Commonly assigned U.S. Pat. No. 5,448,430 is incorporated for its showing of a track following servo system for following servo track edges of dissimilar servo signals, commonly assigned U.S. Pat. No. 5,844,814 is incorporated for its showing of an independent position sensor in a head positioning system, and commonly assigned U.S. Patent Application (Ser. No. 09/413,327) is incorporated for its showing of a servo position detector and a method for detecting and following an index servo position displaced with respect to an edge of a servo track.
This invention relates to recording system track following servos, and, more particularly, to the initialization and calibration of indexed servo positions displaced from servo track edges.
Advancements in technology in the data storage industry often include increases in the data storage capacity of given data storage media. One means of increasing the data storage capacity of data storage media, such as magnetic tape cartridges or magnetic tape cassettes, is to increase the track density of the data storage media.
In a typical magnetic tape, data is recorded in a plurality of parallel, longitudinal data tracks. A tape head may have a plurality of data heads which have fewer numbers of read/write elements than tracks. The data tracks are divided into groups, typically interleaved, and the tape head is indexed laterally with respect to the tracks to access each group of data tracks. In order to properly register the read/write elements with the data tracks, prerecorded servo tracks are provided which are parallel to the data tracks. A servo head located at an indexed position with respect to the read/write elements reads the servo tracks. The servo tracks provide lateral positioning information which, when read by the servo head, can be detected by a servo detector to indicate whether the servo read head is correctly positioned with respect to the servo tracks. Thus, the servo head can be moved laterally to a desired position with respect to the servo tracks so as to properly register the read/write elements with respect to a desired group of data tracks. Then, the servo head can follow the servo tracks as the media and the head are moved longitudinally with respect to each other, so that the read/write elements maintain registration with the data tracks. Typically, the servo head follows servo tracks at an edge, an edge comprising an interface between two dissimilar recorded servo signals.
The incorporated (Ser. No. 09/413,327) application utilizes existing servo tracks having edges, but increases the data track density by employing index servo positions displaced from the edges. Effectively, the index servo position is the lateral position on the tape at which the center of the servo read head is located, and this position is laterally displaced from an edge. The edges are easily tracked in the conventional servo systems by tracking the point at which both the dissimilar signals are sensed by a servo detector as balanced, in effect relying on the photolithography of the recording elements that generated the servo track edges. An example of a servo track following system for tracking edges is illustrated by the incorporated ""430 patent.
As pointed out by the incorporated (TU999049) application, tracking at a displacement from an edge is much more difficult. The index position is at a predetermined displacement distance in a lateral direction from an edge. The servo system servo detector determines the ratio between the two dissimilar recorded servo signals read by the servo head, with the ratio, the ratio representing the lateral position of the servo head with respect to an edge. The servo system then moves the servo head laterally to follow the index position, with the servo detector indicating a ratio of the two dissimilar recorded servo signals that is at an offset from the balanced ratio, the ratio representing the desired index servo position. To track follow at a displaced index position requires a high resolution servo detector that can interpolate the dissimilar recorded servo signals. The dissimilar servo signals are prerecorded onto the media, but are subject to variation in amplitude and possibly placement from one data storage media to the next.
Initializing and calibrating the servo detector to provide a correct interpolation of the recorded dissimilar recorded servo signals therefore becomes difficult.
An object of the present invention is to provide an initialization and calibration sequence that enables a more precise track following alignment of a servo head for following an index servo position displaced laterally from an edge.
Disclosed are a servo system and method for initializing and calibrating at least one index servo track following position substantially parallel to the edges and displaced a predetermined displacement distance in a lateral direction from one of the edges, in accordance with the present invention. A servo head is moveable in the lateral direction with respect to the recording medium and a servo detector is coupled to the servo head for determining a ratio related to the two dissimilar recorded servo signals as read by the servo head, the ratio representing the lateral position of the center of the servo head with respect to one of the edges. A servo track follower is coupled to the servo detector for moving the servo head laterally, the servo track follower, once initialized and calibrated, following an index ratio of the two dissimilar recorded servo signals representing the index servo position. An independent position sensor is provided for determining the mechanical lateral position of the servo head with respect to the recording medium.
In accordance with an embodiment of the present invention, logic, coupled to the servo detector, the servo track follower and the independent position sensor, responds to the servo detector and operates the servo track follower to nominally align the servo head at a lateral position at which the servo detector provided ratio represents one of the edges. The logic measures the mechanical lateral position of the independent position sensor at the nominal alignment of the servo head. The logic then responds to the independent position sensor, operating the servo track follower to reposition the servo head laterally the predetermined displacement distance from the nominal alignment as determined by the independent position sensor. The logic measures the servo detector provided repositioned ratio of the servo signals at the displaced distance, the logic initializing and calibrating the servo track follower to employ the provided repositioned ratio as the index ratio. Thus, track following at the provided index ratio insures that the servo head is at the correct displacement from the edge.
Additionally, if the servo system comprises a plurality of servo heads having the same alignment as a plurality of the prerecorded servo tracks, the nominal alignment of the servo heads at the corresponding edges comprises aligning the servo heads at a lateral position at which the servo detector provided ratio comprises the average of the provided ratios from the plurality of servo heads, the average representing the corresponding edges of the servo tracks. Then, the measurement of the repositioned ratio of the servo signals comprises measuring the average of the servo detector provided repositioned ratios from the plurality of servo heads.
In accordance with another embodiment of the present invention, a check on the edge measurement may be made during the initialization and calibration of index servo positions displaced substantially equidistant in opposite lateral directions from one of the edges. The servo head is again repositioned in the opposite lateral direction from the repositioning step, to the predetermined displacement distance from the nominal alignment as determined by the independent position sensor. The logic measures the again repositioned ratio of the servo signals. Thus, the repositioned ratio represents the displacement at one side of the edge, and the again repositioned ratio represents the displacement at the other side of the edge. The logic determines a midpoint between the repositioned ratio and the again repositioned ratio, compares the midpoint to the nominally aligned ratio, and determines whether the compared ratios are within a predetermined range. If so, the nominal alignment was correct, and the logic initializes and calibrates the servo detector to employ the provided repositioned ratio and the again repositioned ratio as the index ratios for the oppositely displaced index servo positions.
The repositioned locations can also be checked in accordance with the present invention. As a check, the servo head is further repositioned laterally in opposite directions from the repositioned displacement, the further repositioning comprising a substantially equal displacement in the opposite directions, the displacement a fraction of the predetermined displacement distance as determined by the independent position sensor. The logic measures the servo detector provided ratios of the servo signals at the further repositioned displacements, and determines a further midpoint between the opposite further repositioned ratios. The logic compares the further midpoint to the repositioned ratio, determining whether the compared ratios are within a predetermined range of each other. If so, the repositioned ratio is correct, and the logic conducts the initializing and calibrating step.
Hence, the present invention provides an initialization and calibration sequence that enables a more precise track following alignment of a servo head for following an index servo position displaced laterally from an edge.
For a fuller understanding of the present invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings.