Bilateral Dielectric Elastomer Actuators: Paving the Way for Highly Controllable and Multifunctional Soft Robots

By Dr Silpaja Chandrasekar, PhDReviewed by Susha Cheriyedath, M.Sc.Oct 11 2023

In a paper published in the journal Microsystems & Nanoengineering, researchers introduced a novel approach to soft robotics inspired by human muscle groups. They created bilateral actuators using cost-effective dielectric elastomers (DE), enabling versatile control and movement.

By connecting these actuators, they designed a three-dimensional (3D) soft robot capable of crawling in various directions, rolling bidirectionally, climbing slopes, and mimicking mouth-like grabbing motions. This breakthrough offers valuable insights into developing highly controllable and multifunctional soft robots, paving the way for innovative applications in robotics.

Context and Previous Research

The advent of soft robotics signifies a notable departure from conventional rigid robotic designs, drawing inspiration from nature to replicate human and animal behaviors. While traditional robots employ rigid bodies and mechanical actuators, these systems fail to replicate the inherent bionic structure found in living organisms. For instance, the human body comprises tough bones for support, soft muscles, and joints for nuanced control and movement. The development of soft actuators that emulate the construction and kinematics of the animal body has emerged as a promising approach to bridging this gap.

Researchers have harnessed various intelligent materials to craft soft actuators and robots, including DEs, liquid crystalline polymers (LCPs), shape memory alloys (SMAs), and hydrogels. Soft robotics is proving indispensable in diverse fields, such as biomedical engineering, marine detection, medical rehabilitation, and industrial applications, primarily due to the flexibility of their structures. However, achieving precise control and maneuverability remains a paramount challenge for soft robots, which often rely on material deformation for control. 

Methodology Overview

To construct the bilateral soft actuators and gear-shaped soft robots, the researchers initiated the process by laser-cutting 0.18 mm or 0.1 mm thick Polyethylene Terephthalate (PET) film into specific shapes, following designs created using Computer-Aided Design (CAD) software. The PET films were categorized into flexible substrates with elliptical holes and reinforced frames with semicircular recesses. A Very High Bond (VHB) 4910 elastomer (3M 60mm × 60mm) was stretched to 400 × 400% using a pre-stretching tool.

Subsequently, the film was affixed within an acrylic frame and removed from the pre-stretching device. A PET substrate and two reinforced structures, complete with flexible wires, were adhered to the center of a DE-film, with their holes meticulously aligned. Apply a thin carbon grease electrode layer evenly on both sides of the DE-film within the hole area using a soft bristle brush. Finally, the unilateral soft actuator separates it from the DE-film, and the bilateral actuator bonds two unilateral actuators together. The gear-shaped soft robot was assembled by integrating multiple actuators into a ring structure, mirroring the earlier process.

Actuation and Test Method: To control the charging and discharging of each actuator, they designed a multichannel high-voltage control circuit utilizing MOS relays, dry-reed relays, and a microcontroller unit (MCU). This circuit facilitated the management of two-channel voltage control for each actuator: one for charging and the other for discharging. The MCU was employed for programming, enabling precise regulation of each channel voltage's duty cycle and frequency.

For mode one operation of bilateral actuators, both shared a typical negative pole, while the positive pole connected to a four-channel high-voltage control system. Conversely, mode two necessitated only two-channel high-voltage control. Each actuator's actuation in the crawling, rolling, and mouth-like robot occurred synchronously, thus requiring only two-channel high-voltage control. The MCU allowed for the management of frequency in tests involving bilateral actuators, the cycling test, and the actuation frequency of the crawling robot. In the case of the mouth-like robot test, they actively adjusted the weight of the table tennis ball by injecting water. 

Experimental Findings

The research focuses on developing bilateral soft actuators, a crucial component for highly controllable soft robots. These actuators actively assemble two unilateral actuators composed of 0.18 mm-thick PET substrate, reinforcement frames, and DE with carbon grease electrodes. Unlike unilateral actuators, bilateral actuators demonstrate superior stability and control, enabling both unilateral and bilateral bending. Their performance is influenced by actuation frequency, with bilateral actuators maintaining functionality at higher frequencies, making them suitable for underwater applications.

These bilateral actuators offer two distinct actuation modes: Mode one involves active bending in one direction, while Mode two actively creates a gap when both sides are simultaneously activated. By integrating multiple bilateral actuators, a 3D gear-shaped soft robot is created, capable of multidirectional crawling. The robot's crawling speed and step distance vary with the frequency of actuation voltage, with optimal performance achieved at 2 Hz. Additionally, the robot exhibits impressive directional controllability, enabling rapid changes in movement direction. Furthermore, the robot can function as a rolling robot when standing on only two feet, with the ability to adjust its barycenter for forward or backward rolling. It can even roll on a 2° slope, expanding its potential applications. Inspired by the orbicularis oris muscle in the mouth, the robot can mimic grasping actions, adjusting its inner diameter through actuated voltages for practical object grasping, even lifting objects heavier than itself, such as a 31 g table tennis ball. While the gear-shaped soft robot showcases remarkable capabilities, there are opportunities for improvement, such as integrating untethered power sources, remote control, and automatic mode switching to enhance its versatility and practicality.

Conclusion

In summary, this work developed a muscle-inspired soft bilateral actuator using two DE-based unilateral actuators, showcasing its stability and dimensional control advantages compared to unilateral actuators. The study explored the deformation performance of bilateral actuators under two modes. A 3D gear-shaped soft robot integrated with these actuators demonstrated exceptional flexibility and controllability for various tasks, including multidirectional crawling, fast turning, bidirectional rolling, and object manipulation. Overall, this robot design significantly advances the development of highly controllable bionic 3D soft robots.