Disordered multi-target scanning is an important application scenario of lidar target tracking technology. However, when dealing with scanning beam paths steered by cascaded prisms, it presents notable challenges, including highly non-linear and strongly coupled characteristics between the output vectors and prism rotation angles. Additionally, due to prismatic independence, there is no available coordinate system for visually observing the state of multiple prisms and the transitions between these states. In this work, we propose a rotation prism coordinate system, which offers coordinate conversion methods and mathematical expressions for distances. Two scanning modes, the fastest mode, and the shortest mode, along with two criteria, the total distance difference, and path curvature, are proposed for facilitating path selection and adhering to physical restrictions. To tackle the equivalent point-traveling salesman problem within this coordinate system, we employ an ant colony-taboo fusion algorithm. Our experiment results demonstrate a substantial improvement in efficiency, with up to a 2.77-fold enhancement compared to the original technique. This approach presents a promising solution for addressing the complexities of multi-target scanning with cascaded prisms in lidar target tracking. Future applications include environmental detection, rapid scanning and mapping, and identification of assembly parts in chaotic scenarios.
We present a virtual multiview imaging method that incorporates a pair of Risley prisms and a stationary camera coaxially mounted. By rotating double Risley prisms, the virtual array camera (VAC) is generated to capture multiframe images from different perspectives for three-dimensional (3D) reconstruction, which not only extends the field of view (FOV) but also generates rich point cloud data. The VAC imaging process, including the virtual camera imaging theoretical model, automatic virtual optical center calibration, point cloud data acquisition, and target pose estimation, is developed to enable virtual camera field construction. Moreover, an improved iterative closest point registration algorithm by combining the overlapping prepositioning method is proposed to improve the speed of multiframe point cloud stitching. The experiments on real data validate the feasibility and flexibility of our 3D imaging technique capable of realizing large-FOV 3D imaging and accurate pose estimation of the target, which demonstrates a promising application prospect in visual imaging fields.
In this paper, the position error modeling of the three-element Risley-prism beam scan system is established and the exact expressions of the beam pointing errors are derived by ray tracing based on the Snell’s law. The impacts of the tilt error of three-element prisms and the tilt error of the bearing position on the beam pointing accuracy are graphically presented with analytical and numerical results. According to the given pointing accuracy requirements, the allowable limit values of assembly errors are calculated, which is of practical significance for improving the design level of threeelement prisms. The analysis method in this paper is universal for the analysis of the assembly error of other cascade prisms.
This paper presents a passive target tracking method based on the vision information feedback from a variable boresight imaging system. Firstly, the variable boresight imaging system is modelled to demonstrate the adjustment of a camera boresight by Risley prisms. The passive tracking method is then introduced to determine the relative position of dynamic target without any prior information, which can serve as the feedback to steer the camera boresight towards the target of interest. Moreover, an experiment setup is designed with both hardware and software in order to evaluate the feasibility and accuracy of the proposed target tracking method.
Laser Doppler vibrometry (LDV) is a developing and potential vibration measurement technology. This paper gives a review on the fundamental principles and the essential techniques of LDV, put emphasis on comparing and analyzing between scanning laser Doppler vibrometry (SLDV) and continuous scanning laser Doppler vibrometry (CSLDV). Then some crucial issues including the analysis on vibration errors and the required solutions are proposed and commented. Finally, some typical examples applied in the fields of industry, agriculture, medicine, construction and aerospace are introduced to demonstrate the research progress as well as development trend of LDV.
The paper overviews the advantages and disadvantages of various inverse solutions methods for the Risley-prism-based scanner. According to some given target trajectories, the calculation precisions and calculation consuming time by four different inverse solution methods are investigated, respectively, which reflect the computing complexities and condition applicability. Meanwhile, the corresponding scanning precisions for the same target trajectory by four methods are compared to each other. The paper overviews the limits of different inverse solution methods and evaluates different inverse solution methods for specific beam scanning applications.
A pair of wedge prisms can be used to scan a laser beam over a specified angular range with a high resolution. The structural parameters of dual prisms directly influence the optical path and generate a tracking trajectory difference. For a remote target, the influences can be ignored, but for a near field target, the opposite is the case. The paper makes a comparison of rotating dual-prism scanner used in near and far field, as well as a forward solution and a reverse one of a laser beam through the rotating dual-prism system. The conclusion is valuable for a rotating dual-prism scanner to perform the optical tracking and targeting direction.
The paper reviews the recent research progresses of vision measurement. The general methods of the depth detection
used in the monocular stereo vision are compared with each other. As a result, a novel bifocal imaging measurement
system based on the zoom method is proposed to solve the problem of the online 3D measurement. This system consists
of a primary lens and a secondary one with the different focal length matching to meet the large-range and
high-resolution imaging requirements without time delay and imaging errors, which has an important significance for the
industry application.
