HK1104182B - A method and system for testing a micro-actuator in a magnetic tester - Google Patents

Description

Method and system for testing micro-actuator in magnetic tester

Technical Field

The present invention relates to hard disk drives. More particularly, the present invention relates to a method of testing the stroke and frequency response of a micro-actuator used in a hard disk drive.

Background

Hard disk drives are common information storage devices consisting essentially of a series of rotatable disks or other magnetic storage media that are accessed by magnetic read and write elements. These data transfer elements, commonly referred to as transducers, are typically carried by and embedded in a slider body that is held in a close position over discrete data tracks formed on the disk, thereby allowing read/write operations to be performed. To properly position the transducer relative to the disk surface, an Air Bearing Surface (ABS) formed on the slider body experiences a flowing air flow that provides sufficient lift force to "fly" the slider and transducer above the disk data tracks. The high speed rotation of the magnetic disk generates a flowing air stream or wind along its surface in a direction substantially parallel to the tangential velocity of the disk. The air flow cooperates with the ABS of the slider body to cause the slider to fly above the spinning disk. In effect, the suspended slider is physically separated from the disk surface by this self-actuating air bearing.

Some of the primary objectives of ABS designs are to fly the slider and its accompanying transducer as close as possible to the surface of the rotating disk, and to uniformly maintain a constant close distance regardless of the variable flying state. The height or separation gap between the air bearing slider and the rotating magnetic disk is generally defined as the flying height. Typically, the mounted transducer or read/write element only flies about a few micro-inches above the surface of the rotating disk. The flying height of the slider is considered one of the most critical parameters affecting the disk reading and recording capabilities of the mounted read/write element. The relatively small flying height allows the transducer to achieve greater resolution between different data bit locations on the disk surface, thereby increasing data density and storage capacity. With the increasing popularity of lightweight, compact notebook-type computers that utilize relatively small yet powerful hard disk drives, the need for lower and lower flying heights has continued to grow.

FIG. 1 illustrates a hard disk drive design typical in the art. Hard disk drive 100 is a common information storage device consisting essentially of a series of rotatable disks 104 that are accessed by magnetic read and write elements. These data transfer elements, commonly referred to as transducers, are typically carried by the slider body 110 and embedded in the slider body 110, with the slider body 110 being held in a close proximity over discrete data tracks formed on the disk, thereby allowing read/write operations to be performed. The slider is held above the disk by a suspension. The suspension has a load beam and flexure that allows movement in a direction perpendicular to the disk. The suspension is rotated about a pivot by a voice coil motor to provide coarse position adjustment. The micro-actuator couples the slider to the end of the suspension and allows fine position adjustment.

To properly position the transducer relative to the disk surface, an Air Bearing Surface (ABS) formed on the slider body 110 experiences a flowing air flow that provides sufficient lift force to "fly" the slider 110 (and transducer) above the disk data tracks. The high speed rotation of the magnetic disk 104 generates a flowing air stream or wind along its surface in a direction substantially parallel to the tangential velocity of the disk. The air flow cooperates with the ABS of the slider body 110 to cause the slider to fly above the spinning disk. In effect, the suspended slider 110 is physically separated from the disk surface 104 by this self-actuating air bearing. The ABS of a slider 110 is typically disposed on the slider surface facing the rotating disk 104, and greatly affects the ability of the slider to fly above the disk under various conditions. To control in-plane motion of the slider, and in particular to access different data tracks on the disk surface, a Head Suspension Assembly (HSA) typically incorporates a primary actuator. The primary actuator may be a voice coil motor located at the opposite end of the read/write head. Due to the large inertia of the HSA, the primary actuator has a limited bandwidth. The vibration of the suspension makes it difficult to control the position of the read/write head from a distance. There are difficulties in achieving the required speed and position accuracy along the main actuator.

