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A continued trend for greater areal density and faster data transfer rates for hard disk drives (HDDs) places more demand on suspension windage performance. One way to increase areal density is to increase the number of tracks per inch (TPI), which requires a reduction in track mis-registration (TMR). The suspension’s contribution to off-track due to windage excitation must stay within the carefully allotted amount of this ever-tightening TMR budget. One approach to increase the data transfer rate and reduce latency is to increase the disk revolutions per minute (RPM). Higher disk RPM can negatively impact suspension-windage performance due to increased wind energy. For example, if you spin two identical drives at different RPM, the higher RPM drive is going to create a greater amount of wind energy due to increased disk velocity, and therefore, higher TMR. To satisfy the continuing trends of HDDs, tighter TMR budgets will require suspensions that exhibit less off-track due to windage, when exposed to increased levels of windage energy due to increasing disk speeds.
Interactions that determine the suspension’s windage driven off-track can be generalized into three separate variables: (1) source energy, (2) energy extraction, and (3) transfer function (see Figure 1).
Windage off-track occurs due to source energy that originates from fast spinning disks. Turbulent effects of the E-Block and other drive features also contribute to source energy. In terms of windage, the ideal case is to have disks spinning as slow as possible, thus creating minimal turbulence. The second variable is the suspension’s efficiency to extract energy from the source. Different suspension designs extract different amounts of energy from a given source, depending on part length, surface area, rail height, headlift feature, etc. The ideal situation would be a suspension with 0 percent efficiency for extracting wind energy. This way the suspension would not be susceptible to windage driven off-track, no matter how much source energy is present in the system. The third and final variable is the suspension’s transfer function. After a certain amount of wind energy is absorbed into the suspension, the transfer function dictates how it translates to slider off-track. For all suspension modes, the transfer function dictates a given ratio of output-per-input. The ideal goal is to have output minimized as much as possible, by having the ratio as close to zero as possible.
These three separate variables determine the magnitude of the suspension’s windage off-track. Optimization of these variables needs to be included with all of the other critical drive factors. By including windage issues in the optimization of performance tradeoffs, the end result should be a disk drive with the best possible balance of performance.
Dealing with Source Energy
There is a great deal of engineering effort going into designing suspensions that minimize energy extraction and the transfer function to combat windage effects. Some examples include shorter parts with less surface area to extract less energy, higher frequency beams to reduce slider off-track due to the frequency component, and better windage notch control. This article focuses on the third variable of potential improvement, and the source energy and how it impacts suspension excitation. By modifying hard-drive construction and operation, it is possible to reduce the source energy and subsequent negative effects on suspension performance.
One way to describe the magnitude and influence of source energy is with the Bernoulli Equation (see Figure 2). Assume that for any given system, off-track is related to dynamic pressure.
Fluid density and fluid velocity are the two primary factors, with velocity having a squared effect. For a given suspension, an increase in dynamic pressure will result in an increase in windage off-track (an increase in source energy with all else remaining constant).
Standard Flow versus Reverse Flow
In today’s disk drives, media rotates in a direction such that the media passes beneath the E-Block first. As air flows with the spinning media, the airflow encounters the E-Block first, with suspensions and sliders located downstream of this obstruction. In the “between-disk condition,” it is easy to visualize the affects, (see Figure 3A). First, the suspension is located in the E-Block’s wake—any turbulent flow generated by the E-Block can propagate downstream and strike the suspension. Second, the E-Block acts as a funnel, directing more airflow toward the restricted gate where the suspension is located. According to the conservation of mass flow, as the cross-sectional area of the flow region becomes restricted, the fluid density and/or the velocity must increase to account for the smaller cross-sectional area. Increases in these values increase the magnitude of the dynamic pressure acting on the suspension, thus adding to the windage-induced suspension off-track.
If the disks are rotated in the opposite direction, and the slider is rotated 180 degrees to allow for air-bearing flight, it is easy to visualize how airflow over the suspension would be much different (see Figure 3B). In this condition, referred to as reverse flow, the first item the media passes beneath is the slider and suspension, with the E-Block now on the downstream side. The suspension is located upstream of any turbulent effects of the E-Block, which no longer channels air towards the suspension. In this arrangement, suspensions are located in a more laminar and lower-velocity region than in the standard-flow condition. When comparing standard-flow to reverse-flow conditions, lab tests have shown results ranging from a 7 percent to a 49 percent reduction in excitation.
