A fine pointing system for the large space telescope

The large space telescope (LST) developed by NASA requires ultrahigh pointing stability within 0.0005 arc sec rms. A fine guidance system is proposed to body-point the entire spacecraft within this limit. The spacecraft is modeled as a rigid body having reaction wheel actuators and subject to gravitational and magnetic disturbance torques. The fine guidance sensor is cluttered with electronic noise. The disturbance accommodation standard deviation optimal controller (DASOC) is designed to be optimal with respect to the transient and the steady state response to noise, whereas the steady state response to deterministic external torques is exactly zero. Compared with conventional controllers, the fine guidance system with the DASOC offers as much as a factor of 30 improvement in pointing stability, resulting in an optimal performance of nearly 0.0001 arc sec rms. Thus, the required pointing stability can easily be obtained, and a large margin remains for the compensation of possibile deteriorations

Symbolic vector/dyadic multibody formalism for tree-topology systems

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76159/1/AIAA-20780-671.pd

Symbolic computer language for multibody systems

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76245/1/AIAA-20770-590.pd

Coupling dynamics of a geared multibody system supported by Elastohydrodynamic lubricated cylindrical joints

A comprehensive computational methodology to study the coupling dynamics of a geared multibody system supported by ElastoHydroDynamic (EHD) lubricated cylindrical joints is proposed throughout this work. The geared multibody system is described by using the Absolute-Coordinate-Based (ACB) method that combines the Natural Coordinate Formulation (NCF) describing rigid bodies and the Absolute Nodal Coordinate Formulation (ANCF) characterizing the flexible bodies. Based on the finite-short bearing approach, the EHD lubrication condition for the cylindrical joints supporting the geared system is considered here. The lubrication forces developed at the cylindrical joints are obtained by solving the Reynolds’ equation via the finite difference method. For the evaluation of the normal contact forces of gear pair along the Line Of Action (LOA), the time-varying mesh stiffness, mesh damping and Static Transmission Error (STE) are utilized. The time-varying mesh stiffness is calculated by using the Chaari’s methodology. The forces of sliding friction along the Off-Line-Of-Action (OLOA) are computed by using the Coulomb friction models with a time-varying coefficient of friction under the EHD lubrication condition of gear teeth. Finally, two numerical examples of application are presented to demonstrate and validate the proposed methodology.National Natural Science Foundations of China under Grant 11290151, 11221202 and 11002022, Beijing Higher Education Young Elite Teacher Project under Grant YETP1201

Augmented lagrangian and mass-orthogonal projection methods for constrained multibody dynamics

This paper presents a new method for the integration of the equations of motion of constrained multibody systems in descriptor form. The method is based on the penalty-Augmented Lagrangian formulation and uses massorthogonal projections for the solution to satisfy the kinematic constraint conditions. The number of equations being solved is equal to the number of states, and does not depend on the number of constraint conditions. Therefore, the method is particularly suitable for systems with redundant constraints, singular configurations or topology changes. The major advantage of the new method relies on the fact that for a low computational cost, the constraints in positions, velocities and accelerations are satisfied to machine precision during the numerical integration. This process is efficiently done by means of a mass-orthogonal projection without the need for coordinate partitioning or reduction to a minimum set of coordinates. The projection scheme allows for a more accurate and robust integration of the equations of motion since constraint violations constitute one of the primary sources of numerical errors and instabilities during the integration process. The proposed projection is also applied to the classical Lagrangian approach, thus eliminating the need for further stabilization as well as the selection of parameters in Baumgarte's method.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43334/1/11071_2005_Article_BF01833296.pd

A review of friction models in interacting joints for durability design.

This paper presents a comprehensive review of friction modelling to provide an understanding of design for durability within interacting systems. Friction is a complex phenomenon and occurs at the interface of two components in relative motion. Over the last several decades, the effects of friction and its modelling techniques have been of significant interests in terms of industrial applications. There is however a need to develop a unified mathematical model for friction to inform design for durability within the context of varying operational conditions. Classical dynamic mechanisms model for the design of control systems has not incorporated friction phenomena due to non-linearity behaviour. Therefore, the tribological performance concurrently with the joint dynamics of a manipulator joint applied in hazardous environments needs to be fully analysed. Previously the dynamics and impact models used in mechanical joints with clearance have also been examined. The inclusion of reliability and durability during the design phase is very important for manipulators which are deployed in harsh environmental and operational conditions. The revolute joint is susceptible to failures such as in heavy manipulators these revolute joints can be represented by lubricated conformal sliding surfaces. The presence of pollutants such as debris and corrosive constituents has the potential to alter the contacting surfaces, would in turn affect the performance of revolute joints, and puts both reliability and durability of the systems at greater risks of failure. Key literature is identified and a review on the latest developments of the science of friction modelling is presented here. This review is based on a large volume of knowledge. Gaps in the relevant field have been identified to capitalise on for future developments. Therefore, this review will bring significant benefits to researchers, academics and industrial professionals