Advanced hard disk drive designs incorporate a secondary actuator or micro-actuator between the read/write head and the pivot of the HSA. The distance of the stroke or displacement of these micro-actuators in relation to the applied voltage is typically of the order of 1 μm. Figure 2a illustrates a micro-actuator with a U-shaped ceramic frame structure 201. The frame 201 is made of, for example, zirconia. The frame 201 has two arms 202 opposite a base 203. The slider 204 is held by the two arms 202 at the end opposite the base 203. A strip of piezoelectric material 205 is attached to each arm 202. The bonding pad 206 allows the slider 204 to be electrically connected to a controller. Figure 2b illustrates a micro-actuator attached to an actuator suspension flexure 207 and load beam 208. The micro-actuator may be coupled to the suspension tongue 209. Traces 210 coupled along the suspension flexure 207 connect the strip of piezoelectric material 205 to a set of connection pads 211. A voltage applied to the connecting pad 211 causes the strap 205 to contract and elongate, moving the position of the slider 204. The suspension flexure 207 may be attached to a base plate 212 by means of a suspension hinge plate 214, the base plate 210 having a hole 213 for mounting on a pivot. Tooling holes (tolling holes) 215 facilitate handling of the suspension during manufacture, and suspension holes 216 reduce the weight of the suspension.

The read/write heads are routinely tested prior to shipment. Typically, a read/write head is floated on a rotating disk connected to a Dynamic Parameter (DP) tester and a series of read/write activities are performed. DP testing may be performed when the read/write head is assembled into a Head Gimbal Assembly (HGA), HSA, or head disk assembly.

The DP test may include testing the stroke of the micro-actuator at various input voltages. Figure 3 illustrates a method of testing stroke. The quasi-static stroke may be measured by writing the first concentric track 310 and the second concentric track 320 at different constant input voltages and then obtaining a "track profile" of the tracks by reading at successively increasing or decreasing radii. The track profile may compare the radial position of read head 330 to the read back signal 340. The input voltage may be no input, maximum input, or negative maximum input. Each peak of the track profile represents the center of the track. The stroke may be calculated using the distance 350 between adjacent peaks.

The DP test may also include a test for frequency response. The frequency response compares the stroke with the input frequency. As shown in fig. 4, the frequency response is measured by first erasing the surface of the band-shaped disk at each input frequency, and then applying a predetermined ac input voltage at the desired frequency while overwriting this erased band for about one rotation of the disk. After removing the input voltage, the written signal is mapped (map) by reading at successively increasing or decreasing radii, while the amplitude of the read back signal is recorded as a function of radius and angular position. The sinusoid 410 is mathematically adapted to represent the track readings with the location of the peak amplitude of the read-back signal for each track. The amplitude 420 of this sinusoid is the stroke at a given frequency.

These methods are slow. For each desired frequency, the disk surface must be erased and rewritten. At each frequency, the amount of data required to map the written signal is also large because the map (map) of fig. 4 is two-dimensional rather than one-dimensional. Other methods for measuring the frequency response of the microactuator include optical and electrical testing. In optical testing, a laser beam is directed at or near the read/write head. The reflected light is collected and analyzed for velocity or displacement of the reflecting surface. This method requires expensive equipment and precise alignment. Electrical testing is possible for micro-actuators having more than one active element. The input voltage is applied to some but not all of the elements. These elements that do not accept the input voltage are mechanically driven by other elements to generate a smaller output voltage from which the stroke is derived. However, this excitation pattern is different from the pattern in applications where all elements receive an input voltage. Thus, the frequency response belongs to a vibration mode, which is different from the mode of actual interest. Furthermore, when the HSA has multiple read/write heads, it becomes difficult to access only the head of interest without interference from other heads.

Disclosure of Invention

According to a first aspect of embodiments of the present invention, there is provided a test method, including: writing two concentric tracks on a noise-free portion of a magnetic storage medium; positioning a read/write head and a micro-actuator between the two concentric tracks; applying a first oscillating voltage at a first frequency to the micro-actuator while reading back a first signal from the two concentric tracks; and calculating a first stroke characteristic of the micro-actuator based in part on the first signal.

According to a second aspect of embodiments of the present invention, there is provided a test system, comprising: a magnetic storage medium to store data; a read/write head for writing two concentric tracks on a noise-free portion of the magnetic storage medium; a head gimbal assembly for positioning the read/write head and micro-actuator between the two concentric tracks; and a tester to apply a first oscillating voltage at a first frequency to the micro-actuator while reading back a first signal from the two concentric tracks; wherein a first stroke characteristic of the micro-actuator is calculated based in part on the first signal.

Drawings

FIG. 1 illustrates a hard disk drive design as known in the art.

