Ahmet S. Yigit

Fully coupled vibrations of actively controlled drillstrings

Abstract A fully coupled model for axial, lateral, and torsional vibrations of actively controlled drillstrings is presented. The proposed model includes the mutual dependence of these vibrations, which arises due to bit/formation and drillstring/borehole wall interactions as well as other geometric and dynamic non-linearities. The active control strategy is based on optimal state feedback control designed to control the drillstring rotational motion. It is demonstrated by simulation results that bit motion causes torsional vibrations, which in turn excite axial and lateral vibrations resulting in bit bounce and impacts with the borehole wall. It is also shown that the results are in close qualitative agreement with field observations regarding stick–slip and axial vibrations and that the proposed control is effective in suppressing them. However, care must be taken in selecting a set of operating parameters to avoid transient instabilities in the axial and lateral motions.

Stick-Slip and Bit-Bounce Interaction in Oil-Well Drillstrings

Drillstring vibrations and in particular stick-slip and bit-bounce are detrimental to oil-well drilling operations. Controlling these vibrations is essential because they may cause equipment failures and damage to the oil-well. A simple model that adequately captures the dynamics is used to simulate the effects of varying operating conditions on stick-slip and bit-bounce interactions. It is shown that the conditions at the bit/formation interface, such as bit speed and formation stiffness, are major factors in shaping the dynamic response. Due to the varying and uncertain nature of these conditions, simple operational guidelines or active rotary table control strategies are not sufficient to eliminate both stick-slip and bit-bounce. It is demonstrated that an additional active controller for the axial motion can be effective in suppressing both stick-slip and bit-bounce. It is anticipated that if the proposed approach is implemented, smooth drilling will be possible for a wide range of conditions.

Optimizing modular product design for reconfigurable manufacturing

The problem of optimizing modular products in a reconfigurable manufacturing system is addressed. The problem is first posed as a generalized subset selection problem where the best subsets of module instances of unknown sizes are determined by minimizing an objective function that represents a trade-off between “the quality loss due to modularization” and the cost of reconfiguration while satisfying the problem constraints. The problem is then formulated and solved as an integer nonlinear programming problem with binary variables. The proposed method is applied to the production of a modular drive system composed of a DC motor and a ball screw. The study is a first attempt toward developing a systematic methodology for manufacturing modular products in a reconfigurable manufacturing system.

On the Stability of PD Control for a Two-Link Rigid-Flexible Manipulator

Controller design for a rigid-flexible two-link manipulator is considered. Robustness of independent joint PD control is investigated. It has been shown that the stability of independent joint PD control does not depend explicitly on the system parameters. No discretization or linearization of the equations of motion is required to assure the stability. Simulation studies also show that independent joint PD control gives reasonably good results for the flexible system, and is robust to parameter uncertainties.

Characterization of impact in composite plates

The nature of impact response of composite plates is studied through normalization of the governing equations. The key parameters which govern the transition from locally dominated to globally dominated responses are identified and used to characterize the impact response. Numerical and experimental results show that the type of response can be predicted through a functional relationship of the normalized impact force and two nondimensional parameters.

Optimal selection of module instances for modular products in reconfigurable manufacturing systems

A method for optimizing the variety of a modular products, manufactured in a Reconfigurable Manufacturing System, is proposed. The optimization is achieved through appropriately selecting the subsets of module instances from given sets. The problem is formulated as an integer nonlinear programming problem to find a trade-off between the quality loss due to modularity and the cost of reconfiguration for given sets of customer requirements. The proposed formulation is general in the sense that products can have any number of modules. The formulation is an extension to the available formulation that was developed for products with only two modules. Moreover, the current work addresses the effect of different order priorities, customer importance, and demands. The proposed method has been applied to a modular assembly problem and found to be efficient in determining optimum subsets of module instances.

On the impact between a rigid sphere and a thin composite laminate supported by a rigid substrate

Abstract The impact response of a thin composite laminate supported by a rigid substrate is addressed. An impact model based on a static contact law which incorporates damage effects is used in simulations to investigate the effect of impact parameters on the impact response. Damage is found to affect the response significantly. It is shown that the impact energy is not a sufficient parameter to describe impact where the impact parameters other than the impact velocity and the impactor mass are varying. The results show that the impact model presented in the current study completely describes the local impact response of composite laminates.

Low-Velocity Impact Response of Structures With Local Plastic Deformation: Characterization and Scaling

This paper presents a methodology for the characterization and scaling of the response of structures having different shapes, sizes, and boundary conditions that are under impact by spherical objects. The objectives are to demonstrate the accuracy of a new bilinear contact law that accounts for permanent indentation in the contact zone, and to show the efficacy of a characterization diagram in the analysis and design of structures subject to impact. The characterization diagram shows the normalized functional relationship between the maximum impact force and three nondimensional parameters that cover the complete dynamic spectrum for low-velocity impact. The validity of using the bilinear elastoplastic contact law is demonstrated by both finite element (FE) and Rayleigh-Ritz discretization procedures for simply-supported plates. The efficacy of the characterization diagram, which was developed using simple structural models, is demonstrated by the FE simulations of more complicated and realistic structures and boundary conditions (clamped, stiffened plates, and cylindrical panels). All of the necessary parameters needed for the characterization are ‘measured’ using the FE models simulating real-world experiments. Impact parameters are varied to cover the complete dynamic spectrum with excellent results.

Normalized Impact Response and Damage in a Thin Composite Laminate Supported by a Rigid Substrate

The local impact response and damage of thin composite laminates are investigated. The impact model uses a static contact law which incorporates damage effects due to local contact stresses. A major difference between this contact law and the others used previously is that the damage is accounted for during the loading rather than during the unloading phase which is expected to yield a more reasonable prediction of the maximum impact force. Normalization of the governing differential equations and initial conditions yields a single governing parameter, which is a dimensionless group of all the pertinent impact parameters (i.e., impact velocity, impactor mass, material and geometric properties). The introduction of this parameter not only simplifies the problem but also provides the necessary scaling rules for model testing.

Modeling and Analysis of Elastoplastic Impacts on Supported Composites

This paper presents an elastoplastic impact model for a spherical object impacting a supported composite layer or a half-space. The model utilizes a contact law that has been developed based on elastic-plastic and fully plastic indentation theories. For an impact event, the model parameters can easily be obtained analytically, computationally using Finite Elements (FE), and from experiments, by assuming transversely isotropic material behavior. Simulations are compared to those from a nonlinear FE model developed in ABAQUS, and to limited experimental data, with excellent results.