Utilization of stored run-out information in a track following servo system

A servo system for accessing recording data on a rotatable disk having a plurality of concentric tracks, the disk including spaced apart sectors S.sub.N having track following servo data, the tracks having a repeatable error at a determined frequency and being subject to an offset at a frequency lower than said determined frequency, comprising:

• (a) first means, including a recording head which senses said servo data, for generating first signals representing the fine position of said head in relation to the centerline of a desired track, whereby said first signals include information as to said offset;
• (b) second means, external of said head, for generating second signals representing the coarse positions of said head in relation to said desired track, said second signals including information as to said repeatable error;
• (c) first means for storing samples of said second signals corresponding to the positions of said head at sectors S.sub.N ;
• (d) second means, responsive to said first signals, for storing third signals representing said offset;
• (e) means for sampling said second signals and transmitting said samples to said first storing means and for transmitting said first signals to said second means for storing during fine positioning of said head;
• (f) moving means, connected to said head, for moving said head in relation to said desired track, said moving means including means responsive to said stored samples and said third signals during coarse positioning of said head for positioning said head coarsely on said desired track, and means responsive to said first signals and said stored samples when coarse positioning has been completed for positioning said head finely in relation to said track centerline, said second generating means being coupled to said moving means for generating said second signals;
• and
• (g) said moving means including a first subtractor for subtracting said stored samples from said third signals during coarse positioning to produce first error signals and subtracting said stored samples from said first signals during fine positioning to produce second error signals, means for phase compensating said first error signals or said second error signals, and a second subtractor for subtracting said second signals from said second generating means from signals representing said desired track and from said phase compensated first error signals or said phase compensated second error signals to produce third error signals, said phase compensating means phase compensating said first error signals in dependence on any phase difference between said first signals from said second generating means and said phase compensated error signals subtracted by said second subtractor.

A servo system for positioning one member in relation to another movable member having a repeatable error at a predetermined frequency and being subject to offsets at a frequency lower than said predetermined frequency, comprising:

• (a) first means for generating first error signals representing fine positions between said one member and said other member at predetermined locations of said other member;
• (b) second means for generating second signals representing coarse positions of said one member in relation to said other member, said second signals including information of said repeatable error and said offsets;
• (c) means for storing those of said second signals which correspond to said predetermined locations;
• (d) third means for generating second error signals each representing the difference between said first error signals corresponding to one of said predetermined locations and said stored second signals corresponding to a second of said predetermined locations;
• (e) means for phase lagging said second error signals by a predetermined amount;
• (f) fourth means for generating third error signals representing the difference between said second signals from said second generating means corresponding to a third location of said other member and said phase lagged second error signals;
• and
• (g) means, responsive to said third error signals, for moving said one member in relation to said other member.

A servo system for positioning a recording head in relation to a data track on a rotatable disk having a repeatable error at a determined frequency and being subject to an offset at a frequency lower than said determined frequency, comprising:

• (a) means, connected to said head, for moving said head across said disk;
• (b) first means, coupled to said moving means, for generating first coarse positioning signals representing the positions of said moving means;
• (c) second means for generating second signals relating to said offset;
• (d) means for storing a predetermined number N of said first signals, said N first signals corresponding, respectively, to N sectors S.sub.N on said disk;
• (e) third means for generating first error signals representing the difference between said N first signals corresponding to sectors S.sub.N + .DELTA. and said second signals, where S.sub.N + .DELTA. corresponds to sectors spaced a predetermined number of degrees on said disk from sectors S.sub.N ;
• (e) means for phase compensating said first error signals a predetermined degree to produce phase compensated first error signals;
• (f) fourth means for generating second error signals corresponding to the difference between said first coarse positioning signals from said first generating means and said phase compensated first error signals and a signal representing said track;
• and
• (g) means, responsive to said second error signals, for controlling the movement of said moving means, whereby said head is coarsely positioned in relation to said track.