A BRUSHLESS DIRECT CURRENT MOTOR WITH INVERTED MAGNET CUP

Abstract: A brushless direct current motor with the rotating magnet cup (43) permanently attached to the spindle hub (11). After attachment, the combination spindle hub and magnet cup are affixed to a shaft (10) and the resulting subassembly is dynamically balanced. The motor using this balanced subassembly is assembled with no balancing required after assembly. The motor can also be disassembled for repair and reassembled without disturbing the dynamic balance. The motor is especially useful for disk drive applications.

Description: A BRUSHLESS DIRECT CURRENT MOTOR WITH INVERTED MAGNET CUP BACKGROUND OF THE INVENTION This invention relates to a brushless direct current motor, and more particularly to a brushless direct current motor designed for use with rotating disk systems. Even more particularly, the invention relates to a brushless direct current motor wherein all of the rotating components can all be balanced as a unit in one balancing operation. The use of brushless direct current motors in electronic equipment with rotating components, such as a magnetic disk storage system, has become common. There are two reasons for this: (1) the speed of rotation of such devices must be precisely controlled, and the brushless direct current motor, with its electronic control system, allows accurate speed control to be easily accomplished; and (2) the brushless direct current motor can be made an integral part of the mechanical support structure, thereby saving both space and cost. In some uses of the brushless direct current motor, the rotating components of the motor must be balanced to an acceptable level. If the correct level of balance is not achieved, the electronic equipment using the motor may not function as desired, or the life of the motor may be shortened because of the stresses placed on the bearings from the wobble introduced by the imbalance. Balancing of brushless direct current motors is accomplished in the prior art by individually balancing each rotating component of the motor. The components are then assembled and the assembled unit is checked for the correct balance. Quite often, additional balancing is required. If the motor should ever have to be disassembled, e.g., because of maintenance of the motor or the overall system in which the motor is used, additional balancing may be required when the motor is reassembled. This is because the individual components of the motor will probably not reassemble exactly as they were before disassembly. Two approaches have been used in the prior art to avoid this additional balancing step: (1) the individual motor components may be fabricated to very tight tolerances, thereby ensuring that proper balance will be obtained regardless of how the components are reassembled; or (2) keying pins and matching alignment holes may be selectively placed in the components, thereby maintaining a fixed relationship between the components each time reassembly occurs. While the use of tight tolerances and keying pins and alignment holes helps ensure that a motor can be reassembled without needing rebalancing, these approaches also disadvantageously add significantly to the cost of the motor. What is needed therefore is a low cost, simple brushless direct current motor design wherein maintaining the desired balance of the rotating components is no longer a problem. SUMMARY OF THE INVENTION It is an objective of the present invention to provide a brushless direct current motor whose rotating components can be balanced prior to assembly into the motor and which do not need additional balancing after motor assembly. It is another objective of the present invention to provide such a motor that can be disassembled, if necessary, and reassembled without the need of balancing after reassembly. The invention meets these and other objectives through a brushless direct current motor design wherein all the rotating components can be assembled as a subassembly. This subassembly is then balanced and assembled into the stationary components of the motor. Consequently, the rotating components, being in balance prior to assembly into the stationary components, and having no additional parts added, require no further balancing after assembly with the stationary components. Further, because the rotating components of the motor are designed as a balanced subassembly, there is never a need to disassemble the subassembly. Thus, whenever disassembly of the motor is required, the balanced subassembly can be readily removed from, and re-installed into, the stationary components of the motor, yet the balanced subassembly never requires rebalancing. The rotating components included in the balanced subassembly of the present invention comprise a shaft, a spindle hub (on which the rotating disk or disks may be mounted), and a cup assembly (wherein permanent magnetics are mounted). Unlike the prior art, wherein traditional design practice teaches that the cup assembly must be mounted at one end of the shaft using a cup adapter, and the spindle hub must be mounted at the other end of the shaft, the present invention provides a unique subassembly wherein the cup assembly and spindle hub may be advantageously mounted to the same end of the shaft. Once assembled and balanced, therefore, this subassembly need never be disassembled. