Engineering topics

Hamidi, M.

Publications and source records attributed to Hamidi, M..

12 records

NASA NTRS1986

Maximum-Likelihood Parameter-Estimation Algorithm

Efficient version of maximum-likelihood algorithm devised for calculating normal-mode frequencies and damping parameters of vibrating system from experimental data where both process noise and measurement noise present. Method applicable in vibration analysis of such complicated structures as vehicles, aircraft, and spacecraft. New algorithm simplification of existing maximum-likelihood formulation using Kalman filter that allows for both process and measurement noise.

Eldred, D. B.↗

NASA NTRS1985

Control of large antennas based on electromagnetic performance criteria

The electromagnetic (EM) performance of large flexible antennas is traditionally achieved by imposing stringent geometric restrictions on the structural distortions from a nominal optimum configuration. An approach to alleviate the stringency of the geometrical criteria of satisfactory performance is presented. The approach consists of generating a linear optimal control problem with quadratic cost functional where the cost functional is obtained from the EM characteristics of the antenna and the dynamic system constraint is given by the structural model of the antenna. It is established that the EM based optimal controller is considerably more efficient than the traditional geometrical based controllers. The same EM performance can be achieved with a much reduced control effort.

Lin, Y. H.↗

NASA NTRS1985

Space Station Parametric Models

The development of two parametric models for a four-panel planar initial space station is described. The derivations of the distributed parameter model are presented in detail with the hope that the same method and procedures can be employed for stations with different configurations or for changes within the same configuration class. The 19-DOF finite-element model is also described. With the availability of the 19-DOF and a lower-DOF space station models, the frequency characteristics of the various dynamical systems in the space station environment are identified.

Hamidi, M.↗

NASA NTRS1985

Optimization of Controlled Structures

A formulation is presented for the coupled optimal design of a structural system and its control by defining a composite objective function as a linear combination of two components: a structural objective and a control objective. For the case when the structural objective is a function of the structural design variables only, and when the control objective is represented by the quadratic functional of the response and control energy, one can analytically express the optimal control in terms of any set of admissible structural design variables. The expression for the optimal control is used recursively in an iterative Newton-Raphson search scheme, the goal of which is to determine a corresponding optimal set of structural design variables that minimize the composite objective function. A numerical example is given to illustrate the computational procedure.

Salama, M.↗

NASA NTRS1984

Analytic solutions for dual-spin spacecraft during platform motion

This paper presents analytic solutions to the dynamic equations of dual-spinners during platform slews. The rotor is assumed to be an axisymmetric balanced rigid body. The platform is an asymmetric rigid body with small static and dynamic imbalances. The solutions given in this paper extend a previous theory which provided an approximate solution only in the steady state.

Hayati, S.↗

NASA NTRS1983

Distributed system modeling of a large space antenna

A general approach for distributed parameter modeling of complex dynamical systems is described. The method consists of dividing the system in parts which can be modeled by simple partial differential equations and coupling the equations thus obtained by applying Hamilton's variational formalism to the entire system. The modeling of a large, offset-fed, wrap-rib antenna is presented to illustrate the approach. Although such models are perhaps not as precise as finite element models, they can be useful for initial physical insight and parametric design.

Hamidi, M.↗

NASA NTRS1983

Control of large space antennas based on electromagnetic-structural models

A general approach to the optimal control of large space antennas based on their RF/structural characteristics is described. The approach consists of defining a cost functional based on the degradation of the RF performance of the antenna and using the structural model as the dynamic system. The method is applied to the design of an optimal controller for a 55-m, wrap-rib offset-fed antenna. The controller's goal is to minimize the variations of the peak electric field of the antenna due to feed displacements.

Hamidi, M.↗

NASA NTRS1983

Distributed control of large space antennas

A systematic way to choose control design parameters and to evaluate performance for large space antennas is presented. The structural dynamics and control properties for a Hoop and Column Antenna and a Wrap-Rib Antenna are characterized. Some results of the effects of model parameter uncertainties to the stability, surface accuracy, and pointing errors are presented. Critical dynamics and control problems for these antenna configurations are identified and potential solutions are discussed. It was concluded that structural uncertainties and model error can cause serious performance deterioration and can even destabilize the controllers. For the hoop and column antenna, large hoop and long meat and the lack of stiffness between the two substructures result in low structural frequencies. Performance can be improved if this design can be strengthened. The two-site control system is more robust than either single-site control systems for the hoop and column antenna.

Cameron, J. M.↗

NASA NTRS1983

Use of electromagnetic models in the optimal control of large space antennas

A general approach to the optimal control of large space antennas based on their RF/structural characteristics is described. The approach consists of defining a cost functional based on the degradation of the RF performance of the antenna and using the structural model as the dynamic system. The method is applied to the design of an optimal controller for a 55-m, wrap-rib offset-fed antenna. Simulation results show that control energy consumption is reduced to aproximately one third of the energy used to achieve the same RF performance with traditional control strategies.

Manshadi, F.↗

NASA NTRS1982

Optimal control of distributed parameter elastic systems

This paper presents an analytical solution to the Riccati equation for self-adjoint systems such as beams, plates, strings and membranes moving in space, and shows how the optimal control law can be implemented using the given solution. It is then shown that there always exists a self-adjoint operator describing the distribution of potential energy if the state space is appropriately augmented. A beam-like gravity-stabilized satellite moving in a circular orbit around the earth is used to illustrate the main results in this paper.

Juang, J.-N.↗

NASA NTRS1981

Optimal control and controller location for distributed parameter elastic systems

A class of systems governed by second order partial differential equations and driven by controllers located at points r sub i, i = 1, ..., k, is considered. A cost functional quadratic in the time derivative of the state and the control is associated with the system. The optimal controller locations are defined as the ones which minimize the maximum of the cost over all possible initial states. An analytical solution to the associated Riccati equation is presented providing a convenient expression for determining the optimal locations.

Hamidi, M.↗

NASA NTRS1981

On the rigid body motion and shape distortion evaluation for large flexible spacecraft

A procedure is described for evaluating and subtracting the contribution of the rigid body motion from the general displacement of a Large Space Structure. The shape distortions are thus exhibited and their root mean square calculated. It is well known that a rigid body motion is composed of a rotation and a translation. To evaluate the rotation matrix M, use is made of the fact that unitary matrices can be expressed as M = (I-B)/(I+B) where B is computed using the coordinates, before and after the displacement, of three rigidly attached points. The translation vector is then deduced from the same coordinates.

Hamidi, M.↗