Abstract:
In a motor having a number of poles that have nominally equiangularly-spaced positions that in fact deviate from those positions, the actual periods between zero-crossings of the back-EMF generated during pole-pair interactions are measured. The ratios of the various pole periods can then be computed, and the motor drive profile can be adjusted for each pole by applying the respective ratio to fit samples of the back-EMF profile to each respective pole.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application No. 60/826,440, filed Sep. 21, 2006. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a system and method for controlling the spindle of an electric motor, and more particularly to a system and method for controlling the spindle of a motor that rotates the platter of a disk drive. 
     Controlling the speed at which the platter of a disk drive rotates is very important, because even a small error in angular position resulting from an error in speed control may result in an incorrect sector being read or written. It is therefore a nominal goal to determine disk speed as accurately as possible. 
     Position, and therefore speed, of a disk drive platter is commonly determined by detecting the back electromotive force (back-EMF) generated when one of the rotor poles passes one of the stator poles. For example, it is typical for a disk drive motor to have six poles, so that each pole-pair interaction theoretically signifies 60° of motor rotation. However, in practice, it is difficult during manufacturing to accurately position the poles. In particular, the rotor may not be perfectly formed, but even if the rotor is perfectly formed, mechanical differences in the windings of the various poles could result in effective positional differences among the poles. Therefore, from an electrical perspective, some sets of adjacent poles may be closer together than 60°, and other sets of adjacent poles may be further apart than 60°. These offsets may be slight, but may be enough to prevent achieving the desired 0.01% accuracy. 
     Commonly-assigned U.S. Pat. No. 7,098,621, which is hereby incorporated by reference herein in its entirety, describes a method and apparatus for deriving calibration data for a motor, and a method and apparatus for controlling a motor using that calibration data. In accordance with those methods and apparatus, one phase of the motor power supply is suppressed (i.e., tristated) during a time duration when back-EMF is expected to be detected, and at the same time one of the other phases is grounded and the third phase is pulled high. If the back-EMF is detected outside that duration, the duration is expanded. This is iterated until the back-EMF falls within the expanded duration. In addition, commonly-assigned U.S. Pat. No. 7,196,484, which is hereby incorporated by reference herein in its entirety, describes a method and apparatus for adjusting the power supply voltage to minimize current spikes during the tristating operation. 
     The foregoing methods allow clean measurement of back-EMF, and therefore of motor speed. This allows motor speed to be adjusted by adjusting the spindle drive current. However, adjustments occur only once every several revolutions, and between adjustments, the spindle drive current remains constant during motor operation. Therefore, as the rotor rotates with its poles effectively unevenly spaced, the motor speed varies slightly between poles, resulting in potentially unacceptable jitter, even though the average motor speed may be the desired motor speed. 
     Copending, commonly-assigned U.S. patent application Ser. No. 11/347,543, filed Feb. 3, 2006, which is hereby incorporated by reference herein in its entirety, describes a method and apparatus for measuring back-EMF to derive the actual period of each pole and to adjust the drive current accordingly from pole to pole to maintain constant motor speed and reduce jitter. Copending, commonly-assigned U.S. patent application Ser. No. 11/840,460, filed Aug. 17, 2007, which is hereby incorporated by reference herein in its entirety, describes a method and apparatus for measuring back-EMF to derive the back-EMF profile of each pole and to fit the drive profile to the back-EMF profile. However, that system assumes that the measured samples for the back-EMF profile belong to poles whose periods are equal. This could result in a drive profile that causes the motor to operate faster or slower than required in a particular period. Moreover, the system might attempt to compensate based on an assumption that the next pole period will be the same as the current pole period, and because it is not the same, the attempted compensation could actually make the speed variation worse. 
     It therefore would be desirable to be able to control the spindle drive current to minimize motor speed variations. 
