Jj Jan Kok

H/sub /spl infin// control for suppressing stick-slip in oil well drillstrings

We show, applying the linear H/sub /spl infin// control design technique, that the suppression of stick-slip oscillations and transient behavior can be largely improved over a PD-like control system currently applied in oil well drilling. Using rough knowledge (e.g. size and induced oscillation frequency) of the friction disturbance, the appropriate choice of dynamic weighting functions can make the closed loop system robust toward the substantial uncertainty in the knowledge of the nonlinear friction. After an introduction to drilling engineering and a description of the drilling system, a condensed analysis of the stick-slip phenomenon in drillstrings and a brief history of control solutions for the drillstring problem are given, and H/sub /spl infin// control solutions for allied systems are described. Then, the general principles of linear H/sub /spl infin// control design are sketched. Subsequently, the H/sub /spl infin// control design technique is applied to design a controller for the system under study. Next, the improved closed-loop behavior toward stick-slip oscillations is shown in simulations and experiments. Conclusions with respect to the results are drawn.

A numerical-experimental method for a mechanical characterization of biological materials.

For the determination of material parameters, it is a common practice to extract specimens with well-defined geometries. The design of the samples and the choice of the applied load are meant to lead to a homogeneous stress and strain distribution in a part of the sample. When applied to biological materials, this raises a number of problems: homogeneous strains cannot be obtained because the materials have inhomogeneous properties, and the manufacturing of samples is hard or sometimes impossible. In this technical note a different approach is presented, based on the use of a digital image technique for the measurement of nonhomogeneous strain distributions, finite element modeling and the use of a minimum-variance estimator. The method is tested by means of experiments on an orthotropic elastic membrane of a woven and calendered textile. Five parameters are identified using the experimental data of one single experiment.

Grey-box Modeling of Friction: An Experimental Case-study

Grey-box modeling covers the domain where we want to use a balanced amount of first principles and empiricism. The two generic grey-box models presented, i.e., a Neural Network model and a Polytopic model are capable of identifying friction characteristics that are left unexplained by first principles modeling. In an experimental case study, both grey-box models are applied to identify a rotating arm subjected to friction. An augmented state extended Kalman filter is used iteratively and off-line for the estimation of unknown parameters. For the studied example and defined black-box topologies, little difference is observed between the two models.

An Optimal Continuous Time Control Strategy for Active Suspensions with Preview

Abstract A continuous time control strategy for an active suspension with preview, based on optimal control theory, is presented. No approximation is needed to model the time delay between the excitation of the front and the rear wheels. The suspension is applied to a two DOF model of the rear side of the tractor of a tractor-semitrailer. The purpose of the suspension is to reduce either the required suspension working space or the maximum absolute acceleration of the sprung mass, without an increase of the dynamic tire force variation. For a step function as road input, reductions of 65% and 55%, respectively, are possible compared with a passive suspension.

Optimising product recycling chains by control theory

In this paper, a modelling method is described for production chains including recycling. It consists of elementary models of standard production operations, connected by market modules. The models are analysed using methods from control theory. These methods allow us to investigate essential properties of the chain concerning its dynamical behaviour, particularly with respect to stability and controllability. These properties are prerequisites for effective chain management, as in the evaluation of recycling policy. A case study on paper recycling was carried out to demonstrate the applicability of this method to practice.

Preview Estimation and Control for (Semi-) Active Suspensions

SUMMARY An active suspension with preview is tested for rounded pulses and a stochastic road surface, and is compared to a passive suspension. The spectacular performance improvement obtained for a step function as road surface is not achieved but the improvement is still significant. The frequency response of the active suspension is determined for comparison with some suspension systems found in literature An observer to reconstruct the preview information is presented. No model of the road surface is needed. From simulations, it appears that the observer reconstructs both deterministic and stochastic road surfaces satisfactory. However, the influence of measurement noise is not reduced sufficiently.

Nonlinear model predictive control of a laboratory gas turbine installation

The feasibility of model predictive control (MPC) applied to a laboratory gas turbine installation is investigated. MPC explicitly incorporates (input and output) constraints in its optimizations, which explains the choice for this computationally demanding control strategy. Strong nonlinearities, displayed by the gas turbine installation, cannot always be handled adequately by standard linear MPC. Therefore, we resort to nonlinear methods, based on successive linearization and nonlinear prediction as well as the combination of these. We implement these methods, using a nonlinear model of the installation, and compare them to linear MPC. It is shown that controller performance can be improved, without increasing controller execution-time excessively.

Adaptive Computed Reference Computed Torque Control of Flexible Robots

This paper presents a motion control technique for flexible robots and manipulators. It takes into account both joint and link flexibility and can be applied in adaptive form if robot parameters are unknown. It solves the main problems that are related to the fact that the number of degrees of freedom exceeds both the number of actuators and the number of output variables. The proposed method results in trajectory tracking while all state variables remain bounded. Global, asymptotic stability is ensured for all values of the stiffnesses ofjoints and links. To show the characteristics of the proposed control law, some simulation results are presented.

Vibration reduction of a harmonically excited beam with one-sided spring using sliding computed torque control

Vibration reduction of a harmonically excited multi Degree of Freedom (DOF) beam system with a local nonlinearity represented by a one-sided spring is considered. The linear beam system is modeled with a 3-DOF model which is obtained by applying a component mode synthesis method on the linear finite element model of the beam system. The low frequency response of the linear system can be described very accurately while the number of DOFs for the nonlinear analysis is kept small. Vibration reduction is realized in both simulations and experiments by forcing the system at a prescribed excitation frequency from the stable 1/2 subharmonic response of high amplitude towards the coexisting unstable harmonic response of low amplitude using a Sliding Computed Torque Controller (SCTC). The advantage besides the reduction of vibration amplitude is the small control effort needed once the unstable harmonic solution is stabilized. This is due to the fact that the unstable harmonic solution is a natural solution of the uncontrolled system.

Parameter identifiablity in the IAWQ Model No. 1 for modelling activated sludge plants for enhanced nitrogen removal

The IA WQ Model No. 1 is well accepted and used for dynamic modelling of activated sludge plants. The model contains many parameters. Some of the model parameters depend on the wastewater and/or plant. In some applications, the model has to describe a real plant quite accurately and some of the parameters have to be determined. As a consequence, model tuning or calibration has become an active area of research during the last few years. This contribution briefly reviews recent literature on calibration strategies and methods for assessing parameter identifiability of the Model. Some identifiability results for full scale plants are presented obtained by a combined analysis of the parametric sensitivity and the Fisher information matrix.