HP 7936 Support Manual page 56

Functional Description
7936 and 7937
a number of amplifier faults is detected, the
amplifier is disabled and the amplifier crowbar
circuit is activated.
Possible amplifier faults are Power On Reset from
the power supply, Servo Processor Reset from the
controller, Under Voltage from the under voltage
detector, Over velocity fault from the overvelocity
detector, or Missing lOO-kHz PWM Clock from the
missing PWM clock detector. These faults are dis-
cussed in more detail in later paragraphs.
The following paragraphs provide a more detailed
description of the circuit blocks in the servo power
amplifier and associated circuitry.
5-52. Position Loop Compensation. The position
loop compensation circuit provides the frequency
compensation necessary to achieve the desired
phase and gain margins for the position servo loop.
The input to the circuit is Slew Position Error
(SLPES).
The compensator has two zeros at 372
Hz and two poles at 1590 Hz. This, along with the
low-pass filter and notch filters stage, gives a phase
margin of about 30 degrees for the sampled servo
system and 40 degrees for the dedicated servo
system.
The output from the position loop compensation
block is connected into the circuit via the position
loop signal switch when Fine Servo Enable
(FSEN-L) is low.
5-53. Low Pass Filter and Notch Filters. Since
the sampled servo signal has frequency com-
ponents that are mirrored about the sampling
frequency, a low-pass filter is required in order to
remove high frequency components from the
signal.
The low-pass filter is also used to control the out-
put signal from the seek current command gener-
ator. The servo power amplifier has a limited cur-
rent slew rate, since it is driving an inductive load.
In order to ensure that the amplifier output does
not saturate, the rate of change of the input signal
must be within the limits that the amplifier can
follow. The low-pass filter, in addition to a filter
within the seek current command generator, limits
the rate of change of the signal input to the
amplifier.
5-22
Three notch filters in the circuit compensate for
mechanical resonances in the drive. Since these
resonances can be excited by either the signal
derived from Slew Position Error (SLPES) or the
seek command current generator output, both sig-
nals are routed through the notch filters. The
notches are set at frequencies of 1336 Hz, 2200 Hz,
and 6596 Hz.
The output of the low-pass filter and notch fil-
ters block is connected to the input of the servo
power amplifier.
5-54. Seek Current Command Generator. This
circuit provides the input signal for the servo
power amplifier when the actuator seeks between
cylinders. There are three digital inputs to the cir-
cuit. When Positive Acceleration (PACC-L) is low,
the output from the seek current command gener-
ator goes to +6V, which is equivalent to a steady
state current of +3 amperes through the actuator.
When Negative Acceleration (NACC-L) is low, the
output from the seek current command generator
goes to -6V, equivalent to -3 amperes through the
actuator. Both
PACC-L
and
NACC-L
are gated by
Fine Servo Enable (FSEN-L), with the result that
the output from the generator is at zero volts un-
less FSEN-L is high. The seek current command
generator has a low-pass filter in its output stage.
This is part of the filtering which limits the rate of
change of the input signal to the servo power
amplifier. Refer to the low-pass filter and notch
filters circuit description for additional details.
5-55. Position Loop Signal Switch. This analog
switch connects either the output of the position
loop compensator or the seek current command
generator into the input of the low-pass filter and
notch filters block (servo power amplifier input.)
When Fine Servo Enable (FSEN-L) is low, the posi-
tion loop compensator output is selected. This oc-
curs when the servo system is following either the
dedicated or sampled servo code. When
FSEN-L
is
high, the output from the seek current command
generator is used. This occurs when the servo sys-
tem is in the seek mode.