Steffen Opitz

Modeling of carbon nanotube actuators: Part I -Modeling and Electrical properties

The outstanding electrical and mechanical properties of single carbon nanotubes (CNT) are the motivation for an intensive research in various fields of application. The actuation effect constitutes the foundation for any application as a multifunctional material and within the field of adaptronics. The effect is in the majority of cases investigated by a CNT configuration of stochastically aligned CNT, so-called bucky-paper, in an electrolytic environment. The article presents an analytical model for a detailed understanding and investigation of the actuation process. The complete description and parameterization of the model is documented in two parts. In the first part the model is developed, the test setup for the model validation and parameter identification is elucidated, and the electrical parameters are determined. In the second part the mechanical system and the actuating effect will be examined. Finally the applicability of the model will be examined.

Development of active twist rotors at the German Aerospace Center (DLR)

Helicopter main rotors are characterized by complex unsteady aerodynamic conditions, which are causing vibrations and noise in and around rotary wing aircrafts. The aerodynamic conditions also cause increased drag, which leads to higher fuel consumption. Even modern helicopters still suffer from these drawbacks. This is why there are many efforts to influence the flow conditions by passive and active means. Active means have the advantage to adapt to varying demands, which can change significantly. A first attempt is to perform a sinoidal variation of the pitch angle of the blades with a frequency that is an integer multiple of the rotor frequency, the so called higher harmonic frequencies. This can be done by additional displacements of the swash plate or by a variation of the pitch link length using actuated pitch links. Since both designs have several drawbacks, one of the most promising approaches is the realization of a secondary control via the deformation of individual blades. Such an actuation can be realized by blade flaps or by the integration of piezoelectric actuators in the blade itself, which causes the blade to twist. At the DLR such twist blades have been investigated intensively. A series of blades has been built using thin skin integrated actuators. This Paper gives an overview of all active twist blades with skin integrated actuators that have been designed and manufactured at the DLR so far. Different design philosophies have lead to different geometrical setups. A comparison of the blades power consumption for given control laws for noise and vibration reduction is also given. Finally, a unique testing technique for nondestructive measurement of mass distribution is discussed in this paper. are characterized by complex unsteady aerodynamic conditions, which are causing vibrations and noise in and around rotary wing aircrafts. The aerodynamic conditions also cause increased drag, which leads to higher fuel consumption. Even modern helicopters still suffer from these drawbacks. This is why there are many efforts to influence the flow conditions by passive and active means. Active means have the advantage to adapt to varying demands, which can change significantly. A first attempt is to perform a sinoidal variation of the pitch angle of the blades with a frequency that is an integer multiple of the rotor frequency, the so called higher harmonic frequencies. This can be done by additional displacements of the swash plate or by a variation of the pitch link length using actuated pitch links. Since both designs have several drawbacks, one of the most promising approaches is the realization of a secondary control via the deformation of individual blades. Such an actuation can be realized by blade flaps or by the integration of piezoelectric actuators in the blade itself, which causes the blade to twist. At the DLR such twist blades have been investigated intensively. A series of blades has been built using thin skin integrated actuators. This Paper gives an overview of all active twist blades with skin integrated actuators that have been designed and manufactured at the DLR so far. Different design philosophies have lead to different geometrical setups. A comparison of the blades power consumption for given control laws for noise and vibration reduction is also given. Finally, a unique testing technique for nondestructive measurement of mass distribution is discussed in this paper.

Modeling of Carbon Nanotube Actuators: Part II -Mechanical Properties, Electro Mechanical Coupling and Validation of the Model

Carbon nanotubes have the potential to become one of the actuating materials of the future. Since the discovery of the actuating effect a lot of experimental data was collected to describe the electro-mechanical coupling. With this paper a model for the system behavior is available to enhance the understanding of the effect. An analytical model is introduced in the first part of the article. The electrical parameters of the model are determined by impedance spectroscopy. Furthermore, the mechanical parameters and the electro-mechanical coupling have to be examined. The main subject of the second part of the article is to validate the model and to compare measured and simulated responses to several excitations.

Evolution of Active Twist Rotor Designs at DLR

Unsteady flow conditions in the rotor disk are causing intense vibrations and noise in rotary wing aircrafts. Even modern helicopters still suffer from these drawbacks. This decreases the on board comfort, causes material fatigue and reduces the public acceptance of helicopters. For this reason there are many affords towards an active manipulation of the flow conditions to decrease noise and vibrations. This could also lead to a reduction of fuel consumption. One of the most promising approaches to do so is a secondary control by deformation of the individual blades. Such an actuation can be realized by the integration of piezoelectric actuators in the blade itself, which causes the blade to twist. The German Aerospace Center (DLR) started to work with active twist rotor blades in the early 90. Ever since, the technology of actuation has evolved drastically, opening new ways to twist blades by means of actuators. This paper presents the evolution of blade design in the last years.

