HP 7936 Support Manual page 57

5-56. Summing Junction No.1. Here the outputs
from the position loop compensation and the seek
current command generator stages are summed
together. The resulting signal forms the input to
the low-pass filter and notch filters stage.
5-57. Summing Junction No.2. This summing
junction sums the sensed actuator current from the
current-sense amplifier block and the output from
the low-pass filter and notch filters circuit. The
result forms the input to the servo power amplifier
compensation and DC offset compensation stage.
5-58. Power Amplifier Compensation and DC
Offset. This circuit provides the ac compensation
necessary to stabilize the servo power amplifier. It
also contains
a.
resistor divider network which is
used to compensate for DC offset currents in the
servo PCA. During factory testing of the servo
PCA, the DC offset current is measured and the
network adjusted to reduce the actuator offset
current to less than ± 1 milliampere.
5-59. Pulse Width Modulator. The pulse-width
modulator (PWM) section consists of a triangle-
wave generator and a comparator. The frequency
of the triangle-·wave generator is 100 kHz, and the
output is coupled to the negative input of the
comparator. The positive input of the comparator
is connected to the output from the power
amplifier compensation and DC offset compensa-
tion circuit.
The output from the comparator is a pulse-width-
modulated square wave. The duty cycle of the
square wave is: 50 percent when the error signal
input is at 5.l8V. This is equivalent to no move-
ment required from the actuator. As the error sig-
nal voltage increases, the duty cycle increases (stays
high for a longer period), and vice versa. The out-
put from the comparator, labeled Pulse Width
Modulation (PWM-L), is gated with a signal from
the amplifier control and fault indication section.
The gating is required to turn off both output
transistors when the servo power amplifier is shut
off.
Functional Description
7936 and 7937
5-60. Transistor Drivers. The transistor driver
stages change the PWM-L output from the pulse-
width modulator into signals which drive the gates
of the output transistors. The drivers have an op-
tical isolator at their inputs, which translates the
TTL input signal into a signal which switches rela-
tive to the ±45V supply.
Slow turn-on and fast turn-off circuitry incor-
porated in each driver prevents the two output
transistors from being turned on at the same time.
5-61. Output Transistors. The output transistors
are power field-effect transistors (FETs) An
IRF9530 is used for the P-channel device and an
IRF520 for the N-channel device. Power FETs are
used because of their fast switching speed, and the
relatively low power needed to drive the gate
terminal.
5-62. Output Filter. The output filter is basically
a 2-pole low-pass filter. The filter has a series in-
ductor, followed by capacitors connected to
ground. The resulting voltage seen at the actuator
consists mainly of a 100-kHz sine wave, with a
lower frequency component which is generated by
the signal from the power amplifier compensation
circuit. The natural frequency of the output filter
is approximately 15 kHz.
5-63. Current-Sense Resistor. This is a O.1-ohm
precision resistor in series with the actuator. All
current that flows through the actuator also passes
through this resistor. In addition, the current that is
shunted around the gate of the bidirectional triode
thyristor in the actuator crowbar circuit also flows
through this resistor. This offset current is com-
pensated for during factory testing of the servo
PCA. Refer to the paragraphs describing the ac-
tuator crowbar and power amplifier compensation
circuits for additional details.
5-64. Current-Sense Amplifier. The current-sense
amplifier senses the voltage drop across the
current-sense resistor. The amplifier has a gain of
20, so that the equivalent voltage at the output of
the amplifier is 2 volts for every I ampere of cur-
rent in the actuator. The output signal from the
amplifier is labeled Actuator Current (ACUR).
5-23