Abstract:
Approaches for a hard-disk drive (HDD) having a balanced resistive temperature detector (RTD). A HDD includes a head slider comprising a single RTD. A read/write IC comprises a balance resistor having the same resistance as the single RTD when the head slider is not in physical contact with the disk. The same amount of current flows through the single RTD and the balance resistor except when the head slider is in physical contact with the disk. Detecting a voltage change across the single RTD enables physical contact between the head slider and the disk to be accurately detected using a circuit with low noise. Alternately, the head slider may include two RTDs connected in sequence, and the balance resistor may possess the same resistance as the two RTDs. The two RTDs may vary inversely with environmental changes to avoid the need to recalibrate the balance resistor after any environmental change.

Description:
RELATED APPLICATION DATA 
     This application is related to U.S. patent application Ser. No. 13/333,565, entitled “Distributed Temperature Detector Architecture for Head Disk Interface Systems,” invented by John Contreras et al., filed on Dec. 21, 2011, the disclosure of which is incorporated by reference in its entirety for all purposes as if fully set forth herein. 
     This application is related to U.S. patent application Ser. No. 10/691,752, Patent Publication No., 2005/0088772, entitled “Magnetic Recording Disk Drive with Actively Controlled Electric Potential at the Head/Disk Interface for Wear and Durability Control,” invented by Peter Michael Baumgart et al., filed on Oct. 22, 2003, the disclosure of which is incorporated by reference in its entirety for all purposes as if fully set forth herein. 
     FIELD OF THE INVENTION 
     Embodiments of the invention relate to a balanced embedded contact sensor for use in a head disk interface system of a hard-disk drive (HDD). 
     BACKGROUND OF THE INVENTION 
     A hard-disk drive (HDD) is a non-volatile storage device that is housed in a protective enclosure and stores digitally encoded data on one or more circular disks having magnetic surfaces (a disk may also be referred to as a platter). When an HDD is in operation, each magnetic-recording disk is rapidly rotated by a spindle system. Data is read from and written to a magnetic-recording disk using a read/write head which is positioned over a specific location of a disk by an actuator. 
     A read/write head uses a magnetic field to read data from and write data to the surface of a magnetic-recording disk. As a magnetic dipole field decreases rapidly with distance from a magnetic pole, the distance between a read/write head and the surface of a magnetic-recording disk must be tightly controlled. An actuator relies on suspension&#39;s force on the read/write head to provide the proper distance between the read/write head and the surface of the magnetic-recording disk while the magnetic-recording disk rotates. A read/write head therefore is said to “fly” over the surface of the magnetic-recording disk. When the magnetic-recording disk stops spinning, a read/write head must either “land” or be pulled away onto a mechanical landing ramp from the disk surface. 
     Resistor temperature detector (RTD) architectures have been used in the prior art to determine when the read/write head makes physical contact with the magnetic-recording disk based upon the temperature of the read/write head. RTD architectures in the prior art have been implemented using a single temperature sensor that measures temperature based on the amount of voltage across a single temperature sensor. However, prior art approaches exhibit an unsatisfactory amount of noise, which complicates accurate measurements. 
     SUMMARY OF THE INVENTION 
     Approaches described herein teach a balanced embedded contact sensor (bECS) for a head-disk interface (HDI) system. A balanced embedded contact sensor of an embodiment may be implemented by a resistive temperature detector (RTD) that is comprised within a head slider. This RTD is located on an arm of a bridge circuit which has another arm that includes a balanced resistor that is located within the read/write integrated circuit (IC). The balanced resistor is configured to have the same resistance as the RTD in the head slider except when the head slider is in physical contact with the magnetic-recording disk. The bridge circuit of an embodiment allows the affect of noise generated at the head slider to be cancelled at the read/write integrated circuit (IC). As a result, accurate temperature measurements of the head slider may be obtained, thereby enabling embodiments to detect physical contact between the head slider and the magnetic-recording disk with greater precision than prior approaches. 
