Design Principles for Compact, Backdrivable Actuation in Partial-Assist Powered Knee Orthoses

Area of Science:

• Biomedical Engineering
• Rehabilitation Robotics
• Biomechanics

Background:

• Musculoskeletal disorders impair lower-limb function, limiting daily activities.
• Existing orthoses often lack sufficient backdrivability, hindering natural movement.
• Powered knee orthoses offer potential for restoring mobility and reducing muscle strain.

Purpose of the Study:

• To design and validate a backdrivable powered knee orthosis for assisting lower-limb musculature.
• To prioritize backdrivability, output torque, and compactness in actuator design.
• To reduce quadriceps activation in users during functional tasks.

Main Methods:

• Designed a custom Brushless DC motor with encapsulated windings for improved torque density.
• Integrated a 7:1 planetary gearset into the motor stator for compactness.
• Validated actuator performance through benchtop testing (peak/continuous torque, backdrive torque).
• Conducted human subject experiments (N=3) to assess muscle activation during functional tasks.

Main Results:

• The custom actuator achieved 23.9 Nm peak and 12.78 Nm continuous torque.
• Backdrive torque was measured at less than 2.68 Nm under walking conditions.
• Experiments showed reduced quadriceps activation with bilateral orthosis assistance.
• The transmission produced negligible acoustic noise.

Conclusions:

• The designed backdrivable powered knee orthosis effectively assists lower-limb function.
• The actuator design successfully balances backdrivability, torque, and compactness.
• This technology has the potential to improve daily activities for individuals with musculoskeletal disorders.