Optimization of elephant trunk soft pneumatic actuator using finite element method

Soft robots are inherently compliant and adaptive, therefore they are promising candidates for interacting with humans. However robotic manipulators utilizing soft actuators are often constrained by a series of actuator performance limitations. In this work we design a novel linear soft robotic actuator with significantly improved performances over the existing products, achieving 300% deformation ratio, quasi-constant output force over a wide motion range, while maintaining passive compliance and adaptability. Moreover, the novel actuator is less prone to friction, and could be fabricated using inject molding and 3D printing, hence having high repeatability at very low cost. An analytical model was developed to characterize the actuator behavior and provide a guideline for actuator design according to performance specifications. A 6 DOF soft manipulator was designed and fabricated utilizing the novel soft actuator. The manipulator arm had a serial kinematic structure with a biomimetic wrist and was driven by 12 soft actuators mounted onto the arm links. With 1.2m workspace radius and 1kg payload, the working air pressure could be as low as 1bar. Preliminary results have shown the validity of the novel soft actuator and manipulator designs, as well as the strong potential of soft robots in human-oriented applications.

Soft actuators provide highly adaptable actuator options for applications in wearable devices, grippers, and mobile robots due to their inherent compliance. However, this compliance causes soft actuators to have virtually infinite degrees of freedom (DoF) of motion and challenges the accurate prediction of their displacement and interaction forces. While several studies have characterized soft actuators either in blocked condition or in a specific range of motion, the testing conditions often do not match actual loading conditions, which leads to discrepancies between expected and observed mechanical behaviour. Here, we propose a novel multi-DoF experimental protocol for characterizing soft actuator interaction forces by considering three critical aspects-anchoring conditions, displacement boundary conditions, and actuation power. In order to conduct this multi-DoF characterization, we designed a novel reconfigurable robotic test platform for enforcing anchoring conditions and planar displacement boundary conditions, and measuring forces at multiple contact locations. Using this experimental protocol and setup, we dictated three loading conditions-pulling force, tip loading, and three-point bending-and analyzed the interaction forces of a soft bending actuator. Our results show that the three loading conditions produce distinct actuator behaviors. This validates the importance of loading conditions on soft actuator performance.

Soft actuators are gaining significant attention due to their ability to provide realistic tactile sensations in various applications. However, their soft nature makes them vulnerable to damage from external factors, limiting actuation stability and device lifespan. The susceptibility to damage becomes higher with these actuators often in direct contact with their surroundings to generate tactile feedback. Upon onset of damage, the stability or repeatability of the device will be undermined. Eventually, when complete failure occurs, these actuators are disposed of, accumulating waste and driving the consumption of natural resources. This emphasizes the need to enhance the durability of soft tactile actuators for continued operation. This review presents the principles of tactile feedback of actuators, followed by a discussion of the mechanisms, advancements, and challenges faced by soft tactile actuators to realize high actuation performance, categorized by their driving stimuli. Diverse approaches to achieve durability are evaluated, including self-healing, damage resistance, self-cleaning, and temperature stability for soft actuators. In these sections, current challenges and potential material designs are identified, paving the way for developing durable soft tactile actuators.

BackgroundCompliant pneumatic actuators possess many characteristics that are desirable for wearable robotic systems. These actuators can be lightweight, integrated with clothing, and accommodate uncontrolled degrees of freedom. These attributes are especially desirable for hand exoskeletons, where the soft actuator can conform to the highly variable digit shape. In particular, locating the pneumatic actuator on the palmar side of the digit may have benefits for assisting finger extension and resisting unwanted finger flexion, but this configuration requires suppleness to allow digit flexion while retaining sufficient stiffness to assist extension.MethodsTo meet these needs, we designed an actuator consisting of a hollow chamber long enough to span the joints of each digit while sufficiently narrow not to inhibit finger adduction. We explored the geometrical design parameter space for this chamber in terms of shape, dimensions, and wall thickness. After fabricating an elastomer-based prototype for each actuator design, we measured active extension force and passive resistance to bending for each chamber using a mechanical jig. We also created a finite element model for each chamber to enable estimation of the impact of chamber deformation, caused by joint rotation, on airflow through the chamber. Finally, we created a prototype hand exoskeleton with the chamber parameters yielding the best outcomes.ResultsA rectangular cross-sectional area was preferable to a semi-obround shape for the chamber; wall thickness also impacted performance. Extension joint torque reached 0.33 N-m at a low chamber pressure of 48.3 kPa. The finite element model confirmed that airflow for the rectangular chamber remained high despite deformation resulting from joint rotation. The hand exoskeleton created with the rectangular chambers enabled rapid movement, with a cycle time of 1.1 s for voluntary flexion followed by actuated extension.ConclusionsThe developed soft actuators are feasible for use in promoting finger extension from the palmar side of the hand. This placement utilizes pushing rather than pulling for digit extension, which is more comfortable and safer. The small chamber volumes allow rapid filling and evacuation to facilitate relatively high frequency finger movements.

