Ping Hsu

Adaptive control of mechanical manipulators

When an accurate dynamic model of a mechanical manipu lator is available, it may be used in a nonlinear, model-based scheme to control the manipulator. Such a control formula tion yields a controller that suppresses disturbances and tracks desired trajectories uniformly in all configurations of the manipulator. Use of a poor dynamic model with this kind of model-based decoupling and linearizing scheme, however, may result in performance that is inferior to a much simpler, fixed-gain scheme. In this paper, we develop a parameter-adaptive control scheme in a set of adaptive laws that can be added to the nonlinear, model-based controller. The scheme is unique be cause it is designed specifically for the nonlinear, model- based controller and has been proven stable in a full, nonlin ear setting. After adaptation, the error dynamics of the joints are decoupled with uniform disturbance rejection in all ma nipulator configurations. The issues of sufficient excitation and the effect of disturbances are also discussed. The theory is demonstrated with simulation results and also with data from an implementation for an industrial robot, the Adept One.

Grasping and Coordinated Manipulation by a Multifingered Robot Hand

A new avenue of progress in the area of robotics is the use of multifingered robot hands for fine motion manipulation. This paper treats two fundamental problems in the study of multi fingered robot hands: grasp planning and the determination of coordinated control laws with point contact models. First, we develop the dual notions of grasp stability and grasp manipulability and propose a procedure for task modeling. Using the task model, we define the structured grasp quality measures, and using these measures we then devise a grasp planning algorithm. Second, based on the assumption of point contact models, we develop a computed torque-like con trol algorithm for the coordinated manipulation of a multi fingered robot hand. This control algorithm, which takes into account both the dynamics of the object and the dynamics of the hand, will realize simultaneously both the position trajec tory of the object and any desired value of internal grasp force. Moreover, the formulation of the control scheme can be easily extended to allow rolling and sliding motion of the fingers with respect to the object.

Dynamic control of sliding by robot hands for regrasping

The problem of dynamic control of a multifingered hand manipulating an object is considered, under the condition that some of the fingertips slide on the object surface. This work has many useful applications when considered in conjunction with work already done in the area of regrasping. In performing certain tasks with grasped objects, it is often necessary to change the contact locations of the fingers on the object. One method of achieving this is to break and remake the contacts; another method is to slide the fingertips on the object surface. This work provides a dynamic coordinated control scheme for a hand by which one can perform regrasping and reorientation of an object in the planar case. >

Adaptive identification and control for manipulators without using joint accelerations

We present a new scheme for the adaptive control of mechanical manipulators along with proof of convergence. This work is an extension of our earlier work [Craig, Hsu and Sastry] [1]. The new scheme does not require the measurement of joint accelerations and needs less computation. We illustrate the theory with some simulations.

Dynamic regrasping by coordinated control of sliding for a multifingered hand

The authors consider the problem of grasp choice for an object held within a multifingered hand from the viewpoint of avoiding collisions between the manipulator links and the object during trajectory execution. A grasp planner is provided in the form of an algorithm that checks the feasibility of a given object trajectory and provides an envelope of feasible contact positions. During execution of the trajectory, contact positions of the fingertips on the object can be changed by sliding the fingertip along the object surface in a controlled manner. A dynamic control law that achieves this is presented and integrated with the grasp planner to determine a dynamic regrasping algorithm, which is illustrated by simulation.<<ETX>>

Dynamic control of redundant manipulators

Redundant manipulators provide increased flexibility for the execution of complex tasks. The redundancy of such manipulators can be effectively used to avoid obstacles, avoid singularities, and maintain a high degree of manipulability while performing the desired end effector task. The extra degrees of freedom of a redundant manipulator are exhibited as joint velocities that do not contribute to the velocity of the end effector. In this paper, we provide a dynamic control law that guarantees the tracking of a given end effector trajectory and also provides for the control of the redundant joint velocity. The desired redundant joint velocity can then be specified to optimize a cost function over the configurations allowed by the extra degrees of freedom that achieve the given end effector position.The authors provide a dynamic control law that guarantees the tracking of a given end-effector trajectory and also provides for the control of the redundant joint velocity. The desired redundant joint velocity can then be specified to optimize a cost function over the configurations allowed by the extra degrees of freedom that achieve the given end-effector position.<<ETX>>

Control of multimanipulator systems-trajectory tracking, load distribution, internal force control, and decentralized architecture

The author proposes a coordinated control law for a multimanipulator system performing parts-matching tasks. This control law enables the manipulators to perform the preplanned parts-matching maneuver while the entire parts-matching system is driven to follow a desired path. Manipulators are essentially treated as six-degree-of-freedom actuators with some nonlinear dynamics, which exert a set of contact forces on the object so that trajectory tracking is achieved and the desired internal force is realized. When the parts-matching system consists of only a single object, the control law degenerates to an expression that will drive a group of manipulators transporting a single object. A load-sharing scheme minimizes the weighted norm of the force applied to the object. In this way, a heavily weighted direction tends to get less load. This scheme does not require a force sensor. The author also discusses the choosing of the weighting factor and shows that the proposed control law can be implemented in a decentralized fashion.<<ETX>>

On grasping and coordinated manipulation by a multifingered robot hand

Two problems in the study of multifingered robot hands are considered, namely grasp planning and the determination of coordinated control laws with point contact models. using the dual notions of grasp stability and manipulability, and a procedure previously developed for task modeling, the structure grasp quality measures are defined. These measures are then integrated to devise a grasp planning algorithm. Based on the assumption of point contact models, a computed-torque-like control algorithm is developed for the coordinated manipulation of a multifingered robot hand. This control algorithm, which takes into account both the dynamics of the object and the dynamics of the hand, is computationally effective and can be generalized to allow rolling motion of the object with respect to the fingertip.<<ETX>>

The effect of discretized feedback in a closed loop system

When a continuous time control law is implemented using a digital computer, the closed loop system may not have the same stability properties as the system with a true continuous controller due to delay and digitization errors. Using a Lyapunov analysis, this paper shows that, for linear systems and a class of nonlinear systems with discretized feedback, some stability properties can be preserved if the sampling frequency is properly chosen. In particular, we propose a variable sampling interval scheme for linear systems. This scheme is desirable when (1) computer resources are tightly shared by many tasks or (2) power consumption is critical. The effect of truncation error is also studied.

Dynamic Coordination of a Multiple Robotic System with Point Contact

This paper treats two fundamental problems of a multiple robotic system: grasp planning and the determination of coordinated control laws with point contact models. First, using the dual notions of grasp stability and manipulability and a procedure we have previously developed for task modeling we define the structure grasp quality measures These measures are then integrated to devise a grasp planing algorithm. Second, based on the assumption of point contact models, we develop a computed-torque-like control algorithm for the coordinated manipulation of a multifingered robot hand. This control algorithm, which takes into account both the dynamics of the object and the dynamics of the manipulators, is computationally effective and can be easily generalized to allow rolling motion of the object with respect to the fingertip.