BEAM POSITION DETECTOR
BPD-8L
Using the BPD for Position Sensing:
Quadrant photodetectors can be used for high-
resolution position sensing of light beams. They
provide very accurate position measurement using
simple circuitry.
When a light beam of a proper size illuminates a quad
photodiode, it generates a photocurrent in each
quadrant. By comparing those photocurrents, a
change of a position in two axes can be detected.
Figure 1 shows a typical quad-cell with a beam
illuminating its surface.
Figure 2
Beam size too small
Figure 3
Beam size too large
1
2
4
3
y
x
Figure 1
The X and Y axis displacements of the beam relative
to center of the quad cell can be calculated using the
following formulas:
(i
1
+i
2
)−(i
3
+i
4
)
X=
Y=
i
1
+i
2
+i
3
+i
4
(i
i
1
+i
4
)−(i
2
+i
3
)
1
+i2
+i
3
+i
4
The accuracy of measurement depends on the beam
size and the gap between the segments. To optimize
the BPD’s performance, the following design rules
should be observed:
•
The beam diameter must be bigger than the
gap between the segments, such that when
the beam is positioned in the center of the
quad photodiode, it will still illuminate part of
the active area. See Figure 2.
•
•
The beam area cannot exceed the total
active area of the detector in order for the
displacement of the beam to produce a
change in photocurrents. See Figure 3.
The light beam spot size is proportional to the
detection range, and inversely proportional to
the detection resolution. Figure 4 shows the
positional signal vs. beam position for three
different beam sizes. As the beam size
becomes smaller, the slope of the positional
signal becomes steeper, which leads to a
higher position detection accuracy.
•
Linearity of the sensor output is proportional
to the uniformity of the light beam.
2
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