Apparatus and method for performing geometry and magnetic field measurements of a piece with rotational symmetry having a plurality of permanent magnets

Abstract

An apparatus for performing geometry and magnetic field measurements of piece (2) such as a rotor of a permanent-magnet electric motor, having: a stationary first frame (3); a rotating base (4) mounted on the first frame so as to rotate about a rotation axis (4a); a gripping system (6) for holding the piece on the rotating base so as to cause rotation of the piece; a second frame (7) movably mounted on the first frame so as to translate parallel to the rotation axis; an optical detection system (13) fixed to the second frame in order to perform an optical scan of the piece; a slide (14) movably mounted on the second frame so as to translate along a direction (14a) perpendicular to the rotation axis by means of the action of an actuator (15); a magnetic field sensor (16) fixed to the slide so as to be able to be positioned facing a side surface (17) of the piece in order to perform the magnetic field measurements; and a processing and control unit (21) configured to perform the geometry measurements on the basis of the optical scan and to control the actuator so as to position the magnetic field sensor at a suitable measurement distance (D0; D1; D2) from the rotation axis or from the side surface along said direction during the magnetic field measurements. The invention also relates to an associated method for performing geometry and magnetic field measurements of the piece.

Description

[0001] DESCRIPTION

[0002] "APPARATUS AND METHOD FOR PERFORMING GEOMETRY AND MAGNETIC FIELD MEASUREMENTS OF A PIECE WITH ROTATIONAL SYMMETRY HAVING A PLURALITY OF PERMANENT MAGNETS"

[0003] TECHNICAL FIELD

[0004] The present invention relates to an apparatus and a method for performing geometry and magnetic field measurements of a piece which has rotational symmetry, typically cylindrical symmetry, with respect to its axis and has a plurality of permanent magnets arranged around the piece axis, for example a rotor of an electric motor.

[0005] In particular, the present invention may be advantageously, but not exclusively used for the measurement of a rotor of a permanentmagnet electric motor, to which the description below will make specific reference without thereby losing its general character.

[0006] BACKGROUND ART

[0007] The growing use of permanent-magnet electric motors, for example in the automobile sector, requires an increasingly greater need for rapid quality checks of the various parts of the electric motor, and in particular of the rotor of the electric motor. The rotor of a permanentmagnet electric motor comprises a plurality of permanent magnets arranged in a circular manner around an axis of the rotor. The quality of a rotor is defined in general by the mechanical quality, for example the compliance with strict tolerances of various dimensional characteristics, and by the quality of the permanent magnets, namely the strictly magnetic characteristics of each permanent magnet and the distribution of the permanent magnets around the axis of the rotor.

[0008] Apparatuses which are able to perform geometry measurements of mechanical pieces having a cylindrical symmetry or, more generally mechanical pieces able to rotate about their own axis, for example a crankshaft of an internal combustion engine, are known. Said apparatuses comprise means which are able to grip and rotate the mechanical piece about its axis and feeler or optical means for acquiring dimensional parameters of the mechanical piece while it is rotating. Such apparatuses are suitable for measuring the mechanical characteristics of a rotor of an electric motor. Furthermore, there exist other apparatuses suitable for measuring overall magnetic characteristics of the permanent magnets. This means that it is required to perform two measurement sessions on each motor in order to measure separately, i.e. using the two different apparatuses, the geometry characteristics and magnetic field characteristics of the rotor. Consequently, the quality control of the rotor is relatively slow and difficult.

[0009] DISCLOSURE OF THE INVENTION

[0010] The object of the present invention is to provide an apparatus and a method for performing geometry and magnetic field measurements of a rotor of an electric motor, which apparatus and method do not have the aforementioned drawbacks and at the same time can be realized in a simple and low cost manner.

[0011] In accordance with the present invention an apparatus and a method for performing geometry and magnetic field measurements of a piece which has rotational symmetry with respect to its axis and has a plurality of permanent magnets arranged around the piece axis are provided, as defined in the attached claims.