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objectives, advantages and features of the present invention will become more apparent from the following description of the preferred embodiment, which is described with reference to the following drawings, wherein: FIGURE 1 is a cross-sectional drawing of a brushless direct current motor of the prior art, showing the rotating components that may require balancing and how they are assembled with the stationary components of the motor; and FIGURE 2 is a cross-sectional drawing of a brushless direct current motor designed according to of the present invention, showing the same detail as FIGURE 1. DESCRIPTION OF THE PREFERRED EMBODIMENT The following is a description of the best presently contemplated mode of carrying out the present invention. This description is given only for the purpose of describing the general principles of the present invention, and should not be taken in a limiting sense. The true scope of the invention can be ascertained by referring to the appended claims. In order to better appreciate the features and advantages of the present invention, it will first be instructive to describe in more detail a conventional brushless direct current motor as used in the prior art for disk drive applications. Such a motor is shown in the cross-sectional view of FIGURE 1. The rotating components of the motor consist of the shaft 10, spindle hub 11, cup adapter 12, and rotating magnet cup 13. Mounted on the inner edge of the rotating magnet cup 13 are a plurality of permanent magnets, shown in the figure as solid rectangles 14 and 15. The stationary components of the motor consist of the spindle housing 16 with a plurality of motor magnets, shown in the figure as rectangles 17 and 18, arranged around the periphery. These magnets re usually electro-magnets that are excited by applying a direct current of a desired polarity to coil windings associated with the magnets. The shaft 10 of the rotating components is supported in the spindle housing 16 of the stationary components by the ball-bearings 20-27. The ball-bearings can be considered to be part of either the rotating components,' the stationary components, or part of neither since typically the inner races (supports for the bearings) 26-27 rotate with the shaft 10, the outer races 24-25 are stationary against the spindle housing 16, and the ball-bearings 20-23 rotate as necessary between the inner and outer races to support the load of the rotating components. While ball-bearings are shown in the figure, it is not uncommon for any appropriate rotational bearing mechanism, e.g., roller bearing, ball-bearings, tapered pin bearings, etc., to be used. The spindle housing 16 of the stationary components i3 attached to the mount 30 of the mechanical structure of the equipment in which the motor is being used. The manner of mounting is not important to the present invention. It is noted that in a traditional direct current motor, (not a brushless motor) , the magnets are part of the stationary portion of the motor. These magnets may be permanent or electro-magnets which are excited by the application of the proper direct current through coils built into the motor housing. The motor windings are part of the rotating portion of the motor. Direct current is supplied to the windings through a commutator, i.e., slotted, insulated segments of copper (or other conductive material), which is made part of the rotating shaft, through carbon brushes which rub against the commutator. Pairs of segments of the commutator are electrically connected to individual pairs of motor windings. Electrical currents in a pair of motor windings create a magnetic field which opposes the magnetic field of the fixed magnets in the stationary part of the motor, thus creating torsional forces that cause the shaft to rotate. As the shaft, and therefore the commutator, rotates a different pair of motor windings receive the current from the commutator, thereby allowing the torsional forces, and hence shaft rotation to continue. Traditional direct current motors are heavy and bulky, and precise speed control is difficult to achieve. In contrast, the motor windings of a brushless direct current motor are part of the stationary component of the motor and the permanent magnets are part of the rotating components. In order to develop the most torque, the permanent magnets are typically spaced radially from the center line of the shaft as far as possible. Hence, a magnet cup assembly 13, as shown in FIGURE 1, is employed to place the magnets at a maximum radial spacing, rather than merely attaching the magnets to the side of the shaft 10. Thus in FIGURE 1, the permanent magnets 14-15 are placed a maximum distance from the centerline of the shaft 10 and opposite the motor windings 17-18. Direct current is electronically switched to the appropriate pair of motor windings to create a magnetic force which causes the permanent magnets, and therefore all the rotating components of the motor, to rotate. Advantageously, the rotational speed of the brushless motor can easily be measured. For example, a slotted plate could be made part of the rotating portion of the motor. As the slots of such a plate pass between a light source and a photosensitive diode, a pulse train whose frequency is proportional to the rotational speed of the motor is created. This pulse train can be used to control the switching of current to the motor windings and thus accurately control the speed of the motor. Therefore, the ability to precisely control the speed of a brushless direct current motor allows such motors to be used in many applications where a conventional direct current motor would not be acceptable, such as in a disk drive device. Such applications also require, however, a precise balance of the rotating components. As explained earlier, while it is possible to precisely balance a motor such as that shown in FIGURE 1, this balance will be lost if the motor is disassembled. To explain further, the shaft 10 and spindle hub 11 (see FIGURE 1) are fabricated from separate pieces of metal. The spindle hub 11 is then attached permanently, e.g., by means of an interference fit, to the shaft 10. The resultant subassembly is then dynamically balanced. This balancing is typically done by spinning the object at the desired speed, and measuring the amount of imbalance, for example with a strobe light. The object is stopped and an amount of material, estimated to be equal to the imbalance, is removed from the proper area of the object, usually by machining or drilling the surface. The object is again spun and any imbalance is again determined. This process is repeated until the amount of remaining imbalance is within a predetermined limit. The cup adapter 12 and rotating magnet cup 13 must be balanced in a similar manner. When all the rotating components 10-15 are balanced, the rotating components re assembled with the stationary components, and a complete motor assembly is realized. Depending upon the amount of imbalance allowed by the application, the resulting complete motor assembly may require additional balancing. Also, as explained previously, if the motor should ever have to disassembled, a strong possibility exists, dependent upon the care and cost expended in fabricating the rotating parts, that upon reassembly balancing will again be required. FIGURE 2 is a cross-sectional drawing of a brushless direct current motor configured according to the present invention. As can be seen from the figure, a rotating magnet cup 43, along with the permanent magnets 44-45, are attached to a spindle hub 11 instead of to the opposite end of the shaft 10 (as taught in the prior art), and the motor windings 47-48 are located at the spindle hub end of a spindle housing 46. The magnets 44-45 (which may be a single doughnut shaped core piece that is selectively magnetized, or a ' plurality of individual magnetized pieces) are mounted in the cup 43 so as to maintain a selected juxtaposed relationship (depending upon the rotational position of the shaft) an outer surface of the motor windings 47-48. The shaft 10 rotates about an axis 50. A disk 52, typically having a center hole 54 therein, engages the spindle hub 11 when the motor is used for disk drive applications. These features of the invention advantageously allow all the rotating components of the motor, i.e., the shaft 10, the spindle hub 11, rotating magnet cup 43, and permanent magnets 44-45 to be manufactured using conventional manufacturing practices. No additional costs are incurred just because the components are going to be balanced. The rotating components are then assembled into a single compact subassembly. This subassembly is then balanced in the manner described above. The balanced subassembly can then be installed into the stationary components of the motor. A unique feature of the present invention is that once the rotating components are joined together in a subassembly, there is never a need to disassemble them. Thus, once balanced, the rotating subassembly can be installed, and removed and reinstalled., in the motor without requiring additional balancing. This feature significantly reduces the cost of manufacturing and maintaining the motor assembly because only one subassembly need be balanced, and that need only be balanced one time. While seemingly a simple change, placing the rotating magnetic cup 13, permanent, magnets 14-15, and motor windings 17-18 at the spindly hub end of the motor housing as shown in FIGURE 2 represents a significant advance in the art. Prior art disk drive brushless motors known to applicants have all placed the magnetic cup 13 at the opposite end of the shaft from the spindle in order to more evenly distribute the motor inertia along the length of the shaft. Such inertia distribution is necessary, as taught in the art, in order to properly distribute the rotational shaft stresses along the full length of the shaft and in order to maintain a moment of inertia that is more or less centered within the motor housing. The inventors herein where the first to recognize that coupling most of the motor inertia to one end of the shaft as shown in FIGURE 2 does not create a problem as the prior art teachings would suggest. It is believed that the reason for this is that achieving and maintaining a proper dynamic balance, in a simple and inexpensive OM?I_ manner as is done with the configuration of the present invention, results in much less wear and strain of the type that would otherwise make uneven motor inertia distribution (i.e., most of the motor inertia coupled to one end of the shaft) significant. In any event, the results obtained to date by using the motor configuration shown in FIGURE 2 have been surprisingly and unexpectedly good. Accordingly, because of the advantages it provides, the present invention is planned for use in many of the disk drive products manufactured by the inventors' employer. OMPI y ?r.

