Data Recording Fundamentals.
Binary information is created on the disk surface by inducing tiny areas of its recording material to one of two possible magnetic polarities. These tiny areas are called bit cells, each cell consisting of several grains of the disk magnetic material alloy. If all grains in a bit cell are magnetized in the same polarity, it is said to be storing a binary ‘0’ and a transitioned bit cell is considered as storing a binary ‘1’. The bit cells are created by the write head as the disk spins causing the bits to be arranged in concentric circles, known as the tracks. There can be as many as 100,000 tracks on each surface of a 3.2 inch format HDD. Recording (or writing) data as binary conditions and playback (reading) is performed with a write head and a read head, respectively.
Hard Disk Drive Precision Operation.
Each head assembly consists of a bonded matched pair of read and write head transducer elements. The two heads are fabricated on a single slider, which is epoxy-bonded to a stainless-steel or aluminium gimbal at the end of a long and thin structure known as the suspension arm.
Each surface of a hard disk drive is accessed by a dedicated head slider mounted on the tip of a suspension arm. Suspension arms carrying sliders for different surfaces are attached to a single actuator who’s movement is controlled by a motor popularly known as Voice Coil Motor (VCM). The movement of the head sliders between any two tracks is effectuated by the VCM actuator. The VCM also regulates the position of the head over the centre of a track whilst data is being written on or read from that track. It is essential that the heads are positioned above a track with minimum variance from a centre position. The tracking error during regulation of head position must be less than 10% of the track pitch (distance between two adjacent tracks).
When the disk spins at high speed, an air bearing is formed between the slider and the spinning disk that makes the slider lift a small distance above the disk surface. The suspension arm is designed such that it produces precise load force and dampening perpendicular to the disk surface. The movement of the head actuator arm parallel to the disk surface is controlled by a Voice Coil Motor.
Read Write Head Positioning.
With recording densities now greater than 100, 000 tracks per inch (TPI), i.e. centres of two adjacent data tracks separated by 10 µ-inch or 0.25 µ-m, head positioning has to be within nano-meter distances and contend with environmental disturbances that adversely affect the position of the slider and actuator arm. Transitioning between tracks in normal operation is achieved within milliseconds.
The read/write head is expected to fly above the centre of a data track as precisely as possible while writing binary information on the data track or retrieving it from the track. Deviation of the head from this desired position increases the probability of occurrence of erroneous bits by either accidental overwriting on adjacent track or unwanted interference from the adjacent track, producing what is known as bad sectors. Errors permitted are of the order of 1 error per 10, 000, 000, 000 bits read from the disk.
Reference marks that define the centre of the track are created on the disk at the time of servo track writing. These references are special magnetic patterns written in designated areas on the disk surface known as servo sectors. The servo sectors are created at the time of manufacturing and are never overwritten or erased. The closed loop servomechanism uses the feedback signal generated by decoding the information written in these sectors.
The servo information and user data are multiplexed in space around the track. When the disks spin, this spatial multiplexing becomes temporal multiplexing. The feedback signal contains digitally coded track number and Position Error Sensing (PES) information. The PES signal is proportional to the radial distance between the track-centre and the actual position of the read head.
There are separate heads for reading and writing but the two heads are fabricated on a single slider and all heads are moved simultaneously and tracks and sectors are accessed one surface at a time.
The head positioning servomechanism moves the read/write head as fast as possible from one track to another when asked by the host system (Track Seek). Once the head reaches the target track, it is regulated precisely over the track such that the PES is minimized (Track Following). Smooth settling, i.e. transition from “track seek” to “track following” is an important aspect of operational efficiency expected of a HDD servomechanism.
Read Write Head Micro-Jog parameters.
The angle between the slider’s orientation and the track underneath varies as a function of the radial position of the track. The orientation of the read write head assembly with respect to an individual data recording track affects the magnetic patterning and the amplitude of the read back signal.
Data-bits are written using a thin film inductive (TFI) head whilst the read head is a magneto-resistive (MR) sensor. As a consequence there always exists a physical gap between the read sensor and the write head. The read head is also used to sense the servo patterns from the servo sectors which are used to derive the position feedback signal. The read head is used as the position sensor. During the operation of data reading, it is the read head whose position is regulated by the servomechanism, making the sensor and point of control collocated. On the contrary, during write operation, the point of control is the write head but position feedback comes from the read head. If the gap between the read head and the write head is known then this information can be taken into account as an offset value. This offset is known as the micro-jog distance. Since the slider moves in an arc, the micro-jog distances for read and write are different; The micro- jog parameter is needed as a part of the hard disk drive control firmware and varies for specific hard drive models and also the parts used in the manufacturing assembly and initialization process.
Replacing Hard Disk Drive Read Write Heads.
When failed read write heads require replacement sourcing a “micro jog” match is essential. Unfortunately micro-jog parameters for a specific hard drive are not published by manufacturers. This information is extracted from working drives and these drives are then brokered between data recovery practitioners at an added premium. This adds a new level of complexity to Datlabs hard drive data recovery processes, to the scarcity of spare parts, time scales and also uncertainty of the outcomes of our data recovery procedures.