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CESAR demonstrator
CESAR
 
Current and future ESA space missions envisage the use of robots (manipulators and / or mobile robots).

There is a common element in all the robotics systems: the robot controller, whose function and architecture are essentially the same for all missions.

This is the rationale for developing a Common European Space Robot Controller (CESAR), which would constitute a reliable base subsystem of space robotics missions. Based on the controller software developed under earlier ESA contract (SPace Robot COntroller, SPARCO), the hardware and the software architecture of such common controller have been defined and a demonstrator implemented.

The activity has been carried out by Galileo Avionica / Tecnospazio (IT) (Prime Contractor) and Astrium / ST (D) under ESA TRP Contract.
 
Introduction
 
Robotics technology allows solving of manipulation/intervention problems in space by means of a common flexible tool: the robot system.
A robot system is mainly made of two parts:
  • the robot itself: an electro-mechanical assembly
  • the controller: a computer with dedicated peripherals and SW
The structure and required capabilities of the robot are strongly mission dependent (dexterity, reach, operating speed and payload capability drive the design of kinematics structure, size and mass), and therefore no really general purpose robots will exist.

The controller structure, however, is basically always the same. Some control sub-systems may be mission dependent, but its core is not. Considering this and also the fact that the development of a mature robot controller is a multi-million, multi-year effort, ESA has embarked on the development of a common controller which could be used for diverse space robotics missions such as: external or internal servicing of Space Station platforms
  • servicing of geostationary satellites
  • lunar robotics
  • robotics for Mars or cometary applications
  •  
     
    Technological objectives
     
    The objective of this running development is to establish a common Controller for European Space Automation and Robotics (CESAR) in hardware and software.
    CESAR shall have an open architecture, which implies:
    • to be essentially non-proprietary, i.e. to allow any contractor to "application program" it for a specific use or to insert their modules, without needing to involve the original developer
    • to have a clear and modular structure, in order to permit application specific parts to be inserted or added and to allow adaptation and interfacing to the specific implementation environment of the mission (hardware platforms, data handling system, TM/TC system, etc.)
    • to be well documented, to enable different teams to work with it
    CESAR shall also allow lean, streamlined implementations, which implies the ability to optimally tailor the implementation (in hardware and software) to the specific needs. The implementation shall allow the taking off of modules which are not required, but which take away precious resources (computing power and memory occupation for the software, mass, volume and electric power for the hardware).
     
     
    The Starting Point
     
    The implementation of a CESAR system is building upon the SPAce Robot COntroller (SPARCO) which has been developed under Basic TRP funding. SPARCO has added to a commercial controller those control features (impedance and proximity control) which are needed for space use. It has produced a prototype, now installed at ESTEC, which successfully demonstrates these control principles.

    Starting with SPARCO, to arrive to CESAR the following steps are being carried out:
    • software implementation: streamlining, modularization and porting to a space-accepted operating system of the current SPARCO software
    • hardware implementation: design, development and testing of a space qualified hardware

     
     
    CESAR Schematics
    General Architecture
     
    The architecture of CESAR is composed of a Robot Control Unit (RCU), which performs the most computation intensive high level tasks, and a set of more or less intelligent slave modules, named Servo Control Units (SCU), which control the robotic hardware (servo drives, sensors). Due to this strict master-slave structure CESAR does not need any sophisticated multi-processor bus (such as VME). Instead a multi-drop master-slave serial bus is adopted to allow for communication between RCU and SCUs. This serial bus enables both the concentrated and distributed control.

    Depending on the mechanical structure of the robot system the RCU and the SCUs may be concentrated in a single box or distributed in more boxes along the robot structure (distributed arrangement).

    The RCU is a real-time computer equipped with a real-time multi-tasking operating system.
     
     
    Software Implementation
     
    The software architecture features three types of tasks:
    • system tasks: implementing the interface to Telemetry/Telecommands, a monitor shell and some built-in test logic
    • robotic tasks: robot program interpretation, motion control
    • user tasks: to interface to external auxiliary hardware
    The RCU software architecture allows for the easy replacement or addition of tasks to modify/augment the CESAR functionalities.

    The real-time operating system chosen for the CESAR SW is vxWorks, which supports many microprocessors, including the newly available radiation tolerant microprocessor ERC32 and Digital Signal Processors (TSC21020E) developed on behalf of ESA by European industry.
    The software modularity and the wide micro-processor support for the operating System, enable the adoption of HW architectures even different from the CESAR general one.
     
     
    CESAR Hardware
    Hardware Implementation
     
    The CESAR-HW uses two different types of electronic boards to implement the RCU and the SCUs. The RCU uses a Standard PayLoad Computer (SPLC) CPU module fitted with a mezzanine SPLC LAN Adapter and a MIL-Bus Adapter, while the SCUs are developed specifically for the CESAR-HW.
    The SPLC uses a common mezzanine bus for their mezzanine slots (MIL-Bus, LAN). The same bus concept is also used for the SCU Boards.
    A common mezzanine bus concept over the whole CESAR-HW reduces costs since all the elements can make use of existing SPLC Adapters.
    The SCUs are designed in a modular manner. Each SCU consists of a Base-Board, a Core-Board and one or two Mezzanine-Boards.
    A Base Board carries Core- and Mezzanine-Boards and provides the required interfaces to the robot servo amplifiers.
    Core-Boards include a DSP CPU, drivers, non-volatile memory and program/data RAM.
    Mezzanine-Boards are used to interface to the serial bus. They feature a micro-controller, which performs data–communication tasks up to the Application Layer of the ISO/OSI model. With this arrangement the Core-Board is not affected by the specifics of the serial bus used.
    An additional benefit of the arrangement is the ability to change serial bus with an easy exchange of mezzanine boards. While at the moment CESAR-HW makes use of a MIL 1553B serial bus, the mezzanine concept makes possible, in the future, to use faster buses.
    The change of bus will only require the development of a single mezzanine board, which will be replicated for all RCU and SCUs.
     
     
    Conclusion
     
    The hardware and software architecture of a common Controller for European Space Automation and Robotics (CESAR) have been defined, maintaining the well-proven software modules from the earlier SPARCO development. Final implementation has been completed, leading to an engineering-model of advanced robot controller.
     
     
    Last update: 27 September 2006
     


     
     
     
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