Abstract:
A quiet retraction method for regulating constant velocity while parking an arm within a disk drive is described. The method comprises the steps of: driving a motor for the arm using a first drive current for a first period; floating the motor; sampling a back electromotive force (bemf) for a first sampled voltage, while floating the motor; driving the motor with a second drive current during a second period in response to sampling the bemf; determining whether the second drive current exceeds a current limit; estimating the bemf using the first sampled voltage when the second drive current exceeds the current limit; driving the motor with a third current during a third period in response to estimating the bemf; wherein driving the motor with the first, second, and third currents quietly parks the arm, while regulating the constant velocity.

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
     The present application claims priority to jointly owned U.S. Provisional Application corresponding to application No. 61/140,176 entitled “Silent Retract.” This provisional application was filed on Dec. 23, 2008. 
    
    
     DESCRIPTION OF RELATED ART 
     With the evolution of electronic devices, there is a continual demand for enhanced speed, capacity and efficiency in various areas including electronic data storage. Motivators for this evolution may be the increasing interest in video (e.g., movies, family videos), audio (e.g., songs, books), and images (e.g., pictures). Optical disk drives have emerged as one viable solution for supplying removable high capacity storage. Effective writing and reading data from an optical disk mean that an arm should be moved precisely to and from a parked position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The quiet retraction system may be better understood with reference to the following figures. The components within the figures are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts or blocks throughout the different views. 
         FIG. 1  is a system drawing illustrating components within an optical disk drive. 
         FIG. 2  is a block diagram of the QRS of  FIG. 1 . 
         FIG. 3  is a plot illustrating two silent pulse regions where the QRS of  FIG. 1  estimates the Bemf. 
         FIG. 4  shows the flow chart for the retract from a system level. 
         FIG. 5  shows the flow chart for the mechanism to regulate constant Bemf retract. 
         FIG. 6  shows the flow chart for Quiet Retract which is an extension to  FIG. 5 . 
     
    
    
