THORLABS PDP90A, Руководство пользователя

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Lateral Effect Position Sensor Chapter 2: Description
17149-D02 Page 4
2.2.1. Overview
2D lateral effect sensors accurately measure displacement - movements,
distances, or angles – providing feedback from systems that require precise
alignment, such as steering mirrors, microscope focusing, and fiber launch
systems. These sensors are light-controlled variable resistors that work by
proportionally distributing photocurrent to determine position in the X and Y from
the four electrodes when incident light is applied to its surface.
In addition, the PDP90A is a continuous position-sensing detector, which derives
position by dividing photo-generated electrons within their substrate, rather than
by profiling the intensity distribution on the surface as with segmented detectors.
There are two different types of lateral effect sensors available: duo-lateral and
tetra-lateral.
Duo-lateral sensors provide a resistive layer on both the anode and cathode
photodiode connections. This isolates the X and Y positional information of the
sensor and allows the sensor to be highly linear and very accurate. However, the
added resistive layers significantly increase manufacturing cost.
Tetra-lateral sensors use a single resistive layer with a common cathode and an
anode on each side of the detection area. This makes them very inexpensive to
manufacture, but their linearity decreases as the spot moves away from the
center. This is caused by the physical location of the anodes along the sides of
the sensor, specifically in the corners where the anodes approach each other.
Thorlabs’ PDP90A detector uses a variation of the tetra-lateral-type sensor
commonly called a “pin cushion" position-sensing device. This sensor moves the
anodes to the four corners of the sensors and reshapes the sensor area, to
produce linearity comparable to a duo lateral sensor at a significantly lower cost.
The tetra-lateral-type, “pin cushion" has a characteristic form of which we will
used some important values to determine further calculations. The active area is
9x9 mm at the waist of the pincushion; however, at the outer most regions the
area goes to 10mm, therefore the calculations are performed from the widest part
of the sensor.
2.2.2. Calculating the Position
The PDP90A includes circuitry to decode the D
x
, D
y
, and SUM signals as follows
(see Figure 1 ):
∆
∆
Note: A, B, C and D are four electrodes on the upper surface, formed along each
of the four edges