LIMBS International Application Story

LIMBS International, a nonprofit dedicated to helping amputees regain their mobility, together with a United States Naval Academy Capstone Engineering Team, worked on a project with the vision of helping amputee children in developing countries walk again. The project involved designing and testing a low-cost pediatric prosthetic foot made of resin and carbon fiber. To validate the prototype and certify it under ISO 10328 standards, the project underwent fatigue testing using ORCA™ Motors to recreate the heel-to-toe rolling motion of a child walking. The actuators operated over 1.5 million load cycles with variable forces reaching 450 newtons (N), applying controlled, repeatable forces to the heel and toe in sequence.

Definitions

• Compliance: In this context, compliance allows the motor to yield dynamically under force, like a spring, enabling adaptive interactions with objects, surfaces, and people [1]. 
• Fatigue Testing: A form of measurement to evaluate a material’s durability by examining how it performs under continuous cycles of stress, tests typically operate until failure or sample weakening. Fatigue can encompass; tension-tension, tension-compression, compression-compression, and shear-shear cycles. In this application, compression-compression fatigue testing is being studied [2]. 
• Force Feedback: The imitation of real-world physical interaction within simulated environments, accomplished with active motorization or adaptive resistance [3]. 
• Kinematic Mode: A built in mode for ORCA motors, Kinematic Mode is the simplest way to achieve smooth point-to-point position control. It is most useful when a user’s goal is to smoothly move the motor between position targets, allowing you to save complex motion profiles for headless operation [4].
• Sinusoidal Signal: A sinusoidal signal is a smooth, repeating waveform, where the amplitude gradually rises and falls over time [5].

The Challenge: High Force Requirements, 1.5+ Million Cycle Usage, in a Time Constrained Project

An important requirement for the U.S. Naval Academy’s project was fatigue testing the design under conditions that closely replicated the forces a child imparts while walking. This required building a custom test apparatus capable of applying controlled loads over 1.5 million cycles without interruption and continuing operation beyond that. Despite being critical to the project, the team could not invest months in developing a fatigue testing system. They required a proven linear drive system that was quick to integrate and offered plug-and-play functionality. The actuation system needed programmable motion profiles, easy integration with their pre-existing software, and minimal custom programming. With a long-term test unit that has been running continuously for three years and nearly 350 million cycles, the fully integrated ORCA product line was an ideal choice for this test.

Cost-effectiveness was also essential to the project, as the goal was to create accessible pediatric prosthetics. ORCA’s low-maintenance design allowed more testing time and less downtime, giving the team confidence that reliability wouldn’t limit their research. To expose potential weak points in the prosthesis, the team ran long-term cyclic tests that synchronized loads at the heel and toe to simulate natural gait. This required a system capable of applying controlled, phase-accurate loads to each location in an alternating sequence while maintaining tight force and position repeatability. Synchronous motors with pre-programmed motion profiles recreated the heel-to-toe sequence accurately, and closed-loop position control ensured that timing and amplitude errors did not affect fatigue results.

ORCA-15-48V Linear Actuator as an Ideal Solution

Precise, Highly Repeatable Force Control for Long Duration Fatigue Testing

Integrated force sensing and feedback built into each ORCA actuator enabled the team to monitor forces in real time. Repeatable position sweeps with force feedback provided data demonstrating how the maximum and average forces changed over long runs, providing visibility into whether the prosthetic foot was losing stiffness as cycles accumulated. The compliance of the ORCA motors allowed these tests to run millions of cycles while ensuring the protection of the prosthetic during high-load operation. The prosthetic foot, made of carbon and resin, is stiff; therefore, it was important that the testing apparatus applied forces in a compliant manner, as a strictly position-based actuator could break the prototype.

Multiple prosthetic feet were scheduled for testing, each expected to survive more than three million cycles. Repeatability was a critical factor for the success of this testing, to protect the validity of the data and minimize variance in testing parameters. The rugged durability, high repeatability, and closed-loop precision of the ORCA motors provided the confidence the U.S. Naval Academy needed to ensure the tests could run for their months-long duration while providing accurate data.

Technical Specifications

Integration

The integrated ORCA design enabled rapid setup and commissioning. The U.S. Naval Academy designed a custom vertical frame for their fatigue testing, built to mount two ORCA-15-48V actuators and the prosthetic foot. The actuators were positioned at angles relative to the prosthesis, as specified by ISO 10328. Each ORCA delivered controlled loads between 50 and 430 N, reaching peak forces of 450 N within two inches of stroke. Communication and control were handled through MATLAB, where pre-programmed motion profiles synchronized the heel and toe loading sequence.

As the ORCA motors have only a single moving part, the stainless steel shaft, maintenance requirements are minimal and limited to periodic bushing replacement. The complete setup was validated against ISO 10328 standards, ensuring the fatigue test met international certification requirements.

Final Thoughts

The U.S. Naval Academy’s work required actuators that were quick to implement, precise, and repeatable in their operations to withstand millions of test cycles with minimal downtime. ORCA motors provided a balance of high performance and ease of use. Their all-in-one design saved the team valuable development time, both in sourcing, calibrating, and integrating the components and in eliminating the need for custom programming, allowing the system to be operational within weeks. With reliable force feedback, compliance to protect stiff prototypes, and minimal maintenance requirements, the actuators supported accurate, long-duration fatigue testing.