A Methodology Towards Comprehensive Evaluation of Shape Memory Alloy Actuators for Prosthetic Finger Design

Presently, DC motors are the actuator of choice within intelligent upper limb prostheses. However, the weight and dimensions associated with suitable DC motors are not always compatible with the geometric restrictions of a prosthetic hand; reducing available degrees of freedom and ultimately rendering the prosthesis uncomfortable for the end-user. As a result, the search is on-going to find a more appropriate actuation solution that is lightweight, noiseless, strong and cheap. Shape memory alloy (SMA) actuators offer the potential to meet these requirements. To date, no viable upper limb prosthesis using SMA actuators has been developed. The primary reasons lie in low force generation as a result of unsuitable actuator designs, and significant difficulties in control owing to the highly nonlinear response of SMAs when subjected to joule heating. This work presents a novel and comprehensive methodology to facilitate evaluation of SMA bundle actuators for prosthetic finger design. SMA bundle actuators feature multiple SMA wires in parallel. This allows for increased force generation without compromising on dynamic performance. The SMA bundle actuator is tasked with reproducing the typical forces and contractions associated with the human finger in a prosthetic finger design, whilst maintaining a high degree of energy efficiency. A novel approach to SMA control is employed, whereby an adaptive controller is developed and tuned using the underlying thermo-mechanical principles of operation of SMA wires. A mathematical simulation of the kinematics and dynamics of motion provides a platform for designing, optimizing and evaluating suitable SMA bundle actuators offline. This significantly reduces the time and cost involved in implementing an appropriate actuation solution. Experimental results show iii that the performance of SMA bundle actuators is favourable for prosthesis applications. Phalangeal tip forces are shown to improve significantly through bundling of SMA wire actuators, while dynamic performance is maintained owing to the design and implementation of the selected control strategy. The work is intended to serve as a roadmap for fellow researchers seeking to design, implement and control SMA bundle actuators in a prosthesis design. Furthermore, the methodology can also be adopted to serve as a guide in the evaluation of other non-conventional actuation technologies in alternative applications

Development of a SMA-fishing-line-McKibben bending actuator

High power-to-weight ratio soft artificial muscles are of overarching importance to enable inherently safer solutions to human-robot interactions. Traditional air driven soft McKibben artificial muscles are linear actuators. It is impossible for them to realize bending motions through a single McKibben muscle. Over two McKibben muscles should normally be used to achieve bending or rotational motions, leading to heavier and larger systems. In addition, air driven McKibben muscles are highly nonlinear in nature, making them difficult to be controlled precisely. A SMA(shape memory alloy)–fishing–line–McKibben (SFLM) bending actuator has been developed. This novel artificial actuator, made of a SMA-fishing-line muscle and a McKibben muscle, was able to produce the maximum output force of 3.0 N and the maximum bending angle (the rotation of the end face) of 61°. This may promote the application of individual McKibben muscles or SMA-fishing-line muscles alone. An output force control method for SFLM is proposed, and based on MATLAB/Simulink software the experiment platform is set up, the effectiveness of control system is verified through output force experiments. A three-fingered SFLM gripper driven by three SFLMs has been designed for a case study, which the maximum carrying capacity is 650.4 ± 0.2 g

Remote-controlled ambidextrous robot hand actuated by pneumatic muscles: from feasibility study to design and control algorithms

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University LondonThis thesis relates to the development of the Ambidextrous Robot Hand engineered in Brunel University. Assigned to a robotic hand, the ambidextrous feature means that two different behaviours are accessible from a single robot hand, because of its fingers architecture which permits them to bend in both ways. On one hand, the robotic device can therefore behave as a right hand whereas, on another hand, it can behave as a left hand. The main contribution of this project is its ambidextrous feature, totally unique in robotics area. Moreover, the Ambidextrous Robot Hand is actuated by pneumatic artificial muscles (PAMs), which are not commonly used to drive robot hands. The type of the actuators consequently adds more originality to the project. The primary challenge is to reach an ambidextrous behaviour using PAMs designed to actuate non-ambidextrous robot hands. Thus, a feasibility study is carried out for this purpose. Investigating a number of mechanical possibilities, an ambidextrous design is reached with features almost identical for its right and left sides. A testbench is thereafter designed to investigate this possibility even further to design ambidextrous fingers using 3D printing and an asymmetrical tendons routing engineered to reduce the number of actuators. The Ambidextrous Robot Hand is connected to a remote control interface accessible from its website, which provides video streaming as feedback, to be eventually used as an online rehabilitation device. The secondary main challenge is to implement control algorithms on a robot hand with a range twice larger than others, with an asymmetrical tendons routing and actuated by nonlinear actuators. A number of control algorithms are therefore investigated to interact with the angular displacement of the fingers and the grasping abilities of the hand. Several solutions are found out, notably the implementations of a phasing plane switch control and a sliding-mode control, both specific to the architecture of the Ambidextrous Robot Hand. The implementation of these two algorithms on a robotic hand actuated by PAMs is almost as innovative as the ambidextrous design of the mechanical structure itself