SUMMARYThe soft actuator is made of superelastic material and embedded flexible material. In this paper, a kind of soft tube was designed and used to assemble two kinds of pneumatic soft actuators. The experiment and finite element analysis are used to comprehensively analyze and describe the bending, elongation, and torsion deformation of the soft actuator. The results show that the two soft actuators have the best actuation performance when the inner diameter of the soft tube is 4 mm. In addition, when the twisting pitch of the torsional actuator is 24 mm, its torsional performance is optimized. Finally, a device that can be used in the production line was assembled by utilizing those soft actuators, and some operation tasks were completed. This experiment provides some insights for the development of soft actuators with more complex motions in the future.

Development and analysis of a 3D printed hydrogel soft actuator

3-D printed soft magnetic helical coil actuators of iron oxide embedded polydimethylsiloxane

Soft actuators have become central to modern robotics due to their inherent flexibility, enabling safer and more adaptable interactions. Within this domain, omnidirectional actuators are particularly valuable because they enable multidirectional motion suitable for complex robotic tasks and human-robot interaction. However, existing omnidirectional actuators are often bulky and exhibit high design complexity, which limits wearability and often requires high operating pressures. This study presents a novel sleeve-type soft omnidirectional actuator (OSSA), a compact, self-contained sleeve that achieves independent linear extension and contraction and multidirectional bending through differential pressurization of a folded bellows architecture, which lowers the required pressure to 130kPa and enhances wearability and system integration. To enable reliable fabrication at scale, a manufacturing framework for indirect extrusion 3D printing in thermoplastic polyurethane was developed and validated, mitigating common challenges in flexible printing, including nozzle clogging and loss of airtightness. OSSA achieved 60N blocked axial force, 81mm linear displacement, 25mm contraction, and 18N bending force at angles up to 45° under low pressure operation. A parametric analysis revealed key relationships between design geometry and material behavior, providing a framework for actuator optimization. These results advance the development of high-performance omnidirectional soft actuators for applications such as wearable robotics.

Recently, there has been a proliferation of soft robots and actuators that exhibit improved capabilities and adaptability through three-dimensional (3D) bioprinting. Flexibility and shape recovery attributes of stimuli-responsive polymers as the main components in the production of these dynamic structures enable soft manipulations in fragile environments, with potential applications in biomedical and food sectors. Topology optimization (TO), when used in conjunction with 3D bioprinting with optimal design features, offers new capabilities for efficient performance in compliant mechanisms. In this paper, multimaterial TO analysis is used to improve and control the bending performance of a bioprinted soft actuator with electrolytic stimulation. The multimaterial actuator performance is evaluated by the amplitude and rate of bending motion and compared with the single material printed actuator. The results demonstrated the efficacy of multimaterial 3D bioprinting optimization for the rate of actuation and bending.

Stroke is a leading cause of disability worldwide. Rehabilitation is necessary to recover from such disability. However due to growing manpower constraints and greying populations, patients receive limited rehabilitative care. In recent years, soft robotics is slowly becoming more popular as a choice of technology for robot-assisted rehabilitation. Traditional methods of fabricating soft actuators typically require the use of mold-casting, which is a multi-step process. Therefore, this work aimed to adopt a single-step printable pneumatics technique to fabricate soft bending actuators for wrist rehabilitation. The printed pneumatic actuator showed increased bending curvature and force output with greater input air pressure. Based on our preliminary testing, we found that the soft wrist rehabilitation device, based on soft actuators developed using a printable pneumatics approach, is able to guide the wrist model and the human wrist through flexion-extension motions.