Reverse Flow with Downstream Air Dams
Further reduction in suspension-windage excitation can be achieved with the addition of “downstream air dams,” which build from the reduced level of source energy that reverse flow offers. These downstream air dams work by reducing the velocity of the air in the region where the suspension is located (see Figure 4).
The dams work by creating a zone with higher-pressure (lower velocity) air around the suspension. Oncoming air senses the higher pressure and diverts its flow around the pocket. Although this causes fluid density to increase, a reduction in velocity causes an overall reduction in dynamic pressure. Lab test results confirm additional improvements on some 11-millimeter-length suspensions, when used with reverse flow. Longer suspensions have shown limited success because they require larger dams to stagnate a larger pocket of air. Downstream-dam design is a feature that would need to be balanced with other drive factors, such as ID/OD clearance issues, mass, and power consumption.
General Performance
Frequency responses from two typical test setups are shown in Figure 5.
Once the frequency response is obtained, cumulative power (1 sigma off-track) can be calculated for standard flow and reverse flow. Figure 6 shows measured results for several different suspensions where the media size, revolutions per minute, and track location was varied. The first set of values indicates the measured, cumulative 1 sigma off-track, with the second set of values indicating improvements (in percentage of reduction) over standard-flow conditions.
The first section in Figure 6 measures results for 11-millimeter suspensions, beginning with a high-performance, multi-piece TSA suspension. Although it shows improvement in the reverse-flow condition, the addition of dams at the 65-millimeter format cause an increase in windage excitation. The next 11-millimeter suspension is a 4230 TSA, with a partial-etched radius. The final 11-millimeter suspension is a traditional thin-beam suspension, exhibiting the greatest improvement over standard flow, while the addition of downstream dams improves it even further.
The second section of Figure 6 measures results for 14.5-millimeter suspensions, beginning with a traditional thin-beam design (3430 TSA), which performed well in reverse flow for a large-media, slow-RPM application. The next 14.5-millimeter suspension is an actuated suspension (5130 TSA). In all cases, reverse flow offered some improvement over standard flow.
In addition to a reduction in windage off-track at the nominal suspension-offset height, reverse flow also improves the robustness of suspensions to stack-up tolerances. Due to a reduced level of input energy, suspensions show less absolute windage change with changes to stack-up Z-Heights. Figure 7 shows an example where the reverse-flow condition produces lower total variation for first torsion energy across a given Z-Height range. This is true for off-track bending modes as well.
Test Procedure
All tests were conducted in HTI’s development lab and consisted of a single suspension and slider that were glued to an arm to simulate an E-Block arm. The arm was slotted so that the actuator length could be adjusted. The actuator assembly was then attached to the test stand, which had a rotational stage to adjust track location. The test stand was set up so that the actuator was between two disks, with disk spacing determined by the disk-spacer thickness. The spindle motor was air-bearing equipped and the disks were not shrouded, as a true disk drive would be. The “pivot-to-pivot” length was also adjustable, so actual drive dimensions could be replicated on the test stand (see Figure 8).
Conclusions
Test-stand data has shown that reverse flow offers potential reductions in windage-induced suspension off-track. The lower amount of input energy is also seen as a benefit in terms of off-track robustness to product variation. Additional enhancements to windage excitation may be possible with the addition of downstream air dams. Performance testing of reverse flow in a true drive environment will be completed in the near future. HTI looks forward to working with other IDEMA members to help develop industry standards for testing windage.
Michael W. Davis has been with Hutchinson Technology Inc. (HTI) since 1997, initially as a Product Design Engineer and currently as a Senior Engineer in the Advanced Technology Development Group. He has a BS degree in mechanical engineering from Iowa State University. Activities include windage test-stand-correlation studies and suspension-design optimization for windage reduction.
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tel 408.330.8100 / fax 408.492.1425 / www.idema.org
IDEMA is a registered trademark of the International Disk Drive Equipment and Materials Association.
Reprinted with permission from INSIGHT March/April 2001, pp. 8, 10 & 11.
© Copyright 2001 IDEMA. All rights reserved.
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Figure 1. Interactions that determine suspension windage driven off-track.
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Figure 2. Magnitude and influence of source energy.
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Figure 3. Standard- and reverse-flow conditions.
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Figure 4. Reverse flow with downstream air dams.
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Figure 5. Frequency responses from two typical test setups.
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Figure 6. Measured results for several different suspensions, where the media size, RPM, and track location was varied.
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Figure 7. Example showing where reverse-flow condition produces lower total variation for first torsion energy across a given Z-Height range.
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Figure 8. Test procedure.
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