DEVICE FOR DETERMINING AT LEAST ONE ROTATION PARAMETER OF A ROTATION ELEMENT

Description

[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 signal and a rotation sensor capable of detecting said magnetic field.

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

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

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

[0005] To determine a displacement parameter of the moving organ as a function of the evolution of the detected magnetic field, document WO-2006 / 064169 provides for a combination of signals representing 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.

[0006] In particular, the encoder includes an alternating succession of North and South poles defining a polar width L p = πR Npp constant along the reading radius R for a given number Npp of pole pairs, the sensitive elements being equidistant at a distance Lp 2 in order to be able to deliver quadrature signals.

[0007] In some applications, the encoder must have a small number of pole pairs, typically less than 6, so its polar width Lp becomes important, especially on the order of tens of millimeters.

[0008] However, these wide poles deliver a magnetic signal with poor sinusoidality at small reading air gaps, requiring the sensitive elements to be moved away from the magnetic track, which goes against the amplitude of said signal and therefore its good detection by the sensitive elements.

[0009] Furthermore, large poles require a thicker encoder to maintain the sinusoidal nature of the magnetic signal. This hinders the integration of the encoder into compact dimensions and complicates the magnetization process, as a greater thickness must be magnetically saturated.

[0010] Furthermore, encoders are known, notably from document DE-103 09 027, whose magnetic transitions between the North and South poles extend along an Archimedean spiral, each of said spirals being distributed on said encoder by successive rotation through an angle π Npp .

[0011] The advantage of this type of encoder is that the polar width L p of each of the poles along the radius of said encoder becomes independent of the number N pp of pole pairs, thus making it possible to reconcile a small number of poles with an adequate positioning of the sensitive elements relative to the sinusoidality and amplitude of the magnetic signal to be detected.

[0012] However, prior art proposes a positioning of the sensitive elements along the radius of such an encoder, which poses a number of problems.

[0013] In particular, to satisfy the compromise between sinusoidality and amplitude, the sensitive elements are arranged at an air gap distance from the magnetic track that is on the order of Lp 2 . Thus, in particular to avoid risking mechanical interaction between the fixed sensor and the rotating encoder, the polar width L p should typically be between 2 and 6 mm.

[0014] However, to avoid edge effects of the magnetic field delivered by the encoder, the sensitive elements must be positioned relative to the magnetic track with at least one pair of poles on each side, i.e. two L p on each side in addition to the radial space required for the arrangement of the sensitive elements.

[0015] As a result, the encoder must have a significant height, in particular greater than 6.L p, a height which may not be available in some integrations.

[0016] Document EP0927672 discloses a system for determining at least one rotation parameter of a rotating part according to the prior art.

[0017] 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 compromise between the periodicity and the amplitude of the detected magnetic signal can be satisfied without inducing any specific size constraint for the encoder delivering said signal, and this in particular in relation to a magnetic encoder with a low number of pole pairs.

[0018] To this end, the invention proposes a system for determining at least one rotation parameter of a rotating component, said system comprising: an encoder intended to be rotationally associated 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 signal representative of the rotation of said encoder, said track having an alternation of North and South magnetic poles separated by i 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 = Npp . Lp π . θ + θ i , N pp being the number of pole pairs of the magnetic track and L p the polar width of each pole along the radius of said encoder, the angle θ i of rotation of the i th spiral with respect to the first spiral being equal to π Npp . i with i between 0 and 2N pp -1; a rotation sensor capable of detecting the periodic magnetic field emitted by said encoder by means of several magnetic sensitive elements, distributed angularly along the magnetic track, forming between at least two sensitive elements an angle α which is arranged so that the signals delivered by said elements are in quadrature.

[0019] Other features and advantages of the invention will become apparent in the following description, made with reference to the accompanying figures, in which the figures 1 et 2 are schematic representations of a determination system according to respectively an embodiment of the invention, showing in particular the arrangement of the sensitive elements in relation to the encoder.

