FIELD AND SPACE ROBOTICS LABORATORY
Self-Transforming Robotic Planetary Explorers
Olivier Chocron, Postdoc Associate
John Madden, Postdoctoral Associate, Bio-Instrumentation Laboratory, MIT
The exploration and development of the planets and moons of the solar system in the next 10 to 40 years are stated goals of NASA and the international space science community. These missions will require robot scouts to lead the way, exploring, mapping and constructing facilities. The fixed configuration planetary robots of today will not be able to meet the demands of these missions forecast for the coming millennium. This research program has begun to study the concept of self-transforming robotic planetary explorers to meet the needs of future missions. A self-transforming system would be able to change its configuration to overcome a wide range of physical obstacles and perform a wide range of tasks. In order to achieve self-transforming robots for planetary exploration, conventional complex and heavy physical components, such as gears, motors and bearings, must be replaced by a new family of elements. Here, light weight, compliant elements with embedded actuation are proposed. The actuation would be binary in nature, simplifying the control architecture. The physical system would allow the robot to make both geometric and topological configuration changes. It is proposed that the configuration planning would be handled with genetic algorithms. This proposed research would develop the concepts and technologies that will be relevant to the needs of NASA in the 10-40 year period. This program will focus on studying the underlying, fundamental physics and feasibility of self-transforming robotic planetary explorers.
The development of future robotic systems presents a number of important technical challenges, such as in the areas of sensor technologies, communications and artificial intelligence. This research focuses on the problems associated with the design of the physical system and its control. Further, the charge of the NIAC program is to develop technology and concepts that are relevant to the needs of NASA in the 10 to 40 year period. Clearly, a two-year study will not be able to begin to address all the technical issues relevant to the problem in this time frame. It will focus instead on studying the underlying fundamental physics of self-transforming robotic systems.
The approach will be to develop the concept of a self-transforming robotic planetary explorer, called the STX, that could be used in exploration missions in 10 to 15 years. The STX is a hybrid system composed of a combination of conventional system components and ABEs. Phase I of this study demonstrated the addition of small-scale binary actuation (2-4 binary states) to enhance conventional fixed configuration robots with some limited configuration change. The projection into 30 to 40 years would be a system of very large-scale binary actuation (VLSBA; 103 to 104 binary states) which can also deliver the changing topology necessary for truly effective planetary robots. This study will identify the key enabling technologies required for the successful implementation of the STX and to assess the feasibility of the approach, its potential and its fundamental limitations. This study also attempts, consistent with the NIAC charge, to project this technical approach into the 30 to 40 year timeframe.
The focus of the recently completed 6 month Phase I study was to develop concepts of system architectures for planetary robotics in the 10 to 40 year time frame. It was also the goal of this preliminary study to begin to identify and assess enabling technologies which will be instrumental to the development of these system concepts. The identified technologies pertain to all aspects of robotic systems, however, preliminary research is concentrated in the areas of the physical system (structure, actuation, information and power networks), motion control, and configuration planning.
Planning and Control of Binary Robotic Systems
Representative STX Robot Missions
In order to evaluate the STX concept for planetary exploration, a set of robotic missions has been formulated. These missions are representative of the challenges a planetary worker/explorer robot might face in establishing the infrastructure for future human exploration and settlements. The baseline missions selected include locomotion on easy and rough terrain, material transportation, and facilities construction (see below).
These mission representations are being simulated and the STX systems are evaluated against a set of performance criteria. The initial simulations have been focusing on rough terrain exploration. The criteria for this task include overall accessibility (where the robot can or cannot go), average speed, power consumption, and physical stability. Criteria for transporting materials include payload capacity and loading/unloading time, while facility construction will need skills such as dexterity and generation of forces. Several robotic kinematics have been considered and studied at the module level, and are discussed below.
Rough Terrain Mobility Using Low-DOF Binary Robots
The first objective of this part of the study is to determine the fundamental mobility capabilities and limitations of low-DOF binary robots (< 20 actuators) on rough terrain. The quasi-static models, simulation, and analysis being developed in this work may also be used in optimizing system designs and their motions. The results of the static stability analysis are displayed through a simulation of the robot with an OpenGL-based computer graphics user interface (see below). An accessibility map is extracted from the analysis data showing the relative terrain difficulty (see below). This map can then be used as a task-feasibility model for the path planning and control of the binary robotic system.
Control and Planning of Very High Degree-of-Freedom Systems for Manipulation and Locomotion
Because low-DOF systems are limited in achieving tasks, the next phase of this study is to develop planning methods for very high-DOF binary systems. The difficulty of planning grows exponentially with the number of binary actuators, therefore methods used on low-DOF systems tend to be impractical for high-DOF systems.
A serial-parallel kinematic structure is being examined as a robotic device to be used for manipulation, locomotion, etc. (see below). This kinematic mechanism has also been built experimentally. For binary robots, the inverse kinematics problem is a search through the workspace to find the robot configuration whose end-effector most closely matches the target point. Since the number of possible configurations in the workspace grows exponentially with the number of binary actuators, a brute force search is impractical for systems with tens or hundreds of actuators. One focus of this research is to examine algorithms that reduce the search space and can solve the inverse kinematics and trajectory planning problem in real-time.
In simulation studies shown here, both a genetic algorithm
and a combinatorial bit-switching algorithm have been used to solve the
inverse kinematics. The genetic algorithm is a stochastic search;
the combinatorial algorithm is a deterministic search that operates on a
reduced search space. Studies performed here have demonstrated the
computational and analytical feasibility of real-time planning and control
of binary robotic systems with hundreds of actuators and complex parallel
Development of Representative Prototype Devices
The capabilities of future planetary explorers will depend on highly capable actuators. The traditional gears and motors that drive today's planetary explorers are envisioned to be replaced by artificial muscles. There is a wide range of artificial muscle technologies available today. This includes conducting polymers, shape memory alloys, and piezo electrics. While capable of high actuation pressures, they are limited in strain. An emerging artificial muscle technology is EPAM (electrostrictive polymer artificial muscle), which has been developed at the Stanford Research Institute (SRI). These actuators consist of a thin elastomeric film, which is sandwiched between two compliant electrodes. As a voltage is applied to the electrodes, they attract, compressing the film in thickness and expanding it in area (see below). These actuators have the advantage of very large strains (in excess of 100%), which eliminates the need for motion amplification. The ability of the EPAM actuators to meet the needs of future planetary explorers is being investigated.
Prototype Demonstration Devices
Work has been done to experimentally demonstrate and integrate the ideas of elastic hinges, artificial muscle actuation, and bi-stable mechanisms. One such device (which has also been simulated) is the Binary Robotic Articulated Intelligent Device (BRAID, see below). It uses shape memory alloy actuators and uses elastic flexures for hinges.
To be incorporated into the next design iteration will be the idea of bi-stable mechanisms (see below). If a hinge joint is designed so that the mechanism passively locks into the discrete binary states, disturbances can be rejected better and power to actuators can be reduced significantly.
Sujan, V.A., M.D. Lichter, and S. Dubowsky, "Lightweight Hyper-redundant Binary Elements for Planetary Exploration Robots." Proceedings of the 2001 IEEE/ASME International Conference on Advanced Intelligent Mechatronics (AIM '01). Como, Italy, July 2001.
Lichter, M.D., V.A. Sujan, and S. Dubowsky, "Experimental Demonstrations of a New Design Paradigm in Space Robotics." Proceedings of the Seventh International Symposium on Experimental Robotics (ISER '00). Honolulu, Hawaii, December 2000.
This work has also been featured in a popular media article, in the British newspaper the Guardian.
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