Lens molding has become the promising technique to conduct mass produce of aspheric glass lens. It overcomes some
disadvantages of traditional grinding or turning methods, such as high cost, low efficiency, unstable accuracy, and so on.
Up to now, the lens molding process has been looked on as one of the reliable methods in fabrication of aspheric glass
lens. However, in real production, one has found that it’s hard to control the molding parameters, e.g. molding
temperature, molding period, molding speed and pressing pressure, etc. Therefore it’s necessary to develop the specific
molding processes for a certain glass material. In this paper, SCHOTT P-SK57 is adopted to carry out the lens molding
analysis in order to achieve the relative processing parameters. The molding cases are analyzed based on three different
temperatures of 510°C, 520°C, and 530°C, higher than transition point 493°C of P-SK57. Through continuous heating
and pressing simulation, the results show that the best pressing temperature could be about 530°C, at which the residual
stress is only 5.22MPa (with the molding speed of 0.1mm/s).
The paper overviews various supporting ways for large-size movement mirrors, and the
advantages and disadvantages of the support methods are summarized. Some valuable
optimization methods to improve support effects are also introduced. As a case study, a radial
segment-face contact support method is proposed to solve the support problem for a large-aperture
rotating prism, and a two-step optimization method is implemented to improve the support effects.
The surface deformations under different support separated angles are evaluated. The overview
can be as good references for large-size mirror support design in similar opto-mechanical systems
especially under movement conditions.
Success of optical measurement methods such as interferometry, holography and fringe projection methods
mainly depends on the availability of robust phase recovery (PR) algorithms. In presence of some problems such as
strong noise, abrupt phase changes, low fringe contrast and discontinuities, it’s necessary to introduce a highly
reliable algorithm to overcome them. As for the experimental phase map from holographic interferometry of China
coin’s micro-surface, some typical PR algorithms including the proposed (weighted) DCT (Discrete Cosine
Transform) are studied to try to recover the satisfactory continuous phase. The experimental phase map
(13921040 pixels) exhibits the local surface characteristic of one China coin. It features excessive noise and many
bad data points. It’s very difficult to achieve the continuous phase by the general algorithms. So some robust
algorithms are studied to solve this problem. These algorithms include the quality map guided method (QG),
(weighted) least square method (LS or WLS), mask cut method (MC) and (weight) DCT method (DCT or WDCT).
According to the tested results, the fastest algorithm is the LS method, and it only needs about 10s. The clearest
pattern profile results from the DCT and WDCT algorithms that need about 30s and 31s in the consuming time,
respectively. However, it’s very hard to clearly and completely reflect the smooth top of imprinted letters for all the
above algorithms. This could be due to the excessively much noise, the limited resolutions of CCD setup, and so
on.
A novel measurement of laser coarse-fine coupling tracking is proposed for robot trajectory errors, which can not only
meet the requirements of large range, rapid response and dynamic tracking, but also achieve the high accuracy of
submicroradian magnitude. The mathematic model of robot parameters is deduced according to the motion definition. An
experiment platform together with the test system is built to complete the robot trajectory test. The circular and linear
trajectory, as well as the harmonious motion parameters, is tested respectively. Some error factors affecting the test
uncertainty are given to be considered according to the test experiment results.
The laser tracking measurement for space moving targets is the research hotspot in the precision measurement field,
which has attracted many researchers' interests for many years. The tracking mechanism characterized with the
performance of high accuracy and rapid response is the key to solving the dynamic measurement problem. In the paper, a
project with one prism rotating around the rotating axis to attain the linear tracking is proposed, which can not only meet
the requirements of the large range, rapid response and dynamic tracking, but also achieve the high accuracy of
submicroradian magnitude due to the particular opto-mechanical design. Steering the tracking mechanism can well track
the linear motion of space dynamic or static target. In order to study the above system, the technical parameters of the
mechanism are firstly designed, and then the model analysis and simulation calculation is respectively carried out.
Therefore, a new measurement is finally brought about, which points outs a solution for the measurement of space
moving target with large range and high accuracy. The design results show the tracking accuracy is better than 0.5 μrad
within the tracking angle range.
Laser beam angular scan accuracy measurement plays an important role in many fields. A sub-microradian laser beam scan precision measurement method that utilizes the wavefront interference method with a Zygo interferometer is proposed. The experimental results show that the measurement accuracy is approximately 0.04 µrad, and the new method is very effective and feasible.
The original scanner of tilting orthogonal double prisms is studied for testing the tracking performance in inter-satellite
laser communications. Two prisms respectively rotate around the horizontal axle and the vertical one within the
admissible range to determine the corresponding orientation and position of the passing beam, therefore the high
accuracy deviation angle of passing beam can be performed. The test experiments performed with autocollimator and
interferometer, as well as the theoretical analysis, indicates that the scanner can meet the requirements of the deviation
accuracy superior to 0.5 μrad with the deviation range greater than 500 μrad, which accords to our design requirements.