     SUMMARY OF THE INVENTION 
     Assuming constant motor speed and uniform pole spacing, the period of the back-EMF waveform would be expected to be constant. However, if motor speed is constant but the period of the back-EMF waveform is not constant, the varying periods can be correlated to nonuniform pole spacing. Thus, in accordance with the invention, the period (preferably as measured between zero crossings) of the back-EMF waveform is measured over a number of electrical periods corresponding to a single revolution—i.e., a number of periods equal to the number of poles. For example, in a six-pole motor, as is commonly used in disk drives, the back-EMF waveform would be measured over six electrical periods. It does not matter when the measurement is started, as long as it is possible to identify which measurement corresponds to which pole. Thus, an index feature may be provided on one of the poles so that each pole can be identified by the number of pole-pair interactions after the index feature is detected. 
     Once the electrical periods have been measured, the relative pole spacings can be computed by determining the average of the electrical periods, which would be the value of each period in the ideal case where the poles were uniformly spaced, and then computing the ratio of each electrical period to that ideal period. The ratios so computed can be used to adjust the fitting of the measured back-EMF profile to each pole. Then, for any desired motor speed, the drive profile so fitted will achieve a uniform motor speed. It does not matter what speed the back-EMF periods are measured at. Although the absolute values of the periods are inversely proportional to speed, the relative values reflected by the ratios stay substantially the same. The relative values may change with temperature if the motor, and particularly the rotor, does not have a uniform coefficient of thermal expansion. However, for a small motor such as in a disk drive, the coefficient of thermal expansion can be assumed to be substantially constant across the entire motor. Therefore, adjustments based on the initial ratios will be valid substantially at any temperature. 
     Therefore, in accordance with the present invention, there is provided a method for controlling a motor, which motor has a number of poles. The method includes detecting back-EMF from pole-pair interactions, deriving, from that back-EMF detected from pole-pair interactions (a) information regarding periods of respective poles, and (b) a back-EMF profile, and determining a drive profile from the back-EMF profile using the information regarding periods. Apparatus for carrying out the method is also provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other advantages of the invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which: 
         FIG. 1  is a diagrammatic view of theoretical pole placement in a motor, looking along the rotational axis of its rotor; 
         FIG. 2  is a graphical representation of a preferred embodiment of motor drive profiles for an ideal three-phase motor; 
         FIG. 3  is a graphical representation of the phase-to-phase voltages for the drive profiles of  FIG. 2 ; 
         FIG. 4  is a graphical representation of the phase-to-phase voltages for the drive profiles of a non-ideal motor; 
         FIG. 5  is a representation of a method according to the present invention; 
         FIG. 6  is a schematic diagram of motor drive circuitry in accordance with an embodiment of the present invention; 
         FIG. 7  is a block diagram of an exemplary hard disk drive that can employ the disclosed technology; and 
         FIG. 8  is a block diagram of an exemplary digital versatile disk drive that can employ the disclosed technology; 
         FIG. 9  is a block diagram of an exemplary high definition television that can employ the disclosed technology; 
         FIG. 10  is a block diagram of an exemplary vehicle that can employ the disclosed technology; 
         FIG. 11  is a block diagram of an exemplary cellular telephone that can employ the disclosed technology; 
         FIG. 12  is a block diagram of an exemplary set top box that can employ the disclosed technology; and 
         FIG. 13  is a block diagram of an exemplary media player that can employ the disclosed technology. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention will now be described with reference to  FIGS. 1-6 . 
       FIG. 1  shows, schematically, the three phases A ( 11 ), B ( 12 ) and C ( 13 ) of a three-phase motor  10  with which the present invention may be used. It should be remembered that the view of  FIG. 1  is theoretical, notwithstanding that it looks like the rotor of a three pole-pair motor. The number of pole-pairs in the motor is completely independent of the number of power supply phases and the present invention will work with substantially any three-phase motor regardless of the number of pole-pairs. 
