SYSTEM FOR DETERMINING AT LEAST ONE ROTATION PARAMETER OF A ROTATING MEMBER

Abstract

The invention relates to a system comprising a coder that has an alternation of North and South magnetic poles separated by transitions extending along a helix of pitch p and of angle α, the magnetic track having N.sub.pp pairs of North and South poles and a polar width L.sub.p measured along a normal to the transitions which are: N.sub.pp=πa/l, and L.sub.p=p.Math.cos α. The invention also includes at least one sensor able to detect the rotating magnetic field in a plane perpendicular to the magnetic track and to the transitions by means of a mounting of at least two sensitive magnetic elements. The mounting being disposed at a radial reading distance from the magnetic track and being arranged to deliver signals in quadrature.

Claims

1. A system for determining at least one rotation parameter of a rotating member, the system comprising: a coder with a body having a cylindrical periphery with a radius around an axis of revolution (X), the periphery having an alternation of North and South magnetic poles of width l which are separated by transitions, each one of the transitions extending along an helix of pitch p and of angle □ to form a multipolar magnetic track which is able to emit a periodic magnetic field which is rotating in a plane perpendicular to the magnetic track and to the transitions, the track having N.sub.pp pairs of North and South poles and a polar width L.sub.p measured along a normal (N) to the transitions which are: N.sub.pp=□a/l, and L.sub.p=p.Math.cos □; at least one sensor able to detect the rotating magnetic field emitted by the coder by means of a mounting of at least two sensitive magnetic elements, the mounting being disposed at a radial reading distance from the magnetic track and being arranged to deliver signals (V.sub.01, V.sub.02; V′.sub.01, V′.sub.02) in quadrature; wherein the system comprises two sensors of which the mountings are spaced by a distance e measured along the normal (N) to the transitions by delivering respectively signals (V.sub.01, V.sub.02; V′.sub.01, V′.sub.02) in quadrature, the system further comprising a circuit for subtracting the signals in order to form signals (SIN, COS) in quadrature.

2. The system according to claim 1, wherein the mounting comprises two Wheatstone bridge circuits of four sensitive elements, the circuits being disposed in a plane perpendicular to the magnetic track as to detect the magnetic field rotating in the plane which is emitted by the track.

3. The system according to claim 1, wherein each sensitive element comprises at least one pattern with a tunnel magneto resistance material base of which the resistance varies according to the magnetic field detected.

4. The system according to claim 1, wherein the distance e: e=L.sub.p modulo 2L.sub.p.

5. The system according to claim 1, wherein the distance e: 0.55L.sub.p<e<0.82L.sub.p, modulo 2L.sub.p; or 1.18L.sub.p<e<1.45L.sub.p, modulo 2L.sub.p.

6. The system according to claim 6, wherein the distance e is substantially equal to ⅔L.sub.p or 4/3L.sub.p modulo 2L.sub.p.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Other particularities and advantages of the invention shall appear in the following description, made in reference to the accompanying figures, wherein:

[0019] FIG. 1a and FIG. 1b diagrammatically show a coder of a system for determining according to the invention, respectively in perspective (FIG. 1a) and as a side view (FIG. 1b);

[0020] FIG. 2 is a flat representation of the cylindrical periphery of the coder of FIG. 1a and 1b;

[0021] FIG. 3 diagrammatically shows an embodiment of the arrangement at a radial reading distance of a mounting of sensitive elements with respect to a coder according to the invention;

[0022] FIG. 4 is a diagram of a mounting of sensitive elements according to an embodiment of the invention;

[0023] FIG. 5 shows signals in quadrature delivered by the mounting according to FIG. 4;

[0024] FIG. 6 diagrammatically shows an embodiment of the arrangement at a radial reading distance of two mountings of sensitive elements with respect to a coder according to the invention;

[0025] FIG. 7 is a diagram of the integration of the mountings of FIG. 6 in a device for subtracting;

[0026] FIG. 8 is a curve showing the filtering of the 3rd order harmonic according to the distance between the mountings of sensitive elements of the sensors.

DETAILED DESCRIPTION

[0027] In relation with these figures, a system for determining at least one rotation parameter of a rotating member with respect to a fixed structure is described. The parameter of the rotating member can be selected from its position, its speed, its direction of rotation, its acceleration or its direction of movement, axial in particular.

