A disturbance compensator with enhanced frequency tolerance for the micro-position control of magnetic heads

As the data track doubles its density almost in every year, none of the current disk drive platform is free from the various disturbances even under the normal operating environment. A typical example of the disturbance might be repeatable runouts caused by a spin stack. Many well-defined control methods for the control of the repeatable runouts are available and successfully implemented in actual drives. Although most of those methods are effective for the reduction of a disturbance having a fixed fundamental frequency, not many remedies are known to work for a disturbance that changes it's frequency during drive operation. An even slight frequency offset from the original fundamental frequency of a repeatable disturbance could create a wide range of complicate problems. Mechanical property variation of drive components due to a thermal effect such as spindle dynamics change would be one of the typical examples that generates the frequency shift from the original fundamental frequency during drive operation. In this article, a novel robust feedforward control method that effectively works for the drifting frequency disturbances is presented and tested.