Electro hydro dynamic (EHD) and electrostatic propulsion devices has no moving parts and, in the air, operates on electrical energy. It is expected to develop electric propulsion systems without future moving parts of airplanes and helicopters propellers in the future. I had succeeded in hovering EHD device in an arbitrary space for the first time. It means that we can control height at floating in the air. Also, I had succeeded in controlling the speed and acceleration of the EHD device and decreased to a significantly low level. I considered a theory for hovering EHD device. Here, high output voltage of small Tesla coil was modulated at a low repetition frequency, and output voltage of Cock-Walton circuit was controlled in a rectangular pulse shape temporally. Furthermore, I analyzed position, velocity, and acceleration of EHD device for levitation and descent using the optical-flow method. I confirmed that the velocity and acceleration were significantly lower than when a high DC voltage was applied.