Abstract:
A method of detecting contact between a transducing head-carrying slider and a rotatable disc is performed by applying an electrostatic voltage between the slider and the disc and monitoring current flow across an interface between the slider and the disc. The monitored current flow across the interface between the slider and the disc is analyzed to detect contact between the slider and the disc.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a continuation-in-part of U.S. application Ser. No. 10/119,178 filed Apr. 9, 2002 now U.S. Pat. No. 6,700,724 for “Contact Detection And Calibration For Electrostatic Fly Height Control In A Disc Drive” by J. Riddering and L. Knippenberg. 
     This application claims the benefit of Provisional Application No. 60/418,118 filed Oct. 11, 2002 for “In-Situ Technique For The Monitoring Of Head-Disc Contact In Magnetic Disc Files” by J. Hanchi, L. Fu, R. Rao and L. Knippenberg. 
     INCORPORATION BY REFERENCE 
     The aforementioned U.S. application Ser. No. 10/119,178 and Provisional Application No. 60/418,118 are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a system for sensing contact between a slider and a rotating disc in a disc drive, and more particularly to an in-situ technique for detecting slider-disc contact in which an electrostatic voltage is applied between the head and the disc to allow detection of contact. 
     As the density of data recorded on magnetic discs continues to increase, it is becoming necessary for the spacing between the transducing head carried by the slider and the disc to decrease to very small distances. Spacings of well below 10 nano-meters (nm) are required in some applications. In disc drive systems having such small slider-disc spacing, the possibility of contact between the slider and the disc is relatively high, due to factors such as slider manufacturing process limitations and limited air-bearing modeling capabilities. A system for detecting such contacts is useful for a number of diagnostic tests, enabling assessments such as component-level flyability and durability, drive-level reliability, and production-level screening to be made, as well as providing input to fly-height calibration and adaptive-fly-control systems that enable dynamic adjustment of flying height in certain disc drive systems. 
     Existing methods of detecting contact between a slider and a disc typically involve acoustic emission (AE) monitoring by an external AE sensor such as a piezoelectric element having suitable frequency response and sensitivity. While AE sensors are generally effective to detect high intensity and catastrophic slider-disc contact events, their detection abilities are somewhat limited. The use of an external sensor limits the AE sensor&#39;s sensitivity to remotely occurring slider-disc contact events. The physical dimensions of the AE sensor also preclude optimum placement of the sensor in many component-level testing arrangements. Furthermore, the effectiveness of conventional AE sensors may be severely limited by the introduction of polymer-based (“flex”) gimbals, due to the heavy AE attenuation of such gimbals, which act as a high acoustic impedance component between the slider-disc interface and the suspension. 
     There is a need in the art for an improved apparatus and method for sensing contact between a slider and a disc, both in operative disc drive systems and in testing applications. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is a method of detecting contact between a transducing head-carrying slider and a rotatable disc. An electrostatic voltage is applied between the slider and the disc, and current flow across an interface between the slider and the disc is monitored. The monitored current flow across the interface between the slider and the disc is analyzed to detect contact between the slider and the disc. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a portion of a typical, exemplary disc drive illustrating the structure and configuration for mechanically supporting and electrically connecting a slider. 
         FIG. 2  is a simplified diagram of a slider and a disc employing an in-situ slider-disc contact sensing system according to the present invention. 
         FIG. 3  is a graph illustrating the variation of flying height between a slider and a disc as a function of applied voltage therebetween. 
         FIGS. 4A and 4B  are graphs illustrating the results of testing performed in an exemplary slider support configuration to compare the response of the in-situ slider-disc contact detection system of the present invention and an AE sensor according to the prior art. 
         FIG. 5  is a graph illustrating the results of testing performed in an exemplary slider support configuration, having various touchdown RPM and various electrostatic voltages applied between the slider and disc according to the in-situ slider-disc detection system of the present invention. 
         FIG. 6  is a graph illustrating the results of testing performed in an exemplary slider support configuration to compare the response of the in-situ slider- disc contact detection system of the present invention and an AE sensor according to the prior art, in varying slider-disc contact scenarios. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a perspective view of a portion  10  of a typical, exemplary disc drive illustrating the structure and configuration for mechanically supporting and electrically connecting slider  12 . Support arm  14  is connected to load beam  16 , which in turn supports flexure  18  which carries slider  12  over the surface of a rotating disc. Slider  12  is also electrically connected to a drive circuit  20 , with conductive traces  22  connecting drive circuit  20  to read head contacts  24 , and conductive traces  26  connecting drive circuit  20  to write head contacts  28 . Conductive traces  22  and  24  are formed on a polyimide flex circuit, which is typically a structure separate from flexure  18  but could in some embodiments make up flexure  18  itself. The structure supports slider  12  over the surface of disc  30 , allowing data to be transduced therebetween. It will be understood that the configuration shown in  FIG. 1  is only one example of a slider support and connection structure for a disc drive or testing system, and that the example given in  FIG. 1  is intended only to convey the general context for the present invention. 