Hoist-Based Shape Memory Alloy Actuator with Multiple Wires for High-Displacement Applications

Shape memory alloys (SMAs) are smart materials that change their crystalline structures when subjected to heat or tension, resulting in a macroscopic deformation. When applied to actuators, SMAs present a remarkable load–weight ratio and flexibility, making them suitable for diverse applications. However, challenges such as their energy consumption, nonlinear control, and low displacement must be considered. This paper presents a new strategy for improving the total displacement while adding neither supplementary SMA wires nor complex external devices. In addition, a novel control strategy is proposed to improve the nonlinearity of SMAs’ behavior. A hoist system was developed to linearly increase the displacement with the number of pulleys and wire turns used. The design also used parallel actuation to increase the load capacity. The actuator presented a high load capacity with reduced weight, lifting more than 100 times its own mass, with a low-cost and robust external system. The simplicity of the actuator’s control and production and its lightness make it a suitable option for a wide range of applications, including wearable exoskeletons.The study received funding from the “Sistema robótico para propiciar la marcha en niños pequeños con Parálisis Cerebral” under grant PID2019-105110RB-C32/AEI/10.13039/501100011033, which was funded by Agencia Estatal de Investigación (AEI), and from the R&D&I project PLEC2021-007819, which was funded by MCIN/AEI/10.13039/501100011033 and by the European Union’s NextGenerationEU/PRTR

Bionic hand: A brief review

The hand is one of the most crucial organs in the human body. Hand loss causes the loss of functionality in daily and work life and psychological disorders for the patients. Hand transplantation is best option to gain most of the hand function. However, the applicability of this option is limited since the side effects and the need for tissue compatibility. Electromechanical hand prosthesis also called bionic hand is an alternative option to hand transplantation. This study presents a quick review of bionic hand technology

Flexible shape memory alloy actuators for soft robotics: Modelling and control

One of the limitations in the development of really soft robotic devices is the development of soft actuators. In recent years, our research group has developed a new flexible shape memory alloy actuator that provides more freedom of movements and a better integration in wearable robots, especially in soft wearable robots. Shape memory alloy wires present characteristics such as force/weight ratio, low weight, and noiseless actuation, which make them an ideal choice in these types of applications. However, the control strategy must take into account its complex dynamics due to thermal phase transformation. Different control approaches based on complex non-linear models and other model-free control methods have been tested on real systems. Some exoskeleton prototypes have been developed, which demonstrate the utility of this actuator and the advantages offered by these flexible actuators to improve the comfort and adaptability of exoskeletons

Dynamics and Control of Fiber-Elastomer Composites embedded with Shape Memory Alloys

Soft robots have been used in a wide range of applications from robotic and mechanical engineering to medicine and biomededical field. The growing interest in soft robots comes from their good performance in environments which is not best suited for conventional rigid bodies. Soft robots utilize the compliance, adaptability and flexibility of soft materials and actuation methods to develop highly adaptive structures. Among the soft materials, elastomers are specially popular due to their wide range of elasticity and viscoelasticity. Along with elastomers, textile fabrics are also of high interest for soft robotic applications due to their bendable, flexible, and often stretchable nature. The reinforcement of elastomers with textile fibers results in so-called integrated fiber-elastomer composites (IFEC) which offer a wide variety of properties such as flexibility, strength, fracture toughness and damage resistance. The elastic properties of textile reinforced composites require smart actuators which possess adaptability and deformability. Among existing smart actuators, shape memory alloys (SMA) have been frequently adopted in flexible structures including soft robots. SMAs have sensing and actuation capabilities and are characterized by flexibility and lightness which facilitates their integration into these structures. In this dissertation, the modeling and control of soft prototypes made of IFEC are presented. Shape memory alloys are embedded in the composites for the system actuation. First, the mechanical design and production of three IFEC prototypes are described. For each prototype, a test bench including power and control electronics set-up is designed. Next, mathematical models are developed to analyze the dynamic behavior of the prototypes. The IFEC systems exhibit highly nonlinear behaviour due to SMA hysteresis. For modeling, two different approaches, namely physical modelling and system identification are adopted. In physical modeling, the SMA constitutive and heat transfer equations are incorporated with the composite deflection model. To fully develop the equations, thermal and mechanical parameters of SMA wires are identified experimentally. In the second approach, the mathematical model of the systems is derived from experimental identification and unstructured uncertainty models. Two different control techniques are proposed to compensate the nonlinear behavior of the systems and ensure a robust, fast and precise position tracking. In the first control technique, a proportional integral (PI) controller is designed through robust stability analysis. The second controller is a multivariable PI control which is designed for the prototypes that can move in more than one direction. The performance of the controllers are examined experimentally

