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T-Cube Position Aligner
X,Y Display Mode
- The detector head provides top-minus-bottom (Y-Axis alignment)
and left-minus-right (X-Axis alignment) difference signals, together with a signal that
is the sum of signals (total beam power) from all four sensors of the internal
photodiode array. The two difference signals are voltage analogs of the light intensity
difference sensed by the pairs of photodiode elements in the array.
If '
Difference
' is selected, the Main Grid Display plots the XDiff and YDiff signals
from the array.
If '
Position
' is selected, the Main Grid Display plots a graphical representation of
the XOut and YOut values described above (e.g. the piezo positions if a piezo
mirror is being used).
See Section 4.4. for more information.
Grid Display
- shows either the XDiff and YDiff signals from the detector quadrant or
the XOut and YOut position demand outputs, depending on the XY Display Mode
described above.
As previously described, the difference signals indicate the beam position on the
detector in the X and Y axes. The sum signal is used to ‘normalize’ the difference
signals in relation to beam power. For example, consider the situation, where the
beam is positioned in the upper right hand quadrant of the detector. In this case, the
XDiff and YDiff signals may be ‘8’ and ‘2’ respectively, giving a sum value of ‘10’.
If the beam power were to be reduced by half, due perhaps to the fitting of a filter, the
XDiff and YDiff values would then be ‘4’ and ‘1’ respectively. Viewed in isolation, these
values indicate that the spot has moved nearer to the center. It is only when the values
are ‘normalized’ and considered together with the sum value, now ‘5’, that the true
picture becomes apparent.
The display also shows a bar graph, which represents the SUM value from the
detector.
Note
When a PDP series detector head is being used, the accuracy of the
position measurements is affected by the SUM signal level. If SUM is too
low, the measurement error increases due to system offset and gain
errors. If SUM is too high, the sensor and electronics might operate close
to their saturation limits. To warn of this, the SUM bar display turns red
when the SUM value is either below 5% or above 99% of its full range.
