System for determining at least one rotation parameter of a rotating component

Abstract

The invention relates to a system for determining at least one rotation parameter of a rotating member, said system comprising: an encoder (1) forming a magnetic track (2) having Npp pairs of North and South magnetic poles separated by 2Npp transitions (3), each extending along an Archimedean spiral; a rotation sensor comprising two groups (4, 5) of two sensing elements (4a, 4b, 5a, 5b) forming signals SIN and COS respectively, said groups being angularly offset by an angle α arranged so that the SIN and COS signals are in quadrature, the sensing elements (4a, 4b, 5a, 5b) of each of the groups (4, 5) being diametrically opposed, forming an angle of π between them. Figure 1

Description

Title of the invention: System for determining at least one rotation parameter of a rotating member

[0001] The invention relates to a system for determining at least one rotation parameter of a rotating member, said system comprising an encoder emitting a periodic magnetic field as well as a rotation sensor capable of detecting said magnetic field.

[0002] In many applications, it is desired to know in real time and with optimal quality at least one rotation parameter of a rotating member, such as its position, its speed, its acceleration or its direction of movement.

[0003] To do this, document WO-2006 / 064169 proposes the use of an encoder intended to be integral with the moving member and on which a magnetic track is formed which is capable of emitting a pseudo-sinusoidal magnetic field at a reading distance from a sensor comprising several sensitive elements.

[0004] Advantageously, each sensitive element may comprise at least one pattern based on a tunnel magnetoresistive material (TMR in English for Tunnel Magneto Resistance) whose resistance varies according to the detected magnetic field, as for example described in the document WO-2004 / 083881.

[0005] To determine a displacement parameter of the moving member as a function of the evolution of the detected magnetic field, document WO-2006 / 064169 provides a combination of the signals representative of the resistance of each of the sensitive elements, in order to deliver two quadrature signals of the same amplitude which can be used to calculate said parameter. Furthermore, this solution provides a two-by-two subtraction of four signals, in order to obtain two quadrature signals which are free from common noise.

[0006] Document FR-3 078 775 describes an encoder comprising Npp pairs of North and South magnetic poles which are separated by transitions which extend along an Archimedean spiral, each of said spirals being distributed on said encoder by successive rotations of an angle -rr—.

[0007] The advantage of this type of encoder is that the polar width Lp of each of the poles along the radius of said encoder becomes independent of the number Npp of pairs of poles, which thus makes it possible to reconcile a small number of poles with adequate positioning of the sensitive elements relative to the sinusoidality and the amplitude of the magnetic signal to be detected.

[0008] Furthermore, document FR-3 078 775 proposes an arrangement of the elements sensitive axially opposite the magnetic track which makes it possible to filter at least the 3rd harmonic of the magnetic field generated by a spiral encoder on a pair of magnetic poles.

[0009] However, when the rotating member on which the encoder is mounted is subject to axial play, in particular if it is part of a bearingless assembly, for example using plain bearings, the eccentricity of the axis of rotation of the magnetic track relative to the geometric axis of the sensor caused by this play causes the appearance of a significant determination error.

[0010] The invention aims to solve the problems of the prior art by proposing in particular a system for determining at least one rotation parameter of a rotating member, in which the precision of the determination is improved, in particular in the event of eccentricity of the rotation axis of the magnetic track of the encoder relative to the geometric axis of the sensor.

[0011] For this purpose, the invention proposes a system for determining at least one rotation parameter of a rotating member, said system comprising: - an encoder intended to be associated in rotation with the rotating member so as to move jointly with it, said encoder comprising a body on which is formed a magnetic track which is capable of emitting a periodic magnetic field representative of the rotation of said encoder, said track having Npp pairs of North and South magnetic poles which are separated by 2NPP transitions, each of said transitions extending along an Archimedean spiral defined in polar coordinates relative to the axis of rotation by the equation P = -(Q+QiX Lp being the polar width of each of the poles along the radius of said encoder, the angle 0; of rotation of the ith spiral relative to the first spiral being equal to with i between 0 and 2NPP-1; - a rotation sensor comprising two groups of two sensitive elements distributed angularly at a reading distance from the magnetic track, each of said sensitive elements being capable of detecting the periodic magnetic field emitted by said encoder to deliver a signal representative of the rotation of the encoder; - a device for subtracting the signals delivered by the sensitive elements of each of said groups to form signals respectively SIN and COS, said groups being angularly offset by an angle a which is arranged so that the signals SIN and COS are in quadrature, the sensitive elements of each of the groups being diametrically opposed forming between them an angle of jt.

