WO2000004550A2

WO2000004550A2 - Method and apparatus for biasing a magnetoresistive head with constant power dissipation

Info

Publication number: WO2000004550A2
Application number: PCT/US1999/016210
Authority: WO
Inventors: Stefan A. Ionescu; Bernardo Rub
Original assignee: Seagate Technology, Inc.
Priority date: 1998-07-16
Filing date: 1999-07-15
Publication date: 2000-01-27
Also published as: WO2000004550A3

Abstract

Methods and apparatus for biasing a magnetoresistive (MR) head (130) wherein the power dissipated by the MR head (130) is substantially constant are provided. A method of maintaining a constant power dissipation through the MR head (130) is provided wherein the head power dissipation is dynamically maintained at or near a desired level. Also provided is a disc drive (100) which includes a dynamic MR head power dissipation control loop (158) which continuously adjusts the bias current to achieve an essentially constant power dissipation through the MR head (130).

Description

METHOD AND APPARATUS FOR BIASING

A MAGNETORESISTIVE HEAD WITH

CONSTANT POWER DISSIPATION

FIELD OF THE INVENTION

The present invention relates generally to disc drive data storage systems. More particularly, the present invention relates to methods and apparatus for biasing a magnetoresistive data head.

BACKGROUND OF THE INVENTION

A typical disc drive includes a drive controller, an actuator assembly and one or more magnetic discs mounted for rotation on a hub or spindle. The drive controller controls the disc drive based on commands received from a host system. The drive controller controls the disc drive to retrieve information from the magnetic discs and to store information on the magnetic discs. The actuator assembly includes an actuator coupled to the drive controller and an actuator arm for supporting a head-gimbal assembly over each magnetic disc. The head- gimbal assembly carries a data head comprising a hydrodynamic air bearing and a transducer for communicating with the surface of the disc. The actuator operates with the drive controller in a servo system. The actuator moves the data head radially over the disc surface for track seek operations and holds the transducer directly over a desired track on the disc surface for track following operations. The transducer reads data from and writes data to the magnetic disc by sensing or creating a changing magnetic field. A read/ write preamplifier is connected to the transducer at first and second head contacts. The preamplifier includes a read circuit and a write circuit for controlling the read and write operations. The transducer includes a magnetic-field sensing element used to "read" information stored on the magnetic disc. The information is stored on the disc as a series of small magnetic domains which produce a series of small localized magnetic fields. To read the stored information, the sensing element passes over the disc and creates an electrical signal based on the direction of the magnetic fields. The read circuit converts the electrical signal into a digital signal that is decoded to reproduce the stored information.

One type of sensing head, known as a magnetoresistive (MR) head, reads the stored magnetic information by measuring changes in the electrical resistance of a magnetoresistive element, or stripe, within the head as the head passes through the magnetic fields. To convert the changes in resistance into a usable voltage signal, magnetoresistive elements are typically biased with a current that creates a voltage drop across the magnetoresistive element. Typically, the bias current is controlled so that any changes in the voltage drop across the magnetoresistive element are attributed to changes in the resistance of the magnetoresistive element. Thus, when the head is properly biased, the voltage across the magnetoresistive element tracks the changes in the magnetic fields, making the voltage useful as a read signal. Most existing MR head preamplifiers bias the magnetoresistive stripe at a constant bias current. U.S. Patent No. 4,712,144 to Klaasen discloses biasing the MR head with a constant voltage across the MR head.

The higher the current applied to the MR head, the larger the output signal. The magnitude of the current, however, must be limited to avoid overheating the MR element, and to avoid electromigration of the magnetoresistive material. Stripe height, the dimension of the MR element perpendicular to the media, will vary from device to device due to manufacturing tolerances. The width (length in the direction of current flow) and thickness of the MR stripe also vary due to manufacturing tolerances. This dimensional variability results in a variability in the resistance of the MR element within a population of heads. With constant current or constant voltage biasing methods, this variability in the resistance of the MR element gives rise to a variability in the power dissipated by the MR element. This variability in the power dissipation, in turn, gives rise to varying temperature increases.

For example, an MR head with a relatively short stripe height will have a higher resistance than an MR head having a taller stripe height and will therefor dissipate more power. The MR head having the shorter stripe height will therefor operate at a higher temperature than the MR head having the taller stripe height. Product life is inversely related to operating temperature. Thus the life expectancy for the low stripe height and thin MR elements is much shorter than for the higher and thicker MR elements. Thus, in a disc drive having multiple MR heads, the current to all of the MR heads must be reduced in order to accommodate the higher resistance MR heads. Otherwise the useful life of the higher resistance MR heads will expire prematurely. But reducing the current to the MR heads results in a reduced output signal.

