Powered Prosthetic Leg for Restoring Natural Locomotion and Mobility Across Tasks


    Current passive prostheses are used to alleviate the effects of amputation, but they are limited in their ability to completely restore functionality because such mechanisms may only dissipate energy that the user introduces. In the case of above-knee amputees, many passive prosthetic devices aim to mimic normal walking conditions but typically leave the amputee with an asymmetric gait. Significantly, most passive prostheses fail to address functionality across tasks, such as sit-to-stand or stair ascent/descent, which further impedes the amputee's ability to regain quality of life.

    The presented design is a powered knee-and-ankle transfemoral prosthetic leg implementing high-torque, low-impedance actuation that offers several benefits over actuation styles used in modern robotic prosthetic legs. By actively injecting energy into the user's gait, this powered prosthesis is capable of restoring mobility to those who live with limb loss. This powered prosthesis facilitates a more natural user gait by providing such benefits as: free-swinging knee motion, compliance with the ground, negligible unmodeled actuator dynamics, and greater potential for power regeneration through backdrivability. The device is designed towards enhancing functionality with user comfort in mind, minimizing the components used to be lightweight (6.05 kg) while maintaining adjustability for different users and applications.

Figure 1: Final assembly of the prosthetic leg. The image on the left displays a CAD rendering, and the image on the right displays the physical system after assembly. 


Technical Summary:

    High-torque, low-reduction-ratio actuators can have several benefits for control and efficiency of robotics legs. These actuators have a lower mechanical impedance (inertias and frictions) which minimizes the effect of unmodeled dynamics, thereby increasing robustness and forcing the system to behave closer to an ideal model. The low actuator impedance and accurate impedance control together enable the command and control of the system torque without any torque feedback, thus eliminating the necessity for torque sensors from this design. Not only will this simplify the complexity of the mechanical and electrical system, but it can also reduces the overall cost and weight of the prosthetic leg. The transmission design implements a single-stage stepped-planet compound planetary gear transmission (SPC-PGT) coupled with a high torque-density motor - chosen for its efficiency and simplified manufacturability compared with other transmission choices used in robotic legs (e.g. harmonic or cycloid gear drives).

    Benchtop validation of the prosthesis determined the leg to be completely capable of supporting position-based control paradigms for the full range of walking speeds and some running speeds. While the actuator is capable of supporting walking control paradigms based on precise joint position tracking, it also works well for compliant walking control paradigms and is suitable for any kind of compliant control during stance. Further, walking experiments have been successfully performed, demonstrating the viability of the prosthetic leg design as an assistive device.


Figure 2: Position tracking of normative gait trajectories at various frequencies. Blue and red lines denote the desired and measured position, respectively. Plots a), c), and e) present ankle tracking at 0.5, 1.0, and 1.3 Hz respectively. Plots b), d), and f) present knee tracking at 0.5, 1.0, and 1.3 Hz respectively.


Value Proposition:

    The powered knee-ankle prosthesis design implements high-torque density actuators with low-reduction transmissions to enable functionality across various tasks while improving user comfort and quality of life. This compact device is designed using a minimal number of components to support the full range of walking speeds (and some running speeds) with more natural locomotion.



  • Medical devices – Prosthetics & Orthotics
  • Research platform technology - Control prototyping


Key Benefits:

  • Quiet – Transmission style reduces number of gears meshing; produces only 50 dBA of acoustic noise, comparable to a refrigerator, where prostheses with conventional actuation produce approximately 70 dBA, comparable to a vacuum cleaner
  • Adjustable - Desired range of motion and shank lengths may be modified for different use cases; Design accommodates over 91% of the population (based on height)
  • Extended Battery Life – Actuators can be backdriven with low amounts of torque, which may be leveraged for power regeneration from negative work being done on the prosthesis; 35 W of electrical power is consumed at normal walking speeds, compared to 85 W of another powered prosthesis
  • Comfortable – Effective impedance control facilitates compliance with the ground, smoothing touchdown impacts; Allows installation of a cosmetic foot shell
  • Simplified Design – Components selected for ease of manufacturing; Streamlined construction for improved user comfort with lightweight and compact design
  • Natural Gait - Free-swinging knee motion and active energy injection into the user's gait allows for more natural locomotion and improved quality of life
  • Sensorless control - Low actuator impedance and accurate impedance control enable management of the system torque without any torque feedback



Elery, Toby, Rezazadeh, Siavash, Nesler, Christopher, Doan, Jack, Zhu, Hanqi, & Gregg, Robert D. (2018). Design and Benchtop Validation of a Powered Knee-Ankle Prosthesis with High-Torque, Low-Impedance Actuators. IEEE International Conference on Robotics and Automation. Retrieved from http://par.nsf.gov/biblio/10057954

Related Link: Locomotor Control Systems Laboratory - Link

IP Status: Patent pending.

Licensing Opportunity: This technology is available for exclusive or non-exclusive licensing.

ID Number: MP-17029

Contact: otc@utdallas.edu

Patent Information:
For Information, Contact:
OTC Licensing
Robert Gregg
Toby Elery
Siavash Rezazadeh
Christopher Nesler
Energy Storage & Battery
Engineering & Physical Sciences
Life Sciences
Mechanical Systems
Medical Devices
© 2023. All Rights Reserved. Powered by Inteum