1,721,064 research outputs found
"A case of bifid rib from G. Marro" egyptian osteological collection
No full textThe study was conducted on a skeleton belonging to the "G. Marro"Egyptian anthropological collection. The skeleton came from a single grave at Gebelein (Upper Egypt) ; the archaelogical evidence dates it to the Dynastic period. The skeleton is well preserved and complete in all its anatomical parts. The remains are referable to a child of around 4 years. The excellent state of preservation of the skeleton allowed a detailed macroscopic study of all the skeletal regions, which revealed a congenital defect of the right fourth rib : anterior end of the rib is bifucated. The present case documents a skeletal malformation rarely observed in archaelogical materia
H2 optimal decoupling of previewed signals in the discrete-time case.
No full textThe synthesis of a feedforward unit for H2 optimal decoupling of measurable or previewed signals in discrete-time linear time-invariant systems is considered. It is shown that
an H2 optimal compensator can be achieved by connecting a finite impulse response (FIR)
system and a stable dynamic unit. To derive the FIR system convolution profiles an easily
implementable computational scheme based on pseudoinversion (possibly nested to avoid
computational constraints) is proposed, while the dynamic unit is derived by solving a
standard LQR problem, in general cheap or singular
Piezoresistance behavior of silicone-graphite composites in the proximity of the electric percolation threshold
No full textComposites of a silicone matrix charged with graphite powder of micrometric size in volume fractions around the electric percolation threshold (25–35%) have been investigated with regard to their piezoresistance properties. The elastic modulus changes with graphite content, reaching a maximum at 30 vol.%. From measurements of electric resistivity, the percolation threshold was determined as 31 vol.% at a compressive strain of 2%. The threshold value was found to be dependent on the applied compressive strain so that an insulator in the unstrained condition may become a conductor when subjected to a small pressure. The property can be exploited for contact sensors. Further, the electric resistance of a composite, charged a little beyond the percolation threshold, is also strain dependent, according to an equation of the type R = R0exp(βε), where β was found to be about 51.5. This value of corresponds to a very high electric sensitivity of the material to an applied strain and makes it a candidate for application as a logarithmic strain transducer. Owing to the visco-elastic behavior of the elastomer matrix, there is a retardation of the electric response on unloading of about 2 s. The electric response to an applied stress follows an exponential law on loading and undergoes a corresponding retardation on unloading
Regulation transients in discrete-time linear parameter varying systems: a geometric approach to perfect elimination
No full textThis work encompasses the problem of the exact elimination of regulation transients for linear parameter varying systems in a straight geometric framework. Discrete-time, stabilizable systems are specifically addressed. Conditions for problem solvability are proved and the synthesis of the control scheme is illustrated in detail
Structural invariants of the singular Hamiltonian system and non-iterative solution of finite-horizon optimal control problems
No full textA non-iterative solution for a class of discrete-time finite-horizon linear quadratic optimal control problems is obtained through the characterization of a pair of structural invariants of the singular Hamiltonian system associated to the H2 optimal control problem stated for the generic, discrete-time quadruple (A,B,C,D). On the assumption that the final state weighting function in the performance index is represented by a quadratic surface, it is shown that the optimal cost is a function of the initial state with the same structure. Optimal control laws and state trajectories are analytically expressed as functions of the initial state as well. The results hold under rather extensive conditions: those that guarantee the existence and uniqueness of the stabilizing solution of the corresponding discrete algebraic Riccati equation
Geometric methods for invariant zero cancellation in discrete-time non-strictly-proper linear multivariable systems
No full textThis paper presents a geometric procedure for designing a minimal-order dynamic feedforward compensator whose aim is cancelling the minimum-phase invariant zeros of a discrete-time linear multivariable system, non-strictly proper in general. The feedforward compensator also satisfies the condition of being of minimal dynamic order and that of maintaining right invertibility: i.e., if the original system is right invertible, then the cascade of the feedforward compensator and the system is right invertible as well. Special attention is paid to this property since it is a basic property in interesting control problems, like, e.g., reference tracking. Nonetheless, the procedure is developed for non-right-invertible and non-left-invertible systems, in general