     In an another embodiment of the invention, a hard-disk drive (HDD) comprises a head slider that includes a first resistive temperature detector (RTD) and a second resistive temperature detector (RTD). The first RTD and the second RTD are connected in sequence. Any change in temperature causes (a) an increase in resistance of a particular magnitude in the first RTD and (b) a decrease in resistance of the same magnitude in the second RTD. In this way, any environmental change that causes an increase in resistance in one RTD will cause an opposite change in the other resistor, and so the first RTD and the second RTD will provide a constant amount of resistance in the face of any environmental change. This avoids the need to recalibrate the balance resistor located in the read/write IC if the HDD is taken to a different environment having a difference in altitude, pressure, temperature, or humidity. 
     Embodiments discussed in the Summary of the Invention section are not meant to suggest, describe, or teach all the embodiments discussed herein. Thus, embodiments of the invention may contain additional or different features than those discussed in this section. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention are illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which: 
         FIG. 1  is a plan view of an HDD according to an embodiment of the invention; 
         FIG. 2  is a plan view of a head-arm-assembly (HAA) according to an embodiment of the invention; 
         FIG. 3  is an illustration of a read/write circuit within an HDD according to an embodiment of the invention; 
         FIG. 4  is an illustration of a Wheatstone bridge bias and detection circuit employed by embodiments of the invention; 
         FIG. 5  is an illustration of a balanced embedded contact sensor (bECS) architecture having a single RTD according to an embodiment of the invention; and 
         FIG. 6  depicts a balanced embedded contact sensor (bECS) architecture having two RTDs according to another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Approaches for a balanced embedded contact sensor (bECS) for a head-disk interface (HDI) system are described. In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the embodiments of the invention described herein. It will be apparent, however, that the embodiments of the invention described herein may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring the embodiments of the invention described herein. 
     Physical Description of Illustrative Embodiments of the Invention 
     Embodiments of the invention may be used to detect when the head slider makes physical contact with the magnetic-recording disk with greater precision than prior approaches. Embodiments of the invention may be incorporated with a hard-disk drive (HDD). In accordance with an embodiment of the invention, a plan view of a HDD  100  is shown in  FIG. 1 .  FIG. 1  illustrates the functional arrangement of components of the HDD including a slider  110   b  that includes a magnetic-reading/recording head  110   a . Collectively, slider  110 B and head  110   a  may be referred to as a head slider. The HDD  100  includes at least one head gimbal assembly (HGA)  110  including the head  110   a , a lead suspension  110   c  attached to the head  110   a , and a load beam  110   d  attached to the slider  110   b , which includes the head  110   a  at a distal end of the slider  110   b ; the slider  110   b  is attached at the distal end of the load beam  110   d  to a gimbal portion of the load beam  110   d . The HDD  100  also includes at least one magnetic-recording disk  120  rotatably mounted on a spindle  124  and a drive motor (not shown) attached to the spindle  124  for rotating the disk  120 . The head  110   a  includes a write element and a read element for respectively writing and reading information stored on the disk  120  of the HDD  100 . The disk  120  or a plurality (not shown) of disks may be affixed to the spindle  124  with a disk clamp  128 . The HDD  100  further includes an arm  132  attached to the HGA  110 , a carriage  134 , a voice-coil motor (VCM) that includes an armature  136  including a voice coil  140  attached to the carriage  134 ; and a stator  144  including a voice-coil magnet (not shown); the armature  136  of the VCM is attached to the carriage  134  and is configured to move the arm  132  and the HGA  110  to access portions of the disk  120  being mounted on a pivot-shaft  148  with an interposed pivot-bearing assembly  152 . 
