Hard to get an impression of scale, but I'm guessing it's about a foot high.
In the early days while you get the algorithm working I suggest that you slow the system's response down to give you an easier problem to solve. The way to do that is to add weight away from the axis of rotation, for example by adding a piece of wood parallel to the floor and perpendicular to the axle. It needs to be high enough to remain clear of the floor over the range of motion that you are going to support - its purpose is to increase the moment of inertia of the system, not to act as a prop.