A Wearable Device for Real-Time Motion Error Detection and Vibrotactile Instructional Cuing

We have developed a mobile instrument for motion instruction and correction (MIMIC) that enables an expert (i.e., physical therapist) to map his/her movements to a trainee (i.e., patient) in a hands-free fashion. MIMIC comprises an expert module (EM) and a trainee module (TM). Both the EM and TM are composed of six-degree-of-freedom inertial measurement units, microcontrollers, and batteries. The TM also has an array of actuators that provide the user with vibrotactile instructional cues. The expert wears the EM, and his/her relevant body position is computed by an algorithm based on an extended Kalman filter that provides asymptotic state estimation. The captured expert body motion information is transmitted wirelessly to the trainee, and based on the computed difference between the expert and trainee motion, directional instructions are displayed via vibrotactile stimulation to the skin. The trainee is instructed to move in the direction of the vibration sensation until the vibration is eliminated. Two proof-of-concept studies involving young, healthy subjects were conducted using a simplified version of the MIMIC system (pre-specified target trajectories representing ideal expert movements and only two actuators) during anterior-posterior trunk movements. The first study was designed to investigate the effects of changing the expert-trainee error thresholds (0.5 degrees, 1.0 degrees, and 1.5 degrees) and varying the nature of the control signal (proportional, proportional plus derivative). Expert-subject cross-correlation values were maximized (0.99) and average position errors (0.33 degrees) and time delays (0.2 s) were minimized when the controller used a 0.5 error threshold and proportional plus derivative feedback control signal. The second study used the best performing activation threshold and control signal determined from the first study to investigate subject performance when the motion task complexity and speed were varied. Subject performance decreased as motion speed and complexity increased.