1,721,082 research outputs found
A hierarchical procedure for the combination of forecasts
No full textThis paper proposes a strategy to increase the efficiency of forecast combination. Given the availability of a wide range of
forecasts for the same variable of interest, our goal is to apply combining methods to a restricted set of models.With this aim, a
hierarchical procedure based on an encompassing test is considered. First, forecasting models are ranked according to a measure
of predictive accuracy (RMSFE). The models are then selected for combination such that each forecast is not encompassed by
any of the competing forecasts. Thus the hierarchical procedure represents a compromise between model selection and model averaging. The robustness of the procedure is investigated in terms of the relative RMSFE using ISAE (Institute for Studies and
Economic Analyses) short-term forecasting models for monthly industrial production in Italy
An inverse dynamics approach based on the fundamental equations of constrained motion and on the theory of optimal control
No full textIn this investigation, a new algorithm for the nonlinear control of mechanical systems is developed. The method proposed in this paper can be used for the forward and inverse dynamics of nonlinear mechanical systems. For this purpose, the Udwadia-Kalaba equations, also known as the fundamental equations of constrained motion, are combined with the feedback control strategy resulting from the theory of optimal control applied to linear time-invariant dynamical systems. In forward dynamics problems, the fundamental equations of constrained motion allow for explicitly calculating the generalized constraint forces associated with a nonlinear set of kinematic constraints. Furthermore, in inverse dynamic problems, the Udwadia-Kalaba equations can be effectively used for computing the generalized control forces that impose a prescribed dynamic behavior to the mechanical system under consideration. In this dual case, the desired dynamic behavior is described in terms of nonlinear algebraic equations that play the role of the kinematic joints encountered in the direct problem of forward dynamics. Conversely, it is shown in this work that the mathematical tool of the optimal control theory can be employed for the practical design of an effective compensation controller that improves the performance of the nonlinear control laws devised by using the fundamental equations of constrained motion. Employing the new approach proposed in this paper, the compensation controller designed by using the theory of optimal control is fully integrated into the nonlinear set of control laws obtained considering the general form of the Udwadia-Kalaba equations. The method developed in this paper is tested by means of numerical experiments. For this purpose, the nonlinear dynamic equations of a physical pendulum are used in order to exemplify the analytical developments carried out in this work and for assessing in a straightforward manner the performance of the proposed methodology
On the Use of the Udwadia-Kalaba Equations for the Nonlinear Control of a Generalized Van Der Pol-Duffing Oscillator
No full textIn this paper, a new method for controlling nonlinear mechanical systems is proposed. The methodology developed in this work is based on the use of the Udwadia-Kalaba equations in conjunction with the modern techniques of optimal control. The Udwadia-Kalaba equations represent an effective method for solving forward and inverse dynamics problems in the same analytical framework. On the other hand, the optimal control method is used in this work in combination with the inverse dynamic approach based on the Udwadia-Kalaba equations in order to obtain a nonlinear tracking controller. The mechanical system considered in this paper for performing numerical experiments is a nonlinear oscillator which includes in a generalized form the Van der Pol model for the system damping and the Duffing model for the system stiffness. The numerical results presented in this paper demonstrate the effectiveness of the method developed in this investigation
Dynamic analysis and control design of kinematically-driven multibody mechanical systems
No full textIn this investigation, a method for solving the dynamics and control problems of multibody mechanical systems whose time evolution is induced by a kinematically-driven motion is presented. In particular, the motion of a double inverted pendulum is employed as a demonstrative example of the computational procedure developed in this work and the interaction between the pantograph and the catenary is considered as a case study. To this end, the dynamic analysis and the design of a control system are performed. The multibody approach is used for deriving a mechanical model of the double inverted pendulum as well as of the pantograph mechanism. The multibody mechanical model developed in this work is aimed at improving the interaction force between the catenary and the pantograph. The mathematical method devised in this work for constructing multibody models of constrained mechanical systems makes use of a recursive Lagrangian approach. The Lagrangian formulation