Experimental Investigation of an Active Twist Model Rotor Blade Under Centrifugal Loads

Individual Blade Control (IBC) for helicopter rotors promises to be a method to increase flight performance and to reduce vibration and noise. Quite a few concepts to realize IBC Systems have been proposed so far. Some of them have already been tested in wind tunnels or on real helicopters. A drawback of all systems that include discrete mechanical components like hinges, levers or gears is their vulnerability in a helicopter environment with high centrifugal loads and high vibration levels. That’s why the idea of using smart materials that are directly embedded in the rotor blade structure is very attractive for this application. Operating as solid state actuators they can generate a twist deformation of the rotor blade without any friction and wear. In the common DLR-ONERA project “Active Twist Blade” (ATB), DLR designed and build a 1:2.5 mach scaled BO105 model rotor blade incorporating state of the art Macro Fiber Composite (MFC) Actuators. The design of the blade was optimized using a finite element code as well as rotor dynamic simulations to predict the benefits with respect to vibrations, noise and performance. Based on these tools a blade was designed that meets all mass and stiffness constraints. The blade has been intensively tested within some bench- and centrifugal tests. The mechanical properties of the blade obtained within the bench tests showed a good correlation between measured and calculated values. The centrifugal test comprised a measurement of the active twist performance at the nominal rotation speed of 1,043 RPM at different excitation frequencies from 2 up to 6/rev. It was proven, that also under centrifugal loads the predicted twist amplitudes can be achieved.

ACTIVE TWIST ROTOR FOR WIND TUNNEL INVESTIGATIONS

This paper describes the design and manufacturing of active twist rotor blades for the use as secundary control of a helicopter main rotor. These blades have been developed at the DLR in Braunschweig, Germany over the last years. Special features of the blade are the capability to twist individual sections of the blade. It is planned to equip the blade so that the following test methods can be applied: strain gauges and SPR for deformations, optical measurements as a reference of the blade tip twist and pressure sensors in certain sections. I order to reduce the complexity of the blade/rotor shaft interface, blade integrated amplifiers for the strain gauge bridges have been developed and teste. Right now a set of six instrumented blades is being built, in order to test a four bladed rotor in a wind tunnel in 2012.Copyright

Effects of Piezoceramic Actuator in Quasistatic Use

In the field of smart structures, piezoceramic actuators are wildly used for vibration reduction and acoustic manipulation of structures. Those applications typically run at frequencies between 10 Hz and 10k Hz. Prominent examples are the piezoceramic actuators implemented in helicopter rotor blades to twist them dynamically for higher harmonic control (HHC) or individual blade control (IBC). Once the actuators are implemented it would be a great benefit to also use them to statically change the blade twist (higher twist for take-off and landing - for higher lift; lower twist for high speed forward flight - for reduced drag). Staying with this example it can be found that sensing the twist displacement is not an easy task at all (see [1, 2]), so it would be most desirable, to use open loop control. In order to do that, the transfer function has to be known accurately. Unfortunately measurements show that the amplitudes for such very low frequencies behavior behave strongly non linear. This paper presents experimental results investigating the influence of the frequency on the amplitude - especially going for frequencies in the lower mHz region. A variety of piezoceramic actuators has been investigated: from stacks to patch type, d33 as well as d31 effect actuators. A second focus of this paper is the reaction of piezoceramic actuators on the application of a constant DC voltage. The drift that occurs has to be taken into consideration. A third focus of this paper is the dependency of a displacement output of such an actuator at a constant applied DC voltage on the voltages that the actuator had seen before. This topic is of special mportance for aerodynamically effective surfaces that are driven by piezoceramic actuators and should be analyzed (generation of polars) in static conditions.

Experimental investigation of an active twist model rotor blade with a low voltage actuation system

Smart materials that are directly embedded in the rotor blade structure are an attractive concept for active blade control. A promising approach is the use of anisotropic piezoelectric strain actuators embedded in the rotor blade skin. Especially in Europe and the US this concept has been intensively investigated over the past years. A major drawback of all configurations studied so far is the high operation voltage of up to 2,000V of state of the art piezoelectric actuators. Within the Green Rotorcraft Project of the European Joint Technology Initiative Clean Sky, a new approach with a low voltage piezoelectric actuation system is investigated to demonstrate the feasibility of this technology. A first major step in this direction was completed by conducting a centrifugal test with a model rotor blade. The objective of the centrifugal test was to demonstrate the performance of the actuation system and the structural concept under centrifugal loads by showing that the expected twist deformation can be achieved at the nominal rotation speed and different actuation frequencies. It was demonstrated that the new actuation system is capable of operating under representative centrifugal loads. In comparison to state-of-the-art actuators (operation voltage 500V to +1500V) the new actuation system (operation voltage -20V to 120V) exhibits higher active twist performance per active area.

Carbon Nanotube Actuation

The outstanding electrical and mechanical properties of single carbon nanotubes (CNT) are the motivation for an intensive research in various fields of application. The actuation effect constitutes the foundation for any application as a multifunctional material and within the field of adaptronics. The effect is in the majority of cases investigated by a CNT configuration of stochastically aligned CNT, so-called bucky-paper, in an electrolytic environment. The chapter presents an analytical model for a detailed understanding and investigation of the actuation process. The complete description and parameterization of the model is documented. Initial results from experiments with aligned CNT structures and the application of solid electrolytes are presented.