Pneumatic Soft Robots Research Articles

A Chemical Pump that Generates High‐Pressure Gas by Transmitting Liquid Fuel against Pressure Gradient

A pneumatic soft robot can be made autonomous by carrying a liquid chemical fuel. In the existing design, to transmit the fuel, the pressure of the fuel tank must exceed that of the actuator. Consequently, the fuel tank must be sufficiently stiff, which hardens the robot. Herein, inspired by pit membranes in trees, a chemical pump is developed, which is consisting of a nanoporous membrane between the fuel tank and the actuator, and coated with a catalyst on the side of the actuator. The fuel in the fuel tank migrates across the membrane and, on meeting the catalyst, decomposes into a pressurized gas and inflates the actuator. The chemical pump is driven by the free energy of reaction, against the difference in pressure. The pores in the membrane are large enough for the fuel molecules to migrate through, but small enough to block the pressurized gas to tunnel back. In a demonstration, the fuel tank has ambient pressure, and the actuator has a pressure of 350 kPa, comparable to the pressure in a car tire. The chemical pump enables pneumatic robots to be autonomous, powerful, and soft.

Freeform Fabrication of Pneumatic Soft Robots via Multi‐Material Jointed Direct Ink Writing

AbstractPneumatically actuated soft robots have attracted significant attention in recent years due to their non‐linear structures for performing biomimetic motions to enhance human‐machine interactions. However, manufacturing soft robots, especially those featuring complex 3D structures, still faces significant challenges as traditional lithography‐based micro/nanofabrication technologies have some limitations, such as limited material choices and layer‐by‐layer architectures. In this work, a facile multi‐material jointed direct ink writing (MJDIW) printing method is introduced. In comparing with the traditional micro/nanofabrication and other additive manufacturing methods, the method enables truly freeform 3D printing to fabricate entirely soft actuators and robots with complex 3D structures, while integrating materials with different mechanical characteristics to expand the manufacturing capabilities of additive manufacturing methods. The material properties and printing parameters, and conducted finite element analysis (FEA) is systematically investigated to provide the general design guidelines through simulating actuator motions. Several actuators such as linear elongation and bending actuators are manufactured either by employing a single material or multiple materials with different mechanical properties. Finally, a multi‐directional soft manipulator with complex internal channels is printed using different materials to illustrate the versatility of the printing methods.

Design and Characterization of a 3D-Printed Pneumatically-Driven Bistable Valve With Tunable Characteristics

Although research studies in pneumatic soft robots develop rapidly, most pneumatic actuators are still controlled by rigid valves and conventional electronics. The existence of these rigid, electronic components sacrifices the compliance and adaptability of soft robots.} Current electronics-free valve designs based on soft materials are facing challenges in behaviour consistency, design flexibility, and fabrication complexity. Taking advantages of soft material 3D printing, this paper presents a new design of a bi-stable pneumatic valve, which utilises two soft, pneumatically-driven, and symmetrically-oriented conical shells with structural bistability to stabilise and regulate the airflow. The critical pressure required to operate the valve can be adjusted by changing the design features of the soft bi-stable structure. Multi-material printing simplifies the valve fabrication, enhances the flexibility in design feature optimisations, and improves the system repeatability. In this work, both a theoretical model and physical experiments are introduced to examine the relationships between the critical operating pressure and the key design features. Results with valve characteristic tuning via material stiffness changing show better effectiveness compared to the change of geometry design features (demonstrated largest tunable critical pressure range from 15.3 to 65.2 kPa and fastest response time $\leq$ 1.8 s.