Figures 2a-b illustrate a micro-actuator attached to an actuator suspension flexure and load beam as known in the art.

FIG. 3 illustrates one embodiment of the results of a method for testing stroke.

FIG. 4 illustrates one embodiment of results of a method for testing frequency response.

FIG. 5 illustrates one embodiment of a test system as implemented in the present invention.

Fig. 6 illustrates in flow chart form one embodiment of a testing method in accordance with the present invention.

Fig. 7 illustrates an example of a calibration curve generated in accordance with the present invention.

Fig. 8a-c illustrate examples of feedback signals generated according to the present invention.

Fig. 9 illustrates in a graph one profile of stroke compared to frequency.

Fig. 10 illustrates in graph form a profile of gain compared to frequency.

Detailed Description

A system and method for testing the stroke and frequency response of a micro-actuator is disclosed. In one embodiment, the dynamic parameter tester may write two concentric tracks on a noise-free portion of the magnetic storage medium. The read/write head and the micro-actuator may be positioned between two concentric tracks. At a first frequency, an initial oscillating voltage is applied to the micro-actuator while reading back the signal from two concentric tracks. The stroke characteristics of the micro-actuator may be calculated based in part on the read-back signal. The initial oscillating voltage may be determined from previous tests. The first stroke characteristic of the micro-actuator may be based on a time-averaged amplitude of the first signal. Two concentric tracks can be written at a predetermined pitch.

FIG. 5 illustrates one embodiment of a test system as implemented in the present invention. The suspension 501 and micro-actuator 502 of the head gimbal assembly HGA may suspend the slider 503 above a disk 504 storing data. The HGA may be added to a Dynamic Parameter (DP) tester 505. DP tester 505 may have a first electrical connection 506 that controls micro-actuator 502, a second electrical connection 506 that controls the read/write head of slider 503, and a mechanism (not shown) that controls the movement of suspension 501. On most DP testers, the mechanism is standard. A typical mechanism for moving the suspension may include a massive stainless steel platform on which the suspension is mounted. The platform can be mounted to a piezoelectric stage for precise movement and a motorized stage for long distance motion. The DP tester may be used to test the stroke and frequency response of the HGA.

FIG. 6 illustrates, in flow chart form, one embodiment of a testing method. The process begins (block 605) with loading the HGA with slider 503 into the DP tester (block 610). An area of the tape disk 504 is erased (block 615). The read/write head writes two or more concentric tracks on the erased band at a predetermined pitch (2 δ) (block 620). Pitch is the distance between two parallel or concentric lines. Writing is performed when no input voltage is applied to the micro-actuator. The pitch is obtained by moving the mechanism of the DP tester 505. A track profile is obtained on any one of the tracks (block 625). The track profile may be the Track Average Amplitude (TAA) of the readback signal as a function of incremental changes in track radius (r). The function TAA (r) is referenced to the center of the track. Thus, the independent variable (r) typically varies in a range of between plus and minus ten microinches (+ -0.25 μm). The track will then be read back while the micro-actuator sees a variable input voltage (V) with continuously increasing or decreasing frequency (f). The stroke at each test condition is derived from the average amplitude of the readback signal, as defined by a pair of input parameters (e.g., frequency and voltage) and as measured from bottom to peak.

Assuming that the micro-actuator moves sinusoidally under the input voltage, the neutral position of the micro-actuator coincides with the center of the two concentric tracks. Because the test of the frequency response consists of multiple test states, each state can be identified by an index i. Thus, the input frequency and voltage can be represented by f (i) and v (i) and the corresponding stroke and time averaged readback signal amplitudes are represented by s (i) and taa (i), the relationship of which is described by equation 1 below:

a calibration curve for taa (i) versus s (i) may be generated using equation 1 (block 630). Fig. 7 illustrates an example of a calibration curve. The calibration curve in this example compares TAA (millivolts) to the stroke in the microactuator at 30 microinch spacing.

The required parameters taa (r) and taa (i) are a sequence of readback signal amplitudes and are readily available on any standard DP tester 505. In the absence of an excitation microactuator 502, taa (r) is generated in the standard track profile measurement. TAA (r) is used to generate a calibration curve of TAA (i) versus S (i). By definition, taa (i) is the time-averaged readback signal amplitude when the microactuator 502 is energized. As long as the excitation continues for at least one rotation of the disk 504, TAA (i) corresponds to the track average of the readback signal amplitude, which is available in any standard DP tester 504. Thus, the main modification of the DP tester is to provide a means to excite the microactuator, such as adding the second electrical connection 506 shown in FIG. 5.