     As seen in  FIG. 1 , each phase A ( 11 ), B ( 12 ), C ( 13 ) of motor  10  may be modeled as a motor resistance R motor    14 , a motor inductance L motor    15  and back-EMF voltage V BEMF    16  in series between a respective power supply phase SPA ( 110 ), SPB ( 120 ), SPC ( 130 ) and a central tap C tap    17  to which all phases are connected. Although the order of these components  14 ,  15 ,  16  is reversed in phase C ( 13 ) as compared to phases A ( 11 ) and B ( 12 ), the result would be the same if phase C ( 13 ) were identical to phases A ( 11 ) and B ( 12 ). 
       FIG. 2  shows drive profiles  21 ,  22 ,  23  in accordance with above-incorporated application Ser. No. 11/840,460, filed Aug. 17, 2007, applied to phases A, B and C, respectively, of an ideal three-phase motor. Each drive profile  21 ,  22 ,  23  represents voltage applied to a respective phase. As shown in  FIG. 2 , each profile  21 ,  22 ,  23  is applied as a series of discrete samples. In this embodiment, preferably 96 samples are applied during each complete electrical cycle, but another number—e.g., 192 samples—may be applied. Preferably, the number of samples used should be sufficient to approximate a smooth continuous signal. 
     Drive profiles  21 ,  22 ,  23  preferably are determined by measuring the back-EMF across the active phase pairs of the motor during operation and deriving drive profiles  21 ,  22 ,  23 , which are calculated to result in phase-to-phase voltage profiles such as profiles  31 ,  32 ,  33  of  FIG. 3  that match the measured nonsinusoidal back-EMF profiles of the motor. The drive profiles of  FIG. 3  resemble sinusoidal profiles, but are not truly sinusoidal. For example, each profile is flat or truncated at its maxima and minima, representing a “clipped” sinusoid. However, the phase-to-phase voltage profiles derived by plotting the differences between corresponding samples of the two active phases in  FIG. 2 , are substantially truly sinusoidal. This results in substantially sinusoidal drive current in each phase for an ideal motor. 
     If motor  10  is not ideal, and in fact the periods of the poles are not all the same, while the measured back-EMF will track the correct profile, the method of above-incorporated application Ser. No. 11/840,460, filed Aug. 17, 2007, will treat the samples of the measured profile, which are taken at a rate of, e.g., 96 samples per ideal pole period, as being uniformly distributed in time among the pole periods. However, because the motor is not ideal, for a pole whose period is longer than the ideal, 96 samples will represent less than the full profile for that pole, while for a pole whose period is shorter than the ideal, 96 samples will represent more than the full profile for that pole. Therefore, the drive profiles will not match the motor as they should. 
     The speed control circuitry of the motor will exacerbate the mismatch, because it measures the speed after each pole and attempts to correct the next pole accordingly. Thus, if a pole period is shorter than normal, so the motor spins too fast, the speed control circuitry will attempt to slow down the next pole. However, if the next pole period is longer than normal, the motor needs to speed up, rather than slow down, to maintain the correct speed. 
     This discrepancy is accounted for in accordance with the present invention by taking the actual pole periods into account when fitting the drive profile. The actual pole periods can be measured by measuring the electrical periods between zero-crossings of any of the back-EMF traces  41 ,  42 ,  43  in  FIG. 4  in one mechanical period (i.e., one revolution) of motor  10 . As seen in  FIG. 4 , in addition to each trace  41 ,  42 ,  43  having a non-ideal profile shape, the periods T n , as measured between zero-crossings  40  of any back-EMF trace  41 ,  42 ,  43 , appear to be uniform (in  FIG. 4 , only three half-periods are visible), but they may vary slightly from one another (to a degree not visible in  FIG. 4 ) as discussed above. When the back-EMF is measured during a subsequent mechanical period to determine the profile to which the drive current should be fit, the T n  data are used to fit the drive profile to the measured back-EMF profile. Specifically, rather than assuming uniform T n &#39;s and dividing up the samples in the measured back-EMF uniformly, the samples are divided up according the actual T n &#39;s as previously determined.  FIG. 5  shows how each T n  as measured by zero-crossings in one mechanical cycle is carried forward to the same pole during—EMF profile measuring in a subsequent mechanical cycle. 
