Active-Cooling-in-the-Loop Controller Design and Implementation for an SMA-Driven Soft Robotic Tentacle

Abstract

As a classical type of smart materials, shape memory alloys (SMAs) are of high energy density, light weight, and low actuating voltage, and therefore are of great potential to be used as actuators for robots. Major challenges in controlling an SMA-driven soft robot are the limited bandwidth and in cases with external loads. Active cooling has been demonstrated to dramatically increase its bandwidth, but external load may cause severe inaccuracy in the system modeling. Controllers that do not rely on accurate modeling of the SMA-driven soft robot is essential. In this article, we designed an elastomeric soft robotic tentacle actuated by three pieces of SMA springs with both active heating (Joule heating) and active cooling (compressed air). We proposed a multi-input-multi-output controller that directly uses the heating and cooling states of the three SMAs to control the tentacle's bending posture in three-dimensional (3-D) space. The successful implementation of the controller is attributed to a novel dual-channel control algorithm that integrates the bending motion control and swing motion control, and a state-machine controller for coordinating the three SMAs' actuations to achieve robust swing motion control. The system with such hardware and control algorithm was capable of performing bending motions with maximum actuating speed <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ &gt; $</tex-math></inline-formula> 90 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ /$</tex-math></inline-formula> s, deactuating speed <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ &gt; $</tex-math></inline-formula> 25 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ /$</tex-math></inline-formula> s, closed-loop motions with rapidity (6–41 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ /$</tex-math></inline-formula> s for heating, 4–19 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ /$</tex-math></inline-formula> s for cooling), accuracy (steady-state error <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ &lt; $</tex-math></inline-formula> 0.1 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula> for no load, 0.13 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\%$</tex-math></inline-formula> of the full range; steady-state error <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ &lt; $</tex-math></inline-formula> 1.2 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula> with load of 1 bodyweight, 1.6 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\%$</tex-math></inline-formula> of the full range), and load-bearing capability (dynamic load: 1 bodyweight, static load: 8.8 bodyweights). Besides, the tentacle achieved efficient motion tracking by coordinating the bending and swing motion through the controller, for both predefined trajectories and random trajectories. We demonstrated a remotely controlled 360 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula> image scanning of a room using our proposed robotic tentacle equipped with a camera at its top end to intuitively show its performances. We believe this work will advance the design and control of SMA-driven soft continuum robots for potential uses in surgery and explorations of unknown areas.