Position sensing photodiodes are detectors that provide quantitative information about the position of an incident light beam. Laser Components offer a variety of position sensors from component level, to working alignment systems for distance measurement. The position sensor offers a variety of exciting applications from triangulation of distance; angle measurement; to three dimensional vision systems. Applications include:
- Location alignment or centring of components
- Position and motion measurement systems
- Non-contact distance measurement system (triangulation)
- Alignment and surface level measurement
- Optical spectrum analyser
- Three-dimensional machine vision (contour mapping)
- Angle measurement system.
They are two types of position detector offered by Laser Components, multi element photodiodes and the Position Sensing Photodiode (PSD):
Multi element photodiodes are detectors that consist of more than one active area. The most common formats are the bi-cell and quadrant detector (quad cell). Bi-cells are traditionally used to measure light across a barrier in the manufacturing process or in inject printers to detect when paper is in the correct position. Illuminating both detectors with an LED one can measure a drop in signal as one of the sensors is covered up. In the case of a quadrant cell the centroid of an incoming light beam may be determined by comparison of the signals from the four quadrants. In this case the 'x' and the 'y' positions are given by the following equations.
There are several factors that must be considered when designing a system based on a quad cell; the most important is the diameter of the light spot when compared to the detector. Too small and the light spot will be lost between the spaces between the detector elements, whereas too large, and the signal will be truncated by the edges of the detector known as the dead region, resulting in erroneous position feedback. Another spatial consideration is that the response to movement of a circular spot is non linear, because the ratio of the spots movement to the percentage of its area is non linear. Segmented detectors are best used for nulling and centring devices, for such applications repeatability of 0.1µm has been routinely demonstrated.
Position Sensitive Detectors (PSDs) differ to quadrants and bi cells in that they are a single silicon photodiode - there is no dead zone between elements. A PSD derives position by dividing photo-generated electrons within their substrate not by profiling intensity distribution as multiple element photodiodes do. PSDs measure the photometric centroid of the incoming light beam.
A 1-dimensional PSD detects light moving in just one dimension. The device has 3 terminals, 1 on the rear side and two at the front. The photoelectric current flows through the device and can be seen as a pair of currents Y1, and Y2. The relationship between these two output currents is given by:
where L is the length of the PSD.
A 2-dimensional PSD is available in two forms, duo-lateral and tetra-lateral. The duo-lateral type has electrodes on both the front and rear surfaces. Each signal is divided into two parts by two resistive layers. For a tetra-lateral detector have four elements on the front surface of the detector. Consequently the total induced photo current is divided into 4 parts by the same resistive layer. Compared to duo-lateral detectors tetra- lateral detectors are more non-linear from the centre of the detector, however, they produce a faster response with less dark current. The photoelectric current can be seen as two input currents, X1 and X2 and two output currents, Y1 and Y2. Position (x,y) of the beam is given by:
where Lx and Ly is the length of the PSD in x and y directions respectively.
Using a PSD it is possible to measure the x and y position of an incident beam of light over a greater range and it can measure the incident beam right out to the detector's edges. Another advantage over the quadrant or bi-cell detector is the position of the resultant beam being insensitive to light intensity or beam shape. This is because they derive the position of the photometric centroid of the light beam. The resolution is much lower, because of the larger signal to noise ratio from the larger substrate. Thus for centring applications the resolution of a quadrant cell is much higher where as a PSD offers a wider distance range.
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