Aaron D. Price

Design and control of a shape memory alloy based dexterous robot hand

Modern externally powered upper-body prostheses are conventionally actuated by electric servomotors. Although these motors achieve reasonable kinematic performance, they are voluminous and heavy. Deterring factors such as these lead to a substantial proportion of upper extremity amputees avoiding the use of their prostheses. Therefore, it is apparent that there exists a need for functional prosthetic devices that are compact and lightweight. The realization of such a device requires an alternative actuation technology, and biological inspiration suggests that tendon based systems are advantageous. Shape memory alloys are a type of smart material that exhibit an actuation mechanism resembling the biological equivalent. As such, shape memory alloy enabled devices promise to be of major importance in the future of dexterous robotics, and of prosthetics in particular. This paper investigates the design, instrumentation, and control issues surrounding the practical application of shape memory alloys as artificial muscles in a three-fingered robot hand.

A Study on the Thermomechanical Properties of Shape Memory Alloys-based Actuators used in Artificial Muscles

Shape memory alloys (SMAs) are a class of smart material having the unique ability to return to a predefined shape when heated. These materials are employed in a variety of emerging applications, and may potentially be used to avoid traditionally voluminous and heavy prosthetic actuators. The primary focus of this article is to convey the design and evaluation of a compact, lightweight, and high-strain SMA ribbon-based artificial muscle for use in such biomimetic applications. A key factor in the design of such an actuator is a thorough understanding of the thermomechanical response of the shape memory material. As such, a review of the relevant constitutive models is included. A selected hysteresis model is evaluated for potential application to ribbon type elements. The proposed actuator achieves strains of 31.6%; a marked improvement over previously documented SMA-based actuators.

A Simple Phenomenological Model for Magnetic Shape Memory Actuators

This paper presents a new phenomenological model for magnetic shape memory (MSM) alloy actuators. The model was implemented as a lumped element for multi-domain network models using the Modelica language. These network models are rapidly computed and are therefore well suited for MSM-based actuator design and optimization. The proposed MSM model accounts for the 2-D hysteresis of the magnetic field-induced strain as a function of both the applied magnetic flux density and the compressive stress. An extended Tellinen hysteresis formulation was utilized to compute the mechanical strain of the MSM material from measured upper and lower limiting hysteresis surfaces. Two alternative approaches for the computation of the lumped element have been implemented. The first method uses hyperbolic shape functions to approximate the limiting hysteresis surfaces and offers a good balance of simulation accuracy, numerical stability, computational speed, and ease of parameter identification. The second method uses 2-D lookup tables for direct interpolation of the measured limiting hysteresis surfaces, which leads to higher accuracy. Finally, a test case having simultaneously varying compressive stress and magnetic flux density was utilized to experimentally validate both methods. Sufficient agreement between the simulated and measured strain of the sample was observed.

A unified multiphysics finite element model of the polypyrrole trilayer actuation mechanism

Conducting polymer materials have demonstrated new possibilities for low-density active material actuators. This article briefly introduces several existing conducting polymer actuator modelling approaches and identifies limitations on their sole applicability for predictive design. The main contribution of this article is the proposal and development of a new unified multiphysics finite element model of the polypyrrole trilayer actuation mechanism that does not depend on specimen-specific parameters. The model predicts the structural deformation of trilayer actuators using only material properties such that the model itself is sample-independent and thus may have practical use as an electroactive polymer design facility. Comparison with published data indicates that the model’s predictions fall within 95% confidence intervals throughout the entire range of input potentials evaluated.

Evaluation of porous membrane core elasticity and porous morphology for polypyrrole trilayer actuators

Multilayer electroactive polymer actuators consisting of polypyrrole films electropolymerized on a passive polymer membrane core have been harnessed as a source of simple actuation. As an integral component of the actuator, the membrane plays a vital role in the transport of ionic species and largely dictates the stiffness of the layered configuration, yet in past studies the specification of the membrane has remained largely arbitrary. In this investigation, we use quasi-static and dynamic mechanical analysis to investigate the impact of the mechanical properties of the membrane on the actuation response of polypyrrole-based trilayer bending actuators. Candidate materials with distinctly varied microcellular morphologies are identified and include polyvinylidene difluoride, nylon, and nitrocellulose. The quasi-static stress-strain response and the frequency-dependent viscoelastic nature of the candidates are then evaluated. On the basis of mechanical properties these results indicate that polyvinylidene difluoride membranes are superior to the other candidates for application as trilayer actuator cores. Bis(trifluoromethane)sulfonimide doped polypyrrole actuators with polyvinylidene difluoride cores and nylon cores are then fabricated under various synthesis conditions and their electromechanical actuation behavior is reported.

