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PROBA-2 is the second technology demonstration mission within the project for onboard autonomy of the European Space Agency (ESA). Besides other instruments and sensors, the micro-satellite will be equipped with two new types of global positioning system (GPS) receivers. These will support the spacecraft operations and demonstrate recent advances in the field of autonomous real-time navigation and offline orbit determination for microsatellites. The paper provides an overview of the key PROBA-2 navigation elements and discusses their scope and capabilities. Special attention is given to the Phoenix-XNS miniature GPS receiver and its embedded navigation function which are presented along with a discussion of the employed filtering and processing algorithms. The impact of PROBA-2 attitude changes on the GPS tracking is analyzed and the employed strategies for minimizing possible outages are presented. Hardware-in-the loop simulations in a signal simulator testbed are used to demonstrate the feasibility of 1 m level real-time navigation using a single-frequency GPS receiver and to demonstrate the overall robustness of the PROBA-2 onboard navigation.

This paper reports on the control structure of the pneumatic biped "Lucy." The robot is actuated with pleated pneumatic artificial muscles which have interesting characteristics that can be exploited for legged machines. They have a high power-to-weight ratio, an adaptable compliance and they can reduce impact effects. The current control architecture focuses on the trajectory generator and the tracking controller, which is divided into a computed torque controller, a delta-p unit, a PI position controller and a pressure bang-bang controller. The trajectory generator provides polynomial joint trajectories while the computed torque, combined with the delta-p unit, calculates the required muscle pressure levels. The PI and bang-bang controller work at a pressure level to cope with modeling errors and to set the pressures in each muscle. The control design is divided into single support and double support, where specifically the computed torque differs for these two phases. The proposed control architecture is evaluated with a full hybrid dynamic simulation model of the biped. This simulator combines the dynamical behavior of the robot with the thermodynamical effects that take place in the muscle-valves system. The observed hardware limitations of the real robot and expected model errors are taken into account in order to give a realistic qualitative evaluation of the control performance and to test the robustness. A preliminary implementation of the presented controller on the real biped, representing a walking motion of the robot while both feet are in the air, is discussed. This first implementation shows already promising results concerning tracking performance of the proposed control architecture. It confirms that the pneumatic tracking system can be used for a dynamic application such as a biped walking robot.

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In previous work, a method for establishing the equations of motion of open-loop multibody mechanisms was introduced. The proposed forward dynamics formulation resulted in a Hamiltonian set of 2n first order ODE’s in the generalized coordinates q and the canonical momenta p. These Hamiltonian equations were derived from a recursive Newton-Euler formulation. It was shown how an O(n) formulation could be obtained in the case of a serial structure with general joints. The amount of required arithmetical operations was considerably less than comparable acceleration based formulations. In this paper, a further step is taken: the method is extended to constrained multibody systems. Using the principle of virtual power, it is possible to obtain a recursive Hamiltonian formulation for closed-loop mechanisms as well, enabling the combination of the low amount of arithmetical operations and a better evolution of the constraints violation errors, when compared with acceleration based methods.