Stephen Cameron

Collision detection by four-dimensional intersection testing

The collision-detection problem is to decide, given two objects and desired motions, whether the objects will come into collision over a given time span. The solution of this problem is useful, both in robotics and in other problem domains. A method is described for solving collision detection that involves transforming the problem into an intersection-detection problem over space time. The theoretical basis for the solution is given and an efficient implementation described based on describing the objects and motions constructively. The related problems of describing the collision region and detecting collisions when there are more than two moving objects are also described. >

Experiments in automatic flock control

Abstract The Robot Sheepdog Project has developed a mobile robot that gathers a flock of ducks and manoeuvres them safely to a specified goal position. This is the first example of a robot system that exploits and controls an animal’s behaviour to achieve a useful task. A potential-field model of flocking behaviour was constructed and used to investigate methods for generalised flock control. One possible algorithm is described and demonstrated to work both in simulation and in the real world.

A study of the clash detection problem in robotics

To solve the clash detection problem we must decide whether a collision will occur between any pair of objects from a set of objects with known shapes and motions. We have considered three methods for performing clash detection: in the first we sample the motion at a finite number of times and perform interference detection at each time; in the second we create models of the shapes and their motions in space-time, and look for intersections between these four-dimensional entities; and in the third we create models of the volumes swept out by the objects. This paper is a brief, comparative study of these three methods, and includes some details of our experiments with the first two methods as implemented in a geometric modelling system.

The virtual springs method: path planning and collision avoidance for redundant manipulators

Potential-field based methods for path planning have the ad vantage of speed, but also a reputation for getting stuck in local minima when used with complex systems such as re dundant manipulators. We have developed a novel method of setting up a potential-field system that replaces rigid links by stiff (virtual) springs. The method is fast, directly suitable for parallel processing, and experimentally much less prone to the danger of local minima. Although quite general, it is par ticularly suited to use with manipulators whose links are thin compared with their length.

Approximation hierarchies and S-bounds

S-bounds were introduced as a preprocessing step for intersection algorithms using Constructive Solid Geometry (CSG) models, and have been used to obtain order-of-magnitude speedups in interference and collision detection for robotic devices. The key to an S-bound system is a set of initiaJ approximations to prirm”tive shapes in the CSG system; our original implementations used rectangular boxes in space (and prisms in space-time) to obtain an S-bound system in which the preprocessing time is tiny, compared with the time to execute a ‘classical’ in tersection test. However any form of approximation may be used in an S-bound system, and better approximation classes (such as convex hulls) may provide a better (smaller) region to search after preprocessing, at the expense of a larger preprocessing time. We describe an S-bound scheme that uses a hierarchy of two approximation types, namely aligned boxes and convex polyhedra, and provide examples of its use within our geome tric modelling system. The scheme is shown to provide ‘good’ bounds, although the overall utility of the scheme is doubtful in more than two dimensions without hardware support.

Efficient bounds in constructive solid geometry

Testing for intersection between geometric entities in ray casting is normally performed by intersecting a ray (a semi-infinite line) against the surface elements of a geometric model. Simple reasoning about the extent of each geometric entity significantly reduces the time required by such algorithms. If the ray and the geometric entities are boxed, one first tests to see whether the box around the ray and the box around a geometric entity overlap. Only if the boxes overlap does one continue to test to determine whether the ray and the entity overlap. A way to add boxes, called the S-bounds method, is described, and work to data on extending it is summarized. The method is useful for interference-detection and collision-detection problems.<<ETX>>

Robot control of animal flocks

The Robot Sheepdog Project has developed a mobile robot that gathers a flock of ducks and manoeuvres them safely to a specified goal position. This is the first example of a robot system that exploits and controls an animals behaviour to achieve a useful task. A potential-field model of flocking behaviour was constructed and used to aid the design of two novel flock-control methods. These methods are described and evaluated in a series of simulated and real-world experiments.

Obstacle avoidance and path planning

Outlines the state‐of‐the‐art in obstacle avoidance and path planning for industrial robots that is practical on the current generation of computer hardware. Describes practical vehicle planners and planning for manipulators. Summarizes that obstacle avoidance and path planning are techniques with differing goals. Sonar is the standard method of obstacle avoidance systems which is largely limited by the reliability of the sensors used. Path planning however is limited by two things: the algorithms used and the quality of the data available to planners. Concludes that it is now possible to produce path planning and obstacle avoidance systems that can be used in practical robotic systems.

Computing signed distances between free-form objects

Given two sculptured objects, described by a collection of rational Bezier patches, we propose an algorithm that provides the distance between them if the objects are not intersecting, or a measure of penetration otherwise. The algorithm extends the upper-lower bound subdivision approach to the computation of the nearest points by considering the relative orientations between the subdivided patches. All required geometric constructions can be described as rational Bezier patches so that their control points can be precomputed from those of the original patches. Additional operations have been designed to exhibit linear complexity with the total number of involved control points.

ROBMOD: a geometry engine for robotics

A geometric engine called ROBMOD is described that has been designed specifically for intelligent robotics applications. ROBMOD is usable both as a geometric modelling system, based on the constructive solid geometry paradigm, and as a geometry engine that can be interrogated by programs that are reasoning about space. ROBMOD is thus able to reduce the geometric sophistication required in such spatial reasoning programs.<<ETX>>