[0020] In relation to these figures, a system for determining at least one rotational parameter of a rotating element relative to a fixed structure is described. Specifically, the parameter of the rotating element can be chosen from its position, velocity, acceleration, or direction of movement.

[0021] In a particular application, the system can be used in connection with the control of a brushless DC electric motor, allowing in particular to know the absolute angular position on a pair of motor poles of the rotor relative to the stator.

[0022] The determination system comprises an encoder 1 designed to be fixed to the rotating member so as to move in conjunction with it. The encoder comprises a body, in particular annular but which may also be discoidal, on which is formed a magnetic track 2 capable of emitting a periodic magnetic signal representative of the rotation of the encoder. In particular, the emitted magnetic signal may be sinusoidal or pseudo-sinusoidal, that is to say, having at least a portion that can be accurately approximated by a sinusoid.

[0023] Track 2 exhibits an alternation of North and South magnetic poles separated by i transitions 3, each of said transitions extending along an Archimedean spiral defined in polar coordinates (ρ, θ) with respect to the axis of rotation by the equation ρ = Npp . Lp π . θ + θ i , N pp being the number of pole pairs of the magnetic track 2 and L p 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 with respect to the first spiral being equal to π Npp . i with i between 0 and 2.N pp -1.

[0024] Thus, magnetic track 2 delivers a pseudo-sinusoidal magnetic signal whose spatial period is equal to λ = 2Lp. Furthermore, the Archimedean spiral geometry allows, in particular, that the number Npp of pole pairs of magnetic track 2 as well as the polar width Lp can be chosen independently of the radius R of magnetic track 2.

[0025] According to one embodiment, the encoder 1 consists of a magnet on which the multipolar magnetic track 2 is formed. In particular, the magnet can be formed of an annular matrix, for example made from a plastic or elastomer material, in which magnetic particles are dispersed, notably ferrite or rare earth particles such as NdFeB.

[0026] The determination system includes a rotation sensor which is intended to be attached to the fixed structure, said sensor being capable of detecting the periodic magnetic field emitted by the encoder 1. To do this, the sensor includes several magnetic sensitive elements 4 which are arranged at the reading air gap of the magnetic field delivered by the magnetic track 2, each of the sensitive elements being able in particular to be chosen from among the magnetosensitive probes.

[0027] For example, probes based on tunneling magnetoresistances (TMR), anisotropic magnetoresistances (AMR) or giant magnetoresistances (GMR) can measure a component of the magnetic field (normal or tangential to the encoder) or the rotating field (resulting from the normal and tangential components).

[0028] In particular, as described in document WO-2004 / 083881, each pattern 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.

[0029] Advantageously, each sensitive element 4 can comprise at least one motif based on a magnetoresistive material whose resistance varies according to the magnetic field, a sensitive element 4 being able to comprise a single motif or a group of motifs connected in series.

[0030] Alternatively, only the normal component of the magnetic field delivered by encoder 1 can be measured, for example using Hall effect elements. Using only the normal field is advantageous because it is more sinusoidal than the tangential field.

[0031] To determine the rotation parameter of the rotating part, the signals delivered by the sensitive elements 4 must be in quadrature, i.e., out of phase by 90°. In particular, by exploiting such signals in quadrature, 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 by means of a CORDIC type method.

[0032] To achieve this, the sensitive elements 4 are distributed angularly along the magnetic track 2, forming an angle α between at least two sensitive elements 4, arranged so that the signals delivered by said elements are in quadrature. According to the embodiments shown, the angle α formed between the two sensitive elements 4 is equal to π 2 Npp modulo π Npp .

[0033] Thus, the circumferential distribution of the sensitive elements 4 eliminates the edge effects of the magnetic field delivered by the encoder 1, allowing the use of an encoder 1 with a limited height h, in particular less than 6Lp. Specifically, the sensitive elements 4 can be distributed angularly along a radius R, notably the median radius in the figures, of the magnetic track 2 to be as far away as possible from the edges of the encoder 1.

