Following is a brief introduction to the projects that I have done.
Non-jamming Condition and Fixture Loading Planning
We study a rigid body system of one free body (e.g., workpiece) in contact with multiple fixed bodies (e.g., locators). The contacts are unilateral. The investigation is motivated for planning the task of insertion of a workpiece in a fixture. The key issue of the problem is to determine an applied force that can move the workpiece while maintaining all existing contacts with the locators. We first analyze the kinematics of the rigid body constrained by multiple unilateral contacts. The contact constraints are classified into two categories, the configuration constraints and kinematic constraints. We then find a sufficient condition for non-jamming among the multiple contacts in the constrained rigid-body dynamics.; This condition is also a necessary condition when the number of contacts is no less than four. Moreover, a method to find the applied force on the workpiece that results in sliding on all contact points is presented, based on the sufficient condition for non-jamming.
Precision Analysis for Fixture Design
We developed various models to estimate the precision of a fixture. The first model is full-kinematics model. This model incorporates a “virtual” kinematic chain with meshing parameters of contact kinematics in a velocity formulation. Conditions of a deterministic fixture are derived. It is shown that the workpiece position and orientation are completely characterized by the kinematic properties of the locator contacts with the workpiece, including not only the arbitrary locator location errors but also the surface properties at non-prismatic locator-workpiece contacts. Another model is approximate quadratic model. The model is originated from the understanding that the locater error along the contact surface changes not the input of the model, but the structure of it.
Passive Force Closure in Fixture
We study the difference between passive force closure for fixtures and active force closure for robotic hands. Since results of active force closure cannot be used in passive grasping systems, we developed a new theory of passive force closure for fixtures, in which a linear compliant model for contact forces is utilized. The formula to find contact forces for a given load to the workpiece is derived,and properties of passive grasping are analyzed. Passive force closure for a fixture is formally defined, and an algorithm to find the preload that can guarantee passive force closure is developed. It is shown that the range of the preload is a polyhedron convex cone in the wrench space.
Multi-body Contact Dynamics
We consider a system of rigid bodies with multiple simultaneous unilateral contacts. The problem is to predict the velocities of the bodies and the frictional forces acting on the simultaneous multi-contacts. A numerical method based on an extension of explicit time-stepping scheme and an application of the differential inclusion process is developed. From a differential kinematic analysis of contacts, we derive a set of transfer equations in the velocity-based time-stepping formulation. In applying the Gauss-Seidel iterative scheme, the transfer equations are combined with the Signorini conditions and Coulomb's friction law. The contact forces are properly resolved in each iteration, without resorting to any linearization of the friction cone. Its performance is compared with an acceleration-based scheme using linear complementarity techniques.
Off-line Programming for Robotic Welding
This is my master's research, we developed an integrated off-line programming software for articulated robot arc welding tasks. The system includes a feature modeler to generate the welding task information; an expert system to generate welding process specification; and a path planner to generate the robot path. I was in charge of the feature modeler. The system is developed on top of AutoCAD with its C++ API (ObjectARX).