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Content archived on 2022-12-23

Control of automated module for assembly, external maintenance, inspection and scientific experiments for manned space stations

Objective

The aim of this project is to develop the theoretical foundations, structures and algorithms of a high precision control system for a freely-flying automated module (AM) destined for assembly operations, external service, inspection and scientific experiments on manned orbital stations. This is a novel form of space robot. It consists of a standard space station module with six controlled degrees of freedom (three translational and three rotational) carrying special manipulators at one end for capturing, transporting objects and carrying out specialised tasks on the station, including the capture of objects that might accidentally break free. A standard docking interface is provided at the other end for acting as a space tug and assisting with docking of standard space station modules. The platform control actuators comprise a set of on-off thrusters (principally for translational trajectory control) and a set of control moment gyros for orientation.
The determination of an optimal AM configuration, maximising the payload capture envelope and minimising the actuator energy consumption, will be carried out in conjunction with the control law determination. A package of control laws will be derived taking full advantage of modern digital processing technology to support the different operational modes of the AM.
A closed-loop form of minimum-energy trajectory control law will be produced. This is a form of predictive control law in which a 'fast model' of the required manoeuvre is repeatedly run ahead in time, the loop being closed through a) the initial state being set equal to the measured AM body position and orientation co-ordinates and their derivatives and b) the signals to the actuators being made equal to those predicted at the beginning of the last model run. The actuator energy profile will be predicted during each model run and the terminal time of the manoeuvre adjusted, as necessary, to keep the energy profile within prescribed bounds. In this way, the energy optimal control is in a closed-loop form which compensates for inaccuracies in the assumed dynamics parameters of the AM and its payload.
Special on-off thruster control laws will be generated for controlling the AM translational position to high precision, embodying limit cycle control to minimise thruster wear and maximise the specific impulse (equivalent to fuel utilisation efficiency). Novel phase-plane based control techniques will be investigated. Operating the control moment gyro in a switched mode with similar control laws will be considered to maximise the efficiency of the power electronic drives.

Novel use will be made of the manipulators in
a) actively damping vibration modes in flexible objects being carried and;
b) enabling the technique of co-ordinate parametric control to be implemented, where the dynamics parameters of the AM are varied.

The problems of dynamic uncertainty, particularly associated with AM payloads, will be tackled by considering both on-line identification and adaptive control and robust control techniques such as high gain control with output derivative feedback and sliding mode control.
The AM will be equipped with a new optically based relative position and orientation determination system for docking with another object carrying a set of beacons (point like sources of light). The programme includes the determination of a suitable beacon configuration together with an algorithm to transform the measurements from this system (from dedicated sensors at closed range and an on-board camera at a longer range) to orientation and position co-ordinates suitable in form for use by the docking controller.
The control system will be designed for reliability, taking into account sensor and actuator redundancy. The operators will also be provided with a visual display of the working region of the AM, via an on-board camera, so that they may monitor the operation of the automatic control system and take over manually in the rare event of a control system failure. An expert system will be developed to aid the operator in manual control.
The operator interface with the control system will be developed in the form of a virtual control panel implemented on a touch sensitive screen with stereoscopic viewing facilities. This will also be used for operator training and a set of simulations of the various AM operations will be generated for this purpose.

Call for proposal

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Funding Scheme

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Coordinator

Institute for Applied Systems Technology
EU contribution
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Address
Wiener Str. 1
28359 Bremen
Germany

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Total cost
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Participants (4)