Ken Korane • Contributing Editor
Researchers are taking advantage of light, yet powerful pneumatics to improve life for pediatric patients.
The Center for Compact and Efficient Fluid Power (CCEFP), a network of researchers, educators, students and industry experts, is helping advance hydraulic and pneumatic technology and developing fluid-power systems that are smaller, lighter and better-performing than anything currently available.
One CCEFP-sponsored project that aims to meet those goals, Soft Pneumatic Actuator for Arm Orthosis, has researchers at the University of Illinois at Urbana-Champaign (UIUC) and the University of Wisconsin-Milwaukee developing a new type of orthotic support powered by novel, flexible actuators.
Flexible orthosis
According to Elizabeth Hsiao-Wecksler, professor of Mechanical Science and Engineering at UIUC, the overriding goal of the project is to develop a light, soft wrist orthosis for pediatric patients who use crutches for mobility. While walking with crutches, peak loads in the wrist typically approach half of body weight and the wrists are loaded at extreme angles. Repetitive, high loads and poor wrist postures can to lead to joint pain, carpal-tunnel injuries and similar afflictions. Unfortunately, pediatric crutch users often must resort to wheelchairs when their arms cannot support their body weight as they grow and effects of these secondary injuries worsen. Ultimately this reduces mobility, fitness and quality of life.
A light (less than 1 kg), pliable, pneumatically powered wrist orthosis could selectively constrict motion to provide the needed support and transfer loads from hands to forearms during crutch use. That, in turn, would reduce transient loads and associated stresses by 50% and give wrists a more-comfortable posture, therefore lowering the risk for joint injury, explained Hsiao-Wecksler. The device would also allow for normal hand and arm movement when at rest. Such an orthosis is noninvasive and requires no customization, as it readily adapts to the contours of the wearer’s arm.
Simple modifications would also allow the orthosis to be used in auxiliary, or underarm, crutches. Thus, others with acute leg injuries or who face post-surgery recovery would also benefit from the new design.
Novel pneumatic muscles
A key to the project’s success involves the method for powering the device. To that end, researchers have developed innovative pneumatic actuators called Fiber Reinforced Elastomeric Enclosures (FREEs). FREE actuators are similar in construction and operation to pneumatic artificial muscles (PAMs), also called McKibben actuators. PAMs are fiber-reinforced, flexible elastomeric tubes that contract and generate tensile force when pressurized (typically with air) and rebound to their initial length when exhausted. Commercial McKibben actuators include the Fluidic Muscle from Festo and the Air Muscle from Shadow Robot Co.
However, typical PAMs only provide linear motion and force, explained Girish Krishnan, professor of Industrial and Enterprise System Engineering at UIUC. This project expands the capabilities of McKibben actuators by altering their construction to yield different deformation patterns, said Krishnan. Such FREEs permit motions beyond pure extension, including contraction, axial rotation and, more generally, screw motion or simultaneous translation and rotation.
FREEs are made with two sets of inextensible fibers wound in a helical fashion around an elastomer tube. The orientation, or helix angles, of the fibers can vary widely in FREE actuators. Selectively altering the helical angles yields complex deformation patterns when the actuators pressurize. In the orthosis, said Krishnan, the current design involves a spiral design that contracts and holds on to the arm, providing relatively uniform pressure and support on the forearm.
Analysis breakthrough
An important aspect of the project involves fundamental research that expands the knowledge base for understanding how FREEs work and creating modeling tools for designing them, said Krishnan. Current McKibben muscles are based on simple fiber orientations and have well-established, mechanics-based mathematical models to predict their behavior.
To help generalize the construction and operating principles for FREE actuators, the CCEFP group has developed a new method to analyze the deformation behavior of FREEs that involves kinematics and kinetostatics.
As noted above, a FREE is essentially a cylindrical hyperelastic membrane with two sets of inextensible fibers wound helically on its outer surface. When pressurized, the actuator tries to maximize its enclosed volume while the fibers maintain a constant length. Therefore, the analysis considers a volume-maximization problem with constraints due to the inextensibility of fibers, according to the researchers.
After kinematic analysis, they added a material model of the elastic tube for elastokinetic analysis of the actuator. This gives force-versus-displacement and torque-versus-rotation relationships under various pressures. This offers an alternative way to analyze McKibben actuators. But it also analyzes FREE actuators with asymmetric fiber angles for which, previously, there were no known models. This development lets users readily consider various alternatives and design the FREE geometry, construction and material to meet specific stroke and force requirements.
“This is a major contribution to the CCEFP project,” said Krishnan. Prior to this, most literature only considered McKibben actuators, which primarily contract like muscles. “We have extended this to any fiber configuration. It doesn’t limit motions to pure contraction, but also to extension, rotation, and screw and helical motion, which is what we are trying to demonstrate,” he said.
Creating a general model beyond those for McKibben actuators will let users analyze a more-general class of pneumatic actuators, making them useful for various other applications.
“Considering that these actuators are quite soft, the model predicts motion really well, within 3% accuracy,” Krishnan said. However, current methods allow a bit more leeway when predicting force, primarily due to estimated properties of the actuator’s constitutive elements.
Fabrication and testing
To make prototype FREE actuators, technicians use a custom winding machine that can interlace fibers to the elastomer tube at any required angle. Adhesive binds fibers to the tube. Several types of fibers were considered—for instance, common cotton twine gave good performance—but Kevlar proved to be the best option, Krishnan said.
He also noted there are no significant technical hurdles to larger-scale production. The designs, materials and fabrication processes are rather straightforward and can be readily expanded. Commercial ventures like Festo and Shadow have tried-and-tested methods for manufacturing flexible actuators, although those products are mainly used in high-force robotic applications, said Krishnan. “We’re trying to demonstrate more functionality beyond just force, such as different motion patterns.”
The prototype actuators will be integrated into pneumatic sleeve orthoses to assess performance, safety, comfort and efficiency. Testing with patients is slated for this spring.
To power the pneumatic sleeve, Hsiao-Wecksler’s group is also pursuing the possibility of harvesting pneumatic energy during motion by designing a small pneumatic pump that mounts in the tip of the crutch. The resulting pressurized air can be stored and used to replace an external source of pneumatic power. Prototype FREE actuators are about 15 in. long and 0.38 in. in diameter, so inflated volume is quite low, and pressure requirements are in the 30 to 40 psi range. The pumping action might also provide shock absorption and add compliance to the crutch to improve user comfort.
Future applications
Other potential applications for FREE actuators are also on the horizon. These soft, compact, lightweight, high-force and energy-storing pneumatic actuators have the potential to revolutionize portable medical assistive devices such as powered prosthetics and orthotics.
“One straightforward application we foresee is in a powered upper-extremity orthosis, say in a patient-transfer device,” said Krishnan.
Nurses or aides lifting a heavy patient could wear a pneumatic stiffening sleeve that can actually supplement the force their muscles provide. A prototype is under construction.
The capabilities of FREE actuators also lend themselves to use in robots, such as performing pick-and-place tasks, as well as reaching, gripping and twisting. They’re especially suited for applications that involve human contact. Being made of inflatable materials, they can easily interact with humans and cause no harm, even if there is unforeseen impact, said Krishnan.
McKibben-like actuators that spiral or rotate might someday even replace conventional pneumatic motors and rotary actuators. Given the wide range of possible motions, engineers are only scratching the surface of potential applications.
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