Hi Yahia,

To the best of my knowledge, the servo control is a terminology in control engineering, by which means that the desired output is reached by error-correction. For example, we wanna remain a constant stress on a bounding wall in DEM. However, we couldn't set directly the stress (or the goal) on the wall because the real stress is determined by the complex and dynamic interaction between particles and the wall. Thus, we could get the difference between the real stress and the goal to guess a better position of the wall for a better approximated stress.

Generally, the DEM wall does not have a mass so that its motion is not governed by the Newton motion law. In this case, the servo control mechanism is adopted. Of course, you could set a mass to a wall, and then make it join the Newton cycle with a constant stress. However, the inertia force of the wall should be sufficiently small so that we could regard this constant stress as our goal.

Shiwei