Servo positioning system for data storage apparatus

In a servo positioning system for maintaining a data transducer centered on a selected one of a plurality of concentric data tracks on an information bearing disk rotatable in a disk file, said data tracks including interleaved servo position reference sectors and data sectors,

• sampling means for sampling signals from said data transducer at times corresponding to said servo sectors;
• means connected to said sampling means and responsive to said sampled signals to generate a first position error signal indicating deviations of said data transducer from said data track center, said first position error signal being limited in bandwidth by the frequency of occurrence of said servo sectors;
• a servo disk, rotatable coaxially with said information bearing disk, and having continuous servo position reference information recorded thereon in a plurality of concentric servo tracks corresponding to said data tracks;
• a servo transducer, movable conjointly with said data transducer, for sensing said servo information on said servo tracks as said servo disk rotates;
• and
• means connected to said servo transducer and responsive to said sensed servo information signals to generate a second position error signal indicating deviations of said servo transducer from said servo tracks, said deviations corresponding nominally to said deviations of said data transducer from said data tracks but being subject to low frequency differential effects which reduce the accuracy of said second position error signal, the improvement comprising;
• means connected to both said position error signal generating means for combining low frequency components only of said first position error signal with high frequency components only of said second position error signal to provide a hybrid position error signal of increased bandwidth over said first position error signal and of greater accuracy than said second position error signal, and
• actuator means for moving said data and servo transducers, said actuator means being connecting to receive said hybrid position error signal and being responsive thereto to maintain said data transducer centered over said selected data track.