DRIVE ARRANGEMENT WITH ELECTROMAGNETIC BEARING MEANS AND ELASTICALLY PIVOTABLE CENTERING ELEMENT

Abstract: A drive arrangement comprises a first body (1) and a second body (3) which is rotatable relative thereto, electromagnetic means for rotating the second body about an axis of rotation (5), and electromagnetic bearing means (1b, 3a) for supporting the second body relative to the first body without mechanical contact. One of the bodies has a rod-shaped centering element (7) and the other body has a central opening (9) with a centering wall (9a) surrounding the axis of rotation, a centering portion (7a) of the centering element engaging the central opening with clearance. To form an emergency bearing the rod-shaped centering element is elastically pivotable, the centering portion being capable of assuming not only a central position but also positions which are radially offset from this central position as a result of contact with the centering wall during rotation of the second body.

Description: DRIVE ARRANGEMENT WITH ELECTROMAGNETIC BEARING MEANS AND ELASTICALLY PIVOTABLE CENTERING ELEMENT The invention relates to a drive arrangement comprising a first body and a second body which is rotatable relative to said first body, electromagnetic means for rotating the second body about an axis of rotation, and electromagnetic bearing means for supporting the second body relative to the first body without mechanical contact, one of the bodies having a rod-shaped centering element and the other body having a central opening with a centering wall surrounding the axis of rotation, a centering portion of the centering element engaging the central opening with clearance. The invention further relates to an optical unit including the drive arrangement defined above and a mirror for directing a radiation beam to a surface, the mirror being mounted on the second body of the drive arrangement. Such a unit is known from EP-A 0 459 585 (PHN 13.343) and comprises a rotatable mirror, particularly a polygonal mirror, and a drive unit. The drive arrangement comprises a rotor section carrying the mirror and supported so as to be rotatable about an axis of rotation, which rotor section comprises a disc-shaped at least partly permanent- magnetic rotor body. The drive arrangement further comprises a stator section with two coil systems extending into the magnet field of the rotor body. A first one of these coil systems comprises coils for generating electromagnetic driving forces acting on the rotor body to provide the rotary drive of the rotor section and hence the mirror. A second system comprises coils for generating electromagnetic bearing forces acting on the rotor body for electromagnetically, i.e. without mechanical contact, supporting the rotor section relative to the stator section. In order to keep the rotor body and hence the mirror in a more or less centered position in the non-energized condition of the coils, the stator section comprises a centering pin which coincides with the axis of rotation and which projects into a central opening in the rotor section with ample clearance, which opening is bounded by a wall surrounding the axis of rotation. The clearance between the centering pin and the wall bounding the opening should be so large that during normal use no contact occurs between the centering pin and the rotor section in spite of the prevailing tolerances. It has been envisaged to use the centering pin with the wall bounding the central opening as an emergency bearing in the event of a calamity such as a sudden electric power failure or a shock exerted on the scanning unit. However, the known construction is not suitable for this purpose. It has been found that in the event of a calamity the rotor section is initially retained by the centering pin but rapidly adopts a whirling or whipping motion, which ultimately results in violent uncontrolled vibratory excursions of the rotor section, until the rotor section comes to a halt, usually as a result of damaging of parts of the drive arrangement. The invention aims at providing a drive arrangement of the type defined in the opening paragraph with an effectively functioning emergency journal. To this end, the drive arrangement in accordance with the invention is characterized in that the rod-shaped centering element is elastically pivotable, the centering portion being capable of assuming, in addition to a central position, positions which are radially offset from this central position as a result of contact with the centering wall during rotation of the second body. It has been found that in an operating condition the drive arrangement in accordance with the invention is capable of coping with a disruption of the rotation of the rotating body, produced by external causes, such as a power failure or a violent shock, in such a manner that the relevant body can coast to a halt. This favorable effect is obtained as a result of the centering element used, whose elasticity allows it to deflect so as follow a disturbed rotation of the rotating body. This enables the rotating body to virtually roll along the centering portion without losing contact with this portion. Preferably, the dimensioning is such that the mass center of the second body is disposed at the location of the centering portion. Furthermore, it is advisable to make the mass of the pivotable portion of the centering element small with respect to the mass of the second body. Both measures contribute to an effective emergency bearing support for the rotating body. In order to avoid undesired magnetic forces the centering element is preferably made of a non-magnetic material. In a rest condition of the drive arrangement the centering element can keep said bodies in a more or less centered position relative to one another. An embodiment of the drive arrangement in