Patent Publication Number: US-2010118425-A1

Title: Disturbance rejection in a servo control loop using pressure-based disc mode sensor

Description:
BACKGROUND 
     The present invention generally relates to controlling transducer movement and, more particularly, to controlling transducer movement responsive to a position error signal within a servo control loop. 
     A typical data storage disc drive includes a plurality of magnetic recording discs which are mounted to a rotatable hub of a spindle motor and rotated at a high speed. An array of read/write heads is disposed adjacent to surfaces of the discs to transfer data between the discs and a host device. The heads can be radially positioned over the discs by a rotary actuator and a closed loop servo system. 
     The servo system can operate in two primary modes: seeking and track following. During a seek, a selected head is moved from an initial track to a target track on the corresponding disc surface. Upon reaching the target track, the servo system enters the track following mode wherein the head is maintained over the center of the target track while data is written/read. During track following, prerecorded servo information sensed by the head is demodulated to generate a position error signal (PES), which provides an indication of the position error of the head away from a desired location along the track (e.g., the track center). The PES is then converted into an actuator control signal, which is fed back to the actuator that positions the head. 
     As the areal density of magnetic disc drives increases, so does the need for more precise position control when track following, especially in the presence of external vibrations which can cause non-repeatable runout (NRRO) of the position error. Disc drives are being incorporated into increasingly diverse types of electronic devices having widely varying vibrational characteristics. For example, disc drives utilized in music and video playback/recording devices can be subjected to speaker induced vibration. Such speaker induced vibration can exceed the track following capabilities of the servo control loop and result in disruption of the music and video stream and associated skipping and/or stalling of the music and video playback/recording and/or failure of the device operation system. 
     SUMMARY 
     Vibration of a rotatable disc is sensed using a pressure sensor adjacent to and spaced apart from a surface of the rotatable disc. The pressure sensor generates a signal indicative of a pressure variation caused by vibration of the rotatable disc. The pressure sensor may include a polyvinylidene fluoride (PVDF) film, and a pair of electrodes on opposite sides of the PVDF film. 
     A servo control system that controls a position of a read/write head relative to a track on a rotatable disc includes a pressure sensor adjacent to and spaced apart from a surface of the rotatable disc that detects a pressure variation in air caused by vibration of the rotatable disc and generates a vibration sensing signal in response to the pressure variation. An adaptive feed-forward vibration compensation circuit is coupled to the servo control system and to the pressure sensor and generates a feed-forward control signal in response to the vibration sensing signal. The servo control system controls the position of the read/write head in response to the feed-forward control signal. 
     A method of controlling a position of a read/write head of a rotatable disc includes generating a pressure signal indicative of a pressure variation caused by vibration of the rotatable disc using a pressure sensor, and generating a control signal in response to the pressure signal. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of disc drive electronic circuits that include a servo controller that is configured in accordance with some embodiments. 
         FIG. 2  is a block diagram of a servo control loop configured in a track-following mode and which can be partially embodied within the servo controller of  FIG. 1 , in accordance with some embodiments. 
         FIG. 3  illustrates a pressure sensor according to some embodiments. 
         FIG. 4  illustrates a pressure sensor according to some embodiments mounted adjacent a surface of a rotatable disc. 
         FIG. 5  is a graph that illustrates the correlation of disc modes measured with a PVDF sensor and measured using a PES signal. 
         FIG. 6  is simplified diagrammatic representation of a disc drive according to some embodiments. 
         FIG. 7  illustrates a disc ramp assembly including a plurality of pressure sensors according to some embodiments. 
