Abstract:
A stepper motor has electromagnetically driven stator segments facing corresponding rotor segments. In order to reduce vibration of the stator segments, motor body end caps are provided with a stepped annular rim along an inside diameter of a centering sleeve. The stepped rim bears against each axial end of the stator segments, with a radial dimension and an axial dimension bracketing a stator segment end in place.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates to stepper motors and, in particular, to noise reduction in stepper motors, frequently used in robotics, appliances and industrial equipment. 
       BACKGROUND ART 
       [0002]    The problem of mechanical noise in small electrical motors is known in the prior art. In U.S. Pat. No. 5,235,227 C. Fazekas describes the problem of noise in small electrical motors used in the film industry, as well as describing prior art approaches to dampen noise, most involving use of vibration dampening material. Stepper motors have a tendency to be noisy because electrical pulses cause incremental mechanical stepping of a rotor relative to a stator of a degree or so per step. Although mechanical stepping is stop-start motion, when done at high electrical pulse frequencies it appears as smooth motion. Nevertheless, the stop-start characteristic produces noticeable noise due to rotor-stator vibration. 
         [0003]    The rotor-stator vibration arises in stepper motors because fixed stators usually have multiple longitudinal segments with lengthwise teeth arranged around a rotating central rotor with a longitudinal axis of rotation. The stator radially surrounds corresponding longitudinal teeth in a cylinder of rotating iron, with longitudinal teeth of stator and rotor facing each other. The stator segments have electromagnets that are selectively and successively energized by an external control circuit, typically a microcontroller. To make the motor shaft turn, one electromagnet segment is powered, which causes a segment of the rotor&#39;s teeth to be magnetically attracted to a segment of the stator&#39;s electromagnet&#39;s teeth that are energized. When the segment of rotor&#39;s teeth are aligned to the corresponding segment of the electromagnet, they are slightly offset from the next electromagnet. So when the next electromagnet segment is powered on and the first is turned off, the rotor rotates slightly to align with the next electromagnet segment, and from there the process is repeated. Each of those slight rotations is called a step, with an integral number of steps making a full rotation. In that way, the motor can be turned by a precise angular amount by an exact number of steps induced by pulses to electromagnets associated with the rotor segments. 
         [0004]    Stepper motors exhibit more noise than other motor types. One type of noise arises from stator teeth flexing and vibrating against rotor teeth, known as detent torque. Reducing detent torque by varying the pitch angles of the teeth is the most common way to reduce noise. The flexing arises because stator segments are electromagnets that move readily, vibrating under electrical impulses almost like an electromagnetic voice coil in a speaker. The electromagnet segments are typically a coil of wire wound on a plastic spool with inwardly facing teeth. Although plastic spool portions are rigidly held in place, the inwardly facing channels of the rotor will vibrate against nearby portions of the stator. In the prior art, a centering sleeve in a motor end cap has been used as a support for plastic spool edges. The centering sleeve may have a central bearing and axial aperture to support an axis of the rotor. 
         [0005]    An object of the invention is to reduce vibration in stepper motors. 
       SUMMARY OF INVENTION 
       [0006]    The above object has been achieved in a stepper motor wherein stator segments are clamped in place by stator brackets formed in a new centering sleeve having a stepped rim. Stator segments receive electromagnetic pulses from electromagnets wound on spools near segments. The segments ordinarily mechanically behave like voice coils, moving radially inwardly and outwardly with electrical impulses, although the purpose of the electrical impulses is to provide phase offsets that drive the motor. The stepped rims of the centering sleeves in motor end caps of the present invention act like brackets overlapping both axial and radial sides of the stator segments on opposite ends of the segments. The segments are no longer free to move radially in and out, but have opposite ends held tightly in place by the stepped rims, thereby reducing vibration from this source of motor noise, without significant reduction of motor torque. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is an exploded perspective view of a stepper motor having stepped rims associated with centering sleeves in accordance with the present invention. 
           [0008]      FIGS. 2 and 3  are front and back perspective views of motor end caps used in the motor of  FIG. 1 . 
           [0009]      FIG. 4  is a side sectional view of portions of a rotor in the stepper motor of  FIG. 1 . 
           [0010]      FIG. 5  is a side sectional view of the motor of  FIG. 1  in an assembled configuration. 
           [0011]      FIG. 6  is a detail of the motor of  FIG. 5  about the circle  5 - 5 . 
           [0012]      FIG. 7  is an enlarged side sectional view of a servo motor end cap with a centering sleeve, as in  FIG. 5 , but without a rotor in place. 
           [0013]      FIG. 8  is a detail of the centering sleeve shown in  FIG. 7  about the circle  7 - 7 . 
           [0014]      FIG. 9  is a detail of a centering sleeve of the prior art. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    With reference to  FIG. 1 , stepper motor  11  has a steel body  13  that has generally planar sides truncated at corners, so that the overall configuration is octagonal. The body extends along and about a motor axis where the axis is defined by axial shaft  25  associated with rotor  23 . The interior of the body has an octagonal cavity that seats the motor stator  21 . 
