Apparatus and method for removing mechanical resonance using adaptive notch filter

[0001] This application claims benefit from Korean Patent Application No. 2003-6498, filed Feb. 3, 2003, the disclosure of which is incorporated herein by reference.

[0002] 1. Field of the Invention

[0003] The present invention relates to an apparatus and method removing mechanical resonance of a system, and more particularly, to an apparatus and method removing mechanical resonance in a disc drive using an adaptive notch filter and varying its frequency characteristics of the filter based on the mechanical resonance frequency existing in the system.

[0004] 2. Description of the Related Art

[0005] Generally, a hard disc drive (HDD) that is a data storage system includes a head disc assembly (HDA) with mechanical components and electronic circuits. Harmful resonance generated in a head stack assembly (HSA) in the HAD appears directly as a position error signal (PES) degrading stability of the servo tracking control of the HDD.

[0006] In conventional attempts to solve this problem, the resonance frequency having a harmful influence on the HDD is determined from the frequency of the PES and removed by being filtered or screened. The filters used to remove the determined resonance frequency are designed in an initial development stage based on resonance frequency components identified in the PES of a predetermined number of HDDs. The designed filters are applied commonly to all HDDs.

[0007] However, the resonance frequency varies according to the individual characteristics of components in an HSA or the assembly. This variance of the resonance frequency among HSAs having different characteristics results in a problem that cannot be solved by notch filters based on fixed filter coefficients commonly applied to all HDDs.

[0008] A conventional technology developed to attempt to solve this problem uses two notch filters 121 and 122, having different frequency characteristics, to remove a resonance frequency, as shown in FIG. 1.

[0009] A servo controller 110 uses a PES that is the output signal of a voice coil motor (VCM) actuator 140 to generate a control signal moving the position of a transducer to the center of a target track. The notch filter removes a resonance frequency component in a control signal that may stimulate the VCM actuator 140. Since this resonance frequency varies according to individual characteristics of a transducer, the notch filters 121 and 122 are designed to have different central frequencies to remove major resonant points previously designated through experiments using similar transducers. As a result, a multiplexer 130 selects one of the notch filters 121 and 122, according to an operation of the transducer, to attempt to reduce mechanical resonance.

[0010] In the conventional apparatus using two notch filters, each notch filter is also designed to have having characteristics based on a fixed frequency. However, the resonance frequency of an actuator is actually different for each unit. Even in a single unit, resonance varies with respect to a surrounding temperature. Accordingly, due to the fixed central frequency of the notch filters, the related art cannot prevent resonance when the resonance frequency varies according to an operating condition.

[0011] According to an aspect of the present invention, an apparatus and method for removing mechanical resonance is provided using an adaptive notch filter and detecting a resonance frequency whenever a system is used and coefficients of the notch filter are automatically varied, to remove the detected resonance frequency.

[0012] According to an aspect of the present invention, a mechanical resonance removing apparatus is provided using an adaptive notch filter removing a resonance frequency of a system. The apparatus includes an excitation signal generator, which in a predetermined notch filter adjusting mode generates an excitation signal exciting the system and provides the signal to the system. A resonance frequency estimator detects a resonance frequency component from a responding signal from the system corresponding to the excitation signal. The apparatus also includes a notch filter coefficient generator determining coefficients of a notch filter corresponding to the resonance frequency component detected by the resonance frequency estimator, and the notch filter applying the coefficients determined by the notch filter coefficient generator, to remove the resonance frequency of the system.

[0013] According to another aspect of the present invention, a mechanical resonance removing method is provided using an adaptive notch filter to remove a resonance frequency of a system. The method includes providing a predetermined excitation signal to a system to excite the system and estimating a resonance frequency from a responding signal from the system while in a predetermined notch filter adjusting mode. The method further includes determining coefficients of the notch filter corresponding to the estimated resonance frequency, and applying the determined notch filter coefficients to the notch filter, to remove the resonance frequency of the system.

[0014] According to still another aspect of the present invention, a mechanical resonance removing apparatus of a disc drive using an adaptive notch filter to remove a resonance frequency of the disc drive is provided. The apparatus includes a voice coil motor actuator moving a transducer according to a VCM control signal and generating a servo control signal, a switch having the control signal input to an input terminal of the switch, and the output terminal of the switch connected to a resonance frequency estimator. The switching is controlled so that the input terminal and output terminal are connected during a notch filter adjusting mode interval. The apparatus further includes an excitation signal generator generating an excitation signal to excite the disc drive only during the notch filter adjusting mode interval, a mixer mixing the excitation signal with the control signal, and a servo controller using the signal output from the mixer and generating a VCM control signal controlling the voice coil motor. A resonance frequency estimator detects a resonance frequency component from the switch's output signal output from the switch, and a notch filter coefficient generator determines coefficients of a notch filter so that the coefficients have a frequency characteristic used in removing the resonance frequency component detected in the resonance frequency estimator. The apparatus further includes the notch filter. By applying the coefficients determined by the notch filter coefficient generator, the notch filter filters and outputs the servo control signal. The apparatus further includes a multiplexer having a first input terminal connected to the VCM control signal output from the servo controller and a second input terminal connected to the VCM control signal filtered and output from the notch filter. The signal input through the first input terminal is output only during the adjusting mode interval, and the signal input through the second input terminal is selected and output during other intervals.

[0015] Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.

[0026] Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below to explain the present invention by referring to the figures.

[0027] FIG. 2 shows an HDD to which an aspect of the present invention is applied. The drive 10 includes at least one magnetic disc 12 rotatable by a spindle motor 14. The drive also includes a transducer (not shown) positioned close to the surface of a disc.