used in this paper allows for effectively handling the redundancy of the generalized coordinates and leads to a straightforward determination of the nonlinear reaction force fields arising from the presence of the mechanical joints. While the pantograph mechanism is schematized as a multibody mechanism having a geometrically nonlinear structure, the interaction force between the pan-head and the suspended line is modeled in a simplified manner considering a linear elastic element. On the other hand, a force actuator is applied between the upper arm and the pan-head of the pantograph mechanism for controlling the coupled dynamics of the pantagraph system and the catenary cable. The control system is devised with a twofold objective, namely to attenuate the interaction forces generated by the pantograph/catenary contact and to reduce the nonlinear oscillations of the closed-loop mechanism. An optimal law for the control action is obtained by employing a numerical algorithm based on the adjoint method. The numerical results found by means of numerical experiments demonstrate the efficacy of the recursive Lagrangian approach proposed in this work
On the dynamics and control of underactuated nonholonomic mechanical systems and applications to mobile robots
No full textThis paper deals with the dynamics and control of underactuated nonholonomic mechanical systems. It is shown in this investigation that the same analytical methods can be used for effectively solving both the forward and the inverse dynamic problems relative to underactuated mechanical systems subjected to a general set of holonomic and/or nonholonomic algebraic constraint equations. The approach developed in this work is based on the combination of two fundamental methods of analytical dynamics, namely the Udwadia-Kalaba equations and the Underactuation Equivalence Principle. While the Udwadia-Kalaba equations represent a fundamental mathematical tool of classical mechanics, the Underactuation Equivalence Principle is a new method recently discovered in the field of analytical dynamics and is associated with nonholonomic mechanical systems. In the paper, these two important analytical methods are discussed in detail. Furthermore, numerical experiments are performed in this investigation in order to demonstrate the effectiveness of the proposed approach considering as an illustrative example of a dynamic model a mobile robot
Dynamic Analysis of Multibody Mechanical Systems
No full textDynamic analysis of multibody systems (MBSs) is critical in modern mechanical engineering [...
Topology optimization procedure of aircraft mechanical components based on computer-aided design, multibody dynamics, and finite element analysis
No full textIn mechanical engineering, the optimization process is time-consuming because of the lack of communication between design, simulation, and analysis software. In the case of single productions or small quantities, this possibility is not taken into account. In the case of serial productions, on the other hand, the optimization of the design time is of paramount importance due to the large amount of money that can be saved. To address these challenges, this investigation proposes a topological optimization procedure for mechanical parts that have complex geometric shapes using the integration of CAD, MBD, and FEA software. The theory of linear elastodynamics is the basic approach used for the integration process carried out in this paper. In particular, the components analyzed in this work belong to the closing system of the ATR 42/72 cargo door. To explain the software integration procedure devised in the paper using SOLIDWORKS, MSC ADAMS, and ANSYS, a slider-crank mechanism is employed first as a demonstrative example. Subsequently, this computational procedure is applied to a flexible component of the latching system of the door whose loading conditions were previously obtained considering the entire opening mechanism modeled as a rigid multibody system. Finally, the topological optimization of the mechanical part is carried out and a consequential reduction in the amount of material to use is performed. The results obtained are considered significant since they led to considerable advantages in the door opening and closing system as well as a reduction of the total weight of the entire airplane
Multibody Modeling and Dynamical Analysis of a Fixed-Wing Aircraft
No full textThis study is aimed at developing a simplified virtual model capable of simulating the dynamic behavior of a fixed-wing aircraft by applying the multibody approach. The case of an aircraft with simplified aerodynamics, an axial thrust, and without control surfaces is considered. After modeling the aerodynamic actions following a Lagrangian approach, the equations of motion are analytically derived and numerically implemented in a MATLAB computer code to develop a virtual model capable of simulating the dynamic behavior of the aircraft. Finally, the numerical results found are presented and a discussion on the numerical results is provided, paying attention to the Cessna 172 Skyhawk, which is considered as the case study
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