[0012] BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The present invention will now be described with reference to the accompanying drawings, which illustrate a non-limiting example of embodiment thereof, in which:

[0014] - Figures 1 to 4 show the apparatus according to the present invention for performing geometry and magnetic field measurements of a piece during a corresponding number of stages of its operation; and - Figure 5 is a graph showing a measurement of the magnetic field of the piece performed with the apparatus according to Figures 1 to 4.

[0015] BEST MODES FOR CARRYING OUT THE INVENTION

[0016] In Figures 1 to 4, 1 denotes generally, in its entirety, the apparatus according to the present invention for performing geometry and magnetic field measurements of a piece 2 such as a rotor of a permanent-magnet electric motor. The rotor 2 is shown arranged in the apparatus 1 during measurement thereof.

[0017] The apparatus 1 comprises a frame, or first frame, or first stationary frame, 3, which is, for example, able to rest on the ground, a rotating base 4 mounted on the frame 3 so as to rotate about a rotation axis 4a, which for example is substantially vertical, by means of the action of an actuator, or first actuator 5, a gripping system 6 for holding the rotor 2 on the rotating base 4 coaxially with the latter so as to perform rotation of the rotor 2, and another frame, or second frame, 7, movably mounted on the first frame 3 so as to translate, driven by another actuator, or second actuator 8, parallel to the rotation axis 4a. In particular, the first frame 3 comprises a pair of guides 9 parallel to the rotation axis 4a and the second frame 7 is slidably mounted along the two guides 9.

[0018] The gripping system 6 is preferably a live-centre / dead-centre system of the known type. In particular, the gripping system 6 comprises a live centre 10 fixed coaxially to the rotating base 4, a movable head 11 which is mounted on the first frame 3 so as to translate parallel to the rotation axis 4a, and a dead centre 12 fixed to the movable head 11 so as to be coaxial with the live centre 10.

[0019] The apparatus 1 comprises an optical detection system 13 (Figures 2-4) fixed to the second frame 7 so as to focus on the rotation axis 4a in order to acquire images of the rotor 2, and in particular in order to perform an optical scan of the rotor 2 as a result of the translation of the frame 7, typically during rotation of the rotating base 4.

[0020] Preferably, the optical detection system 13 comprises a first component 22, more particularly an emitter, for example an infrared emitter, which is fixed to the frame 7, and a second component 23, more particularly a receiver or sensor, which is fixed to the frame 7 on the opposite side to the rotation axis 4a so as to face the component 22. For example, the component 23 comprises a linear array of photodiodes (CCDs).

[0021] According to a preferred embodiment, the optical detection system is a shadow-casting optical system known per se.

[0022] The apparatus 1 comprises a slide 14, which is movably mounted on the second frame 7 so as to translate, driven by a further actuator, or third actuator, 15, along a direction 14a substantially perpendicular to the rotation axis 4a, and a magnetic field sensor 16 fixed to the slide 14 so as to be positioned, when in use, in front of a side surface 17 of the rotor 2 in order to perform the magnetic field measurements. In particular, a further frame 18 is fixed to the second frame 7 and the slide 14 slides along a guide (not shown) fixed to the further frame 18.

[0023] Advantageously, the magnetic field sensor 16 is a Hall sensor.

[0024] Advantageously, the second frame 7 has a C-shaped structure and is configured to be arranged, when in use, around the rotor 2, so as to enclose it, as shown in Figures 1-4. One of the two components of the optical detection system 13, for example the component 23, is mounted on a first branch 19 of the C shaped structure and the other component of the optical detection system 13, for example the component 22, is mounted on the other branch 20 of the C shaped structure in a position diametrically opposite to that of the component 22 with respect to the rotation axis 4a. The two components 22 and 23 are therefore arranged substantially at the same height along a direction parallel to the rotation axis 4a.

[0025] The magnetic field sensor 16 is mounted on one of the two branches 19 and 20 of the C shaped structure of the frame 7 at a height along the direction parallel to the rotation axis 4a different from the height at which the optical detection system 13 is arranged. In this way, the magnetic field sensor 16 does not interfere with the optical scan performed by the optical detection system 13.