[0028] In one embodiment, the system can be used in relation with the controlling of a brushless direct current electric motor, making it possible to know the absolute angular position on a pair of motor poles of the rotor with respect to the stator.

[0029] The system for determining comprises a coder 1 intended to be integral with the rotating member in such a way as to move jointly with it, the coder comprising a body having a cylindrical periphery with a radius around an axis of revolution X on which is formed a magnetic track 2 which is able to emit a periodic magnetic field representative of the rotation of the coder. The magnetic field emitted can be sinusoidal or pseudo-sinusoidal, i.e. having at least one portion which can be correctly approximated by a sinusoid.

[0030] The track 2 has an alternation of North 2n and South 2s magnetic poles of width l which are separated by transitions 3, each one of the transitions extending along a helix of pitch p and of angle α.

[0031] Thus, the magnetic track has N.sub.pp pairs of North and South poles and a polar width L.sub.p measured along a normal N to the transitions 3 which are such that: N.sub.pp=πa/l, and L.sub.p=p.Math.cos α. The magnetic track 2 delivers a pseudo-sinusoidal magnetic signal of which the spatial period along the normal N is equal to λ=2L.sub.p. The periodic magnetic field delivered by the magnetic track 2 is rotating in a plane perpendicular to the magnetic track and to the transitions 3.

[0032] The magnetic field generated by the coder 1 on a pair of magnetic poles 2n, 2s is the combination of a perfect fundamental sinusoidal component that is sought to be measured in order to determine the parameter, and of several odd-order harmonics (3, 5, etc.).

[0033] If it is assumed that the coder 1 rotates at a constant speed of rotation CO, the magnetic field can be written in the following way:

H(t)=H.sub.1.Math.sin ω+H.sub.3.Math.sin 3ωt+H.sub.5.Math.sin 5ωt+ . . .

[0034] The amplitude H.sub.3 of the 3.sup.rd order harmonic can typically represent 5% of the amplitude H.sub.1 of the fundamental. According to the position of the sensor and the reading distance, this proportion of the amplitude H.sub.3 of the 3.sup.rd order harmonic can be much higher.

[0035] The helical geometry of the magnetic track 2 makes it possible for the number N.sub.pp of pairs of poles 2n, 2s as well as the polar width L.sub.p to be chosen independently of the radius a of the magnetic track 2. In relation with FIGS. 1a and 1 b, the coder 1 comprises four pairs of poles 2n, 2s, which is particularly suitable for the controlling of an electric motor with four pairs of poles, the system providing the absolute position on a pair of motor poles, i.e. 90° mechanical.

[0036] According to an embodiment, the coder 1 is formed from a magnet on the cylindrical periphery of which the multipolar magnetic track 2 is carried out. The magnet can be formed from an annular matrix, for example made from a base of a plastic or elastomer material, in which magnetic particles are dispersed, particles of ferrite or of rare earths such as NdFeB.

[0037] The system for determining comprises at least one sensor that is intended to be integral with the fixed structure, the sensor being able to detect the rotating magnetic field emitted by the coder 1. To do this, the sensor comprises a mounting 4 of at least two sensitive magnetic elements 5, the mounting being disposed at a radial reading distance from the magnetic track 2 in order to deliver signals in quadrature which are representative of the rotation of the coder 1.

[0038] Each one of the sensitive elements 5 can be chosen from magnetically sensitive probes. For example, Hall, tunnel magneto resistance (TMR), anisotropic magneto resistance (AMR) or giant magneto resistance (GMR) probes can measure each one of the two components of the magnetic field (normal and tangential to the coder 1).

[0039] As described in document WO-2004/083881, each element 5 forms a tunnel junction by comprising a stack of a reference magnetic layer, of an insulating separation layer and of a magnetic layer that is sensitive to the field to be detected, the resistance of the stack being according to the relative orientation of the magnetisation of the magnetic layers.

[0040] Advantageously, each sensitive element 5 can comprise at least one pattern with a magneto resistance material base, with a tunnel effect, of which the resistance varies according to the magnetic field, a sensitive element 5 being able to comprise a single motif or a group of motifs connected in series or in parallel.

[0041] In order to be able to determine the rotation parameter of the rotating member, the signals delivered by the mounting 4 of sensitive elements 5 must preferably be in quadrature, i.e. geometrically offset by 90° divided by N.sub.pp. By using such signals in quadrature, in the sensor or in an associated calculator, it is known to determine the angular position of the coder 1, for example through a direct calculation of an arctangent function, using a Look-Up Table (LUT) or a method of the CORDIC type.

