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SERVO
AMP
Fig. 1-2.
Practical ac servo system
Stage/Control
Function
Start Operation
When
the power
switch is turned on,
Q06 in the base circuit of QO7 is cutoff.
As a result, C8 in. the collector circuit
is charged through R27,R8, VR2, R10,
VRI
and R9 when the 33 rpm button
is depressed. Note that VR2 and R10
is shorted
during
45 rpm
operation.
Q07
is forced
into conduction
when
C8 is charged
up to some
specified
voltage.
As
a result
the
following
conditions
exist:
:
Q09 QO10 QOlI
Q3
Q4
Q5
Q6
OFF OFF
ON
ON
ON
ON
ON
and a large enough ac voltage is applied
to the motor
so the motor
starts to
revolve rapidly.
Correct Speed Condition
Frequency
generator
When
the motor starts to revolve, the
frequency
generator
(F.G.) generates
ac voltage
whose
frequency
is pro-
portional to the motor speed. As the
frequency generator is directly coupled
to the shaft
of the drive motor,
it
converts motor speed into frequency.
Differential
Q1 and Q2 forma differential amplifier
amplifier
which amplifies the input FG signal to
the
level required
for the following
Ql, Q2
limiter circuit.
Note that the output is extracted from
collector
circuit
of Ql] and
Q2, and
then fed to the flip-flop stage through
diode limiter D1 and D2.
Stage/Control
Function
Diode limiter
Removes all amplitude variations from
D1, D2
the signal. Each diode conducts when
the signal across it exceeds the barrier
potential (0.6 V) in a forward biased
condition.
Thus, the output signal is
limited
to about
1.2 V peak-to-peak.
Flip-flop
Q01 and QO02 form a flip-flop circuit
circuit
which
generates
square output in ac-
Q01, Q02
cordance
with the input trigger signal
(limiter's output).
The
flip-flop
output
is extracted
at
collector circuit of QO2 and then fed to
the buffer amplifier stages.
Buffer/phase
Q0O3,
Q04
and
QOS
form
a buffer
inverter
amplifier
stage.
Note
that
Q03
and
Q03, Q04,
Q05 are emitter followers but, Q04
Q05
acts as a phase inverter.
Differentiation Square
wave
output
at the emitter
circuit
circuit of QO5 is converted into spike
C7, RO12
pulses through the differentiator circuit
(formed
by C7 and RO12) to trigger
the following saw-tooth wave generator
(Q06).
Saw-tooth
Q06
and
RC
components
(C8,
R9,
wave
VR1, R10, VR2) in the collector circuit
generator
form a saw-tooth wave generator.
Q06, C8, R9,
Note
that the frequency of the saw-
VRI, R10
tooth
wave is determined
by the RC
VR2
time constants in the collector circuit.
Voltage
The
saw-tooth
is fed to the voltage
comparator
comparator formed by QO7 and QO8.
Q07, Q08
Q08
is forward
biased
through
the
FINE control (VR3 paralleled by R12)
R11 and R28. The current flow in Q08
is controlled
by
the
FINE
control
(VR3),
which.
varies
its base-emitter
voltage. Q07 conducts
only when the
base voltage becomes higher than the
emitter voltage which is determined by
the current flow in QO8. Note that the
emitter
voltage
of QO8
serves
as a
reference voltage.
Referring to Fig. 1-3, the comparator
operates as follows: At time T1 a saw-
tooth signal is applied to the base of
Q07.
At time T2 the voltage at the base of
Q07
is sufficient
to
turn
on
QO7
generating a negative pulse.
Note that the pulse width is determined
by saw-tooth signal waveform:
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