Design of a shape memory alloy actuator for soft wearable robots

Soft robotics represents a paradigm shift in the design of conventional robots; while the latter are designed as monolithic structures, made of rigid materials and normally composed of several stiff joints, the design of soft robots is based on the use of deformable materials such as polymers, fluids or gels, resulting in a biomimetic design that replicates the behavior of organic tissues. The introduction of this design philosophy into the field of wearable robots has transformed them from rigid and cumbersome devices into something we could call exo-suits or exo-musculatures: motorized, lightweight and comfortable clothing-like devices. If one thinks of the ideal soft wearable robot (exoskeleton) as a piece of clothing in which the actuation system is fully integrated into its fabrics, we consider that that existing technologies currently used in the design of these devices do not fully satisfy this premise. Ultimately, these actuation systems are based on conventional technologies such as DC motors or pneumatic actuators, which due to their volume and weight, prevent a seamless integration into the structure of the soft exoskeleton. The aim of this thesis is, therefore, to design of an actuator that represents an alternative to the technologies currently used in the field of soft wearable robotics, after having determined the need for an actuator for soft exoskeletons that is compact, flexible and lightweight, while also being able to produce the force required to move the limbs of a human user. Since conventional actuation technologies do not allow the design of an actuator with the required characteristics, the proposed actuator design has been based on so-called emerging actuation technologies, more specifically, on shape memory alloys (SMA). The mechanical design of the actuator is based on the Bowden transmission system. The SMA wire used as the transducer of the actuator has been routed into a flexible sheath, which, in addition to being easily adaptable to the user's body, increases the actuation bandwidth by reducing the cooling time of the SMA element by 30 %. At its nominal operating regime, the actuator provides an output displacement of 24 mm and generates a force of 64 N. Along with the actuator, a thermomechanical model of its SMA transducer has been developed to simulate its complex behavior. The developed model is a useful tool in the design process of future SMA-based applications, accelerating development ix time and reducing costs. The model shows very few discrepancies with respect to the behavior of a real wire. In addition, the model simulates characteristic phenomena of these alloys such as thermal hysteresis, including internal hysteresis loops and returnpoint memory, the dependence between transformation temperatures and applied force, or the effects of latent heat of transformation on the wire heating and cooling processes. To control the actuator, the use of a non-linear control technique called four-term bilinear proportional-integral-derivative controller (BPID) is proposed. The BPID controller compensates the non-linear behavior of the actuator caused by the thermal hysteresis of the SMA. Compared to the operation of two other implemented controllers, the BPID controller offers a very stable and robust performance, minimizing steady-state errors and without the appearance of limit cycles or other effects associated with the control of these alloys. To demonstrate that the proposed actuator together with the BPID controller are a valid solution for implementing the actuation system of a soft exoskeleton, both developments have been integrated into a real soft hand exoskeleton, designed to provide force assistance to astronauts. In this case, in addition to using the BPID controller to control the position of the actuators, it has been applied to the control of the assistive force provided by the exoskeleton. Through a simple mechanical multiplication mechanism, the actuator generates a linear displacement of 54 mm and a force of 31 N, thus fulfilling the design requirements imposed by the application of the exoskeleton. Regarding the control of the device, the BPID controller is a valid control technique to control both the position and the force of a soft exoskeleton using an actuation system based on the actuator proposed in this thesis.La robótica flexible (soft robotics) ha supuesto un cambio de paradigma en el diseño de robots convencionales; mientras que estos consisten en estructuras monolíticas, hechas de materiales duros y normalmente compuestas de varias articulaciones rígidas, el diseño de los