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The interlock loop is part of the digital I/O loop and is sensed by the microprocessor. If the CDRH Interlock
was to become disconnected the ground path for the /CDRHRAW/ signal opens and the signal goes high.
This signal is asserted to the input of U41 pin 13 causing the CDRHBUF signal to go low. The low CDRHBUF
signal is asserted to the input (pin 17) of the Digital Input/Output EPLD U49 which the microprocessor reads,
then displays a fault on the LCD display.
Assuming that the interlock loop is complete and the footswitch is depressed, the shutter will be opened when
the Footswitch Logic EPLD U39 outputs a/FSDOWN1/ or /FSDOWN2/ signal to the Safety Control EPLD
U40. U40 will then output a /SHUT_H_DRV/ and /GATED_SHUT_DRV/ signal to turn on Q2 and Q4 (a
dual MOSFET package).
The first stage of the dual MOSFET package Q4 operates momentarily (
|150ms) each time Q2 turns on to
bypass the shutter solenoid current around resistor R71 as the shutter is moved into place, then turns off to
drop the shutter solenoid current to a lower "holding" level.
Shutter position is monitored by U5 and U6 on the Optics Bench PCB. The two are complementary for the two
shutter positions (opened or closed) one switch is blocked and the other is unblocked. Both devices are
supplied with +5 VDC and return ground from the Optics Bench PCB.
NOTE: Each of these circuits are all interlocked together.
4.4.4 Servo Motor Control Circuit
Refer to pages 8-9 thru 8-10 and 8-21 in Section 8. A rotating mirror is used to multiplex (up to) four Ho:YAG
head outputs into a single beam path. A servo motor is used to precisely position this imaging mirror to any
one of the four possible YAG head outputs. The mirror is mounted to the end of the motor shaft, at a slight
angle, so that as the shaft rotates through 360•, the mirror orientation changes. Between YAG pulses the
mirror must be moved to and stopped at the point in its rotation that aligns it with the output coupler of the
head that will be fired next. After the pulse, it must be moved to the next head to be fired, and so on for as long
as firing continues. 90• x 4 = 360•.
Refer to the Servo Motor Control Circuit Simplified Diagram (Refer to Fig. 4.3) and to the associated
schematics in Section 8. Motor controller U50 receives position commands from the microprocessor over the
Address and Data bus. It outputs direction and pulse commands (which are buffered by U54) to the Servo
Motor Control PCB digital signal buffer U1. U1 outputs a buffered signal to the H Bridge Motor Driver U2
which provides switched DC to the motor, in the polarity indicated by the direction input from the motor
controller, and in time increments as indicated by the pulse input. Note that in the VersaPulse Select III the
motor always moves in the same direction, but the reverse direction signal is still used to providing braking
force as the motor approaches a new stop position.
Motor movement and position information is fed back to the motor controller from an optical encoder located
behind the motor. The channel A and B feedback lines each provide 2000 counts per revolution, and are offset
90• from each other to provide a total of 4000 counts for one rotation. The position sensor also detects a
specific position, referred to as the index position. The index position information is reported to the
microprocessor through a digital I/O input port as SERVO_INDEX_BIT. Once the index position is found, the
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