Electrical noise from motion control components can cause erroneous readings in a system that tests disks for computer hard drives. Here is how engineers minimized the EMI and eliminated these errors.
The insatiable demand for more computer storage has been pushing magnetic disk capacities higher and disk drive sizes smaller. As this trend continues, the need to accurately measure disk characteristics is intensifying. But electromagnetic interference (EMI) from motion control components in the disk verification equipment skews the test results, affecting accuracy and reliability.
These verification systems test computer hard drive disks for bandwidth and glide capabilities, plus they burnish each disk. To test disk attributes such as runout, plus velocity and acceleration capabilities (RVA), a verification system performs closely controlled motions that simulate the disk’s rotation and precisely position sensor heads, usually in micro-inches.
Engineers at ProQuip Inc., Santa Clara, Calif., have continually expanded their verification tester capabilities to accommodate diminishing geometries, plus increasing speeds and media densities. These verification systems incorporate state-of-the-art capacitive gage technology in the precision measurement heads. Because a capacitive gage system can be highly susceptible to EMI, it is imperative that emissions from the motion control components do not cause signal interference.
To maintain the integrity of their measurement systems, the engineers use various techniques to minimize EMI effects. These techniques range from shielded cables, special enclosures, and component relocation, to waveform averaging of test results to eliminate noise-generated readings. Not surprisingly, the most success has been achieved by eliminating the potential for EMI at its source — the motion control components, which include the motor, drive, cables, and power supply.
By incorporating a P325 series microstep motor-control package, engineers achieved the precise head movements required while keeping EMI noise well within FCC Class A specifications that govern electromagnetic interference with radio frequency communications. This motor-control package from American Precision Industries (API), Buffalo, N.Y., which is the basis of the motion control system in ProQuip’s MG150 media (disk) tester, uses twisted pair wiring, shielded motor cables, and an isolated linear power supply to minimize EMI. Because step motor systems require a constant electrical current, linear transistors control the P325’s four-phase unipolar drive output stage so that no electrical chopping noise or mechanical noise is generated at standstill.
The measurement heads in the media tester have minimum signal levels in the 100 to 200 mV range, making it imperative that system noise be held below 100 mV. The drive package from API, with its linear power supply and bi-level voltage drive design, meets the low-EMI requirement without sacrificing the tester’s speed and torque performance demands. Also, the package uses microstepping to obtain finely controlled motor motion. With a microstep resolution of 50,000 steps per revolution, this package achieves maximum positioning accuracy of ±1 microstep (0.0072 deg).