Abstract:
An apparatus for measuring vibration in a article having a rotating member. A motion sensitive transducer produces a time domain analog signal in response to the vibration. An analog-to-digital data acquisition member samples the transducer signal and produces a time domain digital signal therefrom. A timing sensor is adapted to detect an instantaneous speed of the rotating member and preferably triggers the data acquisition member to begin sampling when the rotating member is rotating. A processor processes the time domain digital signal preferably by translation to a frequency domain digital signal and determination of the magnitude and phase of the frequency domain digital signal at a frequency associated with the instantaneous speed of the rotating member.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to the field of vibration measurement for a device having a rotating member and more particularly without limitation to a device and associated method for fast and accurate vibration data collection within a particular frequency associated with the rotational speed of the rotating member. 
     BACKGROUND OF THE INVENTION 
     Devices having rotating members generally require balancing in order to keep vibrations that are caused by the rotating mass below a desired level. In a rotating-disc data storage device, for example, the disc (or disc stack) typically is precision balanced. Otherwise, vibrations associated with an unbalanced condition can impair the operable data transfer relationship between the read/write head and the rotating disc. 
     An offsetting mass is attached to the disc stack in order to balance it. Typically, the discs are attached to the hub of a spindle motor by a clamp ring. The clamp ring itself can be fashioned to provide the offsetting mass. The amount of offsetting mass is determined by the magnitude of the imbalance condition, and the rotational orientation of the offsetting mass is determined by the phase angle of the imbalance condition. There are a number of traditional approaches used to determine the magnitude and phase angle of the imbalance condition. 
     In data storage device manufacturing today, one such approach utilizes a motion-sensitive transducer, such as a piezoelectric transducer, to determine the translational movement imparted to the data storage device housing by the spinning disc stack. In some solutions the analog signal from the transducer is analyzed in the time domain to determine the vibration magnitude and phase. In some solutions the analog signal is translated to a digital pulse stream to facilitate analysis in the time domain. 
     In any event, a problem with these traditional approaches is that they are relatively too slow to keep pace with the station cycle time of high speed manufacturing processes, which in the data storage industry can typically be about four seconds or less. A reason for the slow response is because the vibration measurements must be taken at steady-state conditions. That is, the data collection activity cannot begin until the motor has accelerated the disc stack to operational speed, and until all transient surges associated with the acceleration have dissipated. 
     Another problem associated with traditional approaches is that they require extensive isolation from external vibration sources. This makes it virtually impossible to perform other manufacturing operations while the data collection activity is taking place. Attaching a fastener, for example, can create vibrations that could contaminate the measurements. For this reason it is not unusual to see manufacturing lines designed with dedicated vibration testing stations. 
     It has been determined that by detecting the actual speed of the disc stack and using the speed to trigger data collection activity, transient data can be analyzed during the disc stack acceleration. Furthermore, by translating the analog signal from the transducer to a digital signal in the frequency domain, the vibration analysis can be focused on the frequency associated with the rotational speed of the disc stack. This prevents vibrations not associated with the rotating disc from being included in the data collection activities. These enhancements lend greater speed and accuracy to the manufacturing process, and allow performing simultaneous operations on the data storage device during vibration testing. It is to these improvements and others as exemplified by the description and appended claims that embodiments of the present invention are directed. 
     SUMMARY OF THE INVENTION 
     The embodiments of the present invention contemplate an apparatus and associated method for measuring vibration in an article having a rotating member. The device comprises a motion sensitive transducer attachable to the article comprising an output producing a time domain analog signal in response to the vibration. In one aspect the timing sensor comprises an optic sensor that is responsive to a target feature on the rotating member. The device can comprise two transducers producing simultaneous vibration signals from different planes, such as from orthogonal planes. 
     An analog-to-digital data acquisition member comprising an input connected to the transducer output for sampling the transducer signal and comprising an output producing a time domain digital signal from the sampling. A timing sensor is adapted to detect an instantaneous speed of the rotating member and triggers the data acquisition member to begin sampling when the rotating member is rotating. A processor comprising an input connected to the data acquisition member output for translating the time domain digital signal to a frequency domain digital signal and determining the magnitude and phase of the vibration signal at a frequency associated with the instantaneous speed of the rotating member. The device can perform a Fourier transformation in translating the signal from the time domain to the frequency domain. 
     The device of can further comprise a comparator determining whether the magnitude of the vibration signal at the frequency associated with the instantaneous speed of the rotating member is greater than a preselected threshold. The instantaneous speed can be associated with a transient start up state of the article&#39;s rotating member, which is less than the operating speed of the rotating member. 
