This review comprises a detailed survey of ongoing methodologies for soft actuators, highlighting approaches suitable for nanometer- to centimeter-scale robotic applications. Soft robots present a special design challenge in that their actuation and sensing mechanisms are often highly integrated with the robot body and overall functionality. When less than a centimeter, they belong to an even more special subcategory of robots or devices, in that they often lack on-board power, sensing, computation, and control. Soft, active materials are particularly well suited for this task, with a wide range of stimulants and a number of impressive examples, demonstrating large deformations, high motion complexities, and varied multifunctionality. Recent research includes both the development of new materials and composites, as well as novel implementations leveraging the unique properties of soft materials.

Soft Actuators for Small-Scale Robotics.

Soft robots driven by stimuli-responsive materials have unique advantages over conventional rigid robots, especially in their high adaptability for field exploration and seamless interaction with humans. The grand challenge lies in achieving self-powered soft robots with high mobility, environmental tolerance, and long endurance. We are able to advance a soft electronic fish with a fully integrated onboard system for power and remote control. Without any motor, the fish is driven solely by a soft electroactive structure made of dielectric elastomer and ionically conductive hydrogel. The electronic fish can swim at a speed of 6.4 cm/s (0.69 body length per second), which is much faster than previously reported untethered soft robotic fish driven by soft responsive materials. The fish shows consistent performance in a wide temperature range and permits stealth sailing due to its nearly transparent nature. Furthermore, the fish is robust, as it uses the surrounding water as the electric ground and can operate for 3 hours with one single charge. The design principle can be potentially extended to a variety of flexible devices and soft robots.

https://advances.sciencemag.org/content/advances/3/4/e1602045.full.pdf

Fast-moving soft electronic fish

In this paper, biomimetic underwater robots built using smart actuators, e.g., a shape memory alloy (SMA), an ionic polymer metal composite (IPMC), lead zirconate titanate (PZT), or a hybrid SMA and IPMC actuator, are reviewed. The effects of underwater environment were also considered because smart actuators are often affected by their external environment. The characteristics of smart actuators are described based on their actuating conditions and motion types. Underwater robots are classified based on different swimming modes. We expanded our classification to non-fish creatures based on their swimming modes. The five swimming modes are body/caudal actuation oscillatory (BCA-O), body/caudal actuation undulatory (BCA-U), median/paired actuation oscillatory (MPA-O), median/paired actuation undulatory (MPA-U), and jet propulsion (JET). The trends of biomimetic underwater robots were analyzed based on robot speed (body length per second, BL/s). For speed per body length, robots using an SMA as an actuator are faster than robots using an IPMC when considering a similar length or weight. Robots using a DC motor are longer while their speeds per body length are similar, which means that robots using smart actuators have an advantage of compactness. Finally, robots (using smart actuators or a motor) were compared with underwater animals according to their speed and different swimming modes. This review will help in setting guidelines for the development of future biomimetic underwater robots, especially those that use smart actuators.

Review of biomimetic underwater robots using smart actuators

As a novel biologically inspired underwater vehicle, a robotic manta ray (RoMan-II) has been developed for potential marine applications. Manta ray can perform diversified locomotion patterns in water by manipulating two wide tins. These motion patterns have been implemented on the developed fish robot, including swimming by flapping fins, turning by modulating phase relations of fins, and online transition of different motion patterns. The movements are achieved by using a model of artificial central pattern generators (CPGs) constructed with coupled nonlinear oscillators. This paper focuses on the analytical formulation of coupling terms in the CPG model and the implementation issues of the CPG-based control on the fish robot. The control method demonstrated on the manta ray robot is expected to be a frame- work that can tackle locomotion control problems in other types of multifin-actuated fish robots or more general robots with rhythmic movement patterns.

