Controlling sensation intensity for electrotactile stimulation in human-machine interfaces.

A barrier to practical use of electrotactile stimulation for haptic feedback has been large variability in perceived sensation intensity due to changes in the impedance of the electrode-skin interface, such as when electrodes peel or users sweat. Here, we show how to significantly reduce this variability by modulating stimulation parameters in response to measurements of impedance. Our method derives from three contributions. First, we created a model between stimulation parameters and impedance at constant perceived sensation intensity by looking at the peak pulse energy and phase charge. Our model fits experimental data better than previous models (mean R2 > 0.9) and holds over a larger set of conditions (subjects, sessions, magnitudes of sensation, stimulation locations, electrode sizes). Second, we implemented a controller that regulates perceived sensation intensity by using our model to derive a new current amplitude and pulse duration in response to changes in impedance. Our controller accurately predicts subject-chosen stimulation parameters at constant sensation intensity (mean R2 > 0.9). Third, we demonstrated as a proof-of-concept on two subjects with below-elbow amputations-using a prosthesis with electrotactile touch feedback-that our controller can regulate sensation intensity in response to large impedance changes that occur in activities of daily living. These results make electrotactile stimulation for human-machine interfaces more reliable during activities of daily living.