Exploring planetary surfaces typically involves traversing challenging and unknown terrain and acquiring insitu measurements at designated locations using arm-mounted instruments. We present field results for a new implementation of an autonomous capability that enables a rover to traverse and precisely place an arm-mounted instrument on remote targets. Using point-and-click mouse commands, a scientist designates targets in the initial imagery acquired from the rover's mast cameras. The rover then autonomously traverses the rocky terrain for a distance of 10 - 15 m, tracks the target(s) of interest during the traverse, positions itself for approaching the target, and then precisely places an arm-mounted instrument within 2-3 cm from the originally designated target. The rover proceeds to acquire science measurements with the instrument. This work advances what has been previously developed and integrated on the Mars Exploration Rovers by using algorithms that are capable of traversing more rock-dense terrains, enabling tight thread-the-needle maneuvers. We integrated these algorithms on the newly refurbished Athena Mars research rover and fielded them in the JPL Mars Yard. We conducted 43 runs with targets at distances ranging from 5 m to 15 m and achieved a success rate of 93% for placement of the instrument within 2-3 cm.

We present an overview of the CLARAty architecture which aims at developing reusable software components for robotic systems. These components are to support autonomy software which plans and schedules robot activities. CLARAty modifies the conventional 3-level robotic architecture into a 2-layered design: the functional layer and the decision layer. The former provides a representation of the system components and an implementation of their functionalities. The latter is the decision-making engine that drives the former. It globally reasons about the goals, system resources, and system state. The functional layer is composed of a set of interrelated object-oriented hierarchies consisting of active and passive objects that represent the system abstraction levels. We present an overview of the design of the functional layer. It is decomposed into a set of reusable core components and a set of extended components that adapt the reusable set to different hardware implementations. The reusable components provide interface definitions and implementations of basic functionality, provide local executive capabilities, manage local resources, and support decision layer queries.

Robust navigation through rocky terrain by small mobile robots is important for maximizing science return from missions to Mars. We are addressing this problem at multiple levels through the development of intelligent sequencing, sensor constrained path planning, natural terrain visual localization, and real-time state estimation. Each of these techniques is described and their complementary aspects discussed. Experimental results are provided from implementation on our Mars rover prototype operating in realistic scenarios.

Upcoming missions to the surface of Mars will use mobile robots to traverse long distances from the landing site. To prepare for these missions, the prototype rover, Rocky 7, has been tested in desert field trials conducted with a team of planetary scientists. While several new capabilities have been demonstrated, foremost among these was sun-sensor based traversal of natural terrain totaling a distance of one kilometer. This paper describes navigation results obtained in the field tests, where cross-track error was only 6% of distance traveled. Comparison with previous results of other planetary rover systems shows this to be a significant improvement.

This paper describes the design and implementation at the Jet Propulsion Laboratory of a small rover for future Mars missions requiring long traverses and rover-based science experiments. The small rover prototype, called Rocky 7, is capable of long traverses, autonomous navigation, and science instrument control. This rover carries three science instruments, and can be commanded from any computer platform from any location using the World Wide Web. In this paper we describe the mobility system, the sampling system, the sensor suite, navigation and control, onboard science instruments, and the ground command and control system. We also present key accomplishments of a recent field test of Rocky 7 in the Mojave Desert in California.

This paper provides an overview of a new manipulation system developed for sampling and instrument placement from small autonomous mobile robots for Mars exploration. Selected out of the design space, two manipulators have been constructed and integrated into the Rocky 7 Mars rover prototype. This paper describes the design objectives and constraints for these manipulators, and presents the finished system and some results from its operation.

The port-based object is a new software abstraction for designing and implementing dynamically reconfigurable real-time software. It forms the basis of a programming model that uses domain-specific elemental units to provide specific, yet flexible, guidelines to control engineers for creating and integrating software components. We use a port-based object abstraction, based on combining the notion of an object with the port-automaton algebraic model of concurrent processes. It is supported by an implementation using domain-specific communication mechanisms and templates that have been incorporated into the Chimera real-time operating system and applied to several robotic applications. This paper describes the port-based object abstraction, provides a detailed analysis of communication and synchronization based on distributed shared memory, and describes a programming paradigm based on a framework process and code templates for quickly implementing applications.

This paper provides a system overview of a new Mars rover prototype, Rocky 7. We describe all system aspects: mechanical and electrical design, computer and software infrastructure, algorithms for navigation and manipulation, science data acquisition, and outdoor rover testing. In each area, the improved or added functionality is explained in a context of its path to flight, and need within the constraints of desired science missions.

In 1996, NASA will launch the Mars Pathfinder spacecraft, which will carry an 11 kg rover to explore the immediate vicinity of the lander. To assess the capabilities of the rover, the authors have constructed a new microrover testbed consisting of the Rocky 3.2 vehicle and an indoor test arena containing Mars analog terrain and overhead cameras for automatic, real-time tracking of the true rover position and heading. In this paper, the authors present initial performance evaluation results obtained with this testbed. The authors first decompose rover navigation into four major functions: goal designation, rover localization, hazard detection, and path selection. The authors then describe the Mars Pathfinder approach to each function, present results to date of evaluating the performance of each function, and outline their approach to enhancing performance for future missions. The results show key limitations in the quality of rover localization, the speed of hazard detection, and the ability of behavior control algorithms for path selection to negotiate the rock frequencies likely to be encountered on Mars.

