FR3143733A1 - System for determining at least one rotation parameter of a rotating member - Google Patents

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 which are separated by 2Npp transitions (3) each extending along an Archimedean spiral; a rotation sensor comprising two groups (4, 5) of two sensitive elements (4a, 4b, 5a, 5b) forming signals SIN and COS respectively, said groups being angularly offset by an angle α which is arranged so that the signals SIN and COS are in quadrature, the sensitive 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

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

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.

In many applications, we want 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.

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

Advantageously, each sensitive element can comprise at least one pattern based on a tunneling magnetoresistive material (TMR in English for Tunnel Magneto Resistance) whose resistance varies as a function of the detected magnetic field, as for example described in the document
WO-2004/083881.

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

Document FR-3 078 775 describes an encoder comprising N _pp pairs of North and South magnetic poles which are separated by transitions which extend along an Archimedean spiral, each of said spirals being distributed over said encoder by successive rotations of an angle .

The advantage of this type of encoder is that the polar width L _p of each of the poles following the radius of said encoder becomes independent of the number N _pp of pairs of poles, which thus makes it possible to reconcile a small number of poles with adequate positioning sensitive elements relative to the sinusoidality and the amplitude of the magnetic signal to be detected.

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

However, when the rotating member on which the encoder is mounted is subjected to axial play, particularly if it is part of an assembly without bearing, 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.

The invention aims to resolve 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, particularly in the event of eccentricity. of the axis of rotation of the magnetic track of the encoder relative to the geometric axis of the sensor.

• un codeur destiné à être associé en rotation à l’organe tournant de sorte à se déplacer conjointement avec lui, ledit codeur comprenant un corps sur lequel est formée une piste magnétique qui est apte à émettre un champ magnétique périodique représentatif de la rotation dudit codeur, ladite piste présentant N_pppaires de pôles magnétiques Nord et Sud qui sont séparés par 2N_pptransitions, chacune desdites transitions s’étendant suivant une spirale d’Archimède définie en coordonnées polaires par rapport à l’axe de rotation par l’équation ρ = .(θ+θ_i), L_pétant la largeur polaire de chacun des pôles suivant le rayon dudit codeur, l’angle θ_ide rotation de la i^èmespirale par rapport à la première spirale étant égal à .i avec i compris entre 0 et 2N_pp-1 ;
• un capteur de rotation comprenant deux groupes de deux éléments sensibles répartis angulairement à distance de lecture de la piste magnétique, chacun desdits éléments sensibles étant apte à détecter le champ magnétique périodique émis par ledit codeur pour délivrer un signal représentatif de la rotation du codeur ;
• un dispositif de soustraction des signaux délivrés par les éléments sensibles de chacun desdits groupes pour former des signaux respectivement SIN et COS, lesdits groupes étant décalés angulairement d’un angle α qui est agencé pour que les signaux SIN et COS soient en quadrature, les éléments sensibles de chacun des groupes étant diamétralement opposés en formant entre eux un angle de π.

To this end, 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 N _pp pairs of North and South magnetic poles which are separated by 2N _pp transitions, each of said transitions extending along an Archimedean spiral defined in polar coordinates with respect to the axis of rotation by the equation ρ = .(θ+θ _i ), L _p being the polar width of each of the poles along the radius of said encoder, the angle θ _i of rotation of the i ^th spiral relative to the first spiral being equal to .i with i between 0 and 2N _pp -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 SIN and COS respectively, said groups being angularly offset by an angle α which is arranged so that the signals SIN and COS are in quadrature, the elements sensitive of each of the groups being diametrically opposed forming between them an angle of π.

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

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;

is a schematic representation of the device for subtracting a sensor for a determination system according to the invention;

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;

And

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 ( ) and the arrangement of the sensitive elements relative to the encoder according to respectively an embodiment ( ) and a variant of this realization ( ).

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.

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.

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 can 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 can be sinusoidal or pseudo-sinusoidal, that is to say having at least one portion which can be correctly approximated by a sinusoid.

Track 2 presents N _pp pairs of North and South magnetic poles which are separated by 2N _pp transitions 3, each of said transitions extending along an Archimedean spiral defined in polar coordinates (ρ, θ) relative to the axis of rotation by the equation ρ = .(θ+θ _i ), L _p being the polar width of each of the poles along the radius of encoder 1, the angle θ _i of rotation of the i ^th spiral relative to the first spiral being equal to .i with i between 0 and 2N _pp -1.

