This paper proposes a novel approach for designing an assistance system via haptic joystick force feedback on an electric wheelchair using model-based shared control approach. Assistance system supports wheelchair users through haptic force at joystick which can make the user-wheelchair interaction becomes more intuitive. Firstly Tagaki-Sugeno fuzzy model is used to build an augmented model of user-wheelchair system to deal with non-linear nature of system. Unknown input observers are developed to estimate wheelchair position errors which are considered as user intention based on joystick motions. Fuzzy logic optimal controller is synthesized by Linear Matrix Inequalities method to provide assistance haptic forces feedback to user via joystick. Simulation and experimental results show that this assistance system can predict the desired motion of user and reduce user hand force thanks to haptic force at joystick.