S. Viscuso, S. Pittaccio, CNR IENI Institute for Energetics and Interphases - Italian National Research Council, Lecco, Italy; F. Zappasodi, Università "G. D'Annunzio", Chieti Scalo, Italy; F. Tecchio, CNR-ISTC Institute of Cognitive Sciences and Technologies, Italian National Research Council, Roma, Italy
This work presents a shape-memory rotary actuator, which was designed to passively mobilize body joints. Two plastic shells contain aluminium plates (A and B), which are linked by a stainless steel shaft. This shaft is encastred at the centre of A and connects to the centre of B through a bearing, leaving a clearance of 15mm between the two plates. Six further stainless steel pegs are encastred on plate A, symmetrically with respect to the rotational axis. Each peg holds three pulleys with bearings, on which the NiTi wire is wound, ensuring sufficient lever arm for actuation. The wire describes counter-rotating coils along a spiralling sequence of pulleys. Both ends of the wire are connected to plate A, while the middle is attached to plate B. In this scheme the wire once turns clockwise, than passes through the rotating fixture at B and coils again anticlockwise: note that the magnetic field induced by the injected current is thus automatically counterbalanced. To guide the wires and avoid any short-circuits, the pulleys were designed with two grooves, which can hold wires up to 0.3mm in diameter. Using different wires can help tuning the torque output to the current application. The wire is kept taut by a rotational steel spring placed around the central axis and connected to both plates. The actuator was equipped with a 0.25mm-diameter wire and tested by attaching a series of weights (4N, 5.5N, 6N, 7N, 7.5N, 8N) at a fixed distance of 12cm from the centre. Heating power was provided by electric tension at 30V-dc for 7s. The measured stroke ranged between 31° and 36°. Due to its unique characteristics, this actuator can be applied in the biomedical field and even in neuroscience applications requiring tight electromagnetic noise constraints.
Summary: This work presents a shape-memory rotary actuator, which was designed to passively mobilize body joints. Two plastic shells contain aluminium plates (A and B), which are linked by a stainless steel shaft. This shaft is encastred at the centre of A and connects to the centre of B through a bearing, leaving a clearance of 15mm between the two plates. Six further stainless steel pegs are encastred on plate A, symmetrically with respect to the rotational axis. Each peg holds three pulleys with bearings, on which the NiTi wire is wound, ensuring sufficient lever arm for actuation. The wire describes counter-rotating coils along a spiralling sequence of pulleys. Both ends of the wire are connected to plate A, while the middle is attached to plate B. In this scheme the wire once turns clockwise, than passes through the rotating fixture at B and coils again anticlockwise: note that the magnetic field induced by the injected current is thus automatically counterbalanced. To guide the wires and avoid any short-circuits, the pulleys were designed with two grooves, which can hold wires up to 0.3mm in diameter. Using different wires can help tuning the torque output to the current application. The wire is kept taut by a rotational steel spring placed around the central axis and connected to both plates. The actuator was equipped with a 0.25mm-diameter wire and tested by attaching a series of weights (4N, 5.5N, 6N, 7N, 7.5N, 8N) at a fixed distance of 12cm from the centre. Heating power was provided by electric tension at 30V-dc for 7s. The measured stroke ranged between 31° and 36°. Due to its unique characteristics, this actuator can be applied in the biomedical field and even in neuroscience applications requiring tight electromagnetic noise constraints.
