Methods for improving imaging quality of a small-distance high-speed rotational reflection tracking system.

The imaging quality of a rotational reflection high-speed tracking system is greatly affected by the optical characteristics of the reflector and the depth of field limitations of the imaging system, especially for tracking systems working in small distances. In order to improve the imaging quality, this paper focused on two factors that affect the imaging quality: double vision caused by the optical characteristics of reflectors and blurring caused by the depth of field of imaging systems. This paper quantified the impact of these two factors on imaging through theoretical analysis, proposed a method of changing the hardware position, and conducted a simulation and experiments. The results show that the proposed solution in this paper can effectively improve the imaging quality of the system. The content studied in this paper has certain significance in the field of high-speed tracking of rotating reflectors and can provide reference for relevant researchers.

High-speed target tracking control system based on short-time rotational reflection imaging.

High-speed tracking technology has wide applications in the military and aerospace industry. However, existing approaches, such as camera arrays or Doppler radar systems, suffer from high cost and inconvenience. This paper reports a high-speed target tracking control system based on short-time rotational reflection imaging, specifically aimed at overcoming certain limitations. In the system we designed, a high-speed camera coupled with a rotating reflector is used to achieve reliable high-speed target tracking. This paper first introduces the working principle and mathematical model of the system, then analyzes the key technologies, including motor response delay time and rotational speed curve fitting, and, finally, verifies the feasibility of the system and the correctness of the theory based on a series of experiments. Experimental results demonstrated that our work is efficient and accurate in target tracking and image clarity. The developed system demonstrates significant potential for widespread use across military and aerospace sectors. Furthermore, the insights gained from our investigation into key technologies could act as a reference point for fellow researchers in related scientific areas.

High-performance phase measuring profilometry architecture based on Zynq SoC.

This paper presents a novel high-performance heterogeneous computation architecture, to the best of our knowledge, for stereo structure light using the phase measuring profilometry (PMP) algorithm based on a Zynq UltraScale+ system on chip (SoC). The proposed architecture aims to achieve real-time and high-accuracy 3D shape measurement. The experiment results indicate that the calculation time of a standard four-step PMP algorithm with a resolution of 1280×1024 is 14.11 ms. It is nearly 51 times faster than the well-optimized software implementation running on a Raspberry Pi and nearly three times faster than a high-end PC, with 15 times less power consumption. Consequently, the proposed architecture is deemed suitable for real-time 3D measurements in embedded applications.

Accurate calibration for crosstalk coefficient based on orthogonal color phase-shifting pattern.

The crosstalk coefficient calibration of de-crosstalk in color fringe projection profilometry is an essential step for the high-accuracy measurement. In this paper, a novel approach for calibrating crosstalk matrix of color camera is proposed. The wrapped phase error model introduced by color crosstalk in orthogonal pattern is established. Compared with the existing calibration methods depending on calculating the modulation of the crosstalk channel, the crosstalk coefficients are obtained from phase error in our method. By projecting the designed color orthogonal phase-shifting fringe patterns onto a white plate, the phase-shifting fringe patterns in both horizontal and vertical directions can be separated from captured images. The coefficients between different channels are calibrated by fitting the error relationship between the wrapped phase containing crosstalk and the standard ones. Coefficient fitting simulations and experimental validations including shape measurement of a white plate and distance measurement of a step block are carried out to verify the effectiveness of the proposed method.

Method of high-precision free-space distance measurement for noncooperative targets.

Aimed at high-precision distance measurement for noncooperative targets in free space, a spatial distance measurement method is proposed. Based on the concept of optical carrier-based microwave interferometry, this method extracts distance information from the radiofrequency domain. The interference model of broadband light beams is established, and the optical interference can be eliminated by using a broadband light source. A spatial optical system with a Cassegrain telescope as the main body is designed to effectively receive the backscattered signal without cooperative targets. A free-space distance measurement system is built to verify the feasibility of the proposed method, and the results agree well with the set distances. Long-distance measurements with a resolution of 0.033 µm can be achieved, and the errors of the ranging experiments are within 0.1 µm. The proposed method has the advantages of fast processing speed, high measurement accuracy, and high immunity to disturbances as well as the potential for measurement of other physical quantities.

