and fourth yoke segments4aand4d, while no magnetic flux flows through the gap41between the first and second yoke segments4aand4b. The first and second sensor elements5aand5boutput voltage signals having levels on a broken line II, as illustrated inFIG. 4(d).

When the rotary shaft2further rotates 90° in the clockwise direction from the angular position II to an angular position III, as shown inFIG. 4(c), it causes a maximum magnetic flux density of the positive polarity to be developed in the gap41between the first and second yoke segments4aand4b. The first and second sensor elements5aand5boutput voltage signals having levels on a broken line III, as illustrated inFIG. 4(d).

The amount of magnetic flux flowing from around each of the circumferential centers of the N-pole3aand the S-pole3bis, as described above, substantially constant, thus causing the magnetic flux density within the gaps41between the first and fourth yoke segments4aand4dand between the first and second yoke segments4aand4bduring rotation of the rotary shaft2to change at a constant rate, so that the first and second sensor elements5aand5boutput the voltage signals, as indicated by solid lines inFIG. 4(d).

FIG. 5shows a flowchart of logical steps or program executed by the angular position computing circuit6of the angular position detector1. In the following discussion, voltage outputs of the first and second sensor elements5aand5bare indicated by Va and Vb, respectively, and an output voltage of the angular position computing circuit6is indicated by Vout.

After entering the program, the routine proceeds to step1wherein it is determined whether the voltage output Va is greater than 3.0V or not. If a YES answer is obtained (Va>3.0V), then the routine proceeds to step6wherein the output voltage Vout is determined according to a relation of Vout=1+Vb and returns back to step1

Alternatively, if a NO answer is obtained (Va≦3.0V), then the routine proceeds to step2wherein it is determined whether the voltage output Va is smaller than 2.0V or not. If a YES answer is obtained (Va<2.0V), then the routine proceeds to step7wherein the output voltage Vout is determined according to a relation of Vout=4−Vb and returns back to step1.

Alternatively, if a NO answer is obtained (Va≧2.0V), then the routine proceeds to step3wherein it is determined whether the voltage output Va is smaller than 2.4V or not. If a YES answer is obtained (Va<2.4V), then the routine proceeds to step8wherein the output voltage Vout is determined according to a relation of Vout=Va and returns back to step1.

Alternatively, if a NO answer is obtained (Va≧2.4V), then the routine proceeds to step4wherein it is determined whether the output voltage Vb is greater than 2.6V, and the voltage output Va is smaller than 2.5V or not. If a YES answer is obtained (Vb>2.6V, and Va<2.5V), then the routine proceeds to step9wherein the output voltage Vout is determined according to a relation of Vout=3−Va and returns back to step1.

Alternatively, if a NO answer is obtained in step4, then the routine proceeds to step5wherein it is determined whether the output voltage Vb is greater than 2.6V, and the voltage output Va is greater than or equal to 2.5V or not. If a YES answer is obtained, then the routine proceeds to step10wherein the output voltage Vout is determined according to a relation of Vout=7−Va and returns back to step1.

Alternatively, if a NO answer is obtained in step5, the routine proceeds to step11wherein the output voltage Vout is determined according to a relation of Vout=0 and returns back to step1.

FIG. 6shows the output voltage Vout of the angular position computing circuit6, as derived in the above operations, which changes at a constant rate over a 360° angular range (i.e., −180° to +180°) of the rotary shaft2. Specifically, the angular position computing circuit6works to output an absolute angular position of the rotary shaft2over the full angular range thereof.

The structure of the angular position detector1of this embodiment, as apparent from the above discussion, offers the following effects.

The thickness h of the magnet3is so selected as to decrease from the interfaces3cbetween the ends of the N-pole3aand the ends of the S-pole3bto the circumferential centers thereof, so that the area of the peripheral surface around the circumferential center of each of the N-pole3aand the S-pole3bwill be the smallest. This causes the density of magnetic flux flowing out of the peripheral surface around the circumferential center of each of the N-pole3aand the S-pole3bto be constant, so that the amount of magnetic flux within the gaps41during rotation of the rotary shaft2changes at substantially a constant rate. Specifically, each of the first and second sensor elements5aand5bworks to output a voltage signal as a function of the magnetic flux density within the gap41which exhibits higher linearity.

