Practical Implementation of Attitude-Control Algorithms for an Underactuated Satellite

The challenging problem of controlling the attitude of satellites subject to actuator failures has been the subject of increased attention in recent years. The problem of controlling the attitude of a satellite on all three axes with two reaction wheels is addressed in this paper. This system is controllable in a zero-momentum mode. Three-axis attitude stability is proven by imposing a singular quaternion feedback law to the angular velocity trajectories.Two approaches are proposed and compared to achieve three-axis control: The first one does not require angular velocity measurements and is based on the assumption of a perfect zero momentum, while the second approach consists of tracking the desired angular velocity trajectories. The full-state feedback is a nonlinear singular controller. In-orbit tests of the first approach provide an unprecedented practical proof of three-axis stability with two control torques. The angular velocity tracking approach is shown to be less efficient using the nonlinear singular controller. However, when inverse optimization theory is applied to enhance the nonlinear singular controller, the angular velocity tracking approach is shown to be the most efficient. The resulting switched inverse optimal controller allows for a significant enhancement of settling time, for a prescribed level of the integrated torque

Dynamic replication strategies in data grid systems: A survey

In data grid systems, data replication aims to increase availability, fault tolerance, load balancing and scalability while reducing bandwidth consumption, and job execution time. Several classification schemes for data replication were proposed in the literature, (i) static vs. dynamic, (ii) centralized vs. decentralized, (iii) push vs. pull, and (iv) objective function based. Dynamic data replication is a form of data replication that is performed with respect to the changing conditions of the grid environment. In this paper, we present a survey of recent dynamic data replication strategies. We study and classify these strategies by taking the target data grid architecture as the sole classifier. We discuss the key points of the studied strategies and provide feature comparison of them according to important metrics. Furthermore, the impact of data grid architecture on dynamic replication performance is investigated in a simulation study. Finally, some important issues and open research problems in the area are pointed out

Attitude control of underactuated small satellites

EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Attitude control of underactuated small satellites.

Actuator failures onboard satellites have caused severe and even disastrous consequences on several space missions. In this thesis, the problem of the attitude control of a LEO satellite, subject to actuator failures, is addressed. The three axis stabilisation of a satellite with the two remaining control torques on the principal axes, is a challenging problem since the control system is nonholonomic. Such a system has been proven by Brockett to be non-stabilisable using smooth (continuous and time invariant) control laws. Different non-smooth stabilizing control laws for the underactuated attitude control of a satellite are investigated here using pairs of thrusters, and also using reaction wheels. Using two pairs of thrusters, known singular or time varying approaches are applied with a systematic study of the effects of the torque saturation, PWM, singularity avoidance, noise, external disturbances, sampling and angular velocity tracking that intervene in a realistic case. Using two reaction wheels, a novel control strategy based on a singular nonlinear control approach, is mathematically proven and demonstrated by simulation. The 3-axis stability is proven using Rodriguez parameters and then using quaternions. The study of the symmetrical satellite case using thrusters, and the investigation of the effect of a non-zero total momentum using wheels, are done separately. Practical difficulties of the underactuated attitude control of small satellites using two pairs of on/off thrusters are pointed out. Conversely, using two reaction wheels, the possibility of decisive 3-axis manoeuvres is demonstrated (under realistic assumptions). Indeed, using two wheels, the 3-axis stabilisation is achieved with acceptable torque levels and very satisfactory performance. The activation of the non-smooth controller must be done under small momentum conditions. A complete control strategy, (in case of a high initial bias) including a detumbling phase with magnetorquing, and avoiding the non-smooth controller to start from a singularity, is presented. Following the encouraging results from the SSTL's UoSAT-12 simulator, (accounting for noises and external disturbance torques) in-orbit testing of an underactuated control strategy using two wheels has been successfully achieved on UoSAT-12 (by restricting the attitude to sun tracking due to power consumption problems on UoSAT-12). Another in orbit experiment on UK-DMC, for nadir pointing, has been even more successful. Practical results therefore confirm the possibility of using only two control torques for the 3-axis stabilisation of a satellite. One of many interesting consequences of these results is that a fully redundant 3-axis control can be practically envisaged using a 3-wheel configuration