     With further reference to  FIG. 1 , in accordance with an embodiment of the present invention, electrical signals, for example, current to the voice coil  140  of the VCM, write signal to and read signal from the PMR head  110   a , are provided by a flexible cable  156 . Interconnection between the flexible cable  156  and the head  110   a  may be provided by an arm-electronics (AE) module  160 , which may have an on-board pre-amplifier for the read signal, as well as other read-channel and write-channel electronic components. The flexible cable  156  is coupled to an electrical-connector block  164 , which provides electrical communication through electrical feedthroughs (not shown) provided by an HDD housing  168 . The HDD housing  168 , also referred to as a casting, depending upon whether the HDD housing is cast, in conjunction with an HDD cover (not shown) provides a sealed, protective enclosure for the information storage components of the HDD  100 . 
     With further reference to  FIG. 1 , in accordance with an embodiment of the present invention, other electronic components (not shown), including a disk controller and servo electronics including a digital-signal processor (DSP), provide electrical signals to the drive motor, the voice coil  140  of the VCM and the head  110   a  of the HGA  110 . The electrical signal provided to the drive motor enables the drive motor to spin providing a torque to the spindle  124  which is in turn transmitted to the disk  120  that is affixed to the spindle  124  by the disk clamp  128 ; as a result, the disk  120  spins in a direction  172 . The spinning disk  120  creates a cushion of air that acts as an air-bearing on which the air-bearing surface (ABS) of the slider  110   b  rides so that the slider  110   b  flies above the surface of the disk  120  without making contact with a thin magnetic-recording medium of the disk  120  in which information is recorded. The electrical signal provided to the voice coil  140  of the VCM enables the head  110   a  of the HGA  110  to access a track  176  on which information is recorded. Thus, the armature  136  of the VCM swings through an arc  180  which enables the HGA  110  attached to the armature  136  by the arm  132  to access various tracks on the disk  120 . Information is stored on the disk  120  in a plurality of concentric tracks (not shown) arranged in sectors on the disk  120 , for example, sector  184 . Correspondingly, each track is composed of a plurality of sectored track portions, for example, sectored track portion  188 . Each sectored track portion  188  is composed of recorded data and a header containing a servo-burst-signal pattern, for example, an ABCD-servo-burst-signal pattern, information that identifies the track  176 , and error correction code information. In accessing the track  176 , the read element of the head  110   a  of the HGA  110  reads the servo-burst-signal pattern which provides a position-error-signal (PES) to the servo electronics, which controls the electrical signal provided to the voice coil  140  of the VCM, enabling the head  110   a  to follow the track  176 . Upon finding the track  176  and identifying a particular sectored track portion  188 , the head  110   a  either reads data from the track  176  or writes data to the track  176  depending on instructions received by the disk controller from an external agent, for example, a microprocessor of a computer system. 
     Embodiments of the invention also encompass HDD  100  that includes the HGA  110 , the disk  120  rotatably mounted on the spindle  124 , the arm  132  attached to the HGA  110  including the slider  110   b  including the head  110   a.    
     With reference now to  FIG. 2 , in accordance with an embodiment of the present invention, a plan view of a head-arm-assembly (HAA) including the HGA  110  is shown.  FIG. 2  illustrates the functional arrangement of the HAA with respect to the HGA  110 . The HAA includes the arm  132  and HGA  110  including the slider  110   b  including the head  110   a . The HAA is attached at the arm  132  to the carriage  134 . In the case of an HDD having multiple disks, or platters as disks are sometimes referred to in the art, the carriage  134  is called an “E-block,” or comb, because the carriage is arranged to carry a ganged array of arms that gives it the appearance of a comb. As shown in  FIG. 2 , the armature  136  of the VCM is attached to the carriage  134  and the voice coil  140  is attached to the armature  136 . The AE  160  may be attached to the carriage  134  as shown. The carriage  134  is mounted on the pivot-shaft  148  with the interposed pivot-bearing assembly  152 . 
       FIG. 3  is an illustration of a read/write circuit  310  within an HDD according to an embodiment of the invention.  FIG. 3  depicts hard-disk drive (HDD)  300  which includes enclosure  301  that contains one or more magnetic platters or disks  302 , read elements  304 , write elements  305 , an actuator arm suspension  306 , a transmission line interconnect  308 , a read/write integrated circuit (IC)  310 , a flexible interconnect cable  312 , and a disk enclosure connector  314 . 