The magnet3is so designed as to produce the magnetic flux in the radius direction thereof which changes, as shown inFIG. 3(b), in the form of a rectangular wave, thereby causing the magnetic sensor5to sense the magnetic flux density which changes in the form of substantially a triangle wave. This allows the angular position computing circuit6to determine the angular position of the rotary shaft2correctly using simple operations such as addition, subtraction, multiplication, or division operation or a combination thereof without performing high load operations such as trigonometric function operations.

The yoke4has a thickness, as shown inFIG. 1(a), greater than that of the magnet3. The circumferential center line of the yoke4coincides with that of the magnet3over the entire circumference thereof. This structure allows the rotary shaft2to which the magnet3is affixed to be shifted in the lengthwise direction thereof within a range where the magnet3lies inside opposed end surfaces of the yoke4(i.e., upper and lower end surfaces, as viewed inFIG. 1(a)) in the lengthwise direction of the rotary shaft2, thereby decreasing the amount of magnetic flux leaking outside the yoke4, that is, a change in magnetic flux density to be measured by the magnetic sensor5.

The angular position computing circuit6is designed to combine electric signals outputted from the sensor elements5aand5bof the magnetic sensor5, thereby enabling an angular position indicative analog signal over a range of 90° or more to be produced.

The threshold voltages employed in comparison with the output voltage of the magnetic sensor5in the flowchart ofFIG. 5are merely reference values and changed preferably according to the magnitude of an output of the magnetic sensor5.

FIGS. 7(a) and7(b) show the angular position detector1according to the second embodiment of the invention.

The yoke4is, unlike the first embodiment, made of a one-piece ring which have four recesses42formed at an angular interval of 90° in a circumferential direction thereof to create the gaps41. The formation of the recesses42may be achieved by grinding.

The structure of this embodiment facilitates ease of positioning of the yoke4around the magnet3and results in a decrease in parts making up the angular position sensor.

The grinding of the yoke4to form the recesses42serves to minimize a shift in location of the gaps41in the circumferential direction of the yoke4and/or an error in dimension of the recesses42or the gaps41in the circumferential direction of the yoke4.

Other arrangements are identical with those in the first embodiment, and explanation thereof in detail will be omitted here.

FIGS. 8(a) and8(b) show the angular position detector1according to the third embodiment of the invention.FIG. 8(a) is a sectional view, as taken along the length of the rotary shaft2, which shows the angular position detector1.FIG. 8(b) is a transverse sectional view, as taken along the line VIII-VIII inFIG. 8(a).

The angular position detector1includes an annular magnetic shield7within which the magnet3, the yoke4, the magnetic sensor5, and the angular position computing circuit6are disposed. The distance between the outer periphery of the yoke4and the magnetic shield7is set greater than the length L, as shown inFIG. 8(b), of the gaps41in the circumferential direction of the yoke4, thereby minimizing a leakage of magnetic flux from the yoke4to the magnetic shield7.

The magnetic sensor5, as clearly shown inFIG. 8(a), has terminals which extend in parallel to the length of the rotary shaft2and connect with the angular position computing circuit6. The angular position computing circuit6lead to an external microcomputer (not shown) through a wire harness8.

The magnetic shield7surrounding the yoke4, as shown inFIG. 9, works to protect the yoke4from magnetic flux10flowing around the angular position detector1, thereby eliminating adverse effects of the magnetic flux10on the density of magnetic flux within the gaps41.

The magnetic sensor5may alternatively have terminals, as shown inFIG. 10, which extend in the radius direction of the yoke4and connect with the angular position computing circuit6.

Other arrangements are identical with those in the first embodiment, and explanation thereof in detail will be omitted here.

FIG. 11shows a fourth embodiment in which the angular position detector1of the first embodiment is installed in an electric power steering device11for automotive vehicles which works to assist in steering road wheels of the vehicle manually. Of course, the angular position detector1of one of the second and third embodiments may alternatively be employed in this embodiment.

