# Four-legged robot to balance on two legs

Italian and Belgian engineers have developed a control algorithm to balance a four-legged robot, through which he can not fall, standing on two legs. They showed how real 90-pound robot stands on two legs without falling, and also conducted a simulation where the virtual robot of the same design managed to get a beam width of six inches. Article about the algorithm will be presented in the conference IROS 2020.

Often the four-legged robot is considered as a very passable machine, and partly because their development was stimulated by military, who planned to use them to transport cargo over rough terrain. In many respects this advantage arises from the fact that four-legged robot can adjust the position and height of each leg to the peculiarities of the relief. In some instances, the topography may be such that conventional algorithms can not cope with the calculation of the stable points of footwork and maintaining balance.

Engineers under the leadership of Claudio Semini (Claudio Semini) from the Italian Institute of technology taught the quadruped robot to cope with two special cases of difficult terrain, a control algorithm learned to stand on two support points and to go one by one thin pillar forward.

A new algorithm for balancing in the first stage considers the robot as a flat reverse of the pendulum. It has two degrees of freedom and two main parameters: the maximum growth rate and time zavalivanija, that is the time that will pass from the vertical position before falling on the floor, if the robot does not resist it. This simple model is able to describe the motion of the robot, but it is insufficient to control them, so the developers have also written an algorithm that transforms the model state into commands for a real robot.

Also the robot has an algorithm that is responsible for the movement along a narrow line. It moves one pair of legs situated diagonally, forward, and the second while hanging in the air. The algorithm calculates the symmetric motion, which minimally affect the balancing in the longitudinal plane.

The developers showed a whole bunch of algorithms in simulation, demonstrating how an accurate model of the robot overcame a four-foot beam with a width of six inches (when the diameter of the tip of the leg is four inches). They also demonstrated a real test of balancing algorithm for a quadruped robot HyQ hydraulically operated. The robot was able to not only stand on two legs alone, but to keep from falling, when the little man pushed him in different directions.

Last year, the American engineers demonstrate new skills unusual symmetrical four-legged robot ALPHRED2. In particular, they taught him to balance on two legs and two other legs to raise the box and put it on the table.