[0012] Other features and advantages of the invention will appear in the description which follows, made with reference to the attached figures, in which:

[0013] [Fig.l] schematically represents a determination system according to a first embodiment of the invention, showing in particular the arrangement of the sensitive elements relative to the encoder and an eccentricity of the magnetic track of said encoder relative to said sensitive elements;

[0014] [Fig.2] is a schematic representation of the device for subtracting a sensor for a determination system according to the invention;

[0015] [Fig.3] schematically represents a determination system according to a second embodiment of the invention, showing in particular the arrangement of the sensitive elements relative to the encoder;

[0016] [Fig.4a]

[0017] [Fig.4b] and

[0018] [Fig.4c] schematically represent a determination system according to a third embodiment of the invention, showing the movement of a sensitive element according to the prior art ([Fig.4a]) and the arrangement of the sensitive elements relative to the encoder according to respectively an embodiment ([Fig.4b]) and a variant of this embodiment ([Fig.4c]).

[0019] In relation to these figures, a system is described for determining at least one rotation parameter of a rotating member relative to a fixed structure. In particular, the parameter of the rotating member can be chosen from its position, its speed, its acceleration or its direction of movement.

[0020] In a particular application, the system can be used in connection with the control of a brushless direct current electric motor, making it possible in particular to know the absolute angular position on a pair of motor poles of the rotor relative to the stator.

[0021] The determination system comprises an encoder 1 intended to be integral with the rotating member so as to move jointly with it, said encoder comprising a body, in particular annular but which may also be discoidal, on which is formed a magnetic track 2 which is capable of emitting a periodic magnetic field representative of the rotation of said encoder. In particular, the emitted magnetic field may be sinusoidal or pseudo-sinusoidal, that is to say having at least one portion which can be correctly approximated by a sinusoid.

[0022] Track 2 has Npp pairs of North and South magnetic poles which are separated by 2NPP transitions 3, each of said transitions extending along an Archimedean spiral defined in polar coordinates (p, 0) relative to the axis of rotation by the equation p = NPPLP .(0+00, Lp being the polar width of each of the poles along the JT radius of encoder 1, angle 0; of rotation of the ith spiral relative to the first spiral being equal to ^^.i with i between 0 and 2NPP-1.

[0023] Thus, the magnetic track 2 delivers a pseudo-sinusoidal magnetic signal whose spatial period is equal to X = 2.LP. Furthermore, the Archimedean spiral geometry allows in particular that the number Npp of pairs of poles of the magnetic track 2 as well as the polar width Lp can be chosen independently of the radius of the magnetic track 2.

[0024] According to one embodiment, the encoder 1 is formed of a magnet on which the multipolar magnetic track 2 is produced. In particular, the magnet may be formed of an annular matrix, for example made from a plastic or elastomer material, in which magnetic particles are dispersed, in particular ferrite or rare earth particles such as NdFeB.

[0025] The determination system comprises a rotation sensor which is intended to be integral with the fixed structure, said sensor being capable of detecting the periodic magnetic field emitted by the encoder 1. To do this, the sensor comprises several magnetic sensitive elements 4a, 4b, 5a, 5b, each being capable of detecting the periodic magnetic field emitted by the encoder 1 to deliver a signal representative of the rotation of said encoder, said sensitive elements being arranged with a reading gap for said magnetic field.

[0026] Each of the sensitive elements 4a, 4b, 5a, 5b may in particular be chosen from magnetosensitive probes. For example, probes based on the Hall effect, tunnel magnetoresistors (TMR), anisotropic magnetoresistors (AMR) or giant magnetoresistors (GMR) may measure a component of the magnetic field (normal or tangential to the encoder 1) or the rotating field (resulting from the normal and tangential components).

[0027] In particular, as described in document WO-2004 / 083881, each sensitive element forms a tunnel junction by comprising a stack of a reference magnetic layer, an insulating separation layer and a magnetic layer sensitive to the field to be detected, the resistance of the stack being a function of the relative orientation of the magnetization of the magnetic layers.

