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

Abstract

The invention relates to a system having a coder with a magnetic track that exhibits an alternation of North and South magnetic poles separated by i Archimedean spiral transitions. The invention also includes a rotation sensor able to detect the periodic magnetic field emitted by the coder using several sensitive elements, the sensitive elements being distributed angularly along the magnetic track while forming between at least two sensitive elements an angle α which is designed so that the signals delivered by the elements are in quadrature.

Claims

1. A system for determining at least one rotation parameter of a rotating member, the system comprising: a coder associated in rotation with the rotating member to move together with it, the coder comprising a body on which a magnetic track is formed for emitting a periodic magnetic signal representative of the rotation of the coder, the track exhibiting an alternation of North and South magnetic poles separated by i transitions, each of the transitions extending in an Archimedean spiral defined in polar coordinates with respect to the axis of rotation by the equation $\rho =\frac{\mathrm{Npp}.\mathrm{Lp}}{\pi }.Math.\left(\theta +{\theta }_i\right),$ Npp being the number of pairs of poles of the magnetic track and Lp the polar width of each of the poles along the radius of the coder, the angle θi of rotation of the ith spiral with respect to the first spiral being equal to $\frac{\pi }{\mathrm{Npp}}.Math.i$ with i between 0 and 2.Math.Npp−1; a rotation sensor to detect the periodic magnetic field emitted by the coder using several magnetic sensitive elements; the system wherein the sensitive elements are distributed angularly along the magnetic track while forming between at least two sensitive elements an angle α which is designed so that the signals delivered by the elements are in quadrature.

2. The determination system according to claim 1, wherein the angle α formed between the two sensitive elements is equal to $\frac{\pi }{2\mathrm{Npp}}\mathrm{modulo}\frac{\pi }{\mathrm{Npp}}.$

3. The determination system according to claim 1, wherein the sensitive elements are distributed angularly along a radius R of the magnetic track.

4. The determination system according to claim 3, wherein the sensitive elements are distributed angularly according to the median radius R of the magnetic track.

5. The determination system according to claim 1, wherein the sensor comprises two sensitive elements.

6. The determination system according to claim 1, wherein the coder has a height that is less than 6.Math.Lp.

7. The determination system according to claim 1, wherein the sensitive elements are disposed at a reading air-gap distance from the magnetic track that is approximately $\frac{\mathrm{Lp}}{2}.$

8. The determination system according to claim 1, wherein the number Npp of pairs of poles of the coder is less than 6.

9. The determination system according to claim 1, wherein the polar width Lp of the coder is between 2 and 6 mm.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) Other particularities and advantages of the invention will appear in the following description, made in reference to the appended figures, in which FIGS. 1 and 2 are diagrams of a determination system according to respectively an embodiment of the invention, in particular showing the positioning of the sensitive elements with respect to the coder.

DETAILED DESCRIPTION

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

(3) In a specific use, the system can be used in relation to the control of a brushless direct current electric motor, in particular allowing to know the absolute angular position on a pair of motor poles of the rotor with respect to the stator.

(4) The determination system comprises a coder 1 intended to be rigidly connected to the rotating member in such a way as to move together with it, the coder comprising a body, in particular annular but which can also be discoidal, on which a magnetic track 2 is formed that is capable of emitting a periodic magnetic signal representative of the rotation of the coder. In particular, the magnetic signal emitted can be sinusoidal or pseudo-sinusoidal, that is to say, having at least one portion that can be correctly approximated by a sinusoid.

(5) The track 2 exhibits an alternation of North and South magnetic poles that are separated by i transitions 3, each of the transitions extending in an Archimedean spiral defined in polar coordinates (ρ, θ) with respect to the axis of rotation by the equation

(6) $\rho =\frac{\mathrm{Npp}.\mathrm{Lp}}{\pi }.Math.\left(\theta +{\theta }_i\right),$
N.sub.pp being the number of pairs of poles of the magnetic track 2 and L.sub.p the polar width of each of the poles along the radius of the coder, the angle θ.sub.i of rotation of the i.sup.th spiral with respect to the first spiral being equal to

(7) $\frac{\pi }{\mathrm{Npp}}.Math.i$
with i between 0 and 2.Math.N.sub.pp−1.

(8) Thus, the magnetic track 2 delivers a pseudo-sinusoidal magnetic signal, the spatial period of which is equal to λ=2.Math.L.sub.p. Moreover, the Archimedean-spiral geometry allows in particular for the number N.sub.pp of pairs of poles of the magnetic track 2 as well as the polar width L.sub.p to be chosen independently of the radius R of the magnetic track 2.

(9) According to one embodiment, the coder 1 is formed by a magnet on which the multipolar magnetic track 2 is made. In particular, the magnet can be formed by an annular matrix, for example made from a plastic or elastomer material, in which magnetic particles are dispersed, in particular particles of ferrite or of rare earths like NdFeB.

