Department of Ocean Engineering

Resume of Guoling Shen

Research Assistant
Department of Ocean Engineering, Design Laboratory
Massachusetts Institute of Technology

Current Research Projects

CAD Model Rectificatioin

Current CAD modeling systems often create models containing defects such as gaps, holes and dangling faces. These models may describe unrealizable objects, and may cause severe problems in downstream applications (e.g. volumetric integral computations, finite element meshing and computer-aided manufacturing). The objective of this thesis is to study the nature of CAD model defects and develop model rectification techniques suitable for major applications. A CAD model rectification system using such techniques should generate models which are topologically correct and geometrically consistent, and also preserve the designer's intentions based on knowledge about erroneous models and user input. The major contributions of this thesis will include local and global topological validity verification methods, topological structure reconstruction methods for surface models, and geometric perturbation methods under topological constraints. Issues such as geometric model representation for rectified models will be addressed. The thesis will also include implementation and testing ofthe algorithms developed for complex geometric models.

Previous Research Projects

1. Approximation Measured Data with Interval B-Splines

The advent of accurate 3D laser ranging sensors that provide the capability of producing great amount of precise data sets in extremely short time, has created interest in automated methods for reverse engineering. A typical application of reverse engineering is to build a CAD model from a manufactured object which has no accurate electronic model. However, since measured data always possess uncertainty arising from sensor precision and measurement registration, most algorithms for surface reconstruction may produce an ambiguously defined object and cause a solid modeling system fail due to possible numerical instabilities when manipulating the geometric and topological representation with complex operations. Even in the absence of measurement uncertainty, such a failure could still happen due to the limitation of computation precision. This project attempts to provide an approach towards the robustness of surface reconstruction for reverse engineering. In the approach, measured data are represented as interval data points by involving all sources of errors, and interval surface fitting methods are developed to make it possible to construct interval solid models which with robust solid modeling operations based on rounded interval arithmetic, will build up a robust solid modeling system. The creation of interval surfaces is accomplished by either interpolation or approximation. A linear programming process is used to minimize ranges of the resulting surfaces.

2. Numerical and Geometric Properties of Interval B-splines

As a newly developed geometric representation introduced for increased robustness in solid modeling systems and for uncertainty representation in data fitting problems, interval B-splines have their unique numerical and geometric properties different from classical B-splines.
In order to preserve conservative enclosures, the computation of interval B-splines is implemented in rounded interval arithmetic (RIA). The error propagation in the RIA evaluation of B-spline basis functions is studied and an upper bound of the width of the function values is given. The width of an interval B-spline curve or surface is mainly contributed to the widths of its control points.
Unlike classical B-splines, interval B-spline curves and surfaces are no longer invariant under transformation, if the computation is in RIA. However, both translation and rotation are still conservative, or in other words, in the new coordinate system, the exactly transformed interval curve or surface is contained in the one defined by the transformed (using RIA) control points.
The hodograph curve of an interval curve is defined as its derivative. An interval point on the hodograph curve bounds the range within which the tangent vector can vary. Generally, the hodograph curve intends to inflate in width, and will become meaningless in predicting the tangent direction if the original interval curve has a wide range.
An interval B-spline curve can also be represented as a single valued central curve with a vector-valued error function defined as a piecewise polynomial function in the B-spline basis. This project also studies the extraction of the boundary of the area or volume defined by an interval geometry.
Other properties studied in this project include variation diminishing property, knot insertion, etc.

3. Surface Reconstruction Using Laser Range Scanner

With the advent of triangulation-based laser range scanners which are capable of quickly capturing a large amount of range data with high accuracy, surface reconstruction systems using laser scanners find applications in various areas such as reverse engineering, virtual reality, and inspection of products, etc. In recent years, lots of excellent research work have been done on this topic, and a number of surface reconstruction systems have been developed and entered into application, especially in manufacturing inspection and entertainment industry. However, most of the current systems are application-oriented (designed for specific laser scanners and system configurations), and therefore hard to be generalized for other applications. Some key issues, such as the robustness of data registration, the topological correctness of data integration, and a general next-best-view algorithm, are not well addressed, especially for large workspace.
The ultimate objective of this project is to develop an automated surface acquisition and reconstruction system for complex objects in a large workspace, using laser range scanner. Complex objects mean no constraint on the geometry and the topological features of objects and neither of them is known a priori. Large workspace means that the viewing volume of the system is large compared to the field of view of the scanner, and the driving devices have more degrees of freedom to position the laser scanner and/or the object.
We will study the sufficient conditions of scanning for a successful registration for a specific data registration method, e.g. the iterative closest point algorithm. Meanwhile, we will develop a next-best-view algorithm which is collision-free and feasible in the sense of computation expense.

4. Inspection of Shiphull Using Laser Scanner

In the shipbuilding industry, at the stage of block assembly at the erection site, adjustment work (trimming, hammering...) accounts for half of the total fitting time. In addition, some of the substantial welding (metal melting) time, which accounts for 1/3 of the total welding time, is expended on depositing extra metal into the wide gaps left by inaccurate fitting. Therefore, accuracy control at all stages of shiphull construction is essential to eliminate extra rework during assembly. A successful accuracy control system depends on a reliable, accurate and time-saving measurement method.
Current measurement techniques used at shipyards include photogrammetry, theodolites, digitizers, lasers and CMM. None of these techniques are automated. Some of them need more than one operator and thus are time-intensive, and the measuring result is not real time (need post-processing). Most of them measure a set of points of interest, which are marked on the manufactured structure by retroreflective or some other kinds of labels, and therefore, the measurement is not complete. None of them can be used to inspect a curved shape.
With its high accuracy and data acquisition speed, laser scanner provides the possibility to quickly measure a manufactured structure with a large amount of data (not just a bunch of points). With the use of robots and the development of software, it is also possible to automate the measuring process.

Education

Bachelor of Science in Marine Engineering and Naval Architecture, 1988

Huazhong University of Science and Technology, P.R.China

Thesis: Software Development for Signal Processing

Master of Naval Architecture and Marine Engineering, 1997

Master of Mechanical Engineering, 1997

Massachusetts Institute of Technology

Thesis: Approximation with Interval B-Splines for Robust Reverse Engineering

Research Interests

Publications

  1. Approximation of measured data with interval B-splines. S. T. Tuohy, T. Maekawa, G. Shen and N. M. Patrikalakis. Computer-Aided Design, Vol. 29, No. 11, pp. 791-799, 1997.
  2. Numerical and Geometric Properties of Interval B-Splines. G. Shen and N. M. Patrikalakis. International Journal of Solid Modeling. To appear.

Professional Experience

Research Assistant, Department of Ocean Engineering, MIT
September 1994 to present
Research in the areas of CAD/CAM and computational Geometry; Development of new techniques for various applications; Software implementation of new techniques; teaching assistant of the courses offered by the laboratory staff.
Construction Engineer, Shanghai Shipyard, Shanghai, P.R.China
September 1988 to July 1994
Schedule, coordinate and supervise the constructions in engine room and on decks (equipment installation and testing).

Guoling Shen
Massachusetts Institute of Technology
Department of Ocean Engineering
77 Massachusetts Avenue, Room 5-423a
617-258-7813
glshen@mit.edu

URL: http://deslab.mit.edu/DesignLab/people/Shen.html
Last modified: February 18, 1998