     While the quiet retraction system is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and subsequently are described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the quiet retraction system to the particular forms disclosed. In contrast, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the quiet retraction system as defined by this document. 
     DETAILED DESCRIPTION OF EMBODIMENTS 
     As used in the specification and the appended claim(s), the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Similarly, “optional” or “optionally” means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event or circumstance occurs and instances where it does not. 
     Turning now to  FIG. 1 , is a system drawing illustrating components within an optical disk drive  100 . This disk drive includes an optical media  110  (e.g., a magnetic disk) that stores data, which can be accessed either during a read or write operation. An arm  115  can extend across the disk  110 , for example, during a write operation. A voice coil motor (VCM)  117  can control the movement of the arm  115 . However, the arm  115  may reside in a rest, or parked, position during a read operation. The disk drive  100  includes a quiet retraction system (QRS)  120  that substantially reduces acoustic noise that maybe associated with parking the arm  115 . For example, there may be an acoustic (singing) threshold, or current level that makes noise during retract. In addition, this acoustic threshold may be the point at which a user determines that the acoustic noise becomes apparent. With the QRS  120 , this threshold is programmable and this system reduces acoustic noise for each individual threshold. The QRS  120  assesses when a magnitude of the drive current exceeds a singing threshold and then derives the drive current differently with a much lower frequency, which substantially reduces the acoustic noise. Moreover, this system facilitates this quiet retract, while maintaining a substantially constant back electromotive force (bemf). 
     The disk drive  100  may be a Contact Start/Stop (CSS) typed disk drive where the parking area is in an Inside Diameter, such as the area  123 . Alternatively, this disk drive may be a ramp-typed disk drive where the parking area is on the ramp located toward the Outside Diameter, such as the area  125 . The QRS  120  may use a constant velocity retract. Constant velocity is a technique driving the VCM while maintaining a constant Back Electro Motive Force (Bemf). 
     The velocity of the arm is proportional to the Bemf generated by the VCM. Determining the Bemf can be done by measuring the voltage across the VCM  117  when it is floating, or not driven. The driving current may regulate the velocity of the arm  115 . In the disk drive  100 , a magnet  130  within the VCM  117  strongly pulls this arm when it approaches a crash stop in the area  125 . An alternative implementation may not include this magnet or may include more than one magnet. Floating the VCM  117  and determining the Bemf may take a finite duration and depend on the amplitude of the driven current, the QRS  120  facilitates effectively controlling the velocity of the arm even at relatively high initial velocities by sampling the Bemf when the VCM is floating. 
       FIG. 2  is a block diagram of the QRS  120 . For a specific design, a target velocity may be chosen, which may have units of inches per second, or ips. The relationship between the Bemf and target velocity may be characterized by the following equation: Target_Bemf=Ke*Target_Velocity, where Ke is an electrical motor constant with units of Volts/ips. Therefore, selecting a target velocity enables calculation of a target Bemf with units of Volts. A converter  210  may receive the target Bemf, which may be a target Bemf digital to analog converter (DAC) and transmit a converted signal. 
     A device  220  may have one input terminal  221  that receives the converted signal from the converter  210 , where the device  220  and converter  210  can be referred to as an error circuit. In addition, this device may have other input terminals  223 - 225  for receiving other types of signals. While the device  220  is shown with three input terminals, any number of alternative implementations may occur my varying the number of input terminals. Moreover, the device  220  may be an analog adder/subtractor in one implementation that transmits a first combined signal, such as a Bemf error signal, on terminal  228 . A device  230  receives the first combined signal on the terminal  231  and transmits a second combined signal on a terminal  234 . In addition, this device may have other input terminals, such as input terminal  231 . The device  230  may be an analog adder in one implementation. 
     The QRS  120  also includes two series-connected gain stages. The gain stage  242  receives the second combined signals and increases it in accordance with a first gain factor, such as integrator gain Ki. The gain stage  242  transmits a first gain signal on the terminal  243 . A gain stage  246  receives the first gain signal and increases it in accordance with a second gain factor, such as a proportional gain Kp. As this gain stage transmits a second gain signal on the terminal  247 , a driver stage  250  can receive this gain signal, or current signal. This driver stage can transmit either drive current or a drive voltage that controls the VCM  217 . In one implementation, the device  230 , integrator  270 , gain stage or circuit  242 , and gain stage or circuit  246  may form a current determination block or circuit  273 . 
     As mentioned with reference to  FIG. 1 , VCM  217  may float, or not be driven, for a short duration such that the flyback dies out. When this motor is not floating, a timer  260  may transmit a drive timing signal along terminal  262  that controls how the driver stage  250  drives the VCM  117 . For example, the drive timing signal from this timer may be active for its duty cycle, which would mean that a drive current or drive voltage may be sent to this VCM for this same duty cycle. In addition, this timer transmits a sample timing signal when the VCM  117  is floating along the terminal  261 . 
