Combining Vibrotactile Feedback with Volitional Myoelectric Control for Robotic Transtibial Prostheses

Abstract

In recent years, the development of myoelectric control for robotic lower-limb prostheses makes it possible for amputee users to volitionally control prosthetic joints. However, the human-centered control loop is not closed due to the lack of sufficient feedback of prosthetic joint movement, and it may result in poor control performance. In this research, we propose a vibrotactile stimulation system to provide the feedback of ankle joint position, and validate the necessity of combining it with volitional myoelectric control to achieve improved control performance. The stimulation system is wearable and consists of six vibrators. Three of the vibrators are placed on the anterior side of the thigh and the other three on the posterior side of the thigh. To explore the potential of applying the proposed vibrotactile feedback system for prosthetic ankle control, eight able-bodied subjects and two transtibial amputee subjects (TT1 and TT2) were recruited in this research, and several experiments were designed to investigate subjects' sensitivities to discrete and continuous vibration stimulations applied on the thigh. Then, we proposed a stimulation controller to produce different stimulation patterns according to current ankle angle. Amputee subjects were asked to control a virtual ankle displayed on the computer screen to reach different target ankle angles with a myoelectric controller, and control performances under different feedback conditions were compared. Experimental results indicated that subjects were more sensitive to stimulation position changes (identification accuracies were 96.39 ± 0.86, 91.11, and 93.89% for able-bodied subjects, TT1, and TT2, respectively) than stimulation amplitude changes (identification accuracies were 89.89 ± 2.40, 87.04, and 85.19% for able-bodied subjects, TT1, and TT2, respectively). Response times of able-bodied subjects, TT1, and TT2 to stimulation pattern changes were 0.47 ± 0.02 s, 0.53 s, and 0.48 s, respectively. Furthermore, for both TT1 and TT2, the absolute error of virtual ankle control reduced by about 50% with the addition of vibrotactile feedback. These results suggest that it is promising to apply the vibrotactile feedback system for the control of robotic transtibial prostheses.

Keywords: human-centered closed-loop control; position control; robotic transtibial prostheses; vibrotactile feedback; vibrotactile stimulation; volitional myoelectric control.

Figure 1

(A) Vibrator and (B) placement of vibrators (V1–V6) on the thigh.

Figure 2

(A,B) show the placement of surface EMG electrodes on the residual limb to measure EMG signals from dorsiflexor and plantar flexor muscles, respectively.

Figure 3

Graphic user interface (GUI) for experiment 4. The left graph displays the virtual ankle. Top right part of the GUI shows the selected feedback type and bottom right part of the GUI shows the index and target angle of current experiment trial, as well as the time left for virtual ankle control.

Figure 4

Identification accuracies of discriminating vibration amplitude changes for different vibrators.

Figure 5

(A) Average identification accuracies (mean ± SEM) (%) over eight able-bodied subjects for different combinations of vibration amplitude and different vibrators. (B) Identification accuracies of TT1 for different combinations of vibration amplitude and different vibrators. (C) Identification accuracies of TT2 for different combinations of vibration amplitude and different vibrators.

Figure 6

Response times for different stimulation sequences.

Figure 7

Performance of virtual ankle control under different feedback conditions. (A) Absolute errors of TT1 for different target ankle angles. (B) Absolute errors of TT2 for different target ankle angles.

Figure 8

(A) Concept of closing the human-centered control loop for robotic transtibial prosthesis control. (B) The wearing of a robotic prosthesis (integrated with the systems of volitional myoelectric control and vibrotactile feedback) by a transtibial amputee subject.