GB2460529A

GB2460529A - Stray flux conducting element for rotor position recognition

Info

Publication number: GB2460529A
Application number: GB0909416A
Authority: GB
Inventors: Ulrich Peter; Volker Bosch; Sebastian Laber
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2008-06-03
Filing date: 2009-06-01
Publication date: 2009-12-09
Anticipated expiration: 2029-06-01
Also published as: DE102008002180A1; CN101599666A; GB2460529B; GB0909416D0; DE102008002180B4

Abstract

A rotor assembly for an electric motor 100 comprises a rotor, which rotates with a shaft 12 and comprises a rotor magnet 18, and a coupling element 20 which conducts a magnetic stray field 42 of the rotor 10 to one or more sensors 40 sensitive to magnetic field. The coupling element may be in the form of a sleeve with a collar 28 mounted at the end remote from the rotor. A fixing moulding mass 30 may secure the sleeve and the rotor to the shaft. A coolar may be provided which engages the rotor end.

Description

ROTOR POSITION RECOGNITION

The invention relates to a rotor assembly with a rotor, to position recognition means for rotor position recognition and to an electric motor and an electric tool incorporating such components.

It is necessary for commutation of electric commutating motors (EC motors) to know the exact position of the rotor at every point in time. The rotor position recognition can, as is known, be carried out by methods with or without a sensor.

For rotor position recognition employing a sensor, transmitter magnets arranged at the rotor are aligned with the magnetisation of the rotor magnet. The magnetic field of the transmitter magnets is detected by sensors and the signals delivered are utilised for rotor position recognition. In this manner, rotor position recognition is possible even at standstill of the motor, which, however, necessitates a highly accurate alignment of the transmitter magnets with the magnetisation of the rotor magnets and a stable fixing of the transmitter magnets.

A feedback of the electromotive force ("back EMF") is frequently used for sensor-free rotor position recognition. Although at low rotational speeds this no longer supplies a signal sufficient for evaluation, no additional components for rotor position detection are needed.

However, hand tools, for example electric screwdrivers, are frequently used at low rotational speeds as a consequence of use requirements, so that a sensor-free rotor position recognition is less suitable for tool machines of that kind.

According to a first aspect of the present invention there is provided a rotor of an electric motor, which rotates with a shaft, comprising a rotor magnet, characterised in that provided at the rotor side is a soft magnetic coupling element which conducts a magnetic stray field of the rotor to one or more sensors sensitive to magnetic field.

After magnetisation of the rotor magnet the coupling element can conduct at least a part of the stray flux so that it can be detected by the sensor, which can be mounted in the vicinity and which is sensitive to magnetic field. Since the detected magnetic flux originates directly from the rotor magnet, the exact position of the rotor can always be detected. The sensor can be, in particular, a Hall sensor. The coupling element is preferably of a soft magnetic metallic material. Advantageously, a transmitter magnet can be eliminated and thus also the need for a precise alignment thereof with the magnetisation of the rotor magnet. The soft magnetic coupling element is more economic than a transmitter magnet and requires less effort in mounting. In addition, the magnetisation process of the rotor magnet is not influenced by the coupling element. In the case of a transmitter magnet the risk exists that this, during the magnetisation process of the rotor magnet, is demagnetised; this problem is also eliminated. Furthermore, the soft magnetic coupling element can be mounted at a large spacing from the stator winding and, in particular, from a winding head (at the end face) of the stator, so that the stray field of the winding does not influence the sensor. In principle, any soft magnetic material having a sufficient mechanical stability under the operating conditions, for example 35,000 revolutions per minute, is suitable as material for the coupling element.

Preferably, the coupling element can be constructed as a sleeve which is arranged on the shaft in the region of the stray field of the rotor magnet. Mounting can be carried out simply, in space-saving manner and without complication.

The coupling element can be fixed to the rotor magnet in axial direction. For preference, a fixing or moulding mass can be arranged between shaft and coupling element. The mass can securely fix the coupling element on the shaft, even at high rotational speeds and in the event of temperature changes in operation. A non-ferromagnetic material can be used for the mass. The mass can form a magnetic insulation between the coupling element and the shaft.

