The aim of the thesis is to propose active damping as a potential control strategy for chatter instability in machine tools.<p>The regenerative process theory explains chatter as a closed loop interaction between the structural dynamics and the cutting process. This is considered to be the most dominant reason behind machine tool chatter although other instability causing mechanisms exist.<p>The stability lobe diagram provides a quantitative idea of the limits of stable machining in terms of two physical parameters: the width of contact between tool and the workpiece, called the width of cut and the speed of rotation of the spindle. It is found that the minimum value of the stability limit is proportional to the structural damping ratio for turning operations. This important finding provides the motivation of influencing the structural dynamics by active damping to enhance stability limits of a machining operation.<p>A direct implementation of active damping in an industrial environment may be difficult. So an intermediate step of testing the strategy in a laboratory setup, without conducting real cutting is proposed. Two mechatronic "Hardware in the Loop" simulators for chatter in turning and milling are presented, which simulate regenerative chatter experimentally without conducting real cutting tests. A simple cantilever beam, representing the MDOF dynamics of<p>the machine tool structure constitutes the basic hardware part and the cutting process is simulated in real time on a DSP board. The values of the cutting parameters such as spindle speed and the axial width of cut can be changed on the DSP board and the closed loop interaction between the structure and the cutting process can be led to instability.<p><p>The demonstrators are then used as test beds to investigate the efficiency of active damping, as a potential chatter stabilization strategy. Active damping is easy to implement, robust and does not require a very detailed model of the structure for proper functioning, provided a collocated sensor and actuator configuration is followed. The idea of active damping is currently being implemented in the industry in various metal cutting machines as part of the European Union funded SMARTOOL project (www.smartool.org), intended to propose smart chatter control technologies in machining operations. / Doctorat en sciences appliquées / info:eu-repo/semantics/nonPublished

Indiana University-Purdue University Indianapolis (IUPUI) / In this thesis, a high- fidelity multibody dynamics model of a backhoe for simulating the digging operation is developed using the DIS (Dynamic Interactions Simulator)multibody dynamics software. Sand is used as a sample digging material to illustrate the model. The backhoe components (such as frame, manipulators links,track segments, wheels and sprockets) are modeled as rigid bodies. The geometry of the major moving components of the backhoe is created using the Pro/E solid modeling software. The components of the backhoe are imported to DIS and connected using joints (revolute, cylindrical and prismatic joints). Rotary and linear actuators along with PD (Proportional-Derivative) controllers are used to move and steer the backhoe and to move the backhoes manipulator in the desired trajectory. Sand is modeled using cubic shaped particles that can come into contact with each other, the backhoes bucket and ground. A cubical sand particle contact surface is modeled using eight spheres that are rigidly glued to each other to form a cubical shaped particle, The backhoe and ground surfaces are modeled as polygonal surfaces. A penalty technique is used to impose both joint and normal contact constraints (including track-wheels, track-terrain, bucket-particles and particles-particles contact). An asperity-based friction model is used to model joint and contact friction. A Cartesian Eulerian grid contact search algorithm is used to allow fast contact detection between particles. A recursive bounding box contact search algorithm is used to allow fast contact detection for polygonal contact surfaces and is used to detect contact between: track and ground; track and wheels; bucket and particles; and ground and particles. The governing equations of motion are solved along with joint/constraint equations using a time-accurate explicit solution procedure. The sand model is validated using a conical hopper sand flow experiment in which the sand flow rate during discharge and the angle of repose of the resulting sand pile are experimentally measured. The results of the conical hopper simulation are compared with previously published experimental results. Parameter studies are performed using the sand model to study the e ffects of the particle size and the orifi ces diameter of the hopper on the sand pile angle of repose and sand flow rate. The sand model is integrated with the backhoe model to simulate a typical digging operation. The model is used to predict the manipulators actuator forces needed to dig through a pile of sand. Integrating the sand model and backhoe model can help improving the performance of construction equipment by predicting, for various vehicle design alternatives: the actuator and joint forces, and the vehicle stability during digging.

