Researchers Identify a New Way to Create Lightweight Robots

Spiders have been used as biological models in the research field of soft robotics in more ways than one. These insects apply hydraulic actuation mechanisms to move their limbs when hunting for prey or weaving their web, giving them powers — a trait that has inspired many engineers and roboticists alike.

Now, a research team has identified a new way to leverage the principles of spiders’ joints to power articulated robots without using any huge connectors or components, which weigh down the robot and decrease speed and portability. The research team is from the Max Planck Institute for Intelligent Systems and the University of Colorado Boulder in the United States.

The lightweight, slender and simple structures of spiders’ joints are quite incredible because they allow a robot to jump 10 times its height. The researchers’ study titled, “Spider-inspired electrohydraulic actuators for fast, soft-actuated joints” was published in the Advanced Science journal at the end of May 2021.

Spider-inspired Electrohydraulic Soft-actuated joints — or SES joints for short — enable this excellent performance. These joints can be utilized in several different configurations, and not just when producing an arachno-bot.

In their new article, the team demonstrated a multi-segmented artificial limb, a bidirectional joint and a three-fingered gripper, which can effortlessly pick up fragile objects. All creations have a simple design, are lightweight and display high performance, rendering them perfect for robotic systems that need to move quickly and communicate with diverse surroundings.

The investigators designed their SES joints based on the HASEL technology, which was already developed by the team to construct artificial muscles. SES joints imitate a spider-inspired exoskeletal mechanism composed of both stiff and softer elements, which work similarly to the leg extension of the animal by using hydraulic forces.

The researchers designed a flexible pouch composed of thin plastic films (either polypropylene or polyester) which they later filled with a liquid dielectric — a vegetable-based oil. They subsequently placed electrodes on both sides of the flexible pouch.

These liquid-filled pockets act as actuators, where the hydraulic power is produced via electrostatic forces. This pouch is eventually fixed to a rotary joint. Upon applying a high voltage between the electrodes, the electrostatic forces cause the liquid dielectric to move within the pouch and the joint to flex. The SES joints can rotate up to 70°, causing high torques, and can effortlessly restore to the starting position.

The SES joints are very simple and light, as there are no peripheral components which weigh down the robot. Many applications for soft robots require versatile actuators. These spider-inspired joints allow for high functionality and consume only little power, they are easy and cheap to make — the plastics we are using are for food packaging — and their production is easily scalable.

Christoph Keplinger, Director of Robotic Materials Department, Max Planck Institute for Intelligence Systems

“These are all qualities that are critical for the design of robots, which can move in many different ways and manipulate a variety of objects without breaking them,” added Keplinger.

One application was a three-fingered gripper for which the researchers used SES joints to demonstrate their versatility. If the researchers had fitted the gripper with a muscle-like structure, it would have got in the way of the object that the gripper is grabbing. Using SES joints as the hinges of the gripper needed relatively less space.

The research stands out because we can use a wide variety of materials, even the plastic used to make chips bags to create the pouches. That way we can implement SES in a wide variety of geometries with specifically tuned actuation characteristics.

Nicholas Kellaris, Study First Author, Max Planck Institute for Intelligence Systems

Philipp Rothemund, the second author of the publication, added, “The ultimate goal of our research was not to make a spider robot. We wanted to develop a state-of-the-art, active joint that you can put in any type of robot.”

Particularly, for small-scale robotic systems measuring just a few centimeters in size, where the limited space considerably restricts the choice of actuator technologies, the SES-joints will prove to be highly useful. For the soft robotics community, this breakthrough is indeed a step forward.

Video Credit: Max Planck Institute for Intelligent Systems.

Journal Reference:

Kellaris, N., et al. (2021) Spider-Inspired Electrohydraulic Actuators for Fast, Soft-Actuated Joints. Advanced Science. doi.org/10.1002/advs.202100916.