The electric power steering device11includes an input shaft11a, an output shaft11b, a torsion bar11c, a torque sensor11d, a controller installed in the angular position computing circuit6, an electric motor11e, a torque transmitter11f, and a housing11g. The input shaft11ais connected to a steering wheel of the vehicle. The output shaft11bis connected to steerable road wheels of the vehicle. The torsion bar11cconnects the input and output shafts11aand11btogether. The torque sensor11dworks to measure a steering effort or torque added to the steering wheel. The controller works to determine a target steering assist torque as a function of an output of the torque sensor11d. The electric motor11eworks to produce the target steering assist torque determined by the controller. The torque transmitter11fworks to decrease the speed of an output shaft of the electric motor11eto increase the torque outputted by the electric motor11eand transmit it to the output shaft11b. The housing11gcovers the torque transmitter11f.

The angular position sensor of the angular position detector1is installed round the input shaft11a. The angular position computing circuit6is fixed on the housing11gand receives outputs of the angular position sensor and the torque sensor11d. The angular position computing circuit6works to determine an angular position of the input shaft11a(i.e., a steered angle of the steering wheel of the vehicle) as a function of the output of the angular position sensor (i.e., the magnetic sensor5).

FIGS. 12(a) and12(b) show the magnet3of the angular position detector1according to the fifth embodiment of the invention. The same reference numbers as employed in the above embodiments refer to the same parts.FIG. 12(a) illustrates an example in which the magnet3is elongated along a line extending through the circumferential centers of the N-pole3aand the S-pole3b.FIG. 12(b) illustrates another example in which the magnet3is elongated perpendicular to the line extending through the circumferential centers of the, N-pole3aand the S-pole3b.

The magnet3, as illustrated in each ofFIGS. 12(a) and12(b), is made of an oval-shaped ring and has the width F in the radius direction thereof which decreases gradually from the interfaces3cbetween the N-pole3aand S-pole3bto the circumferential centers of the N-pole3aand the S-pole3b. Specifically, the width F of the circumferential centers of the N-pole3aand the S-pole3bis smaller than that of the interfaces3c, in other words, the volume of the circumferential centers of the N-pole3aand the S-pole3bis smaller than that of the interfaces3c. This causes a total amount of magnetic flux to be almost uniform, like the first embodiment, around the circumferential centers of the N-pole3aand the S-pole3bof the magnet3. Rotation of the magnet3(i.e., the rotary shaft2) will cause the amount of magnetic flux flowing through each of the sensor elements5aand5bof the magnetic sensor5to change cyclically in the form of a wave, as shown inFIG. 3(b). The amount of magnetic flux within a range X (i.e., around the circumferential center of the N-pole3a) is substantially identical with that within a range Y (around the circumferential center of the N-pole3b).

FIGS. 13(a) and13(b) show a modification of the angular position sensor of the angular position detector1. The magnet3has the N-pole3aand the S-pole3bopposed to each other in the thickness-wise direction thereof (i.e., the lengthwise direction of the rotary shaft2). The magnet3has the thickness and width that are uniform over the circumference thereof. Each of the N-pole3aand the S-pole3bis uniform in thickness over the entire circumference of the magnet3. Sub-yokes44aand44bwhich are made of an arc-shaped soft magnetic member having an L-shaped cross section are, as clearly shown inFIG. 13(b), installed on ends of the N-pole3aand the S-pole3bof the magnet3in a diagonally opposed relation so that they surround portions of the periphery of the magnet3. The sub-yokes44aand44bwork to average magnetic flux flowing out of the magnet3in the radius direction thereof to produce a substantially uniform amount of magnetic flux. Other arrangements are identical with those in the first embodiment, and explanation thereof in detail will be omitted here.

FIG. 14shows another modification of the angular position sensor of the angular position detector1. The magnet3is uniform in thickness and width over the circumference thereof. The magnet3has the N-pole3aand the S-pole3bwhich are, like the first embodiment, diametrically opposed to each other across the center of the magnet3. Specifically, the N-pole3aand the S-pole3brange over 180° of the circumference of the magnet3, respectively. The sub-yokes44aand44bare installed on the periphery of the magnet3near the circumferential centers of the N-pole3aand the S-pole3band works to regulate, like the ones inFIGS. 13(a) and13(b), an amount of magnetic flux to be substantially uniform around the circumferential centers of the N-pole3aand the S-pole3bof the magnet3. Other arrangements are identical with those in the first embodiment, and explanation thereof in detail will be omitted here.

FIGS. 15(a) and15(b) show the angular position sensor of the angular position detector1according to the sixth embodiment of the invention.

