Liandong Yu

Black-Box Phase Error Compensation for Digital Phase-Shifting Profilometry

Due to the nonlinear response of digital phase-shifting profilometry (PSP), phase errors are inevitable in the retrieved 3-D profiles and result in negative impacts on the measurement accuracy of the PSP system. A novel method is presented in this paper to reduce the impact. Instead of compensating phase errors in the retrieved 3-D profile, specific harmonics are superimposed upon the initial fringe pattern actively to adjust the amplitudes of the harmonics in the captured fringe image with the intention to suppress the phase error, and all nonlinear response of the PSP system can be compensated in theory. Furthermore, it does not consume additional time in the 3-D profile reconstruction process. Experimental results are shown to demonstrate the validity of the method.

A novel MR device with variable stiffness and damping capability

 This paper proposes a novel device based on the Magnetorheological (MR) fluid which has the capability to change stiffness and damping under control. MR fluid is a type of smart material whose properties could be controlled by the external magnetic field. Most of MR devices are MR dampers, which normally are used as variable damping devices. The presented device consists of two hydro-cylinder-spring structures and one MR valve linking these two structures. The rheological characteristics of MR fluid in the fluid flow channels of MR valve are controlled by the strength of magnetic fields, which directly affect the link conditions. The equivalent stiffness and damping coefficients of the device thus varies with the rheological characteristics of MR fluid simultaneously. A mathematical model is established to describe the properties of the proposed device based on the Bouc-wen model. ADAMS is employed to simulate the dynamic performance of the device. The mathematical model the simulation results indicate that the proposed device can control both the stiffness and damping which has potential to be applied for restrain vibration mitigation efficiently.

A calibration technology for multi-camera system with various focal lengths

Calibration is the basis of three-dimensional (3D) reconstruction for machine vision technology. Nowadays, the most widely used calibration method among computer vision is the technique for binocular stereo measurement. However, binocular stereo vision has limited view field which is difficult to measure large-scale mechanical components synchronously. Thus, enlarging the view field is urgent in need for the large scale measurement. With the application of multi-camera system, the calibration for cameras with different focal lengths is required. In this paper, a method aiming at calibration problems for multi-camera system of different focal lengths is proposed. An imaging model for multi-camera system with various focal lengths is analyzed. The Harris corner detector is applied to determine the relationship between signal camera and checkerboard. Finally, the external parameters of different cameras can be obtained by the link with the checkerboard. The calibration results indicate that the calculation method used in this work can calibrate multi-camera with various focal lengths.

3D reconstruction for sinusoidal motion based on different feature detection algorithms

The dynamic testing of structures and components is an important area of research. Extensive researches on the methods of using sensors for vibration parameters have been studied for years. With the rapid development of industrial high-speed camera and computer hardware, the method of using stereo vision for dynamic testing has been the focus of the research since the advantages of non-contact, full-field, high resolution and high accuracy. But in the country there is not much research about the dynamic testing based on stereo vision, and yet few people publish articles about the three-dimensional (3D) reconstruction of feature points in the case of dynamic. It is essential to the following analysis whether it can obtain accurate movement of target objects. In this paper, an object with sinusoidal motion is detected by stereo vision and the accuracy with different feature detection algorithms is investigated. Three different marks including dot, square and circle are stuck on the object and the object is doing sinusoidal motion by vibration table. Then use feature detection algorithm speed-up robust feature (SURF) to detect point, detect square corners by Harris and position the center by Hough transform. After obtaining the pixel coordinate values of the feature point, the stereo calibration parameters are used to achieve three-dimensional reconstruction through triangulation principle. The trajectories of the specific direction according to the vibration frequency and the frequency camera acquisition are obtained. At last, the reconstruction accuracy of different feature detection algorithms is compared.

Sub-pixel projector calibration method for fringe projection profilometry

Digital projectors are used as standard parts at present in fringe projection profilometry systems to project structured-light patterns onto the object surface to be measured, and the distortion of the projector lens must be calibrated and compensated accurately to satisfy the accuracy requirement of industrial applications. A novel method is proposed to determine the projector pixel coordinates of the marker points of a calibration target accurately in terms of projective transform. With the method, the projector can be calibrated with accuracy of sub-pixel level. The method is applicable for the calibration target with a chessboard pattern or a circle pattern, and the calibration result is independent on the results of camera calibration. Experimental results are shown to demonstrate the effectiveness and validity of the proposed method.