Also, the resistance of an MR element varies with temperature. Thus, as the temperature varies, the resistance of the MR element, and therefor the power dissipated by the MR element, changes. This further affects the temperature of the MR element. Additionally, the resistance of an MR head gradually increases over the life of the device. This phenomenon is related to the leads within the MR head. This increase in resistance results in an increase in power dissipation and a corresponding increase in operating temperature.

Therefor there is a need for a method of biasing the MR heads in such a way that reduces the temperature variations over a population of heads, as well as the temperature variations of a given head over time.

The present invention provides a solution to these and other problems and offers other advantages over the prior art.

SUMMARY OF THE INVENTION The present invention relates to methods and apparatus for biasing a magnetoresistive data head. One embodiment of the present invention is directed to a method of biasing a magnetoresistive head in a magnetic data storage device. A bias current is provided to the magnetoresistive head and a substantially constant power dissipation through the magnetoresistive head is maintained. In one embodiment of the present invention, the power dissipation through the magnetoresistive head is continuously controlled with a dynamic control loop. In one implementation of this embodiment, a desired power dissipation through the magnetoresistive head is stored. During the operation of the magnetoresistive head, the following steps are continuously repeated. First the actual power dissipation through the magnetoresistive head is measured. Next the actual power dissipation is subtracted from the desired power dissipation. Then the bias current provided to the magnetoresistive head is adjusted based on the difference between the actual power dissipation and the desired power dissipation.

Another embodiment of the present invention is directed to a disc drive having a magnetic disc, a magnetoresistive head and a bias circuit. The magnetoresistive head is positioned adjacent the magnetic disc. The bias circuit provides a bias current to the magnetoresistive head and maintains a substantially constant power dissipation through the magnetoresistive head. In one embodiment of a disc drive according to the present invention, the bias circuit includes a power dissipation control loop which dynamically maintains the power dissipation through the magnetoresistive head at a desired level.

These and various other features as well as advantages which characterize the present invention will be apparent upon reading of the following detailed description and review of the associated drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a top view of a disc drive 100 in accordance with one embodiment of the present invention.

FIG. 2 is a block diagram of an MR head bias circuit in accordance with an illustrative embodiment of the present invention.

FIG. 3 is a graph depicting the resistance distribution of a hypothetical population of MR heads. FIG. 4 is a graph depicting the power dissipation distributions of a hypothetical population of MR heads for various biasing methods. FIG. 5 is a block diagram of an MR head bias circuit in accordance with an illustrative embodiment of the present invention.

FIG. 6 is a flow chart representing a method of dynamically controlling the power dissipation in an MR head in accordance with an illustrative embodiment of the present invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS FIG. 1 is a top view of a disc drive 100 in accordance with one embodiment of the present invention. Disc drive 100 includes a disc pack 106 mounted for rotation about spindle 109. Disc pack 106 includes a plurality of individual discs, each of which include concentric tracks, or cylinders, for receiving and storing data in the form of magnetic flux reversals encoded on the tracks. Disc drive 100 also includes an actuator

116 mounted to a base 102 and pivotally moveable relative to discs 106 about pivot shaft 120. Actuator 116 includes an actuator arm assembly

117 which includes a plurality of actuator arms 114. Each actuator arm 114 is attached to one or more flexure arms 112. Each flexure arm 112 supports a data head 110. Data head 110 includes an air bearing, or slider, which supports a transducer for reading information from and encoding information to one of the discs 106. In an illustrative embodiment, actuator 116 includes a voice coil motor, shown generally at 118. Disc drive 100 further includes a drive controller (not shown) which is coupled to a host system or another controller which controls a plurality of drives. In an illustrative embodiment, the drive controller is a microprocessor, or digital computer. The drive controller is either mountable within disc drive 100, or is located outside of disc drive 100 with suitable connection to actuator 116. During operation, the drive controller receives position information indicating a track of the disc 106 to be accessed. The drive controller receives the position information from an operator, from a host computer, or from another suitable controller. Based on the position information, the drive controller provides a position signal to actuator 116. The position signal causes actuator 116 to pivot about pivot shaft 120. In an illustrative embodiment, the position signal comprises a current supplied to the voice coil motor 118, causing actuator 116 to pivot about pivot shaft 120. This, in turn, causes data head 110 to move radially over the surface of the disc 106 in a generally arcuate path indicated by arrow 122.