[0042] To do this, in relation with FIG. 4, the mounting 4 can comprise two Wheatstone bridge circuits of four sensitive elements 5, the circuits being disposed in a plane perpendicular to the magnetic track 2 in such a way as to detect the magnetic field rotating in the plane which is emitted by the track.

[0043] According to the angle γ of inclination of the magnetic field, FIG. 5 shows the signals V.sub.01 and V.sub.02 delivered in quadrature by the bridge which are such that:

V.sub.01=(+V.sub.01)−(−V.sub.01),

V.sub.02=(+V.sub.02)−(−V.sub.02).

[0044] In relation with an application of the system in controlling an electric motor, the good sinusoidality of the signal delivered to the control calculator allows for: [0045] better performance, in particular at start-up, for example the time for reaching the speed or position setting; [0046] a more “gentle” operation, without torque shifts in steady state; [0047] less energy consumption; [0048] a lower operating temperature; [0049] a more substantial maximum torque.

[0050] FIG. 3 shows a mounting 4 in the median position of the periphery of the coder 1 in order to be separated as much as possible from the edges of the coder.

[0051] In relation with FIG. 6, the system for determining comprises two sensors of which the mountings 4, 4′ are spaced by a distance e measured along the normal N to the transitions 3 by delivering respectively signals V.sub.01, V.sub.02 and V′.sub.01, V′.sub.02 in quadrature, the system further comprising a device for subtracting signals in order to form SIN, COS signals in quadrature.

[0052] FIG. 7 shows an embodiment in which the signals formed are:

SIN=(+SIN)−(−SIN);

[0053] +SIN being equal to (+V.sub.01)−(+V′.sub.01),

[0054] −SIN being equal to (−V.sub.01)−(−V′.sub.01);

COS=(+COS)−(−COS);

[0055] +COS being equal to (+V.sub.02)−(+V′.sub.02),

[0056] −COS being equal to (−V.sub.02)−(−V′.sub.02).

[0057] This embodiment allows for a filtering of the noise coming from the outside (for example from the motor or neighbouring interconnections). Indeed, if the magnetic field comprises an identical noise component on the different mountings 4, 4′, the latter will be subtracted from the output signals SIN, COS.

[0058] By positioning the mountings 4, 4′ at the magnetic phases respectively (pi and φ.sub.2, i.e. by spacing them by a distance e measured along the normal N to the transitions 3 which is such that

[00001]${\phi }_1-{\phi }_2=\frac{e}{2L_p}*360,$

the signals V.sub.1=+COS or +SIN and V.sub.2=−COS or −SIN delivered can be written:

V.sub.1(t)=G.Math.H.sub.1.Math.sin(ωt+φ.sub.1)+G.Math.H.sub.3.Math.sin(3ωt+3φ.sub.1)+G.Math.H.sub.5.Math.sin(5ωt+5φ.sub.1)+ . . .

V.sub.2(t)=G.Math.H.sub.1.Math.sin(ωt+φ.sub.2)+G.Math.H.sub.3.Math.sin(3ωt+3φ.sub.2)+G.Math.H.sub.5.Math.sin(5ωt+5φ.sub.2)+ . . .

G being the supposedly identical gain of the mountings 4, 4′, ω being the speed of rotation, H.sub.i being the amplitude of the fundamental for i=1 and of the i-th order harmonics for i=3, 5, etc.

[0059] A subtractor circuit calculates the SIN or COS difference which is then written:

[00002]$V_1\left(t\right)-V_2\left(t\right)=G.H_1.\left[\sin \left(\omega t+{\phi }_1\right)-\sin \left(\omega t+{\phi }_2\right)\right]+G.H_3.\left[\sin \left(3\omega t+3{\phi }_1\right)-\sin \left(3\omega t+3{\phi }_2\right)\right]+G.H_5.\left[\sin \left(5\omega t+5{\phi }_1\right)-\sin \left(5\omega t+5{\phi }_2\right)\right]+.Math.=2.G.H_1.\sin \left(\frac{{\phi }_1-{\phi }_2}{2}\right).\cos \left(\omega t+\frac{{\phi }_1+{\phi }_2}{2}\right)+2.G.H_3.\sin \left(3.\frac{{\phi }_1-{\phi }_2}{2}\right).\cos \left(3\omega t+3.\frac{{\phi }_1+{\phi }_2}{2}\right)+2.G.H_5.\sin \left(5.\frac{{\phi }_1-{\phi }_2}{2}\right).\cos \left(5\omega t+5.\frac{{\phi }_1+{\phi }_2}{2}\right)+.Math.$