Pneumatic System Capable of Supplying Programmable Pressure States for Soft Robots

Pneumatic soft robots are of great interest in varieties of potential applications due to their unique capabilities compared with rigid structures. As a part of the soft robotic system, the pneumatic system plays a very important role as all motion performance is ultimately related to the pressure control in air chambers. With the increasing flexibility and complexity of robotic tasks, diverse pneumatic robots driven by positive, negative, or even hybrid pressure are developed, and this comes with higher requirements of pneumatic system and air pressure control precision. In this study, we aim to propose a simplified pneumatic design capable of generating programmable pressure states ranging from negative to positive pressure in each air branch. Based on the design concept and system configuration, special inflation and deflation strategies and closed-loop feedback control strategy are proposed to achieve precise pressure control. Then, a prototype of the pneumatic system with six independent air supply branches is designed and fabricated. Experimental results show that the pneumatic system can achieve a wide range of pressure from -59 to 112 kPa. The speed of inflation and deflation is controllable. Finally, we demonstrate three robotic applications and design the related algorithms to verify the feasibility and practicability of the pneumatic system. Our proposed pneumatic design can satisfy the pressure control requirements of a variety of soft robots driven by both positive and negative pressure. It can be used as a universal pneumatic platform, which is inspiring for actuation and control in the soft robotic field.

Nanotextured Soft Electrothermo-Pneumatic Actuator for Constructing Lightweight, Integrated, and Untethered Soft Robotics.

In this study, we fabricated a nanofiber-based electrothermo-pneumatic soft actuator (ETPSA) using electrospinning technique. The actuator uses liquid-vapor phase transition. The ETPSA developed in the present study goes beyond the limitations of the existing pneumatic soft actuators. The present ETPSA has a built-in source of heat (Joule heating from an embedded metal wire) and allows the smooth anthropomorphic movement of the actuator and, in particular, eliminates the use of external pumping systems that are indispensable in the existing pneumatic soft actuators and robots. In addition, since the present ETPSA can be operated effectively even using a portable miniature battery, it holds great promise as an adaptable soft actuator for various robotic applications with high energy efficiency and programmable motions.

A Super‐Stretchable and Highly Sensitive Carbon Nanotube Capacitive Strain Sensor for Wearable Applications and Soft Robotics

AbstractThe study of flexible and stretchable strain sensors is growing rapidly owing to the demands for human motion detection, human–machine interaction, and soft robotics. However, super‐stretchable and highly sensitive strain sensors with high linearity and low hysteresis are especially lacking, which therefore limits the use of soft strain sensors in varied practical applications. The stretchability and sensitivity of the capacitive strain sensor are constrained by the material characteristics and structure of parallel plate capacitor (theoretical gauge factor [GF] is 1). To address these limitations, a super‐stretchable and highly sensitive capacitive strain sensor composed of two strips of wrinkled carbon nanotubes‐based electrodes separated by a tape dielectric, is presented. By integrating nanomaterials and wrinkled film structure, this device achieves a GF of 2.07 at 300% strain with excellent linearity and negligible hysteresis. This is the first type of capacitive strain sensors that can achieve super‐stretchability and sensitivity simultaneously. Additionally, the sensor has a fast signal response time of ≈80 ms, and good mechanical durability during 1000 stretching and releasing cycles. The authors demonstrate the use of this sensor as a versatile wearable device for human motion tracking, and as a smart real‐time monitoring device for soft pneumatic robots.

Sensing and Reconstruction of 3-D Deformation on Pneumatic Soft Robots

Real-time proprioception is a challenging problem for soft robots, which have virtually infinite degrees of freedom in body deformation. When multiple actuators are used, it becomes more difficult as deformation can also occur on actuators caused by interaction between each other. To tackle this problem, we present a method in this article to sense and reconstruct 3-D deformation on pneumatic soft robots by first integrating multiple low-cost sensors inside the chambers of pneumatic actuators and then using machine learning to convert the captured signals into shape parameters of soft robots. An exterior motion capture system is employed to generate the datasets for both training and testing. With the help of good shape parameterization, the 3-D shape of a soft robot can be accurately reconstructed from signals obtained from multiple sensors. We demonstrate the effectiveness of this approach on two soft robot designs-a robotic joint and a deformable membrane. After parameterizing the deformation of these soft robots into compact shape parameters, we can effectively train the neural networks to reconstruct the 3-D deformation from the sensor signals. The sensing and shape prediction pipeline can run at 50 Hz in real time on a consumer-level device.

