Shorted turn for moving coil motors

A linear motor of the type used to control the radial position of a write/read transducer relative to a rotating data storage disk comprises an elongated tubular-shaped permanent magnet which concentrically surrounds and is spaced, by an air gap, from an elongated pole piece of highly permeable material (e.g. steel). A selectively energizable coil, concentrically arranged between the magnet and pole piece, is adapted to move axially in response to an applied current. According to the invention, the pole piece supports a plurality of "shorting rings" made of high conductivity, low permeability material. The shorting rings are embedded in the pole piece so as to be coextensive with the surface thereof. The spaced shorting rings function as a "shorted turn" which operates to reduce the rise time of current applied to the coil without significantly reducing the magnetic flux in the air gap through which the coil is linearly driven.

BACKGROUND OF THE INVENTION 
The present invention relates to the field of data recording and playback. 
More particularly, it relates to improvements in radial access apparatus 
for controlling the radial position of a read/write head relative to a 
data storage disk, whereby the head may be moved from one data track to 
another to playback or record data on the disk. 
Linear motors of the moving coil variety are used extensively in 
applications requiring linear force-current characteristics and fast 
response. An example of such applications is the radial access apparatus 
used in a conventional disk drive for moving a read/write head between the 
various data tracks on an optical, magneto-optical, or magnetic data 
storage disk. Such moving coil radial access mechanisms are disclosed, for 
example, in U.S. Pat. Nos. 3,521,092; 4,439,699; and 4,603,270. 
FIGS. 1 and 1A illustrate the essential features of a typical moving coil 
linear motor M. Such device comprises a coil C wound upon one end of a 
tubular bobbin 10 which supports a load, shown as the read/write head 12 
of a data recording system. The coil is suspended in a radial magnetic 
field produced by a tubular permanent magnet 14. The tubular magnet is 
concentrically arranged with respect to the central post 16 of a 
cylindrical housing 18 made of steel or other magnetically permeable 
material. The outer surface of magnet 14 is press fit against the inside 
surface 20a of a peripheral cylindrical sleeve 20 of housing 18, and a 
base portion 22 of the housing provides a return path for the magnetic 
flux F, shown in phantom line. Depending upon the direction of current 
flow within the coil, the coil moves in a linear direction, as indicated 
by the arrows A, in an air gap G defined by the inside surface of the 
magnetic member 14 and the exterior surface of post 16. The coil 
experiences a force directly proportional to the magnetic flux density B 
provided by magnetic member 14, and the current I in the coil, the force 
on the coil being normal to both B and I. As the coil moves linearly in 
the magnetic field, the read/write head 12 moves radially with respect to 
a data storage disk D, such as an optical, magneto-optical, or magnetic 
disk. A motor M operates through a spindle S to rotate the disk D about 
its central axis, while the linear motor operates to move the read/write 
head 12 along the disk radius to access one of the concentrically arranged 
data tracks on the disk. 
Incremental and high speed motion control of the read/write head requires 
rapid changes in the magnitude and direction of the force applied to coil 
C. Since the flux density B is fixed, rapid changes in force imply rapid 
changes in current. In order to achieve a short current rise time, it is 
common practice to enclose the central steel post 16 with a non-magnetic, 
electrically conductive sleeve 24, commonly referred to as a "shorted 
turn." Currents induced in the shorted turn by the change in flux produced 
during movement of the coil act to reduce the magnetic flux diffusing into 
the high permeability post 16. As a result, the back emf seen at the coil 
terminals is reduced, thereby reducing the current rise time. 
While the sleeve-shaped shorted turn shown in FIG. 1 offers the advantage 
of reducing the current rise time, it is disadvantageous from the 
standpoint that it also reduces the available flux density in the air gap. 
Thus, while a shorted turn of this type will increase the rate of rise of 
force on the coil, the final magnitude of such force will be smaller, 
since the magnetic field is less. 
SUMMARY OF THE INVENTION 
In view of the foregoing discussion, an object of this invention is to 
provide a shorted turn configuration which acts to reduce the current rise 
time without substantially reducing the flux density in the gap. The 
moving coil linear motor of the invention is characterized by a shorted 
turn structure comprising a plurality of shorting rings which, according 
to the preferred embodiment, are spaced apart along the central post of a 
steel yoke and imbedded in the surface of such post so that the outside 
surface of each ring does not protrude above the exterior surface of the 
post. According to an alternative embodiment, the respective planes of the 
rings are inclined relative to the central axis of the post to avoid a 
force ripple which occurs when the rings are arranged substantially 
perpendicular to the,post axis. A preferred low cost process for producing 
the shorting rings in the post surface comprises the steps of forming left 
and right hand threads in the post surface, and filling such threads with 
molten copper, aluminum, or other high conductivity, non-magnetic 
material. 
The invention and its various advantages will become more apparent to those 
skilled in the art from the ensuing detailed description of preferred 
embodiments, reference being made to the accompanying drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring to FIG. 2, the steel housing 18 of FIG. 1 has been modified in 
accordance with the present invention to include a plurality of grooves 
16A in the central post portion 16. Each of these grooves is filled with a 
highly conductive, low-permeability material, preferably copper or 
aluminum, to provide a like plurality of shorting rings R. While the term 
"ring" often cannotes a circular configuration, it should be appreciated 
that the post portion may have a polygon cross section in which case the 
"rings" would be similarly configured. 
In the shorted turn configuration of FIG. 2, the magnetic flux travels 
around the shorting rings, i.e., between the rings. This has the effect of 
increasing the surface flux density on the steel post 16, vis-a-vis the 
shorted turn configuration of FIG. 1. Note, however, the overall 
reluctance of the gap is not significantly increased, since the shorting 
rings are embedded in the post so that the outer surface of each ring is 
substantially co-planar with or slightly below the post surface. Thus, the 
shorting rings tend to reduce the current rise time while not 
significantly reducing the air gap flux. 
In the FIG. 2 embodiment, the axial position of the coil with respect to 
the longitudinal axis A of post 16 can introduce an undesirable force 
ripple due to the nonuniform flux along this axis. Such nonuniformity in 
flux, of course, results from the fact that the post regions between the 
shorting rings experience a higher flux density than the regions directly 
underlying the rings. This force ripple can be significantly reduced by 
skewing the rings as shown in FIG. 3 where the rings are skewed at an 
angle .alpha., where .alpha. is at least equal to the arc tangent of D/d, 
where D is the diameter of post 16, and d is the spacing between the 
shorting rings. 
While the multiple shorting rings shown in FIG. 2 may be formed by a 
routine milling operation in which grooves are cut in post 16 by rotating 
the post about its axis A while a cutting tool is moved perpendicular to 
such axis, the skewed shorting rings shown in FIG. 3 are not so easily 
formed. A low cost method for producing skewed shorting rings is to use a 
thread-cutting machine to cut both right- and left-hand threads 30, 32, on 
the post, as shown in FIG. 4. In all three embodiments, the cut grooves 
may be filled with copper or aluminum by simply dipping the post in molten 
metal of the type desired. Alternatively, molten metal may be poured into 
the cut grooves. 
From the foregoing, it will be understood that an improved voice coil, 
linear motor has been disclosed in which multiple shorting rings serve to 
reduce the current rise time without significantly sacrificing air gap 
flux density. Such a motor is readily adapted for use in data recording 
systems for rapidly and precisely locating a read/write head relative to 
the different data tracks on a data storage disk. 
The invention has been described in detail with particular reference to 
certain preferred embodiments thereof, but it will be understood that 
variations and modifications can be effected within the spirit and scope 
of the invention.