     A sample and hold block or circuit  264  has input terminals  265 - 266  that connect to output terminals of the driver stage  250 , while another input terminal receives the sample timing signal. The sample and hold block  264  may be an Analog To Digital converter with a switched cap. Consequently, the voltage across the terminals  265 - 266  approximately equals the sum of the voltage across the motor and the Bemf. Floating the VCM enables effective sampling of the Bemf because the voltage across VCM  117  is approximately the Bemf. The sample and hold block  264  may transmit a sampled signal to a third gain stage or circuit  267 . This gain stage may have an associated factor Katt, which may be approximately one, and transmit a third gain signal to the device  220  via the terminal  225 . In one implementation, the sample and hold block  264  and gain stage or circuit  267  may form a sample block or circuit  275 . 
     The device  220  can determine the Bemf error and transmit as the first combined signal on the terminal  228 . The Bemf error may be the difference between the target Bemf, or converted signal, and the sampled Bemf that leaves the gain stage  267 , or third gain signal. For the nth sample of the Bemf, the following formula may be used in calculating the Bemf error: Error(n)=Target_Bemf-Bemf(n). The device  230  receives the error signal, first combined signal. 
     An integrator  270  also connects to the device  230  so it can determine the second combined signal. Using the integrator  270 , device  230  and gain stage  242 , the Bemf error gets integrated and multiplied by the integrator gain, which produces the second combined signal. The following formula may be used: Integrator(n)=Ki*(Error(n)+Integrator(n−1)). Using the device  246 , the output of the integrator is multiplied by a the proportion gain Kp. Then the driver stage  250  can receive the second gain signal, or the signal from the gain stage  246 . The following formula may be used: Driver_Stage_Input=Kp*Integrator(n). Since the singing threshold is programmable, one can set a corresponding limit. As long as the drive current or drive voltage for the VCM  217  is less than this limit (e.g., predetermined brake voltage limit BRAKE_LIMIT_VOLT), Bemf sampling may continue. 
     As mentioned above, the timer  260  can control how long the driver stage  250  drives, or remains on. The timing duration for the ON time may the time Ton, while the floating time, or timing duration for the OFF may be the time Toff. Both of these may be programmable. 
     The QRS  120  also includes a comparator  280 , compensator  285 , and a matching device or circuit  290 . The comparator  280  may assess a signal value applied to the terminal  247  relative to a limit, such as the voltage limit BRAKE_LIMIT_VOLT. When the signal on this terminal exceeds the limit, this comparator may transmit an update signal to the timer  260 . The comparator  280  may be one of many types of comparators, and the timer  260  and comparator  208  can form at least a portion of a mode selection circuit. In response to receiving the update signal, the timer  260  may extend the drive time, such as a drive time in the range of approximately 10 ms to approximately 20 ms or some other suitable range. This may be done by transmitting a silent pulse with duration between approximately 10 ms to approximately 20 ms. Similarly, when the signal on the terminal  247  is below the limit, the timer  260  may return to the original drive time range of approximately 0.3 ms to approximately 0.5 ms. 
     As the timer transmits the extended pulse, or silent pulse, the compensator  285  estimates the Bemf, since the Bemf is sampled less frequently because of the longer pulse duration. The matching device  290  has input terminals  291 - 292  positioned across a sense resistor Rsense. In one implementation, this matching device may be a differential amplifier that applies a differential signal to the terminal  295 , which is the input of the compensator  285 . In one implementation, the compensator  285  may be a subtraction circuit. It may be implemented with either analog logic or digital logic. In an analog design, the subtraction circuit may be designed as a capacitor with charges added or drained. In the digital design, the subtraction circuit may be designed as an analog to digital converter; other digital circuits may add or subtract the output of this converter. 
     Since this compensator  285  receives both the differential signal on terminal  295  and the third gain signal on the terminal  225 , the compensator  285  can apply a compensated signal to the terminal  223 , which reflects an estimated Bemf. As a result, the device  220  can determine a new first combined, or error, signal based on the target Bemf received on terminal  221 , compensated signal on terminal  223 , and the 3 rd  gain signal on terminal  225 . This produces several other signals as previously described, which eventually results in a drive current or a drive voltage that drives the VCM  117 . In one implementation, the comparator  280 , compensator  285 , and matching device may form an estimation block or circuit  277 . 
     Quiet Retract System in Current Mode 
     How the compensator  285  does Bemf estimation may be understood with reference to either a current mode driver stage or a voltage mode driver stage. This first explanation will focus on the current mode driver stage. When the driver stage  250  is in current mode, the current following through the coil  217  of the VCM  117  may be fixed. For a given Bemf sample, the voltage across the coil is the sum of the Bemf voltage for the sample with the product of the current through the coil, or Ivcm, and the total resistance across this VCM, or Rtotal. But Rtotal may be defined by the following formula: Rtotal=Rfet+Rmotor+Rsense, where Rfet is the resistance of the silicon power device, Rmotor is the resistance of the motor, and Rsense is the resistance of a sense resistor, like sense resistor  218 . 
     Using these relationships, the voltage difference between samples may be governed by the following formulas:
 
