Sven Herold

Electroactive Polymer Actuators in Dynamic Applications

Electroactive polymers (EAPs) have been widely employed as smart material for actuators in recent years. Numerous investigations have focused on static or quasi-static applications. For the use as actuators in the field of active vibration control (AVC); however, the dynamic behavior needs to be studied in detail and the inherent nonlinear effects demand new control concepts. Since AVC applications have only recently been considered for EAP actuators, only a few studies have been published in this area so far . In this paper, the nonlinearities in a dielectric elastomer (DE) actuator and their consequences for dynamic applications are analyzed on a theoretical level first and then shown to be practically relevant in an experimental setup. Afterward, two compensation methods are presented and their improving influence on the dynamic behavior proven. Finally, the DE actuator is included in an active closed-loop control system and its potential for AVC demonstrated. Furthermore, a MATLAB/SIMULINK model of the whole system is presented, its general validity shown, and its potential for future system development processes highlighted.

Transient simulation of adaptive structures

This paper examines an approach used to design adaptive systems by means of Rapid-Prototyping on the basis of a simulation. The vibration behavior of mechanical systems including actuators, sensors, and an adaptive controller in the time domain will be modeled. This approach includes the calculation of the dynamic behavior of the mechanical structure with the help of the Finite Element Method (FEM). The FE model of the mechanical structure is transformed into modal space, then reduced and embedded as a state-space model in the MATLAB/Simulink environment. Thereby, a simple mechanical problem is solved analytically with the FEM as well as with the reduced MATLAB model. The comparison of the results shows a good agreement. Actuators and sensors are attached to the mechanical structure. Based on the resulting dynamical behavior of the mechanical structure, an adaptive control algorithm for the reduction of structural vibration is developed. Experimental tests are performed to verify and update the simulations. The hardware-in-the-loop simulations are carried out with a dSpace system. Some differences (with respect to speed-up, precision) between this simulation and other methods are presented later in the paper. This procedure allows the development and the evaluation of more complex adaptive mechanical structures.

Modeling approaches for active systems

To solve a wide range of vibration problems with the active structures technology, different simulation approaches for several models are needed. The selection of an appropriate modeling strategy is depending, amongst others, on the frequency range, the modal density and the control target. An active system consists of several components: the mechanical structure, at least one sensor and actuator, signal conditioning electronics and the controller. For each individual part of the active system the simulation approaches can be different. To integrate the several modeling approaches into an active system simulation and to ensure a highly efficient and accurate calculation, all sub models must harmonize. For this purpose, structural models considered in this article are modal state-space formulations for the lower frequency range and transfer function based models for the higher frequency range. The modal state-space formulations are derived from finite element models and/or experimental modal analyses. Consequently, the structure models which are based on transfer functions are directly derived from measurements. The transfer functions are identified with the Steiglitz-McBride iteration method. To convert them from the z-domain to the s-domain a least squares solution is implemented. An analytical approach is used to derive models of active interfaces. These models are transferred into impedance formulations. To couple mechanical and electrical sub-systems with the active materials, the concept of impedance modeling was successfully tested. The impedance models are enhanced by adapting them to adequate measurements. The controller design strongly depends on the frequency range and the number of modes to be controlled. To control systems with a small number of modes, techniques such as active damping or independent modal space control may be used, whereas in the case of systems with a large number of modes or with modes that are not well separated, other control concepts (e.g. adaptive controllers) are more convenient. If other elements (e.g. signal amplifiers or filters) in the signal paths have a significant influence on the transfer functions, they must be modeled as well by an adequate transfer function model. All the different models described above are implemented into one typical active system simulation. Afterwards, experiments will be performed to verify the simulations.

Design and Test of a Piezoelectric Inertial Mass Actuator for Active Vibration Control

Active control of vibrations can outperform passive systems in certain applications, e.g. when broadband damping is requested or when several orders of a periodic disturbance have to be cancelled. To generate dynamic forces for the active control system, inertial mass actuators are frequently used. Mainly, they comprise a force generating element driving a single-degree-of-freedom oscillator which is coupled to the structure to be controlled. Thus, those actuators can also be used for retrofitting of existing structures or for prototyping purposes. In this paper, a design for an inertial mass actuator utilizing piezoceramic actuators is studied. Since those actuators integrate both stiffness and force generation into one element, this enables more compact and mechanically robust designs. With respect to the future integration into industrial applications, standard multilayer piezoelectric actuators are considered to decrease the costs of the system and allow for a high reliability. Usually, an inertial mass actuator should possess a low resonance frequency in order to enable operation over a broad frequency range. Since the stiffness of piezoelectric multilayer actuators is rather high, a lever mechanism is designed which transforms the stiffness of the piezoelectric element into the desired range. An analytical model of the inertial mass actuator is derived and parameter studies are performed to investigate the characteristics of the design. A prototype is set up and the main parameters like resonance and block force are experimentally validated. Finally, the integration of the actuator into an active vibration control system for a lightweight structure is described.

