Figure 1. Typical application of a robot controller in space missions.
Résumé
Diverses missions spatiales actuelles et en projet de l'Agence prévoient le recours à des robots, dont des bras manipulateurs et des véhicules automatisés. Un élément commun à tous ces dispositifs est le module de commande, dont les fonctions et l'architecture sont à peu près identiques d'une mission à l'autre. D'où l'intérêt de mettre au point un module de commande commun pour l'automatisation et les robots européens à usage spatial (projet CESAR), qui doit constituer un sous-système fondamental fiable pour les différents robots à employer lors des missions spatiales. L'architecture matérielle et logicielle de ce module commun a été définie d'après le logiciel de commande mis au point dans le cadre d'un contrat antérieur de l'Agence (Sparco), et la réalisation ultime est en cours.
Contractors:
Tecnospazio (I)
Funding:
Basic Technology Research Programme
Robots form the basis of common, flexible, tools for manipulating objects and for other forms intervention in space. A robot system comprises two main parts: the robot itself, which is basically an electro-mechanical assembly, and a controller, normally a computer with its associated software and peripheral devices.
The required capabilities of the robot (e.g. its dexterity, reach, and operating speed) are strongly dependent upon the mission and the payload and these drive the design of its kinematics, its size and mass and thus its overall structure; consequently a general purpose robot does not really exist.
However, the structure of the controller is always basically the same, and although some of its sub-systems may vary from mission to mission, its core remains unchanged. Considering this and also noting that the development of a mature robot controller will take several years and cost several million ECUs1, ESA has embarked on the development of CESAR (common Controller for European Space Automation and Robotics) which could be used for a wide range of missions employing robots in space:
Figure 1 shows an overall scenario of a space mission employing robots.
Figure 1. Typical application of a robot controller in space missions.
The development of CESAR covers both its hardware and its software. CESAR has an open architecture, which implies that its architecture is essentially non-proprietary, allowing a contractor to clone its architectural design and program it for a specific purpose or alternatively to insert his own modules into the design, without recourse to the original developer. Secondly, its structure is modular and clearly defined, permitting application-specific parts to be inserted into the system or to be added onto the system; this allows it to be adapted to, and interfaced with, the particular environment of a specific mission (e.g. the spacecraft and its data handling, telemetry and command systems). Lastly an open system is well documented, which enables different teams to work with it independently.
CESAR also allows a lean and streamlined implementation, whose hardware and software are optimally tailored to the specific needs of the mission, and which are not being forced to be burdened with superfluous modules which consume precious computer resources.
The implementation of CESAR builds upon the Sparco (Space Robot Controller) which has been developed for ESA (and is described in Preparing for the Future December, 1995). Sparco is a combination of a commercial controller to which have been added control features needed for space applications, such as impedance and proximity control. A prototype which successfully demonstrates these control principles has been made and is now in use at ESTEC.
Using Sparco as a starting point, the following steps must be taken to derive CESAR:
CESAR (Figure 2) is composed of a robot control unit (RCU), which performs most of the computationally-intensive high level tasks, and a set of more-or- less intelligent slave modules, called servo control units (SCU), which control the robot's servo drives and sensors. Depending on the mechanical structure of the robot system, the RBU and the SCUs may be concentrated in a single housing or distributed over several boxes, located at various points along the robot's structure.
Figure 2. The hardware architecture of CESAR.
The RCU is a computer, equipped with a real-time multi-tasking operating system. The software architecture distinguishes three types of tasks:
The architecture of the RCU software allows easy replacement or addition of tasks to modify or augment the features of CESAR.
CESAR will employ a newly available radiation-tolerant microprocessor (type ERC32) and a digital signal processor (type TSC-21020E), both developed by European industry to ESA's requirements.
Due to its strict master-slave structure, CESAR does not need a complex multi-processor bus system such as VME; instead a multi-drop master-slave serial bus has been selected for communication between RBU and SCUs. This serial bus supports both the concentrated and distributed control.
The hardware and software architecture of a common Controller for European Space Automation and Robotics (CESAR) have been defined, using as a starting point well-proven software modules developed earlier for Sparco.
Final implementation is under way, to provide a flight-ready version of an on-board controller for space robots, suitable for the wide variety of currently foreseen applications.
Preparing for the Future Vol. 7 No. 2