     One aspect of the embodiments of the present invention contemplates a rotating disc data storage device balancer for measuring vibration. The balancer comprises a motion sensitive transducer attachable to the data storage device comprising an output producing a time domain analog signal in response to the vibration. The balancer can comprise two transducers producing simultaneous vibration signals along different planes, such as along orthogonal planes. 
     A timing sensor is adapted to detect an instantaneous speed of the disc stack. The instantaneous speed can be associated with a transient start up state of the disc and is less than the operating speed of the disc. The timing sensor can comprise an optic sensor that is responsive to a target feature on the rotating member. 
     The balancer is triggered to begin sampling by the timing sensor when the disc stack begins rotating. The balancer further comprises means for processing the transducer signal in determining a magnitude and phase of the signal at a frequency determined by the timing sensor. 
     The means for processing can be characterized by an analog-to-digital data acquisition member comprising an input connected to the transducer output for sampling the transducer signal and comprising an output producing a time domain digital signal from the sampling. 
     The means for processing can be characterized by a digital signal processor comprising an input connected to the data acquisition member output for translating the time domain digital signal to a frequency domain digital signal. The translation can be accomplished by a Fourier transformation. 
     The means for processing can be characterized by a comparator determining whether the magnitude of the vibration signal at the frequency associated with the instantaneous speed of the rotating member is greater than a preselected threshold. 
     One aspect of the embodiments of the present invention contemplates a method for measuring vibration in an article having a rotating member. The method comprises the following: orienting a motion-sensitive transducer on the article for detecting a vibration signal that is proportional to the article vibration along a desired direction; detecting the instantaneous speed of the rotating member; sampling and digitizing the vibration signal in obtaining a time domain digital signal of the vibration; translating the time domain digital signal to a frequency domain digital signal; and determining the magnitude and phase of the frequency domain digital signal at the frequency associated with the instantaneous speed of the rotating member. 
     The sampling and digitizing step can be initiated in response to the detecting step indicating a rotation of the rotating member that is greater than zero. The method can further comprise comparing the magnitude of the signal at the frequency associated with the instantaneous speed of the rotating member with a preselected threshold. 
     These and various other features as well as advantages which characterize the present invention will be apparent upon reading of the following detailed description and review of the associated drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of a data storage device having a disc stack well suited for balancing in accordance with the embodiments of the present invention. 
         FIG. 2  is a diagrammatic view of a balancer device constructed in accordance with embodiments of the present invention. 
         FIG. 3  is an elevational view of a portion of a balancer device constructed in accordance with embodiments of the present invention. 
         FIG. 4  is a block diagram of the imbalance detect circuit of the balancer device of  FIG. 2 . 
         FIG. 5  is a graphical representation of the frequency domain vibration analysis of the imbalance detect circuit of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring to the drawings in general, and more particularly to  FIG. 1 , shown therein is a plan representation of a data storage disc drive  100  constructed in accordance with an embodiment of the present invention. The disc drive  100  includes a base  102  to which various components are mounted, and a cover  104  (partially cut-away) which together with the base  102  forms an enclosure providing a sealed internal environment for the disc drive  100 . 
     Mounted to the base  102  is a motor  106  to which one or more data storage discs  108  are stacked and secured by a clamp ring  110  for rotation at a high speed in direction  111 . A plurality of discs  108  can be stacked with alternating disc spacers  112  ( FIG. 2 ) to form a disc stack. An actuator  113  pivots around a pivot bearing in a plane parallel to the discs  108 . The actuator  113  has actuator arms  116  (only one shown in  FIG. 1 ) that support load arms  118  in travel across the discs  108  as the actuator arms  116  move within the spaces between adjacent discs  108 . The load arms  118  (or “flexures”) are flex members that support data transfer members, such as read/write heads  120  (“heads”), with each of the heads  120  operatively interfacing one of the discs  108  in a data reading and writing relationship. Data read and write signals are transmitted from the head  120  to a preamplifier by electrical traces extending along the actuator  113 . 
     Each of the discs  108  has a data storage region comprising a data storage surface  122  divided into concentric circular data tracks (not shown). Each of the heads  120  are positioned adjacent a desired data track to read data from or write data to the data track. A circular landing zone  124  is provided where the heads  120  can come to rest against the respective discs  108  at times when the discs  108  are not spinning. The landing zone  124  can bound the data storage surface  122  inwardly; alternatively, the landing zone  124  can be located elsewhere. 
     The actuator  112  is positioned by a voice coil motor (VCM)  128  comprising an electrical coil  130  and a magnetic circuit source. The magnetic circuit source comprises one or more magnets supported by magnetic poles to complete the magnetic circuit. When controlled current is passed through the actuator coil  130 , an electromagnetic field is set up which interacts with the magnetic circuit, causing the actuator coil  130  to move. As the actuator coil  130  moves, the actuator  113  pivots around the pivot bearing, causing the heads  120  to travel across the discs  108  within an operable range of movement. 