Claims: CLAIMS What is claimed is: 1. A brushless direct current motor comprising: a housing; a shaft mounted for friction reduced rotation in said housing; motor windings located around an outer surface of said housing; a spindle hub affixed to a first end of said shaft; and a magnet cup affixed to said spindle hub, said magnet cup having at least one permanent magnet mounted therein such that said magnet maintains a selected juxtaposed, non-touching relationship with an outer surface of said motor windings. 2. The motor as defined in claim 1 wherein said shaft, spindle hub, and magnet cup comprise a first mechanical assembly; and said housing and motor windings comprise a second mechanical assembly; and wherein said first assembly is detachably mounted in said second assembly, thereby allowing the selected disassembly and reassembly of said motor. 3. The motor as defined in claim 2 wherein said first assembly is dynamically balanced for rotation about a longitudinal axis of said shaft. 4. The motor as defined in claim 3 wherein said second assembly includes friction reducing bearings for supporting said shaft when said first and second assemblies are mounted together. 5. An improved, low cost, compact brushless direct current motor for use in a disk drive device comprising: a housing having motor windings selectively spaced around an outer surface thereof, and means for mounting said housing to said disk drive device; a shaft rotatably mounted in said housing; a spindle hub affixed to a first end of said shaft, an upper side of said spindle hub being adapted to engage the center of a disk to be driven by said disk drive device; and a magnet cup attached to a lower side of said spindle, said magnet cup including at least one permanent magnet, said magnet cup being positioned such that said magnet maintains a selected juxtaposed, non-touching, relationship with an outer surface of said motor windings. 6. The motor as defined in claim 5 wherein said shaft, spindle hub, and magnet cup are assembled into a first assembly and dynamically balanced about a center axis of said shaft prior to mounting said first assembly into said housing. 7. The motor as defined in claim 6 wherein said first assembly may be selectively detached from said housing, whereby said motor may be disassembled for maintenance or other purposes and reassembled without having to disassemble said first assembly, and further whereby said first assembly need not be dynamically balanced subsequent to an initial balancing operation. 8. A method of manufacturing a compact, low cost, brushless direct current motor, said motor including a housing, stationary windings affixed to an outer surface of said housing so as to lie substantially in a plane orthogonal to a central longitudinal axis of said housing, a shaft, a spindle hub, and a magnet cup having an inside diameter greater than the diameter of said housing and windings, said method comprising the steps of: (a) selectively spacing a plurality or permanent magnets around the inner periphery of said magnet cup; (b) attaching said spindle hub to a first end of said shaft; (c) attaching said magnetic cup to the shaft side of said spindle hub, thereby forming a first assembly comprised of said shaft, hub, and magnet cup; (d) dynamically balancing said first assembly about a central longitudinal axis of said shaft; and (e) rotatably and detachably mounting said first assembly in said housing such that the longitudinal axes of said housing and shaft coincide, whereby the magnets of said magnet cup may rotate substantially in the same plane as said motor windings without coming in physical contact with said windings.

Inventors: HUNT WILLIAM C, WILKINSON RICHARD A JR, HUNT, WILLIAM, C., WILKINSON, RICHARD, A., JR.
Applicants: STORAGE TECHNOLOGY PARTNERS, STORAGE TECHNOLOGY PARTNERS II
Application year: 1983
Main classification: H02K37
IPC codes: G11B19, H02K15, H02K29