System Components 35
Figure 17: Model FSM-CD300B controller/driver front and rear panels
6.6 External Sensor Feedback Control Mode
The Model FSM-CD300B controller/driver can be used in an External Sensor
Feedback Control mode with position feedback from an external sensor, such
as a quad cell or lateral effect cell. This allows the FSM to lock a laser beam
onto a target such as the center of a quad cell. The default alternative is the
Internal Control mode, which utilizes a quad cell sensor built into the mirror
head and feeds the error signal back to the controller/driver via the system’s
15-pin, 3-foot interface cable.
A beam stabilization system with external position feedback sensing is shown
in Figure 18. To switch system operation to External Feedback Control from
the Internal Control, a high TTL-level signal must be applied to the INT/EXT
selector switch (Pin 17) of the 25-pin INTERFACE I/O connector. External
Feedback inputs can then be applied to Pins 11, 12, 15 and 16. Returning the
signal on Pin 17 to a low state will switch the FSM back to Internal Control.
In the illustration in Figure 18, a Logic Unit (supplied by the user) provides
the TTL-level signal for locking the FSM onto the quad cells. The sum of
the outputs from the quad cell determines whether sufficient light is on the
detector. If light is sufficient, the Logic Unit switches the FSM’s to External
Feedback Control and cancels out the tilt errors. If light is insufficient, the
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