Measurement of track eccentricity on bit patterned media
View Patent ↗The eccentricity of tracks defined on a rotating bit patterned media are measured using a readback signal, and the measured eccentricity may be used to control centering of the disk relative to a rotational spindle and/or to control movement of a read/write head relative to a selected track on the disk.
1. An apparatus comprising:
hardware that measures an amount of eccentricity of tracks on a rotating disk in response to amplitude variation in a readback signal from a bit patterned media pattern that includes a plurality of dots arranged in a down-track orientation and in a cross-track orientation;
wherein the hardware counts a number of amplitude variation cycles in the readback signal over a defined time period, and measures the amount of eccentricity in response to the counted number of amplitude variation cycles in the readback signal.
2. The apparatus of claim 1 , further comprising:
a disk positioning unit that is configured to control centering of the disk relative to a rotational spindle in response to the measured amount of eccentricity of the tracks.
3. The apparatus of claim 1 , further comprising:
a servo controller circuit that is configured to control movement of a read/write head relative to a selected track in response to the measured amount of eccentricity of the tracks.
4. The apparatus of claim 1 , wherein the hardware counts the number of amplitude variation cycles limited to within a defined frequency range of the readback signal corresponding to an expected rate of occurrence of the plurality of dots in the cross-track orientation due to track eccentricity.
5. The apparauts of claim 1 , wherein the hardware comprises a delay circuit that triggers output of the counted number of amplitude variation cycles in the readback signal responsive to each revolution of the disk.
6. The apparatus of claim 1 , wherein the hardware measures a range of the eccentricity on the rotating disk as one-half the counted number of amplitude variation cycles in the readback signal.
7. An apparatus comprising one or more hardware components and/or computer readable program code stored on a computer readable storage media configured to form a circuit comprising:
a differentiator to differentiate a readback signal from a bit patterned media including a plurality of dots arranged in a down-track orientation and in a cross-track orientation;
a zero-crossings detector configured to detect zero crossings in the differentiated readback signal; and
measurement circuitry configured to count the zero crossings in the differentiated readback signal and output an eccentricity measurement in response to the counted number of zero-crossings in the differentiated readback signal.
8. The apparatus of claim 7 , and further comprising a low pass filter configured to filter the readback signal before differentiating the readback signal wherein the low pass filter is tuned to substantially attenuate frequencies in the readback signal that correspond to a readback rate of the plurality of dots in the down-track orientation.
9. An apparatus comprising one or more hardware components and/or computer readable program code stored on a computer readable medium configured to form a circuit comprising:
detection circuitry configured to detect peaks in a readback signal from a bit patterned media corresponding to a cross-track orientation over a time; and
measurement circuitry configured to count the number of peaks in the readback signal corresponding to the cross-track orientation and output an eccentricity measurement in response to the counted number of peaks.
10. The apparatus of claim 9 , wherein the detection circuitry comprises a filtering component tuned to attenuate frequencies in the readback signal that correspond to a readback rate of a plurality of dots on the bit patterned media in a down-track orientation to detect the peaks in the readback signal corresponding to the cross-track orientation.
11. A method comprising:
counting a number of amplitude variation cycles in a readback signal from a bit patterned media on a disk corresponding to a cross-track orientation over a time period; and
measuring an amount of eccentricity of tracks on the disk in response to the number of amplitude variation cycles in the readback signal corresponding to the cross-track orientation.
12. The method of claim 11 , further comprising:
controlling a disk positioning unit to center the disk relative to a rotational spindle in response to the measured amount of eccentricity of the tracks.
13. The method of claim 11 , further comprising:
controlling movement of a read/write head relative to a selected track in response to the measured amount of eccentricity of the tracks.
14. The method of claim 11 , further comprising:
filtering the readback signal to attenuate amplitude variation cycles corresponding to an occurrence of dots in a down-track orientation on the bit patterned media.
15. The method of claim 11 , comprising:
measuring a range of eccentricity as a known fraction of the counted number of amplitude variation cycles in the readback signal.
16. The method of claim 11 and comprising the step of:
DC erasing the tracks on the disk prior to measuring the amount of eccentricity.
17. A method comprising:
differentiating a readback signal from a bit patterned media on a disk that includes a plurality of dots arranged in a down-track orientation and in a cross-track orientation to generate a differentiated readback signal;
counting occurrences of zero-crossings in the differentiated readback signal over a defined time period; and
measuring an amount of eccentricity of tracks on the disk in response to the counted number of zero-crossings in the differentiated readback signal.
18. The method of claim 17 , further comprising:
low pass filtering the readback signal before differentiating it to generate the differentiated readback signal, wherein the low pass filter is tuned to substantially attenuate frequencies in the readback signal that correspond to a readback rate of the plurality of dots in the down-track orientation.
19. A method comprising:
filtering a readback signal from a bit patterned media that includes a plurality of dots arranged in a down-track orientation and in a cross-track orientation to substantially attenuate frequencies in the readback signal that correspond to a readback rate of occurrence of the plurality of dots in the down-track orientation;
counting a number of peaks in the filtered readback signal that exceed a threshold value over a defined time period; and
measuring an amount of eccentricity of tracks on the bit patterned media in response to the counted number of peaks.
20. A disk stack alignment apparatus comprising:
a track eccentricity determination circuit configured to measure an amount of eccentricity of tracks on a disk in response to a readback signal, wherein the track eccentricity determination circuit counts a number of amplitude variation cycles in the readback signal over a time period and measures the amount of eccentricity in response to the number of amplitude variation cycles in the readback signal; and
a disk positioning unit that controls centering of the disk relative to a rotational spindle in response to the measured amount of eccentricity of the tracks.