Abstract: A disk drive having a marker for identifying the transition between the first and last written servo wedges on a servo track. The marker indicates the likely position where closure error may occur during the operation of the hard disk drive. The marker is used by the control system of a hard disk drive to provide appropriate position control signals to the transducer assembly to maintain the proper positioning of the transducer about the disk surface. A method for forming the marker on a disk surface is further disclosed.

Abstract: A small, permanent magnet is eccentrically supported on a shaft that is rotated by some means, such as gears, in response to linear change in some quantity, e.g. accumulated water from rainfall. A float on the surface of the water in a receptacle transmits the motion to the gears. At least three sensors, responsive to magnetic flux, are mounted in a circle concentric about the shaft so that the rotating magnet will produce a response as it passes each sensor. Each sensor is connected to an input terminal of two of a plurality of flip-flops (one for each sensor), so that the resultant stored bit of information in each flip-flop is determined by two different sensors. This stored bit comprises a digit of a three digit binary code that is changed whenever a sensor responds. It is available at the output terminal of each flip-flop, which is connected to an AND-OR inverter containing at least three two-input AND gates and an OR gate.