         FIG. 8  illustrates positioning of a disc ramp assembly including a plurality of pressure sensors according to some embodiments adjacent a disc stack in a disc drive. 
         FIG. 9  is a simplified diagram illustrating electrical connection of sensors according to some embodiments. 
         FIG. 10  is a schematic block diagram illustrating positioning of a sensor on an actuator arm according to further embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments of the present invention will now be described more fully hereinafter with reference to the accompanying drawings. However, this invention should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will convey the scope of the invention to those skilled in the alt. 
     It will be understood that, as used herein, the term “comprising” or “comprises” is open-ended, and includes one or more stated elements, steps and/or functions without precluding one or more unstated elements, steps and/or functions. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. The term “and/or” and “/” includes any and all combinations of one or more of the associated listed items. In the drawings, the size and relative sizes of regions may be exaggerated for clarity. Like numbers refer to like elements throughout. 
     Although various embodiments of the present invention are described in the context of disc drives for purposes of illustration and explanation only, the present invention is not limited thereto. It is to be understood that the present invention can be more broadly used for any type of servo control loop that can be subject to vibration. 
     The primary frequency components of NRRO are due to disturbances caused by disc modes. A disc mode is a normal vibration pattern for a data storage disc. A normal mode is a vibration pattern of a physical object that occurs at certain distinct frequencies, depending on the structure and composition of the object. When a disc is vibrating in a disc mode, all parts of the disc move sinusoidally at the same frequency. Costly measures have been used or proposed to reduce NRRO due to disc modes in high capacity drives. These include the use of thicker discs, separator plates between the discs, cover to spindle motor attachment, microactuators or plans to fill the drive with Helium gas. However, these methods may involve increased cost and/or complexity, and/or may lower drive reliability. 
     As described herein, a secondary sensing capability can be provided within a disc drive to facilitate compensation for disc mode disturbances with addition of feedforward control. Some embodiments provide a simple, easily implemented sensor that can be effectively used to sense disc modes to compensate for their effect. 
       FIG. 1  is a block diagram of disc drive electronic circuits  100  which include a data controller  102 , a servo controller  104 , and a read write channel  106 . Although two separate controllers  102  and  104  and a read write channel  106  have been shown for purposes of illustration and discussion, it is to be understood that their functionality described herein may be integrated within a common integrated circuit package or distributed among more than one integrated circuit package. A head disc assembly (HDA)  108  can include a plurality of data storage discs, a plurality of heads mounted to respective arms and which are moved radially across different data storage surfaces of the discs by a head actuator (e.g., voice coil motor), and a spindle motor which rotates the discs. 
     Write commands and associated data from a host device can be buffered by the data controller  102 . The host device can include, but is not limited to, a desktop computer, a laptop computer, a personal digital assistant (PDA), a digital video recorder/player, a digital music recorder/player, and/or another electronic device that can be communicatively coupled to store and retrieve data in the HDA  108 . The data controller  102  carries out buffered write commands by formatting the associated data into blocks with the appropriate header information, and transfers the formatted data via the read/write channel  106  to logical block addresses (LBAs) on a disc in the HDA  108  identified by the associated write command. 
     The read write channel  106  can convert data between the digital signals processed by the data controller  102  and the analog signals conducted through the heads in the HDA  108 . The read write channel  106  provides servo data read from the HDA  108  to the servo controller  104 . The servo data can be used to detect the location of the head in relation to LBAs on the disc. The servo controller  104  can use LBAs from the data controller  102  and the servo data to seek the head to all addressed track and block on the disc (i.e., seek mode), and to maintain the head aligned with the track while data is written/read on the disc (i.e., track following mode). 
