Closed-loop Control using Electrotactile Feedback Encoded in Frequency and Pulse Width
Jakob Lund Dideriksen, Irene Uriarte Mercader, Strahinja Došen
Abstract
Sensory substitution by electrotactile stimulation has been widely investigated for improving the functionality of human-machine interfaces. Few studies, however, have objectively compared different ways in which such systems can be implemented. In this study, we compare encoding of a feedback variable in stimulation pulse width or stimulation frequency during a closed-loop control task. Specifically, participants were asked to track a predefined pseudorandom trajectory using a joystick with electrotactile feedback as the only indication of the tracking error. Each participant performed eight 90 s trials per encoding scheme. Tracking performance using frequency modulation enabled lower tracking error (RMSE: Frequency modulation: 0.27 ± 0.03; Pulse width modulation: 0.31 ± 0.05; p < 0.05) and a higher correlation with the target trajectory (Frequency modulation: 83.4 ± 4.1%; Pulse width modulation: 79.8 ± 5.2%; p < 0.05). There was no significant improvement in performance over the eight trials. Furthermore, frequency-domain analysis revealed that frequency modulation was characterized with a higher gain at lower error frequencies. In summary, the results indicate that encoding of feedback variables in the frequency of pulses enables better control than pulse width modulation in closed-loop dynamic tasks.