A High-Performance Modular SMA Actuator With Fast Heating and Active Cooling for Medical Robotics

Abstract

Shape memory alloy (SMA) has been adopted in medical robotics due to its high power density, compactness, and medical imager compatibility. However, its use in clinical applications remains limited due to its low actuation bandwidth and the lack of proper system integration approaches, leading to poor control accuracy and repeatability. In this article, we report the design, modeling, and characterization of a high-performance SMA actuator for real-time medical robotics application. It uses nichrome wire for fast heating and forced air convection for active cooling to improve the actuation bandwidth, with the SMA spring predisplacements tunable with a simple mechanism. All structures are packaged in a compact module to facilitate practical integration in a robotic system, and enable consistent performance and convenient use. We have also developed an intuitive constitutive model and heat transfer model of the actuator to facilitate selection of its critical parameters, such as SMA predisplacement, heating current, and air pressure, based on the performance requirements. During characterization, the actuator achieved a stroke of 147.2 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mathrm{^{\circ }}$</tex-math></inline-formula> , stall torque of 172.5 Nmm and actuation bandwidth of 0.43 Hz at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pm$</tex-math></inline-formula> 5 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mathrm{^{\circ }}$</tex-math></inline-formula> rotation amplitude. It also demonstrated high control accuracy (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; $</tex-math></inline-formula> 1 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mathrm{^{\circ }}$</tex-math></inline-formula> ) and high repeatability (standard deviation <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$&lt; $</tex-math></inline-formula> 0.16 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mathrm{^{\circ }}$</tex-math></inline-formula> ) under closed-loop control. Torque–speed relationship was experimentally determined to show its performance under external loading. Finally, the actuator was used to build a steerable endoscope manipulator as a demonstration for real-time application in surgical robotics.