25
T-Cube Position Aligner
Fig. 4.7 OPEN LOOP and CLOSED LOOP Schematic Diagram
In ‘Closed Loop’ mode, the signal from the detector is interpreted by the unit, and the
feedback circuit sends position demand signals (XOut and YOut) to the rear panel LV
OUT/XDIFF and LV OUT/YDIFF connectors, which can be used to drive a pair of
positioning elements (e.g. piezo controllers) in order to position the light beam within
the center of the detector array, see Fig. 4.7. For a detailed diagram of the PID loop,
see Fig. 4.8.
When the beam is central, the center LED on the Target display is lit.
Note
In closed loop mode, with the beam central, the X and Y axis difference
outputs from the photodiode array are zero. However, the position
demand signals on the rear panel LV OUT XDIFF and YDIFF SMA
connectors are whatever value is necessary to drive the positioning
elements to centre the beam.
PDQ80A
Quad Detector
A/D
A/D
DAC
DAC
A/D
DSP
XDIFF
YDIFF
SUM
USB
LV OUT XDIFF
LV OUT YDIFF
SUM
OPEN LOOP MODE: The XDIFF and YDIFF signals are routed to the DSP.
The DSP either sets the outputs to zero, or holds them at their last closed loop
voltage (dependent on the setting of the '
Position Demand In Open Loop
Mode
' parameter in the Settings panel).
CLOSED LOOP MODE: The XDIFF and YDIFF signals are routed to the DSP.
The DSP implements a PID control loop for each axis. The resulting position
demand signals (XPos YPos) are routed, via the rear panel SMA connectors,
to the positioning elements to drive the spot to the center of the detector.
