Applied Mechanics and Materials Vol. 706

Abstract: To simplify the designing of electronic components of forklift trucks with regard to their specific operating conditions, a test forklift truck was equipped with industrial-standard instruments to measure the occurring electrical and mechanical stresses. To assess the fatigue potential of the operating load conditions, the acquired measurement data were analyzed in the time and frequency domain and compared with each other with the aid of a rainflow counting algorithm. To compare the failure characteristics of the examined electronic components, fatigue tests were carried out on an electro-dynamic shaker. These experiments were performed on individual components of the electronic drive unit, which were stressed at the same time under typical operating electrical loads. Additionally, the influence of vibration and shock excitation were checked for the control behavior of these components.

Abstract: Comfort is an important topic in the elevator industry, and among the different factors which affect it, vibration in the car is one of the most important. Passengers not only perceive this phenomenon as an unpleasant effect, but also as a signal of the elevator safety level, and therefore it is of great importance to mitigate it. Although mature technologies are available for reducing vibration in low speed elevators, the increased number of high buildings and sky-scrappers, forces the development of new technologies for medium and high speed elevators. Active vibration control techniques are widespread used for damping vibration in different industrial environments, but their application to the elevator industry normally imply expensive solutions, with bulky and heavy actuators which highly burden its application. As alternative, the cheaper and lighter semi-active vibration systems based on passive elements capable of modifying their properties in response to external commands represent an appealing option. In this paper, we describe a design method for a semi-active roller guide based on a commercial magneto-rheological damper. Different control strategies based on low cost acceleration sensors are also analyzed.

Abstract: Vibration phenomena taking place in lifting and hoist installations may influence the dynamic performance of their components. For example, in an elevator system they may affect ride quality of a lift car. Lateral and longitudinal vibrations of suspension ropes and compensating cables may result in an adverse dynamic behaviour of the entire installation. Thus, there is a need to develop reliable mathematical and computer simulation models to predict the dynamic behaviour of suspension rope and compensating cable systems. The aim of this paper is to develop a model of an aramid suspension rope system in order to predict nonlinear modal interactions taking place in the installation. A laboratory model comprising an aramid suspension rope, a sheave/ pulley assembly and a rigid suspended mass has been studied. Experimental tests have been conducted to identify modal nonlinear couplings in the system. The dynamic behaviour of the model has been described by a set of nonlinear partial differential equations. The equations have been solved numerically. The numerical results have been validated by experimental tests. It has been shown that the nonlinear couplings may lead to adverse modal interactions in the system.

Abstract: In predictions of the dynamic behaviour of an elevator car system, it is important to take into account the influence of passengers’ behaviour in the car. In this paper a simulation model to analyse the influence of various loading car conditions on the dynamic response of the elevator system is developed. This involves the investigation of the dynamic response of the car with different loads. An experimental rig with a rectangular elevator platform fixed on the top of four silent blocks attached to a shaker is designed to conduct experimental tests. The car is excited over a range of frequencies and amplitudes. A number of passengers wearing different type of shoes in the car are investigated. The transmissibility measurements are carried out with a harmonic excitation applied first to an empty car and then to the car with a number of passengers. An excellent agreement from experimental tests with the model predictions is achieved. The passenger’s role to act as a dynamic absorber is assessed and recommendations to achieve the best ride quality under load conditions are provided and summarised.

Abstract: In this work we present and discuss a method for measuring the phase of chaotic systems. This method has as input a scalar time series and operates by estimating a fundamental frequency for short segments, or windows, along the whole extension of the signal. It minimizes the mean square error of fitting a sinusoidal function to the series segment. This approach does not require following the trajectory on the attractor, works well over a wide range of adjustable parameters, is of easy implementation, and is particularly appealing for experimental settings with single signal outputs since there is no need of attractor reconstruction. We demonstrate the applicability of this method on experimental time series obtained from two coupled Chua circuits.

Abstract: This work studies the effect of time delayed feedback on stationary solutions in a van derPol type system. We consider the case where the feedback gain is harmonically modulated with a resonantfrequency. Perturbation analysis is conducted to obtain the modulation equations near primaryresonance, the stability analysis for stationary solutions is performed and bifurcation diagram is determined.It is shown that the modulated feedback gain position can influence significantly the steadystates behavior of the delayed van der Pol oscillator. In particular, for appropriate values of the modulateddelay parameters, the existence region of the limit cycle (LC) can be increased or quenched.Moreover, new regions of quasiperiodic vibration may emerge for certain values of the modulatedgain. Numerical simulation was conducted to validate the analytical predictions.

