Laser doppler interferometry on magnetic recording systems

Laser Doppler interferometry is used to determine the six components of velocity of read/write heads in magnetic disk files. These measurements are made on production drives in which the only alteration is the installation of a window so that the laser beams can be directed onto any of the four corners of the slider. A laser Doppler vibrometer (LDV) is used to measure the out-of-plane velocity of the four corners from which we calculate the out-of-plane velocity of the geometric center as well as the pitch and roll of the slider. A laser Doppler anemometer (LDA) measures the in-plane velocity of the four corners, which gives the radial and tangential velocity of the geometric center and the yaw of the slider. We are also using the LDV for the measurement of disk runout and surface topography as well as suspension and actuator vibration. These measurements allow us to determine the natural frequencies of the slider bearing, the rotating disk, the actuator, and the suspension arm. In this paper we summarize some of the results we have obtained using this method on various 5-1/4" and 8" rigid disk drives that employ "Winchester" and "mini-Winchester" sliders together with stepper motor and servo controlled actuators of both the linear and swing arm designs.