A Servo for a Magnetic Disk Drive Head Test System
A technical manager at a large manufacturer of magnetic disk
drives asked me to work with his group to develop a head-positioning
servo for the company's production head test systems. His group,
consisting of both mechanical and electronics engineers, had little
prior experience in control system development, while I had much
experience in this area from my work with optical disk technology.
I immediately identified a system architecture and the major
components. I began the electrical part of the project by working on
the development of the servo demodulator electronics, a board that
extracts head position information from the data on the disk. This
work comprised high-bandwidth, low-noise analog design, integration
of a very complex, high function chip with connecting logic, a
parallel interface with a DSP card, and some rather sophisticated
algorithms and logic for tracking the servo header information
across the surface of the disk and controlling the operation of the
board. I designed most of the custom logic with VHDL and Xilinx
FPGAs.
After the servo demodulator was working, I concentrated on
analyzing the test results to characterize its accuracy. I also
collected frequency response data on the test stand so I could
produce an accurate model of the system's dynamics. I created models
using MATLAB and explored a variety of control strategies. We
demonstrated that the servo removed thermal drift, the major cause
of inaccuracy in the head test, although we were disappointed in the
bandwidth we could obtain. The basic mechanical structure required
some redesign to eliminate resonances that limited the servo's
performance.
This project is a good example of a situation in which I join a
technical group or company as a senior engineer and/or advisor. My
duties, including interaction with other groups and upper
management, are basically the same as those of the other senior
technical people. This project also placed me in the enjoyable role
of mentor to a young engineer.