     Some embodiments of the servo controller  104  provide an adaptive feed-forward control scheme that utilizes a pressure-based sensor to improve the capability of the servo control loop to reject external disturbances while operating in the track settling mode and the track-following mode and subjected to vibration. An adaptive filter generates filter coefficients to filter the vibration signal and generate a feed-forward signal that controls a head actuator to counteract disturbances to head position caused by the vibration. The filter coefficients are tuned in response to the vibration signal and a PES, which is indicative of head position error, to reduce the PES. 
     The filter coefficients may be tuned using a modified filtered-X Least Mean Square (LMS) algorithm. The servo controller  104  attempts to adapt the modified filtered-X LMS algorithm to match the unknown disturbance dynamic effects on the servo control loop, and so that the filter coefficients are thereby tuned to cause the feed-forward signal to cancel the deleterious effects of the external disturbances on head position. Accordingly, this may result in a significant reduction of the non-repeatable runout induced by rotational vibration. An exemplary background servo control loop using a filtered-X LMS algorithm is described in U.S. Pat. No. 6,580,579, the entire disclosure of which is incorporated herein by reference as if set forth in its entirety. 
     Although some embodiments herein will be discussed with respect to a single-input, single-output (SISO) discrete time stochastic system. It will be appreciated that the invention is also applicable to other systems. Moreover, although some embodiments are discussed in the context of the discrete time domain (i.e., digital circuitry), using a sampling time index, k, it will further be appreciated that other embodiments of the invention can be embodied in the continuous time domain (i.e., analog and/or hybrid circuitry). 
       FIG. 2  is a further block diagram of a servo control loop  200  configured in a track settling and track-following mode and which can be partially embodied within the servo controller  104  of  FIG. 1  in accordance with some embodiments. Referring to  FIG. 2 , the HDA  108  can be modeled in the servo control loop  200  as a plant (P)  203  including a digital-to-analog converter (DAC) and power amplifier  202 , a head actuator motor (e.g., voice coil motor)  204 , an actuator  206 , and an actuator arm  208 . The position y m    210  of a read/write head relative to a given track on a disc is sensed (e.g., from servo data on the disc) and compared to a reference position  212  (desired position, r) of the head to generate a position error signal (PES)  214 . The PES  214  is therefore indicative of the difference between the actual and desired positions of the head (i.e., head position error), and is provided to a servo control module  216 . The servo control module (K)  216  responds to the value of PES  214  to generate a servo control signal (U)  218 . 
     The servo control signal  218  is combined with a feed-forward signal (U FF )  220  at a summing node  222  to generate a combined control signal  221 . The combined control signal  221  can be converted by the DAC/power amplifier  202  into an analog signal, and then amplified and provided to the head actuator motor  204 . The head actuator motor  204  is connected to the actuator  206  which moves the actuator arm  208  in response to the amplified control signal supplied to the head actuator motor  204 . The read/write head is connected to the actuator arm  208  (e.g., to an end of the actuator arm  208 ). In this way, servo control module  216  controls the positioning of the read/write bead relative to a selected track on the disc surface during reading/writing of data along the selected track. 
     As shown in  FIG. 2 , the disc mode  230  (Wτ) imparts a disturbance component D 1  to the head through coupling dynamics  234  (G) which are typically unknown to the servo controller  216 . 