(10) The determination system comprises a rotation sensor that is intended to be rigidly connected to the fixed structure, the sensor being able to detect the periodic magnetic field emitted by the coder 1. To do this, the sensor comprises several magnetic sensitive elements 4 that are disposed at a reading air gap from the magnetic field delivered by the magnetic track 2, wherein each of the sensitive elements can in particular be chosen from the magnetosensitive probes.

(11) For example, probes based on tunnel magnetoresistances (TMR), anisotropic magnetoresistances (AMR) or giant magnetoresistances (GMR) can measure a component of the magnetic field (normal or tangential to the coder) or the rotating field (resulting from the normal and tangential components).

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

(13) Advantageously, each sensitive element 4 can comprise at least one pattern made from a magnetoresistive material, the resistance of which varies according to the magnetic field, wherein a sensitive element 4 can comprise a single pattern or a group of patterns connected in series.

(14) Alternatively, the normal component alone of the magnetic field delivered by the coder 1 can be measured, for example via Hall-effect elements. The use of the normal field alone is favorable since it is more sinusoidal than the tangential field.

(15) In order to be able to determine the rotation parameter of the rotating member, the signals delivered by the sensitive elements 4 must be in quadrature, that is to say, out of phase by 90°. In particular, by use of such signals in quadrature, in the sensor or in an associated computer, it is known to determine the angular position of the coder 1, for example by a direct calculation of an arctangent function, using a “Look-Up Table” (LUT) or via a method of the CORDIC type.

(16) To do this, the sensitive elements 4 are distributed angularly along the magnetic track 2 while forming between at least two sensitive elements 4 an angle α which is designed so that the signals delivered by the elements are in quadrature. According to the embodiments shown, the angle α formed between the two sensitive elements 4 is equal to

(17) $\frac{\pi }{2\mathrm{Npp}}\mathrm{modulo}\frac{\pi }{\mathrm{Npp}}.$

(18) Thus, the circumferential distribution of the sensitive elements 4 allows to avoid the effects of edges of the magnetic field delivered by the coder 1, allowing to use a coder 1 having a limited height h, in particular less than 6.Math.L.sub.p. In particular, the sensitive elements 4 can be distributed angularly along a radius R, in particular the median radius in the figures, of the magnetic track 2 in order to be as far as possible from the edges of the coder 1.

(19) Moreover, by disposing the sensitive elements 4 at a reading air-gap distance from the magnetic track 2 that is approximately

(20) 0$\frac{\mathrm{Lp}}{2},$
a good compromise between sinusoidality and amplitude of the signal detected is obtained. In particular, this optimal positioning can be obtained because the polar width L.sub.p can be between 2 and 6 mm, even with a number N.sub.pp of pairs of poles of the coder 1 that is less than 6.

(21) Thus, the circumferential distribution of the sensitive elements 4 has in particular the following advantages: the distance between the two elements 4 is sufficiently large to use discrete components (1D Hall probes) that are not very costly and very linear; the tolerance of circumferential positioning of the elements 4 does not greatly impact the precision of the sensor (since the distance that separates them is large); since the two elements 4 are located on the middle radius R of the coder 1, they are not greatly disturbed by the edge effects; the positioning of the sensitive elements 4 does not depend on the polar width L.sub.p; the reading radius R only very slightly affects the quality of the magnetic signal.

(22) In relation to the figures, a system particularly adapted to the control of an electric motor having four pairs of poles is described below, the system providing the absolute position on a pair of motor poles, or 90° mechanical.

(23) To do this, the coder 1 comprises 4 pairs of poles (N.sub.pp=4), the sensitive elements 4 delivering signals in quadrature on each of the pairs of poles in order for the sensor or the computer for controlling the motor to be able to determine the absolute angular position over an angular sector of 90°.

(24) In relation to FIG. 1, the sensor comprises two sensitive elements 4 forming between them an angle α of

(25) $\frac{\pi }{2\mathrm{Npp}}=22\text{.}5°.$
FIG. 2 shows an embodiment having three sensitive elements 4 separated two by two by an angle α of 22.5°.

(26) In particular, the latter embodiment allows two differential measurements of the magnetic field delivered (the one on the left minus the one in the centre on the one hand, the one in the centre minus the one on the right on the other hand). Thus, if the magnetic field comprises a noise component coming from the outside (for example from the motor or from the neighbouring interconnections) identical on the various sensitive elements 4, it will be subtracted from the output signal.

(27) Because of the good sinusoidality of the signal at a reading air-gap distance of approximately

(28) $\frac{\mathrm{Lp}}{2},$
the system can deliver, to the computer for controlling the motor, the absolute angular position on a pair of motor poles of the rotor in a precise manner, which allows in particular: better performance, in particular on start-up, for example the time for reaching the speed or position setting; “softer” operation, without jumps in torque in a steady state; lower energy consumption; a lower operating temperature; a greater maximum torque.