Table of Contents
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The discussion on Seek operations is divided
'into the following areas:
•
Servo Circuit Operation - Describes the
Servo circuit, which controls the voice
coil positioner.
•
Basic Seek Operation - Provides a gener-
al description of how the drive
~
during a seek operation.
•
Types of Seeks - Describes the different
types of seeks performed by the drive.
•
End of Travel Detection - Describes what
happens when the drive positions the
heads beyond the normal area of travel.
•
Seek Status and Error Conditions - De-
fines certain error conditions related
to seek operations.
SERVO CIRCUIT OPERA nON
The servo circuit is a closed loop servo-
mechanism used to position the read/write
heads.
Figure 3-19 is a simplified schematic
of the servo circuit.
Functions of the major
elements of the system are explained in
Table
3-4.
A
servo loop sums all of the error voltages
imposed on it.
The loop always
attempts
to maintain itself at a null.
If not nulled,
the loop will adjust the correctable device
(in this case, the voice coil positioner) to
achieve this null.
Signals applied to the
loop are called error voltages.
Two major
error voltages are used.
1.
A position error: this is the departure
of the positioner from the desired
position.
2. A feedback signal to modify (or oppose)
the position error to cause a smooth
motion of the positioner.
The position error signal is provided by the
position converter and its allied elements.
The amplitude of the signal is proportional
to the distance from the present position
to the desired position (tracks-to-go). The
major feedback signal is the output of the
velocity transducer.
The amplitude of this
signal is proportional to the velocity of
the positioner while the phase indicates
the direction of motion, forward or reverse.
The loop applies its position and feedback
signals to one summing point, the summing
amplifier.
If the summation of these sig-
nals is not equal to zero, the summing amp-
lifier outputs a signal proportional to the
amplitude of the error voltage (which sig-
nifies the amount of displacement from the
desired position) and the phase of the error
voltage (which indicates the direction of
displacement).
The error output from the summing amplifier
is applied to the actuator assembly.
The
actuator contains a voice coil positioner
that supports and moves the read/write heads.
In turn, the voice coil is located within a
powerful magnet.
Whenever a current passes
through the voice coil windings, the inter-
action of the induced emf and the magnet's
flux field cause the positioner to move.
The acceleration of the motion is propor-
tional to the polarity and amplitude of the
voice coil current.
TABLE
3-4.
SERVO CIRCUIT PUNCTIONS
Circuit Element
Function
Difference Counter
Digital To Analog
Converter
Desired Velocity
Function Generator
83318200
C
Holds the complement of the number of tracks yet to be crossed
.. before reaching the desired track or cylinder.
When On Cylinder
the counter indicates 1023
or 511 for BR3E4/3E5.
An associ-
ated decoding network provides outputs representative of the cur-
rent general content of the counter.
Monitors the 7 lowest order bits of Difference counter to provide
an analog indication of position error the amplitude of which
is proportional to the number of tracks to go.
The amplitude
decreases in discrete steps as last 127 tracks of a seek are
crossed.
Processes Position Error signal at gain levels that vary as
position Error decreases.
The resulting output is the analog
representation of the desired velocity curve to achieve maxi-
mum control of deceleration.
The parallel non-linear feedback
circuit maintains tight loop control
by
increasing gain as the
Position Error signal approaches zero.
This gain control pre-
vents loss of control during the critical deceleration portion
of the seek and is essential to minimize overshoot and settle
out problems.
3-43
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