The transducer reads data from and writes data to the magnetic disc 106 by sensing or creating a changing magnetic field. A read/ write preamplifier is connected to the transducer at first and second head contacts. The preamplifier includes a read circuit and a write circuit for controlling the read and write operations. The transducer includes a magnetoresistive (MR) head 130, shown in FIG. 2, used to "read" information stored on the magnetic disc. The information is stored on the disc 106 as a series of small magnetic domains which produce a series of small localized magnetic fields. To read the stored information, MR head 130 includes a magnetoresistive element, which passes over the disc 106. The resistance of the MR element increases or decreases based on the direction of the magnetic fields. The transducer reads the stored magnetic information by measuring the changes in the electrical resistance of the MR element, or stripe, within the transducer as the MR head 130 passes through the magnetic fields.

FIG. 2 shows a bias circuit 128 in accordance with an illustrative embodiment of the present invention. To convert the changes in the resistance of MR element into a usable signal, in an illustrative embodiment, MR head 130 is biased with a bias current IB produced by bias current generating circuit 132. Bias current IB creates a voltage drop Vo 134 across the MR head 130. The bias current IB is controlled so that any changes in the resistance of MR head 130 can be determined as a function of the output voltage Vo 134 across the MR head 130 and the bias current IB. The read circuit converts the output voltage Vo 134 into a digital signal that is decoded to reproduce the stored information. It should be noted that, in accordance with the present invention, the MR head bias may also be provided by a voltage source and the change in resistance of MR head 130 may be detected by measuring the change in the current through the MR head 130.

According to the present invention, the MR element is biased such that the power dissipated by the MR element remains essentially constant. In an illustrative embodiment, constant power is dissipated in the entire MR head 130, which includes the MR element and the contacts. The power dissipation is a good indicator of the MR head temperature. As a result, maintaining a constant power dissipation reduces the temperature variations that are due primarily to process tolerances over a population of heads.

FIG. 3 illustrates an MR head resistance distribution 140 for a representative population of MR heads. The x-axis 142 represents the resistance in ohms of the population of MR heads. The y-axis 144 represents the number of MR heads. For the purposes of this illustration, the MR stripe height variation, which is the dominant contributor to the resistance of the MR head, is assumed to be the only head parameter that has a non-zero tolerance. This illustration assumes a typical variation in the stripe heights of the various heads. It can be seen that the MR head resistances for this population of heads varies from about 27 ohms to about 40 ohms. It can also be seen that the majority of MR heads have a resistance of between 31 and 35 ohms and that a relatively small number of heads have a resistance approaching the upper limit (~40 ohms) and lower limit (~27 ohms). It should be noted that the resistance numbers used for this illustration are merely hypothetical and used only to illustrate the variance in the resistances of a population of MR heads. That being said, the values used are typical. FIG. 4 shows a power dissipation distribution corresponding to the population of heads represented in FIG. 3 for each of three bias methods: constant current biasing 150, constant voltage biasing 152 and constant power biasing 154. The x-axis 156 represents the power dissipated by the population of MR heads and the y-axis 158 represents the number of MR heads. For the purposes of this comparison, the maximum allowable power dissipation for any given MR head is assumed to be 5 milliwatts (mW). With the constant current biasing scheme 150, it can be seen that the population of heads dissipates anywhere from about 3.5 mW to 5 mW. Because the maximum allowable power dissipation is 5 mW, the magnitude of the constant current provided to the population of heads must be such that none of the MR heads dissipate more than 5 mW. Thus, the MR heads that are at the high end of the resistance distribution 140 (that is, -40 ohms) will dissipate approximately 5 mW. However, at that same constant current, MR heads having resistances of less than -40 ohms will dissipate correspondingly less power. This variation in the power dissipation results in a variation in the operating temperatures of the various MR heads. Also, because the magnitude of the constant current must be chosen to prevent the highest-resistance MR heads from dissipating more than 5 mW, the vast majority of heads are biased with a current having a magnitude that is less than that which the 5 mW power dissipation limit would allow. This decreased bias current level results in a weaker output signal.

The power dissipation distribution of the constant voltage biasing scheme 152 suffers from drawbacks similar to those of the constant current biasing scheme 150. It can be seen that the population of heads dissipates anywhere from about 3.5 mW to 5 mW. This variation in the power dissipation results in a variation in the operating temperatures of the various MR heads. Also, similarly to the distribution of the constant current biasing scheme 150, the vast majority of heads are biased with a voltage having a magnitude that is less than that which the 5 mW power dissipation limit would allow. This decreased bias voltage level results in a weaker output signal.