The magnet3is made of two arc-shaped magnetic members joined to each other each of which has the N-pole3aand the S-pole3bopposed to each other in the thickness-wise direction thereof (i.e., the lengthwise direction of the rotary shaft2). The N-pole3aand the S-pole3bare uniform in thickness over the circumference of the arc-shaped magnetic members. The magnet3also has, as a whole, the N-pole3aand the S-pole3bopposed to each other in the radius direction thereof. The inner diameter of the magnet3is, as can be seen fromFIG. 15(a), greater than that of the yoke4, while the outer diameter of the magnet3is smaller than that of the yoke4. The magnet3is, as clearly illustrated inFIG. 15(b), opposed at an end surface thereof to an end surface of the yoke4. Other arrangements are identical with those in the first embodiment, and explanation thereof in detail will be omitted here.

The yoke4in each of the first to sixth embodiments is made up of four segments, but may alternatively be made up, as shown inFIGS. 16(a) and16(b), two arc-shaped segments4eand4f. The yoke segments4eand4fare opposed at ends thereof to each other through the gaps41located 180° away from each other. A magnetic sensor element5cis disposed within one of the gaps41. The yoke4may alternatively be made up of more than four segments. Other arrangements are identical with those in the first embodiment, and explanation thereof in detail will be omitted here.

The angular position detector1of the seventh embodiment of the invention will be described below with reference toFIGS. 17 to 22.

FIG. 17demonstrates ideal waveforms of the output voltages Va and Vb of the sensor elements5aand5bof the magnetic sensor5.FIG. 18demonstrates an ideal output voltage Vl of the angular position computing circuit6.

The waveform of the output voltage Va of the sensor element5ain an angular range of a complete turn of the rotary shaft2(i.e., −180° to +180°) includes straight segments Val1, Val2, and Val3. The waveform of the output voltage Vb of the sensor element5bin the angular range of a complete turn of the rotary shaft2includes straight segments Vbl1and Vbl2. The output voltages Va and Vb are of substantially a triangular shape and shifted 90° apart in phase. The angular position computing circuit6works to perform the operations, as discussed in the first embodiments, to bring signs of inclinations of the straight segments Val1, Val2, Val3Vbl1, and Vbl2into agreement with each other and move them in parallel to form a straight line, as shown inFIG. 18, made of a combination of the straight segments Val1, Val2, Val3Vbl1, and Vbl2. This enables the absolute angular position of the rotary shaft2to be determined over an angular range of 360° correctly.

The magnetic sensor5, as described above, works to measure the amount of magnetic flux produced from the magnet3as the magnetic flux density. Usually, there is a variation in amount of magnetic flux produced by the magnet3due to a geometrical production error thereof, which will result in variations in the output voltages Va and Vb of the sensor elements5aand5bof the magnetic sensor5. The variations in the output voltages Va and Vb will result in a variation in the output voltage Vl of the angular position computing circuit6. Specifically, the level of voltage appearing at an end of each of the straight segments Val1, Val2, Val3Vbl1, and Vbl2(i.e., at each of joints P1, P2, P3, and P4of the straight segments Val1, Val2, Val3Vbl1, and Vbl2) may not agree with that of an adjacent one of the straight segments Val1, Val2, Val3Vbl1, and Vbl2, thus resulting in shifts between the straight segments Val1, Val2, Val3Vbl1, and Vbl2on the line ofFIG. 18.

In order to avoid the above problem, the angular position computing circuit6of this embodiment is designed to correct the output voltages Va and Vb of the sensor elements5aand5bof the magnetic sensor5, as discussed below, to ensure the linearity of the waveform of the output voltage Vl of the angular position computing circuit6.

FIG. 19shows actual examples of periodic waves of the output voltages Va and Vb of the sensor elements5aand5bof the magnetic sensor5.FIG. 20shows an output voltage LH of the angular position computing circuit6in the event that the straight segments Val1, Val2, Val3Vbl1, and Vbl2(as expressed by La1, La2, La3, Lb1, and Lb2in the drawing) of the waveforms of the output voltages Va and Vb are out of alignment with each other.FIG. 21shows the output voltage LH of the angular position computing circuit6after voltages appearing at the joints P1, P2, P3, and P4of the straight segments La1, La2, La3, Lb1, and Lb2are corrected.FIG. 22is a flowchart of logical steps or a program performed by the angular position computing circuit6to ensure the linearity of the waveform of the output voltage LH.

