A positioning method for hard disk servowriter using an auto focus laser encoder

A new servowriter positioning device for hard disk drives is presented. Existing servowriters usually use laser interferometry for position sensing. Therefore a servowriter is expensive, complicated and large in size. To avoid these problems we propose to use a laser incremental encoder with autofocus operation instead. In the device, an optical read head focuses three beams upon a grating scale. The middle beam is used to keep the read head in focus and two side ones to readout. The grating scale has two sets of the reflective grating displaced relative to each other by one eighth of the pitch width to give 90° out-of-phase signals. The optical read head can be the modification of CD or DVD pickup using the 3-beam principle, and can be naturally designed into the system. Therefore, the cost and size of the servowriter are significantly reduced. At the same time, accuracy can be maintained using an error-correcting grating scale. The positioning device can also be used as a track sensor to directly provide the track position signal of read/write (R/W) heads of HDA instead of a conventional servo pattern