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Theory of Operation
7942 and 7946
head is positioned exactly between two servo
tracks.
In operation, a demodulator circuit
continuously samples the two frequencies from the
servo head, sums and averages the two signals, and
produces a position error signal (PES). This PES
signal is linearly proportional to the error (off
track) amount and also indicates the off track
direction.
During track following, the PES voltage is input to
a transconductance power amplifier which supplies
drive current to the actuator voice coil.
This
causes the actuator to hold the servo head at a
point where the PES voltage is zero -- exactly be-
tween the two servo tracks. Since the servo head
and the read/write heads are fixed on the rigid
rotary arm of the actuator, any movement of the
servo head is translated to all of the read/write
heads.
Signal PES is also used during a seek operation. As
the rotary arm is moving across the discs during a
seek, the PES voltage is a series of positive and
negative peaks which the microprocessor counts to
determine track location. Also, the slope of the PES
voltage is used to determine the velocity of the
rotary arm during the seek operation.
The dual-frequency technique described above is
relatively insensitive to servo surface medium
defects. Also, servo head azimuth alignment is not
a critical factor in the operation of the system.
The actuator servo control circuitry includes a
demodulator,
mode
select
switch,
tachometer,
digital-to-analog converter (DAC), and amplifier.
Details of these circuits are provided in the follow-
ing paragraphs.
3-78. DEMODULATOR. The SERVO+, SERVO-
signal (read from the servo surface via the servo
head and the servo preamplifier IC) consists of a
combination of the frequencies recorded on alter-
nate servo tracks -- 700 kHz and 1020 kHz. This
signal is amplified and then separated into two
frequencies by two identical mixer channels in the
demodulator.
In one channel, the 700 kHz is
mixed with 576 kHz from the clock generator to
produce 124 kHz., In the other channel, the 1020
kHz is mixed with 11 52kHz from the clock gen-
erator to produce 132 kHz. Higher frequencies are
attenuated with active low-pass filters in the
mixer channels. The two outputs from the mixer
channels are rectified, averaged, and fed to the in-
3-18
put of a difference amplifier. The output of this
amplifier is Position Error Signal PES. When the
servo head is positioned exactly between two servo
tracks (read/write head on track), PES is zero. Dis-
placement from this position (read/write head off
track) causes PES to go either positive or negative,
depending on the direction of offset and with an
amplitude equal to the amount of the offset. A
PES level of 1 volt equals a displacement of ap-
proximately 100 microinches.
The rectified and averaged outputs from the two
mixer channels are also fed to a summing and AGC
amplifier which supplies an AGC voltage to the
amplifier at the input of the demodulator.
The demodulator also contains a track zero detec-
tor. This circuit senses the presence of a third
frequency (1100 kHz) recorded on the servo sur-
face to identify track O. The Track 0 output from
the detector is coupled to the microprocessor which
in turn transmits Track Zero signal TRKO-L on
the ST- 506 Interface.
3-79. MODE SELECT SWITCH. The mode select
switch is a solid -state circuit which places the ac-
tuator servo control circuitry in either a seek
(velocity) or track follow mode of operation.
Opera tion of the switch is con trolled by the Seek
signal from the microprocessor.
3-80. SEEK MODE. The seek mode of operation
is selected by the microprocessor when it switches
the mode select switch to the seek position with its
Seek line. This selects a number of circuit blocks
which together comprise a negative feedback servo
control system supplying drive current to the ac-
tuator voice coil. These circuits include a velocity
digital-to-analog converter (DAC), tachometer,
slope selector, current inverter, on -peak detector,
inverter, and power amplifier.
3-81. Velocity DAC. The velocity DAC is a stan-
dard integrated-circuit digital-to-analog convert-
er which outputs a velocity error dc voltage to the
servo system in response to an 8-bit velocity error
command from the microprocessor.
3-82. Tachometer.
The tachometer provides a
measurement of the actuator velocity by integrat-
ing the slope of the PES signal as the actuator
crosses tracks. However, the PES voltage becomes
non -linear at the peaks of its triangle -sha ped
waveform.
Therefore, at this time the power
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