Contents of the Journal of Mechanical Engineering 50, 2 (1999)

J. A. TEIXEIRA DE FREITAS, C. CISMASIU,  Z.-M. WANG: Numerical 
applications with hybrid-Trefftz stress and displacement elements         73

I. BALLO: Improving   effectiveness of an active electropneumatic 
vibration  control system by means of the force generator principle 
(in Slovak)                                                               96

S. KAPUCU, S. BAYSEC: On the applicability of the linear 
discrete-time model and system identification by the least squares 
method                                                                   104

A. TONDL: Vibration quenching of an externally excited pendulum system   119

O. DANEK: Evolutive dynamic continua (in Czech)                          133

The performance of the alternative stress and displacement models of the hybrid-Trefftz finite element formulation is illustrated. Potential problems are used to illustrate the convergence patterns and rates of the finite element solutions and the implementation of a p-adaptive refinement procedure. The insensitivity of the hybrid-Trefftz solutions to mesh distortion and incompressibility is illustrated with three-dimensional elastostatic problems. The direct computation of stress intensity factors is applied to elastic plate stretching problems, which are also used in a shape optimization application. The stress model is applied to the elastoplastic analysis of softening plates based on a gradient dependent plasticity model for localisation. The last test illustrates the application of the hybrid-Trefftz displacement element to the spectral analysis of unbounded media.

In the paper dynamic properties of an improved active vibration control system of the electropneumatic type are studied. In the first part the force generator principle is established and analysed. It is shown how the force generator characteristics can be identified experimentally. The following part analyses the way how to construct the electronic regulator of the improved active system. At the end the amplitude-frequency characteristics of the improved and original active systems were compared with the characteristics of the passive vibration control system of the same mass and stiffness.

Although system identification is a widely used method in obtaining a simple and empirical model for system response, material published on its technological aspects is scarce. This paper intends to compile practical aspects which could be useful in application of identification by looking at the time-step response of the system. An analogue computer model of a second order, linear underdamped system is excited by a pseudo-random binary signal generated by a digital computer and the response of the system is recorded digitally in discrete time. The coefficients of a discrete-time model are calculated as to minimize the cummulative error between the position data recorded experimentally and that calculated by the model. Dependence of the model coefficients on the amplitude and discrete-time step of the excitation is clarified on an analytical basis and verified by experiments. Finally, the sensitivity of the model to variations in excitation frequency is searched in amplitude and phase lag, and sample frequency-response curves are presented.

To an externally excited, elastically mounted pendulum system a dynamic absorber is added which can move in the direction of the pendulum axis. This differs from the previously analysed system (see [1, 2]) where the absorber mass can move perpendicularly to the pendulum axis. The quenching efficiency of this system in question is studied and compared to that of the previously analysed system. The absorber in the system analysed in this contribution can be efficient for slightly damped systems when the dynamic absorber can be correctly tuned, i.e. when the excitation is fully or nearly harmonic with constant frequency. This sensibility to correct tuning is especially required when only an absorber of small mass can be used.

The topic of this contribution is a qualitative analysis of overtuning the initial asymptotic stable dynamic continuum by an evolutive dynamic coupling. In the general equation of free motion, this coupling is expressed by two types of differential operators - a selfadjoint and non-selfadjoint one. The tuning of the couple of complex conjungate eigenvalues through the real dual eigenvalue into two different real ones is followed. The solution is developed by the formula for the roots of the quadratic equation with coefficients given by a scalar product. The real coefficients are obtained for the selfadjoint and complex coefficients for the non-selfadjoint operators. Our attention was aimed mainly at the crossing over to the real dual eigenvalue and also at the dynamic regimes in its narrow surroundings. An essential simplification was obtained by the definition of the changes of dissipation parameter and the discriminant instead of the definition of the type of the evolutive differential operators.