Qixian Zhang

Kinematic control of redundant robots and the motion optimizability measure

This paper treats the kinematic control of manipulators with redundant degrees of freedom. We derive an analytical solution for the inverse kinematics that provides a means for accommodating joint velocity constraints in real time. We define the motion optimizability measure and use it to develop an efficient method for the optimization of joint trajectories subject to multiple criteria. An implementation of the method for a 7-dof experimental redundant robot is present.

Inverse kinematics and dynamics of the 3-RRS parallel platform

This paper presents the formulation of the inverse kinematics and dynamics of the 3-RRS parallel platform with three degrees of freedom. For inverse kinematics, the position analysis is first performed. Then the differential motion constraint equations of the movable platform are established, based on which the velocity and acceleration formulae of leg actuators are derived. In the inverse dynamic analysis, the parallel platform is decomposed into two parts through parting the three spherical joints, and the forces acting on the parted joints are determined according to the moment (and force) equilibriums of the legs and the movable platform. Subsequently, the analytic expressions of the driving moments of the leg actuators are derived by means of the moment equilibrium of the legs. Although only the 3-RRS parallel platform is analyzed, the presented method and process can also be applied to other less DOF parallel platforms.

Optimal grasps for planar multifingered robotic hands

A fast and simple algorithm for determining optimal grasps of a planar multifingered robotic hand is presented. Using a representation for the stiffness of a planar grasp system, the authors derive conditions for static equilibrium, task-oriented grasping, and stability. An optimization model for determining optimal grasps of a planar multifingered hand is established using a quality measure which includes the effect of the external disturbing forces and moments. A constrained gradient descent technique is used to compute the optimal grasps. An example demonstrates the applicability and effectiveness of the theory. >

Optimizing multiple performance criteria in redundant manipulators by subtask-priority control

We consider the simultaneous optimization of multiple performance criteria with one redundant degree of freedom, using the concepts of the optimization expectation and the optimization assignment. A method is described for determining subtask-priority in real time. Activities are divided into a main task having high priority and several subtasks having lower priority, where subtasks can be performed by applying redundant operation to the main task. A modified weighted gradient projection method is developed to simultaneously optimize multiple performance criteria for the redundant manipulator. A simulation example demonstrate the effectiveness of the approach.

Classification of grasps by robot hands

Grasping robotic hands is classified into three categories based on the object connectivity. We decompose the space of contact forces into four subspaces and develop a method to determine the dimensions of the subspaces with respect to the connectivity of the grasped object. The relationships we obtain reveal the kinematic and static characteristics of three categories of grasps. It indicates how contact forces can be decomposed corresponding to each type of grasp. The technique also provides a guideline for determining the distribution of contact forces on grasped objects. We analyze how power grasps are identified from the object connectivity and used to synthesize hand configurations for grasping and manipulation tasks. A physical interpretation of the subspaces and the determination of their dimensions are illustrated by examples.

Real-time control of redundant robots subject to multiple criteria

To realize control of robotic manipulators with redundant degrees of freedom, the concept of a motion optimizability measure is introduced. We derive an optimal solution technique for the inverse kinematics of a redundant robot that achieves real-time control with multiple performance criteria. Using this technique, we propose an efficient method for kinematic control. Experimental results are presented for a 7 DOF manipulator.

Control of redundant robots based on the motion optimizability measure

To realize the control of redundant manipulators, the concept of the motion optimizability measure is introduced. We derive an analytical solution technique for the inverse kinematics of a redundant robot that reduces the computational complexity and does not require the pseudoinverse Jacobian. Based on this technique, we propose an efficient, kinematic optimal control scheme. We show how to incorporate multiple criteria in the formulation. An online implementation of the system that ensures stable operation is presented.<<ETX>>

Singularity avoidance based on avoiding joint velocity limits for redundant manipulators

Redundant robots are characterized by dexterity, to overcome the difficulty of calculation when traditional manipulability is used for dexterity control, the paper researches the inner relation and contradictory conflict of the unity which consists of joint geometrical positions, orientations and joint velocities. A new idea to improve the weighted manipulability ellipsoid by optimizing joint velocities is presented thus geometrical position and orientation of the robot may be indirectly optimized by means of the feedback of joint velocities. Simulation verifies this scheme is very useful for real time control because of its simple arithmetic.

An algorithm for the kinematics of dexterous manipulation

In order for a multi-fingered hand to dexterously manipulate an object, the joint motions must be determined from the desired object motions. In this paper, we use orthogonal decompositions of the spaces of object velocities and joint velocities to develop an algorithm for formulating differential relationships between the motions of the object and joints. The relationships of all possible object velocities and joint velocities satisfying the contact constraints are expressed. The relationships from mobility analysis are also obtained. The method is applicable to all manipulation systems and contact models. Three examples are presented to illustrate the effectiveness of the algorithm.

A new approach to the kinematic control of redundant manipulators

A modified weighted gradient projection method is presented for the optimization of kinematics of redundant robotic manipulators. The method utilizes the matrix weightability measure and the self-motion declinability measure. By these means the weighting matrix and the self-motion amplitude can be determined from the homogeneous solution. Because the optimization method is based on a least-norm solution, it can obtain optimal solutions at low joint velocities. This characteristic has been verified by simulation.