Date of final exam: 22/02/1999

E-mail: argondiz@athena.polito.it

Tutor: Prof.  B. Bona, Politecnico di Torino

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Modeling and Control Design for Hard Disc Drives on Active Magnetic Bearings   ___________________________________________________________________________________________________________

Advisor:

Prof.  B. Bona, Politecnico di Torino

Summary of the thesis

The present work started from an international research cooperation between the Mechatronics Laboratory of the Center for Prototyping Services of the Politecnico di Torino and the group of Prof. Bleuler of the Institut de Microtechnique of the École Polytechnique Fédérale de Lausanne (EPFL). The subject of cooperation is on the active magnetic suspension and its digital control. In particular the research on hard disc drives on active magnetic bearings was commissioned by one of the most important hard disc manufaturer in the world, Seagate.
Several prototypes have being designed, built and tested at the EPFL: the present thesis specially deals with the rotordynamic analysis, the modeling and the digital control design.
The project requires wide variety of knowledges in mechanics, electronics and control science, thus constituiting a real mechatronic design. Furthermore important topics as low power consumption, high level integration, and in particular mass production of a low cost industrial product must be taken into account. This latter constraint introduces tight specifications in the design.
One of the most limiting factors in increasing data storage density on the disks is the presence of non predictable vibrations (non repetitive runout) of the rotating disks. These are caused by the traditional spindle ball bearings and the misalignment of the stack of disks on the rotor hub (disk fluttering). A very good understanding of the rotordynamic behavior of the disc stack is of paramount importance in the design of these devices. Contact free suspension by active magnetic bearings (AMBs) helps to solve some of the major problems caused by spindle ball bearings. Basically, AMBs have no wear and therefore unlimited lifetime. There is no need for lubrication which makes AMBs very suitable for the HDD environment. They can actively damp vibrations occurring in the system.
The multi-channel digital controller for the active magnetic bearings is based on a FE model taking into account the flexural modes of the hard disks at the operating rotational speed of 10000 rpm. The FE model is modally reduced to obtain a pair of models to be used in the control design. A lower order model accounting for the modes to be actively controlled and a higher order model to verify the closed loop stability of the state feedback realized by means of an observer based on the former one. Finally, a digital control board has been designed, based on a digital signal processor (DSP), from Texas Instruments, the TMS320F240, to achieve a realistic (in the meaning of a realization not too different from an industrial one) controller.
 

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