Research On Spot Center Positioning Algorithms For Quadrant Detector

Quadrant detector （QD） is a kind of position detector which takes the "photovoltaic effect" as the core. Because of its high detection sensitivity, simple signal processing and anti-interference capability, QD has been widely used in areas such as military, mapping, astronomy, communications, engineering and many other fields. There are several location algorithms of four quadrant detector, but in different spot models, its linear measurement range and sensitivity are different. Different requirements of measurement properties were needed in practical application, so to simulate offset curves of the actual spot models under different simulation algorithms and compare the results to find the most suitable algorithm which can meet the application requirements to improve the measurement range and precision of the detection system is an important work. The models of the traditional positioning system are always circular light spot with Gauss or flat energy distribution and the algorithm are add-subtraction algorithm and diagonal algorithm. The use of the hardware to realize the algorithms not only increase the cost and limit the use of the algorithm but also make the measurement system have large error and bad flexibility.This paper mainly studies the spot center location algorithms of four quadrant detector and does the following works:（1） Compared the most common spot center location detectors CCD, PSD, QD and analyzed the advantages and disadvantages of them to provide references for the selection of detector. Introduced the main performance parameters of four quadrant detector and supplied the bases for choosing model of QD in specific application.（2） Combined with the performance parameters of the laser and the four quadrant detector been used, designing an optical system that includes a neutral attenuator, spot compression part （inverted Galileo System） and light spot adjusting part （cylindrical lens）. The optical can get an elliptic light spot with Gauss energy distribution, one of its half axis is1.248592mm （side length of four quadrant detector is6mm）, the other half axis length continuous adjustable. It can be the spot model of algorithm simulation.（3） Studied the add-subtraction algorithm, diagonal algorithm, Δ/∑algorithm and logarithmic algorithm of four quadrant detector. On the basis of the study of the algorithms set up center location algorithm of light spot models whose energy distribution is uniform or Gauss, the shape is circular or ellipse.（4） Discuss the circuit connection methods of add-subtraction algorithm, diagonal algorithm, Δ/∑algorithm and logarithmic algorithm of four quadrant detector. Use the existing hardware system to measuring the positioning data of the circle light spot with Gauss energy distribution, and using the fitting method to get the actual position curves.（5）Suppose the radius of the circle spot is r, in the discussion of the four algorithms, the linear measurement range of add-subtraction algorithm is2r, diagonal algorithm will be expanded to22^1/2r. But still cannot satisfy certain requirements which need large linear measurement range. Use the binary optical element to shape the incident light spot on the photosensitive surface to be square with uniform energy distribution to increase the linear range further. The simulation results show that, using this scheme the linear range is4r, the scheme can get the maximum linear measurement range.