5 : T h e o r y o f O p e r a t i o n
CHRISTIE INC. -
CHRISTIE INC. - 35/70 Service/PM Manual
July, 1997
Actual operation of the control loops is somewhat more complex than the introductory description above, and will be
discussed in detail in subsequent paragraphs.
S h u t t e r C o n t r o l C i r c u i t
The primary mechanical elements of the shutter control circuit are the shutter, the shutter encoder, and the shutter motor,
located on the shutter assembly. The shutter encoder provides 20 pulses per revolution of the shutter, and two index mark
indications per revolution, corresponding to the two shutter apertures. The encoder pulses are used to control the speed of
the shutter; the index pulses control frame pulldown in cine mode and are used a reference when shutter phasing is
enabled. The electronic control portion consists of the Shutter PWM Amplifier, 2285, and the shutter control circuitry on
the 2293 Main Electronics Board.
M
E
U22
U42
2285
PWM Signal
Comp
Amp
U30
DAC
U29
U31
UP / DN
CNTR
U22
S-Amp
Sawtooth
Current Limiting
Derived Tachometer Voltage
Speed Control Voltage
SHTR
CPU Speed
Shutter
TACHO
U33
Offset
14
15
16
Figure 5.2: Shutter Motor Control
Shutter operation is diagrammed in Figure 5.2 above. When the shutter is rotating at correct speed, nominally 30 rps,
about 600 pulses per second are sent to the Shutter EPLD, U29, by the 20 slot shutter encoder rotating on the same shaft
as the shutter. Since the shutter is locked to the line frequency, this is either 10 times the 60 Hz rate or 12 times the 50 Hz
rate. Within the EPLD, an edge detector produces a signal of four times this rate, which is sent to the Tacho EPLD U33.
The Tacho circuit produces output signal pulses that can be converted into a tachometer signal. Integrating amplifier U31
produces this analog tachometer signal which is summed into amplifier U22. The speed control voltage into this amplifier
is derived from the negative supply voltage. The output of the summing amplifier U22 is sent to comparator U42, where
the error signal is converted into a pulse-width-modulated (PWM) signal by comparing with a sawtooth input. This output
is sent through photocouplers to the PWM power amplifier, the 2285 card, which drives the shutter motor. The current-
limiting signal from the Power Amplifier turns on the photocoupler, U43, which limits the voltage sent to the pulse width
modulator, U42.
While the circuitry described so far would result in an accurate rate of shutter rotation, it is also desirable that the shutter
rotation be held constant over the long term operation of the system. This requires that any rotational lag or lead that
develops due to mechanical or circuit perturbations be zeroed out. To accomplish this, a positional correction has been
incorporated. The pulses from the Shutter encoder are compared with the control rate by means of an up-down counter in
the shutter EPLD U29. Each pulse from the control rate input causes the counter to count up, each pulse from the encoder
causes the counter to count down. The eight-bit output of this counter is sent to a DAC, U30, where an analog voltage
proportional to the number of pulses by which the shutter lags or leads the reference is generated. This voltage is
amplified by U22 and applied to the summing amplifier U22, along with the control and velocity inputs. Having both the
long term and short term rotational rates of the shutter precisely controlled allows the pulldown operation to be controlled
by the shutter index pulse. Without this control, long term effects would eventually cause the pulldown to over-run the
film loops and stop operation of the projector.
As a further refinement in support of telecine applications, a circuit has been incorporated which ensures the shutter is
synchronized to the vertical blanking time of a video frame. EPLD U19 detects the relative occurrence of the shutter edge
and the 60 Hz reference crossover, and adds or subtracts pulses from the 24 KHz reference to the Shutter EPLD to ensure
synchronization. This also ensures exact frame-per-frame blanking correspondence for multiple projector applications.
