Bio-inspired integrated actuation and variable stiffness for compliant mechanisms

Abstract

Due to advantages, such as a low mechanical complexity, low weight, and the absence of friction of wear, compliant kinetic systems are increasingly used, including for large-scale applications like facade shading. To exploit the advantages also for the actuation, bio-inspired joint-free actuators were developed within two case studies. Both actuation principles proved their potential to actuate 2-dimenional compliant devices within physical prototypes. Additionally, adaptive stiffness concepts were developed to potentially increase the load bearing capability temporarily. Following a biomimetic top-down approach, the leaf folding of the model plant Sesleria nitida caused by turgor variations within large bulliform cells was investigated using a FEA. The turgor pressure opens the leaf against a present pre-stress. Turgor and volume variation within the bulliform cells that result from fluctuations in water availability generate forces high enough to fold and unfold the leaf. This pressurize-based actuation principle is abstracted to a technical cellular structure constructed from GFRP (glass fibre-reinforced plastic) cells with compliant hinges. An increase in inner cell pressure causes a reconfiguration of the cell and an overall bending motion of the actuator. At the same time, thin-walled plant tissues show a strong turgor dependent stiffness. By adding a second, counteracting cell row that decouples deformation from absolute pressure, this can be realized also in the technical actuator. The bending motion is now determined by the pressure ratio, and the stiffness by the pressure magnitude. Within physical and numerical experiments, the stiffness of a cellular actuator increases by a factor of 2.5 at a pressure increase of 1 bar. Within the second case study, a pneumatic actuation that is fully integrated into a GFRP laminate was developed. The wing vein ultrastructure of Graphosoma lineatum italicum inspired the laminate built-up of the GFRP with an integrated pneumatic pouch. By surrounding the pouch with an elastomeric layer, analogous to the resilin bearing endocuticle within the biological model, a delamination of the laminate layers is prohibited. The approach allows a simple fabrication, and slender, homogenous appearance. Upon an internal pressure increase, the eccentric placement of the pneumatic pouch and the greater compliance of the thinner layer results in a rotation into that direction. This way a folding motion is realized by a pouch placed in a hinge zone of greater compliance. A quasi-uniform bending is created by placing a segmented large-surface pouch integrated in a plate of distributed compliance. The adaptive stiffness is added by an antagonistic actuator set-up inspired by opposing muscles used to control and stiffen skeletal joints. For a GFRP plate an increase in stiffness of 60% was achieved at 1.8 bar internal pressure.