Rapid and flexible calibration of DFPP using a dual-sight fusion target.

The parameter calibration of a digital fringe projection profilometry (DFPP) system is a fundamental step and directly related to 3D measurement accuracy. However, existing solutions based on geometric calibration (GC) suffer from the weakness of limited operability and practicality. In this Letter, a novel, to the best of our knowledge, dual-sight fusion target is designed for flexible calibration. The novelty of this target is the ability to directly characterize control rays for ideal pixels of the projector, and to transform the rays into the camera coordinate system, which replaces the traditional phase-shifting algorithm and avoids the error from the nonlinear response of the system. Attributed to the excellent position resolution of a position-sensitive detector within the target, the geometric relationship between the projector and camera can be easily established by projecting only one diamond pattern. Experimental results demonstrated that the proposed method using only 20 captured images is capable of achieving comparable calibration accuracy to the traditional GC method (20 images versus 1080 images, 0.052 pixels versus 0.047 pixels), which is suitable for rapidly and accurately calibrating the DFPP system in the 3D shape measurement field.

Two-step calibration method of the extrinsic parameters with high accuracy for a bistatic non-orthogonal shafting laser theodolite system.

Motivated by the increasing demands on the precision of 3D large-scale measurement, the extrinsic parameters calibration with high accuracy of the bistatic non-orthogonal shafting laser theodolite (N-theodolite) system is required. A two-step method is proposed to achieve the extrinsic parameters calibration with high accuracy in this paper. In the first step, by analyzing and setting the approximate emitted point during the motion of the laser axis in local space, the calculation of the initial extrinsic parameters can be simplified. In the second step, the above results are taken as the initial values of optimization, and the distances between the spatial laser points provided by PSD sensors with high accuracy in global space are used to construct the unconstrained optimal objective function. The proposed method is validated with the measurement experiment of the bistatic N-theodolite system, the average error of 3D coordinate measurement is less than 0.4 mm, and the average error of distance measurement is less than 0.3 mm within 5 m.

Practical zoom camera calibration method for close-range photogrammetry.

Compared to a conventional fixed focus lens, a zoom lens is more flexible and adaptable. However, the challenges involved in precise calibration for a zoom camera prevent its widespread use in close-range photogrammetry. A practical calibration method for a zoom camera is proposed. The zoom-focus model is established through dimension reduction of the setting variables, which is represented as a set of functions of the zoom setting. The zoom and focus settings are updated in real time for objects at different measurement depths. The calibration process only requires the zoom camera to observe the control points distributed in the designed calibration field with several combinations of zoom and focus settings. All the coefficients of the zoom-focus model can be solved by a nonlinear joint optimization. Experimental results have proved that the proposed method is effective for close-range photogrammetry.

Driving system for Mach-Zehnder electro-optic modulators in microwave photonics.

In order to obtain a modulation signal with high bandwidth and highly stable output power without distortion, an electro-optic modulator driving system with quick response to the input signal is designed in this paper. The system mainly includes three parts: power supply module, microwave signal generation module, and automatic gain control (AGC) module. The power supply module provides appropriate voltage and current for other two modules. The microwave signal generation module is used to generate microwave signals. The AGC module adopts the digital AGC control structure and the PID control principle to achieve stable control of the output power of the microwave signal. The hardware is developed, and the software algorithm is integrated into the driving system to verify the effectiveness and stability. The experimental results show that the bandwidth of the AGC module reaches 20 GHz. The proposed AGC module can effectively stabilize the output power of the microwave signal at 10 dBm with fluctuations less than ±1.5 dB, and its stability is improved by 74.95% compared to existing instrument. When the input power changes at 10 KHz, the system can still achieve stable control performance and has exhibited excellent dynamic response characteristics.

Method of high-precision spatial distance measurement based on optical-carried microwave interference.

