Abstract—Paraplegia means immobilization of the lower body as a result of damage to the spinal cord. Rehabilitation of paraplegics become and important research topic to help the patients in restoring and improving their quality of life. This paper focuses on the position control design of an assistive robot’s sit-stand mechanism developed for people with paraplegia to help them in transforming their position from sitting to stand and vice versa. Two control techniques were proposed and compared, including conventional PID and PID integrated with non-linear observer known as sliding perturbation observer (PIDSPO). The control algorithms were designed and then implemented in MATLAB/Simulink environment. The simulation results exemplified that PID is a linear control, therefore, when controlling the system with uncertainties and disturbances, the system response experience oscillations and overshoot. On the other hand, PIDSPO is a robust control with a linear control algorithm integrated with a non-linear compensator shows better performance than PID even in the presence of external disturbances.