Jarir Mahfoud

CONTROL OF THE BREATHING MECHANISM OF A CRACKED ROTOR BY USING ELECTRO-MAGNETIC ACTUATOR: NUMERICAL STUDY

This paper presents a numerical study devoted to the evaluation of the possibility of monitoring and controlling the dynamic behavior of a rotating machine with a cracked shaft by using an Electro-Magnetic Actuator (EMA). The EMA is located at the mid-span of the rotor to provide active control. The opening and closure (breathing) of the crack is determined by the stress field over its cross section resulting from the dynamic bending moment. The system is nonlinear due to the fact that the crack parameters must be determined for each time step and the EMA introduces forces that are inversely proportional to the square of the gap value between the stator and the rotor. The model developed takes into account the behavior of the crack and the in uence of the EMA. Simulations were carried out to access the possibility of controlling the breathing mechanism. The results obtained demonstrate the possibility of using the EMA in order to keep the crack closed along the rotation of the rotor, thus forming a self-healing scheme for the cracked rotor.

On the balancing of flexible rotating machines by using an inverse problem approach

The aim of this work is to propose a methodology for the experimental determination of the matrix relating unbalance forces to measured displacements for a rotating machine. The goal is to obtain an alternative solution to special situations found in the industrial context, especially for highly flexible rotors that may lead to a nonlinear behavior of the system. The trial-weights-based techniques, such as the influence coefficients method or the modal method, cannot be applied in such configurations. The proposed balancing method is based on the use of an identified model for the system. A pseudo-random optimization technique using genetic algorithms was applied for the identification process. The basic idea is to obtain the flexible rotor unbalance response, which is then mimicked by using a finite element method model in which the unbalance masses and their corresponding angular positions are the design variables from the optimization viewpoint. This way, an inverse problem is solved and the corrective masses are obtained. The model characteristics of the system are adjusted experimentally. Experimental investigations were carried out and the results show the efficiency of the method developed; the limitations and improvements are finally discussed.

Experimental Assessment of a New Fuzzy Controller Applied to a Flexible Rotor Supported by Active Magnetic Bearings

The aim of this study was to develop and implement a new control approach dedicated to turbomachinery. The new, fuzzy based controller utilizes inputs expressed in polar coordinates. Its originality is that it manages two significant physical quantities, namely, tangential and radial velocities, associated with steady-state and transient behaviors, respectively. Three controllers are compared for the control of a flexible rotor supported by active magnetic bearings (AMBs): proportional-integral-derivative (PID), single-input and single-output (SISO) fuzzy and the new controller. The assessment was performed using an academic test rig and the results obtained with the new controller show that performances were enhanced with equivalent levels of stability and robustness.

Assessment of the Effectiveness of a Polar Fuzzy Approach for the Control of Centrifugal Compressors

The aim of this study is to assess the possibility of apply a new control approach dedicated to turbomachinery. The controller is fuzzy based using inputs expressed in polar coordinates. The advantage is that it manages two significant physical quantities, namely tangential and radial velocities that are related to steady state and transient behaviors respectively. A synchronous filter is associated to the controller in order to enhance the ratio command force/bearing dynamic capacity. The approach was previously applied experimentally with success for the control of an academic test rig. It is adapted here for the control of an industrial compressor whose flexible rotor is supported by Active Magnetic Bearings (AMB). At this stage, only numerical investigations are performed. The controller has to satisfy the standards and the end users requirements. In addition, it should be easy to implement. The behavior of the machine studied is assessed for several configurations of unbalances. A test that corresponds to usual industrial excitations (subsynchronous excitations at nominal speed) is also carried out. Results obtained are satisfactory and give insight into the potential of the approach. In addition, and as the fuzzy controller parameters are independent from the rotor design, the approach is a first step for the standardization of magnetic bearing controller synthesis

Energy cost assessment of the active control of a rotating machine by using an electromagnetic actuator and a piezoelectric actuator

