Head Actuator Systems The mechanical system that moves the heads across the  disk  is  known  as  the  head  actuator.  These mechanisms have to be extremely precise to position the heads over the proper cylinder. The two types of head  actuators  are  called  stepper  motor  actuators  and voice  coil  actuators. STEPPER MOTOR SYSTEMS.— The stepper motors used in fixed disk systems are very similar to the ones used in floppy disk systems. The stepper motor is generally located outside of the HDA, with just the shaft of the motor penetrating the HDA. Attached to the shaft is a steel band. The other end of this band is attached to  the  head/arm  assemblies.  As  the  motor  moves through its detents, the band will wind or unwind around the shaft and move the heads. A stepper motor in a fixed drive system has two major  disadvantages.  It  is  temperature  sensitive  and  the band can stretch over time. Ambient air temperature can cause minute changes in the size of the disk and stepper band. Since the tracks on a fixed disk can be l/1000th  of  an  inch,  these  size  changes  can  be significant enough to cause a loss of data. A new drive should be allowed to reach operating temperature before it is formatted. This will ensure that the data will be centered on the tracks unless there is a drastic change in temperature. The band that connects the head/arm assembly with the stepper motor shaft is made of steel and can stretch over  time. Again this will cause the heads to be misaligned with the tracks. A good safeguard against losing data to this problem is for you to backup the data and do a low-level format once a year. VOICE COIL HEAD ACTUATOR.—  A  voice coil head actuator works in the same way that an audio speaker does. An electromagnetic coil is connected to the head/arm assembly. As current is applied to the coil, it moves along a track and moves the heads. Movement of the heads in a voice coil actuator is very smooth, but the heads need a signal to tell them when to stop at the right track. One side of one of the disk platters can be dedicated to head positioning by having servo tracks permanently   written   on   it.   The   heads   are   then positioned in a manner similar to the disk memory set. Another method of head positioning used in voice coil actuators is to embed the servo signals in the sector gaps of  the  data  tracks.  This  eliminates  the  need  for  a dedicated surface. Voice coil actuators have several advantages over the  stepper  motor  actuators.  Since  the  heads  are positioned in relationship to the control signal on the disk, they are not temperature sensitive. The heads of a voice coil actuator are self-parking. When power is removed from the drive, the electromagnetic field that positions the heads collapses causing the heads to retract to the park position. Spindle Motor The spindle motor actually spins the disks. A direct drive system is used in all fixed disk drives. Originally, 3,600 rpm was the standard speed used by almost all fixed disk systems. Today, the speeds range from 3,600 rpm to 7,200. The spindle motor is controlled by a tachometer and feedback loop that monitor and adjust the speed of the motor. Logic  Boards All fixed disk drives have at least one logic board. Logic boards provide power to the motors and actuator, and monitor the speed of the disk. They also perform data conversions to a form usable by the controller. DATA ENCODING METHODS Data is stored on the disk by changing the direction of  the  magnetic  field  or  flux  reversals.  The  flux reversals  generate  pulses  when  being  read  from  the disk. Flux reversals are sensed as a positive to negative or  negative  to  positive  pulse.  In  storing  data  in nonretum  to  zero  format,  a  flux  reversal  would  indicate a logic ONE and no flux reversal would indicate a logic ZERO. When reading data from a disk drive, the drive and the disk controller must be synchronized for proper operation. The disk controller uses the flux reversal pulses as timing and synchronization signals in addition to data. Therefore, if a long string of zeros are being read  from  the  disk,  the  possibility  exists  that  the controller could “get lost” because of a lack of pulses. To prevent a loss of synchronization, several methods of encoding data have been developed. These are as follows: l  Frequency  modulation  (FM) .  Modified  frequency  modulation  (MFM) . Run length limited (RLL) 10-25