     To enable the servo control loop  200  to sense and compensate for the effects of the disturbance  230  (WT), a sensor  300  is configured to generate a signal that is indicative of the disc motion due to the disc and disc pack modes. The low level signal from the sensor  300  may be amplified by an optional charge amplifier  301  to generate a signal  240 . The sensor  300  may include a pressure sensor as described in more detail below. The sensor  300  produces an output proportional to pressure variation in air adjacent to the surface of the disc due to gross motion or modes of vibration of the disc. Accordingly, the signal  240  is indicative of the motion of the disc to be correlated with the disturbance D 1  imparted to the head. 
     An adaptive disc mode sensing module  201  is configured to respond to the signal  240  by generating the feed-forward signal  220  (U FF ) to counteract the disturbance D 1  to head position. The adaptive disc mode sensing module  201  can include a Finite Impulse Response (FIR) filter  244  (F), and an adaptation module  250 . 
     The signal  240  is filtered by the adaptive Finite Impulse Response (FIR) filter  244  (F) to generate the feed-forward signal  220  (U FF ). The FIR filter  244  can be configured as a tapped delay line having a plurality of coefficient weights that are applied to respective ones of a plurality of time-delayed taps filtering the sensed signal  240 . The adaptation module  250  tunes the FIR coefficient weights (in the FIR filter  244 ) in response to the signal  240  and error signal or PES  214 . In some embodiments, the adaptation module  250  may use a modified filtered-X LMS algorithm for this timing process. Regardless, the tuning process produces a matching transfer function to estimate the unknown coupling  234  between position of the head and disc motion due to disc modes. 
     The adaptation module  250  tunes the coefficient weights (“FIR Coefficients”) used by the FIR filter  244  in response to the output vibration signal  240  and the PES  214 . The adaptation module  250  may tune the FIR coefficients according to the following equation: 
         W ( n+ 1)= W ( n )+μ* x ( n )*PES( n )  (1) 
     In Eq. 1, W(n+1) represent the next set of coefficients for the adaptive FIR filter  244 , and ti is the constant determining the rate of convergence and the accuracy of the adaptation process. 
     Accordingly, the adaptation module  250  tunes the coefficient weights of the FIR filter  244  in response to the PES  214  and the vibration signal  240  to attempt to match the unknown couplings affecting the servo control loop, and to thereby cause the feed-forward signal  220  (U FF ) to cancel the deleterious effects of the disturbance on head positioning. 
     Some embodiments provide a pressure sensor  300  that uses polyvinylidene fluoride (PVDF) as the transducing material. PVDF is a polymeric material with high piezo- and pyro-electric properties. PVDF films provide a set of attractive properties for the development of simple, reliable disc mode sensor. These properties include fast response time, self-inducing charge (no need for external power), low device cost and simple design. 
     In some embodiments, the sensing element includes a thin segment of PVDF film  302  with two sputtered electrodes  304   a ,  304   b  on opposite surfaces thereof. The film  302  may have a thickness of about 28 micrometers (μm). As shown in  FIG. 3 , the film  302  may be shaped as a small rectangular patch (e.g., having dimensions of about 2 mm by 3 mm). The film  302  and electrodes  304   a,b  are attached to a rigid structure, such as a housing  306 , for mounting above the surface of the disc at a safe distance (e.g. about 400 μm or more) from the disc surface to reduce contact with the disc during shock. For example, the sensor  300  may be spaced far enough apart from the disc that a shock to the disc of less than 300G will not cause the disc to contact the sensor  300 . 
     The film  302  and electrodes  304   a,b  may be mounted directly on the housing  306  so that there is no air between the film  302  and the housing  306 . Thus, film may not deflects like a typical diaphragm in a gas pressure sensor. Transduction occurs when the pressure waves impinge on film  302 , generating stress in the film  302 . The film  302  responsively produces a charge that is proportional to the stress. This charge is sensed as an electric field across the electrodes  304   a,b . Due to the fast dissipation of charge, the film  302  has a low frequency response limit, which prevents/reduces it from acting as a DC sensor. However, the film  302  has a very fast response, which makes it suitable as an AC device. 
       FIG. 4  illustrates a pressure sensor  300  according to some embodiments mounted adjacent a surface of a rotatable disc  320 . As shown therein, a PVDF sensor  300  is mounted on a housing  306  adjacent a data storage surface of a rotatable disc  320  that rotates about an axis of rotation  324 . The housing  306  can be mounted on a disc housing  310  that supports the rotatable disc  320 . 