A pneumatic random-access memory for controlling soft robots.

Pneumatically-actuated soft robots have advantages over traditional rigid robots in many applications. In particular, their flexible bodies and gentle air-powered movements make them more suitable for use around humans and other objects that could be injured or damaged by traditional robots. However, existing systems for controlling soft robots currently require dedicated electromechanical hardware (usually solenoid valves) to maintain the actuation state (expanded or contracted) of each independent actuator. When combined with power, computation, and sensing components, this control hardware adds considerable cost, size, and power demands to the robot, thereby limiting the feasibility of soft robots in many important application areas. In this work, we introduce a pneumatic memory that uses air (not electricity) to set and maintain the states of large numbers of soft robotic actuators without dedicated electromechanical hardware. These pneumatic logic circuits use normally-closed microfluidic valves as transistor-like elements; this enables our circuits to support more complex computational functions than those built from normally-open valves. We demonstrate an eight-bit nonvolatile random-access pneumatic memory (RAM) that can maintain the states of multiple actuators, control both individual actuators and multiple actuators simultaneously using a pneumatic version of time division multiplexing (TDM), and set actuators to any intermediate position using a pneumatic version of analog-to-digital conversion. We perform proof-of-concept experimental testing of our pneumatic RAM by using it to control soft robotic hands playing individual notes, chords, and songs on a piano keyboard. By dramatically reducing the amount of hardware required to control multiple independent actuators in pneumatic soft robots, our pneumatic RAM can accelerate the spread of soft robotic technologies to a wide range of important application areas.

A compact valveless pressure control source for soft rehabilitation glove.

Soft pneumatic robots have shown great promises in hand rehabilitation systems as alternatives to conventional rigid systems. However, their application is limited to clinical rehabilitation programs due to their dependency on large-sized compressors as air suppliers. This disadvantage triggered the search for compact portable pneumatic sources. A compact valveless pneumatic source to control the bending angle of a soft actuator is proposed in this paper. The source incorporates two series of serially connected commercially available microcompressors to provide additional pressure and flowrate in two directions. In the proposed design, an inner-loop controller, controls the output characteristics of the source while an outer-loop controller handles the trajectory tracking of the angular position. Experimental results show that the source is capable of providing up to 160kPa of output. The controller is able to track up to 2rad/sec sinusoidal trajectory with a maximum 0.066rad root-mean-square error. Experimental measurements showed satisfactory results in the maximum ratings and tracking errors whilst relatively low average power was consumed by eliminating control valves.

Bi-Shell Valve for Fast Actuation of Soft Pneumatic Actuators via Shell Snapping Interaction.

Rapid motion in soft pneumatic robots is typically achieved through actuators that either use a fast volume input generated from pressure control, employ an integrated power source, such as chemical explosions, or are designed to embed elastic instabilities in the body of the robot. This paper presents a bi‐shell valve that can fast actuate soft actuators neither relying on the fast volume input provided by pressure control strategies nor requiring modifications to the architecture of the actuator. The bi‐shell valve consists of a spherical cap and an imperfect shell with a geometrically tuned defect that enables shell snapping interaction to convert a slowly dispensed volume input into a fast volume output. This function is beyond those of current valves capable to perform fluidic flow regulation. Validated through experiments, the analysis unveils that the spherical cap sets the threshold of the snapping pressure along with the upper bounds of volume and energy output, while the imperfect shell interacts with the cap to store and deliver the desired output for rapid actuation. Geometry variations of the bi‐shell valve are provided to show that the concept is versatile. A final demonstration shows that the soft valve can quickly actuate a striker.

