19
K-Cube Position Aligner
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 ‘-2’ 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 ‘4’, that the
true picture becomes apparent.
The sum output signal may be used to provide preliminary alignment of the sensor to
the source light beam. First, the beam position is adjusted for maximum sum output
signal. Second, the beam position is adjusted until the X-Axis (XDIFF) and Y-Axis
(YDIFF) difference signals are at minimum. This procedure results in the beam being
centered on the quad photodiode array. See Section 4.6.3.
for a description of how
this procedure is implemented using the KPA101 position aligner cube.
4.2.4
A Note About Bipolar Voltage Outputs
The KPA101 has a ±10V bi-polar output and is generally used with a KPZ101 piezo
driver which has a 0 to +10V output.
The specified 0..+10V input range for the KPZ101 analogue (SMA) input means that,
if there is no other contribution from the other two controlling sources (i.e. the software
controlled DAC and the potentiometer), then a 0 to +10 Volt input will result in a 0…
full range output voltage. So, assuming that the KPZ101 is used in the 150 Volt range,
the 0.. +10V input corresponds to 0…150 Volt output.
However in the KPZ 101 unit, there are three input sources (SMA, DAC and
potentiometer) that are added together to generate an output voltage. So for example,
if the input contribution of the DAC is +5 Volt and you drive the SMA input with -5 Volt,
then the effect of this is zero output. In other words, fro m an electrical point of view
the input accepts both positive and negative voltages and, as long as their sum is a
positive voltage, the KPZ101 will work correctly.
In a closed loop system, the KPA101 acts like the controller and can output a positive
voltage to increase the output voltage of the KPZ101 (make it more positive), or output
a negative voltage to decrease it. Normally the output of the KPZ101 is offset in open
loop to about mid-range and the KPA101 controls the position by incre asing or
decreasing the KPZ101 output to maintai n a fixed position. This requires a bipolar
output voltage, like that produced by the KPA101.
4.2.5
Beam Size Considerations
The light spot applied to the sensor must be smaller than the diameter of the sensor
photodiode array, and the recommended beam diameters are as follows:
PDQ80A - 1 to 3.9 mm, PDP90A - 0.2 to 7 mm, PDQ30C - <0.5 mm.
A decrease in outpu t signal strength is observed as the light spot crosses the
separation boundary of the sensor arrays, usually referred to as ‘the gap’. This effect
is more pron ounced as the diameter of the li ght spot decreases since a larg er
percentage of the light spot’s power falls within the non-active gap. For this reason, it
is recommended that a lens be used to adjust the size of the beam to fall between the
min and max dimensions quoted above.