VCM( n− 1)= I vcm( n− 1)* R total+Bemf( n− 1),for a sample “ n− 1” where V CM is the voltage across the VCM  117  in current mode
 
VCM( n )= I vcm( n )* R total+Bemf( n ),for a sample “n” where V CM is the voltage across the VCM  117  in current mode
 
Since the driver stage  250  is in current mode and the Ivcm is fixed, the voltage difference between these samples is approximately the differences between the Bemfs as illustrated in the following formula:
 
VCM( n )−VCM( n− 1)=Bemf( n )−Bemf( n− 1)=DELTA_BEMF
 
Knowing how the actual Bemf changed between from the last samples enables estimation of the Bemf for a future sample as illustrated in the following formula:
 
Bemf( n )=Bemf( n− 1)+DELTA_BEMF
 
By estimating the Bemf, the frequency of actual, or physical, Bemf samples may be reduced, which correspondingly reduces acoustic noise. In other words, the number of actual Bemf values receive on the input terminals of the sample and hold block  264  may be reduced. Instead, the compensator  285  produces estimated Bemfs using the DELTA_BEMF.
 
     The voltage difference across the VCM  117  between samples may be a large signal voltage, such as a voltage of approximately 5 V. But the voltage across the sense resistor may be amplified with a fixed gain, which facilitates making two measurements. Instead of measuring this difference directly, there may be two separate measurements made as indicated in the following formula:
 
 A =VCM( n− 1)−[ I ( n− 1)* R sense]* R matching,where A is a measurable signal for the sample “n−1”
 
 B =VCM( n )−[ I ( n )* R sense]* R matching,where B is a measurable signal for the sample “n”
 
But Rmatching is programmable and may be selected as approximately the ratio of  R total/ R sense. By subtracting B from A, the Bemf delta may be determined and the Bemf estimated as indicated below:
 
 B−A =VCM( n )−VCM( n− 1)=DELTA_BEMF
 
Bemf( n )=Bemf( n− 1)+DELTA_BEMF=Bemf( n −1)+( B−A ),where Bemf( n− 1)is the last measured Bemf before the singing threshold
 
     As long as the drive current remains above the limit (e.g., singing threshold), the QRS  120  estimates the Bemf using the formula above, which coordinates with the duration of the silent pulse produced by the timer  260 . When this extended pulse expires, the driver stage  230  stops driving and the VCM  117  floats, which facilitates actually sampling the Bemf. And, that process continues until the driven current becomes larger than a set limit, like the singing threshold; the timer  260  sends regular pulses to the driver stage. At this point, the QRS  120  estimates the Bemf; the timer  260  sends extended pulses to the sample and hand hold block  240 . The normal retract pulses and the extended, or Silent Retract, pulses will co-operate until the arm  114  finally parks. 
     Quiet Retract System in Voltage Mode 
     Estimating the Bemf may also be done when the driver stage is in voltage mode. For this mode, the voltage across the coil is fixed and characterized by: VCM=VCMP−VCMN. For two consecutive samples “n−1” and “n” in the voltage mode, VCM(n)=VCM(n−1), which means that the following is true:
 
VCM( n− 1)= I ( n− 1)* R +Bemf( n− 1)
 
VCM( n )= I ( n )* R +Bemf( n )
 
VCM( n− 1)−VCM( n )=0 =R*[I ( n )+ I ( n− 1)]+[Bemf( n )−Bemf( n− 1)]
 
With these relationships, it is possible to determine the delta of the Bemf and the estimated Bemf as indicated below:
 
DELTA_BEMF=Bemf( n )−Bemf( n− 1)=− R*[I ( n )− I ( n− 1)]
 
Bemf( n )=Bemf( n− 1)+DELTA_BEMF
 
     The product of R*I may be a large signal voltage, such as a voltage of approximately 5 V. But the voltage across the sense resistor may be amplified with a fixed gain, which facilitates making two measurements. Instead of measuring this difference directly, there may be two separate measurements made as indicated in the following formula:
 
 A =VCM( n− 1)−[ I ( n− 1)* R sense]* R matching
 
 B =VCM( n )−[ I ( n )* R sense]* R matching
 
But Rmatching is programmable and may be selected as approximately the ratio of Rtotal/Rsense. By subtracting B from A, the Bemf delta may be determined and the Bemf estimated as indicated below:
 
 B−A =−( R sense* R matching)*[ I ( n )− I ( n− 1)]
 
But here, (B−A) is very close to the ideal DELTA_BEMF. This difference depends on effective in matching (Rsense*Rmatching) to Rmotor. The error in the DELTA_BEMF (in percentage) is
 
100*(( R sense* R matching)− R total)/ R total(%)
 
For example, if Rsense=⅓ ohm and R=11 ohm, then ideally, Rsense*Rmatching=R. So, (⅓)*Rmatching=11, or Rmatching=33. If the amplifier designed for Rmatching is 30, the error of the DELTA_BEMF is, 100*((⅓)*30−11)/11=−9%.
 