[0026] For example, in the embodiment shown in the Figures, the magnetic field sensor 16 is mounted on the branch 20 of the C shaped structure of the frame 7 so as to be positioned above the optical detection system 13 with respect to the frame 3.

[0027] It is possible, however, to envisage a shape of the second frame 7 and / or a relative position of the optical detection system 13 and magnetic field sensor 16 different from those illustrated here.

[0028] The apparatus 1 comprises a processing and control unit 21 configured to control the actuators 5, 8 and 15 so as to perform the optical scan of the rotor 2 and then the magnetic field measurements of the rotor 2 in the manner which will be explained in detail hereinbelow.

[0029] The processing and control unit 21 controls the actuators 5 and 8 so as to drive the rotor 2 into rotation with respect to the rotation axis 4a, more specifically to rotate the rotating base 4 and translate the second frame 7 parallel to the rotation axis 4a, for example from the bottom (Figure 1) upwards (Figure 2) in a known manner for performing the optical scan of the rotor 2 by means of the optical detection system 13.

[0030] The processing and control unit 21 is configured to perform the geometry measurements of the rotor 2 on the basis of the optical scan, more specifically by processing the images of the rotor 2 acquired during the optical scan so as to perform the geometry measurements of the rotor 2.

[0031] It is possible to envisage a preliminary step involving repositioning of the optical detection system 13 with respect to the actual position of the rotor 2. Depending on the acquired images of the rotor 2, the processing and control unit 21 controls the actuator 8 to move the frame 7 so as to bring the optical detection system 13 to a suitable height along the direction parallel to the rotation axis 4a, relative to the position of the rotor 2, in order to scan the latter and perform correct geometric measurements.

[0032] Thereafter, irrespective as to the execution of the preliminary step involving repositioning of the optical detection system 13, the processing and control unit 21 controls the actuator 8 to move the frame 7 and bring the magnetic field sensor 16 to a suitable height along the direction parallel to the rotation axis 4a, for example, as shown in Figure 3, to move the frame 7 downwards and bring the magnetic field sensor 16 to a height situated between the heights shown in Figures 1 and 2.

[0033] Preferably, the height along the direction parallel to the rotation axis 4a at which the magnetic field sensor 16 is positioned (Figure 3) is determined depending on the geometry measurements of the rotor 2 obtained by processing the images of the rotor 2 acquired by means of optical scan performed by the optical detection system 13.

[0034] In brief, the geometry measurements are processed so as to determine the actual position of the rotor 2 along the direction parallel to the rotation axis 4a and the frame 7 is translated along the rotation axis 4a depending on the actual position of the rotor 2 so that the optical detection system 13 and / or the magnetic field sensor 16 are positioned at respective suitable heights along said direction parallel to the rotation axis 4a so as to perform correctly the geometry measurements and magnetic field measurements of the piece 2.

[0035] At this point, the processing and control unit 21 controls the third actuator 15 depending on the geometry measurements performed so as to translate the slide 14 and position the magnetic field sensor 16 at a suitable measurement distance from the rotation axis 4a or from the side surface 17 along the direction 14a during the magnetic field measurements (Figure 4).

[0036] The magnetic field measurements are performed by rotating the rotating base 4 through an angle of rotation so as to obtain the angular trend of the magnetic field emitted radially by the rotor 2. Figure 5 shows an example of the trend of the magnetic field B measured around the rotor 2, namely upon variation of the angular position a of the rotating base 4. The peaks in the trend of the magnetic field B correspond to the presence of a magnetic pole, defined by a respective permanent magnet, at a given angular position of the rotor 2.

