Designers of next-generation prosthetic devices and exoskeletons (or orthotics) are faced with the high cost and time requirements of prototyping — a process that can take years and has no way of pretesting candidate designs. One wrong choice (i.e., a spring that’s too stiff, a motor that lacks power or is too heavy, etc.) and you’re back to the drawing board — older and only a little wiser.
Clinicians face a different, but related problem: how to determine the right device for each patient. Today’s options range from those offering basic support to those with high-tech composites and embedded microprocessors but, unfortunately, there is no simple and objective way to compare these options. Clinicians make their best estimate based on their training, experience and knowledge of the patient; the patient then tries the recommended device, and everyone hopes for the best. If the device isn’t a good fit, options for modification or replacement can be limited.
It’s like buying shoes in a shop where everything is “special order.” There aren’t any samples to try on, and you’re limited to buying one style, in one size, at a time. You and the shoe expert look through the catalog together and decide what to order. You know you can probably return them if you don’t like them, but that’s inconvenient, your feet hurt, and there’s no guarantee that your next guess will be right. Plus, you’ll have to repeat the whole ordering/fitting process.
At HuMoTech, we realized that a single, innovative approach could address both problems. Using an emulator — a system that provides users with the physical sensation of wearing a candidate device before it is physically manufactured or fitted — researchers and designers would be able to pretest design concepts before prototyping. An emulation system would also mimic the properties of a variety of commercially available devices, so clinicians and patients could “test drive” products before purchase and fitting.
Of course, it’s one thing to say that a system will solve the problem and another to engineer a system that actually works. That’s where ANSYS Mechanical, which we obtained through the ANSYS Startup Program, came into play.
When we started HuMoTech in 2015, we used Mechanical to transform a prosthetic foot emulator from a research tool built by a graduate student (me) into our first product.
The HuMoTech prosthesis emulator system features a lightweight, high-performance prosthetic foot that attaches to a patient’s prescribed socket. The foot is tethered to an off-board robotic actuation and control system that sends and receives sensor signals and mechanical power to and from the foot through a flexible tether. The system responds to operator-specified parameter changes, allowing rapid, iterative changes and adjustments. The system is used in research settings now for both new product research and clinical trials.
Simulation Solutions Helped Us Put Our Best Foot ForwardSimulation provided direct benefits to the development of the HuMoTech system. The ANSYS tools greatly improved the accuracy of our finite elemental analysis (FEA) simulations and the efficiency of our computer-aided engineering workflow, which in turn enabled us to improve our product’s performance.
Several capabilities were especially important in the development of the emulator:
- Full-assembly nature of the simulations that considered preloads allowed for more realistic results.
- Mesh control and refinement sped up simulation time.
- Contact definitions, which account for interaction of multiple parts, allowed us to improve realistic stresses between parts and confidently predict real-world behavior. This was especially useful for parts with large elastic deformations.
- Fatigue analysis improved estimates of the lifetime of system components, and better predicted where failure may occur and more accurately informed engineers and technicians when parts should be serviced.
ANSYS Mechanical assembly simulations of the robotic prosthesis portion of the emulator system. Multicomponent FEA is essential for accurate assessment of contact stresses between parts, and thus key to our weight-reduction efforts.
In a recent redesign effort, the Mechanical tools enabled a 25 percent weight reduction (about 250 grams) of the prosthesis end-effector, while simultaneously improving manufacturability and reducing costs. The lighter end-effector enables the emulator to better match the weight of lightweight, off-the-shelf prosthetic devices.
We’ve found thathas also increased our productivity by saving time in setup and simulation, which in turn has allowed us to do more thorough analyses. More than ever before, we have a high degree of confidence that our components and assemblies will hold up to years of harsh cyclic loading.