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Zexiang Li

Professor of Department of
Electrical and Electronic Engineering
School of Engineering

Director of Automation Technology Center

Hong Kong University of Science and Technology



Papers Research Students Teaching Contact

Other Activities:

2010 Summerschool on Geometric Methods

International Conference on Robotics and Automation 2011


Publications

Recent publications

More

Full Publication List

Research Interests

  • Dexterous manipulation
  • Multifingered robotic hand
  • Motion planning and control
  • Geometric methods of robotics
  • Parallel mechanisms
  • Electronic manufacturin
  • Computer-aided set up and inspection
  • Precision machining

Recent Projects

Completion of a unified control system architecture for multifingered manipulation

Abstract
To accommodate the need of a diverse range of manufacturing assembly/manipulation tasks, a multifingered robotic hand is designed with a complex structure involving:
  • a number (three or four) of fingers each with a number (three or four ) of joints;
  • a number of sensors of different nature providing information on joint position/torque, contact location/force and geometry/location of the object being manipulated;
  • a multi-processor based control unit. A central issue in the study of multifingered robotic hands is the development of a unified (i.e., hand independent) control system architecture capable of transforming high-level specifications of a task to low-level commands for the fingers.
In early research at the Robot Manipulation Laboratory of HKUST, we developed the HKUST three-fingered robotic hand as a research platform for multifingered manipulation. We proposed a preliminary framework for a unified Control System Architecture for Multifingered Manipulation (CoSAM$^2$). The theory and implementation for several function and sensory modules of CoSAM$^2$ have been completed along with successful demonstration of several important manipulation tasks.
In this proposal, we propose to broaden and enhance the structure of CoSAM$^2$ through the creation of several new modules:
  • Grasp analysis module;
  • Dexterous manipulation module;
  • Object motion estimation module.
Utility of these modules will be demonstrated by a series of well-designed manipulation tasks. It is expected that the integration of these new modules with CoSAM$^2i$ will substantially improve the range of tasks the HKUST hand is capable of performing and thus lead to better understanding of multifingered manipulation.

Automation Technology Cooperative Research Center

Abstract
We propose to establish a university/industry cooperative research centre to develop automation technologies for electronic manufacturing. The mission of the Centre is:
  • to develop critical automation technologies needed for electronic manufacturing and demonstrate the technologies in prototype products.
  • to transfer the technology know-how to local industries,
  • to provide in-depth training to local technical staff and engineers in the areas of automation technologies.
In recent years, Hong Kong's electronics industry has been faced with serious challenges. First, the international market demands high product diversity, rapid product introduction, high product quality and low price.
Second, the electronics industry had to cope constantly with the problems of rising labor and land costs and high turnover of technical staff, problems that have subsequently caused low assembly yield and poor product quality (See Roadmap for Electronics Packaging and Assembly for Hong Kong, produced by the Hong Kong Electronic Industries Association). In response, many electronics firms have become increasingly interested in adopting state-of-the-art automation technologies for their manufacturing operations. But this effort is hindered by several major roadblocks including:
  • lack of technology know-how;
  • shortage of experienced engineers and technical staff capable of designing and maintaining modern manufacturing operations; and
  • lack of local manufacturers of high-end equipment.
By combining the vast technical expertise and other resources of the University and the important product development experience of the industry, the proposed Centre can help overcome these difficulties. First, an Industry Advisory Board (IAB) will be formed with members from all participating companies of the Centre. Engineers from the member companies will be actively involved in deciding new projects. Second, projects will be initiated to develop core technologies for manufacturing automation. These technologies will be demonstrated in the form of semi-product (or "product-like") prototypes. Member companies will be kept informed on the status of these projects and will be provided with full access to these technologies. Finally, seminars, workshops and demonstrations will be organized on a regular basis to disseminate these technologies quickly to member companies. By using these technologies, which are already in semi-product form, member companies can develop their full product prototypes at a substantially reduced cost, cycle-time and engineering complexity.