Analysis of deformation and multi-angle tip contact in a combined mechanism of thin-walled membrane forces and inextensible layers in soft actuators

Soft actuators are a new generation of robotic actuators designed for safer and more adaptable physical human-robot interaction, that can be triggered by various stimulating mechanisms, including pneumatic, electric, electromagnetic, light, magnetic, and thermal sources. Among the different types of soft actuators, thermoresponsive ones that utilize heat as the stimulus show great potential due to their ability to deliver a relatively high force-to-size ratio without the need for external air pumps, tethers, high voltage sources, or complex designs. However, a major drawback of such actuators is their limited bandwidth. Traditional methods rely on Joule heating for actuation, with the actuator deflating when the heat source is turned off and ambient temperature takes over. Recently, the Peltier mechanism has been introduced as an alternative approach for active heating and cooling. This research paper presents a comparative analysis of the Peltier and flexible heater mechanisms in terms of the bandwidth and energy consumption of phase-change thermo-active soft actuators. The study aims to assess the potential of Peltier-based actuation in addressing the bandwidth limitations observed in traditional soft actuators. The findings reveal that Peltier-based actuation can significantly improve actuation speed in thermoresponsive soft actuators. However, it is important to note that the performance of Peltier-based actuators decreases after a few cycles unless additional measures, such as the use of an external fan, are implemented. This increase in performance comes at the cost of higher energy consumption, which should be carefully considered in practical applications.

Soft actuators have grown to be a topic of great scientific interest recently. As the fabrication of soft actuators with conventional microfabrication methods are tedious, expensive, and time consuming, employment of 3-D printing fabrication methods appears promising as they can simplify fabrication and reduce the production cost. Complex structures can be fabricated with 3-D printing such as helical coils can achieve actuation performances that otherwise would not be possible with simpler geometries. In this thesis development of soft magnetic helical coil actuators of iron-oxide embedded polydimethylsiloxane (PDMS) was achieved with embedded 3-D printing techniques. Composites with three different weight ratios of 10%, 20%, and 30% iron nanoparticles to PDMS were formulated. Using iron nanoparticles with 15-20nm size helps preserve viscosity of the printing material low enough that it was possible to print it with small gauge 29 needle (180 micrometers inner diameter). The hydrogel support of Pluroic f-127 bath and the ability to maintain the ratio of the printed fiber’s diameter to coil diameter close to 0.25 approximately resulted in the successful fabrication and release of fabricated helical coil structures. This enabled 3-D printed structures characterized as magnetic actuators to achieve linear and bending actuation of more than 300% and 80°respectively in the case of composites with 30% iron oxide nanoparticles. Moreover, it was shown that the 3D printed helical coils with 10% iron oxide nanoparticles can be utilized as an untethered soft robot that is capable of locomotion on 45 and 90 degrees inclines under an applied magnetic field.

Roll forming can improve the material utilization rate and production efficiency of cups with a curved rotary profile, but there is no basis for the determination of forming limit. The DEFORM-3D software was used to simulate the roll forming of cups. The influence of the billet wall thickness and bottom thickness, coefficient of friction, radius of roller, and the fillet radius of the punch on the forming limit was studied, and the damage value and velocity vector were analyzed. The results showed that the forming limit of the billet’s wall thickness in roll forming for a cup is about 62%. With the increase of the ratio of the formed cup’s wall thickness to the billet’s bottom thickness, the forming limit of wall thickness will be slightly reduced. A larger radius of roller, fillet radius of punch, and friction coefficient between punch and billet and a smaller friction coefficient between roller and billet are good for decreasing the damage value and improving the roll-forming limit. According to the numerical simulation results, the roll-forming limit diagram of cups is established, and the accuracy of the forming limit diagram is verified by experiments.

3D-printed omnidirectional soft pneumatic actuators: Design, modeling and characterization