Matthew T. Mason

Compliance and Force Control for Computer Controlled Manipulators

Compliant motion of a manipulator occurs when the manipulator position is constrained by the task geometry. Compliant motion may be produced either by a passive mechanical compliance built in to the manipulator, or by an active compliance implemented in the control servo loop. The second method, called force control, is the subject of this paper. In particular a theory of force control based on formal models of the manipulator and the task geometry is presented. The ideal effector is used to model the manipulator, the ideal surface is used to model the task geometry, and the goal trajectory is used to model the desired behavior of the manipulator. Models are also defined for position control and force control, providing a precise semantics for compliant motion primitives in manipulation programming languages. The formalism serves as a simple interface between the manipulator and the programmer, isolating the programmer from the fundamental complexity of low-level manipulator control. A method of automatically synthesizing a restricted class of manipulator programs based on the formal models of task and goal trajectory is also provided by the formalism.

Robot hands and the mechanics of manipulation

The abridged contents include: Kinematic and force analysis of articulated hands: contact - freedom and constraint; contacts in groups; force application and velocity analysis; force error analysis. Manipulator grasping and pushing operations: theory of pushing; application; conclusion. Index. This book, based on the doctoral dissertations of the two authors, examines several aspects of manipulating objects. At present, the authors believe that industrial robots are not used effectively. Tasks performed by robot manipulators are now limited to simple packing and stacking operations. By understanding the principles discussed in this book, better industrial robots are presented.

Automatic Synthesis of Fine-Motion Strategies for Robots

Active compliance enables robots to carry out tasks in the presence of significant sensing and control errors. Compliant motions are quite difficult for humans to specify, however. Furthermore, robot programs are quite sensitive to details of geometry and to error characteristics and must, therefore, be constructed anew for each task. These factors motivate the search for automatic synthesis tools for robot program ming, especially for compliant motion. This paper describes a formal approach to the synthesis of compliant-motion strategies from geometric descriptions of assembly operations and explicit estimates of errors in sensing and control. A key aspect of the approach is that it provides criteriafor correct ness of compliant-motion strategies.

Mechanics and planning of manipulator pushing operations

Pushing is an essential component of many manipulator operations. This paper presents a theoretical exploration of the mechanics of pushing and demonstrates application of the theory to analysis and synthesis of robotic manipulator oper ations.

An exploration of sensorless manipulation

An autonomous robotic manipulator can reduce uncertainty in the locations of objects in either of two ways: by sensing, or by motion strategies. This paper explores the use of motion strategies to eliminate uncertainty, without the use of sensors. The approach is demonstrated within the context of a simple method to orient planar objects. A randomly oriented object is dropped into a tray. When the tray is tilted, the object can slide into walls, along walls, and into corners, sometimes with the effect of reducing the number of possible orientations. For some objects a sequence of tilting operations exists that leaves the objects orientation completely determined. The paper describes an automatic planner that constructs such a tilting program, using a simple model of the mechanics of sliding. The planner has been implemented, the resulting programs have been executed using a tray attached to an industrial manipulator, and sometimes the programs work. The paper also explores the issue of sensorless manipulation, tray-tilting in particular, within the context of a formal framework first described by Lozano-Perez, Mason, and Taylor [1984]. It is observed that sensorless motion strategies perform conditional actions using mechanical decisions in place of environmental inquiries.

Stable pushing: mechanics, controllability, and planning

We would like to give robots the ability to position and orient parts in the plane by pushing, particularly when the parts are too large or heavy to be grasped and lifted. Unfortunately, the motion of a pushed object is generally unpredictable due to unknown support friction forces. With multiple pushing contact points, however, it is possible to find pushing directions that cause the object to remain fixed to the manipulator. These are called stable pushing directions. In this article we consider the problem of planning pushing paths using stable pushes. Pushing imposes a set of nonholonomic velocity constraints on the motion of the object, and we study the issues of local and global controllability during pushing with point contact or stable line contact. We describe a planner for finding stable pushing paths among obstacles, and the planner is demon strated on several manipulation tasks.

Two-Dimensional Rigid-Body Collisions With Friction

This paper derives solutions for frictional planar rigid body collisions, using Rouths impact process diagrams, for both Newtonian and Poisson restitution.

Dynamic nonprehensile manipulation: Controllability, planning, and experiments

We are interested in using low-degree-of-freedom robots to perform complex tasks by nonprehensile manipulation (manipulation without aformorforce-closure grasp). By notgrasping, the robot can usegravitational, centrifugal, and Coriolisforces as virtual motors to control more degrees of freedom of the part. The part s extra motionfreedoms are exhibited as rolling, slipping, and free flight. This paper describes controllability, motion planning, and implementation ofplanar dynamic nonprehensile manipukltion. We show that almost any planar object is controllable by point contact, and the controlling robot requires only twvo degrees of freedom (a point translating in the plane). We then focus on a one-joint manipulator (with a two-dimensional state space), and show that even this simplest of robots, by using slipping and rolling, can control a planar object to a fulldimensional subset of its six-dimensional statespace. We have developed a one-jointrobotto perform a variety of dynamic tasks, including snatching an object ftom a table, rolling an object on the surface of the arm, and throwing and catching. Nonlinear optimization is used to plan robot trajectories that achieve the desired object motion via coupling forces though the nonprehensile contact.

Time Optimal Trajectories for Bounded Velocity Differential Drive Vehicles

This paper presents the time optimal trajectories for differential drive vehicles in the unobstructed plane. The wheel angular velocities are bounded, but may be discontinuous. The paper proves the existence of optimal controls, derives the structure of optimal trajectories, and develops an algorithm for producing a time optimal trajectory between any two configurations. Every nontrivial optimal trajectory is composed of straight segments alternating with turns about the robots center. Optimal trajectories may have as many as five actions, but four actions are sufficient—for every optimal trajectory of five actions, there is an equally fast trajectory with four actions.

Posing polygonal objects in the plane by pushing

The authors study the use of pushing actions with a fence to orient and translate objects in the plane. They describe a planner which a guaranteed to construct a sequence of pushing actions to move any polygonal object from any initial configuration to any final configuration. This planner, which utilizes an analysis of the mechanics of pushing an object, generates open-loop plans which do not require feedback sensing. These plans are guaranteed to succeed provided certain physical assumptions are met. Results of experiments conducted to demonstrate the generated plans are presented.<<ETX>>