A method for implementing sliding-window image processing for associative operators such as MIN, MAX, AND, and OR is disclosed. The power adaptor includes a connector, a rectifier, a filter, a regulator and a switching unit coupled between the rectifier and the filter. The oscillator circuit, connected to the controller, wherein the controller controls the oscillator circuit according to the second control signal. The recessed area of the grooves reduce the total contact area of the sheet thereby reducing the drag forces on the sheet but, at the same time, increasing the holding force of the vacuum on the sheet. The current control device produces a current whose phase is shifted such that the magnetic flux of the permanent magnets intensifies.