Forced response analysis of bladed disk assemblies plays a vital role in rotor blade design and has been drawing a great deal of attention both from research community and engine industry for more than half a century. However because of the phenomenon called &lsquo / mistuning&rsquo / , which destroys the cyclic symmetry of a rotor, there have been several difficulties related to forced response analysis ever since, two of which are addressed in this thesis: efficient non-linear forced response analysis of mistuned bladed disks and mistuning identification. On the nonlinear analysis side, a new solution approach is proposed studying the combined effect of non-linearity and mistuning, which is relatively recent in this research area and generally conducted with methods whose convergence and accuracy depend highly on the number of degrees of freedom where non-linear elements are attached. The proposed approach predicts nonlinear forced response of mistuned bladed disk assemblies considering any type of nonlinearity. In this thesis, special attention is given to the friction contact modeling of bladed disks which is the most common type of nonlinearity found in bladed disk assemblies. In the modeling of frictional contact a friction element which enables normal load variation and separation of the contact interface in three-dimensional space is utilized. Moreover, the analysis is carried out in modal domain where the differential equations of motions are converted to a set of non-linear algebraic equations using harmonic balance method and modal superposition technique. Thus, the number of non-linear equations to be solved is independent of the number of non-linear elements used. On the mistuning identification side, a new method is enclosed herein which makes use of neural networks to assess unknown mistuning parameters of a given bladed disk assembly from its assembly modes, thus being suitable for integrally bladed disks. The method assumes that a tuned mathematical model of the rotor under consideration is readily available, which is always the case for today&rsquo / s realistic bladed disk assemblies. A data set of selected mode shapes and natural frequencies is created by a number of simulations performed by mistuning the tuned mathematical model randomly. A neural network created by considering the number of modes, is then trained with this data set for being used to identify mistuning of the rotor from measured data. On top of these, a new adaptive algorithm is developed for harmonic balance method, several intentional mistuning patterns are investigated via excessive Monte-Carlo simulations and a new approach to locate, classify and parametrically identify structural non-linearities is introduced.

In this work, we investigate the active vibration isolation and damping of sensitive equipment. Several single-axis isolation techniques are analyzed and tested. A comparison between the sky-hook damper, integral force feedback, inertial velocity feedback and LagLead control techniques is conducted using several practical examples.<p><p>The study of single-axis systems has been developed and used to build a six-axis isolator. A six degrees of freedom active isolator based on Stewart platform has been designed manufactured and tested for the purpose of active vibration isolation of sensitive payloads in space applications. This six-axis hexapod is designed according to the cubic configuration; it consists of two triangular parallel plates connected to each other by six active legs orthogonal to each other; each leg consists of a voice coil actuator, a force sensor and two flexible joints. Two different control techniques have been tested to control this isolator :integral force feedback and Lag-Lead compensator, the two techniques are based on force feedback and are applied in a decentralized manner. A micro-gravity parabolic flight test has been clone to test the isolator in micro-gravity environment.<p><p>ln the context of this research, another hexapod has been produced ;a generic active damping and precision painting interface based on Stewart platform. This hexapod consists of two parallel plates connected to each other by six active legs configured according to the cubic architecture. Each leg consists of an amplified piezoelectric actuator, a force sensor and two flexible joints. This Stewart platform is addressed to space applications where it aims at controlling the vibrations of space structures while connecting them rigidly. The control technique used here is the decentralized integral force feedback.<p><p> / Doctorat en sciences appliquées / info:eu-repo/semantics/nonPublished

Indiana University-Purdue University Indianapolis (IUPUI) / In this thesis, an explicit time integration code which integrates multibody dynamics and the discrete element method is used for modelling the excavation and moving operation of cohesive soft soil (such as mud and snow) by bulldozers. A soft cohesive soil material model (that includes normal and tangential inter-particle force models) is used that can account for soil compressibility, plasticity, fracture, friction, viscosity and gain in cohesive strength due to compression. In addition, a time relaxation sub-model for the soil plastic deformation and cohesive strength is added in order to account for loss in soil cohesive strength and reduced bulk density due to tension or removal of the compression. This is essential in earth moving applications since the soil that is dug typically becomes loose soil that has lower shear strength and lower bulk density (larger volume) than compacted soil. If the model does not account for loss of soil shear strength then the dug soil pile in front of the blade of a bulldozer will have an artificially high shear strength. A penalty technique is used to impose joint and normal contact constraints. An asperity-based friction model is used to model contact and joint friction. A Cartesian Eulerian grid contact search algorithm is used to allow fast contact detection between particles. A recursive bounding box contact search algorithm is used to allow fast contact detection between the particles and polygonal contact surfaces. A multibody dynamics bulldozer model is created which includes the chassis/body, C-frame, blade, wheels and hydraulic actuators. The components are modelled as rigid bodies and are connected using revolute and prismatic joints. Rotary actuators along with PD (Proportional-Derivative) controllers are used to drive the wheels. Linear actuators along with PD controllers are used to drive the hydraulic actuators. Polygonal contact surfaces are defined for the tires and blade to model the interaction between the soil and the bulldozer. Simulations of a bulldozer performing typical shallow digging operations in a cohesive soil are presented. The simulation of a rear wheel drive bulldozer shows that, it has a limited digging capacity compared to the 4-wheel drive bulldozer. The effect of the relaxation parameter can be easily observed from the variation in the Bulldozer's velocity. The higher the relaxation parameter, the higher is the bulldozer's velocity while it is crossing over the soil patch. For the low penetration depth run the bulldozer takes less time compared to high penetration depth. Also higher magnitudes of torques at front and rear wheels can be observed in case of high penetration depth. The model is used to predict the wheel torque, wheel speed, vehicle speed and actuator forces during shallow digging operations on three types of soils and at two blade penetration depths. The model presented can be used to predict the motion, loads and required actuators forces and to improve the design of the various bulldozer components such as the blade, tires, engine and hydraulic actuators.