A New Twist in Soft Robotics

The sensors and machine learning algorithms that control robots are advancing at a rapid pace, so we should expect to see robots playing an ever larger role in our lives in the years to come. As they become more common, humans will naturally have many more interactions with them than they do today. We may not be there yet, but it is important that we start planning now for that eventuality.

When that time comes, we will certainly hope to find that developers have followed Isaac Asimov's first law of robotics — namely, that a robot must not injure a human being. That is difficult to ensure when working with traditional robotics technologies, however. The rigid components and powerful motors they use can seriously injure a person on accidental contact, even if it is just a minor bump from the perspective of the robot.

The still-nascent field of soft robotics has the potential to alleviate many of these concerns. By making robots soft and flexible, the possibility of serious injuries is significantly reduced. Unfortunately, our most capable technologies are rigid, so there is much work yet to be done before we will have fleets of practical soft robots zooming about.

Soft actuators, in particular, are challenging to develop. Recent work published by a trio of researchers at Northwestern University may soon put an important new tool in the hands of soft robot developers, however. They have created a flexible and practical soft actuator that has many properties similar to natural muscle tissue. It is soft and stretchy, yet when contracted, it stiffens up so that it can lift a heavy load or be used for locomotion. Unlike previous soft actuators, the system is simple to manufacture, inexpensive, small, and lightweight.

The key to the actuator’s success is tiny structures called handed shearing auxetics (HSAs). HSAs have unique properties that cause them to extend or contract, much like a muscle, when they are twisted. Previous efforts have required two to four HSA cylinders, each twisted by its own servo motor. This arrangement made them bulky, heavy, and impractical for many applications. But in this present work, the researchers came up with some clever tricks that enabled them to actuate a single HSA with only one servo motor.

The team first 3D-printed cylindrical objects containing HSAs with a standard desktop 3D printer using a rubber material called thermoplastic polyurethane. This makes them much softer and easier to actuate than traditional HSAs made with plastic resins. Moreover, a flexible bellows shaft was added to the design. The servo turns this bellows shaft, which is in turn connected to the HSA. This enhances the mechanical properties of the device, and enables the transmission of torque from one end of the actuator to the other. This not only enhances performance, but also limits fatigue to extend the life of the system.

The actuator was incorporated into both a worm-like crawling robot and an artificial bicep. By changing the direction of rotation, the actuator can move in two directions. This allowed the worm-like robot to crawl through a pipe-shaped maze, and also go back to return to its starting point. It also allowed the artificial bicep to lift loads like natural muscle tissue.

The lead researcher involved in this project believes that these actuators will inspire innovation, noting that “robots that can move like living organisms are going to enable us to think about robots performing tasks that conventional robots can’t do.” Only time will tell if that pans out in reality, but this is certainly a promising technology.