Lapping Control of Hard Disk Drive Heads

Abstract

Bar lapping is a key process in fabricating computer hard disk drive heads. The purpose is to remove a small amount of material so that the transducer stripe height variation across the bar is minimized. In this article a method of simulating the material removal process is established. The Preston model is identified to be a reasonable constitutive relationship relating lapping rate to pressure and relative velocity. A pressure estimation method is established based on the modified beam on elastic foundation model (MBOEF) to incorporate the effect of surface shape. Simulation is conducted based on the proposed constitutive relationship and MBOEF. Results show good agreement with production observations. Hence the simulation model is used as a tool for identifying the structure of the lapping control model. The pressure dynamics of bar lapping is then identified. The process model is constructed based on the pressure dynamics and the Preston model. A controller is then designed using a trajectory following formulation. Simulation of the controller shows that it is robust to parameter uncertainties. This controller is implemented on a Seagate lapping system. Experiment results show that the proposed method is less sensitive than the existing method to the quality of the bar preparation processes prior to lapping. However, the gains should be appropriately adjusted for optimal performance. With good preparation processes and appropriate gains, performance of the proposed and existing methods are statistically comparable. Otherwise, the proposed method performs better. The proposed method is inferior to the existing method only when its gains are improperly adjusted.