How to enable soft robots to better mimick biological motions

A future version of Star Wars Rogue One's K-2SO robot might look less dorky if Harvard engineers designed his joints and fingers
December 20, 2016

Researchers used mathematical modeling to optimize the design of an actuator to perform biologically inspired motions (credit: Harvard SEAS)

Harvard researchers have developed a method for automatically designing actuators that enable fingers and knees in a soft robot to move more organically, a robot arm to move more smoothly along a path, or a wearable robot or exoskeleton to help a patient move a limb more naturally.

Designing such actuators is currently a complex design challenge, requiring a sequence of actuator segments, each performing a different motion. “Rather than designing these actuators empirically, we wanted a tool where you could plug in a motion and it would tell you how to design the actuator to achieve that motion,” said Katia Bertoldi, the John L. Loeb Associate Professor of the Natural Sciences and coauthor of the paper.

Designing an actuator that replicates a complex input motion. (A) Analytical models of actuator segments that can extend, expand, twist, or bend are the first input to the design tool. (B) The second input to the design tool is the kinematics of the desired motion. (C) The design tool outputs the optimal segment lengths and fiber angles for replicating the input motion. (credit: Fionnuala Connoll et al./PNAS)

The method developed by the team uses mathematical modeling of fluid-powered, fiber-reinforced actuators that can produce a wide range of motions. It optimizes the actuator design for performing specific motions (kinematics), different geometries, material properties, and pressure required for extending, expanding, twisting, and bending.

This soft actuator twists like a thumb when powered by a single pressure source (credit: Harvard SEAS)

The researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and the Wyss Institute for Biologically Inspired Engineering tested the model by designing a soft robot that bends like an index finger and twists like a texting thumb when powered by a single pressure source.

The research was published this week in the journal Proceedings of the National Academy of Sciences. “Future work will also focus on developing a model that combines bending with other motions, to increase the versatility of the algorithm,” the authors note in the paper.

In a future robot design, the model could conceivably be integrated with Cornell University’s design for soft, stretchable optoelectronic sensors in fingers that detect shape and texture.

The new methodology will be included in the Soft Robotic Toolkit, an online, open-source resource developed at SEAS to assist researchers, educators and budding innovators to design, fabrication, model, characterize and control their own soft robots.

Abstract of Automatic design of fiber-reinforced soft actuators for trajectory matching

Soft actuators are the components responsible for producing motion in soft robots. Although soft actuators have allowed for a variety of innovative applications, there is a need for design tools that can help to efficiently and systematically design actuators for particular functions. Mathematical modeling of soft actuators is an area that is still in its infancy but has the potential to provide quantitative insights into the response of the actuators. These insights can be used to guide actuator design, thus accelerating the design process. Here, we study fluid-powered fiber-reinforced actuators, because these have previously been shown to be capable of producing a wide range of motions. We present a design strategy that takes a kinematic trajectory as its input and uses analytical modeling based on nonlinear elasticity and optimization to identify the optimal design parameters for an actuator that will follow this trajectory upon pressurization. We experimentally verify our modeling approach, and finally we demonstrate how the strategy works, by designing actuators that replicate the motion of the index finger and thumb.