accordance with the invention is characterized in that the centering element has been secured to the first body and the central opening has been formed in the second body. This embodiment is of a simple construction and the rotatable body need not be provided with any comparatively slack structural elements. In other words, the second body can be of a stiff construction. Another advantage is that the rotatable body can be light in weight. An embodiment of the drive arrangement in accordance with the invention is characterized in that the centering portion and the centering wall are both circularly cylindrical, the centering portion being radially movable over a distance which is at least substantially equal to half the difference in diameter between the centering wall and the centering portion. Said difference in diameter provides a clearance necessary to allow a slight tilting of the rotatable body during normal operation. Moreover, the clearance permits some eccentricity of the second body, for example as a result of production tolerances, during rotation. If the second body comes into contact with the centering portion owing to a calamity , this portion is off-centered by the resulting frictional forces. The centering element is then given a revolving movement, the centering portion then describing a circular path. This path has a diameter equal to said difference in diameter. Since the unbalanced rotating body cannot react to a surrounding medium, no force on the centering portion is generated which could cause the centering portion to move around another than the central position. The speed with which the centering element revolves in said situation is substantially equal to the speed of rotation of the second body. The centering element mentioned in the present document and the centering wall mentioned in the present document may be regarded, respectively, as a centrally disposed element and a wall surrounding this element, the wall facing a portion of the element, which is also referred to as the centering portion. An embodiment of the drive arrangement in accordance with the invention is characterized in that the centering element is constructed as an elastically deformable rod. Such an embodiment has advantages both as regards its construction and as regards production-engineering. As centering element, for example, a rod of a plastic having suitable anti-friction properties is used. Such a rod is very suitable to be combined with a sapphire centering wall, which guarantees a satisfactory thermal conduction and a low coefficient of friction. An embodiment of the drive arrangement in accordance with the invention is characterized in that the centering element is secured to an end portion so as to be elastically pivotable. This embodiment is preferably implemented by means of an elastic hinge, for example formed by an elastic constraint. An embodiment of the drive arrangement in accordance with the invention is characterized in that the centering element has a constricted portion between a fixing end and the centering portion. The provision of the constricted portion is a simple way of creating an elastic hinge. An embodiment of the drive arrangement in accordance with the invention is characterized in that the centering element has been provided with a shock- absorbing material. The shock-absorbing material assists in damping possible resonances of the centering portion. An embodiment of the drive arrangement in accordance with the invention is characterized in that the centering element has a free end surface which faces a surface of the body having the central opening, the end surface and the last-mentioned surface forming a pivot bearing in a non-energized condition of the electromagnetic bearing means. In this embodiment both bodies are mechanically supported relative to one another in a rest condition. An embodiment of the drive arrangement in accordance with the invention is characterized in that the second body comprises a cylindrical, particularly discshaped, at least partly permanent magnetic rotor body for generating a magnetic field, which magnetic field varies in intensity and/or direction along a circumferential direction of the rotor body, the first body comprising a stator body carrying drive coils, which extend in the magnetic field of the rotor body, for generating electromagnetic driving forces acting on the rotor body, and carrying bearing coils, which extend in the magnetic field of the rotor body, for generating electromagnetic bearing forces acting on the rotor body. In the case of suitable energizing currents high speeds of rotation are attainable in combination with a stable support without mechanical contact. Various configurations of coils are possible, including a configuration in which the drive coils and the bearing coils are implemented as one set of combination coils. The optical unit in accordance with the invention comprises the drive arrangement in accordance with the invention and a mirror for directing a radiation beam to a surface, the second body carrying the mirror. Said mirror is specifically a polygonal mirror. The optical unit is, for example, suitable for use in apparatuses such as laser printers and laser television sets. Therefore, the invention also relates to such an apparatus including the optical unit in accordance with the invention. As a scanning unit the optical unit in accordance with the invention is particularly suitable for scanning information tracks or information strips on an information carrier, such as for example an optical tape. Furthermore, the unit can be used for scanning, for example, bar codes. Consequently, the invention also relates to a bar code reader including the optical unit in accordance with the invention. The invention also relates to a scanning device including the optical unit