Deokhwa Hong

Disassemblability analysis for generating robotic assembly sequences

This paper proposes a mathematical approach to the analysis of disassemblability of a product for generating stable robotic assembly sequences. The disassemblability of a part, which represents a cost for disassembling the part from a base assembly, is derived in terms of both separability of a part from a base assembly and motion instability of the base assembly. Then, stable robotic assembly sequences are found in the list of parts, each of which is sequentially chosen to have a locally maximum disassemblability. To show the effectiveness of the proposed method, case studies are presented for industrial products. The results are compared with those of previous researches. The comparison shows that the proposed method can effectively generate stable robotic assembly sequences and provide essential informations on fixture design.

Optimization of robotic assembly sequences using neural network

This paper presents a neural network based computational scheme to generate the optimize robotic assembly sequence for an assembly product consisting of a number of parts. An assembly sequence is considered to be optimal when it meets a number of conditions: it must satisfy assembly constraints, keep the stability of in-process subassembly, and minimize assembly cost. Currently, various search algorithms have been reported for the purpose, but as the number of the parts increases they often fail to generate assembly sequences due to the explosion of the search space. As an alternative solution to overcome this problem, the authors propose a scheme using both the Hopfield neural network and the expert system. Based on the inferred precedence constraints and the assembly costs obtained from the expert system, the authors derive the evolution equation of the network, and finally obtain an optimal assembly sequence resulting from the evolution of the network. To illustrate the suitability of the proposed scheme, case study is presented for an electrical relay. The result is compared with that obtained by the expert system.

Sensor fusion of phase measuring profilometry and stereo vision for three-dimensional inspection of electronic components assembled on printed circuit boards

Automatic optical inspection (AOI) for printed circuit board (PCB) assembly plays a very important role in modern electronics manufacturing industries. Well-developed inspection machines in each assembly process are required to ensure the manufacturing quality of the electronics products. However, generally almost all AOI machines are based on 2D image-analysis technology. In this paper, a 3D-measurement-method-based AOI system is proposed consisting of a phase shifting profilometer and a stereo vision system for assembled electronic components on a PCB after component mounting and the reflow process. In this system information from two visual systems is fused to extend the shape measurement range limited by 2pi phase ambiguity of the phase shifting profilometer, and finally to maintain fine measurement resolution and high accuracy of the phase shifting profilometer with the measurement range extended by the stereo vision. The main purpose is to overcome the low inspection reliability problem of 2D-based inspection machines by using 3D information of components. The 3D shape measurement results on PCB-mounted electronic components are shown and compared with results from contact and noncontact 3D measuring machines. Based on a series of experiments, the usefulness of the proposed sensor system and its fusion technique are discussed and analyzed in detail.

Generation of robotic assembly sequences with consideration of line balancing using simulated annealing

In designing assembly lines, it is required that the lines should not only meet the demand of the product, but also minimize the assembly cost associated with the line. For such a purpose, numerous research efforts have been made on either the assembly sequence generation or the assembly line balancing. However, the works dealing with both the research problems have been seldom reported in literature. When assembly sequences are generated without consideration of line balancing, additional cost may be incurred, because the sequences may not guarantee the minimum number of workstations. Therefore, it is essential to consider line balancing in the generation of cost-effective assembly sequences. To incorporate the two research problems into one, this paper treats a single-model and deterministic (SMD) assembly line balancing (ALB) problem, and proposes a new method for generating line-balanced robotic assembly sequences by using a simulated annealing. In this method, an energy function is derived in consideration of the satisfaction of assembly constraints, and the minimization of both the assembly cost and the idle time. Then, the energy function is iteratively minimized and occasionally perturbed by the simulated annealing. When no further change in energy occurs, an assembly sequence with consideration of line balancing is finally found. To show the effectiveness of the proposed scheme, a case study for an electrical relay is presented.