After entering the program, the routine proceeds to step100wherein two intersections Xmax and Xmin, as shown inFIG. 19, of the waveforms of the output voltages Va and Vb resulting from a 90° shift in phase thereof are found to determine output voltages VXH and VHL appearing at the intersections Xmax and Xmin.

The routine proceeds to step101wherein a middle voltage VXM between the output voltages VXH and VHL, as determined in step100, is calculated according to an equation below.
VXM=(VXH+VXL)/2

The routine proceeds to step102wherein the same operations as those inFIG. 5are executed to determine locations (i.e., voltages) of ends of the straight segments La1, La2, La3, Lb1, and Lb2, as illustrated inFIG. 20, to be connected together (i.e., the joints P1, P2, P3, and P4). In execution of the program ofFIG. 5, 3.0V in step1is replaced with the output voltage VXH, 2.0V in step2is replaced with the output voltage VXL, and 2.5V in step4is replaced with the middle voltage VXM.

A manner of determining the joints P1, P2, P3, and P4will be described below in detail.

Each of voltage levels P10to P17at ends of the straight segments La1, La2, La3, Lb1, and Lb2, as illustrated inFIG. 20, to be connected together are, as described above, does not identical with that of an adjacent one. Thus, the straight segment Lb1is first moved in parallel until a difference between the voltage P11of the straight segment Lb1and the voltage P11of the straight segment La3decreases to zero. In other words, the straight segment Lb1is moved while keeping an inclination thereof as it is to bring the voltage P11into agreement in level with the voltage P10of the straight segment La3. Similarly, the straight segment La1is moved in parallel to bring the voltage P13thereof into agreement with the voltage P12of the parallel-moved straight segment Lb1. The straight segment Lb2is moved in parallel to bring the voltage P15thereof into agreement with the voltage P14of the parallel-moved straight segment La1. Finally, the straight segment La2is moved in parallel to bring the voltage P17thereof into agreement with the voltage P16of the parallel-moved straight segment Lb2. This makes a single line. The voltage P18at the end of the straight segment La2after moved parallel is illustrated as a maximum voltage P18ainFIG. 21. For ease of visibility, the maximum voltage P18ais illustrated inFIG. 21as being much higher than 4.5V.

Next, a straight line LH is defined, as shown inFIG. 21, which extends between the minimum voltage P9and the maximum voltage P18aat the parallel-moved straight segment La2. A middle voltage value VM between the value VH of the maximum voltage P18aand the value VL of the minimum voltage P9is determined using the following equation.
VM=(VH+VL)/2

Afterwards, the routine proceeds to step103. The middle voltage value VM is not always identical with a middle voltage value 2.5V on the ideal straight line VI, as indicated by a dotted line inFIGS. 20 and 21. Thus, a middle voltage correcting value Vofs is determined in accordance with an equation below to correct the middle voltage value VM to 2.5V.
Vofs=VM−2.5

The routine proceeds to step104. The inclination K of the line LH extending between the maximum and minimum voltages P9and P18ais not always identical with an inclination of the ideal line Vl. Thus, an inclination correcting value Kf is determined in accordance with an equation below to bring the inclination K into agreement with that of the ideal line Vl.
Kf=4/(VH−Vl)

The above correction works to bring the output voltage Vout′ of the angular position computing circuit6nearly into agreement with an ideal output voltage. Specifically, the line made up of the straight segments La1, La2, La3, Lb1, and Lb2in the parallel-moving operation in step102is corrected, as indicated by a solid line inFIG. 21, to almost overlap the ideal line Vl made up of segments VaL1, VaL2, VaL3, VbL1, and VbL2. An output voltage range of the angular position computing circuit6is also adjusted to an ideal output voltage range of 0.5V to 4.5V. The above operations enable the angular position computing circuit6to work to measure an absolute angular position of the rotary shaft2over a full range of 360° with minimum errors.