Novel Dielectric Stack Actuators for Dynamic Applications

In order to realize dielectric elastomer stack actuators suitable for dynamic applications a new actuator design with rigid, perforated electrodes is developed. The low surface resistance of the metal electrodes predestines this concept for dynamic applications where higher currents are present. Detailed numerical analyses are performed to show the potential of this approach, to study the complex material deformation and to optimize the aperture geometry. A multilayer stack actuator is then manufactured and characterized experimentally under various load conditions to gain suitable parameters for a parametrized model. It is subsequently used to attenuate vibrations of a truss structure. By careful adjusting the parameters it functions both as passive absober and as actuator. A comparison of experimental and simulation results proves the high quality of the simulation model. The work shows the great potential of the new design concept for future applications especially in the field of smart structures.Copyright

Dielectric elastomers for active vibration control applications

Dielectric elastomers (DE) have proved to have high potential for smart actuator applications in many laboratory setups and also in first commercially available components. Because of their large deformation capability and the inherent fast response to external stimulation they proffer themselves to applications in the field of active vibration control, especially for lightweight structures. These structures typically tend to vibrate with large amplitudes even at low excitation forces. Here, DE actuators seem to be ideal components for setting up control loops to suppress unwanted vibrations. Due to the underlying physical effect DE actuators are generally non-linear elements with an approximately quadratic relationship between in- and output. Consequently, they automatically produce higher-order frequencies. This can cause harmful effects for vibration control on structures with high modal density. Therefore, a linearization technique is required to minimize parasitic effects. This paper shows and quantifies the nonlinearity of a commercial DE actuator and demonstrates the negative effects it can have in technical applications. For this purpose, two linearization methods are developed. Subsequently, the actuator is used to implement active vibration control for two different mechanical systems. In the first case a concentrated mass is driven with the controlled actuator resulting in a tunable oscillator. In the second case a more complex mechanical structure with multiple resonances is used. Different control approaches are applied likewise and their impact on the whole system is demonstrated. Thus, the potential of DE actuators for vibration control applications is highlighted.

Design and modeling of dielectric elastomer actuators

One of the main technical challenges in the development of dielectric elastomer (DE) stack actuators is the design and realization of suitable electrodes. They must be compliant and be able to undergo large strains without adding too much stiffness. Metal electrodes are therefore normally out of question due to their high stiffness, though their electrical properties are excellent. In this work a new design approach is presented which comprises rigid metal electrodes. Its functionality is proven by means of numerical simulations and experimental tests. It allows the customized tailoring of transducer elements due to the designable electrode structure. A functional demonstrator is built and tested concerning its electrical, mechanical and electromechanical behavior. For this new actuator type a full electromechanical model is developed. It contains all transfer characterisitcs in a nonlinear description and accounts for various physical effects arising from the special actuator design. Due to its standardized interface configuration it can well be used in combination with existing models for mechanical structures and electrical amplifiers to completely model active systems. It is applicable for the realistic simulation necessary in the development of active solutions with EAP devices. A first longterm test with 108 load cycles was performed in order to show the durability of the actuator.

Test rig with active damping control for the simultaneous evaluation of vibration control and energy harvesting via piezoelectric transducers

Piezoelectric transducers can be used to harvest electrical energy from structural vibrations in order to power continuously operating condition monitoring systems local to where they operate. However, excessive vibrations can compromise the safe operation of mechanical systems. Therefore, absorbers are commonly used to control vibrations. With an integrated device, the mechanical energy that otherwise would be dissipated can be converted via piezoelectric transducers. Vibration absorbers are designed to have high damping factors. Hence, the integration of transducers would lead to a low energy conversion. Efficient energy harvesters usually have low damping capabilities; therefore, they are not effective for vibration suppression. Thus, the design of an integrated device needs to consider the two conflicting requirements on the damping. This study focuses on the development of a laboratory test rig with a host structure and a vibration absorber with tunable damping via an active relative velocity feedback. A voice coil actuator is used for this purpose. To overcome the passive damping effects of the back electromagnetic force a novel voltage feedback control is proposed, which has been validated both in simulation and experimentally. The aim of this study is to have a test rig ready for the introduction of piezo-transducers and available for future experimental evaluations of the damping effect on the effectiveness of vibration reduction and energy harvesting efficiency.

Optimization of a tuned vibration absorber in a multibody system by operational analysis

Mechanical vibration in a drive-train can affect the operation of the system and must be kept below structural thresholds. For this reason tuned vibration absorbers (TVA) are usually employed. They are optimally designed for a single degree of freedom system using the Den Hartog technique. On the other hand, vibrations can be used to produce electrical energy exploitable locally avoiding the issues to transfer it from stationary devices to rating parts. Thus, the design of an integrated device for energy harvesting and vibration reduction is proposed to be employed in the drive-train. By investigation of the dynamic torque in the system under real operation, the accuracy of a numerical model for the multibody is evaluated. In this study, this model is initially used for the definition of the TVA. An energetic procedure is applied in order to reduce the multibody in an equivalent single degree of freedom system for a particular natural mode. Hence, the design parameters of the absorber are obtained. Furthermore, the introduction of the TVA in the model is considered to evaluate the vibration reduction. Finally, an evaluation of the power generated by the piezo transducer and its feedback on the dynamic of the drive-train is performed.

Parametric Modeling of Main Excitation Sources on Board Vessels

Efficient analysis methods for predicting the vibro-acoustic behavior of vessels are important, especially during the early design stages. A prediction software tool can help to identify critical points in the ship design and to avoid costly rework subsequent to sea trials. Therefore, an approach based on numerical system-level simulation, to estimate noise, vibration and harshness performance during each step of the design process is proposed. This paper is focused on the development of numerical models of main excitation sources on board. Parametric excitation models for main engines and pumps as well as structural models for power trains and foundations are built up. The sources introduce excitation forces into the foundations or torsional moments into the power train. The overall simulation model is built up modular and contains structural submodels as well as excitation submodels. System-level simulations using Matlab/Simulink are performed and both stationary operational conditions and transient excitations, e.g. engine run-ups and misfiring, are simulated. The numerical results are presented and the accuracy of the model is evaluated by comparing numerical and experimental data.