A Biped Locomotion Strategy for the Quadruped Robot Sony ERS-210

The interest in biped robots has highly increased in the last few years. The availability of cheap and advanced actuators and electronic controllers allowed the realization of many biped systems. There are some very famous humanoid robots, like the SONY SDR3X, or the HONDA Asimo. But there are also a lot of other biped robots at university research level. Many control strategy have been proposed, in a rich and various scientific literature. The various works can be classified using different criteria, but we have found very useful the classification based on the dynamic features of the biped walking style. There are basically four great categories. The first one is that of the passive walking mechanism. These are not real robots, since they don’t have actuators or control hardware, but are simple passive mechanism that walk on a downslope. One of the work most widely known is that of McGeer [1, 2]; these mechanisms are very helpful to understand the basics of biped walking dynamic, since they are very simple and intuitive. The simplest method for biped walking is the one called static walking. This method is based on basic principle of static equilibrium: a rigid body is in static equilibrium if the projection on the ground of its center of gravity is within the area of contact with the ground. The control strategy consists in planning the robot’s movement so that the projection of the center of gravity is always within the area of contact with the ground, that in this case is represented by the foot. The main disadvantage of this kind of robot is that the locomotion is very slow, so there is not much interest in the static walking robots. In order to achieve a faster locomotion it is necessary to abandon the static equilibrium and rely on the dynamic equilibrium principle. The dynamic equilibrium has been applied on the biped locomotion for the first time by M. Vukobratovic, with the introduction of the Zero Moment Point (ZMP) principle [3]. The ZMP theory has then been widely used [4, 5, 6]. This method consists in the planning of the various movement of the robot in a way to assign a desired trajectory to the ZMP. The movement of the torso and of the arms is used to correct this trajectory; the objective is to keep the ZMP within the area of contact of the foot with the ground. If this goal is achieved, then a stable biped locomotion is achieved. With this technique a fast and stable locomotion can be obtained, but the robot needs many degrees of freedom and high performance actuators and control hardware. For example the humanoid robot SONY SDR-3X, based on the ZMP principle, has a total of 24 degrees of freedom. One of the main characteristic of the robots based on the ZMP is the presence of an actuated foot, absolutely necessary for the dynamic equilibrium. There is another category of robot, sometimes referred to as the purely dynamic walkers. These robots do not have an actuated foot, so they surely do not have any configuration of static equilibrium, and cannot implement a walking control based on the ZMP. There are many example of this category [7, 8, 9, 10], and there are many control strategies also. This kind of robots usually have a simpler hardware structure, and sometimes use an external support structure for lateral balancing. The robots belonging to this class are usually cheaper than the others, and walk pretty fast, since the dynamic is fully exploited.