Traditional robots have rigid underlying structures that limit their ability to interact with their environment. For example, conventional robot manipulators have rigid links and can manipulate objects using only their specialised end effectors. These robots often encounter difficulties operating in unstructured and highly congested environments. A variety of animals and plants exhibit complex movement with soft structures devoid of rigid components. Muscular hydrostats e.g. octopus arms and elephant trunks are almost entirely composed of muscle and connective tissue and plant cells can change shape when pressurised by osmosis. Researchers have been inspired by biology to design and build soft robots. With a soft structure and redundant degrees of freedom, these robots can be used for delicate tasks in cluttered and/or unstructured environments. This paper discusses the novel capabilities of soft robots, describes examples from nature that provide biological inspiration, surveys the state of the art and outlines existing challenges in soft robot design, modelling, fabrication and control.

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Soft robotics: Biological inspiration, state of the art, and future research

https://ris.utwente.nl/ws/files/6520436/pp.pdf

Variable impedance actuators: A review

The proliferation of soft robotics research worldwide has brought substantial achievements in terms of principles, models, technologies, techniques, and prototypes of soft robots. Such achievements are reviewed here in terms of the abilities that they provide robots that were not possible before. An analysis of the evolution of this field shows how, after a few pioneering works in the years 2009 to 2012, breakthrough results were obtained by taking seminal technological and scientific challenges related to soft robotics from actuation and sensing to modeling and control. Further progress in soft robotics research has produced achievements that are important in terms of robot abilities-that is, from the viewpoint of what robots can do today thanks to the soft robotics approach. Abilities such as squeezing, stretching, climbing, growing, and morphing would not be possible with an approach based only on rigid links. The challenge ahead for soft robotics is to further develop the abilities for robots to grow, evolve, self-heal, develop, and biodegrade, which are the ways that robots can adapt their morphology to the environment.

Soft robotics: Technologies and systems pushing the boundaries of robot abilities.

In the growing fields of wearable robotics, rehabilitation robotics, prosthetics, and walking k robots, variable stiffness actuators (VSAs) or adjustable compliant actuators are being designed and implemented because of their ability to minimize large forces due to shocks, to safely interact with the user, and their ability to store and release energy in passive elastic elements. This review article describes the state of the art in the design of actuators with adaptable passive compliance. This new type of actuator is not preferred for classical position-controlled applications such as pick and place operations but is preferred in novel robots where safe human- robot interaction is required or in applications where energy efficiency must be increased by adapting the actuator's resonance frequency. The working principles of the different existing designs are explained and compared. The designs are divided into four groups: equilibrium-controlled stiffness, antagonistic-controlled stiffness, structure-controlled stiffness (SCS), and mechanically controlled stiffness.

Compliant actuator designs

Soft fluidic actuators consisting of elastomeric matrices with embedded flexible materials are of particular interest to the robotics community because they are affordable and can be easily customized to a given application. However, the significant potential of such actuators is currently limited as their design has typically been based on intuition. In this paper, the principle of operation of these actuators is comprehensively analyzed and described through experimentally validated quasi-static analytical and finite-element method models for bending in free space and force generation when in contact with an object. This study provides a set of systematic design rules to help the robotics community create soft actuators by understanding how these vary their outputs as a function of input pressure for a number of geometrical parameters. Additionally, the proposed analytical model is implemented in a controller demonstrating its ability to convert pressure information to bending angle in real time. Such an understanding of soft multimaterial actuators will allow future design concepts to be rapidly iterated and their performance predicted, thus enabling new and innovative applications that produce more complex motions to be explored.

Modeling of Soft Fiber-Reinforced Bending Actuators

This article is intended as an introduction to and an overview of Pneumatic Artificial Muscles (PAMs). These are pneumatic actuators made mainly of a flexible and inflatable membrane. First, their concept and way of operation are explained. Next, the properties of these actuators are given, the most important of which are the compliant behavior and extremely low weight. A classification and review is following this section. Typical applications are dealt with in the last but one section and, finally, some concluding remarks are made.