The calibration curve can be qualitatively divided into four regions. Region 701 may be associated with a very small stroke. The read head typically has not reached either of the two tracks, or just reached the inner edges of the two tracks. In addition, the readback signal may be weak compared to background noise. Therefore, the region 701 is generally not suitable for performing stroke measurements. Region 702 may be associated with a medium stroke. The read head of this region can move properly in both tracks, but not out of range. The readback signal may be strong compared to the background noise, and the slope of the calibration curve may be steep. Thus, region 702 may be best suited for stroke measurements. Region 703 may be associated with approximately half of the track pitch. The read head may move near the outer edge of the track, producing a stronger read-back signal. However, the calibration curve may be flat. The resolution of the stroke may be lower than in region 702. In addition, the stroke value may be uncertain because the curve is not monotonic. Region 704 may be associated with a stroke that is greater than half of the track pitch. The slope of the calibration curve may be lower than in region 702. Generally, region 704 may be the second most desirable operating range.

The nature of the calibration curve is important. When the region 702 is wider, the test may be robust. However, during testing, the stroke is typically larger, which results in greater wear of the micro-actuator. The test may be most accurate when region 2 is narrower and the peak of the calibration curve is higher. The spacing between two concentrically written tracks may manipulate the shape of the calibration curve. The peak of the calibration curve typically occurs when the peak-to-peak stroke is approximately equal to the track pitch. The peak is typically highest when the tracks are nearly adjacent to each other. By running some numerical simulations, using a typical track profile, and varying the pitch using equation 1, the user can compare several experimental test conditions.

If V (i) remains unchanged while f (i) is changed, S (i) may span all four regions. To remain within region 702, V (i) may be adjusted following the method shown in FIG. 6. Defining a region 702 on the calibration curve and selecting an "ideal stroke" S in the region 702_ideal. Based on statistics from previous experience, the initial voltage V (1) and frequency f (1) are set (block 635). Read/write head is fixedBetween two concentric tracks (block 640). The read/write head takes a reading while the DP tester 505 energizes the micro-actuator 502 (block 645). The TAA (1) is obtained (block 650) and an initial stroke S (1) is calculated (block 655). If the last frequency is being used (block 660), the process ends (block 665). If the frequency is not the last frequency (block 660), S (1) is compared to the calibration curve. If S (1) is within the area 702 (block 670), then the measurement is acceptable. No retry is required. However, if S (1) is outside of region 702, then V (1) will be adjusted to bring S (1) closer to the "ideal stroke". Assume that in equation 2 is defined as:

G(i)＝S(i)N(i) (2)

is independent of the input voltage, V (1) and S (1) may be proportional to each other as shown in equation 3:

V(i)_new＝V(i)_old×S_ideal/S(i)_old (3)

conceptually, V (1) can be adjusted more than once. V (1) is adjusted according to equation 3 until S (1) is within region 702 (block 685). The iteration stops when S (1) is within region 702. Due to the good linearity of the micro-actuator, more than one adjustment is not actually required.

For subsequent f (i), when i increases (block 675), the default input voltage may be calculated by assuming that the gain does not vary significantly with frequency. Next, as shown in equation 4, V (i +1) is calculated in the same manner as the retries of V (i):

V(i+1)＝V(i)×S_ideal/S(i) (4)

equation 4 may have a "phase lag" because the gain does vary with frequency. However, the region 702 is relatively wide compared to the error associated with the varying gain. Therefore, in practice, equation 4 is usually sufficient. At any given frequency, there is little need to adjust the input voltage by equation 3.

A correction factor may be added to equation 4 that depends on the typical frequency response to predict the gain change from f (i) to f (i + 1). In most applications, this correction factor may not be necessary. The deformation of equation 4 is shown in equation 5:

V(i+1)＝V(i)×[S_ideal/S(i)]×[Gain(i)/Gain(i+1)] (5)

equations 4 or 5 may be used to predict the gain for the next frequency used, if necessary (block 680). The objective is to control v (i) so that the stroke remains within region 702 while the frequency of excitation is swept stepwise within a predetermined range.

