CASSEM: Vibration Control in the Smart Way
by Salim Belouettar
The ambition of the CASSEM project is to define the 'best' models and techniques that will permit us to model, simulate and validate the development of a more efficient vibration control.
In many industrial and defence applications, noise and vibration are important problems. In recent years, the control of sound and vibration has been the subject of much research, and there are now numerous examples of such applications. The most common general classification of vibration control differentiates between passive, active and hybrid passive-active control. Passive control involves the use of reactive or resistive devices that either load the transmission path of the disturbing vibration or absorb vibratory energy. Active control also loads the transmission path but achieves this loading through the use of actuators that generally require external energy. In passive control, the material properties of structure such as damping and stiffness are modified so as to change the response of structure. In active control, the structural response is controlled by adding external effort to the structure.
Combining these two approaches, hybrid control integrates the passive approach with an active control structure, and is intended to reduce the amount of external power necessary to achieve control.
Normally an adaptive or smart structure contains one or more active or smart materials. It is the use of these materials that causes the whole structure to be classified as smart. These materials have the ability to change their shape, rheological properties (eg stiffness and damping), or internal electrical properties (eg dielectric constant or resistivity).
Depending on the relative positions of the viscoelastic layer and the piezoelectric actuator, the viscoelastic passive and piezoelectric active actions can operate either separate or simultaneous actions. Typically, these materials have a sandwich structure, in which a soft, thin viscoelastic layer is confined between identical stiff, elastic layers. These structures yield a superior energy absorption. In particular, they offer the advantage of high damping with low weight addition. The interlayer-damping concept is highly compatible with the laminated configuration of composite structures and with their fabrication techniques, and provides an effective way to reduce vibrations and noise in structures. The damping is introduced by an important transverse shear in the viscoelastic layer. This is due to the difference between in-plane displacements of the elastic layers and also to the low stiffness of the central layer.
The performance of passive and hybrid control systems depends strongly on the viscoelastic material layer and piezoelectric material properties. In this project, numerical identification based on direct/inverse approaches will be developed. An advanced non-contact laser technique (ISI-SYS vibrograph system and Polytec scanning vibrometer) will be applied for the vibration measurements. These experimental data will be used to determine the natural frequencies and corresponding loss factors by the developed modal analysis program.
Three approaches are put forward for retrieving the material parameters. The first approach is the use of neural networks as a regression analysis tool. With the development of neural networks, it has become possible to perform a meaningful parameter extraction without the knowledge of an analytical relationship between the material properties and test values. This allows mechanical parameters to be identified by combining finite element modelling and experimental testing through neural networking.
The second approach considers optimization techniques. An error function, eg the least squares sum of the difference between modelling and experimental results, is minimized by changing the material parameters using different methods. These are either deterministic methods involving semi-analytical or numerical gradient procedures or stochastic methods like genetic algorithms or shooting methods.
The basic idea of the third proposed approach is that simple mathematical models (response surfaces) are determined only by the finite element solutions in the reference points of the experiment design. The function to be minimized describes the difference between the measured and numerically calculated parameters of the response of structure. By minimizing the function, the identification parameters are obtained.
Another issue of this FP6 project is to develop a general analytical and numerical (Finite Element) framework to model: (i) composite structures with piezoelectric sensors and actuators, (ii) thermal and pyroelectric effects in piezoelectric composites, and (iii) piezoelectric shunted damping.
In the context of the FP6 STREP Project 'CASSEM' (Composites and Adaptive Structure: Simulation, Experimentation and Modelling), we will also design a robust controller, which is stable in the presence of uncertainties of modelling and parameters, and ensures optimal disturbance rejection capability. In the implementation of the controller, actuators and sensors are needed. The locations of actuators and sensors over a structure determine the effectiveness of the controller in damping vibrations.
A variety of problems must be clarified before active systems can be implemented within structures. One of these - an important and not fully recognized problem - is the proper positioning of sensors and actuators on structures in the case of active systems, and the location of dampers in the case of passive systems. In active vibration control, actuator and sensor placement is a very significant issue, since it has a direct effect on the control efficiency and cost. For example, large flexible structures require many actuators for active vibration control, and the problem of optimizing their location becomes extremely significant in maximizing system controllability. An arbitrary choice of actuator positions can seriously degrade the system performance. The controllability index, the genetic algorithms, the gradient-based optimization procedure and the heuristic procedures were used to determine the proper sensor or actuator locations. The problem of positioning and size of passive dampers is also important, for similar reasons.
CASSEM is a highly interdisciplinary project combining engineering and physical sciences, for example, experimental material science, numerical modelling methods, mathematics, automation systems and mechatronics. The consortium consists of nine scientific and industrial partners from seven EU countries. The application of the project results will lead to long-term innovations in composites and adaptive structures. The development of vibration control systems will allow the areas of application of multi-functional composite materials to be extended based on the advanced knowledge and understanding of vibration response.
Centre de Recherche Henri Tudor: http://www.tudor.lu
Salim Belouettar, Centre de Recherche Henri Tudor, Luxembourg
Tel: +352 54 55 80 500