Function Test Method of Active Damping for Hard Disk Drive
Original Publication Date: 2000-May-01
Included in the Prior Art Database: 2003-Jun-18
Disclosed is a method to test the function of active damping for hard disk drives using open loop gain of servo control system. It also takes units variance into consideration by way to select measuring points. It can distinguish easily failing drives and suits mass production. Typical hard disk drive's actuator/head system have several structural resonant mode which may cause servo system performance degradation. These modes are excited during track seeking and track following under servo system control. Active damping is one of the useful method to suppress actuator's main resonance by feedback of its vibration detected by a sensor on the actuator. If active damping doesn't work as designed, servo system performance may degrade and system may be unstable. Not only parts level test but also function test after assembled are indispensable. But main resonance frequency is varied due to unit-to-unit manufacturing tolerance such as bearings of the actuator pivot and adhesive and insulator variance. That is why function test of active damping after assembled is difficult. Figure 1 shows a flow chart of the test method. Before operation some points of measuring frequency are selected taking variance into consideration based on statistical distribution of actuator's main resonance frequency. Gain reduction of the servo controller is needed to make servo control system stable. Open loop gain of servo system is measured at each measuring frequency both when active damping on and off by micro code. Ratio of open loop gain when off to on shows the effect of active damping at each measuring frequency. Even if maximum ratio doesn't show significant difference, the drives has some failure in active damping. It uses the feature that active damping suppresses main resonance and measured gain when active damping is on should be much smaller than that when off if active damping works as designed (Fig. 2). Exception process is as follows : The drives which cause seek error when active damping is on have other error reason and should fail, and those which can seek when active damping is on and cause seek error when off should pass in the test. This method can distinguish failing drives easily and suits mass production. Performing this test at initial condition and after long term test under temperature stress condition makes it easy to select failing drives and analyze them.