CMU grad student designs robot leg

As we race toward a futuristic era in which robots may be employed in fire departments and hospitals, not to mention their recent entry into the corporate and domestic spheres, it would not be surprising to see something akin to the Transformers pacing our campus some day.

Carnegie Mellon graduate student Jonathan Hurst has already kicked off the probable legacy; Hurst designed a gigantic robot leg prototype that is an interesting amalgamation of software from the RHex project — a plan to develop a six-legged robot that will walk, run, and even climb stairs — with springs that generate a natural running movement. The software applied includes the QNX real-time operating system, which allows one to assess what computerized actions will take place and when. Furthermore, its prioritizing of tasks deems QNX the ideal operating system for Hurst’s invention.

Hurst said that the motive behind this innovation is to learn about “the dynamic mechanism of running” through robotic motion.

“Seeing a robot run on rough terrains is [something] yet to be explored,” Hurst said.

Popularly known as the Electrical Cable Differential (ECD) Leg in the Robotics Institute, Hurst’s latest project is designed to study the role of compliance in running.

While a conventional robot is made up of mechanical components, electrical hardware, and computer software, the rigidity of this system and sole control through software do not allow for the bouncing movements that are produced by running, Hurst said.

For example, a rotating gear motor — a typical component of the traditional robot — fails to generate leaping running movements. Similarly, a pneumatic actuator that converts potential energy into motion is often hard to control. Therefore, this standard operation cannot possibly determine the rate and motion of running.

Keeping this in mind, Hurst used large fiberglass springs and a series of cables, along with specially designed software, to understand the natural dynamics of running. In Hurst’s prototype, electric motors are attached to the leg joints using steel cables.

The steel cables enfold aluminum pulleys, which work with them in unison to produce the desired effect. Hurst tested his prototype by creating a computer simulation of the robot leg that illustrates the precise running movements of the robot.

“The simulation uses the same graphical interface and same software controllers that are used to design the prototype,” Hurst said.

Down the line, the ECD leg can also be used to explore software control methods for “biomechanically inspired running gaits,” he said.

In the human leg, the fiberglass springs are analogous to tendons or sinews, which link the muscle to bone. The contraction of antagonistic muscles (muscles that counteract each other’s movements) is a possible example of how the human body makes adjustments similar to the springs.

“One potential use of the knowledge we hope to gain from this machine is to build exoskeletons and prosthetic limbs,” Hurst stated in an e-mail.

So far, wearable exoskeletons (similar to those designed for military use) have used large, high-energy actuators that exercise total control over the wearer’s movements. As a result, the exoskeletons turn out to be larger than needed, and not useful enough to achieve their maximum potential. In the case of military application, the exoskeletons are used to help soldiers carry heavier loads while traveling long distances.

“Exoskeletons to assist people with limited mobility is another upcoming application; some people who are currently wheelchair-bound may be able to walk with a relatively low-power assistive device,” Hurst stated.

Yet, the kinetics of running need to be fully understood in order to construct robotic exoskeletons that adapt to the natural running motion.

“By understanding the fundamentals of locomotion, exoskeletons could be designed to work closely with the dynamics of a specific person’s gait, assisting their movement, with relatively low power required to reduce the metabolic cost of the wearer,” Hurst stated. “I do not believe we are limited by the strength or speed of motors, only by our understanding of the gait.”

According to a Carnegie Mellon press release, Hurst works jointly with University of Michigan professor Jessy Grizzle, who received a National Science Foundation grant to study “control methods for legged locomotion.”

“Thumper,” one of the two robots that are equipped with ECD legs, is currently set up in the Robotics Institute. A single robot leg, “Thumper” will be used to investigate the role of compliance in running. The other robot, “MABEL,” stands in Grizzle’s laboratory at the University of Michigan, and will be used to investigate the concept of control in legged locomotion.