robots flexibles se basa en el uso de materiales deformables como polímeros, fluidos o geles, resultando en un diseño biomimético que replica el comportamiento de los tejidos orgánicos. La introducción de esta filosofía de diseño en el campo de los robots vestibles (wearable robots) ha hecho que estos pasen de ser dispositivos rígidos y pesados a ser algo que podríamos llamar exo-trajes o exo-musculaturas: prendas de vestir motorizadas, ligeras y cómodas. Si se piensa en el robot vestible (exoesqueleto) flexible ideal como una prenda de vestir en la que el sistema de actuación está totalmente integrado en sus tejidos, consideramos que las tecnologías existentes que se utilizan actualmente en el diseño de estos dispositivos no satisfacen plenamente esta premisa. En última instancia, estos sistemas de actuaci on se basan en tecnologías convencionales como los motores de corriente continua o los actuadores neumáticos, que debido a su volumen y peso, hacen imposible una integraci on completa en la estructura del exoesqueleto flexible. El objetivo de esta tesis es, por tanto, el diseño de un actuador que suponga una alternativa a las tecnologias actualmente utilizadas en el campo de los exoesqueletos flexibles, tras haber determinado la necesidad de un actuador para estos dispositivos que sea compacto, flexible y ligero, y que al mismo tiempo sea capaz de producir la fuerza necesaria para mover las extremidades de un usuario humano. Dado que las tecnologías de actuación convencionales no permiten diseñar un actuador de las características necesarias, se ha optado por basar el diseño del actuador propuesto en las llamadas tecnologías de actuación emergentes, en concreto, en las aleaciones con memoria de forma (SMA). El diseño mecánico del actuador está basado en el sistema de transmisión Bowden. El hilo de SMA usado como transductor del actuador se ha introducido en una funda flexible que, además de adaptarse facilmente al cuerpo del usuario, aumenta el ancho de banda de actuación al reducir un 30 % el tiempo de enfriamiento del elemento SMA. En su régimen nominal de operaci on, el actuador proporciona un desplazamiento de salida de 24 mm y genera una fuerza de 64 N. Además del actuador, se ha desarrollado un modelo termomecánico de su transductor SMA que permite simular su complejo comportamiento. El modelo desarrollado es una herramienta útil en el proceso de diseño de futuras aplicaciones basadas en SMA, acelerando el tiempo de desarrollo y reduciendo costes. El modelo muestra muy pocas discrepancias con respecto al comportamiento de un hilo real. Además, es capaz de simular fenómenos característicos de estas aleaciones como la histéresis térmica, incluyendo los bucles internos de histéresis y la memoria de puntos de retorno (return-point memory), la dependencia entre las temperaturas de transformacion y la fuerza aplicada, o los efectos del calor latente de transformación en el calentamiento y el enfriamiento del hilo. Para controlar el actuador, se propone el uso de una t ecnica de control no lineal llamada controlador proporcional-integral-derivativo bilineal de cuatro términos (BPID). El controlador BPID compensa el comportamiento no lineal del actuador causado por la histéresis térmica del SMA. Comparado con el funcionamiento de otros dos controladores implementados, el controlador BPID ofrece un rendimiento muy estable y robusto, minimizando el error de estado estacionario y sin la aparición de ciclos límite u otros efectos asociados al control de estas aleaciones. Para demostrar que el actuador propuesto junto con el controlador BPID son una soluci on válida para implementar el sistema de actuación de un exoesqueleto flexible, se han integrado ambos desarrollos en un exoesqueleto flexible de mano real, diseñado para proporcionar asistencia de fuerza a astronautas. En este caso, además de utilizar el controlador BPID para controlar la posición de los actuadores, se ha aplicado al control de la fuerza proporcionada por el exoesqueleto. Mediante un simple mecanismo de multiplicación mecánica, el actuador genera un desplazamiento lineal de 54 mm y una fuerza de 31 N, cumpliendo así con los requisitos de diseño impuestos por la aplicación del exoesqueleto. Respecto al control del dispositivo, el controlador BPID es una técnica de control válida para controlar tanto la posición como la fuerza de un exoesqueleto flexible que use un sistema de actuación basado en el actuador propuesto en esta tesis.Programa de Doctorado en Ingeniería Eléctrica, Electrónica y Automática por la Universidad Carlos III de MadridPresidente: Fabio Bonsignorio.- Secretario: Concepción Alicia Monje Micharet.- Vocal: Elena García Armad