Thermo Servo for Track Centering on a Disk

There are two basic methods of positioning read/write heads in magnetic disk drives using closed loop feedback circuits. The first method, called dedicated servo, dedicates one head and one disk in the disk drive to the task of acquiring position information. The dedicated head is referred to as the servo head and the dedicated disk is referred to as the servo disk. In this method, all the data heads and disks must maintain their positions relative to the servo head and the servo disk or off-track error will result. Off-track errors reduce read margins and may cause data errors. The servo head is in a closed loop feedback system with the servo disk. The other data heads and other disks in the disk drive do not have the benefit of a separate closed loop feedback system. Thus, in order to function properly the other disks and heads must maintain their original position with respect to the servo head and the servo disk over time, through temperature extremes and after mechanical wear.

The second method of positioning read/write heads is called embedded servo. Embedded servo uses position information written on the surface of each disk in the disk drive. Each disk and head have loop feedback circuit. This eliminates the requirement of the dedicated servo method that each data head maintains its position with respect to another data head through temperature extremes, over time and after mechanical wear.

However, many other disadvantages are associated with the embedded servo method. For example, embedded servo reduces soft sectored formatting and requires position information to be recorded on each disk. The position information on each disk reduces data capacity and increases drive complexity. Therefore, it is desirable to develop an apparatus and method in which all the benefits of dedicated servo can be maintained by compensating for slight distortions caused by wear over time and by temperature fluctuations.

Disclosed is a thermally compensating fixed arm which is capable of maintaining a data head over the center of a data track in the presence of environmental changes or mechanical wear. An arm made of a material capable of thermal expansion carries a transducer. One side of the arm carries a heating element. Varying the amount of power to the heating element causes linear expansion or contraction in the arm which moves the transducer across the track. A feedback loop circuit monitors a read parameter and when it varies from a desired state, the power to the heating element is varied to move the transducer to a position over the center of the track.

For a better understanding of the present invention, reference may be had to the accompanying drawings, in which:

FIG. 1 is a top view of a thermally compensating fixed arm tracking over a magnetic disk.

FIG. 2 is a top view of the thermally compensating fixed arm which shows several exaggerated positions of the arm.

FIG. 3 is a schematic diagram showing a feedback circuit for the thermally compensating fixed arm.

These drawings are not intended as a definition of the invention but are provided solely for the purpose of illustrating the preferred embodiment of the invention, described below.

Illustrated in FIG. 1 is a preferred embodiment of the present invention. A thermally compensating fixed arm 10 is shown tracking over a disk 12. The disk has a plurality of tracks 14. The arm 10 has a free end 16 and a pivotally attached end 18. Attached to the free end 16 of the arm is a transducer 20 for reading data. Preferably, the transducer tracks over the center of one of the tracks 14 during reading or recording of data. The end 18 of the arm is attached to an actuator (not shown) which pivots the arm to move the transducer 20 from track to track.

The arm 10 also includes a heating element 22 such as a resistor, which is carried by one side of the arm 10. In the preferred embodiment, the heating element 22 is embedded in the arm 10. The arm 10 is made of a material which expands upon heating and contracts upon cooling such as metal, plastic or a composite material. When the heating element 22 heats, the side of the arm 10 having the heating element 22 expands as a function of the coefficient of linear expansion for the particular material and the transducer 20 moves in a direction across the track. When the heating element cools and the arm 10 cools, the arm contracts as a function of the coefficient of linear expansion and the transducer moves across the track in the opposite direction. The range of movement, typically in terms of microinches, and the type of movement depends on the geometry of the arm 10, material from which the arm is made and temperature rise or drop in the arm.

FIG. 2, illustrates the motion of the arm 10 during its operation. In FIG. 2, the arm 10 is shown as a triangle 24 having a fixed base 26, a fixed length leg 28 and a variable length leg 30. The transducer 20 is positioned near the intersection of the fixed length leg 28 and the variable length leg 30. The fixed base 26 is assumed since the actuator (not shown) does not move after positioning the transducer 20. The heating element 22 is embedded in the variable length leg 30. The resulting motion is essentially along an arc 32 resulting from the fixed length leg 28 pivoting about the point 34 where the fixed length leg 28 attaches the fixed base 26. The centered position is shown in solid lines. Two additional positions of the arm 10 are shown in dashed lines. The amount of actual movement is very slight. The two additional positions shown in FIG. 2 are exaggerated in order to illustrate the motion. Furthermore, in order to keep FIG. 2 from becoming cluttered, dashed lines along only the free end of the triangle 24 show the two additional positions rather than showing dashed lines along the entire length of the fixed length leg 28 and the variable length leg 30.

Now turning to FIG. 3, the thermally compensating fixed arm 10 includes a circuit 38 having a heating element power supply 36, a read parameter amplifier 40 and a comparator 42. The circuit 38 detects an actual read parameter and compares it with the desired read parameter of the data. When the transducer 20 is off the center of the track, the actual read parameter will not match the desired read parameter. When the actual read parameter is compared to the desired read parameter at the comparator 42, the comparator 42 produces an error signal 44. The error signal 44 is input to the power supply 36. The power supply 36 either provides more or less power to the heating element 22 thereby moving the transducer 20 over the center of the track. With less power, the heating element 22 cools, the variable length leg 30 shortens and the transducer 20 moves one way. With more power, the heating element 22 heats, the variable length leg 30 elongates and the transducer 20 moves the other way.

Once the transducer 20 is again centered over the track 14, the desired read parameter and the actual read parameter are equal and no error signal 44 is produced. In the absence of an error signal 44, the circuit 38 supplies the heating element 22 with enough power to maintain the transducer position. In operation, heat is applied to the arm 10 at a predetermined rate and the transducer 20 is centered over the track 14. When the heating element 22 is a resistor, a constant amount of heat is typically applied to arm 10 by supplying the resistor with a constant amount of power equal to half of a pre-determined maximum power value. The actual read parameter is monitored during tracking and compared to the derived read parameter by the circuit 38. When the transducer 20 deviates from a position over the center of the track, the comparator 42 produces the error signal 44. The error signal 44 is input to the heating element power supply 36 which increases or decreases the amount of power to the heating element 22 to center the transducer.

The present invention and the best mode of practicing it have been described. It is to be understood that the foregoing description is illustrative only and that other means and techniques can be employed without departing from the full scope of the invention as described in the appended claims.