[0028] Advantageously, each sensitive element 4a, 4b, 5a, 5b may comprise at least one pattern based on a magnetoresistive material whose resistance varies as a function of the magnetic field, a sensitive element 4a, 4b, 5a, 5b being able to comprise a single pattern or a group of patterns connected in series.

[0029] Alternatively, the normal component alone of the magnetic field delivered by the encoder 1 can be measured, for example using Hall effect elements. The use of the normal field alone is favorable because it is more sinusoidal than the tangential field.

[0030] The circumferential distribution of the sensitive elements 4a, 4b, 5a, 5b makes it possible to to overcome the edge effects of the magnetic field delivered by encoder 1, allowing the use of an encoder 1 of limited height, in particular less than 6.LP.

[0031] Furthermore, by arranging the sensitive elements 4a, 4b, 5a, 5b at a reading air gap distance from the magnetic track 2 which is of the order of , a good compromise is obtained between sinusoidality and amplitude of the detected signal. In particular, this optimal positioning can be obtained because the polar width Lp can be between 1 and 6 mm, even with a number Npp of pole pairs of the encoder 1 which is less than 12.

[0032] The sensor comprises two groups 4, 5 of two sensitive elements 4a, 4b, 5a, 5b distributed angularly at a reading distance from the magnetic track 2, the determination system comprising a device for subtracting the signals V4a, V4b and V5a, V5b delivered by the sensitive elements 4a, 4b, 5a, 5b of each of said groups to form signals SIN and COS respectively ([Fig.2]).

[0033] The groups 4, 5 of sensitive elements 4a, 4b, 5a, 5b are angularly offset by an angle a which is arranged so that the signals SIN and COS are in quadrature, i.e. phase shifted by 90°.

[0034] Advantageously, the sensitive elements 4a, 4b, 5a, 5b are distributed angularly along the magnetic track 2, forming between the two groups 4, 5 an angle a which is equal to modulo .

[0035] In particular, by exploiting such quadrature signals, in the sensor or in an associated computer, it is known to determine the angular position of the encoder 1, for example by a direct calculation of an arctangent function, using a “Look-Up Table” (LUT) or using a CORDIC type method.

[0036] The sensitive elements 4a, 4b, 5a, 5b of each of the groups 4, 5 are diametrically opposed, forming an angle of jt between them. In relation to FIG. 1, the track 2 comprises a pair of poles which makes it possible to determine an absolute angle over 360° as soon as the system is powered up, the groups 4, 5 of sensitive elements 4a, 4b, 5a, 5b being offset by an angle a which is equal to

[0037] The signals Si, S2 detected by the sensitive elements 5a, 5b of group 5 are then written:

[0038] Sj = Asin(è);

[0039] S2 = AsinO + 7T)

[0040] where A is the amplitude of the signal and b is the rotation angle of encoder 1.

[0041] Thus, for an eccentricity ex along the X axis containing the sensitive elements 5a, 5b, the signals are written:

[0042] S^Asin^ + ç?);

[0043] 52 = Asin( / ? + tt - (p)

[0044] m — being the phase shift caused by the eccentricity ex.

[0045] For example, for a polar length Lp= 2mm and an eccentricity of ex=0.1mm, we obtain a phase shift q>=9°, which could directly have an angular error impact on the determination of the rotation parameter.

[0046] Thus, when the rotating member on which the encoder 1 is mounted is subject to axial play, in particular if it is part of a bearingless assembly, for example using plain bearings, the eccentricity of the axis of rotation of the magnetic track 2 relative to the geometric axis of the sensor caused by this play causes the appearance of a significant determination error.

[0047] But, insofar as the two sensitive elements 5a, 5b of group 5 are diametrically opposed, the difference of the two signals S1 and S2 gives:

[0048] Sj- S2 = Asin(b + <p) - Asin(^ + 7r- (p) = 2Asin (^-- ) .cos(h + 7 )

[0049] = 2A.cosç?.sin / ?

[0050] The amplitude of this difference changes slightly (by 1.3% in our example) but its phase does not change. On the other hand, the effect of the eccentricity ex on the difference of the signals of the sensitive elements 4a, 4b is minimal because, along this axis X, the sensitive elements 4a, 4b substantially follow the curvature of the spiral which supports them. Their respective phase is only slightly modified.

[0051] Thus, overall, the effect of the eccentricity ex on the angular determination is minimized. The complete analytical proof for compound eccentricities along the X and Y axes is more complex to obtain, but this result is confirmed by numerical simulations with different eccentricities.

