Research On Tracking And Scanning Characteristics Of Servo System Based On Dual Liquid Crystal Polarization Gratings

The dual liquid crystal polarizing gratings can realize the beam deflection control by co-axial independent rotation,which has good beam pointing performance and good scanning dynamics.The beam pointing control mechanism has compact structure,low power consumption and fast response,which can realize a wide range of beam deflection control.It is an important research topic to explore the intrinsic relationship between the rotation angle of the two liquid crystal polarizing gratings and the pointing position of the emitted beam.The application of target tracking based on dual liquid crystal polarizing gratings requires the establishment of a target tracking device model based on dual liquid crystal polarizing gratings,and the relationship between the miss distance,the target position,and the rotation angle of liquid crystal polarizing gratings needs to be obtained,and the tracking process of the target needs to be described using the given relationship.Since the relationship between the pointing of the outgoing beam and the position of the two gratings rotation angles is nonlinear,resulting in a nonlinear relationship between the steering rate of the beam,i.e.,the angular velocity of the beam steering,and the grating rotation speed.Therefore,it is necessary to analyze the nonlinear range of the dual liquid crystal polarization gratings when it is applied to the beam scanning field,and this topic is centered on the above issues in a progressive manner.For this reason,this thesis carries out a study of the servo system tracking and scanning characteristics based on dual LCPPGs,and the main research contents are as follows.(1)In using a rotating dual liquid crystal polarizing gratings for beam scanning,this paper developed a geometric model based on the rotating dual liquid crystal polarizing gratings to describe the mechanism of the dual liquid crystal polarizing gratings operation.Since there is a nonlinear relationship between the outgoing beam steering and the grating rotation angle,this paper defines the beam steering rate as the beam steering angle divided by the beam steering time.The study of scanning characteristics is based on the angular position of the two grating sets solved by the inverse of the beam pointing,and the directional derivative is used to express the ratio of the calculated grating speed to the beam steering rate along the direction of the outgoing beam movement,to analyze the grating driving and control requirements for different initial pointing of the outgoing beam moving in different directions,and to be able to give its observation field center and edge singularity range according to the given grating control parameters.The results of the simulations show that for a defined rotating dual-grating beam scanning system,the grating rotation drive and control requirements for the outgoing beam steering process depend only on the current outgoing beam pointing deflection angle and the angle between the target moving direction and the radial direction;the outgoing beam moving in the tangential or radial direction will lead to grating rotation control singularities for the outgoing beam in the center and edge regions of the system observation field,respectively;there is no significant difference in the requirements of the dual grating rotation drive and control for the outgoing beam based on the two sets of inverse solutions.This paper introduces two new liquid crystal polarization gratings based on the rotating dual grating,discuss the beam scanning method that can be achieved before and after the introduction of liquid crystal polarization grating three and liquid crystal polarization grating four,verify the availability of the four-grating beam scanning pointing and describe the advantages of the beam scanning method based on the rotating four liquid crystal polarization gratings.(2)This paper proposes for the first time the relationship between miss distance,target position,and rotation angle of liquid crystal polarization grating,give the derivation process of this relationship,introduce the incident vector into the direction cosine of the outgoing beam passing through the dual rotating liquid crystal polarization gratings at vertical incidence,establish a target tracking system based on dual rotating liquid crystal polarization gratings,and describe the closed-loop target tracking process based on this relationship;this paper establish the optical structure of the dual rotating liquid crystal polarization gratings,and simulate and compare the difference of the deflection angle and azimuth angle between the target position derived from the proposed formula and the target position derived from the ZEMAX simulation.In the simulation verification,this study compares and analyzes the proposed new formula with the formula of vertical incidence to show the necessity of introducing the miss distance in the relation.In this paper,the coarse tracking device of the target was assembled,the tracking process based on the proposed formula was designed and implemented,and the coarse tracking accuracy of the system by processing the collected experimental data was obtained.The obtained results satisfy our proposed technical specification of less than or equal to 0.45 mrad,which can provide a reference for researchers in related fields.