Soft Robotics: a Review of Recent Developments of Pneumatic Soft Actuators
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actuators Review Soft Robotics: A Review of Recent Developments of Pneumatic Soft Actuators James Walker 1, Thomas Zidek 1, Cory Harbel 1, Sanghyun Yoon 1, F. Sterling Strickland 2, Srinivas Kumar 1 and Minchul Shin 1,* 1 Department of Mechanical Engineering, Georgia Southern University, Statesboro, GA 30460, USA; [email protected] (J.W.); [email protected] (T.Z.); [email protected] (C.H.); [email protected] (S.Y.); [email protected] (S.K.) 2 Department of Manufacturing Engineering at Georgia Southern University, Statesboro, GA 30460, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-912-478-5759 Received: 29 September 2019; Accepted: 11 December 2019; Published: 10 January 2020 Abstract: This paper focuses on the recent development of soft pneumatic actuators for soft robotics over the past few years, concentrating on the following four categories: control systems, material and construction, modeling, and sensors. This review work seeks to provide an accelerated entrance to new researchers in the field to encourage research and innovation. Advances in methods to accurately model soft robotic actuators have been researched, optimizing and making numerous soft robotic designs applicable to medical, manufacturing, and electronics applications. Multi-material 3D printed and fiber optic soft pneumatic actuators have been developed, which will allow for more accurate positioning and tactile feedback for soft robotic systems. Also, a variety of research teams have made improvements to soft robot control systems to utilize soft pneumatic actuators to allow for operations to move more effectively. This review work provides an accessible repository of recent information and comparisons between similar works. Future issues facing soft robotic actuators include portable and flexible power supplies, circuit boards, and drive components. Keywords: soft robotics; soft pneumatic actuators; control systems; sensors; soft materials; modeling 1. Introduction Robots are becoming ubiquitous in modern society. Robots have changed the way humanity manufactures our goods, performs surgery, and transport ourselves along with our possessions. However, the boundary between robots and humans remains hazardous [1] as the modern day hard robot requires a lot of precise control and while they excel at repetitive massive production functions, they struggle to interact with uncertain environments [2]. This is where the new field of soft robotics excels. A soft robot is primarily composed of stretchable, flexible materials, such as silicon rubber. While these softer materials have a lower rigidity when compared to metals used in hard robots, these softer robots have provided more flexibility and adaptability to the workspace creating safer environments, particularly in a human-robot integrated workspace. There have been many exciting developments and research around soft robotics recently, which will be chronicled in this review study. The most important physical aspect of a robot is movement [3]. For soft robots, movement is conducted by using variable length tendons [3], by pneumatic actuations, or even more recently by twist-and-coil actuators [4], as seen in Figure1. Many of the modern actuation methods have been designed to replicate biological movements, such as fish fins or octopus tentacles [5–10]. Actuators 2020, 9, 3; doi:10.3390/act9010003 www.mdpi.com/journal/actuators Actuators 2020, 9, 3 2 of 26 Actuators 2020, 9, 3 2 of 27 Figure 1. 1. VariousVarious soft soft robots robots with with various various actuation actuation methods methods (a) Actuator (a) Actuator with multi-material with multi-material 3D printing 3D printing[11]. (Open [11 Access);]. (Open (b Access);) Soft linear (b) Softactuator linear [12]. actuator (Open [Access);12]. (Open (c) Design Access); for (c )bidirectional Design for bidirectionalbending [13]. bending(Copyright [13 2015,]. (Copyright Elsevier); 2015, (d) Snake-modeled Elsevier); (d) Snake-modeled robot [14] (Copyright robot [14 2012,] (Copyright Taylor & 2012, Francis). Taylor & Francis). Many soft robots use pneumatics or hydraulics to actuate their bodies, but another way actuation has beenbeen implementedimplemented into soft robots isis throughthrough thethe developmentdevelopment ofof electroactiveelectroactive polymerspolymers whichwhich electrically activate soft actuators in order to cause movement [[15–18].15–18]. The flexibleflexible nature of softsoft robotsrobots necessitates the the development development of of stretchable stretchable electronics electronics such such as sensors as sensors and andpower power sources. sources. The Thecompliance compliance and morphology and morphology of soft