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C o n s t a n t V e l o c i t y M o t o r C o n t r o l
As seen in Figure 5.1, the main mechanical elements of the Constant Velocity Motor circuit are the Motor, encoder,
tachometer and constant velocity drive sprocket contained on the constant velocity drive assembly. The electronic portion
of the control circuit is the Constant Velocity PWM amplifier 2285 and the CV control circuit on the 2293 Main
Electronics Board.
UP / DN
CV Encoder
CNTR
Direction
SERVO9
DRM Speed
DRM Speed
Direction
Constant Velocity operation is diagrammed in Figure 5.3. A signal with frequency proportional to the desired film rates is
generated by the Digital Rate Multiplier EPLD U14 and associated Programmable Timer Modules. The rates generated
for various film speeds, in pulses per second, are given below in Table 5.4.
A direction signal is also supplied by the CPU. These signals are processed by the Tacho EPLD U33, whose output is
integrated by U44 into an analog speed control voltage. An offset derived from the plus voltage is also summed by U44.
This speed control signal is compared to the CV tachometer signal by U23, and the absolute value of the error signal is
derived by U23 in combination with U28. This unidirectional signal is converted to a PWM signal by comparator U12,
and sent to the Constant Velocity PWM Amplifier 2285. Comparator U38 derives a direction signal which is also sent to
the PWM Amplifier. The current-limiting signal from the power amplifier turns on the photocoupler, U11, which limits
the voltage sent to the pulse width modulator, U12.
In a manner similar to the case of the shutter motor control, a signal representing the long-term position error is also
summed into U23, along with the speed control voltage and the tachometer signal. This signal is derived from the CV
encoder pulses and the DRM speed pulses, using the SERVO9 EPLD U15 as an up-down counter. The eight-bit output of
this counter is converted into an analog voltage by DAC U52, and amplified by U18 before being summed into U23. To
CHRISTIE INC. - 35/70 Service/PM Manual
CHRISTIE INC. -
July, 1997
U15
U52
U18
19
DAC
AMP
U33
17
U44
Offset
Speed Control Voltage
TACHO
Figure 5.3: Constant Velocity Motor Control
Frames
per
Second
24 FPS
30 FPS
48 FPS
60 FPS
120 FPS
Table 5.4: Constant Velocity Rates for Film Speeds
CV Tach
ABS VAL
Current Limiting
AMP
18
U23
U23/U28
20
Sawtooth
SUMMING
AMP
U38
COMP
Pulses per
Second
240 PPS
300 PPS
480 PPS
600 PPS
1200 PPS
5 : T h e o r y o f O p e r a t i o n
U12
PWM Signal
2285
21
COMP
Direction
22
E
T
M
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