Advantageously, the coupling element comprises a sleeve and has a collar at its end remote from the rotor magnet. The stray flux of the rotor magnet can thus be focused towards the sensor, with the advantage of higher sensitivity in the case of low rotational speeds.

Alternatively or additionally, the coupling element can comprise a sleeve and have a collar at its end facing the rotor magnet. The collar can preferably abut the rotor magnet. In this manner the stray flux of the rotor magnet can be better detected and the measuring accuracy increased.

The coupling element can also be constructed as a cylinder without a collar.

According to another aspect of the invention there is provided a device for rotor position recognition of an electric motor, with a rotor which rotates with a shaft, comprising a rotor magnet and a least one sensor sensitive to magnetic field, the device comprising a coupling element which conducts a magnetic stray field of the rotor to the sensor.

The coupling element can preferably extend axially at least up to the sensor. A secure recognition of the rotor position can thus be guaranteed. By virtue of the simple geometry of the coupling element, for example a cylindrical sleeve without a collar or a sleeve with a collar at one or both ends of the sleeve, the coupling element can be easily adapted to the different sizes of the motor.

According to yet another aspect of the invention there is provided an electric tool machine, particularly an electric screwdriver, with a device of that kind.

A preferred embodiment of the present invention will now be more particularly described with reference to the accompanying drawings, in which: Figs. la andib are a schematic longitudinal section and schematic cross-section, respectively, of a first rotor assembly embodying the invention, showing a direction of magnetisation; Figs. 2a and 2b are a schematic perspective view and a schematic longitudinal section, respectively, of a second rotor assembly embodying the invention; Figs. 3a and 3b are a schematic perspective view and a schematic longitudinal section, respectively, of a third rotor assembly embodying the invention; Figs. 4a and 4b are a schematic perspective view and a schematic longitudinal section, respectively, of a fourth rotor assembly embodying the invention; and Fig. 5 is an elevation of an electric tool machine incorporating rotor position recognition means embodying the invention.

Referring now to the drawings, Fig. 1 shows a rotor assembly of an electric motor (not illustrated), the assembly comprising a rotor 10, which rotates with a shaft 12, and a device 14 for position recognition of the rotor. Arranged on the shaft 12 of the rotor 10 is a rotor magnet 18, the magnetisation 44 of which runs transversely to the axis 32 of rotation, as is indicated by the arrows in Figs. 1 a and 1 b in a side view and a cross-section of the magnet 18. A sensor 40 sensitive to magnetic field, for example a Hall sensor, is mounted on the motor stator (not illustrated) in the vicinity of an end 16 of the magnet 18. In the case of brushless motors, usually several, for example three, sensors are employed as position transmitter sensors.

In the illustrated example the rotor magnet 18 is preferably constructed as an integral hollow cylinder, which has diametral two-pole magnetisation, thus parallelly and transversely through the magnet, as illustrated by the arrows. The invention functions in unchanged form even if the magnet is of multi-part construction in the form of individual rings or in the form of shells. Equally, the invention is not confined to a two-pole arrangement, but can also work with higher-poled magnets, for example four-poled, six-poled, eight-poled, etc. In the case of more than two poles, the magnetisation should no longer be carried out diametrally, but laterally.

In a first embodiment of the invention a soft magnetic coupling element 20 is provided at the rotor side in longitudinal direction 34 near the rotor magnet 18 and conducts a magnetic stray field 42 of the rotor 10 to the sensor 40, which is arranged in the vicinity, for example at the stator. The coupling element 20 is with advantage arrangement remotely from the winding head of the stator so that interference signals, which can originate from the winding head, can be minimised.

The coupling element 20 is constructed as, for example, a sleeve which is arranged on the shaft 12 in the region traversed by the stray field 42 of the rotor magnet 18. The sensor 40 passes on the detected signals by way of a signal line 46 to a usual electronic system (not illustrated), which determines therefrom, in a manner known per se, the rotor position.

The coupling element 20 is fixed to the rotor magnet 18 in axial direction in that a fixing or moulding mass 30 is arranged between the shaft 12 and coupling element 20. The coupling element 20 is pressed onto the mass 30 or the mass 30 is pressed into an intermediate space between the coupling element 20 and shaft 12. In the illustrated embodiment the mass 30 also fixes the magnet 18. In the fixing of the magnet 18 on the shaft 12 the mass 30, for example a plastics material mass, is injection-moulded or press-moulded between the magnet 18 and the shaft 12. In this working step the coupling element 20 can be simultaneously assembled together with the magnet 18. However, it is also conceivable for the shaft 12 to be injection-moulded or press-moulded around by the mass 30 in a first step and the rotor magnet 18 and coupling element 20 pressed or glued on only subsequently.