     Electrical signals are communicated between the read/write elements and read/write integrated circuit  310  over transmission line interconnect  308 . Read/write integrated circuit  310  conditions the electrical signals so that they can drive write element  305  during writing and amplifies the electrical signal from read element  304  during reading. Signals are communicated between read/write integrated circuit  310  and disk enclosure connector  314  over flexible cable  312 . Disk enclosure connector  314  conducts signals with circuitry external to disk enclosure  301 . In other embodiments, read/write integrated circuit (IC)  310  is located elsewhere than depicted in  FIG. 3 , such as on flex cable  312  or on printed circuit board (PCB) within the hard-disk drive. 
     Balanced Embedded Contact Sensor (BECS) 
       FIG. 4  is an illustration of a Wheatstone bridge bias and detection circuit  400  employed by embodiments of the invention. The circuit of  FIG. 4  will be discussed below to illustrate certain operational principles of embodiments. In  FIG. 4 , tunable bias resistors  410  and  412  have resistances values of R B  and R′ B  respectively, while resistive temperature detectors (RTDs)  414  and  416  have resistances values of R +  and R −  respectively. The sum of the resistances of R B  and R′ B  is much larger than the sum of the resistances of R +  and R − . 
     A common voltage V Bias  is applied to tunable bias resistors  410  and  412  at source  420  to current bias resistive temperature detectors (RTDs)  414  and  416 . Current will flow to ground  430  through shared terminal  422  of RTDs  414  and  416 . Shared terminal  422  is also coupled to TFC ground  432 . In this configuration, noise from voltage source  420  is common mode and is not sensed by differential amplifier  440 , which provides immunity to external noise. In addition, the noise from tunable bias resistors  410  and  412  are negligible (factors of R + /R B  and R − /R′ B  smaller than the thermal noise from R +  and R − ). Therefore, the system noise may be given by the thermal noise as expressed by 4 k B  T (R + +R − ) plus noise from amplifier  440 . 
       FIG. 5  depicts a balanced embedded contact sensor (bECS) architecture  500  according to an embodiment of the invention. The embodiment of  FIG. 5  employs the low noise circuit depicted in  FIG. 4 , where a first arm of the circuit is comprised within head slider  510  and a second arm of the circuit is comprised within read/write IC  550 . The circuit arms of bECS architecture  500  may be arms of a Wheatstone bridge type circuit.  FIG. 5  depicts head slider  510  and the relevant elements therein, namely RTD  520  and resistors  522  and  524 . RTD  520  has a resistance value of R ECS  and resistors  522  and  524  each have a resistance of R SB . 
     In an embodiment, RTD  520  may be embodied as a thermistor. RTD  520  may be composed of, but not limited to, metallic (e.g., NiFe) and semiconductor materials. RTD  520  may measure temperature based on the voltage drop associated therewith. Changes in temperature cause a change in the amount of resistance provided by a resistive temperature detector. A small increase in temperature may result in an increase or decreases in voltage across a resistive temperature detector. Thus, the amount of voltage across a resistive temperature detector may be used to identify the temperature associated with the resistive temperature detector. 
     RTD  520  is located on or proximate to the air bearing surface of head slider  510 . When physical contact is made between head slider  510  and the magnetic-recording disk when the magnetic-recording disk is rotating, the resulting friction causes an increase in temperature within head slider  510  originating at the point of contact. The change in temperature resulting from the physical contact will be a gradient as a function of distance from the point of contact. The increase in temperature will cause a measurable change in the voltage across RTD  520 . 
       FIG. 5  also depicts read/write IC  550 . Read/write IC  550  comprises balance resistor  530  having a resistance of R BAL . Read/write IC  550  also includes resistors  532  and  534  and amplifiers  540  and  542 . 