     In typical systems such as the one shown in  FIG. 1 , an acoustic emission (AE) sensor, such as a piezoelectric sensor, is attached to the slider support system or to slider  12  itself in order to detect contact between slider  12  and disc  30 . The potential limitations of AE sensors are discussed above. The present invention proposes the use of an in-situ sensor for detecting contact between slider  12  and disc  30  by applying an electrostatic voltage across the head-disc interface and measuring the change in current flowing across that interface due to contact events between slider  12  and disc  30 . 
     In order to implement the in-situ head-disc contact system of the present invention, few structural changes need to be made to a typical slider support configuration. A voltage difference must be applied between slider  12  and disc  30  to induce current across the head-disc interface. In typical disc drives, both slider  12  and disc  30  are grounded. Thus, a voltage must be applied to either slider  12  or to disc  30 . Typical sliders are already provided with a ground path through which a voltage can be applied, thus requiring no additional structure and making this option attractive. Alternatively, the voltage could be applied to the disc while leaving the slider grounded, which would require some additional structure because of the need for the disc to retain its path to ground. However, the incorporation of this structure is well within the abilities of one skilled in this art. 
       FIG. 2  is a simplified diagram of slider  12  and disc  30  employing an in-situ slider-disc contact sensing system according to the present invention, illustrating that the sensing system can be implemented without substantial addition to conventional slider and disc structures. Slider  12  is connected to sensing circuitry  32 , which is operable to generate control voltage V at electrical connection  34  so that sensing circuitry  32  can sense the current flowing across the interface between slider  12  and disc  30 . Disc  30  is connected at disc electrical connection  36  to a common conductor as shown. Stray electrical paths from slider  12  to the common conductor are reduced or eliminated so that slider  12  (or a portion of it) can be charged relative to disc  30 . Slider  12  acts as a first capacitor plate, and the portion of disc  30  that faces slider  12  acts as a second capacitor plate. The two capacitor plates are separated by air gap  37  (which includes a lubricant film and a diamond-like carbon (DLC) overcoat between the plates in an exemplary embodiment), and a current flows across air gap  37  between the plates when a voltage is applied to slider electrical connection  34  and to disc electrical connection  36 . 
     In one embodiment, first electrode  38  is formed on at least a portion of air-bearing surface  39  of slider  12 , and second electrode  40  is formed on a portion of disc  30  that faces first electrode  38 . Second electrode  40  has a shape that is defined by the facing shape of first electrode  38  that overlies it. Control signal V is generated by sensing circuitry  32  and is coupled by electrical conductors to first electrode  38  along line  42 . First electrode  38  can be a metallization that is insulated from the main body of slider  12  by insulating layer  44 . Alternatively, slider  12  itself can be connected to sensing circuitry along line  46  and entire slider  12  can serve as the first electrode. The voltage difference V between the first and second electrodes causes a current to flow therebetween, which will change when slider  12  contacts disc  30  to enable detection of slider-disc contact events. 
       FIG. 3  is a graph illustrating the variation of flying height between a slider and a disc as a function of applied voltage therebetween. Applying an electrostatic voltage between a slider and disc will cause a Coulomb force of attraction between them, governed by the following equation: 
                 F   c     ⁢           ⁢     (   h   )       =     0.5   ⁢     E   2     ⁢           ⁢       δ   ⁢           ⁢   C   ⁢           ⁢     (   h   )         δ   ⁢           ⁢   h                 (     Eq   .           ⁢   1     )               
where F c  is the Coulomb force, E is the applied electrostatic voltage, C is the slider-disc capacitance, and h is the flying height between the slider and the disc. Thus, for the in-situ sensing technique of the present invention to be effective, the voltage applied between the slider and the disc must not be so large that the flying height is significantly affected. The graph of  FIG. 3  shows that for an exemplary configuration, an electrostatic voltage of up to 500 milli-Volts (mV) results in a minimal flying height variation. Specifically, the flying height variations for given value of applied electrostatic voltage shown in  FIG. 3  are as follows:
 
                                           TABLE 1                   Flying height (nano-   Change in flying height       Applied voltage (Volts)   meters (nm))   from nominal (nm)                                0.0   13.3   0.0       0.5   13.2   −0.1       1.0   13.0   −0.3       1.5   12.5   −0.8       2.0   11.8   −1.5       2.5   10.8   −2.5       3.0   9.4   −3.9                    
These results indicate that higher applied voltages result in greater flying height variations. The level of flying height variation that is tolerable will depend on the specific application in which the in-situ head-disc contact detection system of the present invention is employed.