The performances for controlling a rotating machine by using either an Electromagnetic Actuator or a Piezoelectric Actuator are compared in this work. The aim is to establish selection criteria based on environmental impact. Life Cycle Analysis shows that the operating stage has a considerable impact. In this study, only the operating stage is considered. The energy consumed by the actuators seems to be the appropriate indicator for the same “mechanical” performances. Numerical studies are carried out in order to quantify the energy consumed in each case. Modal control strategy with a fuzzy controller is used. The controller inputs are displacements and velocities. The system studied is modeled by using finite element method and the electrical circuit of each actuator is modeled by using basic electricity and electromagnetism theories. Several configurations are assessed and defined by using the chosen Functional Unit. The results obtained show that both controllers are efficient and enable recommendations for optimal control procedures design for the energy consumed.Copyright

Modeling and identification of electromagnetic actuator for the control of rotating machinery

This work deals with the design and the assessment of electromagnetic actuators (EMAs) for the control of rotating machines. The system studied has a hybrid bearing that exhibits nonlinear behavior. The system is composed of a horizontal flexible shaft supported by two ball bearings at one end and a roller bearing that is located in a squirrel cage at the other end. Four identical EMAs supplied with constant current are utilized. The EMAs associated to the squirrel cage constitutes the hybrid bearing. The aim is to develop a strategy in order to define and to identify a reliable model necessary for the control of rotating machinery in the presence of localized non-linearity. The identification strategy consists in modeling the system with as many sub-models as needed that are identified separately. This enables obtaining a straightforward modeling of rotating machinery even in the case in which system components are frequency or time dependent. For the system studied, two sub-models were necessary. First the EMAs were modeled by using classical equations of electromagnetism and then identified experimentally. Then, a linear model of the shaft mounted on its bearings was defined by using the finite element method and was identified successfully. The model of the system was adjusted after assembling the different identified sub-models. The identification is carried out by using a pseudo-random search algorithm. The model of the system is then assessed for different configurations. The results obtained demonstrate the effectiveness of the developed strategy.

Experimental assessment of a new fuzzy controller applied to a flexible rotor supported by Active Magnetic Bearings

The aim of this study was to develop and implement a new control approach dedicated to turbomachinery. The new, fuzzy based controller utilizes inputs expressed in polar coordinates. Its originality is that it manages two significant physical quantities, namely tangential and radial speeds, associated with steady state and transient behaviours respectively. Three controllers are compared for the control of a flexible rotor supported by Active Magnetic Bearings (AMB): PID, SISO fuzzy and the new controller. The assessment was performed using an academic test rig and the results obtained with the new controller show that performances were enhanced with equivalent levels of stability and robustness.

Rotating Machine Critical Speed Suppressing by Using Electromagnetic Actuators

The possibility of suppressing critical speeds by using electromagnetic actuators (EMAs) is assessed numerically and experimentally in this paper. The system studied is composed of a horizontal flexible shaft supported by two ball bearings at one end and a roller bearing that is located in a squirrel cage at the other end. Four identical EMAs supplied with constant current are utilized. The EMAs associated to the squirrel cage constitutes the hybrid bearing. Results obtained, show that the constant current, when applied to the EMAs, produces a shift of the first critical speed toward lower values. Moreover, the application of constant current for a speed interval around the critical speed enables a smooth run-up or run-down without crossing any resonance.Copyright

Monitoring the Dynamic Behavior in Polar Domain for an Easier Diagnostic of Rotating Machinery

The observation of the measurements in the polar domain can lead to an easier interpretation of the dynamic behavior, where the steady state and the transient behaviors could be directly distinguished. This transformation is simple and widely used in robotic. Numerical and experimental investigations are performed and presented in this paper. Experiments were performed on a test bench dedicated to the assessment of the aerodynamic behavior of a compressor impeller. The impeller is mounted at the free end of a rotor supported by two tilting pad bearings. Besides the unbalance excitations, the impeller is subject to low frequency aerodynamic perturbations. The results obtained show the effectiveness of this methodology for an easier monitoring and for the control of rotating machinery.