GB2366871A

GB2366871A - A high resolution position sensor utilising a rotary magnetic encoder

Info

Publication number: GB2366871A
Application number: GB0110152A
Authority: GB
Inventors: Oliver Andrieu; Jean-Baptiste Yvon
Original assignee: Electricfil SA
Current assignee: Electricfil SA
Priority date: 2000-04-26
Filing date: 2001-04-25
Publication date: 2002-03-20
Anticipated expiration: 2021-04-25
Also published as: DE10118174A1; ITTO20010397A1; BR0101605A; FR2808325B1; GB0110152D0; ITTO20010397A0; FR2808325A1; GB2366871A8; GB2366871B; SE0101453L; SE0101453D0

Abstract

The invention relates to a sensor comprising at least one rotary magnetic encoder of the type comprising at least one circular magnetic track (2<SB>i</SB>) formed by a series of elements (5) for generating a varying magnetic field, the elements being distributed to form P pairs of elements at a mechanical period (T) equal to 360{/P, the magnetic track being designed to travel past at least one detector cell (3<SB>i</SB>) delivering a periodic electrical signal corresponding to variation in the intensity of the magnetic field generated by the elements. According to the invention, the encoder comprises either a set of n circular magnetic tracks (2<SB>i</SB>) each being designed to travel past a respective detector cell (S<SB>i</SB>) and mounted in such a manner that said elements (5) of the tracks are mutually angularly offset, or else a circular magnetic track for travelling past n detector cells that are angular offset relative to one another, the relative angular offset between the generator elements and the cells being adapted to obtain n electrical signals (S<SB>i</SB>) that are electrically phase-offset relative to one another by a value equal to 1/2n times the value of the period (T), thereby enabling the resolution to be increased by n times.

Description

2366871 A HIGH RESOLUTION POSITION SENSOR The present invention relates to

the technical field of magnetic sensors of the type comprising an encoder element that moves in the vicinity of a detector cell and that is adapted to identify at least one angular position, in the broad sense.

5 The invention is particularly advantageous in the automotive industry where such a sensor is used, for example, for ignition timing functions.

In the above-mentioned preferred field, it is known to implement a magnetic sensor suitable for measuring the change in the intensity of a magnetic field whenever an encoder provided with a series of varying magnetic field generator elements moves

10 past a measurement or detector cell. The detector cell which can be a Hall effect probe or a magneto-resistive probe, for example, delivers a periodic electrical signal corresponding to variation in the intensity of the magnetic field as generated by the elements. The detector cell is associated with a comparator having hysteresis, e.g. a Schmitt trigger, so as to obtain clear-cut transitions in output voltage at magnetic field

15 values that are distinct, depending on whether the field is increasing or decreasing.

In order to make a speed-detecting sensor, it is known to provide an encoder having elements for generating a varying magnetic field arranged in regular manner around a circumference.

In a first embodiment, the generator elements are constituted by elements for 20 disturbing a magnetic field created by a stationary magnet located close to such disturbing elements. For example, such disturbing elements can be constituted by teeth provided in a ferromagnetic ring.

In a second embodiment, the elements for generating a varying magnetic field are formed by magnetic poles that are regularly spaced apart at a given pitch. Such an 25 encoder is thus in the form of a multi-pole magnetic ring.

To enable at least one position to be detennined, e.g. corresponding to top dead-center for ignition in a cylinder, it is known for the magnetic encoder to carry a mark. Thus, two teeth of the ferromagnetic ring can be omitted, for example. In a variant embodiment of an encoder implemented in the form of a multi-pole magnetic 30 ring, either a plurality of magnetic poles can be omitted so as to leave a gap, or else one or more poles of given sign can be replaced by one or more poles of opposite sign. Thus, a pole of given magnetization is caused to be present between its two adjacent poles of opposite sign at a spacing that is different relative to the pitch of the other poles.

35 In order to improve the resolution of such a sensor, it is appropriate to increase the number of generator elements provided around a circumference. Nevertheless, a technical limit arises associated with making ferromagnetic teeth or magnetic poles since they must be of dimensions that are large enough to be detected by the measurement cell. In addition, in many applications it is not possible to increase the circumference of such an encoder, because of constraints on available space.

