Figure 1. The chassis of the IARES vehicle.
Résumé
Un système de démonstration au sol des technologies de l'exploration lunaire et de leur adaptation aux missions à destination de Mars est à l'étude dans le cadre du programme Eureka. Cet "Illustrateur Autonome de Robotique mobile pour l'Exploration Spatiale" (IARES) est constitué d'un véhicule tous terrains extrêmement maniable, équipé de caméras stéréoscopiques, d'un système de navigation inertielle et de moyens de traitement des données à bord. Il peut aussi communiquer avec son secteur terrien, lequel est doté d'une interface d'opérateur pour la conduite des opérations et des essais. Un prototype est en construction et les essais sont prévus pour 1998.
Contractors:
CEA (F), CNRS F), INRIA (F),
ONERA (F) and the Eureka partners: Matra Marconi Space (F),
SAGEM (F), ITMI (F), Alcatel Espace (F), Cybernetix (F),
Ikerlan (E), VNII Transmash (Rus) and KFKI (H),
Funding:
Funded under the Eureka Programme by France, Spain, Hungary and Russia.
The IARES project is sponsored by the Eureka Programme of the European Communities and managed by the French national space agency, CNES. Its main objectives are to demonstrate the feasibility of a planetary rover vehicle and to collect data needed to evaluate the vehicle's characteristics and performance. With definition of future operational rovers in mind, a flexible prototype has been designed to allow quantitative evaluation of different vehicle configurations.
Although the prototype (demonstrator) is representative of a planetary vehicle in terms of functionality, mass and architecture, it will be made of standard ground equipment whose technology is capable of space qualification at a later date. The demonstrator will be tested in a realistic environment.
The vehicle chassis (Figure 1), designed in Russia by VNII Transmash, has advanced capabilities. Its six wheels are independently powered and steerable through angles of ±40 degrees. An articulated frame allows it to adapt passively along its transverse axis to obstacles in its path. A combination of passive and active longitudinal deformation together with active equalisation of wheel loading is used to traverse an arbitrary slope. A two-speed gearbox provides a maximum velocity of 0.10 m/s in first gear and 0.35 m/s in second gear. Varying the separation between the wheels allows the vehicle to "walk" on its wheels.
Figure 1. The chassis of the IARES vehicle.
This vehicle will be able to travel over various types of cohesive and non-cohesive soil and climb over obstacles up to 0.5 m high; it will negotiate slopes of 30 degrees over non-cohesive soil or 40 degrees over cohesive and hard soil.
The vehicle chassis measures 1235x1200x730 mm., weighs 80 kg and can carry a 70 kg payload. Potentiometers sense angles in the articulated chassis and incremental encoders sense wheel rotation. Acquired data is sent to an on-board computer.
The surrounding terrain is viewed by stereoscopic CCD cameras (288x384 pixels) which have a 90-degree field-of-view. The electronic image is captured in memory and transferred via a transputer link to the main computer. Estimated mass of the cameras is less than 1 kg. The cameras are mounted on a pan-and-tilt turret and their height can be adjusted from 1.5 to 2.1 metres above ground. The turret also houses the servo electronics and serial data link to the main computer.
The attitude and position of the rover is determined by an inertial navigation system whose software corrects for drift during the time the rover is still. Position data from external sources may also be used.
The manipulation subsystem has been developed by Ikerlan, IAI and INTA laboratories. The manipulator is designed to collect samples within an 80 cm radius, to deposit and operate scientific experiments and to measure the soil resistance. It can also be used to partly inspect the rover. The manipulator arm has six degrees-of-freedom and a total extension of 1.3 m. The end of the arm is fitted with a two-finger gripper, a camera, a laser range finder and a force-torque wrist sensor. The movements of the robot are coordinated by a transputer and micro-controllers control the joint servos.
The 28 V, 700 Wh batteries permit autonomous operation for up to 1.5 hours. For long-duration tests an external power supply may be used.
A star distribution system has a separate line to each sub-system. DC-DC convertors separate primary and secondary power lines. Current monitoring and safety protection and switching are centralised under control of a PC604 processor. Input-output is performed by M-modules and a wireless ethernet link provides communications to the ground station. The locomotive subsystem has its own processing capability, designed by KFKI, which employs micro-controllers and a transputer; the manipulative subsystem also does its own processing.
The ground segment provides the software and MMI needed to operate the rover and its arm under remote control and sends a schedule of task sequences for autonomous execution. In addition, it can monitor the status of the robot, and store sampled data and analyse the results of tests.
The rover has an on-board autonomous navigation system which guides it to a location, selected by a human operator from an image of the surroundings, captured by the on-board cameras. The command GOTO (x,y) is then sent to the navigation system, which pilots the rover to its target. The following sequence is used:
The longest path to the goal which avoids dangerous and unknown areas is then computed, allowing for any uncertainties and safety margins. The computed trajectory is then followed. This cycle is then repeated until the rover reaches its objective. The cycle can be executed in discrete steps or continuously during a single manoeuvre during which progress is monitored and a future path is defined (or updated) whilst the current segment is being travelled.
The operator can also command the manipulator arm to collect a set of samples or to analyse the soil at a set points, leaving image processing, path determination and sequencing to be executed automatically by the rover.
The rover can be steered by a remote operator. To do this, the ground segment is supplied with a digital terrain model of the environment 8 to 10 metres in front of the vehicle, generated periodically on board from the stereoscopic images taken during the manoeuvre, and displayed to the operator. To take account of the processing and other delays, the view is artificially advanced to predict the scene pertaining at the time the signal reaches the operator.
The manipulator arm can also be controlled by an operator who sends sequences of elementary functions for execution by the controller. In addition, the operator has access to the low level control of all subsystems for remote fault-finding and maintenance.
The feasibility of the control concepts have already been tested on the EVE vehicle in 1996 and their implementation on IARES will benefit from this experience. The chassis and its equipment will allow testing to be extended to cover very difficult terrains to better know the limits of the possible missions.
The chassis was delivered in August 1996 and the electrical subsystems have been fitted. A new stereoscopic vision system and turret are being manufactured. The inertial unit and the arm will be installed in mid-1997 and end-1997 respectively. The software is under development at CNES. Testing will take place in 1998.
A follow-on phase of the project named IARES L2 is being planned to provide the increased autonomy needed for exploration of the planet Mars. This phase will start in 1999.
Preparing for the Future Vol. 7 No. 2