Abstract: This study focuses on analysing the effects of nonlinear torsional stiffness on the dynam-ics of a slender elastic beam under torsional oscillations, which can be subject to helical buckling.The helical buckling of an elastic beam confined in a cylinder is relevant to many applications. Someexamples include oil drilling, medical cateters and even the conformation and functioning of DNAmolecules. A recent study showed that the formation of the helical configuration is a result of onlythe torsional load, confirming that there is a different path to helical buckling which is not related tothe sinusoidal buckling, stressing the importance of the geometrical behaviour of the beam. A lowdimensional model of an elastic beam under torsional oscillations is used to analyse its dynamical be-haviour with different stiffness characteristics, which are present before and after the helical buckling.Hardening and softening characteristics are present, as the effects of torsion and bending are coupled.With the use of numerical algorithms applied to nonlinear dynamics, such as bifurcation diagramsand basins of attraction, it is shown that the nonlinear stiffness can shift the bifurcations and inducechanges in the stability of the desirable and undesirable solutions. Therefore, the proper modellingof these stiffness nonlinearities seems to be important for a better understanding of the dynamicalbehaviour of such beams

Abstract: This paper presents some approaches to the design and some experimental results for current and speed control loops of a reaction wheel (RW). Reaction wheels are largely employed in satellite attitude control due to its large range of torque capability, small power consumption and high reliability. However, to achieve such performance the RW design shall deal with several restrictions, such as to support the space environment hazards (radiation, vacuum, high and fast temperature variation), and launch requirements (vibration, noise and choke). In this work some experimental results of an air-bearing table attitude control equipped with a Fiber Optics Gyro (FOG), a reaction wheel and a small fan will be presented. The RW is controlled by speed reference, and a second speed mode control similar to the first one was implemented in an external computer. Both were then compared by means of the air-bearing attitude control performance during the wheel zero-speed crossing. The results showed that the controllers have similar performance, as expected, and the maximum attitude pointing error remained below 0.08 degrees, which complies with the attitude requirements of Earth pointing satellites.

Abstract: his work is applied to the dynamics of rotational motion of artificial satellites, that is, itsorientation (attitude) with respect to an inertial reference system. The attitude determination involvesapproaches of nonlinear estimation techniques, which knowledge is essential to the safety and controlof the satellite and payload. Here one focuses on determining the attitude of a real satellite: CBERS-2(China Brazil Earth Resources Satellite). This satellite was launched in 2003 and were controlled andoperated in turns by China (Xi’an Control Center) and Brazil (Satellite Control Center). Its orbit isnear polar sun-synchronous with an altitude of 778km, crossing Equator at 10:30am in descendingdirection, frozen perigee at 90 degrees, and providing global coverage of the world every 26 days.The attitude dynamical model is described by nonlinear equations involving the Euler angles. Theattitude sensors available are two DSS (Digital Sun Sensor), two IRES (Infra-Red Earth Sensor), andone triad of mechanical gyros. The two IRES give direct measurements of roll and pitch angles with acertain level of error. The two DSS are nonlinear functions of roll, pitch, and yaw attitude angles. Thegyros furnish the angular measurements in the body frame reference system. Gyros are very importantsensors, as they provide direct incremental angles or angular velocities. They can sense instantaneousvariations of nominal velocities. An important feature is that it allows the replacement of complexmodels (different torques acting on the space environment) by using their measurements to turn thedynamical equations into simple kinematic equations. However gyros present several sources of errorof which the drift is the most troublesome. Such drifts yield along time an accumulation of errorswhich must be accounted for in the attitude determination process. Herein one proposes to estimatethe attitude and the drift of the gyros using the Least SquaresMethod. Results show that one can reachaccuracies in attitude determination within the prescribed requirements, besides providing estimatesof the gyro drifts which can be further used to enhance the gyro error model.

Abstract: An artificial satellite subject to the attraction of the Earth is disturbed due to nonsphericaldistribution and non-symmetrical Earth mass. This uneven distribution of mass is expressed by the socalledspherical harmonic coefficients of the Earth potential. For a faster computation, the accelerationderived from the potential is obtained by a series expansion in terms of these harmonics, the fullynormalized Legendre polynomials and their derivatives, and several recursions associated with thelongitude, geocentric latitude and altitude of the center of mass of the satellite. This paper analyzesthe detailed aspects of disturbances in artificial satellites, related with the modeling of the Earth'sgravitational potential as well as numerical implementation of a recursive algorithm to calculate theacceleration of the geopotential based on the Clenshaw summation. In general, one uses recursiveequations of high degree and order to calculate the Legendre polynomials in order to obtain fasterprocessing and numerical accuracy. However, the recursions can yield numerical errors at each stepof the recursion so that higher orders and degrees of harmonics, the accumulated error may be quitepronounced. The computational implementation of the algorithm is carried out by a PC computer.With the implementation of this algorithm it is possible to calculate the geopotential accelerationfor different orbits and different situations. Such approach aims at mitigating the numerical problemsarising from use of extended series expansion when computing recursively the Legendre polynomials.Once the favorable numerical properties are proven, the algorithm can be used in the solution ofpractical problems of orbital space mechanics and for the Brazilian Space Mission.