     The motor  106  spins the discs  108  at a high speed as the head  120  reads data from and writes data to the data storage surface  122 . The head  120  includes an aerodynamic slider portion (not shown), that engages the fluid flow to fly the head  120  away from the data storage surface  122  during data reading and writing operations. 
     Vibration caused by the rotating discs  108  can adversely affect the data reading and writing relationship between a head  120  and its respective disc  108 . To keep the vibration at an operable level, the data storage device  100  typically is spin balanced during manufacturing. In doing so, the magnitude and phase angle of the imbalance condition is measured in order to determine the amount and placement of an offsetting mass that is attached to the disc stack to balance it in rotation. In the data storage device  100  of  FIG. 1 , the clamp  110  has a segment  123  of increased mass, thereby forming a nonconcentric mass distribution around the disc  108  axis of rotation. The amount of mass in the mass segment  123  can be varied depending upon the magnitude of the unbalance condition, and the angular placement of the mass segment  123  can be varied depending upon the phase angle of the unbalance condition. The following discusses the manner of determining this magnitude and phase angle of the unbalance condition so that a balancing process can be successfully accomplished. 
       FIG. 2  is a diagrammatic view of a balancer device  200  that is constructed in accordance with the embodiments of the present invention. The balancer device  200  has a structural fixturing block  202  (shown partially cutaway) to which the base  102  is secured. A controller  204  provides top level control of the balancer device  200  including activation of an energizing means  206  and initialization and operation of an imbalance detection circuit  208 . 
     The energizing means  206  provides power to rotate the spindle motor  106  at a desired speed. Where the spindle motor is a multi-phase direct-current inductive type motor, then the energizing means can be electrically connectable to the spindle motor&#39;s coil terminals so as to electrically commutate the motor to rotate the hub  210  to which the discs  108  are clamped. 
     The imbalance detection circuit  208  is responsive to a transducer  212  that produces a time domain analog signal  214  that is proportional to vibration in the fixturing block  202  imparted by the spinning discs  108  in the data storage device  100 . In an illustrative embodiment the transducer  212  can comprise a piezoelectric transducer construction. As discussed below, the data storage device  100  and the transducer  212  can be selectively positioned in order to measure the vibration along a desired direction. Alternatively, two or more transducers  212  can be used to measure vibration along multiple axes. 
     The imbalance detection circuit  208  is also responsive to a timing sensor  216  that sends a signal  218  indicating the rotational speed of the discs  108 . The timing sensor  216  can comprise an optical transducer that emits a light beam against a selected portion of the rotating hub  210  of the spindle motor  106 . An index mark, such as an aperture or void, can be disposed in the hub  210  to instantaneously vary the reflectivity sensed by the timing sensor  216  on each revolution. An amplifier  220  conditions the output from the timing sensor  216 , such as providing a frequency modulated pulse to the imbalance detection circuit  208 . One suitable timing sensor and amplifier is commercially available model FS-V1 from Keyence Corporation, Woodcliff Lake, N.J. 
       FIG. 3  is an elevational view of a balancer  200 A constructed in accordance with the embodiments of the present invention. The data storage device  100  is fixed on edge to the fixturing plate  202 , which is spatially separated from a base plate  250  by a number of upstanding column members  252 . The column members  252  are fashioned of sufficiently small diameter to allow first order vibration between the fixturing plate  202  and the base plate  250 . A transducer  212  has a base portion  256  supported by an upstanding member  257  depending from the base plate  250 , and an actuator portion  258  supported by an upstanding member  260  depending from the fixturing plate  202 . The actuator  258  can threadingly engage the upstanding portion  260  so that it can be longitudinally positioned in order to preload the transducer  212 . 
     The edgewise placement of the data storage device  100  upon the fixturing plate  202  transfers vibration into the fixturing plate  202  that lies in a direction that is transverse to the plane in which the discs  108  rotate. Translation of the data storage device  100 , and hence the fixturing plate  202 , due to vibrations along this direction is denoted by arrow  262 . Translation of the discs  108  and the read/write heads  112  along direction  262  are of particular interest because it can adversely affect the operable fly height of the head  120 . It will be noted that the transducer  212  is positioned in parallel with the direction  262  in order to measure the vibration in that direction. Although not shown, additional transducers  212  can be placed between the fixturing plate  202  and the base plate  250  to measure vibration along other desired directions. 
