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# Head Actuator Assembly with Zero Lateral Force on Pivot Bearing

IP.com Disclosure Number: IPCOM000118275D
Original Publication Date: 1996-Nov-01
Included in the Prior Art Database: 2005-Apr-01
Document File: 6 page(s) / 133K

IBM

## Related People

Moser, MA: AUTHOR

## Abstract

Disclosed is a design and methodology for a rotary actuator which induces no lateral force on the pivot bearings. Design conditions are stated in terms of distributions of mass and forces induced by the Voice Coil Motor (VCM), and an implementation is shown for a planar coil VCM.

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Head Actuator Assembly with Zero Lateral Force on Pivot Bearing

Disclosed is a design and methodology for a rotary actuator
which induces no lateral force on the pivot bearings.  Design
conditions are stated in terms of distributions of mass and forces
induced by the Voice Coil Motor (VCM), and an implementation is shown
for a planar coil VCM.

Lateral forces on the pivot bearings of a rotary magnetic head
actuator introduce a parasitic mode which degrades the
track-following capabilities of the disk drive.  Elimination of the
lateral force on the pivot prevents the parasitic mode from being
excited.

By placing the center of mass between the pivot bearing and the
voice coil motor (Fig. 1), mathematical conditions can be derived to
ensure that there is no lateral force on the pivot bearing.  For
devices that do not match these conditions, modifications that lead
to meeting  the condition are: (1) redistribution of the mass, (2)
modification of  the voice coil motor geometry, and (3) a combination
of these.

A free body diagram of the actuator system is shown in
Fig. 2.  The center of mass CM is offset from the pivot point P by a
distance &scriptl.; the force and moment produced by the motor act at
a point O which is at a distance L from the pivot point.  For
convenience, all three points have been assumed to be colinear and
the motor is taken  to have symmetry such that no force is produced
in the x-direction. The  objective is to produce a design in which
the system naturally rotates  about the pivot point (i.e., ue sub p =
'0'.).  This is achieved if the  following relation is satisfied:
F sub Oy (I sub cm + m scriptl(scriptl - L)) - m scriptl
M sub O = 0.                                                 (1)

A planar voice coil motor consists of a coil with multiple
turns of wire, which is acted upon by a permanent magnetic field with
alternating polarity (Fig. 3).  A typical coil consists of two
sections (1 and 3 in figure) that lie along radial lines, with two
circular arcs (2 and 4) connecting them.  When a current is applied
to the coil, the sections along a radial line produce a force and a
moment that rotates the actuator; the circular arcs do not produce
forces or moments if they are outside of the permanent magnetic
field.  Using the nomenclature from Fig. 3, the force provided by the
motor is:
F sub Oy = 2 integral from r sub i to r sub o
f ' ' cos ' ' theta ' ' dr                                   (2)
= 2 ' ' f ' ' ( r sub o - r sub i ) ' ' cos ' ' theta ,
where f is the force per unit length.  The moment provided by the
coil is
M sub O = 2 integral from r sub i to r sub o f ' ' r ' ' dr  (3)
= 2 ' ' f ' ' ( r sup 2 sub o - r sup 2 sub i )
= 2 ' ' f ' ' ( r sub o - r sub i ) ( r sub o + r sub i )

Substitution of the force and moment relations...