Positioning System of HDD Magnetic Heads
The rotary mechanism of the positioning system resembling the tower crane in design is put on an axis. On the one of this axis there is a long thin light supporting frame ending by magnetic heads and turned to disks, and on the other side of it, there is short and more massive shank with the electromagnetic drive coil which is surrounded by a permanent magnet. At supplying the coil with a current of individual strength and polarity, the rocker arm starts to turn in the necessary side with corresponding acceleration, providing moving of magnetic heads on an arch between the center and periphery of a magnetic disk. Dynamically changing the current in the coil positioning system, it is possible to set magnetic heads on the necessary track of the magnetic disk. The electromagnetic drive moving the block of heads was given the name "Voice Coil" owing to direct analogy to the device of an electrodynamic loudspeaker (interaction of fields of a permanent magnet and winding). The maximum angle of rotation of the positioner is about 30°. Extreme positions of the positioner are limited by mechanical fixing devices (stops).
Management of the servo driver is optimized according to the time of moving to the block of heads to a demanded track, i.e. if a deflection from the prime position is significant, the large current causing the fast acceleration and quick flight of the block of record/reading heads is supplied. While the calculated track is being approached, the current decreases. For compensation of inertia at the end of the flight the current changes a direction, providing electric braking.
In hard drives the positioning system has no discreteness, and placing on track is made according to servo marks that provide a substantial growth of the accuracy of positioning and record density and also reduces the influence of temperature and other adverse factors to the accuracy of reading of the information. Servo marks are marked out by electronics of an HDD from a signal which is continuously being read out by a magnetic head from a magnetic surface of the rotating disk; it allows defining their position against the magnetic disk and provides keeping on a required magnetic track.