     The resistance (R SB ) of each of resistors  522  and  524  is much greater than the resistance (R B ) of each of resistors  532  and  534 . Resistors  522  and  524  are used to set the voltage for the head slider body  512  to the desired value (V SB ) by using a feedback loop to control the voltage at the V +  terminal  560  and the V −  terminal  562  (note that the feedback loop is not shown in  FIG. 5 ). Resistors  532  and  534  resistors are used as bias resistors to regulate the current bias along the two arms of the circuit. 
     The first arm and second arm of the circuit are shown in  FIG. 5 . Since resistors  522  and  524  have a much greater resistance value than RTD  520 , the total value of the resistance from the V+ terminal  560  to the V− terminal  562  along the first arm of the circuit is the resistance value of RTD  520  (which is R ECS )+the resistance value of resistor  532  (which is R B ). Along the second arm of the circuit, the total value of the resistance from the V+ terminal  560  to the V− terminal  562  is the resistance value of resistor  534  (R B )+the resistance value of balance resistor  530  (R BAL ). Therefore, by adjusting the resistance R BAL  of balance resistor  530 , it is possible to balance the bridge circuit (i.e., the first arm and the second arm of the circuit have the same resistance when the circuit is balanced) and remove the baseline signal not related contact between the head slider and the disk. Unless there is physical contact between head slider  510  and the magnetic-recording disk, the same amount of current will flow through both the first arm and the second arm of the circuit. Only changes in the resistance value of RTD  520  (R ECS ) will be detected by amplifier  540  to generate a voltage signal that mostly contains contact information. Noise generated by the first arm of the circuit is cancelled out by noise generated by the second arm of the circuit, thereby allowing detection of physical contact between head slider  510  and the disk using a low noise circuit. 
     The resistance (R BAL ) of balance resistor  530  is calibrated or configured to be equal to the resistance of RTD  520 . In an embodiment, the resistance of balance resistor  530  may be calibrated or configured by setting certain electrical switches to place a portion of a plurality of resistors in series to form balance resistor  530  so that the portion of the plurality of resistors forming balance resistor  530  have the desired resistance, i.e., the amount of assistance of RTD  520  when head slider  510  is not in physical contact with the disk. For example, if the total amount of resistance of balance resistor  530  is determined to be 200 ohms when head slider  510  is not in physical contact with the disk, then certain electrical switches may be configured to place a certain number of resistors in series so that the total amount of resistance of the resistors in series is 200 ohms. The calibration or configuration of balance resistor  530  may be performed once during manufacturing or assembly of the hard-disk drive. Optionally, the hard-disk drive may contain a sensor that detects certain environmental changes, such as a change in altitude, humidity, pressure, or temperature. If the hard-disk drive detects an environmental change that exceeds a certain threshold, then the resistance value of balance resistor  530  may be recalibrated or reconfigured as explained above. In this way, if the hard-disk drive is taken to a new environment that may affect the resistance of RTD  520 , the resistance of balance resistor  530  may be updated so that it is equal to the resistance of RTD  520  in the new environment. 
     In an embodiment, amplifier  542  may optionally be connected directly across the ECS terminals (labeled ECS+ and ECS− in  FIG. 5 ) to measure the absolute resistance of ECS. In this configuration, the noise for contact detection is given by 4 K B  T R ECS  plus the amplifier noise, where T is temperature and K B  is Boltzmann&#39;s constant. 
     In addition, the embodiment depicted in  FIG. 5  enables independent control of the head slider body  512  potential (by adjusting V SB ) and the bias current through RTD  520  by adjusting current I B . In this way, the voltage level associated with ground at head slider  510  may be higher than the voltage level of ground at the magnetic-recording disk. Advantageously, such independent control enables the interface voltage control (IVC) feature discussed in U.S. patent application Ser. No. 10/691,752 to be on or off regardless of the value of I B  chosen to operate RTD  520 . 
