Engagement Enhancement Based on Bayesian Optimization for Adaptive Assist-as-Needed Controller

Assist-as-needed (AAN) control strategy is applied in robot-aided therapy to promote the subject's engagement with minimal assistance. Current methods are based on physical measurements or employ a model to estimate the subject's engagement, which indirectly reflects the physiology of the subject or needs a complicated modeling process. Besides, manual adjustments are required to tune the parameters of the AAN controller to adapt to the estimated engagement of subjects. Such two drawbacks of previous studies have limited their effectiveness for promoting the subject's engagement during the training. In this paper, we propose an engagement enhancement method based on Bayesian Optimization for the adaptive AAN controller. Firstly, we evaluate the subject's engagement by using sEMG-based muscle activation, which directly relates to the physiology of the subject and eliminates the need for complicated modeling of engagement. Next, Bayesian optimization, which is tolerant of noisy measurements and human adaptation, is used to search for the optimal parameter of the AAN controller for each training trial. Finally, we conduct a study with sixteen healthy human subjects to verify the effectiveness of the proposed method. The experimental results showed that subjects' engagement in the experiment group improved more significantly and was maintained at a higher level than that in the control group during the training. The performance improvement after the training obtained in the experiment group was higher than that in the control group (45.42% v.s. 30.40%). The experimental results indicate that the proposed method may be promising for transferring to the rehabilitation of post-stroke patients.