       FIG. 6  shows an embodiment of control circuitry  60  for practicing the invention. Control circuitry  60  includes a motor controller  61 , a motor control interface  62 , and a power supply  63  that provides the three voltage phases  630 ,  631 ,  632  to motor  10 . Motor controller  61  is sometimes referred to as a Pcombo or power combo chip, and is normally mounted at or near spindle motor  10 , controlling both spindle motor  10  and the voice-coil motor (not shown) that moves the read/write head. Motor control interface  62  is sometimes referred to as the device system-on-a-chip or “SoC,” and is normally removed from motor  10  itself, as it is the main controller and interface of the device (e.g., a disk drive) of which motor  20  is a part. In the present invention, the drive profile fitting preferably is carried out in motor control interface  62  by processor  64  thereof, using memory  65  thereof. 
     Preferably, controller  61  includes back-EMF detection circuitry  66  which preferably detects during motor operation the back-EMF profiles across the various phase pairs and the back-EMF zero-crossings that determine the periods T 1 -T 6 , and preferably stores both the periods T 1 -T 6 , and the beck-EMF profiles, in memory  67  or registers  68 . Processor  64  of motor control interface  62  then uses those stored periods and profiles from memory  67  or registers  68  to compute drive profiles  630 ,  631 ,  632  for each phase, such that application of those profiles  630 ,  631 ,  632  to the three phases  11 ,  12 ,  13  causes the drive voltage across active pairs of phases  11 - 12 ,  11 - 13  or  12 - 13  to match the stored back-EMF profiles. 
     As mentioned above, the relative periods are substantially constant for a motor once manufactured, and therefore need be measured only once, and associated in memory with their respective poles. To allow the control circuitry to know which pole is the present pole, so that the correct stored values are used, a marker, which may be an optical or magnetic mark, or any other suitably detectable mark, can be placed on the rotor adjacent a particular pole as a reference. Control circuitry  60  ( FIG. 6 ) preferably can count cycles from the detection of that marker—e.g., by a suitable optical or inductive detector, identify the current pole, look up the appropriate T n , and adjust accordingly the sampling of the back-EMF profile to derive the drive profile that will maintain constant motor speed. 
     Thus it is seen that a method and apparatus for controlling motor speed to avoid jitter, notwithstanding pole position deviations, has been provided. 
     Referring now to  FIGS. 7-13 , exemplary implementations of the present invention are shown. 
     Referring now to  FIG. 7  the present invention can be implemented in a hard disk drive  600 . The present invention may implement either or both signal processing and/or control circuits, which are generally identified in  FIG. 7  at  602 . In some implementations, the signal processing and/or control circuit  602  and/or other circuits (not shown) in the HDD  600  may process data, perform coding and/or encryption, perform calculations, and/or format data that is output to and/or received from a magnetic storage medium  606 . 
     The HDD  600  may communicate with a host device (not shown) such as a computer, mobile computing devices such as personal digital assistants, cellular telephones, media or MP3 players and the like, and/or other devices, via one or more wired or wireless communication links  608 . The HDD  600  may be connected to memory  609  such as random access memory (RAM), low latency nonvolatile memory such as flash memory, read only memory (ROM) and/or other suitable electronic data storage. 
     Referring now to  FIG. 8  the present invention can be implemented in a digital versatile disk (DVD) drive  700 . The present invention may implement either or both signal processing and/or control circuits, which are generally identified in  FIG. 8  at  702 , and/or mass data storage of the DVD drive  700 . The signal processing and/or control circuit  702  and/or other circuits (not shown) in the DVD drive  700  may process data, perform coding and/or encryption, perform calculations, and/or format data that is read from and/or data written to an optical storage medium  706 . In some implementations, the signal processing and/or control circuit  702  and/or other circuits (not shown) in the DVD drive  700  can also perform other functions such as encoding and/or decoding and/or any other signal processing functions associated with a DVD drive. 
