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Section IV
Principles of Operation
plates, and a grounded rotor with apertures to permit pas-
sage of the 200 kHz excitation signal. The rotor is attached
to the galvanometer shaft. The bottom plate is divided into
four sections, connected diagonally (Figure 4-4) so that the
excitation is received differentially. This differential action
keeps the load on the excitation oscillator constant, to aid
the oscillator regulator in maintaining a constant output
current. Excitation signal amplitude is important since it
affects position transducer sensitivity.
4-15. The demodulator circuit senses the unbalance be-
tween the two portions of the position sensing capacitor,
caused by the amount of energy from the oscillator that is
coupled through the apertures in the rotor on the galva-
nometer shaft. If the circuit is balanced, both halves of the
position capacitor receive equal amounts of 200 kHz radi-
ation from the oscillator output plate. Refer to Figure 44.
On the p osi tive excursion of the excitation signal
(200 kHz), diode CR2 conducts, turning on transistor Q2
and charging Cl. Diode CRI is back-biased, and keeps Ql
cut off during this period. On the negative excursion of the
excitation signal, the opposite current flow takes place,
with Ql cut off during this period. On the negative excur-
sion of the excitation signal, the opposite current flow
takes place, with Ql conducting the same amount as Q2
conducted previously. When the circuit is balanced, Cl thus
reflects a net zero voltage output. During an unbalanced
condition, either Ql conducts more or Q2 conducts more
to produce an average positive or negative voltage on Cl.
4-16.
An
example of unbalance is shown in Figure 44. As
the positive signal causes the galvanometer to move the
stylus as shown, the rotor moves so that more of the oscil-
lator output is felt on the shaded pair of split capacitor
plates. These plates are connected to the CRI-Q 1 half of
the demodulator, so that more negative voltage is impressed
upon Cl. The unshaded plates receive proportionately less
of the oscillator output, and so CR2-Q2, the positive side of
the demodulator, produces less positive output for Cl. Cl,
then, sends a negative feedback voltage to the driver ampli-
fier, which tends to return the stylus toward the center of
the chart. The feedback voltage is aided by a torsion spring
that facilitates setting of the stylus mechanical center. For
maintenance purposes, note that one volt of position volt·
age corresponds to 10 divisions of stylus movement.
4-17. Galvanometer Damping.
4-18. The position feedback signal is fed back to the driver
amplifier through resistor R36. Part of the position voltage
is fed back through C6 as velocity information and C4 and
C5 as acceleration information. The amount of velocity
feedback controls the damping, which is varied with R30.
4-19. DAMPING. Damping is a force that is
(1) propor-
4-2
Model 7754A/7414A
tional to galvanometer velocity and (2) opposite to the
direction of pen motor velocity. Figure 4-5 shows the
effects of damping on frequency response and transient
response, where underdamping produces peaked and oscil-
latory waveforms, and overdamping diminishes response.
Optimum damping leaves about 71 % of the original signal
strength, so that the frequency response is about 3 dB
down at the galvanometer's natural frequency and the
response to a square wave (step function) input shows
about 4% overshoot.
4-20. Heat Control Circuit.
4-21. Stylus heat is controlled from the Heat Pot Board,
A2A2, on the front panel. The heat control voltage is
applied to a simple feedback amplifier located on the driver
amplifier assembly, Qll, Q12, and Q13, which has a cur-
rent limiting circuit similar to that used for the galva-
nometer. The amplifier output drives the resistive stylus
heat element. A good stylus should have about 34 ohms
resistance.
4-22. Power Control Circuits.
4-23. Power is controlled from the recorder front panel.
Figure 4-6 shows recorder power switching and fuses to-
gether with chart motor control circuits and speed control
solenoid circuit.
4-24. line common reaches the chart drive motor through
SI, the power switch and S4, the voltage selector, whenever
SI is on. The high side of line power is applied to the chart
drive motor through motor relay Kl, which is actuated by
the control switch RUN button through interlock S3 or by
a remote run signal (Figure 2-8). The motor is described
further in Paragraph 4-29. Kl also turns on stylus heat
through the control switch.
4-25. The motor drives the gearbox, the speeds of which
are controlled by speed selection solenoids Ll, L2, L3, and
IA. The speed control action of these solenoids is described
in
Paragraph 4-31. The segments of the con trol switch are
so arranged that the solenoids are energized in the correct
combination for each speed desired (Table 4-1). The switch
is shown in the Imm/sec position.
As
an example of how
the switch works, the -24V supply voltage, applied to the
switch via the main feeder line at the top of Figure 4-6
energizes the center contacts of the switch segment. When
the pushbutton marked "I" is depressed, -24V is applied
to solenoids Ll and L2 through resistors R8 and R7 respec-
tively, selecting the proper gear combination. In speeds of
2.5mm/sec and higher, the heat control voltage is aug-
mented by fixed voltages applied to the heat control poten-
tiometers by the control switch. The heat control poten-
07754-1
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