[0037] Controlling the position of the magnetic field sensor 16 along the direction 14a has the function of positioning the latter at a distance from the side surface 17 of the rotor 2, also called measurement distance, such as to allow correct magnetic field measurements upon variation of the dimensions of the rotor 2. In fact, the strength of the magnetic field, and therefore the amplitude of the signal detected by the magnetic field sensor 16, depends greatly on the distance from the source of the field. In other words, controlling the position of the magnetic field sensor 16 along the direction 14a allows to automatically adapt the magnetic field measurements to the transversal dimensions of the rotor 2.

[0038] According to a first embodiment, the processing and control unit 21 is configured to calculate a measurement distance DO from the rotation axis 4a depending on the nominal dimension values of the rotor 2.

[0039] Controlling the position of the magnetic field sensor 16 along the direction 14a also allows to compensate errors in the measurement of the magnetic field due mainly to the non-ideal aspects indicated below, that is aspects of the rotor 2 to be checked and / or of its arrangement that differ with respect to the theoretical, "ideal" aspects.

[0040] A first non-ideal aspect is a possible offset of the rotor 2 with respect to the gripping system 6, owing to mechanical imprecision of the live centre 10 and dead centre 12 and / or of the gripping holes of the rotor 2, which are situated on the longitudinal axis of the rotor 2 and are engaged by the live centre 10 and dead centre 12. The offset error results in a slight oscillation of the rotor 2 with respect to the rotation axis 4a during rotation of the rotating base 4. In other words, the offset error produces an oscillation, namely a periodic variation, of the distance between the magnetic field sensor 16 and the side surface 17 of the rotor 2 during the rotation of the rotating base 4 and variation in distance introduces errors in the magnetic field measurement.

[0041] Another non-ideal aspect is the difference between the nominal values and the real values of the dimensions of the rotor 2. Said difference often results in an asymmetry of the side surface 17 of the rotor 2 with respect to its longitudinal axis. Said asymmetry results in a variation of the distance between the magnetic field sensor 16 and the side surface 17 of the rotor 2 during the rotation of the rotating base 4 and the variation in distance introduces errors in the magnetic field measurement.

[0042] In accordance with a second embodiment, the processing and control unit 21 is configured to determine transversal dimensions of the rotor 2 and the offset of the rotor 2 with respect to the rotation axis 4a, namely the arrangement of the longitudinal axis of the rotor 2 with respect to the rotation axis 4a, depending on the geometry measurements, and to compensate for the magnetic field measurements, by means of suitable signal processing, depending on the offset. Furthermore, the processing and control unit 21 is configured to calculate a measurement distance DI from the rotation axis 4a, depending on the transversal dimensions of the rotor 2, and, to control the third actuator 15 so as to position the magnetic field sensor 16 and to keep it stationary at the measurement distance DI from the rotation axis 4a along the direction 14a during the magnetic field measurements.

[0043] In greater detail, the processing and control unit 21 is configured to process the geometry measurements so as to determine the transversal dimensions of the rotor 2, reconstruct the longitudinal axis of the rotor 2, calculate the variation in distance between magnetic field sensor 16 and side surface 17 upon variation of the angular position of the rotating base 4 on the basis of the arrangement of the reconstructed longitudinal axis with respect to the rotation axis 4a, and transform the variation in distance into a magnetic field oscillation to be algebraically added to the measured magnetic field.

[0044] Therefore, the magnetic field sensor 16 remains stationary at the measurement distance DI from the rotation axis 4a during the magnetic field measurements and the offset of the rotor 2 is corrected thus compensating for the magnetic field measurements. The measurement distance DI takes into account the transversal dimensions of the rotor 2 and a predetermined distance between magnetic field sensor 16 and the side surface 17 of the rotor 2.

[0045] According to a third embodiment, the processing and control unit 21 is configured to:

[0046] - determine the transversal dimensions of the rotor 2, in particular at the height at which the magnetic field sensor 16 is positioned, depending on the geometry measurements;

[0047] - determine the offset of the rotor 2 with respect to the rotation axis 4a, namely the arrangement of the longitudinal axis of the rotor 2 with respect to the rotation axis 4a, depending on the geometry measurements; and

[0048] - depending on the so determined transversal dimensions and offset of the rotor 2, dynamically controlling the actuator 15 during complete rotation of the rotating base 4 so as to keep the magnetic field sensor

[0049] 16 at a predetermined measurement distance D2 from the side surface

[0050] 17 along the direction 14a.

