Patent Document

RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/150,141 filed on Aug. 20,1999. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to improvements in the operation of data channels, and more particularly to calibrating data channels more effectively to compensate for thermal fluctuations in transducers such as read/write heads. 
     BACKGROUND OF THE INVENTION 
     High performance data handling systems frequently make use of delicate transducers. In many wireless communications, for example, antennas and sensors must be exposed directly to the air. This can cause temperature fluctuations that will disrupt their calibration and can degrade or even prevent their performance. 
     Modern disc drives position transducers adjacent to a spinning disc, the close positioning required for reading the weak signals received from the closely spaced tracks. Disc drives are the primary devices employed for mass storage of computer programs and data. The advantages of disc drive technology over other means of data storage include a lower cost per unit of storage capacity and a generally higher transfer rate. 
     In a typical “Winchester” magnetic disc drive, digital data is written to and read from a thin layer of magnetized material on a surface of one or more discs. A load beam supports a hydrodynamic air bearing slider close to each data surface of a rotating magnetic disc. The load beam supplies a downward force that counteracts the hydrodynamic lifting force developed by the slider&#39;s air bearing. The slider carries a magnetic transducer for communication with individual bit positions on the rotating magnetic disc. Even a small thermal variation resulting from ambient temperature shifts can disrupt the magnetic properties and physical positioning of a transducer head. Also, moving disc surfaces contact the tiny disc drive heads, frequently causing localized temperature changes in their sensitive read or write transducers. 
     Disc drives including thermal calibration have existed for several years. For example, U.S. Pat. No. 5,005,089 entitled “HIGH PERFORMANCE, HIGH CAPACITY MICRO-WINCHESTER DISK DRlVE” issued Apr. 2, 1991 to Thanos et al. discloses a disc drive with a temperature sensor that is periodically checked to facilitate recalibration. Unfortunately, systems relying on periodic calibration have inherent weaknesses. Such systems respond poorly to thermal changes that occur soon after a calibration, for example. In a typical system, thermal calibration is performed every ten minutes. Reducing this interval would improve performance, but is somewhat impractical because of the processing demands it would impose. 
     A need therefore exists in the art for data handling systems with a more responsive thermal calibration. It is desirable that this be achieved, moreover, without a substantial increase in processing overhead. The present invention solves these problems and offers other advantages over the prior art. 
     SUMMARY OF THE INVENTION 
     The present invention differs from prior systems because it obtains and uses temperature readings aperiodically, on demand. Rather than merely obtaining temperature readings on each expiration of a fixed timer, systems of the present invention update one or more operating parameters in response to a fault detected in the data path. Data handling systems of the present invention each include at least one data path having a thermal range within which it operates best. Applicant notes that some “key” operating parameters typically have a substantial effect on this thermal range. (As used herein, a “substantial” effect on a range is one that can potentially cause a shift of at least 10° C. in either the maximum or the minimum of the range.) Temperature sensors positioned near the transducers provide a temperature reading to control circuits, which update “key” operating parameter values in response to indications of faults in a primary data path. 
     Systems of the present invention respond to temperature shifts, reducing the need for more frequent temperature monitoring and calculation-intensive recalibration. Other features and advantages of various embodiments will become apparent upon a review of the following figures and the accompanying explanations. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic diagram of a data handling system of the present invention. 
     FIG. 2 shows an oblique, exploded view of a disc drive implementing the present invention. 
     FIG. 3 plots the allowed values of a binary register (as an example of an operating parameter) against temperature. 
     FIG. 4 plots the allowed values of another binary register against temperature. 
     FIG. 5 depicts a calibration method of the present invention. 
     FIG. 6 depicts a read error recovery table known in the disc drive industry. 
     FIG. 7 depicts a method of the present invention particularly suited to read error recovery in a disc drive. 
    
    
     DETAILED DESCRIPTION 
     Numerous aspects of data handling and thermal calibration that are not a part of the present invention, or are well known, are omitted for brevity. These include (1) deciding what kind of transducer is best suited to a particular application; (2) defining what range of temperatures a given system should accommodate; and (3) deciding exact formulae to be used for updating key operating parameters. Although each of the examples below shows more than enough detail to allow those skilled in the art to practice the present invention, subject matter regarded as the invention is broader than any single example below. The scope of the present invention is distinctly defined, however, in the claims at the end of this document. 
