Abstract:
An automatically acting braking system for a stepper motor ( 10 ) of the type having a rotor ( 18 ) that is radially centered within a stationary stator ( 12 ) and which rotates about the central axis of a central shaft ( 30 ). The rotor ( 18 ) is allowed to axially slide back and forth, biased in one direction continually by a resilient means ( 40 ). When the stator ( 12 ) is energized, it pulls the rotor ( 18 ) to an axially centered position within stator ( 12 ), but when it is de energized, the resilient means ( 40 ) pushes the rotor ( 18 ) to one side, lockingly engaging its teeth ( 24 ) with the teeth ( 44 ) of a fixed locking ring ( 42 ).

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to stepper motor drive assemblies in general, and specifically to a positively locking brake for such an assembly.  
         BACKGROUND OF THE INVENTION  
         [0002]    The air flow control valves used automotive air conditioning and ventilation systems have conventionally been swinging door type valves, turned back and forth over less than a full turn by a conventional electric motors. A conventional electric motor has an inherent resistance to being back driven. Furthermore, to step down the speed and elevate the effective torque of these motors, reduction gears are typically used, which are inherently difficult to back drive. The combination of motor and reduction gear resistances generally provide sufficient holding force to keep the valves in place at any given position, when the motor is turned off.  
           [0003]    Swinging door type valves are, in some applications, being replaced by so called film valves, in which a roll of flexible film with vent openings is rolled back and forth between rollers, somewhat like a window shade with a roller at both the top and bottom. Typically, one roller will be powered and turned, while the other is spring loaded to take up, or wind out, the film roll, maintaining the belt in tension. Instead of conventional motors, stepper motors may be used to advance and wind up a film valve, since they provide the potential for precise control of the film position. An electric stepper motor comprises a primary member and a secondary member which move relative to each other. The primary member, usually the stator, is wound with a plurality of regularly angularly spaced drive coils having central iron cores which energized by drive pulses in some switched sequence. The secondary member, usually the rotor, is axially and radially centered within the stator, supported for rotation by suitable bearings. Typically, the rotor consists of two toothed discs of magnetic material separated by a permanent magnet between them. As the drive pulses are switched to different drive coils, the teeth on the rotor discs move to maintain alignment with the magnetic axes of those coils which are energized and thus relative rotation between the primary and secondary members takes place in a precise manner. When the stator drive coils are de energized, the rotor&#39;s magnetic poles still tend to take on a stable, pre determined angular position within the stator, with the teeth of the rotor aligned with the stator coils. This stable position, while predictable, is not a strongly maintained, and does not strongly resist turning of the rotor.  
           [0004]    While it is precisely controllable, and efficient, a stepper motor generally is slower than a conventional electric motor by a factor of ten, and proportionately easier to back drive. Film valves must move through multiple turns to be effective, unlike flapper door valves, and any reduction gears used to step down the already lower motor speed will have a much lower step down ratio. The net effect, then, with a stepper motor and its reduction gears is a far smaller inherent resistance to back driving when the motor is de energized, lower by as much as a factor of 100. The film valve tension spring can even potentially over power the stepper motor assembly when it is turned off, and pull the film valve out of its desired position. Therefore, some other means of holding the film valve in position when the motor is de energized may be needed.  
         SUMMARY OF THE INVENTION  
         [0005]    The subject invention provides a means of positively holding or braking a stepper motor rotor relative to the stator, automatically, when the motor is de energized, and releasing it automatically when the motor is re energized. Any mechanism driven by the stepper motor is therefore also braked and held when the motor is de energized.  
