Induced strain actuator

Fig. 2.1. The bio-inspired approach to adaptronic structures a active materials, b induced-strain actuators, c integrated active sensors d multifunctional composites, e microcontrollers...

The adaptronic approach would be one that would borrow directly from the biological world. Materials that behave more or less like muscles can be used in adaptronic structures and are called induced strain actuators. When energy is applied to the actuators, they attempt to expand/contract and work against any load that is applied to them. The actuators are typically bonded to the surface of a structure, or embedded within the material. This means that the artificial muscles must now work against the inherent structural impedance of the component, just as hmnan muscles are parallel to the skeletal structure or bone. However, whereas the arm has discrete joints about which rotation occurs, the adaptronic structure may be a continuum, thereby necessitating a distributed actuation system. For example, the tip motion of a beam will not occur by rotating the beam about a joint but by inducing its deformation by means of induced strain actuators placed on the beam. 

Induced-strain Actuation for Aeroelastic and Vibration Control... 

Fig. 2.3. Mitigation of the input/output requirements and induced-strain actuation capabilities...

Induced-strain Actuation of Fixed-Wing Aircraft. The feasibility of using active piezoelectric control to alleviate vertical tail buffeting was inves-... 

P.C. Chen, I. Chopra, A feasibihty study to build a smart rotor Induced-strain actuation of airfoil twisting using piezoceramic crystals, in Smart Structures and Intelligent Systems (Proceedings), vol. 1917 (SPIE, BeUingham, 1993), pp. 238-254... 

P.C. Chen, I. Chopra, Hover testing of smart rotor with induced-strain actuation of blade twist. AIAA J. 35(1), 6 16 (1997)... 

E.F. Crawley, K.B. Lazarus, Induced strain actuation of isotropic and anisotropic plates. AIAA J. 29(6), 944-951 (1991)... 

Crytal chemitry. The effect of solid solution on the transition behavior of perovskite (ABX3) structures has been intensively scrutinized for more than 50 years. These materials have merited continuous attention because of their enormous technological versatility. As multilayer capacitors, piezoelectric transducers, and positive temperature coefficient (PTC) thermistors they generate a market of over 3 billion every year (Newnham 1989, 1997). In addition to ease of fabrication, these compounds exhibit a number of attributes required of ideal actuators (1) They display very large field-induced strains (2) They offer quick response times and (3) Their strain-field hysteresis can be chemically controlled to be very large or negligibly small, depending on the application. Details of their technical applications can be found in Jaffe et al. (1971) and Cross (1993). 

Actuators are among the latest applications for thin sheets of piezoelectric materials. Actuators are small displacement elements for the precision positioning of optical and other motors and machinery. All of these piezoelectric devices operate on the principle of electric field-induced strain. 

The magnitude of the induced strain x by an external electric field E is represented by tbis figure of merit (an important figure of merit for actuator applications) ... 

Actuator materials are classified into three categories piezoelectric, elec-trostrictive and phase-change materials. Modified lead zirconate titanate [PZT, Pb(Zr,Ti)03l ceramics are currently the leading materials for piezoelectric applications. The PLZT [(Pb,La)(Zr,Ti)03l 7/62/38 compound is one such composition [31], The strain curve is shown in Figure 4.1.19a (left). When the applied field is small, the induced strain x is nearly proportional to the field E (x = dE, where d is called the piezoelectric constant). As the field becomes larger (i.e. greater than about IkV/cm), however, the strain curve deviates from this linear trend and significant hysteresis is exhibited due to polarization reorientation. This sometimes limits the use of such materials for actuator applications that require non-hysteretic response. 

The second category of actuators is based on electrostriction as exhibited by PMN [Pb(Mg 3Nb2/3)03] based ceramics. Although it is a second-order phenomenon of electromechanical coupling (x = ME, where M is called the electrostrictive coefficient), the induced strain can be extraordinarily large (more than 0.1%) [33], An attractive feature of these materials is the near absence of hysteresis (Fig. 4.1.19b). The superiority of PMN to PZT was demonstrated in a scanning tunneling microscope (STM) [34]. The STM probe was... 

Solid-state actuation signifies the use of the induced-strain effect present in active materials to achieve actuation without any moving parts, i.e., in a solid-state manner. Already, solid-state actuation has found niche application in the aerospace industry. The aero-servo-elastic control of vibrations and flutter with solid-state actuated flaps, tabs, vanes, etc. for helicopter rotor blades and aircraft wings is currently being experimented on. 

For electromechanical and actuation applications, the following parameters are critical maximum electric-induced strain response (Xm), block force (Fb) and elastic energy density, including the volumetric elastic energy density (Wy) and gravimetric elastic... 

Ferroelectric polymer materials like PVDF or its derivatives are mentioned, since they behave as ferroelectric materials (see Fig. 2.2) - They have crystallinity and the crystals show polymorphism by controlling the preparation method. Much detailed work has been carried out on piezoelectric and/or pyroelectric properties, together with their characteristics as electroactive actuators. These materials have long been mentioned as typical electroactive polymers. Through these materials, it is considered that the strain induced in the polymer materials is not large. The electrostrictive coefficient is known to be small for polymers. These are non-ionic polymers and the induced strain originates from the reorientation or the deformation of polarized crystallites in the solid materials. 

When d is substituted as outlined above and the compliance coefficients associated with the induced strain coefficients d are used, then the formulation turns into the upper part of the constitutive equations given on the right-hand side of Eqs. (4.10a). In addition, the actual thermal coefficients may be taken into consideration by the vector a. Thus, supplying specialized finite elements also capable of capturing anisotropic thermal effects with the constitutive coefficients and electric field strength of the electromechanically coupled problem, as given by Eq. (4.16), is a convenient procedure for the case of static actuation. 

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