of robots soft robots prevent prevent the use the of usemany of conventional many conventional sensors sensors seen in seenhard inrobots, hard so robots, there sohas there been has active been research active research into stretchable into stretchable electronic electronic sensors and sensors curvature and curvature sensors. sensors.Elastomer Elastomer sensors allow sensors for allow minima forl minimal impact to impact the actuation to the actuation of the robot of the [19]. robot Along [19]. Along with these with thesesensors, sensors, soft robots soft robots also alsohave have the thecapability capability of ofhosting hosting chemical chemical and and biological biological sensors sensors to sensesense environmental signals which could eventually allow them to be human-centeredhuman-centered tools for the idealideal workspaceworkspace between humans and robotsrobots [[5].5]. Power sources have also been a bigbig pointpoint ofof emphasisemphasis withwith generalgeneral compressors compressors using using a lota lot of electricity,of electricity, alternative alternative methods methods such such as chemical as chemical reaction-based reaction- sourcesbased sources and combustible and combustible fuels have fuels been have looked been atlooked for solutions at for solutions [6,20,21]. [6,20,21]. In turn, manyIn turn, soft many robotics soft designsrobotics havedesigns been have made been using made ‘2.5D’ using designs, ‘2.5D’ limited designs, primarily limited byprimarily 3D printed by 3D capabilities printed capabilities and model complexity,and model orcomplexity, use abstract or approachesuse abstract with approaches the current with software the current available software [22–24 ].available Fabrication [22–24]. has alsoFabrication been an has area also of interest;been an however,area of interest; thanks howe to rapidver, growth thanks into digitalrapid growth design andin digital fabrication design tools, and therefabrication are now tools, many there ways are now to develop many ways soft robotsto develo withp soft diff erentrobots material with different selections material and asselections well as embeddedand as well needed as embedded electronics needed into electronics channels of into the ch materialannels of for the system material use [for11 ,system25–28]. use [11,25–28]. The movements of of soft soft bodies bodies can can be be described described by by an an infinite infinite number number of ofdegrees degrees of freedom. of freedom. Its Itsintrinsic intrinsic deformation deformation such such as asbend, bend, twist, twist, stretch, stretch, compress, compress, buckle, buckle, wrinkle, wrinkle, etc. etc. makes makes the the control of soft robots very challenging [[7].7]. The muscle analogyanalogy of soft organisms is introduced to understand thethe workingworking principle principle of softof soft bodies. bodies. An understandingAn understanding of the principleof the principle of octopus of hasoctopus led to inspirationhas led to forinspiration modeling for and modeling control and [7]. control [7]. Due to itsits absentabsent ofof rigidrigid structure,structure, thethe kinematicskinematics andand dynamicsdynamics ofof soft-roboticsoft-robotic systemssystems are didifferentfferent fromfrom thatthat ofof conventionalconventional systems.systems. Therefore,Therefore, researchers have developed a new static, dynamic, andand kinematic models for soft-robotic system.system. Designers modelmodel thethe kinematics of soft robots using piecewisepiecewise constant curvature (PCC) [[8].8]. In approaches of developingdeveloping methodsmethods toto connectconnect thethe actuation space to the configurationconfiguration space,space, the morphologymorphology of the robot bodybody andand thethe characteristicscharacteristics of the actuation system is studied and and combined. combined. Some Some forward forward kinematics kinematics basis basis approaches approaches are are a ause use of of Bernoulli-Euler Bernoulli-Euler beam beam mechanics mechanics to to predict predict deformation deformation [8], [8], a a relationship between the jointjoint variablesvariables andand thethe curvature arcarc parametersparameters ofof highhigh andand mediummedium pressurepressure robot.robot. Existing solutions of the inverse-kinematics problem are neglecting the consideration of the pose of the end effector [9,29]. This limited computation creates difficulty in avoidance of autonomous obstacles and movement Actuators 2020, 9, 3 3 of 26 the inverse-kinematics problem are neglecting the consideration of the pose of the end effector [9,29]. This limited computation creates difficulty in avoidance of autonomous obstacles and movement under restrained environment. Without consideration of the entire robot body including the end effector, task-space planning