     Referring momentarily back to  FIG. 2 , the imbalance detection circuit  208  is illustrated in accordance with the embodiments of the present invention by the block diagram of  FIG. 4 . An analog-to-digital data acquisition card  300  has an input  302  connected to the signal  214  ( FIG. 2 ) from the transducer  212  ( FIG. 2 ). The data acquisition card  300  is triggered by the signal  218  ( FIG. 2 ) when the timing sensor  216  ( FIG. 2 ) detects rotation of the discs  108 . The data acquisition card  300  samples and digitizes the analog signal  214  ( FIG. 2 ) and at an output  304  produces a time domain digital signal  306  that is proportional to the vibration. 
     The imbalance detection circuit  208  further comprises digital signal processing means  308  comprising a signal processor  312  having an input  310  connected to the signal  306  from the data acquisition card  300 . The signal processor  312  translates the time domain digital signal  306  to a frequency domain digital signal  314 . In an illustrative embodiment the signal processor  312  employs Fourier transform analysis in performing the translation. 
     The signal processing means  308  further comprises signal analysis means  316  receiving the frequency domain digital signal  314  and determining the magnitude and phase of the vibration signal at a frequency associated with the instantaneous speed of the disc  108 , the instantaneous speed being provided by the timing sensor signal  218 .  FIG. 5  is a graphical representation of the frequency domain digital signal  314 , having constituent phase graph  320  and corresponding amplitude graph  322  plotted against the frequency spectrum. A vertical intercept  324  is calculated by the signal analysis means  316  at the frequency associated with the instantaneous speed of the discs  108 . At block  326  ( FIG. 4 ) it is determined whether the amplitude  328  at the intercept  324  exceeds a predetermined threshold. If the amplitude exceeds the threshold, then a reject signal  330  notifies the controller  204  ( FIG. 2 ) that retesting or rework is required. Rework may include balancing the disc stack. It will be noted the phase  332  at the intercept  324  is useful to the balancing operation in orienting any offsetting mass. If the amplitude does not exceed the threshold, then an approved signal  328  notifies the controller  204  ( FIG. 2 ) that the vibration is within operational tolerances and the data storage device  100  is approved for further downstream processing. 
     Because the imbalance detection circuit  208  is responsive to the timing sensor signal  218 , transient vibration measurements can be taken during spindle motor  106  acceleration. That is, the instantaneous speed used in determining the vibration amplitude can be less than the operational speed of the spindle motor  106 . This reduces the amount of station time that must be budgeted for vibration testing. This also makes it more possible to perform two or more balance tests during the station time if, for instance, a retest is necessary. 
     Erroneous vibration readings are discarded by filtering the frequency domain signal  314  so as to only consider a relatively small bandwidth  340  ( FIG. 5 ) enveloping the intercept  324 . Accordingly, other activities occurring simultaneously with vibration testing, such as shuttling and clamping the data storage device  100 , are not included in the vibration analysis if those vibrations lie outside the preselected bandwidth  340  of consideration. 
     The foregoing discussion primarily involves a static imbalance analysis wherein one transducer is employed, such as the transducer  212  of  FIG. 3 . The magnitude and phase determined from the FFT is proportional to the actual drive imbalance condition. In order to calculate the actual imbalance condition, a magnitude scaling factor and a phase angle offset must be applied. These are determined by calibrating the balancer. 
     To calibrate the balancer, first a calibration data storage device  100  is measured and the results are recorded. A known weight is then applied at a known radius and angle. The calibration data storage device  100  is then measured again and the change in imbalance is calculated. The change in imbalance condition must be equal to the calibration weight and its applied angle. In order to reach this equivalency, the appropriate scaling factor and angle offset are calculated and stored by the controller  204 . These calibration factors are subsequently applied in converting FFT magnitude and phase measurements to units of static imbalance magnitude and angle. 
     This calibration procedure can be extended in principle to a dynamic balance system where two or more transducers  212  are employed. The signals from the transducers  212  are individually acquired and translated to a frequency domain signal as above. Unlike a static balance system, however, the measurements are not necessarily taken at a plane coincident with the preferred balance correction plane. Rather, the offsetting weights of known mass and angle are empirically positioned to find the correction planes whereat the vibration at the measurement planes is minimized. 
     It is to be understood that even though numerous characteristics and advantages of various embodiments of the present invention have been set forth in the foregoing description, together with details of the structure and function of various embodiments of the invention, this disclosure is illustrative only, and changes may be made in detail, 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 vibration detection and analysis members may vary while maintaining substantially the same functionality without departing from the scope and spirit of the present invention. In addition, although the preferred embodiment described herein is directed to use with a moving disc data storage device, it will be appreciated by those skilled in the art that the teachings of the present invention can be applied to other work pieces as well without departing from the scope and spirit of the present invention.