[0051] In this way differences between nominal values and real values of the dimensions of the rotor 2 and / or any offset of the rotor 2 are compensated for. In fact, if the magnetic field sensor 16 remains stationary, any offset of the rotor 2 also results in a variation in distance between the magnetic field sensor 16 and the side surface 17 during a complete rotation of the rotating base 4.

[0052] According to an embodiment, not shown, the apparatus 1 comprises a contact detection probe of the known type for performing measurements in addition to those described above. For example, the contact detection probe is mounted in the second frame 7 in a position facing the magnetic field sensor 16 with respect to the rotation axis 4a, and in particular is arranged in a housing of the frame and mounted on a slide movable along a direction parallel to the direction 14a so as to exit and enter back inside said housing, moving towards and away from the rotor 2. For example, said housing is arranged in the first branch 19 of the frame 7 above the component 23.

[0053] An advantage of the apparatus 1 described above is the automatic positioning of the magnetic field sensor 16 at a distance from the side surface 17 of the rotor 17 such as to allow correct magnetic field measurements, owing to the control of the position of the magnetic field sensor 16 along the direction 14a depending on the geometry measurements of the rotor 2. Another advantage is the automatic compensation of certain non-ideal aspects of the measurement environment, i.e. an offset of the rotor 2 and / or differences between nominal values and real values of the dimensions of the rotor 2.

[0054] It is pointed out that the apparatus 1 described above can be used for the geometry and magnetic field measurement of any piece (not necessarily a rotor of a permanent-magnet electric motor as described above by way of example) which has rotational symmetry, typically cylindrical symmetry, with respect to its axis and has a plurality of permanent magnets arranged around the piece axis.

Claims

CLAIMS1. Apparatus for performing geometry and magnetic field measurements of a piece (2) which has rotational symmetry, for example cylindrical, with respect to its axis and has a plurality of permanent magnets arranged around the piece axis, for example a rotor of an electric motor, the apparatus (1) comprising: a first stationary frame (3); a rotating base (4) which is mounted on the first stationary frame (3) and rotates, driven by a first actuator (5), about a rotation axis (4a); a gripping system (6) to hold the piece (2) on the rotating base (4) coaxially with the latter to drive the piece (2) into rotation; a second frame (7) which is movably mounted on the first frame (3) so as to translate, driven by a second actuator (8), parallel to the rotation axis (4a); an optical detection system (13) fixed to the second frame (7) to perform an optical scan of the piece (2); a slide (14) movably mounted on the second frame (7) so as to translate, driven by a third actuator (15), along a direction (14a) substantially perpendicular to the rotation axis (4a); a magnetic field sensor (16) fixed to the slide (14) so as to be positioned, when in use, in front of a side surface (17) of the piece (2) to perform the magnetic field measurements; and a processing and control unit (21) which is configured to perform said geometry measurements on the basis of the optical scan and to control the third actuator (15) so as to position the magnetic field sensor (16) at a suitable measurement distance (DO; DI; D2) from the rotation axis (4a) or from said side surface (17) along said direction (14a) during the magnetic field measurements.

2. Apparatus according to claim 1, wherein the processing and control unit (21) is configured to control the third actuator (15) depending on the geometry measurements so as to position and hold the magnetic field sensor (16) at said measurement distance (DI; D2) during the magnetic field measurements.

3. Apparatus according to claim 1 or claim 2, wherein theprocessing and control unit (21) is configured to determine an offset of the piece (2) with respect to the rotation axis (4a) and transversal dimensions of the piece (2) depending on said geometry measurements, to calculate said measurement distance (DI) depending on the transversal dimensions of the piece (2), to control the third actuator (15) so as to position the magnetic field sensor (16) and keep it stationary at said measurement distance (DI) from the rotation axis (4a), and to compensate said magnetic field measurements depending on said offset of the piece (2).

