Project — Medical robotics

ELSA powered ankle prosthesis

A powered ankle prosthesis that delivers 150 Nm in a foot-sized package weighing 1.2 kg, designed by AlpineLab from a blank sheet to small-series production.

Total weight: 1.2 kg
Ankle torque: 150 Nm
Envelope: Foot-sized

Challenge

An ankle prosthesis has to deliver the torque of a human ankle at every step, yet stay light enough to wear all day and small enough to fit inside a shoe. Powered prostheses usually trade one of these against the others.

Approach

AlpineLab designed ELSA from a blank sheet: actuator, transmission, structure, electronics and control. The motor drives the ankle through a belt and screw transmission. A spring mechanism in parallel with the actuator assists it during the step and disengages automatically at toe-off, while the main actuator continues the push-off. The electronics are integrated within the foot.

Exploded view of the ELSA prosthesis, showing the belt drive, screw transmission, parallel spring, electronics and foot structure ELSA prosthesis with its cover removed, showing the actuator, spring and electronics integrated in the foot

Custom dashboard

To interact with the device, AlpineLab also developed a dedicated dashboard that connects to the prosthesis over Bluetooth. It lets clinicians and engineers:

select the activity mode (standing, walking, sitting, stairs, cycling), automatically or manually
tune the controller live, such as stiffness, damping, push-off threshold and assistance level, and save the settings as user profiles
follow the gait phase and controller state in real time, with a live torque/position plot
stream and record data for analysis
calibrate the torque sensor, encoder and inertial sensors
monitor faults and stop the device instantly with an emergency stop

ELSA dashboard showing activity mode selection, controller tuning sliders, gait state and a live torque/position plot

Result

ELSA delivers 150 Nm of ankle torque for a total weight of 1.2 kg, all within the volume of a foot. The design was taken through to small-series production.

Publication

The design and validation of ELSA are published in IEEE Transactions on Robotics:

F. Heremans, J. Evrard, D. Langlois and R. Ronsse, “ELSA: A Foot-Size Powered Prosthesis Reproducing Ankle Dynamics During Various Locomotion Tasks,” IEEE Transactions on Robotics, vol. 41, pp. 415–429, 2025. doi:10.1109/TRO.2024.3508314

Exploded view of the ELSA prosthesis and its removable foot cover.

Walking sequence showing the mechanism through each phase of the gait cycle.

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