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Pneumatic artificial muscles: actuators for robotics and automation

This paper describes the design of a new type of Pneumatic Artificial Muscle (PAM), namely the Pleated Pneumatic Artificial Muscle (PPAM). It was developed as an improvement with regard to existing types of PAM, e.g. the McKibben muscle. Its principle characteristic is its pleated membrane. It can inflate without material stretching and friction and has practically no stress in the direction perpendicular to its axis of symmetry. Besides these it is extremely strong and yet very lightweight and it has a large stroke compared to most other designs. A general introduction on PAMs is given together with a short discussion and motivation for this new design. The concept of the PPAM is explained and a mathematical model is derived. This model proves its principle of operation. From the model, several characteristics, such as developed force, maximum contraction, diameter, volume and membrane tensile stress, are obtained. Material choices and dimensions of a typical PPAM are next discussed and its measu...

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The Concept and Design of Pleated Pneumatic Artificial Muscles

This paper reports on the bipedal robot Lucy which is actuated by pleated pneumatic artificial muscles. This novel actuator is very suitable to be used in machines which move by means of legs. Besides its high power to weight ratio the actuator has an adaptable passive behavior, meaning the stiffness of the actuator can be changed on-line. This allows to change the natural frequency of the system while controlling angular joint positions. The main control concept intended for Lucy is joint trajectory control while selecting appropriate actuator compliance characteristics in order to reduce control efforts and energy consumption which is of great importance towards the autonomy of legged robots. Presently Lucy has made her first steps with the implementation of basic control strategies.

The Pneumatic Biped “Lucy” Actuated with Pleated Pneumatic Artificial Muscles

For a robotic system that shares its workspace with humans and physically interacts with them, safety is of paramount importance. In order to build a safe system, safety has to be considered in both hardware and software (control). In this paper, we present the safe control of a two-degree-of-freedom planar manipulator actuated by Pleated Pneumatic Artificial Muscles. Owing to its low weight and inherent compliance, the system hardware has excellent safety characteristics. In traditional control methods, safety and good tracking are often impossible to combine. This is different in the case of Proxy-Based Sliding Mode Control (PSMC), a novel control method introduced by Kikuuwe and Fujimoto. PSMC combines responsive and accurate tracking during normal operation with smooth, slow and safe recovery from large position errors. It can also make the system behave compliantly to external disturbances. We present both task- and joint-space implementations of PSMC applied to the pneumatic manipulator, and compare their performance with PID control. Good tracking results are obtained, especially with the joint-space implementation. Safety is evaluated by means of the Head Injury Criterion and by the maximum interaction force in the case of collision. It is found that in spite of the hardware safety features, the system is unsafe when under PID control. PSMC, on the other hand, provides increased safety as well as good tracking.

Proxy-based Sliding Mode Control of a Planar Pneumatic Manipulator

Reports on a type of pneumatic artificial muscles (PAMs) that was developed at the Vrije Universiteit Brussel, Department of Mechanical Engineering. Its distinguishing feature is its pleated design. Due to this, it has a very high contraction force and an equally high travel. The weight of these pleated PAMs is very low: a muscle of only 60 gr can pull up to 3500 N and contract by 42%. Furthermore, dry friction and associated hysteresis, typical of many other designs, is avoided by the folding-unfolding action. This significantly simplifies position control using these actuators. Although the force-displacement characteristics of our actuators are non-linear, they can be effectively controlled using basic linear PI techniques. Another advantage of these actuators is their inherent and controllable compliance, making them ideally suited for walking/running machines or whenever delicate tasks, e.g. handling fragile objects, have to be performed. In view of all characteristics pleated PAMs are very well suited for automation and robotic applications.

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Frank Daerden

Papers

Pneumatic artificial muscles: actuators for robotics and automation

The Concept and Design of Pleated Pneumatic Artificial Muscles

The Pneumatic Biped “Lucy” Actuated with Pleated Pneumatic Artificial Muscles

Proxy-based Sliding Mode Control of a Planar Pneumatic Manipulator

Pleated pneumatic artificial muscles: actuators for automation and robotics