The Open-source Robotic Leg Prosthesis: Specifications

The overarching objective of the Open-source Robotic Leg Prosthesis project is to provide a ubiquitous hardware system that will facilitate growth in the area of prostheses controls. To achieve this objective, the OSL needed to have high performance coupled with wide accessibility, which led to five main characteristics being developed to guide the design for the OSL:

  1. Simple: components of the leg do not require high-precision machining and are easily assembled and disassembled. All components are machined by a single manufacturer.
  2. Portable: the prosthesis is lightweight and does not require a tether to a power supply. Onboard batteries can be mounted on each joint such that testing can take place outside the laboratory.
  3. Scalable: the knee and ankle joints operate independently, enabling researchers to test each joint individually. Control strategies can be implemented in a single embedded system that operates both joints.
  4. Customizable: the knee contains a series elastic actuator, which allows varying amounts of series elasticity. The ankle can use both low-profile and flat feet, giving researchers flexibility while still providing a common platform.
  5. Economical: the prosthesis is estimated to cost $10,000-$25,000, depending on degrees of freedom, series-elastic configuration, and sensing options.

Features of Both Joints

  • Aluminum housings contain timing pulleys and provide structural support for the prosthesis
  • To ensure that belts stay in tension, custom end caps for gear shafts were designed with non-concentric circles such that each shaft is shifted away from the other two. Go to step XX on the How to Assemble page for a more in depth description of how the caps function. 
  • Include space for all electronics, creating a self-contained, portable prosthesis

Features of the Knee Joint Prosthesis

Transmission:

  • Transmission ratio of 49.4:1, which consists of a three-stage belt drive containing a 2 mm pitch stage and two 5 mm pitch stages
  • Mechanical hard stops incorporated into housing to prevent rotation into biomechanically unsafe positions

Selectable Series Elasticity:

  • Selectable series elasticity (SEA) feature of knee joint allows for varying knee torques for a given angle
  • Up to six custom torsional spring disks can be inserted into the knee to change the knee torque for a given angle.
  • Each disk is 4.3 mm thick with 24 radially cantilevered beams that can deflect up to 15°
  • Peak von Mises stress of approximately 250 MPa (about half of yield stress) when disk is maximally deflected
  • Each spring disk has a mean stiffness of 97±20 Nm/rad, resulting in a total stiffness between about 100-600 Nm/rad in the system, depending on how many springs are used. 
  • Spring disks are stacked inside 3rd stage pulley for no added volume

Features of the Ankle Joint Prosthesis

  • Consists of two 3 mm pitch belt stages and a four-bar linkage with a 30° range of motion
  • Utilizes a kinematically varying transmission ratio, as can be seen in the figure to the right
  • Stays within desired transmission ratio range of 40:1 – 60:1 for most range of motion
  • Rotation of the linkage mechanism is driven by the second stage pulley and results in the rotation of the ankle
  • Angle of linkage rocker corresponds to angle of ankle joint
  • Linkage rocker couples transmission to prosthetic foot
  • Ankle does not contain series elasticity, although linkage rocker can be coupled to carbon fiber foot, which provides some of the benefits of series elasticity