Abstract
In order to meet the urgent need of large-scale search and accurate target recognition， an infrared zoom imaging system with large zoom ratio is developed. Two independently moving zoom lenses and one compensating lens are designed， the large zoom ratio can be obtained by the cascade of two zoom lenses. According to the characteristics of multiple moving lenses and complex zoom curves， the zoom motion is realized by linear motion mechanism， and driven by linear motor integrated encoder and thread screw rod. The mechanical analysis of the system is carried out by the finite element simulation， and the maximum displacement of the lenses is 3.04×1
China is a country with more than 40，000 kilometers of border and coastal defense lines and 1.4 billion people. It is under great pressure in various fields such as aerospace， military defense and counter-terrorism. As a passive detection method， an infrared imaging system has the advantages of strong anti-interference ability， good concealment， all-weather work and so on， and has played an increasingly important role in these fields. It is also being used more and more widely in the field of civil surveillance identificatio
In 2004， Yoram A et al. designed a 30 times continuous zoom optical system through the thrice-imaging structur
According to the results of literature investigation， the research results of infrared continuous zoom imaging system with large zoom ratio， especially the imaging system and the imaging effect of the large zoom ratio are less public. Compared with a few available infrared zoom imaging systems， the zoom ratio is not high and most of the system structures are complex and cumbersome. In this paper， two zoom lenses and one compensating lens were innovatively designed for zoom motion. At the same time， according to the characteristics of many moving lenses and complex zoom curve， the linear motion mechanism was used to realize the zoom motion of the lenses. The linear guide was used as the motion support and the linear motor was used as the drive， which made the system structure simple， the imaging quality high and realize self-locking in the vibration environment. The infrared zoom imaging system developed in this paper has a range of continuous zoom imaging from 6 mm to 330 mm and a zoom ratio of 55 times. The results of laboratory imaging and outfield imaging show that the system can realize infrared continuous zoom imaging from short focal length to long focal length， and the imaging effect is good.
In order to realize the infrared continuous zoom imaging with the large relative aperture and large zoom ratio， the optical model innovatively designed two independently moving zoom lenses and one independently moving compensation lens based on the classical four-component mechanically compensated zoom syste
Fig. 1 The optical structure of the infrared zoom lens
图1 红外变焦镜头光学结构
In the actual alignment， the zoom distance had been adjusted according to the machining and assembly errors. The zoom motion curves are shown in
Fig. 2 The zoom motion curves
图2 变焦运动曲线
The traditional zoom system usually uses a zoom cam to move the zoom lens and compensation len
In recent years， the linear motion technology has been greatly developed， and the accuracy and integration of linear guides and linear motors have been greatly improved. In addition to the application of ball screw， there are also productized linear motors with integrated encoders and thread screw rod
The structure principle of the infrared zoom imaging system with large zoom ratio based on the linear motion mechanism is shown in
Fig. 3 The principle of the system structure
图3 系统结构原理
The zoom ratio of the infrared continuous zoom imaging system with large relative aperture and large zoom ratio was 55 times， the F number was 2， and the first front fixed lens was the largest lens with an aperture of 168 mm. The system detector was an area-array cooled infrared detector. The imaging system parameters are shown in
| Parameter | Indices |
|---|---|
|
Zoom range Imaging field angle |
6-330 mm 91.4º-2.1º |
| Detector pixel number | 640×512 |
|
Detector pixel size Band range |
15 μm×15 μm 3.7-4.8 μm |
The system optical path diagram is shown in
Fig. 4 The system optical diagram
图4 系统光路图
The system used the linear motor as the drive， the photoelectric switch as the limit position feedback， and the cylindrical linear guide rail as the linear motion support. The system overall dimension was 622 mm （length） × 360 mm （width） × 222 mm （height）. The overall structure of the imaging system is shown in
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Fig. 5 The general structure of the imaging system, (a) the imaging system model, (b) the front view of the imaging system, (c) the cutaway view of the imaging system
图5 成像系统总体结构,（a）成像系统模型，（b）成像系统前视图，（c）成像系统剖视图
Zoom lens seat 1， the compensating lens seat and zoom lens seat 2 each installed a limit plate. When the zoom motion made the system in the long focal length state， zoom lens seat 1 and the compensation lens seat were limited by the front limit seat， and zoom lens seat 2 was limited by the rear fixed imaging lens seat. At this time， limit disks mounted on zoom lens seat 1， the compensation lens seat， and zoom lens seat 2 occluded the detection groove of the photoelectric switch to generate a signal， and the control system took this state as the system zero. The zoom imaging system drove zoom lens 1， the compensation lens and zoom lens 2 to perform the continuous zoom motion at the initial zero position. The high-precision position feedback was achieved through the linear motor encoder， and the imaging system had infrared zoom imaging capability with large zoom ratio.
