*Article* **Flexible Fingers Based on Shape Memory Alloy Actuated Modules** †

#### **Daniela Ma**ffi**odo \* and Terenziano Raparelli**

Department of Mechanical and Aerospace Engineering, Politecnico di Torino, 10129 Torino, Italy; terenziano.raparelli@polito.it


Received: 9 May 2019; Accepted: 7 June 2019; Published: 10 June 2019

**Abstract:** To meet the needs of present-day robotics, a family of gripping flexible fingers has been designed. Each of them consists of a number of independent and flexible modules that can be assembled in different configurations. Each module consists of a body with a flexible central rod and three longitudinally positioned shape memory alloy (SMA) wires. When heated by the Joule effect, one to two SMA wires shorten, allowing the module to bend. The return to undeformed conditions is achieved in calm air and is guaranteed by the elastic bias force exerted by the central rod. This article presents the basic concept of the module and a simple mathematical model for the design of the device. Experimental tests were carried out on three prototypes with bodies made of different materials. The results of these tests confirm the need to reduce the antagonistic action of the inactive SMA wires and led to the realization of a fourth prototype equipped with an additional SMA wire-driven locking/unlocking device for these wires. The preliminary results of this last prototype are encouraging.

**Keywords:** shape memory alloy; SMA wires; flexible actuator; modular actuator; mathematical model; experimental test
