1. Introduction 1.1 Gait rehabilitation 1 1.2 Body weight support(BWS) system 2 1.3 Interactive treadmill(ITM) 4 1.4 Body weight support(BWS) system for interactive treadmill 5 1.5 Control strategy for conventional body weight support system 6 1.5.1 zeroG body weight support system 6 1.5.2 FLOAT body weight support system 7 1.6 Purpose of the research 8 2. Background 2.1 Series elastic actuator(SEA) 10 2.1.1 Concept of the series elastic actuator 10 2.1.2 Characteristic of series elastic actuator 11 2.2 Time delay control(TDC) 12 2.3 Time delay control(TDC) based impedance control 14 2.4 TDC based force tracking impedance control 15 3. Method 3.1 Hardware 17 3.1.1 Frame of the body weight support system 17 3.1.2 Motor and motor driver setup & specification 18 3.1.3 Load cell setup & specification 19 3.2 Proposed controller 20 3.2.1 Concept of impact absorption 20 3.2.2 Applying the concept to two-cable driven mechanism 22 3.2.3 Applying switching control method 23 4. Results 4.1 Non-human experiment, barbell test 24 4.1.1 Comparison with TDC based impedance control and PID 24 4.2 Human walking experiment, asymmetric gait 26 4.2.1 Comparison with proposed controller and PID(conventional treadmill) 26 4.2.2 Comparison with proposed controller and PID(interactive treadmill) 27 5. Conclusion and Discussion 28 5.1 Hardware and control limitation 28