     FIG. 1 shows a schematic diagram of a data handling system  100  of the present invention. Data flows through a data path  110  between a medium  146  and a cache  115 . The transducer  160  may be an antenna able to send and receive data through a medium  146  such as air. Alternatively, the transducer  160  may be a read/write head configured to read data from a medium  146  such as a moving data storage disc or tape. In any event, the necessity of the present invention arises because the performance of transducer  160  depends upon its temperature and whether its operating parameters are adequately calibrated to that temperature. For this reason, a temperature sensor  130  is positioned near enough to transducer  160  so that sensor  130  effectively measures the temperature of transducer  160 . Gap  135  between transducer  160  and sensor  130  desirably contains a thermal conductor (e.g. a metal) so that the thermal coupling is very strong. 
     Alternatively, the temperature sensor  130  can optionally be constructed integrally into the transducer  160 . In either case, it is desirable that the sensor  130  be separated from the medium  146  by a separation  136  large enough that localized, medium-induced variations in transducer temperature will be attenuated by 50% or more, as sensed by temperature sensor  130 . 
     Printed circuit board assembly (PCBA)  167  includes cache  115 , clock/counter  140 , and two integrated circuits (channel IC  120  and controller IC  150 ). Several signal lines are provided so that controller IC  150  can request and receive data from fault detector  122  (of channel IC  120 ) or from temperature sensor  130 . Controller IC  150  contains several registers with values  151 , 152 , 153  that control the operation of the transducer  160  and of the channel  110 . As is conventional in the art, each parameter control value is encoded in a binary form that can only assume a finite number of values. For example, value  151  is an 8-bit code that corresponds to a predetermined level of bias current. Bias current is important for magneto-resistive heads common in disc drives, and typically takes values between 2and 20 milliamperes. 
     Value  152  is a lower-resolution code (primarily for simplicity) indicative of a write current (typically 10 to 100 milliamperes). The register values shown also include a write precompensation value  153 , an off-track threshold value  154 , and a nonlinearity correction value  155  as are known in the art. In general, the types of parameters selected for thermal calibration will depend on the type of transducer  160  used. Because some channel operating parameters are highly sensitive to transducer temperature variation and are controllable, it is important that at least some of them are updated when the transducer temperature changes. In a preferred embodiment, only 1 to 5 “most sensitive” operating parameter values are recalibrated in response to a fault detected. 
     When the temperature of transducer  160  changes, controller IC  150  must sense the change relatively soon so that the time during which the data path  110  is not operating well is minimized. As transducer  160  changes temperature, the performance of data path  110  declines. This causes fault detector  122  to send a fault condition signal to controller IC  167 , which responds by recalibrating some of the registers  151 , 152 , 153  accessible to controller circuit  150 . Note that register values  151 , 152 , 153  are desirably accessible so that they can be recalibrated even if they reside in the channel circuit  120 . 
     Fault detector  122  is configured to transmit a fault indication when any of several simple criteria in the data path  110  are met. For example, fault detector  122  is optionally configured to indicate a fault condition (a) whenever 1000 consecutive blocks each have at least one erroneous bit, (b) whenever a gain control circuit in a block of the data path indicates a gain change larger than 50% in one gain adjustment cycle, or (c) whenever a block transmission is retried 20 times. Note that each criterion of these three included an “problem indicator” value and at least one corresponding quantitative “threshold.” A “problem indicator” is desirably selected for convenience, using problem-indicative signals or values already existing in the error correction subsystem (not specifically shown). Corresponding thresholds are desirably selected so that “fault condition” indicators will usually occur less often than once per hour in normal operation at a stable temperature. Because error correction requires more complex fault detection criteria than is required for present purposes, such thresholds are easy to guess and to verify for many readily-obtained problem indicators. In some disc drive designs, a suitable “fault condition” value or signal is already available, thus allowing fault detector  122  to be implemented merely by extending the signal line, and without adding any components. 
     FIG. 2 shows an oblique, exploded view of a disc drive  200  implementing the present invention. Disc drive  200  includes a base member  242  to which all other components are directly or indirectly mounted and a top cover  220  which, together with the base member  242 , forms a disc drive housing which encloses delicate internal components and isolates these components from external contaminants. 
