Of course, there’s a lot more to drive testing than environmental test chambers. In this room, we encountered two machines dedicated to testing vibration. You see the first in the image below, a thigh-high barrel-like structure mounted on heavy-duty casters and tethered to a surprisingly large heat exhaust pipe given the size of the equipment. The reason soon becomes apparent once engineers turn it on and crank it up. The machine literally sounds like a jet engine revving for take-off. (Click through to our impromptu video clips to get a better idea.) The vibe table applies energy to the abused, attached drive in three dimensions, all controlled by the nearby workstation system, which also reads performance data from the drive during testing. As you can tell from the lack of attached power and data cables in most of the images below, engineers did not have drives in active testing while we were present.
In the screen shot below, we see the shake pattern sent to the vibration deck. Testing can be run at or above the specified product vibration frequency and pulse duration. It may not look like much in the videos, but it’s definitely “destructive” for a vibration test, particularly with a random vibe pattern. Passing and failing criteria are defined by the drive’s ability to read and write data during testing (operational) or after testing (non-operational). Different drive classes are expected to retain certain performance levels under given ambient vibration conditions. Finely controlled tests such as these are what confirm that performance.
Sometimes, engineers want to examine the impact of a specific axis of motion on drives. Enter the “slip table,” which confines motion to a single plane.
Not least of all, Seagate uses several rotary vibration tables. Clearly, these are smaller than the other machines, but they don’t have to be very large to mount a single drive and shake it about its central z-axis (perpendicular to the table surface). Rotary energy has a particularly pronounced impact on hard drives due to their internal spinning disks and the heads attempting to move across those platters in a rotary arch with minute accuracy. Rotary energy tends to kick those heads off-track, which results in no data being written or, worse, writing on an adjacent track. According to engineers, the importance of these smaller vibe tables is actually greater than their much larger cousins.
If you think that a drive mounted in a rack or PC won’t experience rotary energy interference, think again.
“Most energy gets converted to rotary because of the mechanics of the chassis and all the parts that are connected,” noted one technician. “Another source of rotary vibration can be the hard drive itself. It's doing random seeks, right? Well, that energy gets thrown into the drive base plate, which gets transmitted to the mount. That mounting scheme—it could be a hard mount, it could be isolators—will then kick that energy back into the base plate, and it can cause its own throughput hit, because it's creating a lot of rotary energy.”
This is one of the many reasons why nearline drives have accelerometers built into their PCBs. They measure rotary energy and feed the results into servo loops able to compensate and keep the drive literally on-track. Consumer class drives have no such countermeasures and will pay the performance price in the presence of vibration.