The invention seeks to remedy the above-specified drawbacks by proposing a 5 sensor that possesses high resolution while presenting an encoder that is of limited diametral size.

To achieve this object, the invention provides a sensor comprising at least one rotary magnetic encoder of the type comprising at least one circular magnetic track formed by a series of elements for generating a varying magnetic field, the elements

10 being distributed to form P pairs of elements at a mechanical period equal to 360'/P, the magnetic track being designed to travel past at least one detector cell delivering a periodic electrical signal corresponding to variation in the intensity of the magnetic field generated by the elements.

According to the invention, the encoder comprises either a set of n circular 15 magnetic tracks each being designed to travel past a respective detector cell and mounted in such a manner that said elements of the tracks are mutually angularly offset, or else a circular magnetic track for travelling past n detector cells that are angular offset relative to one another, the relative angular offset between the generator elements and the cells being adapted to obtain n electrical signals that are 20 electrically phase-offset relative to one another by a value equal to 1/2n times the value of the period, thereby enabling the resolution to be increased by 11 times.

The invention also provides a sensor, characterized in that the detector cells are connected to means for processing phase-offset electrical signals so as to deliver digital signals on the basis of which a digital signal is obtained at a period equal to 25 3600/2P.

According to an advantageous characteristic, for a number of cells or tracks equal to 2, the processor means comprise:

means for taking the difference between two phase-offset electrical signals in order to obtain a differential signal; and 30. means for digitizing the differential signal and at least one electrical signal so as to obtain digitized signals from which it is possible to obtain the digitized signal of period equal to 360'/2P.

Various characteristics appear from the following description given with reference to the accompanying drawings which show embodiments of the invention as 35 non-limiting examples.

Figure I is a diagrammatic view showing a first embodiment of a sensor of the invention.

Figure 2 is a waveform. diagram showing the electrical signals obtained by a position sensor provided with the encoder shown in Figure 1.

Figure 3 shows a preferred example of how the signals delivered by the sensor of the invention can be used.

5 Figure 4 shows another diagram of a sensor constituting a first variant embodiment.

Figure 5 is a diagrammatic view showing a second variant embodiment of a sensor of the invention.

Figures 6 and 7 are views showing how an encoder of the invention can be 10 mounted.

Figures 8 and 9 are respectively an elevation view in section and a perspective view of a complete sensor including both high resolution and low resolution encoders.

Figure 1 shows a first embodiment of a rotary magnetic encoder I of the invention. The encoder I has n circular magnetic tracks 2i (where i = 2 to a) each 15 designed to travel past a detection or measurement cell 3i in order to form a position sensor 4. In the example shown in Figure 1, the encoder I has two magnetic tracks 21, 22 each travelling past a respective detector cell 31, 32.

Each magnetic track 2i has a series of elements 5 for generating a varying magnetic field, which elements are distributed around a circular track to form P pairs

20 of elements 5 at a mechanical period T equal to 360'/P. In the embodiment shown in Figure 1, the varying magnetic field generator elements 5 are formed by alternating magnetic poles so that each magnetic track 2i is constituted by a multi- pole magnetic ring. In this embodiment, each magnetic track 21 of the encoder I has a series of south poles and of north poles arranged to present pairs of adjacent poles at a regular 25 pitch.

Naturally, each magnetic track 2i could be made using variable magnetic field generator elements 5 formed by elements for disturbing a magnetic field created by a stationary magnet placed close to said magnetic tracks. For example, the disturbing elements could be constituted by teeth formed in a ring of ferromagnetic material.

30 According to a preferred characteristic of the invention, at least one of the magnetic tracks 2i includes at least one "irregular" generator element 51 at a spacing that differs from the pitch at which the other generator elements 5 are spaced apart. In the example shown in Figure 1, the irregular generator element 51 has an angular pitch that is twice that of the other generator elements 5. The presence of such an 35 irregular generator element constitutes a marker for the magnetic encoder that makes it possible to determine at least one angular position. Naturally, one or more irregular generator elements 51 could be provided on one or more of the magnetic tracks 2i.

