Appendix D
58
HA0165T Rev 4K Sept 2017
However, piezo transducers are unipolar and to achieve a positive and negative
adjustment either side of center, the piezo must be at its center of travel when the
error signal is zero. This is achieved by adding an offset voltage to the error signal.
Now, the range of the piezo is typically 20 to 30 µm, while the fiber is oscillated by 2
or 3 microns. Therefore, it can be seen, that by varying this offset voltage we can
adjust the position at wh ich the oscillation of the fiber take s place - see Fig . D.4
detail A.
Fig. D.4 Offset voltage and fiber position
In reality, the fiber is oscillated both horizontally and vertically, with the modulating
signal for Y phase shifted by 90° with re spect to X - see F ig. D.4 detail B, which
produces a circular oscillation. The detected signal is compared with both the
horizontal and vertical phases of the modulating signal which results in both horizontal
and vertical error signals.
As mentioned previously, the polarity of the error signals corresponds to the relative
position of the two fib ers and consequently, the signals can be used to d rive the
piezos until the fibers are aligned and the error voltages fall to zero.
Once the NanoTrak has found the position of maximum power, if one of the fibers is
moved slightly (due to mechanical creepage or thermal drift), the NanoTrak detects
piezo drive voltage
0.0
75
offset = 37.5V (mid position)
fiber position in X axis (μm)
mid position (37.5V)
increased offset V
time
piezo drive voltage
0.0
75
time
detail A
detail B
x axis
y axis
0
30
X axis (μm)
Y axis (μm)
0
