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Head Stack Assembly

< Prev (Start)   Next > (Heads)

This section shows a close-up of the head stack assembly and other related parts.

The main assembly is called a head stack assembly and consists of

1. Actuator Magnetic Flux Circuit
   This forms a vital part of the Voice Coil Motor (2 below) and consists of two strong magnets that encapsulate the VCM. When an electric current is applied to the coil, it functions in exactly the same way as a motor and as the coil moves it moves the entire head stack assembly over the platters.
2. Voice coil motor (VCM) or actuator
   When a command is sent to the drive to read or write data, first of all the drive needs to position the heads over the correct area of the drive. To do this a voltage is applied to the coil. The voltage applied is proportional to the position on the platters where the located on the disk. As the electric current is applied to the coil, it 'rotates' and positions the heads over the correct area on the drive. Most modern drives use a feedback mechanism so that tiny changes in voltage can position the heads precisely (see description below).
3. Actuator Axis

This consists of a shaft and a bearing and is the axis on which the actuator arm roates when power is applied to the VCM.
4. Actuator Arm

The far end of the arms is where the head suspension arms are attached.
5. Head suspension

Made from spring steel or nickel plated beryllium-copper so as to be flexible to allow them to take off and land as the power is applied to the motor.
6. Parking Ramp
   Technically this is not a part of the head stack assembly. This mechanism is where the heads park when the drive is not in use. This type of parking technology is becoming more common and means that the heads should never come into contact with the platter, therefore prolonging the life of the drive.
7. Connector

Connects the internal head preamplifier circuit and VCM to the external electronics board.
8. Head Preamplifier
   This can just about be seen on the edge of the assembly. Its function is to amplify the signal from the read heads to remove 'noise' so that the data can be clearly identified by the main amplifier on the circuit board

The function of the VCM and Heads

The VCM serves as the head stack assembly positioning system and is controlled by the VCM controller on the circuit board. As each seek is made to read specific data on the drive, the processor on the circuit board calculates the exact position on the platters to read the data. The processor then sends the necessary commands to the VCM controller to get it to set the heads in position ready to read the data. The mechanics of this operation is;

The coil is mounted on the actuator assembly in close proximity to a strong permanent magnet (1 above). When current is applied to the coil, an electromagnetic field is created that forces the actuator arm - and therefore the heads - to move to a position according to the amount of current applied. By precise control of the voltage/current, the heads can be moved to the exact position needed ready to read or write the data.