[0052] In relation to figures 3 and 4, the sensitive elements 4a, 4b, 5a, 5b of each of the groups 4, 5 are positioned at a reading distance from the magnetic track 2 so as to detect a magnetic field of phase respectively q>i and cp2, the phase shift ô = q>2 -q>i being such that:

[0053] 0.55ir < ô.Npp < 0.83ir, modulo 2ir; Or

[0054] 1.17jt < ô.Npp < 1.45jt, modulo 2ir.

[0055] In this configuration, the signals Vj, V2 delivered by the sensitive elements 4a, 4b, 5a, 5b of the same group 4, 5 can be written:

[0056] = Gif b sin(an + + G.H3.sin(3an + 3(p { ) + G^sin(5an + 5^) + ...

[0057] v2(t)=GHpsinf œt + <p^ + GJ / 3.sin ( 3œt + 3^j + GJl 5.sin(5œt + 5^2j + ...

[0058] co étant la vitesse de rotation, H; étant l’amplitude de la fondamentale pour i=l et des harmoniques de rang i pour i=3, 5, etc.

[0059] Let us take a subtraction of the signals Vj, V2 delivered by the two sensitive elements 4a,

[0060]

[0061]

[0062]

[0063]

[0064]

[0065]

[0066]

[0067]

[0068]

[0069]

[0070]

[0071]

[0072] 4b, 5a, 5b magnetically out of phase by an angle δ which is written: V2(t) = ) -sin( + ^,)] + CZÆf^Lsinf 3cc'it4-) -.810(3^ + 3^.,^ + 0 / 75^11(51^ + 5^) -sin(5w + 5ç>,)] + ... = ).cos (,,f+^ ) +2.G.7 / ?sin(3^ )xos (3^ + 3^) +2.GJ / ,S1n(5.Ç )xos (5^ + 5.^ ) + ... Considering a filtering of at least 3 dB of the harmonic of rank 3, it is therefore necessary that: / X < r— sin(W) • / \ — "> sm (J Therefore, the filtering of the third harmonic is satisfactory for a magnetic phase shift ô which is such that: 0.55ir < ô.Npp < 0.83ir, modulo 2ir; Or 1.17jt < ô.Npp < 1.45jt, modulo 2ir. The suppression, or at least the attenuation, of the harmonic of rank 3 in the SIN, COS signals processed to determine the rotation parameter is beneficial in relation to the precision of said determination, but also for the signal processing algorithms which carry out: - the removal of the offset of the SIN and COS channels; - equalization of the amplitudes of the SIN and COS channels; - phase correction between SIN and COS channels. Optimally with respect to filtering, the phase shift ô is such that ô.Npp is substantially equal to 2ir / 3 or 4ir / 3 modulo 2ir, since the harmonic of rank 3 is then cancelled: Vj( t) -VG. t) — fë-GH^cos (wth—j + 0 -^3.GJ^^.cos 5œt + 5."y" j + ... However, this phase shift ô leads to a gain of ^3 of the SIN, COS signals delivered by the groups 4, 5 of sensitive elements 4a, 4b, 5a, 5b, which can lead to varying said phase shift in the ranges mentioned above to optimize the filtering-gain pair. In relation to Figure 3, track 2 comprises a pair of poles which makes it possible to determine an absolute angle over 360°. In addition to the shifting of groups 4, 5 by an angle a equal to y, arrows F4 and F5 indicate the displacement of a sensitive element 4c, 5c of each of groups 4, 5 from a position forming an angle y of 120° with the sensitive element 4a, 5a of its group 4, 5, to a configuration according to the invention in which the sensitive elements 4a, 4b, 5a, 5b of each of groups 4, 5 are diametrically opposed while maintaining a phase shift δ of 120° to filter the harmonic of rank 3.

[0073] In particular, each sensitive element 4c, 5c of a group 4, 5 can be moved anywhere along the Archimedean spiral A4, A5 passing through its original angular position of angle y as determined above, or on the equivalent spiral of another magnetic period, angularly offset by a multiple of 360° / Npp, until the sensitive elements 4a, 4b, 5a, 5b of each of the groups 4, 5 are diametrically opposed.

[0074] If possible, positions will be preferred where the measured magnetic fields have a good sinusoidal shape and the same amplitude, in particular positions rather close to the median reading radius RL.

