Preliminary study of the effect of user intent recognition errors on volitional control of powered lower limb prostheses

Previously developed user-intent-recognition (UIR) systems have demonstrated promising accuracy for identifying the user's locomotion mode, which is potentially useful for volitional control of powered artificial legs in ambulation. The fundamental question facing us now is whether or not the prosthesis users are safe when the UIR system is directly integrated with the intrinsic controller to operate powered artificial legs. In this preliminary study, we aimed to address this question by investigating the effect of UIR errors on the walking stability of users, wearing a UIR-controlled powered transfemoral (TF) prosthesis. First, a novel control of powered prosthesis was developed, which hierarchically connected our designed UIR system with an intrinsic controller. Three types of errors were purposely added into the UIR output at different gait phase while an able-bodied subject walked on a treadmill with the powered prosthesis. Subjective opinions were obtained to evaluate the effect of applied UIR errors on the user's walking balance. The kinematics and kinetics of the prosthetic knee were quantified while the errors occurred. The preliminary results showed that not all the UIR errors applied caused a subjective feeling of balance instability. The effects of UIR errors on the prosthesis control and user's balance depended on the gait phase when the errors happened and the amount of mechanical work applied to the knee joint caused by the errors. The results of this study could aid the future design of true bionic prostheses that enable lower limb amputees to perform various activities intuitively and safely.