  Bipedal robots adapt to the environment of the modern society due to the
similarity of movement to humans, and therefore they are a good partner for
humans. However, maintaining the stability of these robots during
walking/running motion is a challenging issue that, despite the development of
new technologies and the advancement of knowledge, does not yet have a
satisfactory solution. In most of the proposed methods by researchers, to
maintain the stability of walking bipedal robots, it has been tried to ensure
the momentary stability of motion by limiting the motion to multiple
constraints. Although these methods have good performance in sustaining
stability, they leave the robot away from the natural movement of humans, with
low efficiency and high energy consumption. Hence, many researchers have turned
to the walking techniques that follow a certain motion limit cycle, in which we
can consider the overall stability rather than momentary. In this paper, a
method is proposed to maintain the stability of the limit cycle against
disturbance. For this purpose, the dynamical model of the biped robot is
extracted in the space of total momentum variables and, according to the
desired step length and speed, the motion limit cycle is designed.
Subsequently, a motion stabilizer is proposed based on the idea of length
shift, which is a natural human strategy for sustaining the balance in case of
impact. The simulations show that this technique has a good performance in
maintaining the stability of motion and has similar responses to human
response.