Because the distance between the lenses in the long focal length and the short focal length states in the optical zoom structure was very small， the structure design of lens seat was that the lens was mounted in front of the supporting structure， so that the two ends of the lens seat could be installed suitable supporting surface length linear bearings， thus ensuring the motion stability of the system zoom structure. The zoom motion mechanism needed to ensure the concentricity between the lenses during the zoom process. The linear guide rail and the inner ring of the linear bearing were matched with a small gap within 0.005 mm at the bearing fit， and the outer ring of the linear bearing and the lens seat were matched with a small gap within 0.005 mm to prevent the bearing from the radial moving. The system used the custom linear motor that integrated the encoder and the thread screw rod， and the axial displacement of the thread screw rod was eliminated by applying the pre-tightening method of the eliminate clearance nut. The minimum thrust of linear motor was 3.5 kg， the step length was 0.006 35 mm， and the step angle was 1.8°.
The imaging system as a whole model was analyzed by finite element simulation analysis. The model was simplified in detail without affecting the simulation results， and the matching relationship was added to the model. Material parameters such as alloy steel， aluminum alloy and optical material were added to the parts， and the fixed constraint was added at the bottom of the system. The gravity load was added to the model， and the motor thrust was added to zoom lens seat 1， the compensation lens seat and zoom lens seat 2 respectively. The zoom imaging system would be mounted on a two-dimensional turntable for imaging， and would be affected by the pitch moment T1 and the azimuth moment T2. The pitch moment of inertia J1 was 1 kg×
| ， | (1) |
| ， | (2) |
T1 and T2 were exerted to the simulation model， and the stress and displacement simulation analysis were carried out after meshing， as shown in
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Fig. 6 The finite element analysis of the system structure, (a) the mesh generation, (b) the structural stress, (c) the structural displacement
图6 系统结构有限元分析,（a）网格划分，（b）结构应力，（c）结构位移
The infrared zoom imaging system with large zoom ratio based on the linear motion mechanism designed in this paper had been actually processed and assembled， the cross wire imaging was used for infield test after optical calibration with a two-dimensional turntable in the laboratory. The infrared zoom imaging system continuously performed continuous zoom imaging on the cross wire of the optical calibration experiment， and we compared the imaging images of the focal length of 6 mm， 40 mm， 80 mm， 120 mm， 168 mm， 210 mm， 250 mm， 290 mm and 330 mm. The imaging results are shown in
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Fig. 7 The laboratory imaging of the imaging system, (a) the laboratory imaging, (b) focal length 6 mm, (c) focal length 40 mm, (d) focal length 80 mm, (e) focal length 120 mm, (f) focal length 168 mm, (g) focal length 210 mm, (h) focal length 250 mm, (i) focal length 290 mm, (j) focal length 330 mm
图7 成像系统实验室成像,（a）实验室成像，（b）焦距6毫米，（c）焦距40毫米，（d）焦距80毫米，（e）焦距120毫米，（f）焦距168毫米，（g）焦距210毫米，（h）焦距250毫米，（i）焦距290毫米，（j）焦距330毫米
After the system imaging verification was completed in the laboratory， the imaging system was mounted on a high-rise rooftop for outfield infrared zoom imaging of distant urban buildings. The imaging results are shown in
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Fig. 8 The outfield imaging of the imaging system, (a) the outfield imaging, (b) focal length 40 mm, (c) focal length 168 mm, (d) focal length 250 mm, (e) focal length 330 mm
图8 成像系统外场成像, （a）外场成像，（b）焦距40毫米，（c）焦距168毫米，（d）焦距250毫米，（e）焦距330毫米
It can be seen from the above， the imaging system can realize the infrared continuous zoom imaging from the large field of view with the focal length of 6 mm to the small field of view with the focal length of 330 mm， with clear imaging and good image quality.
In this paper， a large zoom ratio infrared continuous zoom imaging system with a focal length from 6 mm to 330 mm is designed. The F number of the system is 2， and the zoom ratio is as high as 55 times. It is suitable for medium-wave infrared cooled 640×512 focal plane area-array detector. The large zoom ratio was obtained by cascading two zoom lenses. In view of the fact that there were three moving lenses and unsmooth zoom curves due to the large zoom ratio， we used the linear guides as the motion support and the linear motors as the drive， and innovatively used the linear motion mechanism to complete the zoom motion of the lenses. In the zoom system with the linear motion mechanism， the problem of concentricity and positioning accuracy in lens linear motion needs to be mainly solved. According to the results of laboratory imaging and outfield imaging， the system has clear imaging and good image quality in the process of continuous zoom， which proves that the system design is reasonable and reliable.
The research findings of this paper can be applied to the new infrared search and tracking system. Through the linear motion technology， the complex and even reciprocating zoom motion of multiple moving lenses can be realized， so as to realize the continuous zoom imaging with large zoom ratio. In the large field of view imaging， a short time airspace coverage can be realized， and a wide range of scenery information can be obtained. At the same time， it can obtain long-distance detection ability in the small field of view imaging and improve the resolution of key targets. It has broad application prospects in search， tracking， reconnaissance， and surveillance.
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