Design and Locomotion Control of a Biomimetic Underwater Vehicle With Fin Propulsion

Electroactive polymer (EAP) actuators are electrically responsive materials that have several characteristics in common with natural muscles. Thus, they are being studied as 'artificial muscles' for a variety of biomimetic motion applications. EAP materials are commonly classified into two major families: ionic EAPs, activated by an electrically induced transport of ions and/or solvent, and electronic EAPs, activated by electrostatic forces. Although several EAP materials and their properties have been known for many decades, they have found very limited applications. Such a trend has changed recently as a result of an effective synergy of at least three main factors: key scientific breakthroughs being achieved in some of the existing EAP technologies; unprecedented electromechanical properties being discovered in materials previously developed for different purposes; and higher concentration of efforts for industrial exploitation. As an outcome, after several years of basic research, today the EAP field is just starting to undergo transition from academia into commercialization, with significant investments from large companies. This paper presents a brief overview on the full range of EAP actuator types and the most significant areas of interest for applications. It is hoped that this overview can instruct the reader on how EAPs can enable bioinspired motion systems.

https://iopscience.iop.org/article/10.1088/1748-3182/6/4/045006/pdf

Electroactive polymer actuators as artificial muscles: are they ready for bioinspired applications?

In this paper, a physics-based model is proposed for a biomimetic robotic fish propelled by an ionic polymer-metal composite (IPMC) actuator. Inspired by the biological fin structure, a passive plastic fin is further attached to the IPMC beam. The model incorporates both IPMC actuation dynamics and the hydrodynamics, and predicts the steady-state cruising speed of the robot under a given periodic actuation voltage. The interactions between the plastic fin and the IPMC actuator are also captured in the model. Experimental results have shown that the proposed model is able to predict the motion of robotic fish for different tail dimensions. Since most of the model parameters are expressed in terms of fundamental physical properties and geometric dimensions, the model is expected to be instrumental in optimal design of the robotic fish.

Modeling of Biomimetic Robotic Fish Propelled by An Ionic Polymer–Metal Composite Caudal Fin

Small (decimeter-scale) robotic fish are promising mobile sensor platforms for aquatic environments. Fine-grained localization for dense networks of such robotic fish presents a challenge because of noisy underwater environment, required submeter accuracy, and constraints on onboard processing power and hardware complexity. In this paper, we present an efficient time-of-flight-based acoustic ranging system for localization of robotic fish with limited onboard resources. The system involves simple hardware: a single pair of monotone buzzer and microphone. The distance between two nodes is determined by the time it takes for an acoustic signal generated by the buzzer on the first node to reach the microphone on the second node. The arrival of the signal is detected with the sliding discrete Fourier transform (SDFT) algorithm, where the rise dynamics of the signal is modeled and used for compensation of detection latency. The algorithm is implemented onboard a small biomimetic robotic fish, and experiments in an indoor pool have shown that the compensated SDFT algorithm results in an underwater ranging error of 1.9 wavelengths (1 m), and is thus promising for localization of dense aquatic networks.

An Efficient, Time-of-Flight-Based Underwater Acoustic Ranging System for Small Robotic Fish

Fine-grained (sub-meter) ranging and localization is critical to the deployment of dense, mobile sensor networks in aquatic environments. However, such a task is faced with a number of challenges, including noisy underwater environments, limitation on size and complexity of localization hardware, and constraints on computing capabilities of sensor platforms. In this paper we present a sliding-window discrete Fourier transform (DFT)-based algorithm for precise detection of the arrival of a monotone acoustic signal, as a key enabling step in measuring the time of flight (TOF) of the acoustic signal for localization of the sensor node. The algorithm accommodates the rise dynamics of the signal and compensates for the latency in detection given the signal model, detection threshold, and steady-state signal amplitude. The algorithm is implemented onboard a small biomimetic robotic fish, and experiments in an indoor pool have shown that the proposed method results in an underwater ranging error of 1.4 wavelengths (74.3 cm), and is thus promising for localization of dense aquatic networks. The proposed method has also shown robustness in comparison with other tested methods including a matched filter-type method.

/pdf/a-compensated-sliding-window-dft-algorithm-for-fine-grained-4azyfr34q8.pdf

Stephan Shatara

Papers

Modeling of Biomimetic Robotic Fish Propelled by An Ionic Polymer–Metal Composite Caudal Fin

An Efficient, Time-of-Flight-Based Underwater Acoustic Ranging System for Small Robotic Fish

A compensated sliding-window DFT algorithm for fine-grained underwater acoustic ranging