Thus, the magnetic track 2 delivers a pseudo-sinusoidal magnetic signal whose spatial period is equal to λ = 2.L _p . In addition, the Archimedean spiral geometry allows in particular that the number N _{p p} of pairs of poles of the magnetic track 2 as well as the polar width L _p can be chosen independently of the radius of the magnetic track 2.

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 can be formed from an annular matrix, for example made from a plastic or elastomeric material, in which magnetic particles, in particular ferrite or rare earth particles such as NdFeB, are dispersed.

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 at a reading gap for said magnetic field.

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

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

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 may comprise a single pattern or a group of patterns connected in series.

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

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

Furthermore, by arranging the sensitive elements 4a, 4b, 5a, 5b at a reading gap distance from the magnetic track 2 which is of the order of , we obtain a good compromise between sinusoidality and amplitude of the detected signal. In particular, this optimal positioning can be obtained because the polar width L _p can be between 1 and 6 mm, even with a number N _pp of pairs of poles of the encoder 1 which is less than 12.

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, determination system comprising a device for subtracting the signals V_4a,V_4b and V_5a,V_5bdelivered by the sensitive elements 4a, 4b, 5a, 5b of each of said groups to form signals SIN and COS respectively ( ).

Groups 4, 5 of sensitive elements 4a, 4b, 5a, 5b are angularly offset by an angle α which is arranged so that the signals SIN and COS are in quadrature, that is to say phase shifted by 90°.

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 α which is equal to module .

In particular, by exploiting such quadrature signals, in the sensor or in an associated calculator, 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.

The sensitive elements 4a, 4b, 5a, 5b of each of the groups 4, 5 are diametrically opposed, forming an angle of π between them. In relation to Figure 1, 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, groups 4, 5 of sensitive elements 4a, 4b, 5a, 5b being offset by an angle α which is equal to .

The signals S ₁ , S ₂ detected by the sensitive elements 5a, 5b of group 5 are then written:

;

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

Thus, for an eccentricity e _x along the axis X containing the sensitive elements 5a, 5b, the signals are written:

;

being the phase shift caused by the eccentricity e _x .

For example, for a polar length L _p = 2mm and an eccentricity of e _x =0.1mm, we obtain a phase shift φ=9°, which could directly be reflected in angular error on the determination of the rotation parameter.

Thus, when the rotating member on which the encoder 1 is mounted is subjected to axial play, in particular if it is part of an assembly without bearing, 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.

But, to the extent that the two sensitive elements 5a, 5b of group 5 are diametrically opposed, the difference of the two signals S ₁ and S ₂ gives:

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 e _x on the difference in the signals of the sensitive elements 4a, 4b is minimal because, along this axis . Their respective phase is only slightly modified.

Thus, overall, the effect of the eccentricity e _x 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.

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 φ ₁ and φ ₂ , the phase shift δ = φ ₂ – φ ₁ being such that:

0.55π < δ.N _pp < 0.83π, modulo 2π; Or

1.17π < δ.N _pp < 1.45π, modulo 2π.

In this configuration, the signals V ₁ , V ₂ delivered by the sensitive elements 4a, 4b, 5a, 5b of the same group 4, 5 can be written:

ω being the rotation speed, H _i being the amplitude of the fundamental for i=1 and of the harmonics of rank i for i=3, 5, etc.

Or a subtraction of the signals V ₁ , V ₂ delivered by the two sensitive elements 4a, 4b, 5a, 5b magnetically phase shifted by an angle δ which is written:

Considering a filtering of at least 3 dB of the third-order harmonic, it is therefore necessary that:

Consequently, the filtering of the third order harmonic is satisfactory for a magnetic phase shift δ which is such that:

0.55π < δ.N _pp < 0.83π, modulo 2π; Or

1.17π < δ.N _pp < 1.45π, modulo 2π.

• la suppression de l’offset des voies SIN et COS ;
• l’égalisation des amplitudes des voies SIN et COS ;
• la correction de phase entre les voies SIN et COS.

The removal, or at least the attenuation, of the third-order harmonic 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;
• equalizing the amplitudes of the SIN and COS channels;
• phase correction between the SIN and COS channels.

Optimally with respect to filtering, the phase shift δ is such that δ.N _pp is substantially equal to 2π/3 or 4π/3 modulo 2π, since the harmonic of rank 3 is then canceled:

However, this phase shift δ leads to a gain of signals SIN, COS delivered by 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 couple.