     DVD drive  700  may communicate with an output device (not shown) such as a computer, television or other device, via one or more wired or wireless communication links  707 . The DVD drive  700  may communicate with mass data storage  708  that stores data in a nonvolatile manner. The mass data storage  708  may include a hard disk drive (HDD). The HDD may have the configuration shown in  FIG. 7 . The HDD may be a mini-HDD that includes one or more platters having a diameter that is smaller than approximately 1.8″. The DVD drive  700  may be connected to memory  709  such as RAM, ROM, low-latency nonvolatile memory such as flash memory, and/or other suitable electronic data storage. 
     Referring now to  FIG. 9 , the present invention can be implemented in a high definition television (HDTV)  800 . The present invention may implement either or both signal processing and/or control circuits, which are generally identified in  FIG. 9  at  822 , a WLAN interface and/or mass data storage of the HDTV  800 . The HDTV  800  receives HDTV input signals in either a wired or wireless format and generates HDTV output signals for a display  826 . In some implementations, signal processing circuit and/or control circuit  822  and/or other circuits (not shown) of the HDTV  800  may process data, perform coding and/or encryption, perform calculations, format data and/or perform any other type of HDTV processing that may be required. 
     The HDTV  800  may communicate with mass data storage  827  that stores data in a nonvolatile manner such as optical and/or magnetic storage devices. At least one HDD may have the configuration shown in  FIG. 7  and/or at least one DVD drive may have the configuration shown in  FIG. 8 . The HDD may be a mini-HDD that includes one or more platters having a diameter that is smaller than approximately 1.8″. The HDTV  800  may be connected to memory  828  such as RAM, ROM, low-latency nonvolatile memory such as flash memory, and/or other suitable electronic data storage. The HDTV  800  also may support connections with a WLAN via a WLAN network interface  829 . 
     Referring now to  FIG. 10 , the present invention implements a control system of a vehicle  900 , a WLAN interface and/or mass data storage of the vehicle control system. In some implementations, the present invention may implement a powertrain control system  932  that receives inputs from one or more sensors such as temperature sensors, pressure sensors, rotational sensors, airflow sensors and/or any other suitable sensors and/or that generates one or more output control signals such as engine operating parameters, transmission operating parameters, and/or other control signals. 
     The present invention may also be implemented in other control systems  940  of the vehicle  900 . The control system  940  may likewise receive signals from input sensors  942  and/or output control signals to one or more output devices  944 . In some implementations, the control system  940  may be part of an anti-lock braking system (ABS), a navigation system, a telematics system, a vehicle telematics system, a lane departure system, an adaptive cruise control system, a vehicle entertainment system such as a stereo, DVD, compact disc and the like. Still other implementations are contemplated. 
     The powertrain control system  932  may communicate with mass data storage  946  that stores data in a nonvolatile manner. The mass data storage  946  may include optical and/or magnetic storage devices for example hard disk drives HDD and/or DVDs. At least one HDD may have the configuration shown in  FIG. 7  and/or at least one DVD drive may have the configuration shown in  FIG. 8 . The HDD may be a mini-HDD that includes one or more platters having a diameter that is smaller than approximately 1.8″. The powertrain control system  932  may be connected to memory  947  such as RAM, ROM, low latency nonvolatile memory such as flash memory, and/or other suitable electronic data storage. The powertrain control system  932  also may support connections with a WLAN via a WLAN network interface  948 . The control system  940  may also include mass data storage, memory and/or a WLAN interface (none shown). 
     Referring now to  FIG. 11 , the present invention can be implemented in a cellular telephone  1000  that may include a cellular antenna  1051 . The present invention may implement either or both signal processing and/or control circuits, which are generally identified in  FIG. 11  at  1052 , a WLAN interface and/or mass data storage of the cellular phone  1000 . In some implementations, the cellular telephone  1000  includes a microphone  1056 , an audio output  1058  such as a speaker and/or audio output jack, a display  1060  and/or an input device  1062  such as a keypad, pointing device, voice actuation and/or other input device. The signal processing and/or control circuits  1052  and/or other circuits (not shown) in the cellular telephone  1000  may process data, perform coding and/or encryption, perform calculations, format data and/or perform other cellular telephone functions. 