The corner frequency of the low pass filter may be at least several times greater than the frequency of the microactuator excitation when measuring taa (i) on DP tester 505. This prevents the envelope of the readback signal from being artificially flattened and prevents the "average" amplitude taa (i) from being exaggerated. Fig. 8a shows the original feed. Fig. 8b illustrates the envelope of the readback signal with the correct low pass filter and fig. 8c illustrates the envelope of the readback signal with the wrong low pass filter.

If equation 3 is invoked during the measurement, in other words if the microactuator is excited by more than one input voltage at the same frequency f (i), only the last input voltage value and the last time-averaged readback signal amplitude value are retained as V (i) and TAA (i). Measurements outside of region 2 may be discarded. For each head, one calibration curve is valid at all frequencies. At each frequency f (i), one v (i) and one taa (i) are used as raw data. Using the calibration curve, TAA (i) yields the stroke S (i). Fig. 9 illustrates a profile of stroke (in microinches) compared to frequency (Hz). G (i) is generated by equations 2, S (i) and V (i). Fig. 10 illustrates a profile of gain (microinches/volts) compared to frequency (Hz).

Although several embodiments are specifically illustrated and described herein, it will be appreciated that modifications and variations of the present invention are covered by the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the invention.

Claims (12)

1. A method of testing, comprising:

writing two concentric tracks at a predetermined pitch 2 δ on a noise-free portion of a magnetic storage medium;

positioning a read/write head and a micro-actuator between the two concentric tracks;

applying a first oscillating voltage v (i) at a first frequency f (i), where i ═ 1, to the microactuator while reading back a first signal from the two concentric tracks; and

based on the time-averaged amplitude taa (i) of the first signal, where i ═ 1, a first stroke characteristic s (i) of the micro-actuator is calculated according to the following equation, where i ═ 1,

where the test state index i is 1, taa (r) is the track average amplitude of the readback signal, and r is the radius measured from the track center.

2. The test method of claim 1, wherein the noise-free portion of the magnetic storage medium is generated by erasing a portion of the magnetic storage medium.

3. The test method of claim 1, further comprising determining the first oscillating voltage from a previous test.

4. The test method of claim 1, wherein the first stroke characteristic of the micro-actuator is based on a time-averaged amplitude of the first signal.

5. The test method of claim 1, further comprising:

applying a second oscillating voltage v (i) at a second frequency f (i), where i-2, to the microactuator while reading back a second signal from the two concentric tracks; and

based on the time-averaged amplitude taa (i) of the second signal, where i-2, a second stroke characteristic s (i) of the micro-actuator is calculated according to the following equation, where i-2,

wherein i is 2.

6. The test method of claim 5, further comprising calculating the second voltage by multiplying the first voltage by an ideal stroke characteristic and dividing by the first stroke characteristic.

7. The test method of claim 6, further comprising using a typical frequency response to predict a gain change between the first frequency and the second frequency.

8. A test system, comprising:

a magnetic storage medium to store data;

a read/write head for writing two concentric tracks at a predetermined pitch 2 δ on a noise-free portion of the magnetic storage medium;

a head gimbal assembly for positioning the read/write head and micro-actuator between the two concentric tracks; and

a tester for applying a first oscillating voltage v (i) at a first frequency f (i), where i ═ 1, to the microactuator while reading back a first signal from the two concentric tracks;

wherein a first stroke characteristic s (i) of the micro-actuator is calculated based on a time-averaged amplitude taa (i) of the first signal, where i-1,

where the test state index i is 1, taa (r) is the track average amplitude of the readback signal, and r is the radius measured from the track center.

9. The test system of claim 8, wherein the first stroke characteristic of the micro-actuator is based on a time-averaged amplitude of the first signal.

10. The test system of claim 9, wherein the tester applies a second oscillating voltage at a second frequency to the micro-actuator while reading back a second signal from the two concentric tracks.

11. The test system of claim 10, wherein the second oscillating voltage is calculated by multiplying the first voltage by an ideal stroke characteristic and dividing by the first stroke characteristic.

12. The test system of claim 10, wherein a typical frequency response is used to predict gain variation between the first frequency and the second frequency.