Figs. 2a, 2b, 3a, 3b, 4a and 4b show different forms of the rotor assembly as a perspective view (respectively Figs. 2a, 3a, 4a) and a longitudinal section (respectively Figs. 2b, 3b, 4b). The embodiment of Figs. 2a and 2b corresponds with that of Fig. la, to which, for avoidance of unnecessary repetition with respect to the description, reference is made.

The coupling element 20 in the first embodiment is preferably constructed as a sleeve and arranged in longitudinal direction near the rotor magnet 18 to abut, by its end 22 facing the rotor, the magnet.

As shown in Figs. 2a and 2b, the coupling element 20 can also have a collar which is preferably constructed to encircle the coupling element 20, again constructed as a sleeve.

The collar 28 is provided at the end 24 of the coupling element 20 remote from the magnet 18. Thus, the magnetic stray field can be conducted in particularly effective, focused manner to the sensor 40, which is located in the vicinity (see Fig. la).

Figs. 3a and 3b show an alternative embodiment in which the coupling element 20, once more constructed as a sleeve, has a collar 26 at its end 22 facing the rotor magnet 18, wherein the collar 26 can abut the rotor magnet 18. The magnetic stray field can thus be coupled into the coupling element 20 particularly effectively.

A combination of the embodiments of Figs. 2a, 2b and 3a, 3b is obviously also possible, in which a respective collar 24, 26 is arranged at each of the two ends 22, 24 of the coupling element 20, again constructed as a cylindrical sleeve.

Fig. 5 shows, by way of an example, a preferred electric tool machine 100, particularly an electric screwdriver, with a device 14 of one of the embodiments described in the foregoing and illustrated in, for example, Fig. 2a, 2b, 3a, 3b, 4a or 4b.

Claims

CLAIMS1. A rotor assembly for an electric motor, comprising a rotor which is rotatable with a shaft and comprises a rotor magnet, and a soft magnetic coupling element arranged to be influenced by a magnetic stray field of the rotor and to conduct the field for evaluation byfield-sensitive sensor means.
2. An assembly as claimed in claim 1, wherein the coupling element comprises a sleeve arranged on the shaft in a region through which the stray field passes.
3. An assembly as claimed in claim 1 or claim 2, wherein the coupling element is fixed to the magnet in the axial direction of the rotor and shaft.
4. An assembly as claimed in any one of the preceding claims, comprising a fixing mass between the shaft and the coupling element.
5. An assembly as claimed in any one of the preceding claims, wherein the coupling element comprises a sleeve provided with a collar at an end thereof remote from the magnet.
6. An assembly as claimed in any one of the preceding claims, wherein the coupling element comprises a sleeve provided with a collar at an end thereof facing the magnet.
7. An assembly as claimed in claim 6, wherein the collar at the sleeve end facing the magnet abuts the magnet.
8. An assembly as claimed in any one of the preceding claims, wherein the coupling element is substantially cylindrical.
9. Position recognition means for recognition of the rotational position of a rotor of an electric motor, the rotor being rotatable with a shaft and comprising a rotor magnet, the position recognition means comprising sensor means sensitive to magnetic field and a coupling element arranged to be influenced by a magnetic stray field of the rotor and to conduct the field to the sensor means for evaluation to recognise therefrom the rotational position of the rotor.
10. Position recognition means as claimed in claim 10, wherein the coupling element is arranged to extend axially of the shaft up to the sensor means.
11. An electric motor comprising a rotor assembly as claimed in any one of claims 1 to 8 and sensor means for evaluating the stray magnetic field conducted by the coupling element.
12. An electric tool comprising a rotor assembly as claimed in any one of claims 1 to 8.
13. An electric tool comprising position recognition means as claimed in claim 9 or claim 10.
14. An electric tool comprising an electric motor as claimed in claim 11.
15. A tool as claimed in any one of claims 12 to 14, the tool being an electric screwdriver.