Fast Passive Auto-Focus For Opto-Electrical Theodolites

The opto-electrical theodolite is a large ground-based surveying equipment,which can realize the real-time tracking and recording of the target's image and the azimuth and elevation angle at the same time.It is widely used in surveillance of the moving targets in the fields of aerospace,defense and scientific research.And a fast and accurate automatic focusing(AF)control is the key to ensure the clear imaging of the theodolite.Generally,the AF system in most theodolites is based on distance-temperature detection.It is an active approach,which needs a laser rangefinder(LR)to measure the distance between the target and the theodolite.Once the measurement fails,it will not be able to perform autofocus.Besides,the ranging signal is likely to be sensed by the target,for instance,a hostile aircraft.In recent years,passive AF methods based on image processing have been studied extensively.They have the advantages of low cost and high precision,but it always comes to a dead end when seeking for high speed.Therefore,the following research work has been done to improve the speed and accuracy of the AF system of opto-electrical theodolites.First,a defocus model of optical system is set and factors are analyzed for bringing defocusing to it.Three different optic-based AF principles have been studied.On this basis,a phase difference focus detection system based on a dual-wedge prism is designed.Compared with the conventional micro lens setup,this design has a higher sensitivity,simpler structure and easier processing and alignment.This optical approach is fast and direct,which meets the requirement of theodolite's AF system.Then,a thorough investigation of image-based autofocus has been done.Via massive theoretic analysis and experimental verification,a space domain evaluation function called SML is selected for the image-based AF of theodolite.This function is of good efficiency and high sensitivity.Combining with the curve fitting search strategy,it is suitable for the accurate automatic focusing of the theodolite.Afterwards,a hybrid method combining optic-based AF and image-based AF is proposed to achieve fast and accurate autofocus for opto-electrical theodolites.In this approach,the Phase Difference Autofocus(PDAF)provides a fast initial focus.Then,by analyzing the images nearby using Depth from Focus(DFF),the best focus can be achieved.At last,based on the structure of a modern theodolite,a passive autofocusing prototype with a focal length of 1000 mm and a diameter of 200 mm is designed,fabricated and an experimental tested.The autofocusing experiment is conducted in the laboratory.A collimator is used to simulate the targets of different distances and a computer is employed to collect and process images.The results show that the prototype can achieve the best focus in 4 steps with a precision of 0.02 mm,which is significantly faster than the existing AF system based on image processing.In this work,the principle,structure,and performance of passive AF system of opto-electric theodolites are systematically investigated.A new hybrid AF method is proposed,and the system design and experimental verification are carried out.This research may provide a theoretical or practical reference to the AF system design of the opto-electrical theodolites in the future.