Geometric Algorithms for Hybrid Localization and Tolerance Verification

Abstract
Manufacturing enterprises today are confronted by an international market which demands high product diversity, rapid product introduction, high part quality, and low price. These demands create a need for manufacturing systems that are more flexible and agile enough to respond quickly to product and demand changes. In response to this need, computing technologies are increasingly being applied to many aspects of manufacturing.
Over the last five years, research activities in the Machine Intelligence Laboratory of HKUST have been centered on:
  • design of flexible open-architecture machine tool control systems; and
  • development of efficient geometric algorithms for localization of workpieces.
These technologies address important current manufacturing bottlenecks: the presently excessive costs of setup, refixturing and dimensional inspection. Our overall aim is to extend current on-machine probing inspection capabilities to reduce process time, improve machine tool flexibility, and increase part quality.
In the present proposal, which supplements our on going laboratory work in an important way, we propose to study efficient algorithms for hybrid localization/envelopment problems, i.e., how to align a part model with a workpiece such that all points measured on finished surfaces of the workpiece match closely to corresponding surfaces on the model while all unmachined surfaces lie outside the model to guarantee the presence of material to be machined at a later time. An important application of hybrid localization/envelopment problems is found in setting up for machining of a partly finished workpiece having both finished and unmachined surfaces, e.g., a casting or forging for post machining in such a way that the model to be cut is inside the material envelope. In parallel, we propose to study geometric techniques for formulating tolerancing notions contained in ANSI Y14.5M, and develop efficient tolerance verification algorithms. By integrating these algorithms with an open-architecture machine tool environment we hope to complete a Computer-Aided Setup and Inspection (CASI) system leading to simplified and accelerated machining cycle with improved product quality.

Geometric Analysis and Implementation of Workpiece Localization Algorithms

Abstract
Manufacturing enterprises today are confronted by an international market which demands high product diversity, rapid product introduction, high part quality, and low price. These demands create a need for manufacturing systems that are more flexible and agile enough to respond quickly to product and demand changes. In response to this need, computing technologies are increasingly being applied to many aspects of manufacturing.
Over the last three years, research activities in the Machine Intelligence Laboratory of HKUST have been centered on:
  • design of open-architecture machine tool controllers,
  • development of algorithms for workpiece localization, and
  • formulation in precise mathematical terms various geometric notions of tolerancing and study suitable approaches for tolerance verification.
These technologies address important current manufacturing bottlenecks: the presently excessive costs of setup, refixturing and dimensional inspection. Our overall aim is to extend current on-machine probing inspection capabilities to reduce process time, improve machine tool flexibility, and increase part quality.
In the present proposal, which supplements our on going laboratory work in an important way, we propose to study and analyze the geometric properties of two promising algorithms that have been developed by us and others for workpiece localization. These algorithms are the H-T algorithm of Hong and Tan and the variational algorithm developed in one of our early works. We will use gradient flow techniques to study convergence properties and
convergence rates of these two geometric algorithms. This study will lead to highly efficient localization algorithms which will possess the advantages of both the variational algorithm and the H-T algorithm but none of their shortcomings. In parallel, we will develop envelopment algorithms for localizing partially finished workpieces as they proceed through successive stages of machining, and reliability analysis algorithms which can determine after an initial data acquisition step if tool paths can be positioned reliably enough relative to possible errors in the position of a part being cut, and if not can indicate where further probes should be made.
We will also develop algorithms for automatic probing of three-dimensional workpieces. By integrating these algorithms with an open-architecture machine tool environment we propose to develop a Computer-Aided Setup and Inspection (CASI) system. The use of a CASI system will eliminate the need of having an operator fixture a workpiece accurately, thus simplifying and accelerating the machining cycle. Furthermore, it will ensure product quality by performing on-linedimensional inspection.

