Tape drive servo operation by use of servo pattern width measurements

1. A method for measuring the width of a servo pattern on a magnetic tape, said servo pattern providing input to a head position servo to maintain the position of a magnetic tape head relative to said magnetic tape, comprising the steps of:

injecting a periodic signal into said head position servo to displace said magnetic tape head relative to said magnetic tape;

determining a first measurement position;

determining a second measurement position;

measuring a displacement of said magnetic tape head between said first measurement position and a said second measurement position; and

determining said width of said servo pattern as equal to said displacement of said magnetic tape head.

2. The method of claim 1 , wherein the step of determining a first measurement position further comprises the steps of:

determining a first total displacement of a servo read element when said servo read element is positioned in proximity of an upper servo pattern edge and a position error signal is valid; and

identifying said first measurement position as the value of said position error signal corresponding to placement of said servo read element at the midpoint of said first total displacement.

3. The method of claim 1 , wherein the step of determining a second measurement position further comprises the steps of:

determining a second total displacement of a servo read element when said servo read element is positioned in proximity of a lower servo pattern edge and a position error signal is valid; and

identifying said second measurement position as the value of said position error signal corresponding to placement of said servo read element at the midpoint of said second total displacement.

4. The method of claim 1 , wherein a fundamental frequency of said periodic signal is greater than a fundamental tape guiding frequency of said magnetic tape.

5. The method of claim 1 , wherein a fundamental frequency of said periodic signal is not harmonically related to a fundamental tape guiding frequency of said magnetic tape.

6. The method of claim 1 , wherein the measuring step further comprises the steps of:

(A) adjusting said periodic signal to position a servo read element between an upper servo pattern edge and a lower servo pattern edge during the time of the largest rate of change of said periodic signal;

(B) adjusting said periodic signal to provide a range of movement of said servo read element that exceeds a distance between said upper servo pattern edge and said lower servo pattern edge;

(C) providing a position error signal to said head position servo during the time said position error signal is determined to be valid;

(D) providing a independent position sensor signal to said head position servo during the time said position error signal is determined to be invalid;

(E) determining a first independent position sensor value when said magnetic tape head is located at said first measurement position;

(F) determining a second independent position sensor value when said magnetic tape head is located at said second measurement position;

(G) determining a position difference by subtracting said second independent position sensor value from said first independent position sensor value;

(H) repeating steps E, F and G to obtain multiple values of said position difference; and

(I) determining a measurement of said displacement of said magnetic tape head between said first measurement position and said second measurement position by averaging said multiple values of said position difference.

7. The method of claim 6 , further comprising the steps of:

Obtaining said position error signal from said servo read element when said servo read element is positioned in proximity of said upper servo edge and said during the time said position error signal is determined to be valid; and

Obtaining said position error signal from said servo read element when said servo read element is positioned in proximity of said lower servo edge and said during the time said position error signal is determined to be valid.

8. A data storage apparatus comprising:

A magnetic tape head having multiple servo elements;

an independent position sensor (IPS) coupled to said magnetic tape head to provide positioning information for said magnetic tape head;

an actuator coupled to said magnetic tape head to position said magnetic tape head relative to a magnetic tape;

a position error detection controller to generate position error signals (PESs) each indicating a distance between a servo element and a servo edge of a servo pattern on said magnetic tape;

a head position servo to maintain the position of said magnetic tape head relative to said magnetic tape;

a control unit coupled to said IPS, said actuator, said head position servo and said position error detection controller, said control unit programmed to perform steps for measuring the width of said servo pattern on said magnetic tape, comprising the steps of:

injecting a periodic signal into said head position servo to displace said magnetic tape head relative to said magnetic tape;

determining a first measurement position;

determining a second measurement position;

measuring a displacement of said magnetic tape head between said first measurement position and a said second measurement position; and

determining said width of said servo pattern as equal to said displacement of said magnetic tape head.

9. The data storage apparatus of claim 8 , wherein the step of determining a first measurement position further comprises the steps of:

determining a first total displacement of a servo read element when said servo read element is positioned in proximity of an upper servo pattern edge and a position error signal is valid; and

identifying said first measurement position as the value of said position error signal corresponding to placement of said servo read element at the midpoint of said first total displacement.

10. The data storage apparatus of claim 8 , wherein the step of determining a second measurement position further comprises the steps of:

determining a second total displacement of a servo read element when said servo read element is positioned in proximity of a lower servo pattern edge and a position error signal is valid; and

identifying said second measurement position as the value of said position error signal corresponding to placement of said servo read element at the midpoint of said second total displacement.

