Optimum Mechanical Design of Binary Actuators Based on Shape Memory Alloys

Abstract

Many SMAs applications have been studied in recent years. Kuribayashi [5] proposes a rotary joint based on a bending SMA actuator, while, in [6] design and applications of SMA Smart Actuation and Sensing Systems -Recent Advances and Future Challenges 4 actuators are presented. Microrobots can be developed using SMA as shown in [7] where there is a basic method to design the SMA spring based on a thermo-electromechanical approach. Reynaerts et al. [8] present design considerations concerning the choice of the active element to evaluate SMA actuator efficiency. Lu et al. [9] design a high strain shape memory actuator taking into account pseudoplasticity and compared its performance with traditional actuators. Due to SMA high non linearity, design curves relying on experiments are proposed in [10] to assess the SMA actuator geometry. A comprehensive review of applications of SMAs in the field of mechanical actuation has recently been published in [11]. Jansen et al [12] develop a linear actuator used as a drive module in an angular positioning mini-actuator. This architecture allows both large force and long strokes to be obtained. Strittmatter et al. [13] propose a SMA actuator for the activation of a hydraulic valve, biased by a conventional spring. Bellini et al. [14] propose a linear SMA actuator able to vary the air inflow for internal combustion engines, improving gas combustion and leading to higher efficiency. Haga et al. [15] propos a mini-actuator to be used in Braille displays. Elwaleed et al.[16] develop a SMA beam actuator able to amplify the SMA actuator strain using elastically instable beams. Among the proposed actuator there is lack of simple design instruments to provide basic information to the designer, either due to specific constraints of application or due to the high complexity of the thermomechanical material models used. In order to answer for an analytical design methodology, the author described in several technical publications a set of equations useful for linear [18] and rotary application [19]. The authors developed the design equations both for SMA actuators under a general system of external forces [20] and produced the design formulae to increase the output stroke thanks to negative stiffness compensators [21]. Moreover two peculiar systems were designed and developed: a telescopic actuator [22] and a wire on drum system [23]. The present work reviews and improves the design rules developed by the authors and set them in a coherent formal analytical framework. Design examples are provided to illustrate the step-by-step application of the design optimization procedures for realistic case studies. Challenges and issues in SMA actuators designThe three main challenges in SMA actuator design are: obtaining a simple and reliable material model, increasing the stroke of the actuator and finding design equations to guide the engineer in dimensioning the actuator. To design SMA actuators, a material model must describe the mechanical behaviour of the alloy in two temperature ranges: b...

Smart materials: Properties, design and mechatronic applications

“…[111][112][113] In the case of two antagonistic SMA springs ( Figure 10(c)), the primary and the backup springs are activated alternatively in order to maximize the stroke S. In general, for a given primary SMA element and external force, the highest stroke is achieved by a SMA backup spring (Figure 10(c)), followed by the constant force backup (Figure 10(a)) and the elastic backup spring (Figure 10(b)). [114][115][116] In recent papers Scire`Mammano and Dragoni have developed the theory for a novel backup system based on the concept of elastic compensation, 117,118 which can be applied to augment any of the classical backup systems in Figure 10. The compensation unit works like a spring with negative stiffness and allows greater strokes to be achieved with respect to all classical arrangements.…”

Experimental characterization and modelling validation of shape memory alloy Negator springs

“…Applying directly, Votta's model would be correct only for initial region of the austenitic regime. In order to model the martensitic behaviour of the SMA, a pseudoplastic law is considered, according to previous authors' work (Scire`Mammano and Dragoni, 2011a;Spaggiari et al 2012a).…”

“…The second mechanical asset is that output force is constant, the best condition for an actuator. Unless elastic compensation system is considered (Spaggiari et al, 2012a), the typical mechanical characteristic of an SMA actuator is linear regardless of the backup element. The linear force-stroke characteristic often leads to an oversizing of the actuator in order to guarantee the desired force over the entire travel.…”

Design of a 2 DOFs Mini Hollow Joint Actuated with SMA Wires

“…A traditional DC or a piezoelectric motor [ 12 , 13 , 14 , 15 , 16 , 17 , 18 ], would not be the ideal if the design involves embedding each motor within the robot because of weight and size issues. In contrast, SMA wires, widely used to design joints with multiple DOFs provide a better solution [ 19 ]. They can be used in various configurations, including: (i) antagonistic [ 20 , 21 , 22 ], (ii) with elastic joint [ 23 ] and (iii) mixed antagonistic-elastic joint [ 24 , 25 ].…”