Service manual

5: Theory of Operation
CHRISTIE INC. - CHRISTIE INC. - 35/70 Service/PM Manual
July, 1997
Constant Velocity Motor Control
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.
U18
AMP
U52
UP / DN
DAC
U15
CNTR
CV Encoder
SERVO9
Direction
DRM Speed
19
U23
CV Tach
SUMMING
17
U38
COMP
18
22
21
M
U12
COMP
2285
PWM Signal
Sawtooth
Current Limiting
Direction
E
T
U23/U28
AMP
ABS VAL
20
DRM Speed
Direction
TACHO
Offset
U44
AMP
Speed Control Voltage
U33
Figure 5.3: Constant Velocity Motor Control
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.
Frames
per
Second
Pulses per
Second
24 FPS 240 PPS
30 FPS 300 PPS
48 FPS 480 PPS
60 FPS 600 PPS
120 FPS 1200 PPS
Table 5.4: Constant Velocity Rates for Film Speeds
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