In contrast to the power dissipation distributions for the constant current 150 and constant voltage 152 biasing schemes, the constant power biasing scheme of the present invention produces a power dissipation distribution 154 in which all of the MR heads in the population have a power dissipation substantially equal to 5 mW. Because all of the heads in the population dissipate substantially the same amount of power, the variation in the operating temperatures of the various heads is mi irnized. Also, since all of the heads are biased with a constant power dissipation that is at or near the 5 mW power dissipation limit, the strength of the output signal is maximized. In an illustrative embodiment of the present invention, maintaining a constant power dissipation through the MR head 130 is accomplished by dynamically controlling the power dissipation through the MR head 130. In an illustrative embodiment, this is achieved by including a continuously active control loop in the read/ write preamplifier so that the desired power dissipation can always be attained without any software intervention. This approach has several advantages. Firstly, it allows the constant power dissipation to be maintained even as the MR head resistance changes as a function of temperature. Secondly, the continuous constant power control loop is implemented in hardware and is totally transparent to the operation of the drive. It does not require any software support. Also, it is not necessary to store a bias current for each MR head 130 based on power dissipation. FIG. 5 shows a bias circuit 128 in accordance with an illustrative embodiment of the present invention in block diagram form. Bias circuit 128 includes an illustrative embodiment of a continuously active constant power control loop 158 according to the present invention. Constant power control loop 158 includes power dissipation measurement circuit 160, which measures the MR head power dissipation. Such power dissipation measurement functionality is necessary to continuously control the head power dissipation with hardware. In an illustrative embodiment, the internal measurement of power is obtained by multiplying internal signals proportional to the MR head bias current and the voltage across the MR head 130. In one illustrative embodiment, a signal proportional to the bias current is derived from the current generating circuits. In an alternative embodiment, a signal proportional to the bias current is generated by sensing the current through the MR head 130.

Internal register 164 holds the value of the desired head power dissipation. Register 164 communicates with one input of comparator 162 and provides comparator with the desired value of the head power dissipation. Another input of comparator 162 receives the value of the actual head power dissipation from power dissipation measurement circuit 160. Comparator 162 subtracts the actual head power dissipation from the desired head power dissipation thereby creating an error signal. Filter 166 and amplifier 168 collectively generate a current command signal as a function of the error signal. This current command signal is provided to bias current generating circuit 132, which generates an MR head bias current proportional to the current command. Thus, constant power control loop 158 dynamically varies the MR bias current as necessary to match the desired head power dissipation. By running this loop continuously, the desired head power dissipation is achieved for a wide distribution of head resistances and maintained over a wide range of operating temperatures.

FIG. 6 is a flow chart representing a method of dynamically controlling the power dissipation in an MR head. At step 180, a bias current is provided to the MR head. At step 182, the actual power P_act dissipated by the MR head is measured. At step 184, the actual power Pact dissipated by the MR head is subtracted from the desired power dissipation Pdes- At step 186, the magnitude of the bias current is adjusted according to the difference between the actual power P_act dissipated by the MR head and the desired power dissipation Pdes. Returning to step 180, the adjusted amount of bias current is then provided to the MR head and the process is repeated. This process repeats continuously during the operation of the MR head.

In a first alternative implementation of biasing MR head 130 with constant power, external equipment is used to measure the DC voltage across MR head 130. This measurement is possible with read/ write preamplifiers that provide a buffered version of the MR head voltage. The voltage is measured as a function of the programmed bias current. The voltage measurement is used to determine the power dissipation as a function of the MR bias current, the power dissipation being equal to the product of the head voltage and the MR bias current. A software algorithm is implemented during manufacture of the disc drive which determines the magnitude of the MR bias current which produces a power dissipation which is approximately equal to the desired value. The selected bias current for each MR head is permanently stored in a table that can be read by the drive at power-up.

In a second alternative implementation of biasing MR head 130 with constant power, resistance measurement capability available in some read/ write preamplifiers is used to determine the MR head power dissipation as a function of the programmed bias current. The resistance measurement is used to determine the power dissipation as a function of the MR bias current, the power dissipation being equal to the product of the MR head resistance and the square of the MR bias current. A software algorithm is implemented to determine the magnitude of the MR bias current which produces a power dissipation which is substantially equal to the desired value. Illustratively, the desired bias current is computed at power-up, making it unnecessary to maintain a table of bias currents for each MR head. Also, with this illustrative embodiment, it is possible to measure the head resistance in the field in order to modify the bias current to compensate for resistance changes due to temperature variations. Alternatively, the software algorithm can be implemented during manufacture of the disc drive and the selected bias current for each MR head stored in a table. This reduces the software complexity.