           [0006]    In the preferred embodiment disclosed, a stepper motor has a conventional stator, but the rotor is co axially supported within the stator by a central shaft by bearings that allow the rotor to shift to one side, out of axial alignment with the stator. A constantly acting resilient means, such as a spring, tends to push the rotor in one axial direction, out of its axial centered position within the stator. When the stator is energized, however, its electro magnetic force is sufficient to overcome the mechanical resilient force, and pull and keep the rotor axially centered within the stator. The resilient means can therefore actually shift the rotor to the side only when the stator is de energized. A toothed locking ring of non magnetic material is provided, fixed relative to the stator, that registers with the teeth of the rotor so as to lockingly engage with them when the rotor shifts to the side at the time of stator de energization. This is possible because of the pre determined, stable angular position that the rotor teeth take on relative to the stator (and locking ring) when the stator is de energized. The rotor is thereby positively locked and prevented from turning, and any load or mechanism driven by the motor is thereby held stationary, as well. When the stator is re energized, the pulling of the rotor back to center within the stator disengages and frees the rotor, also automatically. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 is a cross sectional view of a housing containing a preferred embodiment of a stepper motor and brake assembly made according to the invention, shown in a de energized and locked position;  
         [0008]    [0008]FIG. 2 is a view like FIG. 1, but showing the energized and un locked position;  
         [0009]    [0009]FIG. 3 is a perspective view of just the rotor and locking ring, in the disengaged, un locked position,  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0010]    Referring first to FIG. 2, a stepper motor indicated generally at  10  includes a stator, indicated generally at  12 , which is comprised of a plurality of drive coils  14 , each of which has a typical central iron core, not illustrated. The drive coils  14  are evenly spaced circumferentially or angularly about a generally cylindrical envelope. Stator  12  is fixed within a housing  16  that contains other components as well. Centered within stator  12  is a rotor, indicated generally at  18 , which contains two toothed disks of magnetic material, typically iron, a first disk  20  and second disk  22 . The terms “first” and “second” have no significance here other than locational, and the two are basically identical although, as indicated above, they may be offset a half step relative to each other in order to create a greater possible number of indexed positions of the rotor  18  within the stator  12 . Here, first disk  20  has a particular role in the novel brake mechanism of the invention, specifically through its regularly spaced peripheral teeth  24 . Second disk  22  has similar teeth  26 , but they perform no function in the brake of the invention, although they could in another embodiment. Fixed between the disks  20  and  22  is a permanent magnet  28 , which provides magnetic flux continually permeating disks  20  and  22 . Disks  20 ,  22  and magnet  28  are all fixed to a central shaft  30 , co axially maintained within stator  12 . Shaft  30  is supported within housing  16  on plain bearings  32  that allow it to rotate, and also to slide back and forth along the central axis of shaft  30 , for a purpose described below. Also fixed to shaft  30  is a first gear  34 , which meshes with a second gear  36 , to power a drive shaft  38  that exits housing  16 . Drive shaft  38  would operate any desired mechanism, such as a film valve roller, and the size relationship of the two gears  34  and  36  would determine the torque multiplication created, if any. The components described so far, with the exception of the allowed axial sliding motion of shaft  30 , are conventional to a stepper motor like  10 , as is the basic motor operation. As the stator  12  is energized, as shown in FIG. 2, drive coils  14  are energized in a desired pattern, the rotor  18  is indexed from one pre determined angular position to another, as the rotor teeth  24  and  26  align with the cores in the coils  14 . That pre determined angular position of the rotor teeth  24  and  26  remains when the coils  14  are de energized, a stable position naturally achieved because of the interaction of the magnetized rotor teeth  24  and  26  with the iron cores of the drive coils  14 . In addition, when the stator  12  is energized, there is a centering force that acts to keep rotor  18  stable and axially centered, side to side, within stator  12  as rotor  18  turns. Normally, this axial centering force is invisible, since rotor  18  would be supported on bearings that kept it axially centered within stator  12  at all times, anyway. Both of these characteristic features of the operation of rotor  18 , the axial centering force and the stable, pre determined angular aligned positions, are taken advantage by the brake mechanism of the invention, described next.  
