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 Open-source Leg:
- 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.
- 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.
- 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.
- 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.
- Economical: the prosthesis is estimated to cost $10,000-$25,000, depending on degrees of freedom, series-elastic configuration, and sensing options.
Open-source Leg v1.0
Knee
- 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 (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
Ankle
- Utilizes a kinematically varying transmission ratio within the range of 40:1 – 60:1 for most range of motion.
- Consists of two 3 mm pitch belt stages and a four-bar linkage with a 30° 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
Open-source Leg v2.0
The knee and ankle of the Open-source Leg v2.0 are mechanically identical and support series elasticity. The transmission ratio is 45.0 and has an efficiency of around 77%. The knee has a range of motion of 120 degrees and the ankle has a range of motion of 60 degrees (+- 30 degrees). They can be configured with either 5mm or 3mm GT3 belts.
More information will be added soon.
Open-source Leg Dataset
Our mission is to foster research on prosthetic control strategies through the use of an open and common hardware platform. In addition, we seek to provide data available from our testing to help researchers gain a deeper understanding of use of the OSL. Dr. Levi Hargrove and our collaborators at the Shirley Ryan AbilityLab tested the OSL on three people with transfemoral amputation, ambulating through a circuit with different activities. These activities included level ground walking, ramp ascent, ramp descent, stair ascent, and stair descent, as well as seamless transitions between activities. The OSL was used with impedance control and tuned by prosthetists and physical therapists to meet a set of desired clinical ambulation goals. The captured data provide a window into what can be recorded with the OSL, and how a researcher-tuned impedance control will govern leg mechanics during gait.
Click here to download the data from these tests. We have also included a MATLAB script that will automatically plot all of the sensor data. Sensors include: IMUs, joint angles/velocities, shank/thigh angles, vertical ground reaction force, loadcell, motor current, and impedance control parameters.
More information is available in our publication:
A F Azocar, L M Mooney, J F Duval, A M Simon, L J Hargrove, and E J Rouse, “Design, Control, and Clinical Implementation of an Open Source Bionic Leg,” Nature Biomedical Engineering.
