BIPOLAR MOTOR CONTROL

A disc drive system is disclosed comprising a data storage medium in the form of a disc (21) having a plurality of data storage tracks. The medium further includes servo sectors (50), the servo data in each said sector including a plurality of first servo data stored at a position on one side of the data track centerline and one-half a track space distance from said centerline, and a plurality of second servo data stored at the other side of said data track centerline and one-half of said track space distance from said centerline. Positionable access means are provided for reading the servo tracks and said tracks and for generating servo signals representing said first and second servo burst data (40, 42), said access means including a transducer (23) for accessing said servo and data information, and actuator (24, 25) for supporting and positioning said transducer, and a stepper motor (Figure 1) having a plurality of stator coils (A, B, C, D, E) subject to selective simultaneous energisation at full current with currents of selected polarities of all or all but one of said coils for defining a plurality of motor detent positions. The servo data is located at positions defined by said motor detent position. The data track is reached by said transducer by applying approximately one-half of said energisation current to one coil not energised to reach an adjacent motor detent servo track position, the motor detent position on the other side of said track centerline being reached by energisation of all said coils with currents of selected polarities.