in accordance with the invention, an evacuated chamber being provided which accommodates at least the mirror and the second body carrying the mirror. The scanning device in accordance with the invention utilizes the advantages of the drive arrangement in the optical unit in accordance with the invention in an optimum manner. Since the rotatable body and the mirror are disposed in the evacuated chamber high speeds of the mirror, particularly a polygonal mirror, can be attained. The vacuum in the chamber mitigates noise production as well as erosion and pollution of the drive arrangement and the mirror. Moreover, the loss of energy in operation is particularly small owing to the absence of mechanical friction and the negligible or at least low air resistance. The invention will now be described in more detail, by way of example, with reference to the drawings, in which Fig. 1 is a perspective, partly sectional, view showing an embodiment of the drive arrangement in accordance with the invention included in an optical unit, Fig. 2 is a diagrammatic sectional view showing a first variant of a constructional part of the drive arrangement in accordance with the invention, Fig. 3 is a diagrammatic sectional view showing a second variant of said constructional part, Fig. 4 is a diagrammatic sectional view showing a third variant of said constructional part, and Fig. 5 is a diagrammatic sectional view showing an embodiment of the scanning unit in accordance with the invention. The optical unit in accordance with the invention shown in Figure 1 includes a drive arrangement in accordance with the invention, which arrangement has a stator section, which comprises a first body or stator body 1, and which further has a rotor section, which comprises a second body 3 with a permanent magnetic rotor body 3a. The stator body 1, which in the present embodiment is made of a non-magnetic material, particularly aluminum, carries two systems of coils, which are disposed in a magnetic field of the rotor body 3a. The rotor body 3a is constructed as a flat ring-shaped axially magnetized permanent magnet configured in such a manner that the magnetic field varies in intensity and/or direction along a circumferential direction A of the rotor body 3a. A first one of the systems of coils comprises drive coils la for generating electromagnetic driving forces acting on the rotor body 3a and second system comprises bearing coils lb for generating electromagnetic bearing forces acting on the rotor body 3a. In the present example the coils la and lb are constructed as segmental coils, in particular arcuate banana-shaped segmental coils, each of the systems comprising four uniformly spaced coils, viewed in the circumferential direction A of the rotor body 3a. When the drive coils la are energized they cooperate with the rotor body 3a for the rotary drive of the second body 3 about an axis of rotation 4. When energized, the bearing coils lb can exert such forces on the rotor body 3a that the second body 3 is supported to float freely. When suitably driven, the coils lb can be used for translating and/or tilting the axis of rotation 5 relative to the first body 1. Furthermore, the coils lb can assist in driving the first body 1. The electromagnetic drive means and bearing means created by the coils la and lb and the rotor body 3a are described comprehensively in the afore-mentioned EP-A 0 459 585 (herewith incorporated by reference). In order to keep the second body 3 in a more or less centered position with respect to the first body 1 in the non-energized condition of the coils la and lb the first body 1 has a rod- shaped centering element 7 and the second body has a central opening 9, in which a round centering portion 7a of the centering element 7 engages with clearance. The opening 9 is bounded by a circular centering wall 9a of the second body 3, the difference in diameter between the centering wall 9a and the centering portion 7a being approximately 40 μm in the present example. The drive arrangement in accordance with the invention comprises an emergency bearing which in the event of a calamity, such as a sudden failure of the power supply to the coils during operation, should ensure that the rotating second body 3 can come to a standstill without causing any damage. For this purpose the centering element 7 is elastically pivotable, in such a manner that the centering portion 7a, apart from the central or neutral position shown in Figure 1 , in which the central axis of the centering portion coincides with the axis of rotation 5, can assume other positions which are radially offset from said neutral position, if the centering portion 7a is struck by the centering wall 9a during rotation of the second body 3. Preferably, the centering element 7 is pivotable in such a manner that the centering portion 7a can deflect radially over a distance which is at least equal to but is not much greater than half the difference in diameter between the centering wall 9a and the centering portion 7a. In order to support the two bodies 1 and 3 with respect to one another in the rest condition, the drive arrangement in accordance with the invention comprises a pivot bearing. This bearing is formed by a free end surface 7b of the centering element 7 and a facing surface 15 of the second body 3. There is a small clearance between the surfaces 7b and 15 while the second body 3 rotates. The optical unit in accordance with the invention shown in Figure 1 comprises a mirror 13, particularly a polygonal mirror, having ten mirror facets. The mirror 13 is disposed on a circumferential surface of the second body 3, which takes the form of a comparatively flat disc. The mirror 13 can be used for directing and aiming a radiation beam at a surface. If high speeds of rotation of the second body 3 are required, this body can be