Each annular magnetic track 2i is designed to travel past a stationary detector cell 3i which delivers a periodic electrical signal corresponding to variation in the intensity of the magnetic field delivered by the generator elements 5. In conventional manner, each detector cell 3i is associated with processor means, such as a level 5 comparator exhibiting hysteresis (not shown but known per s) that delivers an electrical signal Si presenting clean transitions at magnetic field values that are distinct depending on whether the field is increasing or decreasing.

In accordance with the invention, the generator elements 5 of the n magnetic tracks 2 are mounted in such a manner as to be angularly offset from one another so 10 as to obtain n electrical signals Si that are mutually phase-offset electrically by an angle equal to 1/2n times the mechanical period T.

In the example shown in Figure 1, the electrical signals delivered by the cells are offset by a value equal to one-fourth of the period T, i.e. 90', as can be clearly be seen in Figure 2. When both magnetic tracks 21 and 22 have identical numbers of 15 pairs of poles P, the magnetic tracks 21 and 22 are mutually offset by a value corresponding to half a pole. Similarly, for an encoder 1 having three of four tracks, the generator elements 5 are mutually offset so as to obtain, respectively, three or four electrical signals Si that are mutually electrically phase-offset respectively by 60' (one-sixth of a period) or by 45' (one-eighth of a period).

20 In conventional manner, each cell 31, 32 delivers an analog signal of period T respectively referenced SM I, Sm2, which signals after processing, i.e. after digitizing, give rise to respective digital signals S 1, S2. Rising and falling fronts are detected in the digital signals S 1, S2 in order to provide an output signal Ss of period T/2.

It should be understood that by implementing R magnetic tracks 2i, it is 25 possible to increase the resolution of the sensor n times, without increasing the diametral size of the encoder. Thus, in the example shown in Figure 1, where an encoder has two magnetic tracks 21, 22, each comprising, for example, 180 generator elements 5, it is possible to detect 360 events per revolution, if both the rising and falling fronts of the signals are used. This makes it possible to obtain a sensor 30 presenting resolution of P.

Figure 2 shows the conventional way of using the signals delivered by the detector cells. Figure 3 shows a preferred way of using the two analog signals SMI, Sm2 that are delivered by the cells. In this preferred technique, the processor means take the difference between the two analog signals Sml, Sm2 so as to obtain a 35 differential analog signal Smd, which is then digitized in order to obtain a digital differential signal Sd- One of the analog signals, e.g. S.1, is digitized so as to obtain a digital signal S 1. By detecting the rising and falling fronts in the digital signals Sd and S 1, it is possible to obtain an output signal Ss of period T/2. The use of a differential signal Sind makes it possible to overcome instability in the analog signals and to smooth out errors that appear between the half periods of the signals. This technique of using the signals makes it possible to increase the accuracy of the sensor.

5 In the example shown in Figure 1, the magnetic tracks 2i are made using a band of elastomer or plastics material filled with magnetized particles to constitute the magnetic poles. The elastomer band can be mounted on a support ring (not shown).

In the embodiment shown in Figure 4, magnetic poles 5 can be provided that are inclined relative to a direction D parallel to the axis of rotation D of the encoder.

10 The angle of inclination of the magnetic poles relative to the direction D serves to define the angular offset between the two tracks 21, 22 travelling past the detector cells 31, 32- In this variant embodiment, the poles are made to extend continuously from one track to the next.

In the examples described below, the magnetic tracks 2i have identical 15 numbers P of generator element pairs 5. It should be observed that the magnetic tracks 2i could be provided with different numbers P of pairs of generator elements.

Figure 5 shows a second variant embodiment of the sensor 4 of the invention in which the sensor comprises an encoder I having a single circular magnetic track 21 made up of a series of generator elements 5 as described above. The magnetic track 20 21 is designed to travel past n detector cells 3i (where n > 2), that are angularly offset from one another in such a manner that the relative angular offset between the generator elements 5 and the cells 3i is adapted to obtain n electrical signals Si that are electrically phase-offset relative to one another by an amount equal to 1/2n times the value of the period T. The detector cells 3i are thus offset in a direction parallel to 2S a direction perpendicular to the axis of rotation D of the encoder. In the preferred circumstance where the sensor has two cells 31, 32, the angular signals SMI, Sm2 are made use of in application of the technique described with reference to Figure 2, or preferably with reference to Figure 3.

