project specification

Cyberlegs

An actuated knee-ankle prosthesis based on a variable stiffness actuator with energy transfer from the knee to the ankle. This project allows a good approximation of the joint torques and the kinematics of the human gait cycle while maintaining compliant joints and reducing energy consumption during level walking. This prototype has a passive knee and an active ankle, which are energetically coupled to reduce the power consumption. Cyberlegs (CYBERnetic Lower-Limb Cognitive Ortho-prosthesis ) provide the necessary torques at the ankle and the knee for a 80kg person walking on level ground and transfers energy from the knee to the ankle to reduce the work of the motor.

Specifications

Maximum weight<5 kg (w/o batteries/control unit)
Maximum assistance50 % normal-cadence torque
Peak hip flexion-extension torque35 Nm
Link inertia<10 % of human thigh inertia
Target user weight80-85 kg
Pelvis width350 - 440 mm
Backside support120 - 175 mm
Thigh link length310 - 370 mm

Overview

This project allows a good approximation of the joint torques and the kinematics of the human gait cycle while maintaining compliant joints and reducing energy consumption during level walking. This prototype has a passive knee and an active ankle, which are energetically coupled to reduce the power consumption.

Cyberlegs (CYBERnetic Lower-Limb Cognitive Ortho-prosthesis ) provide the necessary torques at the ankle and the knee for a 80kg person walking on level ground and transfers energy from the knee to the ankle to reduce the work of the motor. 

The stiffness of the ankle joint can be adjusted to better fit the needs of different amputees. The total weight of the prosthesis is under 5kg, electronics included but without batteries. This is less than the weight of an actual leg and can be further reduced by optimizing the design. 

Ankle

A human ankle produces energy during walking. To mimic this behavior with the prosthesis, the ankle must also have an energy source.  A solution to this is to connect an electric actuator to the joint. For this prosthetic a MACCEPA actuator (see images), a variation on previous designs of this variable compliance actuator used for biologically inspired robots, is designed for the ankle joint.

Knee

When a person walks, a knee joint primarily dissipates energy. One of the periods of time where energy dissipation occurs is when the knee slows the lower leg down at the end of the leg swing phase. In most passive knee-prostheses, this energy is dissipated by using a damper. If this energy is stored, for example in a spring, it is not dissipated and it can be usedin an other phase of the gait cycle. This is the purpose of theenergy transfer mechanism in the knee joint of the prosthesis. The stored energy will be used in the ankle to reduce the torquethat the motor has to provide. The prosthetic knee should also provide a good approximation of the torque behavior of a healthy knee. 

The knee behavior can be subdivided  in  two  parts:  

  • the weight acceptance phase, characterized by a high joint stiffness
  • the  flexion  phase, where there is a high knee flexion of about 60◦ and a low torque to prevent the leg from collapsing. 

The knee behavior can roughly be approximated by using two springs placed between the lower leg and the upper leg. One spring provides all of the negative torques in a way that the other spring will only be loaded in one direction, which aids mechanical construction. The stiff spring used for the weight acceptance must be disengaged after the weight acceptance phase so the knee can flex and provide enough ground clearance for the swing phase. This requires a locking-mechanism to unlock the spring at one side so that it no longer exerts a torque around the knee. 

Between the end of the weight acceptance and maximum flexion, a higher torque is needed around the knee joint to prevent the knee joint from collapsing at this point during stance phase. A second locking mechanism locks in another stiff spring, placed between the knee and the ankle. This  energy transfer mechanism provides the necessary stiffness at the knee and, because it is connected to the ankle, transfer stored energy to the ankle where it can be used for push-off.

Prosthesis design

The prosthesis has a 200W Maxon motor and 14:1 gearhead which are placed in parallel with the lower leg shank. A helical gearing with another 10:1 reduction connects the motor to the two ankle joint moment arms. The spring is placed centrally between the moment arms for a symmetrical load. The power of the motor is this high to be able to compare operation with and without energy transfer, and to maintain a high enough speed during the swing phase despite the high gear ratio. The  8W precompression motor from Maxon and 1621:1 gearhead are placed below the spring. It is attached to an ACME lead screw with a 3 mm pitch which compresses the spring and is located in the ankle axis.

References

Describes the working principle and simulation of the ankle actuator, and does the same for the knee mechanism and the actual realization of the concept is explained,

J. Geeroms, L. Flynn, R. Jimenez-Fabian, et al.

Describes the background of the project, its system design, and experiments with amputees. Describes the results and discussion.

Luka Ambrozic, Maja Gorsic, Joost Geerom, et al.

Describes the project, the knee design and simulation principles, preliminary data, conclusion and future work.

Joost Geeroms, Bram Vanderborght, Rene Jimenez Fabian, et al. 2015