Compliant mechanisms with selective compliance In numerous applications, such as airfoil or rotor blade shape adaptation, a smooth change of the structure’s geometry is necessary in order to operate efficiently. Even if the loads are not exactly predictable, a defined shape is required. In this context, compliant mechanisms with selective compliance are being developed at Empa.
In contrast to conventional mechanisms consisting of rigid members and classical hinges, compliant mechanisms exploit structural flexibility to produce controllable large deformations. They present several advantages, ranging from the absence of wear and backlash, lower noise level, clean operation, easier maintenance, and reduced manufacturing effort. However, the kinematic behavior of compliant mechanisms is mostly load-dependent, i.e. a large number of possible deformation patterns can be obtained by changing the load acting on the system. This does not happen in the case of conventional mechanisms, which are only provided with a small number of degrees of freedom (called mobility of the mechanism). Compliant systems with selective compliance allow the realization of freely configurable mechanisms with a defined mobility. The realization of systems with selective kinematics and single mobility is achieved by means of a mathematical procedure based on a modal representation of the structure’s response to external loads. This procedure essentially manipulates the system’s stiffness matrix by enforcing a predetermined eigenshape, and minimizes the related eigenvalue (a measure of the selective flexibility of the system in terms of strain energy) with respect to the other eigenvalues of the system. This new class of mechanisms is particularly promising for applications requiring smooth deformation patterns, which are not conceivable with conventional means or only at the price of high complexity and structural weight. Compliant mechanisms with selective compliance are therefore a key technology for so-called morphing wings, i.e. airfoil structures with a virtually unrestricted geometrical adaptability. Due to the interaction of the main constraints of high strength, high geometrical adaptability and low weight, morphing structures constitute one of the most challenging tasks of applied mechanics.
Empa Activities 09/10 Civil and Mechanical Engineering
The described technology was exemplarily applied to so-called belt-rib airfoil structures. Belt-rib structures are fully compliant, lightweight, airfoil structures with a controllably adaptable aerodynamic profile. The core of this concept is the belt rib, a planar mechanism with selective compliance optimized for one single family of profile shapes (mobility equal to one). The choice of a single profile within the family is realized by controlling the strain energy transferred to the system. A prototypical implementation of the above mentioned modal design procedure is shown in Figure 1. As an illustration of the load-independent kinematics, two different loads were applied to the airfoil profile. In both cases the deformed structure fits with the desired shape (Fig. 2).
Alexander Hasse, Flavio Campanile
Fig.1: Belt-rib prototype.
Fig. 2: Desired (red) and deformed shapes under different loads.
Support: Gebert Rüf Stiftung Links: www.empa.ch/abt119 > Smart Structures
www.kompliant.ch
Contact: alexander.hasse@empa.ch Reference: A. Hasse, F. Campanile, Smart Mater. Struct. 18 115016, 10 pp (2009)
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