         [0011]    Referring next to FIG. 1, it will be recalled that the bearings  32  allow the central shaft  30  to slide axially back and forth, as well as rotate, which is not a conventional feature of a stepper motor. There would normally be no reason to allow the central shaft  30  to slide, nor to axially locate the rotor  18  anywhere other than centered within stator  12  at all times. Here, however, a resilient means in the form of a coil spring  40  continually presses between housing  16  (specifically against left bearing  32 ) and first gear  34  (or any other stop member fixed to central shaft  30 ) so as to force shaft  30  and rotor  18  continually axially in one direction, to the right as shown. However, the spring  40  is able to do so only when the stator  12  is de energized, as shown in FIG. 1, because when stator  12  is energized, (referring back to FIG. 2) the axial centering force noted above is strong enough to counteract spring  40  and pull rotor  18  back to the normal axially centered position within stator  12 , compressing spring  40 . This centering action is similar to the motion of a solenoid plunger within a solenoid coil. The purpose of the pre loaded, axial sliding of shaft  30  and rotor  18  is described next.  
         [0012]    Referring next to FIGS. 1 and 3, the other extra component added by the invention, in addition to the bearings  32  that allow the shaft  30  to slide axially and the spring  40  that causes it to slide axially, is a locking ring  42 . Locking ring  42  is formed from a suitably rigid but non-magnetic material, nylon, for example. Locking ring  42  is fixed relative to stator  12  so as to be co-axial to central shaft  30  and, as best seen in FIG. 3, has a series of locking teeth  44  that match the teeth  24  on first rotor disk  20 . Ring  42  is angularly aligned such that its locking teeth  44  are in a position that registers with the stable position that the first rotor disk teeth  24  characteristically achieve when the stator  12  is de energized. When that de energization occurs, the compressed spring  40  is now able, automatically, to expand and push the first rotor disk  20  to the right far enough to lockingly engage the rotor disk teeth  24  with the locking ring teeth  44 , as shown in FIG. 1. The engagement of teeth  24  with teeth  44  is a positive, locking engagement in the sense that some component, such as the teeth, would actually have to be forced strongly enough to break in order for the rotor  18  to be released, as contrasted to a less positive braking force, such as a brake pad, in which only the frictional force acts. After engagement, rotor  18  is locked relative to stator  12  and housing  16 , central shaft  30  is locked against turning, and so are the gears  34 ,  36  and the drive shaft  38 . Any mechanism driven by shaft  38  will be effectively braked, and far more solidly than a simple friction pad type of brake. For example, if the mechanism driven by the drive shaft  38  is a winding roller of a film valve, the film valve will be held firmly in position, and will not be back driven by its tensioning spring.  
         [0013]    Referring again to FIG. 1, when the stator  12  is re energized, the rotor  18  is pulled back leftward to center, as shown, recompressing spring  40 . The rotor disk teeth  24  are pulled axially out of engagement with the locking ring teeth  44 . The rotor  18  is then able to index and turn conventionally. Both the locking and unlocking occur automatically and fairly quickly, because of the cooperation of the axial sliding or rotor  18  allowed by the bearings  32 , the uni directional axial bias provided by the coil spring  40 , and the oppositely acting centering action of the rotor  18  within the energized stator  12 . The positively locking brake is provided very cost effectively, only at the cost of the bearings  32 , the spring  40  and the locking ring  42 , none of which are expensive components.  
         [0014]    Variations in the embodiment disclosed could be made. The axial sliding of shaft  30  and rotor  18  could be provided by a two part shaft in which one section telescoped within the other, rather than by sliding bearings  32 . A resilient means other than a coil spring  40  could provide the constant, unidirectional axial bias on rotor  18 , and in either axial direction, acting either in tension or compression. For example, a telescoping shaft  30  could have its own internal biasing spring. If the direction of axial bias were reversed, then a locking ring like  42  could be made to engage with the teeth  26  on the other side of rotor  18 , instead. The teeth on either rotor disk  20  or  22  make a convenient locking surface to engage a locking member like the teeth on ring  42 , since they are there anyway. However, another type of locking member could be provided, fixed relative to stator  12 , so long as it was able to register with and automatically lockingly engage with some part of the rotor  18  as it axially shifted at the time of stator de energization. A stepper motor like  10  could, conceivably, be made to turn a mechanism directly, with no intervening reduction gear set like  34  and  36 . In that case, the spring  40  or other resilient means could press against some purposefully provided solid stop on shaft  30 , rather than the conveniently located first reduction gear  34 .