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Motion Controller Helps Test for Hard Disk Defects

Hard disks were originally intended for computers in ultra-clean, ultra-safe data offices; by the 21st century, hard disks had moved beyond the office, finding their way into digital cameras, video recorders, PDAs, and cell phones. With the ubiquity of hard disks comes the risk that any number of electronic devices can fail if their hard disk crashes. To detect hard disk defects while still in the assembly stages, THôT Technologies (Campbell, CA) developed the Model 42000, a laser-based test and measurement system for hard drives’ disks, using the DMC 1832 3-axis controller from Galil Motion Control (Rocklin, CA) to help monitor and regulate various systems of the Model 42000 as it conducts testing.

altThe DMC 1832, part of Galil’s larger 18x2 PCI bus series (with units controlling 1 – 4 axis formats available), controls three components of the Model 42000: the spindle (to rotate the disk), the Polytec Laser Doppler Vibrometer (LDV), and the microscope carriage. A 32-bit microcomputer provides PID compensation, with an uncommitted I/O for synchronizing external events. THôT specifically chose the DMC 1832 due to the 3-axis control; previous models had each axis controlled by a separate unit. THôT developed the Model 42000 test on the idea that as disks have come to store more information and at the same time shrink in size, the standard 4-point test of disks was obsolete. The tester consists of spinstand consisting of the Galil-controlled spindle, the laser slide to slowly scan the laser across the surface (to produce a spiral scan), and a microscope for doing failure analysis. An electronic console houses the various stepper motor and spindle driver, and computer processes the data acquisition system, while an electronics drawer houses the signal processing electronics.

Disk defects can cause the read-write head of the disk drive to crash into the surface of the platter. Revolving at almost 75 mph, such an impact can cripple or completely destroy the disk as the head skids across the magnetic film. Power loss, physical shaking, age, corrosion, contamination, or inherent defects of the disk can set off a “head crash” that precipitates potential data loss.

A disk, or “platter,” has certain topographical features that are characteristic, loosely referred as “surface morphology.” The Model 42000 uses information gathered by the LDV to categorize these features using wavelength as the discriminator. Some characteristics are pronounced, including cracks, pits, scratches, and morphological deformities due to waviness or roughness of the platter’s surface. Roughness describes tiny bumps on the surface of the platter (typically less than 20 microns in wavelength between the lumps. Waviness describes when the surface of the disk itself is rippled, and refers to irregular distances between the “crests” on the millimeter scale (two to five mm wavelengths). Both can occur on a flawed disk, and can confuse the read-write head into impacting the disk’s surface. The target morphology for roughness and waviness by the Model 42000 are 0.5 – 1.5 Å and 3 Å respectively. No disk is absolutely flat; the Model 42000 tester searches for those flaws that are too great for proper function to be assured.

Disks are placed on the spindle and then spun at 20,000 rpm. The LDV optical measuring system then conducts a non-contact spiral-track laser scan of the disk. The laser signal is then split into relevant wavelengths needed to do multiple measurements. Height values are registered on a computer monitor as a color gradient. The inspection maps the full surface of the platter. By spinning the disk in the test, the degree of disk flutter may also be observed for structural lopsidedness.