In summary, one embodiment of the present invention is directed to a method of biasing a magnetoresistive head 130 in a magnetic data storage device 100. A bias current is provided to the magnetoresistive head 130 and a substantially constant power dissipation through the magnetoresistive head 130 is maintained.

In one embodiment of the present invention, the power dissipation through the magnetoresistive head 130 is continuously controlled with a dynamic control loop 158. In one implementation of this embodiment, a desired power dissipation through the magnetoresistive head 130 is stored. During the operation of the magnetoresistive head 130, the following steps are continuously repeated. First the actual power dissipation through the magnetoresistive head 130 is measured. Next the actual power dissipation is subtracted from the desired power dissipation. Then the bias current provided to the magnetoresistive head 130 is adjusted based on the difference between the actual power dissipation and the desired power dissipation.

Another embodiment of the present invention is directed to a disc drive 100 having a magnetic disc 106, a magnetoresistive head 130 and a bias circuit 128. The magnetoresistive head 130 is positioned adjacent the magnetic disc 106. The bias circuit 128 provides a bias current to the magnetoresistive head 130 and maintains a substantially constant power dissipation through the magnetoresistive head 130.

In one embodiment of a disc drive 100 according to the present invention, the bias circuit 128 includes a power dissipation control loop 158 which dynamically maintains the power dissipation through the magnetoresistive head 130 at a desired level.

It is to be understood that even though numerous characteristics and advantages of various embodiments of the present invention have been set forth in the foregoing description, together with details of the structure and function of various embodiments of the invention, this disclosure is illustrative only, and changes may be made in details, especially in matters of structure and arrangement of parts within the principles of the present invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed. For example, the bias to the MR head may be provided by a voltage source rather than a current source and dynamically controlling the power dissipated by the MR head may be achieved by dynamically adjusting the magnitude of the voltage source according to the difference between the actual and desired power dissipation. Other modifications can also be made.

Claims

WHAT IS CLAIMED IS:

1. A method of biasing a magnetoresistive head in a magnetic data storage device, the method comprising steps of:

(a) providing a bias current to the magnetoresistive head; and (b) maintaining a substantially constant power dissipation through the magnetoresistive head.

2. The method of claim 1 wherein the maintaining step (b) comprises controlling the power dissipation through the magnetoresistive head with a dynamic control loop.

3. The method of claim 2 wherein the maintaining step (b) comprises steps of:

(b)(i) storing a desired value for the power dissipation through the magnetoresistive head; and

(b)(ii) during the operation of the magnetoresistive head, repeating steps of: measuring an actual power dissipation through the magnetoresistive head, thereby obtaining an actual power dissipation value; subtracting the actual power dissipation value from the desired power dissipation value; and adjusting the bias current provided to the magnetoresistive head based on a difference between the actual power dissipation value and the desired power dissipation value.

4. The method of claim 1 wherein the maintaining step (b) comprises steps of:

(b)(i) measuring a resistance of the magnetoresistive head; (b)(ii) calculating a power dissipation through the magnetoresistive head as a function of the bias current and the measured resistance; and (b)(iii) adjusting the bias current provided to the magnetoresistive head such that the power dissipation through the magnetoresistive head is substantially equal to a desired power dissipation.

5. A disc drive implementing the method of biasing a magnetoresistive head set forth in claim 1.

6. A disc drive comprising: a magnetic disc; a magnetoresistive head adapted to be positioned adjacent the magnetic disc; and a bias circuit adapted to provide a bias current to the magnetoresistive head and adapted to maintain a substantially constant power dissipation through the magnetoresistive head.

7. The disc drive of claim 6 wherein the bias circuit comprises a power dissipation control loop adapted to dynamically maintain the power dissipation through the magnetoresistive head at a desired level.

8. The disc drive of claim 7 wherein the power dissipation control loop is adapted to dynamically vary the bias current in order to maintain the power dissipation through the magnetoresistive head at the desired level.

9. A disc drive comprising: a magnetoresistive head adapted to be positioned adjacent a magnetic disc; and means for providing a bias current to the magnetoresistive head such that a substantially constant power dissipation is maintained through the magnetoresistive head.