     Referring to  FIGS. 3 and 4 , the housing  306  may be formed of a lightweight material capable of supporting the film  302  and electrodes  304   a,b . In some embodiments, the housing  306  may include aluminum, ceramic, and/or plastic. In some embodiments, the material of the housing  306  may be chosen to limit or reduce reflection of electromagnetic energy to/from the film  302 . 
     The pressure sensor  300  detects vibration  330  of the rotatable disc  320  in a direction normal to a plane defined by the surface of the rotatable disc  320  in response to pressure variation of a gaseous atmosphere surrounding the rotatable disc  320  caused by the disc vibration. 
     The use of a pressure sensor  300  to detect disc modes may provide significant benefits relative to the use of other types of sensors, such as capacitive sensors. For example, a capacitive sensor may have to be positioned relatively closely to the disc surface (e.g. 50 μm or so) in order to be useful for disc mode detection. At such a distance, undesirable contact may occur between the sensor and the disc surface even at relatively low shocks. Furthermore, capacitive sensors require external power and may require complicated circuitry to detect changes in capacitance due to disc mode vibration. In contrast, the sensor  300  can be positioned a safe distance from the disc surface to reduce the possibility of contact with the disc surface. Furthermore, because the sensor  300  generates a voltage directly in response to pressure variation adjacent the disc  320 , an external power source will not be needed, and only a charge amplifier  301  may be needed to generate a voltage signal that can be used to generate the feed-forward control signal  220 . 
     Sensing transduction is based on the stress, due to air pressure, caused by disc vibration, on the film  302  attached to the structure  306 . The spatially integrated charge induced within the film  302  is sensed as voltage between the top and bottom electrodes  304   a ,  304   b . The induced field (E) across the electrodes equals the product of stress (ρ) and the largest PVDF strain constant (g 33 ) as follows: 
         E=ρg   33 ( V/m ) 
     Accordingly, a PVDF film  302  including electrodes  304   a ,  304   b  is mounted on a sensor housing  306 . The housing  306  is positioned such that the surface of the film  302  is at a safe distance from the surface of the disc. 
     A PVDF film according to some embodiments may have dimensions of length (l)=about 1 mm to about 3 mm, width (w)=about 0.5 mm to about 2 mm and thickness=about 10 μm to about 50 μm. A PVDF film according to some embodiments may have dimensions of length (l)=3 mm, width (w)=2 mm and thickness=28 μm. The PVDF material has three dimensional strain constants as follows: 
         g   31 =0.216 V/m/N/m 2    
         g   32 =19 V/m/N/m 2    
         g   33 =−339 V/m/N/m 2    
     Accordingly, the PVDF film  302  in the sensor  300  may be oriented to take advantage of the high g 33  strain constant. 
     Other methods of implementation may include installation of the film on a housing that extends over each surface of the each disc in a multi-disc drive. Alternatively, a single sensor  300  may be used for each disc  320 . Accordingly, a disc drive according to some embodiments may include one pressure sensor  300  per disc surface and/or one pressure sensor  300  per disc. At most, two conductors are needed to receive the output of the sensor  300 . A single conductor may be used for each sensor  300  when using a common ground attached to one electrode of each sensor  300 . 
       FIG. 5  is a graph that illustrates the correlation of observed disc modes measured with a PVDF sensor  300  and measured using the PES signal for a high capacity disc drive. In particular, a PVDF sensor was positioned adjacent a disc in a disc drive having the following modes (in Hz): 806.3, 1275.0, 2150.0, 2537.5, 3362.5, and 4318.8.  FIG. 5(A)  is a graph of sensor output and PES versus frequency.  FIG. 5(B)  is a graph illustrating coherence between the sensor output and the PES, and  FIG. 5(C)  illustrates the phase relationship between the sensor output and the PES. As shown in  FIG. 5  and Table 1, the output of the PVDF sensor shows high correlation to PES for the disc modes. The strong correlation of the observed disc modes measured with the PVDF sensor and PES demonstrates the ability of the sensor  300  to accurately identify disc modes. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 PES/Sensor Coherence 
               