In relation to Figure 3, track 2 includes a pair of poles which makes it possible to determine an absolute angle over 360°. In addition to the shift of groups 4, 5 by an angle α equal to , the arrows F4 and F5 indicate the movement of a sensitive element 4c, 5c of each of the groups 4, 5 from a positioning forming an angle γ of 120° with the sensitive element 4a, 5a of its group 4, 5, towards a configuration according to the invention in which the sensitive elements 4a, 4b, 5a, 5b of each of the groups 4, 5 are diametrically opposed while maintaining a phase shift δ of 120° to filter the harmonic of order 3.

In particular, we can move each sensitive element 4c, 5c of a group 4, 5 anywhere along the Archimedean spiral A ₄ , A ₅ passing through its original angular position of angle γ as determined here -above, or on the equivalent spiral of another magnetic period, angularly offset by a multiple of 360°/N _pp , until the sensitive elements 4a, 4b, 5a, 5b of each of groups 4 , 5 are diametrically opposed.

If possible, we will prefer positions where the measured magnetic fields have a good sinusoidal shape and the same amplitude, in particular positions rather close to the median reading radius R _L.

In relation to Figures 4, we describe below a system particularly suited to controlling an electric motor with four pairs of poles, said system providing the absolute position on a pair of motor poles, i.e. 90° mechanical.

To do this, the encoder 1 comprises 4 pairs of poles (N _pp = 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 in order to that the sensor or the motor control computer can determine the absolute angular position over an angular sector of 90°.

In relation to Figure 4a, two sensitive elements 4a, 4c, 5a, 5c form between them an angle γ of = π/6, the angle α between groups 4, 5 being = π/8.

Likewise on the , the arrow F5 indicates the movement of a sensitive element 5c of a group 5 from a positioning forming the angle γ of π/6 with the sensitive element 5a of its group 5, towards a configuration according to the invention with a angle of 180° while maintaining a phase shift δ of π/6 to filter the third-order harmonic.

By doing the same for the other group 4 of sensitive elements 4a, 4b, we obtain the configuration according to the in which the groups 4, 5 are angularly offset by an angle α = π/8, the sensitive elements 4a, 4b, 5a, 5b of each of the groups 4, 5 being diametrically opposed by being magnetically phase shifted by an angle δ = π/6.

As a variant shown on the , a group 4 of sensitive elements 4a, 4b can be shifted at isophase by an angle of π/2 relative to the configuration of the .

Claims (8)

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 representative periodic magnetic field of the rotation of said encoder, said track having N _pp pairs of North and South magnetic poles which are separated by 2N _pp transitions (3), each of said transitions extending along an Archimedean spiral defined in polar coordinates with respect to the axis of rotation by the equation ρ = .(θ+θ _i ), L _p being the polar width of each of the poles along the radius of said encoder, the angle θ _i of rotation of the i ^th spiral relative to the first spiral being equal to .i with i between 0 and 2N _pp -1;
• a rotation sensor comprising two groups (4, 5) of two sensitive elements (4a, 4b, 5a, 5b) angularly distributed at a reading distance from the magnetic track (2), each of said sensitive elements being capable of detecting the magnetic field periodic 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 α 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 an angle of π between them. Determination system according to claim 1, characterized in that the angle α formed between the two groups (4, 5) of sensitive elements (4a, 4b, 5a, 5b) is equal to module . Determination system according to one of claims 1 or 2, characterized in that 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 φ ₁ and φ ₂ , the phase shift δ = φ ₂ – φ ₁ being such that:
0.55π < δ.N _pp < 0.83π, modulo 2π; Or
1.17π < δ.N _pp < 1.45π, modulo 2π. Determination system according to claim 3, characterized in that the phase shift δ is such that δ.N _pp is substantially equal to 2π/3 or 4π/3 modulo 2π. Determination system according to any one of Claims 1 to 4, characterized in that the encoder (1) has a height which is less than 6.L _p . Determination system according to any one of claims 1 to 5, characterized in that the sensitive elements (4a, 4b, 5a, 5b) are arranged at a reading gap distance from the magnetic track (2) which is the order of Determination system according to any one of claims 1 to 6, characterized in that the number N _pp of pole pairs of the encoder (1) is less than 12. Determination system according to any one of claims 1 to 7, characterized in that the polar width L _p of the encoder (1) is between 1 and 6 mm.