Fault Latches; Servo Power Amplifier - HP 7936 Support Manual

activator is moving at high speed and the time
interval between track crossings is to short for the
servo controller to process. For this reason, track
crossings are divided by five when the heads are
more than 20 tracks from the addressed track.
5-50. FAULT LATCHES. The fault latches
retain servo errors detected by the servo system
fault detection circuitry for action by the servo
controller and read/write controller. Inputs to the
latches are TimIng Error
1 (TERR1-L)
and Timing
Error 2 (TERR1-L), from the sampled servo timing
block; and Off track Unlatched (OFTU-L), from the
off-track detector. Latch outputs are Servo Timing
Fault (SRTF-L) and Off track Latched (OFTRK-H)
to the servo controller, and Servo Error
(SRVER-L) to the read/write controller. The
latches are reset by Clear (CLER-L) and Clear
Servo Error
(CLSER-L)
from the servo controller.
Servo system error detection and reporting are dis-
cussed in later paragraphs of this chapter.
5-51. SERVO POWER AMPLIFIER. The servo
power amplifier and associated circuitry provides
the power to move the actuator. See figure
~
.
The input to the power amplifier portion of the
circuit comes from the output of the low-pass fil-
ter and notch filters block. When the actuator is
doing a track to track seek, input to this block is
from the seek current command generator and
when the actuator is track following, the input is
derived from Position Error signal (PES). The,
selection of the input is performed by the position
loop signal switch, which is controlled by Fine
Servo Enable
(FSEN-L).
The servo pow,er amplifier converts the voltage
from the output of the low-pass filter and notch
filters block into a current through the actuator.
The amplifier generates I ampere DC through the
actuator for every 2 volt DC input.
The input signal to the amplifier is compared with
sensed Actuator Current (ACUR) at the power
amplifier compensation and DC offset compen-
sator stage. The sensed current signal comes from
the current-senSt! amplifier block. The resulting er-
ror signal from the amplifier compensation circuit
is proportional to the difference between the input
signal and the sensed current, with the frequency
effects from the compensator.
Functional Description
7936 and 7937
The error signal from the amplifier compensation
circuit is input to the pulse-width modulator
(PWM) block. Here the error signal is converted
into a lOO-kHz pulse-width-modulated TTL square
wave, labeled Pulse Width Modulation
(PWM-L).
The square wave is routed through gate logic in
the amplifier control and fault indication block.
The output from the gating logic goes to the tran-
sistor driver stages.
The transistor drivers shift the level of the gated
signal to the level required to drive the output
transistors 'and also provide the gate drive voltage
needed to switch the output transistors on and off.
The output devices are power field effect transis-
tors (FETs). One FET is connected to the positive
power supply and the other is connected the the
negative power supply.
The voltage seen at the common connection of the
FET (drain) is the same waveform generated at the
PWM block, except that the voltage level is shifted
from +32/+37V to -32/+37V.
The signal at the common FET connection is fil-
tered by the output filter. This filter removes the
high frequency components of the square wave
and changes the pulse-width-modulated square
wave into a signal resembling a 100-kHz sine wave
with a lower frequency component that is propor-
tional to the original error signal from the power
amplifier compe'nsation circuit. The filtered volt-
age is applied to the actuator and the resulting cur-
rent is sensed through the current-sense resistor
using the current sense amplifier.
The combined action of the pulse-width
modulator, transistor drivers, output transistors,
and output filter is to power amplify the error sig-
nal from the power amplifier compensation block.
Using pulse-width m(;dulation gives the amplifier a
relatively high efficiency of over 80 percent
during the high current portion of a track to track
seek.
The servo power amplifier is enabled/disabled
through the amplifier' control and fault indication
block. If there are no detector faults, the amplifier
is enabled when Amplifier Enable (AMPEN-H)
from the servo controller goes high, and is disabled
when AMPEN-H goes low. However, if anyone of
5-21