Spacecraft formation reconfiguration with collision avoidance

In this paper we present a behavioral control solution for reconfiguration of a spacecraft formation using the Null-Space Based (NSB) concept. The solution is task based, and aims to reconfigure and maintain a rigid formation while avoiding collisions between spacecraft. A model of relative translation is derived, together with a passivity-based sliding surface controller which globally stabilizes the equilibrium point of the closed-loop system. The NSB control method is implemented by giving each task different priorities and then calculating desired velocity and a Jacobian matrix for each spacecraft and each task. The velocity vector for each task is then projected into the null-space for higher prioritized tasks to remove conflicting velocity components. Simulation results are presented, showing that each spacecraft moves into the predefined formation without breaking any rules for the higher priority tasks, and all collisions are avoided.     

Occurrence of fragmentation during compression of pellets prepared from a 4 to 1 mixture of dicalcium phosphate dihydrate and microcrystalline cellulose

The occurrence of pellet fragmentation during the compression of pellets prepared mainly from a hard pharmaceutical filler material was investigated. The pellets consisted of 4 parts dicalcium phosphate dihydrate (generally considered as a hard material) to 1 part microcrystalline cellulose (used as a pellet forming material). Pellets of two porosities, 36% and 55%, were prepared. Compacts formed at 100 MPa applied pressure were fractured and the fracture surfaces were then visually examined. Lubricated pellets were also compacted in order to obtain tablets which could be easily deaggregated. Pellets retrieved from deaggregated tablets were compared with uncompacted pellets with respect to size and fracture resistance. The results showed that many pellets exposed in the tablet fracture surface were fractured, especially those with higher porosity. However, the lubricated pellets retrieved from deaggregated tablets were similar in size to the uncompacted pellets, i.e. fragmentation of these pellets was minimal. Furthermore, these retrieved pellets were more resistant to fracture than the original uncompacted pellets, indicating that the formation of cracks or flaws in the pellets during compaction was also minimal. It was thus concluded that deformation and probably densification, and not fragmentation, was the dominant compression mechanisms of this pellet formulation.     

The relationship between chord length and rime icing on wind turbines

Numerical simulations of ice accumulation on four different wind turbine blade profiles, from 450 kW, 600 kW, 1 MW and 2 MW, fixed speed, stall controlled, wind turbines, were performed to determine how wind turbine size affects atmospheric icing. The simulations indicate that dry rime icing is less severe for larger wind turbines both in terms of local ice mass and in terms of relative ice thickness. Copyright © 2009 John Wiley & Sons, Ltd.     

Multivariate methods in the development of a new tablet formulation: optimization and validation

In a previous study of the development of a tablet formulation approximately 100 excipients were characterized in screening experiments using multivariate design. Acceptable values for important responses were obtained with some of the formulations. The relationships between the properties of the excipients and the responses were evaluated using PLS. In this study additional experiments were performed in order to validate models obtained from the screening study and to find a formulation of suitable composition with desired tablet properties. A formulation with the desired disintegration time was found with the additional experiments and the agreement between observed and predicted values was fair for the tablets that did disintegrate. A limitation of this study was that tablets from four experiments did not disintegrate within the set time limit. The lack of agreement between observed and predicted values of these four experiments was probably due to the nature of one of the factors in the design. Considering the reduced experimental design the results are still encouraging.     

Performance losses due to ice accretion for a 5 MW wind turbine

A numerical study of power performance losses due to ice accretion on a large horizontal axis wind turbine blade has been carried out using computational fluid dynamics (CFD) and blade element momentum (BEM) calculations for rime ice conditions. The computed aerodynamic coefficients for the normal and iced blades from the CFD calculations were used together with the BEM method to calculate the torque, power and curves of the wind turbine for both normal and icing conditions. The results are compared with the published data. It is shown that icing results in a reduced power production from the turbine and that changing the turbine controller could improve the power production with iced blades. Copyright © 2011 John Wiley & Sons, Ltd.     

TORQUE AND FLUX TRACKING OF INDUCTION MOTORS

The problem of torque tracking and rotor flux norm regulation of induction motors perturbed by an unknown constant load torque was recently solved with an observer-based controller in Reference 5. In this paper we extend this result to treat the practically important case when the rotor flux norm is required to follow a time-varying reference. The controller design follows the passivity-based approach and proceeds in two steps: first, we define a target closed-loop dynamics compatible with the physical model of the motor that delivers the desired rotor flux and torque. Second, we propose a nonlinear dynamic output feedback controller that ensures this behaviour is asymptotically achieved. A proof of global tracking is given under the assumption of known motor parameters. Some key features of our physically based design are that the control law does not require measurement of rotor variables, is always well defined and does not rely on (intrinsically nonrobust) nonlinear dynamics cancellation. A corollary of our main result is the proof that a slight modification of the classical indirect field-oriented controller ensures global tracking for current-fed machines. © 1997 by John Wiley & Sons, Ltd.     

Effect of atmospheric temperature and droplet size variation on ice accretion of wind turbine blades

A numerical study of ice accretion and the resultant flow field characteristics of a 5 MW pitch controlled wind turbine blade profile (NACA 64618) have been carried out to understand the effects that atmospheric temperature and droplet size variations have on the rate and shape of ice growth. Resultant aerodynamic characteristics of the blade profile were analysed at different angles of attack ranging from −10° to +20°. Results show an increase in the ice growth with the increase of droplet size; whereas change in atmospheric temperature significantly affects the shape of accreted ice. Streamlined ice shapes were observed for low temperatures, whereas horn shape ice accretion was found at higher temperatures. Results show that for the iced blade profiles, changes in the aerodynamic characteristics are least prominent for the case of rime ice as compared to glaze ice.     

Flow Properties of Compressible Lactose Containing Small Quantities of Drug Substances

The flow behaviour of direct compressible lactose after addition of small quantities of drug substances was studied by different laboratory methods. Determination of the limiting unconfined yield pressure, [f_c], in an annular shear cell was found to be a material saving technique suitable for use in early formulation work. Estimation of the flow behaviour in a multiple punch machine by means of coefficient of tablet weight variation at high tabletting speed in a multiple punch machine was also performed with a material saving technique and was found to reflect the differences in flow behaviour as measured with the shear cell technique and also angle of repose. Water adsorption to the added drug substances was found to be an important factor for explaining the sometimes significant changes in flow behaviour.     

Passivity-based control of a class of Blondel-Park transformableelectric machines

Concerns the extension to the general rotating electric machine model of the passivity-based controller method for induction motors. The motor's passivity properties are used at 2 levels. First, we prove that the motor model can be decomposed as the feedback interconnection of two passive subsystems (essentially, the electrical and mechanical dynamics). Then, we design a torque-tracking controller that preserves passivity for the electrical subsystem and leaves the mechanical part as a passive disturbance. This leads to the cascaded controller structure which is typically analyzed involving time-scale separation. Our aim is to characterize a class of machines for which such a passivity-based controller solves the output feedback torque-tracking problem. The class consists of machines whose nonactuated dynamics are damped and whose dynamics can be decoupled. This requires that the air-gap magnetomotive force must be suitably approximated by the first harmonic in its Fourier expansion. These conditions have a clear physical interpretation in terms of the couplings between its dynamics and are satisfied by many machines. The passivity-based controller presented reduces to the well-known indirect vector controller for current-fed induction machines. Our developments constitute an extension to voltage-fed machines of this de facto standard in industrial applications