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Monday, May 17, 2010 - 1:15 PM

Elastic Compensation of Linear Shape Memory Actuators for Greater Stroke and Output Force

G. Scirč Mammano, University of Modena and Reggio Emilia, Reggio Emilia, Italy; E. Dragoni, Universitą degli Studi di Modena e Reggio Emilia, Reggio Emilia, Italy

Although actuators based on Shape Memory Alloys (SMAs) can generate considerable output forces, they typically exhibit small displacements. The total displacement (stroke) is chiefly limited by the maximum strain that the shape memory material can withstand for the desired service life. A further loss of stroke comes from the contrasting action of the backup element (weight, classical spring, second SMA spring), which prevents the active material from fully recovering the memorized shape.

The usual force-displacement curve of shape memory actuators is linear with peak force at one end of the stroke and zero force at the other end.This characteristic does not fit well with the fact that the external load is often constant over the stroke. As a rule, for given output force the overall displacement of the actuator increases as the stiffness of the backup element decreases. Thus, a constant backup force (dead weight) is more efficient than a conventional spring. This trend suggests that a compensation system with negative slope would promote even higher displacements. Alternatively, given the stroke, an actuator with negative compensation would provide greater output forces than traditional systems.

This paper examines the improvements produced by the elastic compensation concept on stroke and output force of SMA actuators.

The presentation is divided in two parts. The first part describes a step-by-step procedure for finding the elastic characteristics of the compensator starting from the prescribed values of output force and stroke of the actuator. The second part presents the kinematic design of specific compensating mechanisms based on two families of bi-stable devices. These solutions are widely used on mechanical manipulators (for weight compensation) and as pipeline supports (to provide nearly-constant reaction forces).

It is shown that with simple compensated architectures, improvements of two times of stroke and output force are easily achieved over non-compensated alternatives.


Summary: The actuators based on shape memory alloys represent an interesting class of actuating devices due to the high power to mass ratio, the tolerance to harsh environments and the simple construction. The main limitations of this technology are the reduced stroke and the unfavourable force output characterized by a linear dependence between force and displacement. This paper aims at reducing these disadvantages by introducing an elastic compensation system that improves the stroke and balances the output . In the first part, the paper presents a new methodology for the integrated design of the SMA elements and the compensation element. In the second part, two special mechanisms embodying the compensation system are presented together with their kinematic design.