         [0016]    The stator  21  is segmented using spools  41 ,  43 ,  45 , and so on, that seat electromagnets that are coils wound on the spools. Each spool has a radially outer shoulder and a radially inner shoulder and a spool body between the shoulders. The radially outer shoulders are formed by a unitary plastic octagon  42  adhered to the octagonal cavity of the motor body  13 . Inner shoulders of each spool are circumferentially spaced stator segments joined to the outer shoulders and having a coil  26  made of multiple turns of fine wire wrapped around an interior core of the spool body for the purpose of generating a magnetic field for each stator segment with field lines extending inwardly toward the rotor axis. The wire receives electrical pulses from wires  28  that extend from the motor body. Each inner shoulder carries a piece of steel with axial or longitudinal teeth  24  forming the stator teeth. Inner spool shoulders  22  are seen to extend axially further than the steel pieces forming the stator teeth. The stator teeth face corresponding rotor teeth  28  on rotor  23 . The inner spool shoulders guide a centering sleeve, such as centering sleeve  32  whereby the sleeve abuts the stator segments as described below, although the centering sleeve maintains a slight radial clearance relative to the axially extending inner shoulders. In other words, the centering sleeve fits within the inner shoulders but abuts the stator segments. 
         [0017]    End caps  15  and  17  close the body  13  at opposed ends using screws  33  to connect the end caps through the motor body. End cap  17  is machined so that it has a unitary centering sleeve  32  projecting towards stator  21 . With reference to  FIGS. 2 and 3 , end cap  15  is seen to have a central hole  35  for accommodating a motor axial shaft. End caps on opposite sides of the motor body are the same. An invisible outline  36  of a centering sleeve is seen in  FIG. 2 . With reference to  FIG. 3 , end cap  15  is seen having a centering sleeve  45  between a cutout region  43  and wall  41 . The centering sleeve is annular, with a stepped inner rim, described below. The centering sleeve is integral with the end cap and may be machined or forged if metal, and molded if plastic. A ferromagnetic material is preferred for the end caps and the motor body in order to contain magnetic field lines and prevent electromagnetic interference. A central hole  35  supports the motor axial shaft by means of a bearing support wall  49  radially within a collar  47 , a flat recessed annulus, between sleeve  45  and the hole  35 . The wall  41  is octagonal in order to seat an octagonal stator as explained below. 
         [0018]    With reference to  FIG. 4  stator  21  has an outer annular rim divided into segments, such as segment  54 , one of eight segments, having a radially inward shoulder  56  a spool core  58  and a radially outward shoulder  60  extending between dashed lines  62  and  64  that designate the angular extent of a stator segment. The shoulders and core define a spool supporting a number of turns of fine copper wire  64  that generate a magnetic field with field lines generally parallel to the spool, then captured by stator bar  66 . The magnetic field saturates a bar of soft iron, termed a stator bar  66 , a stator segment adhered to inward shoulder  56 . The soft iron has a defined number of stator teeth  68  facing generally corresponding teeth in a rotor, not shown. Activation of different magnetic fields in different spools, in a precisely timed pattern, cases rotor rotation in a well known manner. 
         [0019]    In  FIG. 5  the stator  21  is seen to surround rotor  23 . Axial shaft  25  is seen extending through end cap  15  and end cap  17 . Centering sleeve  45  has a stepped rim  51  with a portion of the step extending axially and holding an end of stator bar  66  in place with a small amount of axial and radial contact by a stepped rim, i.e. contact in horizontal and vertical directions forming a bracket. The same structure exists at the opposite end of the rotor so that each stator bar is held in place at both ends. 
         [0020]    In  FIG. 6 , the rotor  23  is shown to be in close proximity to stator bar  66  with stator teeth  68  being slightly exposed. The end cap  15  has a centering sleeve  145  with a stepped inner rim  71 , the step having an axial portion  73  and a radial portion  75  supporting the axial end region of stator bar  66 . A portion of the wire spool in spool  54  having a radially inward shoulder  56  and a radially outward shoulder  60 . The stepped rim dampens vibration of stator bar  66  which is mechanically driven to behave vibrationally like a voice coil, although only magnetization is intended. By dampening vibration of the stator bars, noise is reduced in stepper motors. 
         [0021]    In  FIG. 7  end cap  15  is again shown with centering sleeve  145  having a stepped inner rim  71 . Stator bar  66  is shown to be held in place by the axial portion  73  and the radial portion  74  of a rim in the centering sleeve  145 . Although the centering sleeve overlaps a small amount of the axial portion of the stator bar, blocking full stator bar magnetic linkage with the rotor, the loss of torque is thought to be negligible by the small overlap which may result in shorting of magnetic field lines from the overlap region.  FIG. 8  shows the centering sleeve  145  with stepped inner rim  71  and the step with axial portion  73  and radial portion  74  retaining stator bar  66 . 
         [0022]    In  FIG. 8  a centering sleeve  245  of the prior art has no stepped rim. While the centering sleeve may or may not make axial overlapping contact with stator bar  66 , there is no radial contact and, consequently, the stator bar is not held firmly in place in two dimensions.