Multimaterial Pneumatic Soft Actuators and Robots through a Planar Laser Cutting and Stacking Approach

Pneumatic soft robotic systems show remarkable potentials in producing versatile locomotion and manipulations, owing to their flexibility in structural design and material selections. However, fabrication of pneumatic soft robotic systems with complex 3D structures and material distribution still remains elusive. Herein, a mode‐free fabrication approach, called planar laser cutting and stacking fabrication (PLCSF), is proposed to create pneumatic soft robotic systems with multi‐material and complex structure, which involves the following steps: 1) slicing the 3D model of desired pneumatic soft robotic systems into 2D layers; 2) fabricating each layer via laser cutting corresponding 2D membrane; 3) stacking all layers together to finish the fabrication. With the PLCSF approach, various prevalent pneumatic soft actuators are fabricated with complex structures (including fiber‐reinforced and pneu‐net) and different actuation modes (such as bending, elongation, twisting, abduction, contraction, and grabbing). The scalability of the PLCSF approach to fabricate multiple degrees‐of‐freedom (DOFs) pneumatic soft actuators with integrated structures, such as twisting‐bending pneumatic soft actuators for delivering and bending‐elongation‐bending pneumatic soft actuators for crawling, is also demonstrated. It is further demonstrated that the PLCSF approach also enables creating a bio‐inspired soft hand with nine DOFs, capable of various dexterous motions and manipulations.

Passive Flow Control for Series Inflatable Actuators: Application on a Wearable Soft-Robot for Posture Assistance

This paper presents a passive control method for multiple degrees of freedom in a soft pneumatic robot through the combination of flow resistor tubes with series inflatable actuators. We designed and developed these 3D printed resistors based on the pressure drop principle of multiple capillary orifices, which allows a passive control of its sequential activation from a single source of pressure. Our design fits in standard tube connectors, making it easy to adopt it on any other type of actuator with pneumatic inlets. We present its characterization of pressure drop and evaluation of the activation sequence for series and parallel circuits of actuators. Moreover, we present an application for the assistance of postural transition from lying to sitting. We embedded it in a wearable garment robot-suit designed for infants with cerebral palsy. Then, we performed the test with a dummy baby for emulating the upper-body motion control. The results show a sequential motion control of the sitting and lying transitions validating the proposed system for flow control and its application on the robot-suit.

Pneumatic soft robots take a step toward autonomy.

A four-legged soft robot walks, rotates, and reacts to environmental obstacles by incorporating a soft pneumatic control circuit.

Development of an expansion mechanism for a pneumatic soft mobile robot in a confined space

Development of an expansion mechanism for a pneumatic soft mobile robot in a confined space

Data-Driven Control of Soft Robots Using Koopman Operator Theory

Controlling soft robots with precision is a challenge due to the difficulty of constructing models that are amenable to model-based control design techniques. Koopman operator theory offers a way to construct explicit dynamical models of soft robots and to control them using established model-based control methods. This approach is data driven, yet yields an explicit control-oriented model rather than just a “black-box” input-output mapping. This work describes a Koopman-based system identification method and its application to model predictive control (MPC) design for soft robots. Three MPC controllers are developed for a pneumatic soft robot arm via the Koopman-based approach, and their performances are evaluated with respect to several real-world trajectory following tasks. In terms of average tracking error, these Koopman-based controllers are more than three times more accurate than a benchmark MPC controller based on a linear state-space model of the same system, demonstrating the utility of the Koopman approach in controlling real soft robots.