     The disc drive includes a plurality of discs  246  which are mounted for rotation on a spindle motor (not shown). The discs  246  include on their surfaces a plurality of circular, concentric data tracks  248  on which data are recorded via an array of vertically aligned heads (one of which is shown at  260 ). Heads  260  are supported by head suspension tabs, which are attached to arm portions of actuator  210 . Actuator  120  is mounted for rotation about a pivot shaft  132 . Power to rotate the actuator  210  is provided by a voice coil motor (VCM). The VCM consists of a coil  254  which is supported by actuator  210  within the magnetic field of an array of permanent magnets  275  fixedly supported by the base member  242 , all in a manner well known in the disc drive industry. A controller board  267  contains most of the electronic circuitry controls the operation of the disc drive  200 . Conventionally, this includes data signals to and from the heads  260  via interface block  268 , flexible printed circuit cable (PCC)  264 , and preamplifier chip  213 . 
     Like the system of FIG. 1, disc drive  200  of FIG. 2 includes a data path with circuitry both on and off the primary controller board  267 . Preamp chip  213  includes several registers that are desirably updated based on a temperature reading. Unfortunately, this may require more sophisticated computation than is practical for implementation in a preamp chip. To minimize the demands upon a processor tasked with updating, disc drive  200  performs such updating only during an initialization routine and when a fault detector (like item  122  of FIG. 1) indicates a fault condition in the data path. This is particularly advantageous when disc drive  200  is to be used in a server of a network. Servers have very high performance demands and very stable temperatures in their normal application. 
     FIG. 3 plots the allowed values  302  of a binary register against temperature  301 . The binary register is one of the registers resident in preamp chip  213  of FIG.  2 . Each of the allowed values corresponds to a respective bias current value between 2 and 20 milliamps. Bias current is a parameter relating to reading data. If the register value is  11 b, FIG. 3 indicates that the read data channel will have a range  330  of optimal performance extending between 14° C. (range minimum  334 ) and 38° C. (range maximum  335 ). 
     The calibration of disc drive  200  will now be explained with reference to temperature thresholds  391 , 392 . Thresholds  391 , 392  determine which write current register value  302  is most appropriate for the head&#39;s current temperature  301 . When disc drive  200  is powered on, thermal calibration is performed at a time when head  260  is at 28° C. Because this is below the lowest threshold of 31° C., the register value is set to 11 binary. After a short time head  260  warms up past 40° C., which is above the optimal performance range maximum  335 . A degradation in the performance of the read channel is detected by a fault detector on controller board  267 , triggering a calibration routine. 
     The calibration routine begins with receiving an updated temperature measurement, which indicates that the head temperature is approximately 41° C. The calibration routine decides which of the allowed values  302  to assign based on the measured temperature in relation to the thresholds  391 , 392 . Because 41° C. is between the lower threshold  391  and the upper threshold  392 , the write current register receives a value of 10 binary. This causes a lesser amount of write current through the transducer head  260  when writing. For a measured temperature above 44° C. (threshold  392 ), the calibration routine will cause an even lower write current level to be used (corresponding to a register value  302  of 01 binary). 
     Note that if the temperature ranges  310 , 320 , 330  of optimal performance were vertically aligned for a given register, thermal calibration of that register would be unnecessary. Accordingly, devices of the present invention desirably perform temperature calibration only for parameter values  302  having a significant effect on thermal ranges  310 , 320 , 330  of optimal performance. By a “significant” effect, it is meant that the highest and lowest thermal ranges  310 , 330  are offset by at least 10° C. In the present case, the maximum  315  of range  310  is 68° C. and the maximum  335  of range  330  is 38° C. These differ by more than 20° C., so this parameter has a very significant effect on the thermal range of optimal performance. 
     FIG. 4 plots the allowed values  402  of a binary register against temperature  401 . The binary register is one of the registers resident in preamp chip  213  of FIG. 2, one that pertains to the data channel operating in a read mode. Each of the allowed values corresponds to a respective nonlinearity correction mode numbers. There are six allowed values: 010b, 011b, 100b, 101b, 110b and 111b. Note that the “X” indicates that the third bit has is a “don&#39;t care” bit, indicating that it has no significant effect on the thermal ranges of operability  410 , 420 , 430 . Thus, calibration/mode selection may leave the third bit unchanged. 
