Balance mechaism of a biped robot.....

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Hello friends. I had choosen to develop a biped robot(human walk simulation robot, two legs) as my project for my current semester and I am collecting some informations on how to make it. Can you please help me?? Can you please tell me an effective, not too dynamic, and a cheap balance mechanism of a biped robot with detection of unstable condition too. I want to only simulate the legs so the design of my robot only includes legs(no upper body parts). I do have some ideas like: 1)a ball filled with mercury and calculating the voltage at different points but its too dynamic. 2) using strain gauge below its feet. Please suggest me some ideas??