11. The data storage apparatus of claim 8 , wherein a fundamental frequency of said periodic signal is greater than a fundamental tape guiding frequency of said magnetic tape.

12. The data storage apparatus of claim 8 , wherein a fundamental frequency of said periodic signal is not harmonically related to a fundamental tape guiding frequency of said magnetic tape.

13. The data storage apparatus of claim 8 , wherein the measuring step further comprises the steps of:

(A) adjusting said periodic signal to position a servo read element between a upper servo pattern edge and a lower servo pattern edge during the time of the largest rate of change of said periodic signal;

(B) adjusting said periodic signal to provide a range of movement of said servo read element that exceeds a distance between said upper servo pattern edge and said lower servo pattern edge;

(C) providing a position error signal to said head position servo during the time said position error signal is determined to be valid;

(D) providing a independent position sensor signal to said head position servo during the time said position error signal is determined to be invalid;

(E) determining a first independent position sensor value when said magnetic tape head is located at said first measurement position;

(F) determining a second independent position sensor value when said magnetic tape head is located at said second measurement position;

(G) determining a position difference by subtracting said second independent position sensor value from said first independent position sensor value;

(H) repeating steps E, F and G to obtain multiple values of said position difference; and

(I) determining a measurement of said displacement of said magnetic tape head between said first measurement position and said second measurement position by averaging said multiple values of said position difference.

14. The data storage apparatus of claim 13 , wherein said position error signal is obtained from said servo read element when said servo read element is positioned in proximity of said upper servo edge and when said servo read element is positioned in proximity of said lower servo edge and said during the time said position error signal is determined to be valid.

15. An article of manufacture comprising a data storage medium tangibly embodying a program of machine-readable instructions executable by a digital processing apparatus to perform method steps for measuring the width of a servo pattern on a magnetic tape, said servo pattern providing input to a head position servo to maintain the position of a magnetic tape head relative to said magnetic tape, said method steps comprising:

injecting a periodic signal into said head position servo to displace said magnetic tape head relative to said magnetic tape;

determining a first measurement position;

determining a second measurement position;

measuring a displacement of said magnetic tape head between said first measurement position and a said second measurement position; and

determining said width of said servo pattern as equal to said displacement of said magnetic tape head.

16. The article of manufacture of claim 15 , wherein the step of determining a first measurement position further comprises the steps of:

determining a first total displacement of a servo read element when said servo read element is positioned in proximity of an upper servo pattern edge and a position error signal is valid; and

identifying said first measurement position as the value of said position error signal corresponding to placement of said servo read element at the midpoint of said first total displacement.

17. The article of manufacture of claim 15 , wherein the step of determining a second measurement position further comprises the steps of:

determining a second total displacement of a servo read element when said servo read element is positioned in proximity of a lower servo pattern edge and a position error signal is valid; and

identifying said second measurement position as the value of said position error signal corresponding to placement of said servo read element at the midpoint of said second total displacement.

18. The article of manufacture of claim 15 , wherein a fundamental frequency of said periodic signal is greater than a fundamental tape guiding frequency of said magnetic tape.

19. The article of manufacture of claim 15 , wherein a fundamental frequency of said periodic signal is not harmonically related to a fundamental tape guiding frequency of said magnetic tape.

20. The article of manufacture of claim 15 , wherein the measuring step further comprises the steps of:

(A) adjusting said periodic signal to position a servo read element between an upper servo pattern edge and a lower servo pattern edge during the time of the largest rate of change of said periodic signal;

(B) adjusting said periodic signal to provide a range of movement of said servo read element that exceeds a distance between said upper servo pattern edge and said lower servo pattern edge;

(C) providing a position error signal to said head position servo during the time said position error signal is determined to be valid;

(D) providing a independent position sensor signal to said head position servo during the time said position error signal is determined to be invalid;

(E) determining a first independent position sensor value when said magnetic tape head is located at said first measurement position;

(F) determining a second independent position sensor value when said magnetic tape head is located at said second measurement position;

(G) determining a position difference by subtracting said second independent position sensor value from said first independent position sensor value;

(H) repeating steps E, F and G to obtain multiple values of said position difference; and

(I) determining a measurement of said displacement of said magnetic tape head between said first measurement position and said second measurement position by averaging said multiple values of said position difference.

21. The article of manufacture of claim 20 , wherein said position error signal is obtained from said servo read element when said servo read element is positioned in proximity of said upper servo edge and when said servo read element is positioned in proximity of said lower servo edge and said during the time said position error signal is determined to be valid.