[0034] Furthermore, by arranging the sensitive elements 4 at a reading air gap distance from the magnetic track 2 which is on the order of Lp 2 A good compromise is obtained between sinusoidality and the amplitude of the detected signal. In particular, this optimal positioning can be achieved because the polar width Lp can be between 2 and 6 mm, even with a number Npp of pole pairs of encoder 1 that is less than 6.

[0035] Thus, the circumferential arrangement of the sensitive elements 4 offers the following advantages in particular: the distance between the two elements 4 is large enough to use inexpensive and very linear discrete components (1D Hall probes); the circumferential positioning tolerance of the elements 4 has little impact on the accuracy of the sensor (because the distance between them is large); the two elements 4 being located on the mid radius R of the encoder 1, they are little disturbed by edge effects; the arrangement of the sensitive elements 4 does not depend on the polar width L p; the reading radius R has very little influence on the quality of the magnetic signal.

[0036] In relation to the figures, a system particularly suited to controlling an electric motor with four pairs of poles is described below, said system providing the absolute position on one pair of motor poles, i.e. 90° mechanical.

[0037] To do this, the encoder 1 comprises 4 pairs of poles (N pp = 4), the sensitive elements 4 delivering quadrature signals on each of the pairs of poles so that the sensor or the motor control computer can determine the absolute angular position over an angular sector of 90°.

[0038] In relation to the figure 1 The sensor comprises two sensitive elements 4 forming an angle α between them. π 2 Npp = 22 , 5 ° . There figure 2 represents an embodiment with three sensitive elements 4 separated two by two by an angle α of 22.5°.

[0039] In particular, this latter implementation allows for two differential measurements of the delivered magnetic field (the left field minus the center field on the one hand, and the center field minus the right field on the other). Thus, if the magnetic field contains a noise component coming from outside (for example, from the motor or nearby interconnections) that is identical on the different sensing elements 4, this will be subtracted from the output signal.

[0040] Due to the good sinusoidal shape of the signal at a reading air gap distance on the order of Lp 2 The system can accurately deliver the absolute angular position of a pair of rotor motor poles to the engine control unit, which notably allows: improved performance, particularly at start-up, for example the time to reach the set speed or position; smoother operation, without torque jumps at steady speed; lower energy consumption; lower operating temperature; higher maximum torque.

Claims

1. System for determining at least one rotation parameter of a rotating member, said system comprising: - a coder (1) intended to be associated in rotation with the rotating member in such a way as to move together with it, said coder comprising a body on which a magnetic track (2) is formed that is capable of emitting a periodic magnetic signal representative of the rotation of said coder, said track exhibiting an alternation of North and South magnetic poles separated by i transitions (3), each of said transitions extending in an Archimedean spiral defined in polar coordinates with respect to the axis of rotation by the equation ρ = Npp . Lp π . θ + θ i , Npp being the number of pairs of poles of the magnetic track (2) and Lp the polar width of each of the poles along the radius of said coder, the angle θi of rotation of the ith spiral with respect to the first spiral being equal to π Npp . i with i between 0 and 2.Npp-1; - a rotation sensor able to detect the periodic magnetic field emitted by said coder by means of several magnetic sensitive elements (4); said system being characterised in that 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 said elements are in quadrature, the angle α formed between the two sensitive elements being equal to π 2 Npp modulo π Npp ., the sensitive elements (4) being distributed angularly along a radius R of the magnetic track (2).

2. Determination system according to claim 1, characterised in that the sensitive elements (4) are distributed angularly according to the median radius R of the magnetic track (2).

3. Determination system according to one of claims 1 or 2, characterised in that the sensor comprises two sensitive elements (4).

4. Determination system according to any one of claims 1 to 3, characterised in that the coder (1) has a height that is less than 6.Lp.

5. Determination system according to any one of claims 1 to 4, characterised in that the sensitive elements (4) are disposed at a reading air-gap distance from the magnetic track (2) that is approximately Lp 2 .

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

7. Determination system according to any one of claims 1 to 6, characterised in that the polar width Lp of the coder (1) is between 2 and 6mm.