6300
Functionally, the magnet structure and the ways that the carriage rides
on
are the
stationary portion
of
the linear motor.
The
moving structure consists
of
the carriage, the
positioner coil, the heads mounted into the carriage, the bearings which support the
carriage
on
the ways,
and
the moving portions of the Velocity Transducer and the Position
Transducer. The force that is necessary to move the moving-mass portion
of
the Positioner
is a function
of
the interreaction of the magnetic fields produced by the permanent magnet
structure
and
the magnetic field resulting from the current in the positioner coil. This force
may
be
used for purposes of accelerating or decelerating the moving-mass, or
as
a
restoring force for the purposes of holding the moving-mass
in
a given position. To a first
approximation, the force developed is proportional to the current in the positioner
coil.
The
Velocity Transducer provides
an
electrical signal that is proportional to the velocity
of
the moving-mass and consists of a permanent magnet moving inside
of
the specially
wound
coil. This coil is attached to the positioner baseplate and is stationary; the magnet
is mounted
on
a shaft which is attached to the carriage. The amplitude
of
the signal
derived from the Velocity Transducer is approximately equal to the actual moving-mass
velocity.
The
magnitude of this signal is indicative
of
the speed of the mass and its
polarity indicates the algebraic sign
of
the velocity.
The
Position Transducer is a photoelectric sensor that develops four electrical signals,
each serving a specific function for use
in
controlling the positioner.
An
incandescent
lamp, mounted in the position transducer body, supplies illumination to a group of
photodiodes located opposite the lamp and within the body of the Position Transducer.
The
transducer body is mounted stationary to the positioner baseplate
by
the use of a
mounting strip.
Attached to the carriage is a precision scale which is part of the moving-mass. This scale
is interposed between the lamp
and
the photodiode and consists of opaque
and
transparent areas in specific patterns. Also interposed between the scale and the
photodiodes is a precision reticle which is also made
up
of opaque and transparent areas
in specific patterns.
The
combination of the reticle patterns and the scale patterns
are
used to control the amount of illumination reaching the photodiode group.
Two
of
the signals developed
by
the Position Transducer
as
a function of carriage position
are
level change type
of
signals. One signal, the Heads Retracted signal, changes state
when the carriage is approximately
1/4-inch
from the fully retracted position and is used to
indicate the gross carriage position, specifically, the retracted position of the heads.
The
other Signal
is
the Position Transducer Index signal which is used to define the legal range
of the carriage
pOSition. This signal is a multi-change-of-state Signal. One
of
the
transitions
of
this signal is used in the initialization process during a head loading
operation.
The
other two signals derived from the Position Transducer
are
referred to
as
X + 0
and
X +
90.
These signals
are
linear Signals that
are
cyclic
as
a function
of
cylinder pOSition,
and bi-polar in terms of polarity. They
are
displaced in electrical phase
by
approximately
90
degrees.
All of the signals from the Position Transducer are current signals proportional
in
amplitude to the illumination
of
the specific photodiode associated with that signal.
The
X + 0
and
X +
90
signals
are
actually derived from photodiode pairs rather than a single
photodiode.
4-6
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