Yu-Che Chen

ICRA Conference 2000 Conference Paper

Instability of Pseudoinverse Acceleration Control of Redundant Mechanisms

• Kevin A. O'Neil
• Yu-Che Chen

Resolved acceleration motion control using the pseudoinverse or weighted pseudoinverse of the Jacobian matrix is a well-known algorithm for control of redundant robotic manipulators. Several authors have observed instabilities in simulations using this algorithm. In this paper the cause of the instability is determined, and the corresponding growth of joint velocities and acceleration is characterized in terms of the smallest singular value of the Jacobian matrix of the kinematic function, and a nearly-conserved quantity analogous to angular momentum. The analysis is supported by simulations. Based on the analysis, a stabilizing modification to the control scheme is derived that does not rely on the simple addition of a kinematic component, but rather addresses the cause of the instability directly. Simulations show that the new control algorithm stabilizes the motion in agreement with the analysis.

ICRA Conference 1999 Conference Paper

Using Redundancy to Reduce Accelerations Near Kinematic Singularities

• Kevin A. O'Neil
• Yu-Che Chen

Solutions of the inverse kinematic problem for redundant manipulators are studied for workspace paths that pass near, but not through, kinematic singularities. Widely-used algorithms such as pseudo-inverse velocity control may compute solution paths that have low joint velocities but very high joint accelerations, which may not be physically desirable or even feasible. In this paper we show the existence of solutions with low joint space acceleration, which use the redundancy of the mechanism to track the workspace trajectory by means of a more natural joint motion. We derive a redundancy resolution algorithm at the velocity level that is computationally undemanding and tracks the desirable low-acceleration solution path. Simulations illustrating the feasibility of the proposed algorithm are presented.

IROS Conference 1998 Conference Paper

On the fixturing of non-prismatic workpieces under frictionless contact models

• Yu-Che Chen

The goal of this paper is to provide guidelines for the fixturing of nonprismatic workpieces based on the mechanics of frictionless contact model. Geometries of stable fixtures are studies in this analysis. Previous works on the fixturing of prismatic workpieces are extended to the fixturing of nonprismatic workpieces.

IROS Conference 1998 Conference Paper

On the fixturing of polyhedral workpieces under frictional contact models

• Yu-Che Chen
• C. L. Philip Chen

Most analyses in fixture designs are conducted for prismatic workpieces based on the assumption that the contacts between a workpiece and its surrounding fixels are frictionless. To improve on the existing analyses, the goal of this paper is to study the fixturing of polyhedral workpieces under frictional point contact and soft-finger contact models. Geometries of stable fixtures are investigated based on the mechanics of these contact models.

ICRA Conference 1997 Conference Paper

A predictive algorithm for rate control of mechanisms near singularities

• Yu-Che Chen
• Jiaqing Seng
• Kevin A. O'Neil

A new kinematic algorithm for the control of mechanisms is described, which produces smoother and more accurate control near kinematic singularities than the usual damped least-squares method. The new method predicts the occurrence of singularities, evaluates the singular terms of the inverse kinematic function using a nonsingular expression, and smoothly interpolates between the singular and nonsingular forms. The desingularized expression makes use of the rates of change of the singular vectors and values of the Jacobian matrix; formulas for these derivatives are presented. Simulations of the control of two and six joint serial manipulators moving through and near singular configurations are given to illustrate the method.

ICRA Conference 1997 Conference Paper

Lowest-order rate control of mechanisms near singularities

• Yu-Che Chen
• Jiaqing Seng
• Kevin A. O'Neil

Resolved motion rate control, which uses a linear approximation to the kinematic function to find joint velocities, can fail at kinematic singularities. Pohl and Lipkin (1991) proposed rate control using a quadratic approximation to the full kinematic function, but this algorithm has disadvantages. Here we propose a hybrid algorithm that works at singularities, is computationally efficient, and finds all physically meaningful solution trajectories. Furthermore, it is shown that this algorithm finds smoother solutions than, the damped least squares algorithm for paths passing through singular points.

ICRA Conference 1996 Conference Paper

Desingularization of resolved motion rate control of mechanisms

• Kevin A. O'Neil
• Yu-Che Chen
• Jiaqing Seng

Resolved motion rate control is an algorithm for solving the path-tracking problem in robotic control which can fail at singular points of the kinematic function. In this paper we find a new second-order condition which, when satisfied, ensures the existence of a solution path with continuous, bounded joint rates. The condition is related to the curvature of the path at the singular value. As an application, we give a sufficient condition for the existence of self-motion for redundant manipulators at singular points. We derive a simple formula for the rate of recovery of the manipulability measure along the path at the singularity, and prove that the usual resolved motion rate control algorithm can be modified to compute these solution paths. Several realistic simulations are presented.

ICRA Conference 1996 Conference Paper

The importance of sequence in clamping prismatic workpieces in fixturing processes

• Yu-Che Chen
• C. L. Philip Chen

The purpose of this paper is to demonstrate the importance of clamping sequences in fixturing processes. We analyze the stability of fixturing prismatic workpieces during different stages of clamping. Sequences of clamping described in this research assure the stability of the workpiece during clamping processes and relax the stringent requirement on the positioning accuracy of placing the clamps.

ICRA Conference 1995 Conference Paper

A Sufficient Condition of Force Closure and Force Distribution for Grasping Solid Objects

• Yu-Che Chen

This paper presents an analysis on the mechanics of grasping general solid objects under frictional point contact model. Any disturbing force/moment pair at the mass center of an object can be balanced (or force-closured) if there exist a particular set of internal forces falling inside the friction cone of the contact points. Based on this principle, sufficient conditions for the geometrical properties of the contact points guaranteeing the existence of such internal contact forces are investigated.

IROS Conference 1995 Conference Paper

Escapability of singular configuration for redundant manipulators via self-motion

• Jiaqing Seng
• Kevin A. O'Neil
• Yu-Che Chen

The goal of this paper is to study the behavior of general redundant manipulators near singular configurations, and investigate the escapability of redundant manipulators at a singularity. Escapability means that the manipulator can actually reconfigure itself from a singular posture to a nonsingular posture via self-motion. Criteria for the classifications of escapable and inescapable singularities are established. Examples are given to demonstrate the use of these criteria.

ICRA Conference 1994 Conference Paper

A Consistent Approach to the Instantaneous Kinematics of Redundant, Non-Redundant and In-Parallel Manipulators

• Yu-Che Chen
• Ian D. Walker

A unified method for the instantaneous kinematics of redundant, nonredundant and in-parallel manipulators is developed. For redundant arms, we investigate the relationship between kinematic and algorithmic singularities when the method of extended Jacobian is used for task optimization. Under the same model, it is found that, for serial manipulators to move in a point-to-point fashion and for general motion of in-parallel manipulators, their instantaneous kinematics take a similar format as the method of extended Jacobian for redundant arms. Thus, our method of solution can be applied to these manipulators. Several features for the manipulators are demonstrated based on the concept of task separation in the row space of the Jacobian of these manipulators. >

IROS Conference 1993 Conference Paper

An analysis of contact forces decomposition for multi-fingered grasping

• Yu-Che Chen
• Ian D. Walker

Presents an analysis of the mechanics for multi-fingered grasps of planar objects. The main feature in the approach followed is a novel decomposition of the contact finger forces. At each grasp point, a contact force is resolved into components along the normal and tangential directions. This concept of decomposition gives deeper physical insights into the roles of the normal and tangential finger force components for multi-fingered manipulation.

IROS Conference 1992 Conference Paper

Technology For Dexterous Remote Manipulation

• Yu-Che Chen
• Ian D. Walker
• John B. Cheatham