[0060] In relation with FIG. 6, e=L.sub.p modulo 2L.sub.p, i.e. the mountings are offset 180° modulo 360°, this difference is written:

[00003]$V_1\left(t\right)-V_2\left(t\right)=2.G.H_1\cos \left(\omega t+\frac{{\phi }_1+{\phi }_2}{2}\right)-2.G.H_3.\cos \left(3\omega t+3.\frac{{\phi }_1+{\phi }_2}{2}\right)+2.G.H_5.\cos \left(5\omega t+5.\frac{{\phi }_1+{\phi }_2}{2}\right)+.Math.$

[0061] It can be seen that the 3.sup.rd and 5.sup.th order harmonics are retained and have the same gain 2 as the fundamental after the subtraction operation.

[0062] In order to obtain a precise determination of the rotation parameter, it is sought to measure the filtered signal of at least the 3.sup.rd order harmonic. However, any fixed compensation of the error generated by the harmonics is difficult to carry out, in that it depends on the measurement conditions (gap, position of the sensor). Moreover, a calibration is also difficult to consider for large volume and low cost application.

[0063] FIG. 8 shows the filtering of the 3.sup.rd order harmonic according to the value of the offset φ.sub.1−φ.sub.2.

[0064] When the distance e is substantially equal to ⅔L.sub.p or 4/3L.sub.p modulo 2L.sub.p, the difference is written:

[00004]$V_1\left(t\right)-V_2\left(t\right)=\sqrt{3}.G.H_1\cos \left(\omega t+\frac{{\phi }_1+{\phi }_2}{2}\right)+0-\sqrt{3}.G.H_5.\cos \left(5\omega t+5.\frac{{\phi }_1+{\phi }_2}{2}\right)+.Math.$

[0065] In this case, the 3.sup.rd order harmonic is cancelled, the fundament and the 5.sup.th order harmonic have a gain of 1.73 after the subtraction operation. A 3.sup.rd order harmonic spatial filter was then carried out, while still retaining 86.5% of the fundamental.

[0066] Generally and in relation with FIG. 8, considering that the filter of the 3.sup.rd order harmonic plays its role if it removes at least 3 dB from its value without filtering in relation to the amplitude of the fundamental, it is therefore required that:

[00005]$.Math.\frac{2.G.H_3.\sin \left(3\frac{{\phi }_1-{\phi }_2}{2}\right)}{2.G.H_1.\sin \left(\frac{{\phi }_1-{\phi }_2}{2}\right)}.Math.\le \frac{\sqrt{2}}{2}.Math..Math.\frac{H_3}{H_1}.Math.\iff .Math.\frac{\sin \left(3\frac{{\phi }_1-{\phi }_2}{2}\right)}{\sin \left(\frac{{\phi }_1-{\phi }_2}{2}\right)}.Math.\le \frac{\sqrt{2}}{2}\iff {\phi }_1-{\phi }_2\in \left[99°;148°\right]\mathrm{modulo}360°\mathrm{ou}{\phi }_1-{\phi }_2\in \left[212°;261°\right]\mathrm{modulo}360°$

[0067] Expressed in distance, in order to obtain a filtering of the 3.sup.rd order harmonic, it is therefore required that the mountings 4, 4′ are spaced by a distance e measured along the normal N to the transitions 3 which is such that:

0.55L.sub.p<e<0.82L.sub.p,modulo 2L.sub.p; or

1.18L.sub.p<e<1.45L.sub.p,modulo 2L.sub.p.

[0068] The distance e between the mountings 4, 4′ can vary within the ranges mentioned hereinabove in order to optimise the couple filtering-gain. Moreover, according to the space available, the mountings 4, 4′ can be aligned along the normal N to the transitions 3, along the axis X or offset circumferentially (FIG. 6).

[0069] The suppression, or at least the attenuation, of the 3.sup.rd order harmonic in the processed signals to determine the rotation parameter is beneficial relative to the precision of the determination, but also for the processing algorithms of the signal that carry out: [0070] deletion of the offset of the signals; [0071] balancing of the amplitudes of the signals; [0072] phase correction between the signals.