 
       FIGS. 4A and 4B  are graphs illustrating the results of testing performed in an exemplary slider support configuration to compare the response of the in-situ slider-disc contact detection system of the present invention and an AE sensor according to the prior art.  FIG. 4A  shows the results of a test performed with an electrostatic voltage of 18 mV applied between the slider and the disc, where the onset of continuous slider-disc contact occurs at the time denoted by reference numeral  50 .  FIG. 4B  shows the results of a test performed with an electrostatic voltage of 500 mV applied between the slider and the disc, where the onset of continuous slider-disc contact again occurs at the time denoted by reference numeral  50 . The test results shown in  FIGS. 4A and 4B  illustrate that virtually equal sensitivity to the onset of continuous slider-disc contact exists for all applied electrostatic voltages. Although the magnitude of the change in current flowing across the slider-disc interface varies slightly, the time at which an appreciable change in current flow occurs is consistent, and can be detected to indicate slider-disc contact. Thus, applied electrostatic voltage does not have a significant effect on the sensitivity of the in-situ slider-disc contact detection system of the present invention with respect to continuous contact events. 
       FIG. 5  is a graph illustrating the results of testing performed in an exemplary slider support configuration, having various touchdown RPM and various electrostatic voltages applied between the slider and disc according to the in-situ slider-disc detection system of the present invention. These test results indicate that while continuous contact between the slider and disc (occurring at 1000 RPM) is detectable at all applied voltages, “ultra-light” intermittent contact events (and even “near-proximity” events in some cases) prior to the onset of continuous slider-disc contact (represented by RPM of up to 2000) are only detectable for higher applied voltages, such as 500 mV in the testing whose results are shown in  FIG. 5 . 
       FIG. 6  is a graph illustrating the results of testing performed in an exemplary slider support configuration to compare the response of the in-situ slider- disc contact detection system of the present invention and an AE sensor according to the prior art, in varying slider-disc contact scenarios. The current sensed by the in-situ system of the present invention (with an applied electrostatic voltage of 500 mV) is shown in the top portion of the graph, and the voltage output from an AE sensor is shown in the bottom graph. The test involves a regular flying (non- contact) scenario during time  60 , “near-proximity” and/or “ultra-light” contact between the slider and disc during time  62 , and continuous contact between the slider and disc during time  64 . The test was terminated by unloading the slider at time  66 . This test shows that while both the in-situ sensing system of the present invention and the prior art AE sensor system are able to detect continuous contact between the slider and the disc (occurring during time  64 ), only the in-situ sensing system of the present invention has enough sensitivity to detect ultra-light contact between the slider and the disc (occurring during time  62 ). 
     The present invention provides an in-situ sensing system for detecting contact between a slider and a rotating disc. The system is able to detect contact events by applying an electrostatic voltage between the slider and the disc, and sensing the current flowing across the head-disc interface (which will change as the slider contacts the disc). The voltage applied between the slider and the disc is selected so that the Coulomb force of attraction therebetween does not significantly affect the flying height of the slider. This sensing system has excellent sensitivity to light contact and “near-contact” or “near-proximity” phenomena between the slider and the disc, since a change in current flow across the slider-disc interface can occur when slider-disc spacing becomes sufficiently small without continuous or violent contact. The sensing system of the present invention therefore has greater sensitivity that prior AE sensors, and does not suffer from many of the limitations of such sensors. The sensing system of the present invention also has numerous advantages over “readback” type sensing systems, particularly because it does not require the slider to have the read/write transducing elements completed in order to assess the mechanical “flyability” and durability of components and disc drives. 
     The sensing system of the present invention may be used in a number of disc drive-related applications. It may be employed in a spinstand tester for assessing component-level flyability and durability. It might also be used for drive-level reliability assessment of disc drives, both in their early mechanical phases and in fully functional drives. Screening of head gimbal assemblies (HGAs) in pre-production phases as well as production phases is possible with the present invention, whether the HGA employs a conventional metal gimbal or a “flex” (polymer-based) gimbal. The system of the present invention may also be employed in disc drive systems which adaptively control the flying height of the slider, as a mechanism to ensure that the adjustment of flying height does not result in undesirable levels of slider-disc contact. Those skilled in the art will recognize that still further applications may exist for the system of the present invention due to its versatility and broad level of efficacy. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.