Amel Bouaziz’s research while affiliated with University of Sfax and other places

There are several types of renewable energy, produced from different sources. One of the most efficient renewable today is the wind energy. In the case of wind power, the kinetic energy of the wind drives a generator which produces electricity. The drive train is an important component in realizing reliable and robust wind turbines. This research work investigates the dynamic behavior of a horizontal axis wind turbine drivetrain under different excitation sources. The present paper developed an 11° of freedom (DOF) dynamic model of wind turbine geared transmission system. This model includes the main factors such as the time-varying mesh stiffness, bearing stiffness and damping. The aerodynamic driving is also defined taking into account the size of the blades and the average wind speed. Using the implicit Newmark’s method, the dynamic response of the bearing and the transmission error of the wind turbine gearbox is presented with the time signal and Fast Fourier Transform (FFT) spectrum. The results show the different vibration levels of the bearings and the effect of the wind speed and the gear ratio on the dynamic behavior of the system. Finally, a time-frequency representation is used to have more information about our system dynamic behavior.

In modern production, milling is considered the widespread cutting process in the formatting field. It remains important to study this manufacturing process as it can be subject to some parasitic phenomena that can degrade surface roughness of the machined part, increase tool wear and reduce spindle life span. In fact, the best quality work piece is obtained with a suitable choice of parameters and cutting conditions. In another hand, the study of tool vibrations and the cutting force attitude is related to the study of bearings as they present an essential part in the spindle system. In this work, a modeling of a High Speed Milling (HSM) spindle supported by two pair of Active Magnetic Bearings (AMB) is presented. The spindle is modeled by Timoshenko beam finite elements where six degrees of freedom are taken into account. The rigid displacements are also introduced in the modeling. Gyroscopic and centrifugal terms are included in the general equation. The bearings reaction forces are modeled as linear functions of journal displacement and velocity in the bearing clearance. A cutting force model for peripheral milling is proposed to estimate the tool-tip dynamic responses as well as dynamic cutting forces which are also numerically investigated. The time history of response, orbit, FFT diagram at the tool-tip center and the bearings dynamic coefficients are plotted to analyze dynamic behavior of the spindle.

The high speed milling represents the most important process to produce parts in different fields such as aeronautics, automotive and mould. It allows obtaining parts with complex form and with the best surface quality. So, it remains essential to understand the impact of both the cutting force model and its parameters on the tool tip response. Consequently, the mastering of chatter vibration in milling and surface properties of the work piece is essential. In this paper, the simulation of machining is applied to determine the cutting forces distribution. A spindle system modeling is presented using a new approach: Both rigid and flexible modes of the spindle's shaft are taken into account. The shaft is discretized with the Timoshenko beam finite elements with different circular sections. Nonlinear electromagnetic loads exerted by AMBs are computed in terms of the nominal air gap between bearings and spindle, the control current and the displacement of each node. A parametric study is performed to determine the influence of some parameters, such as the feed rate, the tangential cutting coefficient, the spindle speed and the axial depth of the cut on the cutting forces and the chatter vibrations in milling.

The use of Active Magnetic Bearings (AMBs) in the High Speed Machining (HSM) field is increasing. In fact, they have important advantages such us: high speed which can reach 150000 rpm, high stiffness and absence of lubrication. Indeed, these bearings operate through magnetic interaction generated between the stator and the rotor. The spindle, principal component in the HSM, supported by AMBs, is considered as a source of vibration in machine tool spindles when it is affected by some defects during their functioning. These defects can affect either the rotor of the spindle like the unbalance defect, or the AMBs such us misalignment of bearings, failure of actuators, sensors, amplifier and also the eccentricity defect. This paper studies the influence of spindle or AMBs defects on the dynamic behavior of a spindle supported by two AMBs. The system modeling is based on the finite element method. Four degrees of freedom is used and the coupling between elastic and rigid displacements is taken into account.

High speed machining (HSM) is a new technology used to product parts in different sectors. This technology permits time and processing cost reduction. One of the principal components of the HSM is the spindle. It is generally supported by active magnetic bearings (AMBs), in order to guarantee the greatest speed without affecting the dynamic characteristics of the machine. In this paper, a spindle supported by two AMBs is modelled using the finite element method. It is discretised with Timoshenko beam finite element with different circular sections. Both flexible and rigid movements are taken into account. The non-linear electromagnetic loads exerted by bearings are computed in terms of the nominal air gap between bearings and spindle, the control current and the degrees of freedom of each node. A parametric study is performed to determine the influence of some parameters on the magnetic forces and subsequently the dynamic behaviour of the spindle.