[0075] In relation to Figures 4, a system particularly suitable for controlling an electric motor with four pairs of poles is described below, said system providing the absolute position on a pair of motor poles, i.e. 90° mechanically.

[0076] To do this, the encoder 1 comprises 4 pairs of poles (Npp = 4), the groups 4, 5 of sensitive elements 4a, 4b, 5a, 5b delivering SIN, COS signals in quadrature on each of the pairs of poles so that the sensor or the engine control computer can determine the absolute angular position over an angular sector of 90°.

[0077] In relation to figure 4a, two sensitive elements 4a, 4c, 5a, 5c form between them an angle y of = jt / 6, the angle a between the groups 4, 5 being = ir / 8.

[0078] As in [Fig.3], the arrow F5 indicates the displacement of a sensitive element 5c of a group 5 from a position forming the angle y of jt / 6 with the sensitive element 5a of its group 5, towards a configuration according to the invention with an angle of 180° while maintaining a phase shift ô of ir / 6 to filter the harmonic of rank 3.

[0079] By doing the same for the other group 4 of sensitive elements 4a, 4b, we obtain the configuration according to [Fig.4b] in which the groups 4, 5 are angularly offset by an angle a = jt / 8, the sensitive elements 4a, 4b, 5a, 5b of each of the groups 4, 5 being diametrically opposed while being magnetically phase-shifted by an angle ô = jt / 6.

[0080] In a variant shown in [Fig.4c], a group 4 of sensitive elements 4a, 4b can be offset isophase by an angle of ir / 2 relative to the configuration of [Fig.4b].

Claims

1. Claims System for determining at least one rotation parameter of a rotating member, said system comprising: - an encoder (1) intended to be associated in rotation with the rotating member so as to move jointly with it, said encoder comprising a body on which is formed a magnetic track (2) which is capable of emitting a periodic magnetic field representative of the rotation of said encoder, said track having Npp pairs of North and South magnetic poles which are separated by 2NPP transitions (3), each of said transitions extending along an Archimedean spiral defined in polar coordinates relative to the axis of rotation by the equation p = Npp.Lp .(0+0^, Lp being the polar width of jT each of the poles following the radius of said encoder, the angle 0; of rotation of the ith spiral relative to the first spiral being equal to .i with i between 0 and 2NPP-1; - a rotation sensor comprising two groups (4, 5) of two sensitive elements (4a, 4b, 5a, 5b) distributed angularly at a reading distance from the magnetic track (2), each of said sensitive elements being capable of detecting the periodic magnetic field emitted by said encoder to deliver a signal representative of the rotation of the encoder (1), - a device for subtracting the signals delivered by the sensitive elements (4a, 4b, 5a, 5b) of each of said groups to form signals respectively SIN and COS, said groups being angularly offset by an angle a which is arranged so that the signals SIN and COS are in quadrature; said determination system being characterized in that the sensitive elements (4a, 4b, 5a, 5b) of each of the groups (4, 5) are diametrically opposed, forming between them an angle of ji, the sensitive elements (4a, 4b, 5a, 5b) of each of the groups (4, 5) being positioned at a reading distance from the magnetic track (2) so as to detect a magnetic field of phase respectively q>i and q>2, the phase shift ô = q> 2 - q>i being such that: 0.55ir < ô.Npp < 0.83ir, modulo 2ir; Or 1.17jt < ô.Npp < 1.45ji, modulo 2ir.

2. Determination system according to claim 1, characterized in that the angle a formed between the two groups (4, 5) of sensitive elements (4a, 4b, 5a, 5b) is equal to .....modulo . 7 ° 2 A pp h pp

3. Determination system according to one of claims 1 or 2, characterized in that the phase shift ô is such that ô.Npp is substantially equal to 2ir / 3 or 4ir / 3 modulo 2ir.

4. Determination system according to any one of claims 1 to 3, characterized in that the encoder (1) has a height which is less than 6.LP.

5. Determination system according to any one of claims 1 to 4, characterized in that the sensitive elements (4a, 4b, 5a, 5b) are arranged at a reading gap distance from the magnetic track (2) which is of the order of ££

6. Determination system according to any one of claims 1 to 5, characterized in that the number Npp of pairs of poles of the encoder (1) is less than 12.

7. Determination system according to any one of claims 1 to 6, characterized in that the polar width Lp of the encoder (1) is between 1 and 6 mm.

Images