Position sensitive device

A position sensitive device or position sensitive detector (PSD) is an optical position sensor (OPS) that can measure the one or two-dimensional position of a point of light.

Principles

PSDs work on different principles. Basically, these can be divided into two classes: On the one hand, the analog sensors, which have an isotropic sensor surface and provide continuous position information, and, on the other hand, discrete sensors, whose surface is structured like a grid and which therefore provide discrete location information.

Analog sensors

Basic structure of a PSD based on a pin diode

In technical applications, the term position sensitive device (PSD) usually stands for large-area photodiodes (mostly a pin diode ) that are contacted on two or four opposite sides of the cover electrode . The further interpretation of the abbreviation PSD as position sensitive diode is used for this .

The top electrode has a comparatively high, uniform sheet resistance . The diode is exposed point-like. In the area of exposure, a photocurrent is generated which, depending on the position of the light, flows off in a certain ratio via the contacts a – c or b – d located opposite at the edges. From the currents , , and the location of the exposure can be calculated from the following formulas. ${\ displaystyle I_ {a}}$ ${\ displaystyle I_ {b}}$ ${\ displaystyle I_ {c}}$ ${\ displaystyle I_ {d}}$

{\ displaystyle x = k_ {x} \ cdot {\ frac {I_ {b} -I_ {d}} {I_ {b} + I_ {d}}}}

and

{\ displaystyle y = k_ {y} \ cdot {\ frac {I_ {a} -I_ {c}} {I_ {a} + I_ {c}}}}

Are and scaling factors that z. B. allow an approximate transformation into metric coordinates. ${\ displaystyle k_ {x}}$ ${\ displaystyle k_ {y}}$

The advantage of the method is the continuous measurement of the position with measuring rates of over one hundred kilohertz. The position measurement is largely independent of the size of the light point. In addition to the position, the intensity of the signal can also be measured. For example, carrier frequencies or information can also be transmitted.

Disadvantages are the dependence of the location measurement on the shape of the light point and the non-linear relationship between currents and location, which can be partially compensated for by special electrode shapes.

The active area can be 2 x 2 mm to 45 x 45 mm.

Discrete sensors

Serial evaluation

The most widely used sensor principle for applications with a measuring rate of less than 1 kHz is the CCD or CMOS camera. The sensor is divided into individual fields, so-called pixels , whose exposure value can be read out sequentially. The position of the light point can be calculated directly from the brightness distribution using the methods of photogrammetric image measurement .

Parallel evaluation

Functional principle of a discrete PSD according to ( Lit .: Massari) with parallel evaluation. The yellow circle marks the illuminated area.

Matrix sensors with parallel evaluation have been developed for faster applications. The illuminance of each pixel is compared with a global threshold value, both in rows and in columns. The comparison results are linked in rows and columns with a logical OR . From all columns and all rows that contain an element that is lighter than the specified threshold value (marked in red in the picture), the mean value of the coordinates is calculated and output: This is the searched coordinate of the light point.

literature

Anssi Mäkynen: Position-Sensitive Devices and Sensor Systems for optical Tracking and Displacement Sensing Application. Dissertation, Faculty of Technology, University of Oulu, 2000, ISBN 951-42-5780-4 ( abstract and PDF ).
Nicola Massari, Lorenzo Gonzo, Massimo Gottardi, Andrea Simoni: High Speed Digital CMOS 2D Optical Position Sensitive Detector. In: Proceedings of the 28th European Solid-State Circuits Conference. ESSCIRC 2002. 2002, pp. 723-726 ( PDF ).
JT Wallmark: A New Semiconductor Photocell Using Lateral Photoeffect . In: Proceedings of the IRE . tape 45 , no. 4 , 1957, pp. 474-483 , doi : 10.1109 / JRPROC.1957.278435 .

Web links / individual references / footnotes

Position measurement methods - PSD and CCD in comparison (PDF file; 152 kB)

↑ Note: Solar cells and other photodiodes, on the other hand, should have the lowest possible surface resistance of the transparent cover electrode
↑ Hamamatsu Photonics data sheet PSD type S1880 / S2044

[1] Note: Solar cells and other photodiodes, on the other hand, should have the lowest possible surface resistance of the transparent cover electrode

[2] Hamamatsu Photonics data sheet PSD type S1880 / S2044