A Friction Compensation Strategy for an Upper Limb Exosuit∗

Cable-driven exosuits are gaining attention in the upper limb assistive scenario thanks to their portability, comfort, and low cost. In order to guarantee an effective and transparent use of the device, the interaction force between the arm of the user and the supporting cable needs to be carefully tracked. In this work, we proposed and identified a Bowden cable transmission model and we implemented three different direct force controllers: SL) sensorless model-based controller, PI) proportional-integral controller, PIM) combining the PI and the model-based terms. The three control strategies were tested on a motorized mannequin and results show that the lowest tracking error when supporting the mannequin arm with 50% gravity compensation was achieved by the PIM strategy (RMSE = 6.9 N). The SL strategy also achieved a good performance (RMSE = 8.8 N) while minimizing oscillations around the reference force. The PI controller achieved the worst performances both in terms of RMSE (18.2 N) and oscillating behaviour. This work outlined two possible strategies for two different use-cases: 1) a sensor-based strategy (PIM) when the use-case requires high tracking precision, 2) a sensorless strategy (SL) when the use-case requires a more robust, simple, and cost-effective device.