Actuator and head positioning system

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b2ap3_thumbnail_iStock_000014700978XSmall.jpgEvery modern HDD actuator is equipped with the actuator coil. Such actuator is resistant to thermal expansion. It is also faster and more reliable than a stepper motor actuator. All modern HDDs use a solenoid actuator.

A closed-loop feedback system used in such actuator is called a servo positioning system. In this system, target approach process can be described in stages: approach to target, direction adjustment, distance judgment, travel check. After the final stage is over, the process repeats itself. Thus target approach process is accompanied by adjustments based on various factors.
Servo system functions as a car driver. If a car starts to run off the course, a driver twists the steering wheel to adjust driving direction.

The key element of a closed-loop feedback system is a measuring device. In case of a car driver, a measuring device is his eyes. In case of HDD, it is a read/write head. To enable the head to perform its measuring function, magnetic platters store special codes. The codes inform the HDD of the heads’ position at any given time irrespective of the HDD’s work mode. These codes are usually called servo codes. They are read by the heads and fed back to the actuator control logic, where they are decoded to locate the heads. Since all servo codes differ from each other and contain precise information about the track number, servo code number, etc., the disk controller always “knows” heads’ exact position.

There are three different ways of HDD servo mechanism implementation.

1. Wedge servo: This implementation was used in early HDDs. The servo information was recorded in the “wedge” of each platter. It is like a thin slice of a cake while the rest of the cake is the data. A drawback of this technique is that the servo information is stored only in one place on the whole track. This causes slow positioning since to get information about the head’s position one must wait a full rotation of the disk platter until the servo information occurs under the heads. As a result, this technique is no longer in use.

2. Dedicated servo: In this implementation of the servo mechanism, the entire surface of one disk platter is “dedicated” to the servo information while the rest of the disk platters contain only data. This technique considerably reduces positioning time. Though it has drawbacks too. Firstly, the entire disk platter is wasted since it cannot contain data. Secondly, the head reading servo information may not necessarily be lined up with the other heads. Thus compensation technique must be applied. Thirdly, if the temperature of the disk platter containing servo information is different from that of the disk platters containing data, positioning errors as well as read/write errors will take place. To solve this problem, manufacturers equipped HDDs with the thermal recalibration technology. Such servo systems were used in HDDs until the mid-90s.

3. Embedded servo: In this technique, servo information is interspersed with data. Servo information and data are read by the same heads. Contrary to the wedge servo, one doesn’t need to wait a full platter rotation period to position the head. Unlike the dedicated servo, this technique doesn’t provide constant access to positioning information. However, thermal recalibration is no longer required since the servo information and the data are at the same distance from the disk center. When the temperature rises, the servo information and the data move away from the center together. This technique is implemented in all modern HDDs.

In modern HDDs, the servo information is recorded to the disk platter at the time when it is manufactured. This is done with the help of sophisticated and very expensive equipment called a servowriter. The servo information is never changed through HDD’s life. The controller blocks heads from writing to the areas where the servo information is stored. Creation of this information is the low-level formatting of the HDD. The low level formatting of a modern hard disk drive can be done only at the factory. A user can damage the servo information only if they deliver physical shock to the HDD.

The picture illustrates the difference between the dedicated servo and the embedded servo. To the left is the dedicated servo: only one disk platter contains servo information, the rest of disk platters contain only data. To the right is the embedded servo: the data and the servo information are stored together. For clarity, the picture on the right shows only one track. All tracks have similar structure and may differ from each other only by the number of servo codes, which depends on the distance of a track from the disk center.


Last modified on Friday, 22 May 2015 14:18
Data Recovery Expert

Viktor S., Ph.D. (Electrical/Computer Engineering), was hired by DataRecoup, the international data recovery corporation, in 2012. Promoted to Engineering Senior Manager in 2010 and then to his current position, as C.I.O. of DataRecoup, in 2014. Responsible for the management of critical, high-priority RAID data recovery cases and the application of his expert, comprehensive knowledge in database data retrieval. He is also responsible for planning and implementing SEO/SEM and other internet-based marketing strategies. Currently, Viktor S., Ph.D., is focusing on the further development and expansion of DataRecoup’s major internet marketing campaign for their already successful proprietary software application “Data Recovery for Windows” (an application which he developed).


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