Modular SMA Ankle Rehabilitation System
Current research line
Modular SMA actuation for early ankle rehabilitation
This project investigates whether Shape Memory Alloy wires can provide compact, quiet, and biomimetic actuation for controlled ankle mobilization. The system uses two artificial muscles in an agonist-antagonist arrangement to generate dorsiflexion and plantarflexion through tendon-based transmission.
The intended use is controlled mobilization during early rehabilitation while a patient is seated or in bed. The prototype is not designed for gait assistance and has not undergone clinical validation.
Why this problem matters
Early rehabilitation can require repetitive joint mobilization before a patient is ready for standing or walking. Conventional electric actuation can provide accurate motion, but may add mass, rigid transmission, and mechanical complexity close to the body.
SMA wires offer a different design space: high force-to-weight ratio, silent operation, and muscle-like contraction. Their use is challenging because heating and cooling introduce hysteresis, changing dynamics, and the possibility of thermal accumulation. The project therefore treats thermal recovery and actuator coordination as central control requirements.
System architecture
Mechanical layer
Modular tibial and foot units, sagittal-plane ankle rotation, adjustable fit, tendon routing, and controlled pretension.
Actuation layer
An antagonistic pair of SMA actuators reproduces the functional relationship between opposing muscle groups.
Supervision layer
State-based activation coordinates EXT, FLEX, and IDLE periods while embedding safety and recovery rules.
Research workflow
The validation strategy progresses from component behavior to system endurance:
- Establish mechanical pretension and usable angular response.
- Characterize relaxed and activated actuator behavior.
- Evaluate cyclic consistency under repetitive reference tracking.
- Test long-duration operation and analyze performance batch by batch.
- Separate tracking evidence from safety claims and clinical interpretation.
Current evidence
The latest experimental suite evaluates global tracking error, signed bias, maximum error, state occupancy, switching behavior, and long-duration stability. In the updated master’s-thesis analysis of the 40-minute endurance experiment, the system maintained a global RMSE of approximately 0.220° over the final active analysis window.
0.220° updated thesis RMSE
0.167° endurance MAE
−0.029° signed bias
1.384° maximum error
These results support the feasibility of the control architecture on the current laboratory prototype. They do not establish clinical effectiveness or safety for patient use.
The earlier Bilbao publication used the analysis window and processing available when the conference paper was prepared. Its reported values therefore remain distinct from these updated thesis metrics and should not be treated as interchangeable.
Research context
The work is developed at Universidad Carlos III de Madrid within the RoboticsLab research environment. It is part of the context of Robótica blanda para la rehabilitación del tobillo (SRAR), R&D grant PID2023-149141OB-I00, funded by MCIN/AEI/10.13039/501100011033 and FEDER/EU.
The project page intentionally documents my research contribution and prototype development rather than serving as an official SRAR project page.
Next steps
- Extend sensing while preserving a clear safety hierarchy.
- Characterize thermal behavior more directly.
- Evaluate broader motion profiles and participant-specific fitting.
- Progress from engineering validation toward appropriately approved human studies.