Design and preliminary testing of a handheld antagonistic SMA actuator for cancellation of human tremor

Essential Tremor is a debilitating disorder that in the US alone is estimated to affect up to ten million people. Unfortunately current treatments (i.e. drug therapy and surgical procedures), are limited in effectiveness and often pose a risk of adverse side-effects. In response to this problem, this paper describes an active cancellation device based on a hand-held Shape Memory Alloy (SMA) actuated stabilization platform. The assistive device is designed to hold and stabilize various objects (e.g. eating utensils, tools, pointing implements, etc.) by sensing the user's tremor and moving the object in an opposite direction using SMA actuators configured in biologically inspired antagonistic pairs. To aid in the design, performance prediction and control of the device, a device model is described that accounts for the device kinematics, SMA thermo-mechanics, and the heat transfer resulting from electrical heating and convective cooling. The system of differential equations in this device model coupled with the controller gain can be utilized to design the operation given a frequency range and power requirement. To demonstrate this, a prototype was built and experimentally tested under external disturbances in the range of 1-5 Hz, resulting in amplitude reduction of up to 80%. The extent of cancellation measured for both single-frequencies and actual human tremor disturbances demonstrate the promise of this approach as a broadly used assistive device for the multitudes afflicted by tremor.