     Suppose that the increments of temperature  401  shown on FIG. 4 are at least about 5° C., so that the nonlinearity correction (mode number) register value has a significant effect on temperature. In that case, temperature calibration is desirably performed on register value  402 . The thermal calibration is performed similar to that of FIG.  3 . If a temperature reading is below threshold  491 , the thermal calibration routine will set the upper two bits of value  402  to “10”. If a temperature reading is above threshold  492 , the upper two bits are set to “01”. If a temperature reading is between the thresholds  491 , 492 , the upper two bits are set to “11”. 
     FIG. 5 depicts a calibration method  500  of the present invention comprising steps  505  through  565 . The present invention is most useful for heavily utilized processors. In a disc drive, large numbers of time-critical positioning computations must be performed  515  for high performance track following. A processor must occasionally suspend this processing to determine whether a data path fault condition has been detected  520 , and if so then must determine how recent the last thermal calibration update was  525 . If it was sufficiently recent (e.g. less than about 10-1000 seconds ago), a new temperature measurement is obtained  540 . If the measurement has changed, at least one channel operating parameter is updated  550 . It will be understood that this method  500  is performed iteratively, so that the processor quickly returns to positioning computations  515  following the parameter update step  565 . 
     FIG. 6 depicts a read error recovery table known in the disc drive industry. This table is described more fully in U.S. Pat. No. 5,721,816 entitled “ADAPTIVE RECOVERY OF READ AND WRITE ERRORS IN A DISC DRIVE” issued Feb. 24, 1998 to Kusbel et al. and commonly assigned with the present invention. The table describes a series of corrective operations optimally sorted to minimize the time required to recover a data sector that cannot be read successfully because the read channel is suboptimally configured. The Kusbel et al. patent teaches how to reorder such a table so that more successful operations are advanced in the table so that they occur earlier. 
     FIG. 7 depicts a method  700  of the present invention for read error recovery comprising steps  705 - 770  in an optimal order. The method is begun  705  in response to a failure to read a data block. The actuator is moved away from the target data block and returned, and then the data block is reread  710 . If the reread is successful  720 , the method  700  ends. Otherwise, the error correction code (ECC) level is changed, and the data block is reread again  730 . If the reread is successful  740 , the method  700  ends. Otherwise, the temperature sensor&#39;s output value is latched  745 . Read channel parameters are updated  750  by methods similar to those described above in conjunction with FIG.  1 . Next, the block is reread again  760 . Note that mechanical manipuation operations such as “off track center” and “wiggle retry” are generally more time-consuming than electrical manipulation operations. For this reason, additional corrective activity  770  desirably includes mechanical manipulation. 
     Characterized more particularly, a first contemplated embodiment of the present invention combines all of the features disclosed in conjunction with FIGS. 1&amp;2. A disc drive  200  includes several rotatable data storage discs  246  each having top and bottom surfaces. A transducer head  160 , 260  is positioned for interacting with the top surface of one disc  246 . The head  260  has several controllable operating parameter values including a bias current value  151 , a write current value  152 , a write precompensation value  153 , an off-track threshold value  154 , and a nonlinearity correction value  155 . Each of these operating parameter values  151 , 152 , 153  has a corresponding binary representation suitable for storage in a register of a controller circuit  150 . Each binary representation has a range of allowed values  302  (exemplified as 01b, 10b, and 11b in FIG.  3 ). 
     This first embodiment also includes a read data path including the transducer head  160 , 260 . The read data path  110  has a thermal range  330  that will shift by at least 10° C. (e.g. to range  310 , which is optimal for higher temperatures) in response to the bias current value traversing the range corresponding to the bias current value. Calibrated values of bias current, write current, and nonlinearity correction are each highly temperature sensitive—i.e., able to shift the data path&#39;s thermal range of optimal performance by at least 10° C. in response to changes in any one of these parameters. 
