Numerical Investigation of Shallow Depth Sloshing Absorbers for Structural Control

A liquid sloshing absorber consists of a container, partially filled with liquid. The absorber is attached to the structure to be controlled, and relies on the structure’s motion to excite the liquid. Consequently, a sloshing wave is produced at the liquid free surface within the absorber, possessing energy dissipative qualities. The behaviour of liquid sloshing absorbers has been well documented, although their use in structural control applications has attracted considerably less attention. Generally it is accepted that sloshing absorbers with lower liquid levels are more effective energy dissipaters than those with higher levels, although there has not yet been a study to reveal an ‘optimum’ design mechanism. The main limitation of numerically modelling such circumstances is the inherent complexity in the free surface behaviour, predictions of which are limited when using grid-based modelling techniques. Considering such limitations, Smoothed Particle Hydrodynamics (SPH) is used in this study to model a 2-dimensional rectangular liquid sloshing absorber. SPH is a Lagrangian method of solving the equations of fluid flow that is suitable to model liquid sloshing due its grid-free nature, and inherent ability to model complex free surface behaviour. The primary objective of this paper is to numerically demonstrate the effect of tuning a container's width, to complement previous work [6] on the effect of liquid depth. This study is in an attempt to reveal geometry that enables both effective energy transfer to sloshing liquid and to dissipate this energy quickly

Vibration absorbers for chatter suppression: A new analytical tuning methodology

Vibration absorbers have been widely used to suppress undesirable vibrations in machining operations, with a particular emphasis on avoiding chatter. However, it is well known that for vibration absorbers to function effectively their stiffness and damping must be accurately tuned based upon the natural frequency of the vibrating structure. For general vibration problems, suitable tuning strategies were developed by Den Hartog and Brock over 50 years ago. However, the special nature of the chatter stability problem means that this classical tuning methodology is no longer optimal. Consequently, vibration absorbers for chatter mitigation have generally been tuned using ad hoc methods, or numerical or graphical approaches. The present article introduces a new analytical solution to this problem, and demonstrates its performance using time domain milling simulations. A 40-50% improvement in the critical limiting depth of cut is observed, compared to the classically tuned vibration absorber. © 2006 Elsevier Ltd. All rights reserved

IMECE2005-79838 ENERGY DISSIPATION WITH SLOSHING FOR ABSORBER DESIGN

ABSTRACT Sloshing is the low frequency oscillation of the free surface of a liquid in a partially full container. Due to its detrimental effects, efforts are usually made in the direction of suppressing sloshing. In addition, intentionally induced sloshing may be employed as an effective energy sink to provide protection for resonant structures exposed to excessive vibration levels. It is generally reported that sloshing absorbers with shallow levels of liquid are more effective energy dissipators than those with deep levels. However, there has not yet been a study to reveal the mechanism of energy dissipation for practical applications, although there has been ample empirical proof for effectiveness. One of the limitations from a numerical perspective lies with the difficulty in predicting extreme free surface behaviour by traditional grid based computational methods. The objective of this paper is to report initial observations in this direction using Smoothed Particle Hydrodynamics (SPH). SPH is a Lagrangian method of solving the equations for fluid flow, that is suitable for modeling free surface phenomena such as sloshing due to its grid-free nature. Results are reported in this paper in the form of numerical case studies

Effective vibration suppression of a maneuvering two-link flexible arm via an event-based stiffness controller

Vibration control of a maneuvering flexible robotic arm is a challenging task in the presence of changing structural dynamics which has to deal with measurement inaccuracies and complex modeling efforts. This paper presents an effective and versatile controller for a maneuvering flexible arm. Proposed Variable Stiffness Control (VSC) is stable, due to its being dissipative in nature. The technique is suitable to be implemented as an add-on controller to existing robots, and it requires no additional hardware. Control is based on the detection of a kinematic event, peak relative displacement, rather than an accurate knowledge of structural dynamics. Hence, although there may not be a claim for the suggested control to be the most effective, it certainly represents significant practical advantages for cases where there may be structural uncertainties

Tool chatter in turning with a two-link robotic arm

Robotic operations have undeniable advantages of speed and precision in machining. However, such operations are generally limited by the self-excited tool chatter problem. When uncontrolled, chatter leaves a rough machined surface, accelerates wear of the cutter and creates unacceptably loud noise levels. A conventional approach to suppress chatter is to slow the waste removal rate. Such an action is usually successful to avoid chatter, but causes increased production time and cost. Therefore, it is desirable to maintain a reasonably fast rate of production and employ a chatter control measure. A simple semi-active parameter control technique is investigated numerically in this study. The proposed method is effective and requires no additional hardware to implement at the actuated joints such as those in robotic structures