This paper is focused 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. The balanced posture set-points (joint trajectories) under ZMP criterion are generated offline. The ZMP based set points are provided as the desired postures to ALEX-I. Fuzzy Logic Controller (FLC) determines the modified set points based on postures balancing sensed by loadcells. Ground Contact Point (GCP) is used to find the "ZMP-like in real time". GCP data is obtained by placing 4 loadcells forming a force plate on each foot of ALEX-I. This GCP data is then compared with the reference ZMP. Uncertainties of the model parameters, backlash, and joint tolerance are considered as disturbance. The differences of ZMP and GCP on x-z plane are used as the inputs to the FLC. The 4 outputs from FLC are the compensated angles of left and right ankles joints in roll and pitch axes that make the actual ZMP locate in the convex hull of the supporting area.

This paper proposes an approach of generating the gait pattern that Asian Institute of Technology's Leg EXoskeleton (ALEX) and its wearer can walk safely with the passing criteria of Zero Moment Point (ZMP) theorem for static and dynamic considerations, respectively. ALEX has 12 DOF (6 DOF for each leg: 3 at the Hip, 1 at the knee and 2 at the ankle), controlled by 12 DC motors. The CAD drawing and assembly of ALEX are exported directly to MATLAB's Simulink/SimMechanics simulation environment to assure accurate positioning of center of gravity (CG) and moment of inertia of each of the links that make up this 12 DOF robot. The gait pattern is visually observed in virtual environment (VR) using 3D VRML interpreter while ZMP trajectory is monitored using MATLAB's 2D graphics representation. With this developed simulation of ALEX, the robot can be tested to confirm its balance gait motion prior to the real implementation on the physical system that could cause serious damages to the robot itself and its fragile electronic devices.

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