            
           
           
               
               
               
            
               
                   
                 f(Hz) 
                 PES/Sensor Coherence 
               
               
                   
                   
               
               
                   
                  806.3 
                 0.43 
               
               
                   
                 1275.0 
                 0.85 
               
               
                   
                 2150.0 
                 0.89 
               
               
                   
                 2537.5 
                 0.83 
               
               
                   
                 3362.5 
                 0.90 
               
               
                   
                 4318.8 
                 0.83 
               
               
                   
                   
               
            
           
         
       
     
     Furthermore, a pressure sensor according to some embodiments may have the sensing capability to identify different types of time invariant or impulsive disturbance events inside a disc drive. Examples of such disturbances include motor pure tones (RRO disturbance), ramp contact detect, latch opening, coil popping, shock, external vibration and excitation due to sounds/music, etc. That is, both the output of the sensor  300  and the PES include many matching frequency components not due to disc modes. This indicates that the sensor  300  can be used to identify many types of steady state disturbances or impulsive events inside a disc drive. 
     Some embodiments provide a disc mode sensor for a disc drive that includes a PVDF film as a sensing element. A PVDF-based disc mode sensor according to some embodiments can have a relatively simple design that is inexpensive to manufacture and incorporate within a disc drive housing. Furthermore, the circuitry required to implement an adaptive feed-forward control system may be simplified, because a PVDF-based disc mode sensor may not require an external power source and may generate an output voltage signal directly in response to pressure variation adjacent a disc surface. 
     A simplified diagrammatic representation of a disc drive, generally designated as  10 , is illustrated in  FIG. 6 . The disc drive  10  includes a disc stack  12  (illustrated as a single disc in  FIG. 6 ) that is rotated about a hub  14  by a spindle motor mounted to a base plate  16 . The disc drive includes a housing  44  that surrounds and protects the disc stack  12  and associated hardware and electronics of the disc drive  10 . 
     The disc stack  12  includes a plurality of discs. An actuator arm assembly  18  is also mounted to the base plate  16 . The disc drive  100  is configured to store and retrieve data responsive to write and read commands from a host device. A host device can include, but is not limited to, a desktop computer, a laptop computer, a personal digital assistant (PDA), a digital video recorder/player, a digital music recorder/player, and/or another electronic device that can be communicatively coupled to store and/or retrieve data in the disc drive  100 . 
     The actuator arm assembly  18  includes one or more read/write heads (or transducers)  20  mounted to a flexure arm  22  which is attached to an actuator arm  24  that can rotate about a pivot bearing assembly  26 . The heads  20  may, for example, include a magnetoresistive (MR) element and/or a thin film inductive (TFI) element. The actuator arm assembly  18  also includes a voice coil motor (VCM)  28  which radially moves the heads  20  across the disc stack  12 . The spindle motor  15  and actuator arm assembly  18  are coupled to a controller, read/write channel circuits, and other associated electronic circuits  30  which can be enclosed within one or more integrated circuit packages mounted to a printed circuit board (PCB)  32 . The controller, read/write channel circuits, and other associated electronic circuits  30  are referred to below as a “controller” for brevity. The controller  30  may include analog circuitry and/or digital circuitry, such as a gate array and/or microprocessor-based instruction processing device. 
       FIG. 7  illustrates a disc ramp assembly  50  for use in a 4-disc/8-head disc drive including a plurality of pressure sensors according to some embodiments, while  FIG. 8  illustrates positioning of a disc ramp assembly including a plurality of pressure sensors according to some embodiments adjacent a disc stack in a disc drive. 
     Disc ramp assemblies are commonly used in disc drives to provide a location to receive and secure, or park, the transducers when the disc is not in use. Referring to  FIGS. 7 and 8 , the disc ramp assembly  50  includes eight ramps provided in respective ramp pairs  60 A,  60 B so as to provide a ramp on either side of each disc (i.e., one ramp per head  20 ). In some embodiments, the ramps may be positioned on opposing sides of an opening  62  that is positioned over an edge of a disc  12 A- 12 D. As shown in  FIG. 7 , a plurality of sensors  300 A- 300 D are positioned on inner surfaces of the openings  62 , so that each sensor  300 A- 300 D is positioned adjacent a surface of a respective disc  12 A- 12 D. In some embodiments, one sensor  300 A- 300 D may be provided per disc while in other embodiments, one sensor  300 A- 300 D may be provided per disc surface (i.e., two sensors per disc  12 A- 12 D). For example, a second sensor  300 A′ may be mounted within the opening  62  adjacent the ramp  60 A and across the opening  62  from the sensor  300 A. 