A Multimode Flow Control Valve for Simplifying the Driving System of Pneumatic Soft Robots

This study aims to simplify the driving system of pneumatic soft robots by developing a lightweight and small-sized multimode flow control valve (MMFCV). It can be embedded in robots as the components, and thus it requires no conventional valves with large size and heavy mass, which significantly reduced the volume and mass of the driving system. The valve is comprised of an input port, an output port, a positive port, and a negative port. By designing the connection of these ports to the actuators, the MMFCV can work in three different modes: one-direction delay mode, delay deflating mode, and double-port control mode. The pressure output characteristics of the MMFCV are modeled, and the validity is verified through experiments. In addition, a robot that comprised of two radial expansion modules, one axial expansion module, and four MMFCVs are fabricated, and the body is driven by the air pressure from the air supply through only one tube. The results of the pipe crawling experiments demonstrate that the MMFCV with multimode increased the flexibility of circuit and time sequence of the air flow, which can reduce the number of the tubes from the air supply to the robot. As a result, it reduced the load pulled by the tubes, and prevented the mission failure from the risk of entanglement of the tubes.

A variable structure pneumatic soft robot

In this paper, a variable structure pneumatic soft robot is proposed. Its structure is variable in that when it grasps irregular objects, it can adapt to different sizes by active expansion or contraction. Its expansion range is from diameter 200 to 300 mm, its four soft pneumatic actuators (SPAs) can be rotated independently to adapt to different shapes, and it has high flexibility. The active compliant grasping method enables it to capture at the best position, which can improve the success rate of capture and reduce damage to the object being grasped. The experiment proves the effectiveness of the variable structure mechanism, and the proposed soft robot has low cost and a simple manufacturing process, so the mechanism has great application prospects.

Design and Fabrication of a Multi-motion Mode Soft Crawling Robot

This article proposes a novel pneumatic soft actuator, which can perform bending in different directions under positive or negative air pressure. The actuators are composed of multiple airbags, and the design of the airbags is analyzed. A pneumatic soft robot based on these soft actuators is designed and fabricated by 3D printing technology. This robot consists of three soft multi-bladder actuators, one soft sensor, middle layer, bottom layer, front barb, front feet and rear feet. According to the different positive or negative pressure control of the three soft multi-bladder actuators, the robot can perform both linear, crossing and climbing movements. The soft robot has excellent environmental adaptability and can pass through complex environments by combining three modes of motion. Then, we establish the closed-loop automatic control system using soft sensor. The soft sensor can be stretched and compressed as the soft robot’s movement. Finally, the automatic control system is verified by linear, crossing and climbing movement experiments. Results indicate that the robot can pass through complex environments under the closed-loop control system.

Fluid Mechanics of Pneumatic Soft Robots.

Pressurization of gas within embedded channels and cavities is a popular method for actuating soft robots. Various previous works examined the effects of internal fluid mechanics on this actuation approach, as well as on leveraging viscous effects to extend the capabilities of soft robots. However, no existing works studied the combined effects of fluid viscosity and compressibility, relevant to miniaturized configurations, which is the aim of the current work. We derive a general model for compressible viscous flow in an elastic media representing a simplified miniaturized soft robot. We illustrate applying this model to periodic configurations, simplifying it via a long-wave approximation. Steady, and time-dependent solutions are obtained, allowing to model the flow and to provide insight into the actuation dynamics of miniaturized pneumatic soft robots.

Status of and trends in soft pneumatic robotics

Soft robots are receiving increasing attention and have great potential in the fields of health care, education, service, rescue, exploration, detection, and wearable devices owing to their inherent high flexibility, good compliance, excellent adaptability, and safe interactivity. Soft pneumatic robots are a well-known type of soft robot with characteristics such as light weight, high performance, pollution-free operation, and strong environmental adaptability. In recent decades, the development of soft pneumatic robots is closely related to the application of new materials, novel designs, and new manufacturing processes. In this study, the literature review and main advances in soft pneumatic robots’ design and fabrication technology are reviewed. In addition, the trends of multifunctionalization, modularization, and the bionic design of soft pneumatic robots are analyzed and discussed. Furthermore, the problems and challenges associated with soft pneumatic robots’ system reliability, air supply, and motion control are analyzed. Finally, the future development outlook and significance of soft pneumatic robots are summarized.