     The cellular telephone  1000  may communicate with mass data storage  1064  that stores data in a nonvolatile manner such as optical and/or magnetic storage devices—for example hard disk drives (HDDs) and/or DVDs. At least one HDD may have the configuration shown in  FIG. 7  and/or at least one DVD drive may have the configuration shown in  FIG. 8 . The HDD may be a mini-HDD that includes one or more platters having a diameter that is smaller than approximately 1.8″. The cellular telephone  1000  may be connected to memory  1066  such as RAM, ROM, low-latency nonvolatile memory such as flash memory, and/or other suitable electronic data storage. The cellular telephone  1000  also may support connections with a WLAN via a WLAN network interface  1068 . 
     Referring now to  FIG. 12 , the present invention can be implemented in a set top box  1100 . The present invention may implement either or both signal processing and/or control circuits, which are generally identified in  FIG. 12  at  1184 , a WLAN interface and/or mass data storage of the set top box  1100 . Set top box  1100  receives signals from a source  1182  such as a broadband source and outputs standard and/or high definition audio/video signals suitable for a display  1188  such as a television and/or monitor and/or other video and/or audio output devices. The signal processing and/or control circuits  1184  and/or other circuits (not shown) of the set top box  1100  may process data, perform coding and/or encryption, perform calculations, format data and/or perform any other set top box function. 
     Set top box  1100  may communicate with mass data storage  1190  that stores data in a nonvolatile manner. The mass data storage  1190  may include optical and/or magnetic storage devices for example hard disk drives HDD and/or DVDs. At least one HDD may have the configuration shown in  FIG. 7  and/or at least one DVD drive may have the configuration shown in  FIG. 8 . The HDD may be a mini-HDD that includes one or more platters having a diameter that is smaller than approximately 1.8″. Set top box  1100  may be connected to memory  1194  such as RAM, ROM, low-latency nonvolatile memory such as flash memory, and/or other suitable electronic data storage. Set top box  1100  also may support connections with a WLAN via a WLAN network interface  1196 . 
     Referring now to  FIG. 13 , the present invention can be implemented in a media player  1200 . The present invention may implement either or both signal processing and/or control circuits, which are generally identified in  FIG. 13  at  1204 , a WLAN interface and/or mass data storage of the media player  1200 . In some implementations, the media player  1200  includes a display  1207  and/or a user input  1208  such as a keypad, touchpad and the like. In some implementations, the media player  1200  may employ a graphical user interface (GUI) that typically employs menus, drop down menus, icons and/or a point-and-click interface via the display  1207  and/or user input  1208 . Media player  1200  further includes an audio output  1209  such as a speaker and/or audio output jack. The signal processing and/or control circuits  1204  and/or other circuits (not shown) of media player  1200  may process data, perform coding and/or encryption, perform calculations, format data and/or perform any other media player function. 
     Media player  1200  may communicate with mass data storage  1210  that stores data such as compressed audio and/or video content in a nonvolatile manner. In some implementations, the compressed audio files include files that are compliant with MP3 format or other suitable compressed audio and/or video formats. The mass data storage may include optical and/or magnetic storage devices for example hard disk drives HDD and/or DVDs. At least one HDD may have the configuration shown in  FIG. 7  and/or at least one DVD drive may have the configuration shown in  FIG. 7 . The HDD may be a mini-HDD that includes one or more platters having a diameter that is smaller than approximately 1.8″. Media player  1200  may be connected to memory  1214  such as RAM, ROM, low-latency nonvolatile memory such as flash memory, and/or other suitable electronic data storage. Media player  1200  also may support connections with a WLAN via a WLAN network interface  1216 . Still other implementations in addition to those described above are contemplated. 
     It will be understood that the foregoing is only illustrative of the principles of the invention, and that the invention can be practiced by other than the described embodiments, which are presented for purposes of illustration and not of limitation, and the present invention is limited only by the claims which follow.