Paolo Gasbarri

A two-dimensional approach to multibody free dynamics in space environment

Abstract The equation of motion of a multibody system, described here as a chain of rigid bars and revolute joints orbiting around the Earth, is derived. For each bar two translational and one rotational equilibrium equations are written. The forces acting on each body are the gravitational forces and the reaction forces (unknown) acting on its end joints. The complete set of equilibrium equations consists of N X differential equations, where N X is the order of the state vector. The total number of unknowns is N X + N R where N R =2 N J and N J is the number of joints. The N R additional equations, to make the system determinate, are provided by the nondifferential compatibility equations. The resulting system is a set of differential algebraic equations (DAE) for which the well-known method of reducing the system to ordinary differential equations (ODE) is applied. Since the internal forces are associated with the relative displacements between the bodies, which are small fractions of the distance of the multibody spacecraft from the center of the Earth, the task of obtaining these forces from inertial coordinates, from a numerical viewpoint, could be impossible. So the problem is reformulated in such a way that the equation of motion of the system, contains global quantities where no internal forces appear, and local equations where internal forces do appear. In the latter one, only quantities of the same order of the spacecraft dimensions are present. Numerical results complete the work.

A variational approach for solving an inverse vibration problem

The present investigation is focused on the solution of a dynamic inverse problem which is concerned with the assessment of damage in structures by means of measured vibration data. This inverse problem has been presented as an optimization problem and has been solved through the use of the Variational Approach, i.e. the conjugate gradient method (CGM) coupled with the adjoint equation. The results have been presented in a satisfactory form when a small structure with few degrees of freedom (DOF) is considered, however, when a higher DOF structure is studied, the simple application of the variational approach is not any more satisfactory, being necessary the application of an additional methodology. In order to solve this difficulty, a new approach based on the use of the genetic algorithm (GA) method has been proposed. The GA method is used to generate a primary solution which is employed as the initial guess for the CGM. The application of this new approach has shown that better results can be achieved, although the computational time for the application analyzed here could be increased. The damage estimation has been evaluated using noiseless and noisy synthetic experimental data, and the reported results are concerned with both truss and beam-like structures, which have been modeled through a finite element technique. Moreover, in order to take into account the reduced set of experimental data to be employed in the optimization algorithm, a Guyan reduction technique has been adopted on the finite element formulation. ¶Selected paper from Inverse Problems, Design and Optimization Symposium, 2004.

Adaptive Thrust Vector Control during On-Orbit Servicing

On-orbit servicing missions often include a final propulsive phase where a spacecraft pushes the other one towards a different orbit. Specifically this is the case of the debris grasping mission where the chaser, after capturing the target by means of robotic arms, has to perform a de-orbit operation. The large thrust involved needs a perfect alignment with respect to the center of mass or the system composed by chaser and target, in order to avoid attitude changes. Such accurate alignment is quite difficult to achieve especially when the characteristics of the target are not perfectly known. A procedure is proposed in this paper, allowing a complete estimation of the center of mass position and of the moments of inertia of the system, starting from the data obtained by the gyros mounted on board of the spacecraft. The output is used to design a maneuver for correcting the target and chaser relative position by moving the robotic arms. Numerical simulations show the proficiency and the applicability of the estimation algorithm and of re-alignment maneuver to a selected mission scenario.

Analysis and tests of visual based techniques for orbital rendezvous operations

Advanced visual based techniques are proposed in this paper for the dual task of identifying a target and evaluating the relative position and attitude between a chaser and the target during a space rendezvous. The algorithm is first tested on a given image, and then implemented onboard a free floating platform. The experimental results of this rendezvous performed with the testbed in the lab are reported, showing remarkable performance that offers the basis for future developments of close proximity control strategies.

A Comparative Overview of Strain Actuated Surfaces' Design

In this paper two different approaches to design of strain actuated lifting surfaces are compared with each other and with a classical aeroelastic tailoring method. Piezoelectric actuation is accomplished by piezo-patches embedded in the wings composite lay-up and piezoelectric ribs (piezo-ribs) placed spanwise. If the piezoelectric patches are isotropic in their plane they can not directly induce any twist effect in the wing. Thus, in this case, the wing can undergo twisting only as an effect of the coupling (due to tailoring and wing sweep angle) between bending and twisting torques. When piezoelectric patches are used, this configuration is not yet the most effective one. In fact, the largest electromechanical coupling coefficient, which is the one in the direction of the applied electric field, is not thoroughly exploited. On the other hand, piezo-ribs are able to induce not only bending but also pure twisting deformation and exploit the largest electromechanic coupling effect so they may provide the designer with a higher control authority. However, this kind of control involves larger weight. In this paper, aeroelastic wing performance for these different configurations is analyzed. Moreover, the comparison is also made in terms of energy consumption. Numerical examples complete the work.