The encoder 1 as described above is for mounting on a rotary target 6, in the 30 broad sense, from which at least one position is determined.

In a particular feature, the encoder I is designed to be mounted on a drive pulley mounted at the outlet from a motor vehicle engine, i.e. a distribution pulley or one of the auxiliary pulleys.

In an advantageous characteristic, as shown in Figures 6 and 7, the encoder 1 35 is mounted on the drive pulley 6 that lies on the axis of the crank shaft, thus making it possible to detect the top dead-center or ignition point of a cylinder. In the example shown, the encoder I is constituted by a multi-pole magnetic band mounted on the inner radial wall or hub 7 (Figure 6) or on the outer radial wall 8 (Figure 7) of the drive pulley 6. The multi-pole magnetic elastomer band is mounted either directly on the pulley 6 whose radial wall provides a support ring, or else indirectly via its own support ring which is fixed by any suitable means to the pulley. As can be seen in 5 Figures 6 and 7, the detector cells 31, 32, e.g. Hall effect cells, are mounted in a setback defined between the radially outer and inner walls 8 and 7 of the pulley.

In the embodiment shown, the position sensor has as its drive pulley a crank shaft pulley provided with a magnetic annulus adapted to mark a single position. It should be observed that the invention can optionally be applied to making a sensor 10 having a magnetic encoder I provided with a plurality of irregular poles 51, thus making it possible to identify a plurality of positions. Advantageously, the magnetic encoder I has four irregular poles 5 1, for example, thus making it possible to identify the positions of the cylinders in an engine. Under such circumstances, the encoder I is secured to the cam shaft of a motor vehicle engine. Naturally, the encoder 1 can be 15 mounted on the cam shaft while having only a single irregular pole.

In a preferred embodiment, the encoder I of the invention is for mounting inside a support plate for a dynamic sealing gasket for a transmission shaft between the crank shaft and the gear box of a motor vehicle engine. The encoder I is rotated by the transmission shaft and is mounted in the vicinity of n detector cells 3i mounted 20 on the support plate for the sealing gasket, so as to constitute a position sensor.

In another preferred embodiment, the encoder I of the invention is rotated by the crank shaft or the cam shaft of a motor vehicle engine, being mounted inside the engine unit of such a vehicle, close to n detector cells 3i so as to constitute a position sensor.

25 The sensor 4 of the invention as described above is a "high resolution" sensor.

In another feature of the invention, such a sensor 4, as shown in Figures 8 and 9, can also include a low resolution sensor V forming a multi-pole magnetic annulus having a series of varying magnetic field generator elements 5 at a spacing that is different so as to form P irregular element pairs. The low resolution encoder V is for travelling 30 past a detector cell 2'. Naturally, and as explained above, the generator elements 5 can be formed by elements for disturbing a magnetic field or by magnetic poles.

In a preferred feature of the invention, the low resolution encoder 1' is mounted on the target 6 that already has the high resolution encoder I mounted thereon. This target is generally implemented in the form of a band having a radial or 35 peripheral wall 61 and a transverse wall 62, each being suitable for receiving a respective encoder 1, 1'.

The invention is not limited to the embodiments described and shown since numerous modifications can be made thereto without going beyond the ambit of the invention.