To visually determine the position and orientation of a mobile robot from a fixed location in its vicinity, the authors have employed a cylindrical target which has different colors in each of its four quadrants. By judicious selection of the colors, segmentation of imagery from the fixed location can determine the size and centroid of the cylinder, as well as the visible color quadrants. Both the cylinder size in monocular images, and the centroid disparity in stereo pairs, are shown to provide a measure of distance. The angle of the cylinder is determined by analyzing which color quadrants are visible and to what degree. Implementation and experimental testing of this technique shows that it provides accurate localization data to within one or two pixels of error.

This paper provides an overview of the authors' selection process for proximity sensors for manipulator collision avoidance. Five categories of sensors have been considered for this use in space operations: intensity of reflection, triangulation, time of flight, capacitive, and inductive. From these categories, the most promising commercial and mature laboratory prototype sensors have been selected and tested. After reviewing the selection process and the experimental results, conclusions are drawn about which sensors are best and why.>

This report details the authors' development of a computer controlled spotlight with four actuated degrees-of-freedom for pan, tilt, linear movement, and beam focusing. The authors review the mechanical structure, kinematic model, calibration data, and control algorithm. The resultant system enables the spotlight to maintain constant intensity of the illumination of a surface, while changing the incident angle of the light. In this way, the authors effectively simulate the changing illumination experienced in low earth orbit. The simulation system is currently used to test human and computer inspection of orbital platform mockups for the Space Station freedom.>

The authors present results of experimental tests of a broad spectrum of basic force control strategies: proportional with feedforward, integral, filtered feedback proportional-derivative, and second-order low-pass filtering. The experiments were performed on the CMU Direct Drive Arm II so that the results would reflect the nature of the controller and not the unmodeled aspects of the system which comprises the plant. The commonality amongst the experiments was such that it was possible to compare and contrast the strategies objectively and to draw conclusions about the efficacy of each. The results support previous analysis and show the superiority of integral force control for force trajectory tracking.>

Abstract—In this paper we develop a framework for integrating real-time software modules that comprise a reconfigurable multisensor based system. Our framework is based on the proposed concept of a global database of state information through which realtime software modules exchange information. This methodology allows the development and integration of reusable software in a complex multiprocessing environment. A reconfigurable sensor-based control system consists of many software modules, each of which can be modelled using a simplified version of a port automaton. Our new state variable table mechanism can be used in both statically and dynamically reconfigurable systems, and it is completely processor independent. Individual modules may also be combined into larger modules to aid in building large systems, and to reduce bus and CPU utilization. An efficient implementation of the state variable table mechanism, which has been integrated into the Chimera II Real-Time Operating System, is also described. 1 Keywords—reconfigurable sensor-based control systems, reusable software, real-time operating systems, interprocessor communication, spin-locks, real-time software modelling. I.

This paper presents a complete overview of basic strategies that have been proposed for force control of robot manipulators. First, the model of the plant to be controlled is reviewed. Next, the strategies are divided into force-based and position-ba9ed categories, according to previously reported implementations. Each of the controller types within these categories are analyzed, and predictions of stability and efficacy are made. Then it is shown that these two categories are actually the same, and this recognition leads to the concept of a novel second order low pass filter controller. All of the predictions that result from this analysis have been experimentally confirmed. Among the contributions presented here is a comprehensive explanation of why integral gain control is the best method of force control for manipula t om.

The authors present experimental results from a strategy for stable hard-on-hard impacts of a robot manipulator, and show how this method fits into the framework of force control. Previously proposed methods of impact control are reviewed, and their inadequacies are revealed. A fourth-order model of an arm/sensor/environment system is reviewed. It is pointed out how proportional gain force control with feedforward and impedance control are equivalent when the environment is stiff. An analysis of the model and the controllers shows the gain selection that provides the best impact response. Extensive experimental results confirm the correctness of the model, the equivalence of the controllers, and the efficacy of this impact control strategy.>

A potential function based on superquadrics is presented that closely models a large class of object shapes. This potential function also prevents the creation of local minima when it is added to spherically symmetric attractive wells. Two compatible forms of the superquadric potential function are introduced: one for obstacle avoidance, and another for obstacle approach. The avoidance and approach potentials are implemented in simulations. In these simulations the end effector of the manipulator experiences an attractive force from a global spherical well, while the end effector and each of the links experience repulsive forces from all of the objects. The authors have also experimentally implemented the avoidance potentials on the CMU DDARM II system. The results demonstrate successful obstacle avoidance and approach, and exhibit an improvement over existing schemes.>

An obstacle-avoidance potential based on superquadrics is discussed. The superquadric formulation is a generalization of the elliptical potential function method and therefore is viable for a much larger class of object shapes. As with elliptical potentials, a modified form of the superquadric potential provides safe approach objects. The avoidance and approach potentials are implemented in simulations and the results exhibit an improvement over existing potential schemes. The simulations also use an algorithm that eliminates collisions with obstacles by calculating the repulsive forces exerted on links, based on the shortest distance to an object.>