A printed circuit board on which are laid out detection windings and an excitation magnetic flux detection winding formed of conductor patterns is disposed so as to be spaced a predetermined gap quantity from a binary cyclic random number sequence code plate which is rotatably disposed. Once an initial flaw or crack is produced, a full body crack can be propagated across a dimension of the brittle material with a laser beam that is substantially absorbed proximate the surface of the sheet to produce sub-sheets. The mounting system is also advantageous in that the lawnmower operator need not specifically align the mower with any objects that present an obstacle for mowing, but may instead move the accessory to the object.