     Another advantage provided by the embodiment depicted in  FIG. 5  is that circuit  500  may be embodied using a head slider  510  constructed without requiring any special builds since one RTD is employed within head slider  510 . 
       FIG. 6  depicts a balanced embedded contact sensor (bECS) architecture  600  according to another embodiment of the invention. The circuit architecture of  FIG. 6  may be that of a Wheatstone bridge type circuit, thus noise generated by the first arm of the circuit may be cancelled out by noise generated by the second arm of the circuit. Unless there is physical contact between head slider  510  and the magnetic-recording disk, the same amount of current will flow through both the first arm and the second arm of the circuit. 
     The embodiment of  FIG. 6  comprises two RTDs, namely RTD  610  and RTD  612 . RTDs  610  and  612  have resistance values R ECS−  and R ECS+  respectively. RTD  610  is constructed using a material that has an opposite temperature coefficient of resistance (TCR) than the material used to construct RTD  612 . Resistor  612  may be placed close to the air bearing surface (ABS) of head slider  510  while resistor  610  is placed well inside of head slider  510 , e.g., resistor  610  may be offset from the air bearing surface or embedded within head slider  510 . When physical contact is made between head slider  510  and the magnetic-recording disk when the magnetic-recording disk is rotating, the resulting friction causes an increase in temperature within head slider  510  originating at the point of contact. The change in temperature resulting from the physical contact will be a gradient as a function of distance from the point of contact. 
     Resistor  630 , having a resistance of R BAL  and residing in read/write IC, is used to cancel the baseline resistance of both RTDs  610  and  612 . For example, if RTDs  610  and  612  each have a resistance of 100 ohms, then resistor  630  will have a resistance of 200 ohms. 
     A change in temperature caused by physical contact between head slider  510  and the magnetic-recording disk will affect resistor  612  more than RTD  610  since RTD  612  is closer to the point of contact. As a result, the resistance of RTD  610  will change by a different magnitude than any change in resistance of RTD  610 , since RTD  610  is further away from the point of contact. Therefore, the voltage across the first arm of circuit  600  will be different than the voltage across the second arm of circuit  600  when physical contact is made between head slider  510  and the magnetic-recording disk. 
     Note that environmental changes, such as a change in temperature, pressure, humidity, or altitude will affect RTDs  610  and  612  equally. Therefore, if the hard-disk drive contains a sensor which detects a change in the environment of the HDD, such as a change in temperature, pressure, humidity, or altitude, then it would not be necessary to recalibrate resistor  630 , as the environmental change will have no affect on the total amount of resistance of RTD  610  and  612 . This is so because any change in resistance in one of RTD  610  and  612  caused by such an environmental change will be cancelled out by an opposite change in resistance by the other of RTD  610  and  612 . 
     Any common mode temperature changes will be partially cancelled since RTD  610  and  612  will vary in opposite directions with any ambient (non-contact related) temperature change. This embodiment offers the same noise advantages as the embodiment depicted in  FIG. 5 , but provides better cancellation of thermal signatures not related to contact between head slider  510  and the magnetic-recording disk. The embodiment of  FIG. 6  also allows independent control of interface voltage control (IVC) and embedded contact sensors (ECS), but it does not allow absolute slider temperature measurements and requires special slider builds since two RTDs are employed within head slider  510 . 
     The embodiments depicted in both  FIG. 5  and  FIG. 6  may both include an auto-zeroing power up and/or periodic calibration to zero-out the amplifier&#39;s input voltage. 
     In the foregoing specification, embodiments of the invention have been described with reference to numerous specific details that may vary from implementation to implementation. Thus, the sole and exclusive indicator of what is the invention, and is intended by the applicants to be the invention, is the set of claims that issue from this application, in the specific form in which such claims issue, including any subsequent correction. Any definitions expressly set forth herein for terms contained in such claims shall govern the meaning of such terms as used in the claims. Hence, no limitation, element, property, feature, advantage or attribute that is not expressly recited in a claim should limit the scope of such claim in any way. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.