The read/write heads and the servo head are mounted on the end of the rotary arm actuator. When the disk unit is
turned off, these heads are resting on the outer edge of the disk platter; an area where data is not recorded. When the
disk unit is turned on and the drive motor begins to spin the disk platters, the heads slide on the lubricated surface of the
disk platters. As the speed of the platters increases, air pressure, and the aerodynamic characteristics of the head
design, cause an air-bearing to form which causes the heads to take off from the disk surface and "fly." The head
assemblies are designed such that between the air-bearing force that tries to lift the heads and the spring force that
causes them to land on power-off, they fly at approximately 19-microinches above the disk surface.
The rotary arm actuator positions the heads over the disk surface. The heads are mounted on one end of the arm and
the other end, a permanent magnet, is within a linear voice coil. Changes of current in the voice coil, caused by the servo
system, make the permanent magnetic end of the arm move and hence cause the heads to move. The rotary arm
actuator is statically balanced to make it less sensitive to shock, vibration, and physical orientation.
All electrical connections to the sealed volume are made through a small PCB on the top of the casting. This PCB also
contains the preamplifiers that amplify the weak read signals from the heads before sending them to the electronics PCB.
There are no active components within the sealed cavity of the HDA subassembly. The bottom cover and the small PCB
seal the cavity and they must not be removed unless the HDA is in a clean-room; otherwise, the cavity will become
contaminated. The legs of the component sockets on the PCB, when soldered in place, are also part of the seal; they
also must not be removed except in a clean-room. The interior of the cavity was designed such that the rotation of the
disk platters causes high- and low-pressure areas. The resulting circulating air flow is directed through a 0.3 micron
absolute filter within the sealed cavity. Thus, the air within the cavity is being continually filtered. Another 0.3 micron
absolute filter on the bottom cover is used to allow the cavity pressure to equalize with the outside ambient pressure.
Exterior of the HDA Subassembly. The exterior of the HDA, formed by the exposed area on the top of
the casting, contains the drive motor, motor-starting resistor, brake solenoid, and Commutator PCB.
The drive motor for the HDA is a brushless dc motor. The rotating portion of the motor consists of the permanent
magnets, while the stationary portion consists of the coils that cause the permanent magnet to move. The dc
commutated voltage to the coils is provided by electronic circuitry on the Commutator PCB. The rotating portion of the
motor consists of the permanent magnets and an aluminum hub with fins to create air movement for cooling. The
aluminum hub acts as a flywheel and counter balances the weight of the disk platters within the sealed unit.
The motor starting resistor is a high-wattage resistor, which is used to limit the current to the dc motor as it starts up.
Approximately 20 seconds after the unit is turned on, circuitry on the Motor Control PCB will switch the starting resistor
out of the motor circuit and thus apply the full +24V commutated voltage to the motor coils.