accommodated in a sealed housing which has been evacuated or in which at least a sub- atmospheric pressure prevails. In the following description of further embodiments parts which have already been described will bear the same reference symbols. Figure 2 shows a variant of the emergency bearing of the drive arrangement shown in Figure 1. In this variant the centering element 7 is a rigid rod of, for example, non-magnetic steel or bronze, which has an end portion 7c secured in the first body 1 by means of an elastic material 15, such as rubber, so as to be elastically pivotable. Figure 3 shows a second variant of the emergency bearing. In this variant the rod-shaped centering element 7 has a constricted portion 7d. This portion 7d, which extends between a fixing end 7e, by which the centering element 7 is fixed to the body 1, and the centering element 7a, enables the centering element 7 to deflect elastically. Figure 4 shows a third variant of the emergency bearing. In this variant the centering element 7 is provided with a shock-absorbing material, such as rubber, in the present example at the location of the constricted portion 7d. The element is clamped in at its fixing end 7e. It is obvious that more variants are conceivable with the scope of the invention. The scanning device in accordance with the invention shown in Figure 5 includes an optical unit as shown in Figure 1 , the unit being accommodated in a vacuum chamber 17 formed by a partly transparent housing 19. The optical unit will not be described because it has already been described comprehensively in this document. However, the corresponding reference numerals have been inserted in Figure 5. The use of a vacuum chamber 17 makes it possible to obtain a drastic reduction of the resistance presented to the rotating system comprising the second body 3, the rotor body 3a and the mirror 13, in the present example a polygonal mirror, by the gaseous medium in the chamber 17, generally air. This is of particular importance if very high speeds of rotation (for example 180.10 3 rpm) of the mirror 13 are required.

Claims: CLAIMS: 1. A drive arrangement comprising a first body and a second body which is rotatable relative to said first body, electromagnetic means for rotating the second body about an axis of rotation, and electromagnetic bearing means for supporting the second body relative to the first body without mechanical contact, one of the bodies having a rod-shaped centering element and the other body having a central opening with a centering wall surrounding the axis of rotation, a centering portion of the centering element engaging the central opening with clearance, characterized in that the rod-shaped centering element is elastically pivotable, the centering portion being capable of assuming, in addition to a central position, positions which are radially offset from this central position as a result of contact with the centering wall during rotation of the second body. 2. A drive arrangement as claimed in Claim 1, characterized in that the centering element has been secured to the first body and the central opening has been formed in the second body. 3. A drive arrangement as claimed in Claim 1 or 2, characterized in that the centering portion and the centering wall are both circularly cylindrical, the centering portion being radially movable over a distance which is at least substantially equal to half the difference in diameter between the centering wall and the centering portion. 4. A drive arrangement as claimed in Claim 1, 2 or 3, characterized in that the centering element is constructed as an elastically deformable rod. 5. A drive arrangement as claimed in Claim 1, 2, 3 or 4, characterized in that the centering element is secured to an end portion so as to be elastically pivotable. 6. A drive arrangement as claimed in Claim 1, 2, 3 or 4, characterized in that the centering element has a constricted portion between a fixing end and the centering portion. 7. A drive arrangement as claimed in Claim 1, 2, 3, 4, 5 or 6, characterized in that the centering element has been provided with a shock-absorbing material. 8. A drive arrangement as claimed in any one of the preceding Claims, characterized in that the centering element has a free end surface which faces a surface of the body having the central opening, the end surface and the last- mentioned surface forming a pivot bearing in a non-energized condition of the electromagnetic bearing means. 9. A drive arrangement as claimed in any one of the preceding Claims, in which the second body comprises a cylindrical, particularly disc-shaped, at least partly permanent magnetic rotor body for generating a magnetic field, which magnetic field varies in intensity and/or direction along a circumferential direction of the rotor body, the first body comprising a stator body carrying drive coils, which extend in the magnetic field of the rotor body, for generating electromagnetic driving forces acting on the rotor body, and carrying bearing coils, which extend in the magnetic field of the rotor body, for generating electromagnetic bearing forces acting on the rotor body. 10. An optical unit comprising a drive arrangement as claimed in any one of the preceding Claims and a mirror for directing a radiation beam to a surface, the second body carrying the mirror. 11. An apparatus including an optical unit as claimed in Claim 10. 12. A scanning device including an optical unit as claimed 10, a vacuum chamber being provided which accommodates at least the mirror and the second body carrying the mirror.

Inventors: VAN LEEST JOHANNES HENRICUS FR, VAN ROSMALEN GERARD EDUARD, VAN LEEST, JOHANNES, HENRICUS, FRANSISCUS, MARIA, VAN ROSMALEN, GERARD, EDUARD
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1997
Main classification: H02K7
IPC codes: G02B26, H02K7, H02K21