Researchers at Vanderbilt University have invented a nitrogen gas-powered thruster that is worn on the back that can correct falls as they start, restoring balance and averting potential injury.

Falls can lead to severe and fatal injuries, particularly for elderly people. According to the U.S. Centers for Disease Control and Prevention, millions of people fall each year, with one in five suffering serious injury.

Michael Goldfarb, a professor of mechanical engineering at Vanderbilt University, has first-hand experience dealing with this health risk. “As my parents grew older, both had experiences with fall risk that really limited their quality of life,” he said. “My father, in particular, had Parkinson’s disease.”

Goldfarb began thinking about new ways to correct occasional states of imbalance. Robotic exoskeletons can offer balance assistance, but have design challenges, such as having to coordinate a recovery response with the user. To avoid requiring a large frame or other structure (which would impede independence), he decided to focus on building a backpack-worn system that can apply forces directly to a person’s body using a thruster.

“This research challenge seemingly has not received much attention and therefore there were a lot of unknowns,” Goldfarb said. “Correcting a fall is a shared task between the person falling and the device helping to arrest the fall. What is the distribution of shared effort? How much force is appropriate to correct an impending fall? For how long is the force required? Can the magnitude of assistance required be provided by a compact cold-gas thruster?”

Goldfarb and fellow researchers Almaskhan Baimyshev and Michael Finn-Henry then set out to create a lightweight, cold-gas thruster (CGT) jetpack that senses when a fall has been initiated and can provide a restoring force to help the person recover his or her balance.



The prototype consists of four key features:

  • Gas thruster subsystem: The compressed gas tank and converging-and-diverging nozzle use nitrogen because the non-toxic gas has a high molecular weight (which increases thrust) and is available at a low cost.
 
  • Gas valve subsystem: A custom high-throughput, fast-acting solenoid valve activates the CGT in the event of an impending fall.
 
  • Nozzle servo subsystem: A servo system controls the angle of the nozzle (and therefore the thrust vector) about the axis of rotation.
 
  • Cold-gas thruster embedded system and control hardware: The embedded system provides sensing and control of the CGT prototype, including a microcontroller and six-axis inertial measurement unit.
 

To test the system, the team created a “rocking block” model as a simple approximation of a standing human. The CGT was able to restore the rocking block to its original stability when released from all initial angles well outside of its unassisted basin of stability, indicating promise as a wearable device for preventing or mitigating falls for people at fall risk.

Among the design challenges the team faced was developing a low-power, compact, very-high-throughput, fast-acting valve that can release 1,500-psi cold gas with a response time of a few milliseconds. Another challenge was detecting the onset of a fall and predicting when (and eventually in what direction) to fire the thruster to arrest the fall.

“We also wanted to confine the sensing and actuation elements of the system to the backpack,” said Goldfarb. “All sensing required to make these decisions should ideally be on the backpack—for example, detection should not depend on external cameras or sensors worn on other parts of the body.”

The CGT is a first-of-its-kind device. The team was pleasantly surprised by the control authority the thruster provides (the ability to provide substantial corrective assistance) and its soft but firm correction when arresting a fall.

“One thing I really appreciate about it is that it combines what are sometimes considered disparate fields of mechanical engineering, including fluid mechanics, nozzle design, electropneumatic valve design, dynamics of multi-link rigid-body systems, and control theory,” Goldfarb said. “The work has also entailed substantial aspects of mechatronic design, particularly associated with the onboard sensing and control of the CGT.”

The team published their results in the paper, “Feasibility of a Wearable Cold-Gas Thruster for Fall Prevention” in the August 2022 issue of the ASME Journal of Dynamic Systems, Measurement, and Control.

Future Development

Goldfarb is currently developing a single degree-of-freedom system, which is directed at arresting backward falls. 

“For what it’s worth, we were told by people who are experts in satellite thrusters that we would never be able to get the magnitude of thrust we were seeking,” said Goldfarb. “Our experiments proved them wrong.” 

The team recently tried the device on human subjects and it was effective in correcting balance. A current challenge is determining what algorithm to use for detecting an impending fall in a human. “Much research has been conducted to detect a fall that has occurred, but almost none for how to detect an impending fall,” said Goldfarb. “We have enjoyed this research to date and will be delighted if our work eventually contributes to interventions that improve independence and quality of life for people who are at fall risk.”