Claims

I/ A sensor comprising at least one rotary magnetic encoder of the type comprising at least one circular magnetic track (2i) formed by a series of elements (5) for generating a varying magnetic field, the elements being distributed to fonn P pairs of elements at 5 a mechanical period (T) equal to 360'/P, the magnetic track being designed to travel past at least one detector cell (3i) delivering a periodic electrical signal corresponding to variation in the intensity of the magnetic field generated by the elements, the sensor being characterized in that the encoder comprises either a set of n circular magnetic tracks (2i) each being designed to travel past a respective detector 10 cell (31) and mounted in such a manner that said elements (5) of the tracks are mutually angularly offset, or else a circular magnetic track for travelling past n detector cells that are angular offset relative to one another, the relative angular offset between the generator elements and the cells being adapted to obtain n electrical signals (Si) that are electrically phase-offset relative to one another by a value equal 15 to 1/2n times the value of the period (T), thereby enabling the resolution to be increased by n times.

2/ A sensor according to claim 1, characterized in that the detector cells (3i) are connected to means for processing phase-offset electrical signals so as to deliver 20 digital signals on the basis of which a digital signal (Ss) is obtained at a period equal to 360'/2P.

3/ A sensor according to claim 2, characterized in that for a number of cells or tracks equal to 2, the processor means comprise:

25 means for taking the difference between two phase-offset electrical signals in order to obtain a differential signal (Sd); and means for digitizing the differential signal (Sd) and at least one electrical signal so as to obtain digitized signals from which it is possible to obtain the digitized signal (Sd) of period equal to 360'/2P.

4/ A sensor according to claim 1, characterized in that at least one of the magnetic tracks (2i) has at least one irregular generator element (51) at a spacing that is different from the pitch at which the other generator elements are spaced apart.

35 51 A sensor according to claim 1, characterized in that the n magnetic tracks (2i) of the set have identical or different numbers (P) of pairs of generator elements (5).

6/ A sensor according to claim 1, characterized in that said set comprises two, three, or four magnetic tracks (2i) each presenting 90, 60, or 45 pairs respectively of magnetic field generator elements.

5 7/ A sensor according to claim 1, characterized in that each magnetic track (2i) comprises varying magnetic field generator elements (5) formed by alternating magnetic poles.

8/ A sensor according to claim 1, characterized in that each magnetic track (2i) comprises varying magnetic field generator elements (5) formed by elements for disturbing a magnetic field created by a stationary magnet placed in the vicinity.

9/ A sensor according to claim 7, characterized in that each magnetic track (2i) is formed by a band of elastomer or plastics material filled with magnetized particles to constitute the magnetic poles, the band being mounted on a support ring.

10/ A sensor according to any one of claims 5 to 7, characterized in that in the set of tracks, the magnetic poles which are mutually angularly offset are constituted by magnetic poles that are inclined relative to a direction (D) parallel to the axis of rotation (D) of the encoder.

11 / A position sensor according to any one of claims 1 to 10, characterized in that it comprises a low resolution encoder (P) forming a multi-pole magnetic annulus having a series of varying magnetic field generator elements (5) at a spacing that is different so as to form P irregular element pairs, the low resolution encoder (P) being designed to travel past a detector cell (2).

12/ A position sensor according to any one of claims I to 11, characterized in that the encoder (1) and optionally the low resolution encoder (F) are mounted on a target (6) constrained to rotate on a shaft of a motor vehicle engine, the n detector cells (3i) and optionally the detector cell (2') for the low resolution encoder being mounted relative to said encoders.

13/ A position sensor according to claim 12, characterized in that the target (6) comprises a peripheral wall (61) and a transverse wall (62) each being designed to receive a respective encoder (1, P).

141 A position sensor according to claim 12 or claim 13, characterized in that the target (6) is fitted to or forms an integral portion of a drive pulley.

151 A position sensor according to claim 14, characterized in that the drive pulley is a 5 crank shaft pulley.

16/ A position sensor according to claim 14, characterized in that the drive pulley is a cam pulley.

10 17/ A position sensor according to claim 12 or claim 13, characterized in that the target (6) is mounted inside a support plate for a dynamic sealing gasket mounted close to the n detector cells (3i) and between the crank shaft and the gear box of a motor vehicle engine.

15 18/ A position sensor according to claim 12 or claim 13, characterized in that the target (6) is rotated by the crank- shaft or the cam shaft, being mounted inside the engine unit of a motor vehicle and close to n detector cells (3i).

19/ A sensor substantially as hereinbefore described with reference to 20 the accompanying drawings.