As apparent from the above discussion, correction values used to connect the straight segments La1, La2, La3, Lb1, and Lb2at the joints P1, P2, P3, and P4in the parallel-moving operation in step102are different from each other, thus resulting in an increased operation load on the angular position computing circuit6. Usually, a steering wheel of automotive vehicles stays at a neutral position (angular position of zero (0°)) for the longest period of time. Therefore, in a case where the angular position detector1is installed in the automotive electric power steering device11ofFIG. 11, a decrease in number of operations to determine the joints P1, P2, P3, and P4, in other words, a decrease in operation load on the angular position computing circuit6is accomplished by setting angular positions of the joints P1, P2, P3, and P4to any angular positions other than the neutral position of the steering wheel. The setting of the angular positions of the joints P1, P2, P3, and P4to any angular positions other than the neutral position of the steering wheel is achieved by moving the magnet3in a circumferential direction of the rotary shaft2(i.e., the steering shaft) upon installation on the rotary shaft2.

The waveforms of the output voltages Va and Vb of the sensor elements5aand5b, as illustrated inFIG. 19, are unchanged. A further decrease in operation load on the angular position computing circuit6may be achieved by rotating the steering wheel over 360° one time in either of clockwise and counterclockwise directions to determine the middle voltage correcting value Vofs and the inclination correcting value Kf as fixed initial values.

The angular position detector1of the eighth embodiment will be described below with reference toFIG. 23.

FIG. 23demonstrates actual and ideal periodic waveforms of the output voltages Va and Vb of the sensor elements5aand5bof the magnetic sensor5.

Usually, the amount of magnetic flux produced by the magnet3decreases gradually with a rise in ambient temperature, thus resulting in a decrease in magnetic flux density to be measured by the sensor elements5aand5b. This will cause the output voltages Va and Vb of the sensor elements5aand5bto drop. In order to eliminate this problem, the angular position detector1of this embodiment is designed to compensate for the drops in output voltages Va and Vb of the sensor elements5aand5barising from a rise in ambient temperature. This compensation will be described below in detail.

InFIG. 23, broken lines Vam and Vbm represent the ideal waveforms of the output voltages Va and Vb, respectively. Solid lines Vaj and Vbj represent the actual waveforms of the output voltages Va and Vb, respectively.

The magnetic sensor5is equipped with a temperature sensor (not shown) which works to measure the temperature around the magnetic sensor5. Some of available Hall sensors are equipped with a temperature compensating function. The first and second sensor elements5aand5bof this embodiment are each implemented by the Hall sensor. Specifically, the first and second sensor elements5aand5bhave installed therein a temperature-to-correction value map for use in brining the actual output voltages Vaj and Vbj of the sensor elements5aand5binto agreement with the ideal output voltages Vam and Vbm, respectively. The temperature-to-correction value map is preselected in terms of type of the magnet3and/or the amount of magnetic flux generated by the magnet3.

Specifically, the magnetic sensor5monitors the ambient temperature, selects correction values from the temperature-to-correction value maps, and corrects the actual output voltages Vaj and Vbj into agreement with the ideal output voltages Vam and Vbm, respectively, thereby compensating for the drops in output voltages Va and Vb of the sensor elements5aand5bdue to a temperature characteristic of the magnet3.

Instead of compensating for the drops in output voltages Va and Vb of the sensor elements5aand5bin themselves, the angular position computing circuit6may be designed to perform operations, as discussed below, to bring the actual output voltages Vaj and Vbj into agreement with the ideal output voltages Vam and Vbm, respectively. Specifically, maximum values Vammax and Vbmmax and minimum values Vammin and Vbmmin of the ideal output voltages Vam and Vbm are prestored in the angular position computing circuit6. The angular position computing circuit6calculates differences Hmax between a maximum value Vajmax of the actual output voltage Vaj and Vammax and between a maximum value Vbjmax of the actual output voltage Vbj and between Vbmmax and differences Hmin between a minimum value Vajmin of the actual output voltage Vaj and Vammin and between a minimum value Vbjmin of the actual output voltage Vbj and Vbmmin and corrects the maximum values Vajmax and Vbjmax and the minimum values Vajmin and Vbjmin of the actual output voltages Vaj and Vbj using the differences Hmax and Hmin so as to bring the actual output voltages Vaj and Vbj into agreement with the ideal output voltages Vam and Vbm.