4. Apparatus according to claim 1 or claim 2, wherein said measurement distance (D2) is predetermined and the processing and control unit (21) is configured to determine an offset of the piece (2) with respect to the rotation axis (4a) and transversal dimensions of the piece (2) depending on said geometry measurements, and to control dynamically the third actuator (15) depending on the offset and of the transversal dimensions of the piece (2) so as to keep the magnetic field sensor (16) at the measurement distance (D2) from the side surface (17).

5. Apparatus according to any one of the preceding claims, wherein said gripping system (6) comprises a live centre (10) fixed coaxially to the rotating base (4), a movable head (11) which is mounted on the first frame (3) so as to translate parallel to the rotation axis (4a), and a dead centre (12) fixed to the movable head (11) so as to be coaxial to the live centre (10).

6. Apparatus according to any one of the preceding claims, wherein said magnetic field sensor (16) is a Hall sensor.

7. Apparatus according to any one of the preceding claims, wherein said second frame (7) has a C-shaped structure and is configured to be arranged, when in use, around the piece (2); said optical detection system (13) comprising a first component (22) mounted on a first branch (19) of the C-shaped structure and a secondcomponent (23) mounted on the other branch (20) of the C-shaped structure and said magnetic field sensor (16) being mounted on one of the two branches (19, 20) of the C-shaped structure of the second frame (7) at a height along a direction parallel to the rotation axis (4a) which is different from the height at which the optical detection system (13) is arranged.

8. Apparatus according to claim 7, wherein said optical detection system (13) is a shadow-casting optical system, and said components are an emitter (22) and a sensor (23), respectively.

9. Method for performing geometry and magnetic field measurements of a piece (2) which has rotational symmetry, for example cylindrical, with respect to its own axis and includes a plurality of permanent magnets arranged about the axis of the piece (2), for example a rotor of an electric motor, the method comprising the steps of:- driving the piece (2) into rotation with respect to a rotation axis (4a);- performing an optical scan of the piece (2) by means of an optical detection system (13);- processing images of the piece (2) acquired during the optical scan so as to perform geometry measurements of the piece (2);- positioning and holding a magnetic field sensor (16) at a suitable measurement distance (DO; DI; D2) from the rotation axis (4a) or from a side surface (17) of the piece (2) along a direction (14a) perpendicular to the rotation axis (4a) during the magnetic field measurements; and- obtaining an angular trend of the magnetic field emitted radially by the piece (2) to perform magnetic field measurements.

10. Method according to claim 9 and comprising the steps of:- processing the geometry measurements so as to determine transversal dimensions of the piece (2) and an offset of the piece (2) with respect to the rotation axis (4a),- calculating the measurement distance (DI) from the rotation axis (4a) depending on the transversal dimensions of the piece (2);- keeping the magnetic field sensor (16) stationary at the measurement distance (DI) from the rotation axis (4a) along said direction (14a) during the magnetic field measurements; and- compensating the acquired magnetic field measurements as a function of the determined offset of the piece (2).

11. Method according to claim 9, wherein said measurement distance (D2) is predetermined; the method comprising the steps of:- processing the geometry measurements so as to determine transversal dimensions of the piece (2) and an offset of the piece (2) with respect to the rotation axis (4a); and- dynamically controlling the position of the magnetic field sensor (16) depending on the offset and on the transversal dimensions of the piece (2) so as to hold the magnetic field sensor (16) at the measurement distance (D2) from the side surface (17) of the piece (2) along said direction (14a) during the magnetic field measurements.

12. Method according to any one of the claims 9 to 11, further comprising the steps of:- processing the geometry measurements so as to determine the actual position of the piece (2) with respect to a direction parallel to the rotation axis (4a); and- positioning the optical detection system (13) and / or the magnetic field sensor (16) at a suitable height along said direction parallel to the rotation axis (4a) depending on the actual position of the piece (2) to perform said geometry measurements and / or said magnetic field measurements of the piece (2).

Images