     The data path  110  is characterized as “write data path” or a “read data path,” depending on whether the disc drive  200  is writing or reading. A monitoring circuit  122  monitors the data path, and is configured to generate a read fault indication when a series of blocks of data in the read data path each contain more than a permissible number of errors. The temperature sensor  130  is preferably provided on the preamplifier chip  213  or on the data head  260 . In either case, the temperature sensor  130  is separated from the transducer  160  by a gap  135  small enough so that the temperature sensor  130  can adequately measure the temperature of the transducer head  160 . A control circuit  150  is coupled to the temperature sensor  130  and to the monitoring circuit  122 . The control circuit  150  is triggered periodically by clock counter  140  and whenever a fault indication is received from the monitoring circuit  122 . 
     In a preferred embodiment, the controller circuit disregards fault indications within about 1-60 seconds after updating the values  151 , 152 , 153 . Also, the controller circuit includes a buffer  159  that continuously receives an analog signal from temperature sensor  130 , the analog signal being sampled, held, and converted to a digital value as a step of a calibration routine. The calibration routine desirably derives updated operating parameter values  151 , 152 , 153  based upon additional parameters such as gain values from an automatic gain control circuit (in addition to the temperature reading in buffer  159 ). 
     Alternatively characterized, a second contemplated embodiment of the present invention is a data handling system  100  with a data path  110  with a thermal range of optimal performance  310 , 320 , 330 , 410 , 420 , 430 . At least one operating parameter value  302 , 402  substantially affects the thermal range of optimal performance (i.e., enough so that the parameter can shift an endpoint of the range by at least 10° C.). The second embodiment further includes a temperature sensor  130  constructed and arranged to measure a temperature of a portion of the data path  110 . A control circuit  150  is operatively coupled to the data path  110  and to the temperature sensor  130 . The control circuit  150  is configured to update the operating parameter value  302 , 402  in response to an indication of a fault (e.g. via fault detector  122 ) in the data path. 
     The data handling system of this second embodiment is optionally a magnetic disc drive  200  with a transducer head  260  that floats adjacent to a rotatable disc  246 . The data path is a “read path” or a “write path,” depending on whether data flows from or toward the disc. In either case, the data path will operate optimally only when the transducer head  260  is within the thermal range  330  for which it was calibrated. Many of the calibration parameters significantly affect the path&#39;s thermal range of optimal performance. In this second embodiment, it so happens that such parameters include a bias current register value, a write current register value  302 , or a nonlinearity correction register value  402 . In some cases, also, “the thermal range of optimal performance”  330  may be necessary for the disc drive  200  to operate reliably. 
     The control circuit of this second embodiment optionally includes a monitoring circuit  122  configured to indicate fault condition (i.e. transmit a code or signal indicative of a fault) when a predetermined number of consecutive data blocks each contain more than a permissible number of errors. The selection of suitable limits is within the skill of an ordinary practitioner with the benefit of teachings herein. 
     This second embodiment preferably includes temperature sensor  130  either on the transducer head  160  (for more accuracy) or on the preamp chip  213  (for lower cost). The construction and arrangement of suitable locations to allow the sensor  130  to measure the temperature of transducer head  160  cost-effectively is a matter of design choice for an ordinary practitioner with the benefit of teachings herein. 
     This second embodiment optionally includes the ability to update channel operating parameters in rotation, so that a temperature sensor reading need not perform the entire block of register updates each time a temperature measurement becomes available. Optionally, a clock/counter  140  is provided so that partial or total update computation routines are executed periodically based on the most recent temperature sensor measurement available. In this way, the clock/counter  140  can trigger a periodic update of the first operating parameter value based on an aperiodic measurement from the temperature sensor  130 . Note that the temperature sensor  130  itself need not be calibrated to an absolute temperature, but can instead provide composite measurements that depend on manufacturing variation as well as temperature. 
     All of the structures described above will be understood to one of ordinary skill in the art, and would enable the practice of the present invention without undue experimentation. 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. Changes may be made in the 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 particular elements may vary depending on the particular application for the present system while maintaining substantially the same functionality, without departing from the scope and spirit of the present invention. In addition, although the preferred embodiments described herein are largely directed to disc drives, it will be appreciated by those skilled in the art that the teachings of the present invention can be applied to other data handling systems such as wireless communication without departing from the scope and spirit of the present invention.

Technology Category: 3