3D information detection with novel five composite fringe patterns

A novel phase unwrapping method with five composite fringe patterns and phase-shifting method was proposed for three-dimensional information detection. The composite fringe patterns were composed of phase-shifted sinusoidal and stair phase-coding fringe patterns. The fringe patterns are five step phase-shifted. The relative phase and the stair code-words can be obtained simultaneously based on the five step phase-shifting algorithm. The monochrome composite structure has the improvement on the measurement speed and has the potential of solving the problem of color coupling and color imbalance. Combined with the modified minimum phase map theory, this method can provide more code-words for the full field measurement. Simulation and experimental results were presented in the final part to verify the flexibility and feasibility of the proposed method.

Non-uniform fringe patterns for compensating the projector distortion in absolute phase map

Digital projector is a main component in Fringe projection profilometry (FPP). To acquire precise 3D profiles, it is crucial to accurately calibrate and compensate projector distortion. In this paper, we propose a new method to improve the accuracy of projector compensation. Coefficients of projector distortion are first accurately calibrated. And based on the calibration results, non-uniform fringe patterns are designed to compensate the projector distortion. With the non-uniform fringe patterns, phase errors in absolute phase map can be automatically and effectively offset. Experimental results are shown to demonstrate the validity of the proposed method.

Errors analysis of dimensional metrology for internal components assembly of EAST

The precision of dimensional measurement plays an important role in guaranteeing the assembly accuracy of its internal components during the upgrading phase of EAST device. In this paper, the experimental research and analysis were done based on three dimensional combined measurement systems, combining Laser Tracker, flexible Measure ARM and measurement fiducials network, which are used for alignment and measurement of EAST components during the assembly process. The error sources were analyzed, e.g. temperature, gravity, welding, and so on. And the effective weight of each kind of error source was estimated by the simulation method. Then these results were used to correct and compensate the actual measured data, the stability and consistency of the measurement results was greatly improved in different measurement process, and the assembly precision of the EAST components was promised.

Multi-cameras calibration from spherical targets

Multi-cameras calibration using spheres is more convenient than using planar target because it has an obvious advantage in imaging in different angles. The internal and external parameters of multi-cameras can be obtained through once calibrat ion. In this paper, a novel mult i-cameras calibration method is proposed based on multiple spheres. A calibration target with fixed multiple balls is applied in this method and the geometric propert ies of the sphere projection model will be analyzed. During the experiment, the spherical target is placed in the public field of mult i-cameras system. Then the corresponding data can be stored when the cameras are triggered by signal generator. The contours of the balls are detected by Hough transform and the center coordinates are determined with sub-pixel accuracy. Then the center coordinates are used as input information for calibrat ion and the internal as well as external parameters can be calculated by Zhangs theory. When mult iple cameras are calibrated simultaneously from different angles using multiple spheres, the center coordinates of each sphere can be determined accurately even the target images taken out of focus. So this method can improve the calibration precision. Meanwhile, Zhangs plane template method is added to the contrast calibrat ion experiment. And the error sources of the experiment are analyzed. The results indicate that the method proposed in this paper is suitable for mult i-cameras calibrat ion.

The design of an energy harvesting device for prolonging the working time of DC equipment

Energy harvesting (EH) derives from the idea of converting the ambient energy into electric energy, which can solve the problem of DC supply for some electronic equipment. PZT is a typical piezoelectric material of inorganic, which has been developed as EH devices to transfer ambient vibration energy into electric energy. However, these PZT devices require relatively violent excitation, and easy to be fatigue fracture under the resonance condition. In this paper, PVDF, which is a kind of soft piezoelectric polymer, is adopted for developing transducer. The PVDF devices are flexible and have longer life time than PZT devices under the harmonic environment. The EH researches are mainly focused on the development of energy transfer efficiency either by the mechanical structure of transducer or the improvement of circuit. However, the practicality and stability of the EH devices are important in the practical engineering applications. In this paper, a charge amplifier is introduced in the circuit in order to guarantee the stability of the battery charging under small ambient vibration conditions. The model of the mechanical structure of PVDF and the electric performance of circuit are developed. The experimental results and simulation show that the stability of battery charging is improved and the working time of DC equipment is prolonged.