BALANCING CONTROL OF LEG EXOSKELETON USING ZMP-BASED JACOBIAN COMPENSATION

Narong Aphiratsakun and Manukid Parnichkun

Keywords

ALEXI, exoskeleton, GCP, Jacobian, ZMP

Abstract

This paper focuses on the use of zero moment point (ZMP) concept for balancing control of the Asian Institute of Technology Leg EXoskeleton-I (ALEX-I). ALEX-I has been developed to assist patients who suffer from paraplegia or immobility due to the loss of power on lower limbs. ALEX–I has 12 degree of freedom (DOF): 6 DOF at each leg: 3 at the hip, 1 at the knee and 2 at the ankle, controlled by 12 DC motors. The balanced posture joint trajectories (set points) under ZMP criterion are generated offline. The ZMPbased set points are provided as the desired postures to ALEX-I, while the proportional, integral, and derivative (PID) control is used for compensating errors in Cartesian space based on balancing postures sensed by loadcells. Jacobian compensation control (JCC) on the other hand converts the compensated set point data into joint space. Ground contact point (GCP) is used to find the “ZMP-like in a real time. Force plate which form by placing four loadcells This GCP data is on each foot is used to obtain GCP data. then compared with the reference ZMP. Uncontrollable movement of the upper part of wearer, uncertainties of the model parameters, backlash, and joint’s gap are considered as disturbance. The joints gap, occurring because of material wearing after heavy movement of ALEX-I for long period of time, is found as significant disturbance. The difference between ZMP and GCP on x − z (frontal-sagittal) plane is used as inputs to the JCC. The 12 outputs from JCC acts as the compensated angles for the 12 joints in the roll, pitch, and yaw axes that retain the actual ZMP in the convex hull region of the supporting area.

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