A modified Laplacian-pyramid method and system filters the Gaussian image at each level of the pyramid, and uses the filtered Gaussian image to produce the Laplacian-pyramid images. The land has an upper surface which is entirely curved upward as a transverse sectional configuration along a belt width direction. In this case, a first signal, which describes a voltage across a rotor winding, and a second signal, which describes a current flowing through the rotor winding, are measured simultaneously.