     Vibration of the discs  12 A- 12 D causes pressure variation in the gas (e.g., air) adjacent the discs  12 A- 12 D, which pressure variation is sensed by the sensors  300 A- 300 D. It will be appreciated that although disc drives generally include air within the drive housing  44 , other gases could be provided within the disc drive housing  44 . 
     The high sensitivity of the PVDF film in the sensors  300 A- 300 D facilitates a response due to extremely small pressure fluctuations. This in turn allows measurement of disc modes, as perturbation in the air pressure, at relatively large distances away from the discs  12 A- 12 D. This attribute is attractive since it allows installation of the sensor  300 A- 300 D at a relatively large distance (e.g. 400 μm) away from the discs  12 A- 12 D to prevent contact between the discs  12 A- 12 D and the sensors  300 A- 300 D during a shock event. The large distance that the sensors  300 A- 300 D can be mounted from the discs  12 A- 12 D is important for mass production and installation of the sensor  300 A- 300 D in single or multi-platter disc drives. With a larger allowed gap, manufacturing tolerances for positioning the sensor  300 A- 300 D at the edge of the discs  12 A- 12 D will be relaxed, thereby reducing the cost for fabrication and installation of the sensor housing. 
     The PVDF film of the sensors  300 A- 300 D may be pre-assembled in/on a sensor housing designed to be installed in single- or a multi-platter platter drive. The housing of the sensor  300 A- 300 D may resemble the structure of disc head ramps, such as are typically used in disc drives. In some embodiments, the structure of the ramp can also serve as the housing for the sensor. For example,  FIG. 7  shows a possible location for installation of the PVDF film sensors  300 A- 300 D oil a ramp  50  used in a 4-disc/8-head drive. Such installation would be possible due to the small area of the PVDF film needed. This approach will allow precise positioning of the sensors  300 A- 300 D above the edge of the discs  12 A- 12 D using a part that is already in use in disc drives. 
     Installation of the sensor in/on the ramp assembly  30  may allow precise positioning of the sensors  300 A- 300 D at a desired distance from the surface of each disc  12 A- 12 D using an existing part. 
     The ramp  50  part may be modified to 1) optimize the area of the film for the sensor  300 A- 300 D, 2) provide a conduit for electrical traces from each film, and 3) include the charge amplifier and an optional switching circuit to rout data from a single sensor at one time. 
       FIG. 9  is a simplified diagram illustrating electrical connection of sensors  300 A- 300 D according to some embodiments. As shown therein, each sensor  300 A- 300 D is connected through a switch  70  to a charge amplifier  301 . Each sensor  300 A- 30 D may be coupled to the switch  70  by respective signal lines  71 A-D that include at most two traces (one trace if a common electrical ground can be established). The switch  70  may, for example, sequentially connect the sensors  300 A- 300 D to the charge amplifier  301  via analog time division multiplexing. A single charge amplifier  301  may be used, since the output of only one sensor  300 A- 300 D will be used for the matching head under track follow or settle mode control. The switch  70  and the charge amplifier  301  may be positioned next to or on the sensor housing (such as on the ramp assembly  50 ) to reduce noise and/or improve signal to noise ratio (SNR). This may reduce component cost while reducing the number of electrical traces that extend from the sensor assembly to two. 
     The output of the sensors  300 A- 300 D will have some variation with temperature. In particular, there will be some reduction in the output of the PVDF film at higher temperatures. Within the disc drive, the reduction in the output of the sensor can be accounted for using an adjustable gain that can be modified by the drive electronics  30  based on the sensed temperature of the drive. Accordingly, an adjustable gain amplifier  302  can be provided between the charge amplifier and the adaptive disc mode sensing amplifier  201 . It will be appreciated that although the adjustable gain amplifier  302  is illustrated as a separate block, the adjustable gain amplifier  302  could be implemented within software in the servo controller  104 . 
     Referring to  FIG. 10 , in other embodiments, a sensor  300  may be positioned at the distal end (tip) of the flexure arm  22 , with the film of the sensor  300  facing the surface of the disc  12 . This approach provides the ability to co-locate the sensor  300  with the head  20 , as well as the ability to position the sensor  300  at different disc radii. In these embodiments, the sensor electrical conduit may be added to the existing head trace assembly. 
     The output of the sensor(s)  300 A- 300 D may be used within a closed feedback control loop using the filtered-x LMS algorithm. The LMS algorithm minimizes the error (PES) based on the sensed data representing the amplitude of the disc modes or other disturbances. Such error minimization (LMS; i.e. minimization of the least mean square of the error) will not require very precise calibrated sensor data but acceptable SNR to allow correlation between the frequency content of PES and the sensor data. 
     In the drawings and specification, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.