Optimal Parallel Manipulator Design as Linear Matrix Inequality Problems

Abstract
This research deals with the problem of optimal design of parallel manipulators which have no singularity, have high stiffness and manipulability and are the most economic. By observing that those requirements can be cast into linear matrix inequalities (LMIs), we formulate the design problem as a convex optimization problem subject to LMIs with either a linear function or a max-det function as its objective function. The variables x associated with LMIs are nonlinear functions of some key kinematic parameters /spl alpha/. If the dimension of x is equal to the number of independent kinematic parameters, a two-level algorithm can be applied to solve for a set of optimal kinematic parameters:
  • applying the interior-point algorithm for solving of x;
  • applying the Newton method to a set of nonlinear algebraic equations for solving of /spl alpha/. If the dimension of x is greater than the number of independent kinematic parameters (i.e., x are not linearly independent), we consider the constrained semi-definite programming problems and the constrained max-det problems by taking account of an additional set of nonlinear constraints. We propose a simplified constrained gradient algorithm for solving of x in such cases, /spl alpha/ derives from x using Newton method. Simulation results verify the effectiveness of the proposed algorithms.

A Geometric Theory for Analysis and Synthesis of sub-6 DoF Parallel Manipulators

Abstract
This research developed a rigorous and precise geometric theory for the analysis and synthesis of sub-6 DoF parallel manipulators. We give a rigorous definition for the parallel manipulator synthesis problem, and introduce a general method for specifying the corresponding subchains which will result in the desired parallel manipulator. Following this, a procedure for solving the parallel manipulator synthesis problem is proposed when the set of desired end-effector motions is in the form of Lie subgroup or a regular submanifold of SE(3). Numerous examples are used to illustrate the generality and effectiveness of the proposed synthesis method.

Parallel Manipulator Design for Optimal Workspace

Abstract
This work intends to deal with the optimal kinematic synthesis problem of parallel manipulators under a unified framework. Observing that regular (e.g., hyper-rectangular) workspaces are desirable for most machines, we propose the concept of effective regular workspace, which reflects simultaneously requirements on the workspace shape and quality. The effectiveness of a workspace is characterized by the dexterity of the mechanism over every point in the workspace. Other performance indices, such as manipulability and stiffness, provide alternatives of dexterity characterization of workspace effectiveness. An optimal design problem, including constraints on actuated/passive joint limits and link interference, is then formulated to find the manipulator geometry that maximizes the effective regular workspace. This problem is a constrained nonlinear optimization problem without explicit analytical expression. Traditional gradient based approaches may have difficulty in searching the global optimum. The controlled random search technique, as reported robust and reliable, is used to obtain an numerical solution. The design procedure is demonstrated through examples of a Delta robot and a Gough-Stewart platform.

Students

H. Shen(Ph D)
Winnie Leung(Ph D)
G.F. Liu(Ph D)
J.J. Xu(Ph D)
Jie Li( Post doc)
Y.S. Chan (RA)
Tim Wu(Ph D)
K.T. Yuen(M Phil)
D.J. Zhang(Ph D)
Y.K. Yiu(Ph D)
J. Meng(Ph D)
Loginov A. (RA)
Lin Shi (Junior RA)
Toby (M Phil)
N. Chen(M Phil)
Y. Lu(M Phil)
Y.J. Lou(Ph D)
A. Kojokarou(RA)
KangKo A. (Post doc)
H. Cheng(M Phil)
Khrapko(RA)

Teaching

The list below is the courses that I have taught at HKUST. The course links will take you to the (current) course homepage.

  • Elec 374: Introduction to Robotics
  • Elec 564: Robot Manipulation
  • Elec 560: Linear Systems Theory
  • Elec 562: Nonlinear Systems
  • Elec 112: Linear Circuit Theory
  • Elec 692A: Special Topics in Robot Motion Planning

Contact

Mailing Address

Zexiang Li
Electronic and Computer Engineering
Hong Kong University of Science and Technology
Clear Water Bay, Sai Kung
New Territory, Hong Kong, China

Contact information

E-mail: eezxli@ust.hk
Tel:(852) 2358 7051
Fax:(852) 2358 1485

Other links

Automation Technology Center
2010 Summerschool on Geometric Methods
HKUST

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