High-precision spatial ranging plays a significant role in both scientific research and industrial practice. However, it is difficult for existing equipment to achieve high speed, high precision, and long distance simultaneously. Inspired by the concept of optical carrier-based microwave interferometry (OCMI), this paper reports a method of high-precision spatial distance measurement. A microwave-modulated broadband optical signal is sent to the interferometer whose measuring arm is an optical echo receiving system in free space. By scanning the microwave frequency, the measured distance can be resolved from the interferogram. Since the processing of the interference spectrum is performed in the microwave domain, this method is insensitive to the types of optical waveguides and states of optical polarizations. The experimental results show that the root mean square error (RMSE) of ten repeated measurements at 0.5 m is 0.016 µm, the RMSE is 0.023 µm within a 1 m distance, which can effectively represent the length measuring capability of the proposed system.

Registration of 3D point clouds using a local descriptor based on grid point normal.

The coarse-to-fine method is the prime technology for point cloud registration in 3D reconstruction. Aimed at the problem of low accuracy of coarse registration for the partially overlapping point clouds, a novel, to the best of our knowledge, 3D local feature descriptor named grid normals deviation angles statistics (GNDAS) for aligning roughly pairwise point clouds is proposed in this paper. The descriptor is designed by first dividing evenly the local surface into some grids along the x axis and y axis of the local reference frame, then making the statistics of the deviation angles of normals at grid points. Based on the correspondences generated by matching descriptors and a transformation estimation method, the initial registration result is obtained. The coarse registration result is used as the initial value of the iterative closest point algorithm to achieve the refinement of the registration result. Experimental comparisons on two public datasets demonstrate that our GNDAS descriptor has high descriptiveness and strong robustness to noise at low level and varying mesh resolution. The registration results also confirm the superiority of our registration approach over previous versions in accuracy and efficiency.

Calibration method of a laser beam based on discrete point interpolation for 3D precision measurement.

A laser beam used as a visualizing measuring axis is an important technique in 3D shape measurement. A highly accurate calibration method of a laser beam based on discrete point interpolation is proposed in this paper. A flexible control field constructed by a laser tracker, a theodolite and a target plane with 5 high-precision machining holes is presented. The discrete point interpolation model is established by the coordinates of holes measured by a laser tracker and the angles of holes measured by a theodolite. The coordinates of laser spots on the target plane are obtained based on the angles and discrete point interpolation model, and the direction vector of the laser beam is obtained by linear fitting. The optimal measurement pose of a theodolite is analyzed by the simulation results. The experimental results show that the RMSE of linear fitting of laser beams is no more than 14 µm within a 5 m distance, the RMSE of the spatial points is 0.09 mm and the RMSE of the reconstructed distance is 0.09 mm.

Dynamic pose estimation of uncooperative space targets based on monocular vision.

A novel algorithm of dynamic pose estimation for monocular visual sensor is proposed in this paper. The sensor is principally composed of two 1D turntables, one collimated laser, and one industrial camera. In particular, the proposed algorithm is suitable for the cases of uncooperative targets. By analyzing the motion of a laser beam based on quaternion, the functional detection algorithm is derived from the position information of multiple scanning points. Furthermore, the depth recovery based on a nonparametric model is a key step in the pose calculation, which is unnecessary to make use of the calibration parameters of an industrial camera. It is, however, effective to avoid the influence of camera distortion and calibration error. After establishing a test platform, simulation and experiments for pose estimation are carried out. The experimental results show that the maximum error is 0.98° at a range of 500 mm, which proves that the proposed algorithm is accurate and effective.

A Novel Calibration Method of Articulated Laser Sensor for Trans-Scale 3D Measurement.

The articulated laser sensor is a new kind of trans-scale and non-contact measurement instrument in regular-size space and industrial applications. These sensors overcome many deficiencies and application limitations of traditional measurement methods. The articulated laser sensor consists of two articulated laser sensing modules, and each module is made up of two rotary tables and one collimated laser. The three axes represent a non-orthogonal shaft architecture. The calibration method of system parameters for traditional instruments is no longer suitable. A novel high-accuracy calibration method of an articulated laser sensor for trans-scale 3D measurement is proposed. Based on perspective projection models and image processing techniques, the calibration method of the laser beam is the key innovative aspect of this study and is introduced in detail. The experimental results show that a maximum distance error of 0.05 mm was detected with the articulated laser sensor. We demonstrate that the proposed high-accuracy calibration method is feasible and effective, particularly for the calibration of laser beams.