A Novel Stability Control System for Two-Wheeled Robotic Wheelchairs
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Most conventional robotic wheelchairs contain four wheels (two active driving wheels and two passive casters) which makes them statically stable. In comparison, a two-wheeled robotic wheelchair (TWRW) offers much better maneuverability, while without the support of casters, it is inherently unstable and requires a stability control. Majority of stability controllers rely on the driving torques of the wheels which are high in magnitude and results in the increase of energy consumption. Various disturbances in the system also affect the performance of the controller. In this research, these issues will be resolved through a novel control approach where the stability is kept by the motion of a pendulum-like movable mechanism added to the TWRW. The control schemes including PID control, Computed torque control (CTC), Sliding mode control (SMC), and Second-order sliding mode control (SOSMC) are developed for stability control. The model-based controllers (CTC, SMC, and SOSMC) are developed from the dynamic model established through the Euler-Lagrangian method in which the disturbances caused by model uncertainties and rider’s motion are considered. Simulation results show the stability is achieved through the proposed system with much less torque, power, and energy consumption than the conventional control system. Stability control becomes more challenging when a TWRW is also required to move in a desired direction. To rely on the wheels’ motions to achieve both stability and direction control tend to impose a large burden on the wheels’ driving motors or other types of actuators in terms of their driving torque and energy consumption. To solve these problems, the added movable mechanism is used to assist the wheels to produce control actions. The simulation results validate the effectiveness of the proposed system, where the TWRW can achieve stability and direction control in a similar pattern to the conventional system. However, the input torque, input power, and energy consumption of motors in the proposed system are much smaller than those required in the conventional approach. To verify the simulation results, the experimental results are provided, where a scaled-down TWRW is designed and modelled to evaluate the stability control systems. The experimental results confirm the results obtained from the simulation.