16
NanoTrak Autoalignment Controller
If the Diameter Mode is set to '
LUT
', the circle diameter is adjusted automatically,
using a table of range dependent circle diameter values (in NanoTrak units).
When automatic LUT diameter adjustment mode is enabled, the system uses
values in this LUT to modify circle diameter in relation to the input range currently
selected.
This LUT diameter adjustment mode allows appropriate circle diameters to be
applied on an application specific basis. Typically, the circle diameter LUT will be
configured to generate a reducing circle diameter for an increasing range setting.
This fits an assumption that higher input signal readings correspond to a smaller
FWHM of the feedback signal peak being tracked, and so a smaller circle diameter
is required to prevent NanoTrak instability.
Enter a circle diameter (0 to 5 NT units) for each input range of the NanoTrak (1 to
14, corresponding to 3nA to 10mA respectively)
Frequency
- the frequency of the scanning circle. The circle scanning frequency lies
in the range 15 to 200Hz. The factory default setting for the scanning frequency is
43.75Hz. This means that a stage driven by the NanoTrak makes 43.75 circular
movements per second. Different frequency settings allow more than one NanoTrak
to be used in the same alignment scenario. Refer to Section 3.10.3. for more
information
Control Options
Control Mode - Typically, the NanoTrak is operated in Open Loop mode because the
piezo amplifiers have a higher bandwidth operation in open loop, thereby ensuring
best bandwidth performance. If the NanoTrak is latched, the piezo actuator position
is best maintained if the drift is minimized. This is achieved by placing the piezo
channels into Closed Loop mode.
