Position Sensitive Device

A Position Sensitive Device and Position Sensitive Detector (PSD ) is an optical position sensor ( OPS), which can measure the one - or two-dimensional position of a light spot.

Principle

PSDs work according to different principles. Basically, you can divide them into two classes: first, the analog sensors, which have an isotropic sensor surface and provide continuous position information to other discrete sensors whose surface is a grid-like structure and provide therefore a discrete location information.

Analog sensors

In the industrial application is generally referred to with the term PSD the design principle, the (usually a PIN diode ) is used, a plated on two opposite sides the top electrode of a large area photodiode. For this purpose, the further interpretation of the acronym PSD ( position sensitive diode) is in use.

The top electrode has a relatively high, uniform sheet resistance. The diode is exposed dot-shaped. In the range of the exposure results in a photo current which flows depending on position of the light at a certain ratio on the lying at the edges of contacts. From the currents, and the location of the exposure can be calculated from the following formulas.

And

In this case, and easy scaling factors that enable an approximate coordinate transformation metric.

Advantage of the method is the continuous measurement of the position with sampling rates up to more than hundred kilohertz. The position measurement is substantially independent of the size of the light spot.

A disadvantage is the dependence of the position measurement of the shape of the light spot and the non-linear relationship between currents and place that can be compensated by special electrode forms part.

Discrete sensors

Serial evaluation

The most widespread principle of the sensor for applications with less than 1 kHz sampling rate, the CCD or CMOS camera. The sensor is with them, divided into individual fields, called pixels, the exposure value can be read out sequentially. The position of the light spot can be calculated using the methods of photogrammetric image measurements directly from the brightness distribution.

Parallel evaluation

For faster applications matrix sensors were developed with parallel evaluation. This is compared both row-wise and column-wise the light intensity of each pixel with a global threshold. The comparison results are in rows and columns associated with a logical OR. For all columns and all rows that contain an element that is lighter than the predetermined threshold (pictured in red) the average of the coordinates is calculated and output, respectively: This is the sought- coordinate of the point of light.