A geometric perspective on H2-optimal rejection by measurement feedback in strictly-proper systems: the continuous-time case
No full textIn this work, we develop a geometric method for solving the problem of H2-optimal rejection of disturbance inputs in continuous-time linear systems without feedthrough terms from the control input and the disturbance input to the controlled output and the measured output. A necessary and sufficient condition for the solvability of the problem is expressed in terms of a pair of subspaces, a controlled-invariant subspace and a conditioned-invariant subspace, derived from the Hamiltonian systems associated with the problem. The if-part of the proof shows how to synthesize the feedback regulator, which is non-strictly-proper in general
A nested computational approach for l2 optimization of regulation transients in discrete-time linear parameter varying systems
No full textThis work deals with the optimization, expressed as the minimization of the l2 norm of the tracking error, of regulation transients caused by instantaneous, wide parameter variations occurring in discrete-time linear systems. The regulated system switching law is assumed to be completely known a priori. A feedback regulator, designed according to the internal model principle, guarantees closed-loop asymptotic stability and zero error in the steady-state condition. The compensation scheme for the minimization of transients includes a feedforward action on both the plant and the feedback regulator and a switching policy for the states of the exosystem and the feedback regulator. The feedfoward action and the state switching policy are computed off-line, by means of a two-level nested algorithm. The lower level includes a sequence of finite-horizon optimal control problems (each one corresponding to a time interval between two subsequent switches), while the upper level combines relevant data from the lower-level problems into a global l2 optimization problem. A substantial feature of this contribution is that the approach to the lower-level problem relies on an original procedure which provides the solution of a discrete-time finite-horizon optimal control problem in closed form as a function of time. Thus, discrete-time intervals with a large number of samples can easily be handled
Regulation transients in discrete-time linear parameter varying systems: l2 optimization with preview
No full textThis work introduces a methodology for the minimization, in terms of the l2 norm of the tracking error, of the regulation transients in discrete-time linear systems subject to instantaneous, wide parameter variations. A feedback regulator designed according to the well-known internal model principle guarantees closed-loop asymptotic stability and zero error in the steady-state condition. The compensation scheme for the minimization of transients includes a feedforward action on both the plant and the feedback regulator and a switching policy for the states of the exosystem and of the feedback regulator itself. The feedfoward action and the state switching policy are computed off-line, on the basis of the complete preview of the regulated-system switching law in a given time interval
Exact Unknown-State, Unknown-Input Reconstruction: A Geometric Framework for Discrete-Time Systems
No full textThe complete solution of the unknown-state, unknown-input reconstruction problem in systems with invariant zeros is intrinsically limited by the fact that for any invariant zero, at least one initial state exists, such that, when the mode associated to the invariant zero is suitably injected into the system, the corresponding output is zero. Although in the awareness of this restriction, the problem of reconstructing the initial state and the inaccessible inputs from the available measurements is the object of a fair amount of research activities because of its impact on a wide range of applications, specifically those dealing with the synthesis of enhanced-reliability control systems. In this context, the present paper contributes a geometric method aimed at solving the exact unknown-state, unknown-input reconstruction problem in discrete-time linear time-invariant multivariable systems with nonminimum-phase zeros. The case where all the system invariant zeros lie in the open set outside the unit disc of the complex plane is regarded as the basic one. The difficulties related to the presence of those invariant zeros are overcome by allowing a reconstruction delay commensurate to the invariant zero time constants. The same technique also applies to the case of systems without invariant zeros. In the latter circumstance, however, the reconstruction delay is related to the number of iteration required by the algorithm for the computation of a specific subspace to converge. Finally, the more general case where the problem is stated for a system whose invariant zeros lie both inside and outside the unit disc of the complex plane is reduced to the basic problem referred to a new system, derived from the original one through a procedure aimed at replacing the minimum-phase zeros with their mirror images with respect to the unit circle
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