     For the voltage mode, the DELTA_BEMF estimate has an error introduced by the matching of the designed amplifier (Rmatching) versus the ratio of the total resistance and the sense resistance (R/Rsense). With this, the Bemf may be estimated as indicated below:
 
ESTIMATE_DELTA_BEMF=−( R sense* R matching)*[ I ( n )− I ( n− 1)]DELTA_BEMF=−( R )*[ I ( n )− I ( n− 1)]Bemf( n )=Bemf( n− 1)+ESTIMATE_DELTA_BEMF=Bemf( n− 1)+( B−A ),where Bemf(n−1) is the last measured Bemf before the singing threshold
 
     As long as the drive current remains above the limit (e.g., singing threshold), the QRS  120  estimates the Bemf using the formula above, which coordinates with the duration of the silent pulse produced by the timer  260 . When this extended pulse expires, the driver stage  230  stops driving and the VCM  117  floats, which facilitates actually sampling the Bemf. And, that process continues until the driven current becomes larger than a set limit, like the singing threshold; the timer  260  sends regular pulses to the driver stage. At this point, the QRS  120  estimates the Bemf; the timer  260  sends extended pulses to the sample and hand hold block  240 . The normal retract pulses and the extended, or Silent Retract, pulses will co-operate until the arm  114  finally parks. 
     Turning now to  FIG. 3 , this figure is a plot illustrating two silent pulse regions where the QRS  12  estimates the Bemf. For the silent pulse associated with the region  310 , the actual sampled Bemf is shown with the curve  313  that has “o” for each data point, while the estimated Bemf is shown with the curve  315  that has “+” for each data point. As illustrated in this figure, the QRS effectively estimates the Bemf, during the silent pulse region. The same applies for the silent pulse associated with region  320 . The width of the Silent Retract pulse is programmable. The duration of the Silent Retract affects the accuracy of the estimator. The longer the duration, the more error may be introduced to the estimator and vice versa. While  FIG. 3  illustrates the voltage mode, a similar drawing may be done for the current mode. In voltage mode, the accuracy of the estimator depends on the ability of matching Rmatching to the ratio Rtotal/Rsense. This error will introduce an offset into the closed loop. Purposely choosing the gain Rmatching difference from the ratio Rtotal/Rsense will compensate for the non-linearity of the motor constant (Kt). 
     Quiet Retract System Process 
       FIG. 4  shows the flow chart for the retract from a system level.  FIG. 5  shows the flow chart for the mechanism to regulate constant Bemf retract.  FIG. 6  shows the flow chart for Quiet Retract which is an extension to  FIG. 5 . In  FIG. 5  there is a calculation for Vout; this calculation has the main variable as the Bemf. Quiet Retract, implemented in  FIG. 6 , will replace the Bemf with the Estimate Bemf. 
     Parts of the quiet retraction system  120  may be implemented within software as an ordered listing of executable instructions for implementing logical functions that can be embodied in any computer-readable medium. This medium may be for use by or in connection with an instruction execution system, apparatus, or device, such as a computer-based system, processor-containing system, or other system that can fetch the instructions from the instruction execution system, apparatus, or device and execute the instructions. In the context of this document, a “computer-readable medium” can be any means that can contain, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device. The computer-readable medium can be, for example, but, not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, device, or propagation medium. More specific examples (a non-exhaustive list) of the computer-readable medium can include the following: an electrical connection (electronic) having one or more wires, a portable computer diskette (magnetic). Note that the computer-readable medium can even be paper or another suitable medium upon which the program is printed. The program can be electronically captured, via for instance optical scanning of the paper or other medium, then compiled, interpreted or otherwise processed in a suitable manner if necessary, and then stored in a computer memory. 
     While various implementations of the quiet retraction system have been described, it may be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible that are within the scope of this system. Although certain aspects of the quiet retraction system may be described in relation to specific techniques or structures, the teachings and principles of the present system are not limited solely to such examples. All such modifications are intended to be included within the scope of this disclosure and the present quiet retraction system and protected by the following claim(s).