KEYWORDS: Acquisition tracking and pointing, Receivers, Satellites, Laser communications, Collimators, Near field optics, Transmitters, Optical scanning, Optical simulations, Near field diffraction
The propagation of laser beam in inter-satellite laser communications belongs to the far-field diffraction, but in the optical test and verification of pointing, acquisition and tracking (PAT) function on the laboratory the beam from a terminal propagates within the near-field. In this paper, in terms of the Fresnel diffraction theory the inherent difference is found that in the far-field diffraction the optical tracking position error is resulted from both the mutual movement between two laser communication terminals and the tilting of the receiver terminal, but the tilting of the transmitter has no effect on the error position; and that in the near-field diffraction the position error is caused by the tilting of the transmitter or the receiver, but the mutual movement has no effect. It is furthermore found that the use of a beam scanner in the test in the near-field can simulate exactly the mutual movement of satellites in the far-field, and the trajectory formula for the beam scanning is deduced that is the same as the mutual angular trajectory from one satellite to another. Therefore a practical PAT test bed of a double-focus laser collimator, a beam scanner and a fine beam steering device is developed by us to test and verify the PAT function of inter-satellite laser communication terminals. The optical aperture is about φ440mm for this use. And a test bed for concurrent test and verification of both PAT function and communication performance is also demonstrated. The test bed consists of a conventional laser collimator, an optical scanner and a far-field beam transmission simulator, which is a combination of a Fourier-transform lens an a followed multiple-stage imaging amplifier. The details of configurations are given. It is clear that these test beds can be also used to test and verify the functions of laser radar, passive optical tracker, and so on.
The paper comprehensively analyzes and evaluates the mechanical property and thermo-structure distortions of the assembly of circular wedge prism in the device by method of finite element analysis. Regarding the prism assembly as a finite element model, the optical-mechanical-thermal integrating analysis is done. In terms of the principle of structure statics, the structure design and intensity of the prism assembly is verified and checked, and the analysis of surface deformation of the prism is correspondingly provided under static loading; then the thermal elasticity distortions of the prism are analyzed and the estimation of optics performance of the circular wedge prism is given. The analysis results show: the maximal distortion of the prism assembly is 10nm magnitude and the maximal stress is 0.403Mpa, which has much tolerance to the admissible stress of material and the precision requirement of structure; By comparing thermal-structure coupling analysis with statics analysis, the influence of heat effect on the prism surface deformations is proved far greater than the influence of static loading, so the strict temperature-controlled measures must be taken when the device is used.
The scanner of orthogonally tilting double prisms is researched for testing the performance of tracking performance in inter-satellite laser communications for the first time. With the reduction ratio of more than a hundred times from the change rate of deviation angle of beam to that of tilting angle of each prism, the scanner can reach the scanning accuracy of sub-microradian order but facilitates the mechanical structure design. The theoretical analysis performed, as well as the validation experiment, indicates the scanner can meet the requirements of the scanning accuracy superior to 0.5 μrad with the scanning range greater than 500 μrad.
A scanning system of a high accuracy double-wedge is presented, which allows us to perform very small angle deviation of a passing beam in a simple way. The first wedge's principal section is perpendicular to the horizontal axis, and the second is to the vertical axis correspondingly. They respectively rotate around the horizontal axis and the vertical axis as they work. So different small rotation angles of two wedges determine the corresponding orientation and position of the passing beam, and then high accuracy and very small-angle beam deviation can be performed. According to the design result: when the wedge angle is 5°, the refraction light beam will change about 1μrad if the wedge is rotated 1arcmin; the scanning range of light beam in the horizontal direction and the vertical direction can be not less than 600μrad, and the scanning precision of the device can be superior to 0.2 micro μrad
For on-ground performances testing and verification for intersatellite laser communication systems, a compact and multi-function optical testbed and a 2-D sub-micro-radian beamsteerer were designed. The testbed consists mainly of an optical simulator for long-distance propagation of laser beams, two CCD detectors, an autocollimation system, an interferometer, and a noise light source, for the uses is to measure far-field beam characteristics, transmitting powers, the wavefront errors and to evaluate the communication performances of any lasercom terminals with a aperture less than 280 mm in link level. The range of beam propagation distance of the optical simulator is from 2.8km to 3,5000km. An angular deviation yielded by the beamsteerer is to simulate the pointing error caused by all error sources. And a light source is to provide the noise-like illumination of changeable levels. Combined the testbed with the beamsteerer, it can measure the pointing errors and the tracking errors of the lasercom terminals, and estimates dynamic communication performances in the presence of random angular jitter. So the testbed has the potential to become all-purpose and tests the optical, communication and track performances for intersatellite lasercom terminals (ISLTs).
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