Flexible depth-of-field imaging system using a spatial light modulator

A major problem of optical microscopes is their small depth-of-field (DOF), which hinders automation of micro object manipulation using visual feedback. Wavefront coding, a well-known method for extending DOF, is not suitable for direct application to micro object manipulation systems based on visual feedback owing to its expensive computational cost and due to a trade-off between the DOF and the image resolution properties. To solve such inherent problems, a flexible DOF imaging system using a spatial light modulator in the pupil plane is proposed. Especially, the trade-off relationship is quantitatively analyzed by experiments. Experimental results show that, for low criterion resolution, the DOF increases as the strength of the mask increases, while such a trend was not found for high criterion resolution. With high criterion resolution, the DOF decreases as the mask strength increases when high-resolution images are required. The results obtained can be used effectively to find the optimum mask strength given the desired image resolution.

Visual gyroscope: Integration of visual information with gyroscope for attitude measurement of mobile platform

In this paper, fusion of visual and inertial information is proposed for attitude measurement of remotely-operated mobile robots. The primary objective is to eliminate the drift error of the inertial sensors using visual information for exact and fast sensing of attitude. By this fusion each output is not only combined together but also helps the other yield better result. The environment is assumed unknown and corner features are utilized for rotation matrix calculation, for corners are better extracted in nature scene than lines. The measured information is to be used for stabilizing the image sent to the operator and for stabilizing the robot body itself in bumpy terrain. In the paper, the fusion algorithm based on Kalman filter is utilized, and its performance is evaluated by experiment. For the experiment, high performance camera is used with low-quality gyro sensors. The experimental results show that the vision algorithm well tracks the attitude change of the camera, and by fusing that with gyro sensor output, we could get high rate angle measurement without drift error.

Measurement of partially specular objects by controlling imaging range

When specular surface is imaged on a group of CCD cells, the image usually suffers from saturation and blooming problem. This problem is a serious obstacle to applying optical profiling methods based on structured light to specular objects. In this paper, a phase-based profiling system combined with a spatial light modulator in the imaging part is proposed to measure three dimensional shape of partially specular surfaces. The spatial light modulator prevents the image sensor from being saturated. In this way, projected fringes are well imaged and their phase information is correctly extracted. The system configuration and transmittance control scheme are explained in sequence. This idea is verified by experimental results, in which phase information is successfully extracted from the areas which are not normally measurable due to saturation.

The design of active vision system for variable view imaging of micro objects

Insufficient vision information such as occlusion and low resolvability is one of the important issues that limit the micromanipulation and microassembly. In this paper, we proposed the active vision system that can interact with the environment by changing optical system parameters such as spatial position, orientation and focus plane. As an optomechatronic system, the proposed system integrates a pair of wedge prism, a scanning mirror, a deformable mirror and off-the-shelf optics. The compact double wedge prisms can change the view direction, however the aberration induced by wedge prisms can be corrected by deformable mirror. Combining with a scanning mirror, active optical system can observe the micro object in different view. Owing to the orthogonality of the Zernike polynomials, the proposed deformable mirror control algorithm can correct the aberration in each Zernike mode instead of controlling each actuator, which simplifies the control issue of deformable mirror. The preliminary experiment setup was built, and initial experiments were demonstrated to investigate the validity of the concept of the proposed system.

Variable view imaging system with decoupling design

Insufficient vision information such as occlusion, low resolvability, and a small field of view (FOV) represent important issues in microassembly and micromanipulation. In previous research, an active optical system was designed to supply a compact flexible view. However the complex kinematics makes the system operation and calibration much difficult. In this paper, a decoupling design for the variable view image system with a telecentric lens group is proposed to decouple the view angle and scanning mirror angle. The proposed design increases the range of zenith angle. The forward kinematics is analyzed with the help of vector diffraction theory. The singularity of Jacobin is analysis and the singularity configurations are identified. In order to verify the proposed system, a prototype system is built up. A series of experiments on the prototype system shows the validity of the new design.

Depth of field extension using variable annular pupil

The DOF (depth of field) of an imaging system is closely related to the pupil function that describes the characteristics of the pupil such as size and shape. Shallow DOF is especially problematic in microscopic optical imaging systems because they have relatively large aperture size for high optical resolution. In this paper, a new DOF extension method is proposed. This is to acquire an image by integrating the incoming light while changing the size of annular pupil. By divining the pupil into annular form, the effect of the defocus aberration is reduced. Theoretical background and preliminary experimental results are shown. The results show that the DOF can effectively be extended by this method.