Experimental investigation of on-off control modulation for rigid and flexible free floating platforms

Abstract A study is performed on the effects of a discrete modulation of the control signal for an attitude re-orientation manoeuvre of a satellite, applying the Bang-Bang technique, the Pulse Width Modulation and the Pulse Width Pulse Frequency Modulation. The performance of the three techniques are first compared by means of a detailed mathematical model, taking into account sensors’ noise and actuators’ delays and inaccuracies. Then, an experimental free floating platform is used to actually test the different modulation techniques. The experiments are performed by means of two versions of the floating platform, i.e. a rigid and a highly flexible one, where two elastic appendages are added. The interaction of the thrusters firing sequence with the flexible dynamics is analysed in detail, highlighting which of the control modulations is mostly affected by this interaction.

A novel technology for thermal control for ISP module for space applications

Thermal control is one of the most important themes in space package technology such as Integrated Sub-system Package technology. At present the technical solutions reducing heat stagnation are thermal dissipation and thermal cooling. In this paper a novel approach to increase thermal dissipation, based on the use of the pyroelectric materials, is proposed. The pyroelectric materials are able to convert the thermal energy into electric energy spontaneously. This characteristic is employed to increase thermal flux inside the microelectronic units and to reduce the thermal overstocking. In order to evaluate the temperature distribution inside the component a numerical evaluation of the transient thermal problem inside a multi-layered wall electronic device is performed and a preliminary design of an adaptive dissipation device based on pyroelectric material is presented.

Visual based Navigation of a Free Floating Robot by Means of a Lab Star Tracker

A visual based navigation for a free floating platform has been realized. The basic principle is the same as for the star trackers used in space operations for attitude determination, with the remarkable difference that also the position with respect to an inertial reference frame is evaluated. Both the working principles and the algorithms for increasing the robustness of the device will be reported. The design and realization of the prototype is illustrated. Finally, the performance of the navigation system will be tested both in a numerical environment and in a dedicated experimental setup, showing a satisfactory level of accuracy for the intended operations.

Flight Dynamics Numerical Computation of a Sounding Rocket Including Elastic Deformation Model

The atmospheric flight of an aerospace vehicle is, in general, subjected to high level of loads (propulsive, aerodynamics, etc.) acting over the vehicle structure which can deform it and, consequently, changes its dynamics during the flight. Depending on the geometry complexity, this deformation, which is associated to the structural elastic behavior, can be modeled by analytic or finite element methods (FEM). In the general case of rockets, which can be considered as a slender body, it is common to find in the literature modeling that assumes the vehicle as a free-free beam, following Euler-Bernoulli or Timoshenko approaches. Therefore, the aim of this paper is to present the development of a three degree-of-freedom (3-DOF) dynamic model of a sounding rocket that includes the elastic deformation parameters in the differential equations of motion solved during the trajectory computation. To accomplish this objective, the model assumes the vehicle as a mass-varying Euler-Bernoulli free-free beam whose modal parameters are solved by FEM code and, in the sequence, included in the set of differential equations written in a properly way. The verification of the results for elastic deformation obtained using this approach is made by comparison with results obtained from solving the static aeroelastic problem using an workflow CFD (computational fluid dynamics)/CSD (computational structural dynamics). The verification of the flight dynamic results consistency is performed by comparison with results obtained using a code for ROcket SImulation (ROSI). The preliminary analyzes have shown a good consistency between results, which indicates a reliable and stable solution when using the methodology developed in this paper.

An experimental testbed to simulate space manipulators GNC

The space environment is very demanding and every mission requires careful assessment and preparation. This is especially true when operations are rather challenging as in the case of a multibody spacecraft and grasping mission. Therefore, the design and set-up of specific ground simulation techniques is mandatory in order to define and study the control law strategies to be later applied in orbit. The paper depicts the experimental activities carried out at the University of Rome “La Sapienza” to experimentally simulate planar, controlled operations of orbiting manipulators. The architecture of the set-up, including the choice for limiting the friction, is discussed. Hardware selection and assembly issues, with reference to performance expected by simulation set-up, are detailed. Special attention is devoted to the kinematic state determination and trajectory reconstruction, to be obtained by sensors and devices located onboard the manipulators and validated by external means like webcams, which become a part of the simulation testbed itself. Relevant performance of each subsystem is described, and verified in robust closed loop grasping maneuvers. A second application of the same set-up, which is also helpful and currently used in formation flying studies, is introduced.