Integrated Design Optimization of Dielectric Elastomer Actuators in High-Performance Switchgear

Although the performance of stacked dielectric elastomers has been demonstrated experimentally, improved simulation tools are required to accurately predict their dynamic performance in demanding applications in the power and automation industries. This investigation presents an integrated design optimization system that elucidates the relationship between various design variables and resulting electromechanical system performance in the context of a dielectric elastomer actuator driven electrical contactor.The system comprises several specially developed software modules that interact to automatically determine an optimal design. The core module consists of a user material subroutine that models electro-viscoelastic elastomers, the development of which is presented herein. This material behavior is then integrated within a coupled multi-body simulation implemented in the ABAQUS commercial finite element software. This module captures the mechanical dynamics of the switchgear for a given actuator design candidate. Finally, a fully-integrated multi-objective optimization workflow that automatically generates and evaluates design candidates is presented.Copyright

Biologically inspired anthropomorphic arm and dextrous robot hand actuated by smart-material-based artificial muscles

Shape memory alloys (SMA) are a class of smart material having the unique ability to return to a predefined shape when heated. SMA based actuators have the potential to be very compact and low weight. As a result, much research has been devoted to the design of SMA based actuators; however, commercialization has been largely impeded by the small strain capacity inherent to SMA. To address this deficiency, this paper conveys the design of a large-strain SMA actuator (in excess of 30%) whose feasibility is investigated by integrating the actuators as artificial muscles in a two link anthropomorphic arm. The ensuing experimental results indicate that the actuators show great potential for a variety of emerging applications. Finally the design of an SMA based dextrous robotic hand evaluation facility is proposed, and provides a case study illustrating how smart structures provide a superior alternative to conventionally voluminous and heavy prosthetic actuators.

Electroactive polymer actuators for active optical components

Adaptive optical systems incorporate active components that compensate for wavefront aberrations introduced by optical defects. The quality of optical compensation is largely determined by the stroke of the adaptive component’s underlying actuating mechanism. Development of compact polypyrrole trilayer actuator arrays may deliver superior performance over conventional active technologies such as electrostatic electrodes or piezoelectric actuators. This study introduces a novel piston–tilt mirror apparatus that utilizes low-voltage electroactive polymer actuators to reorient a plane mirror. The design of the mirror and its ancillary systems are first reported, followed by the polymer synthesis procedure and actuator fabrication method. Finally, laser beam steering results are provided in the context of an experimental retinal imaging system. The outcomes indicate a promising future for electroactive polymer-enabled devices in adaptive optical systems with technological implications ranging from more powerful astronomical telescopes to improved retinal tissue diagnosis.

Optimization of porous membrane core morphology for polypyrrole trilayer actuators

Large-strain electroactive polymer bending actuators may consist of two conductive polymer layers electropolymerized on opposing faces of a porous membrane core that facilitates ion migration and electrolyte storage. Although several studies have been devoted to the conditions of the polymerization process, far less effort has been devoted to the systematic selection of the core material and the impact of the cores cellular morphology on the resulting actuation characteristics. This study introduces our initial work towards elucidating the relationship between the material properties of the porous core and the underlying actuation mechanism by reviewing how the physical characteristics of the membrane core are intrinsically captured by existing conductive polymer actuator models. The electromechanical response of polypyrrole trilayer actuators having poly(vinylidene fluoride) and Nylon cores is also explored.

Photopolymerization of 3D conductive polypyrrole structures via digital light processing

The intrinsically conductive polymer polypyrrole is conventionally synthesized as monolithic films that exhibit significant actuation strains when subjected to an applied electric potential. Though numerous linear and bending actuators based on polypyrrole films have been investigated, the limitations inherent to planar film geometries inhibit the realization of more complex behaviours. Hence, three-dimensional polypyrrole structures are sought to greatly expand the potential applications for conductive polymer actuators. This research aims to develop a novel additive manufacturing method for the fabrication of three-dimensional structures of conductive polypyrrole. In this investigation, radiation-curing techniques are employed by means of digital light processing (DLP) technology. DLP is an additive manufacturing technique where programmed light patterns emitted from a dedicated source are used to selectively cure a specially formulated polymer resin. Successive curing operations lead to a layered 3D structure into which fine features may be incorporated. Energy dispersive spectroscopy (EDS) is subsequently employed to examine the unique microstructural features of the resultant 3D printed polymer morphology in order to elucidate the nature of the conductivity. These polymer microstructures are highly desirable since actuation response times are highly dependent on ion transport distances, and hence the ability to fabricate fine features offers a potential mechanism to improve actuator performance.