The angular position detector1according to the ninth embodiment will be described below with reference toFIGS. 24(a),24(b), and25.FIG. 24(a) shows the angular position sensor of the angular position detector1equipped with the magnet3, as illustrated inFIG. 12(b).FIG. 24(b) shows the angular position sensor of the angular position detector1equipped with the magnet3of this embodiment. InFIG. 24(b), a broken line indicates a profile of the magnet3ofFIG. 24(a).FIG. 25demonstrates a periodic wave indicating a change in amount of magnetic flux φ which is generated by the magnet3ofFIG. 24(b) and measured by the sensor elements5aand5bof the magnetic sensor5as a function of an angular position θ of the rotary shaft2.

The magnet3, as illustrated inFIG. 12(b), has a circular inner periphery and an oval outer periphery. In a case, as illustrated inFIG. 24(a), where the magnet ofFIG. 12(b) is disposed within the yoke4whose inner periphery is circular, the distance G between the circumferential centers of the N-pole3aand the S-pole3bof the magnet3and the inner periphery of the yoke4is much greater than the distance between interfaces between the N-pole3aand the S-pole3band the inner periphery of the yoke4. This causes the amount of magnetic flux which is generated around the circumferential centers of the N-pole3aand the S-pole3band leaks out of the yoke4to increase, thus resulting in a decreased density of magnetic flux flowing through the yoke4, which leads to a decrease in output of the magnetic sensor5.

In order to avoid the above problem, the magnet3has the interfaces between the N-pole3aand the S-pole3bground or cut to have flat side surfaces130aand130b. This permits the width of the circumferential centers of the N-pole3aand the S-pole3bin the radius direction of the magnet3to be increased more than that of the magnet3, as illustrated inFIG. 24(a). Specifically, the gap G between the circumferential centers of the N-pole3aand the S-pole3band the inner periphery of the yoke4is allowed to be smaller than that inFIG. 24(a). This results in a decreased leakage of magnetic flux outside the yoke4.

The magnet3ofFIG. 24(b) is, as apparent from the above, substantially circular as compared with that ofFIG. 24(a). The width of the circumferential centers of the N-pole3aand the S-pole3band the width of the interfaces between the N-pole3aand the S-pole3bare so selected as to have a relation therebetween which produces the magnetic flux changing in the form of a wave, as illustrated inFIG. 25, upon rotation of the rotary shaft2. Specifically, the amounts of magnetic flux within a range X (i.e., around the circumferential center of the N-pole3a) and a range Y (around the circumferential center of the N-pole3b) are substantially uniform.

FIG. 26shows the angular position sensor of the angular position detector1according to the tenth embodiment of the invention.

The yoke4in each of the first to ninth embodiments is, as described above, made of a metallic soft magnetic material. The yoke4of this embodiment is made up of four soft magnetic plates4a,4b,4c, and4dwhose thickness in the radius direction of the yoke4is smaller than that of the yoke segments4ato4din each of the first to ninth embodiments. The magnetic plates4ato4dof this embodiment are each formed by press such as punching or bending and smaller in weight than the yoke segments4ato4din each of the first to ninth embodiments by the volume S.

The magnetic plates4ato4dmay alternatively be formed by grinding metallic blocks.

FIGS. 27(a) and27(b) show the angular position sensor of the angular position detector1according to the eleventh embodiment of the invention.

The magnet3ofFIG. 27(a) has a profile identical with that ofFIG. 24(a) orFIG. 24(b), but different therefrom in that the thickness B is greater than the thickness C of the yoke4. Specifically, corners31of the magnet3are located outside the yoke4in the lengthwise direction of the rotary shaft2. This causes the magnetic flux to flow from the corners31out of the yoke4which serves to attract incoming iron powders to avoid sticking thereof to the inner periphery of the yoke4and an opposed portion of the outer periphery of the magnet3, thus ensuring the stability of flow of magnetic flux from the magnet3to the inner periphery of the yoke4for an extended period of time.

The magnet3may be made integrally with the rotary shaft2using insert-molding techniques. This improves cocentricity of the magnet3and the rotary shaft2.

FIG. 27(b) shows a modification of the magnet3ofFIG. 27(a).

The magnet3is affixed to the rotary shaft2through a ring-shaped resinous magnet holder9. The magnet holder9may be formed integrally with the rotary shaft2using insert-molding techniques. The magnet3may be made of a ferrite magnet or a plastic bonded magnet.