[0028] The transducer senses and magnetizes a magnetic field of each disc 12 such that information is read from or written to the rotating disc 12. The transducer is coupled with the surface of each disc. The transducer includes a writing transducer magnetizing the disc 12 and a reading transducer sensing the magnetic field of the disc 12. The reading transducer is implemented using magneto-resistive (MR) devices.

[0029] The transducer may be integrated into a head 16. The head has a generating air bearing between the transducer and the disc surface and is coupled to a head stack assembly (HSA) 22. The HSA 22 is attached to an actuator arm 24 having a voice coil 26. The voice coil 26 is disposed close to a magnetic assembly 28 defining a voice coil motor (VCM) 30. The current supplied to the voice coil 26 generates torque rotating the actuator arm 24 about a bearing assembly 32. The rotation of the actuator arm 24 moves the transducer across the surface of the disc.

[0030] Information is stored in annular tracks on the disc 12. Generally, each track 34 contains a plurality of sectors. Each sector includes a data field and an identification field. The identification field contains a gray code identifying the sector and the track (cylinder). The transducer moves across the disc surface reading information from or write information on another track.

[0031] As shown in FIG. 3, a mechanical resonance removing apparatus using an adaptive notch filter according to an aspect of the present invention includes an excitation signal generator 310, a mixer 320, a servo controller 330, a VCM actuator 340, a band pass filter 350, a resonance frequency estimator 360, a notch filter coefficient generator 370, and a notch filter 380.

[0032] The notch filter 380 removes a resonance frequency component in a control signal and is operable as a second-order infinite impulse response (IIR) filter corresponding to the following equation (1): $\begin{array}{cc}{H}_n\left(z\right)=\frac{{\alpha }_0{z}^2+{\alpha }_{1}z+{\alpha }_2}{z^2+{\beta }_{1}z+{\beta }_2}& \left(1\right)\end{array}$

[0033] Filter coefficients α₀, α₁, α₂, β₁, and β₂ are defined according to equation (2): $\begin{array}{cc}{\alpha }_0=\frac{1+2{\mathrm{\xi \alpha \Omega }}_n+{\Omega }_n^2}{1+2{\mathrm{\xi \Omega }}_n+{\Omega }_n^2},{\alpha }_1=\frac{2\left({\Omega }_n^2-1\right)}{1+2{\mathrm{\xi \Omega }}_n+{\Omega }_n^2},{\alpha }_2=\frac{1-2{\mathrm{\xi \alpha \Omega }}_n+{\Omega }_n^2}{1+2{\mathrm{\xi \Omega }}_n+{\Omega }_n^2},{\beta }_1=\frac{2\left({\Omega }_n^2-1\right)}{1+2{\mathrm{\xi \Omega }}_n+{\Omega }_n^2},{\beta }_0=\frac{1-2{\mathrm{\xi \Omega }}_n+{\Omega }_n^2}{1+2{\mathrm{\xi \Omega }}_n+{\Omega }_n^2}& \left(2\right)\end{array}$

[0034] Parameters α and ξ, representing a damping coefficient, determine a gain and a notch width at a central frequency (f_c). Parameter Ω_n is determined by a central frequency and a sampling time (T_s) according to equation (3): $\begin{array}{cc}{\Omega }_n=\tan \frac{2\pi f_c{T}_s}{2}& \left(3\right)\end{array}$

[0035] FIG. 5 shows the frequency gain characteristic of an IIR notch filter and that the notch width increases in proportion to the increase of parameter ξ value. FIG. 6 shows the frequency phase characteristic of an IIR notch filter and that the phase delay amount increases in proportion to the increase of parameter ξ value and therefore the phase margin of the control system decreases. Accordingly, to minimize a phase loss, it is necessary to use a notch filter having a low ξ value. However, when the notch width decreases and the resonance frequency varies, the resonance restriction is less effective.

[0036] An excitation signal x_ext(n) generated in the excitation signal generator 310 is expressed in the following equation (4):
X_ext(n)=sin(ω₁T_sn)+sin(ω₂T_sn)+sin(ω₃T_sn)+ . . . +sin(ω_NT_sn)  (4)

[0037] In equation (4), ω₁, ω₂, ω₃, ω_N denote resonance frequency candidates. Accordingly, the excitation signal of equation (4) corresponds to a signal obtained by synthesizing all resonance frequency components expected in the system. As an example, an excitation signal obtained by synthesizing frequency components at 8 kHz is shown in FIG. 7.

[0038] Thus, the excitation signal generated in the excitation signal generator 310 is mixed in the mixer 320 with a servo control signal generated in the VCM actuator 340 and provided to the servo controller 330. According to an aspect of the present invention, the servo control signal is a PES.

[0039] The servo controller 330 generates a VCM control signal controlling a voice coil motor by using the servo control signal, in which the excitation signal is synthesized, positioning the transducer at the center of a target track and providing the VCM control signal to the VCM actuator 340.

[0040] The VCM actuator 340 is driven by the current provided to the voice coil motor and vibrates by the excitation signal generated in the excitation signal generator 310. The excitation signal is reflected in an PES generated in the VCM actuator 340 and output. An example of the PES in which the excitation signal is reflected is shown in FIG. 8.

[0041] The band pass filter 350 is used to extract only the frequency band component of the excitation signal from this PES.

[0042] According to an aspect of the present invention, the band pass filter 350 is used as a second-order filter and is designed to have the frequency characteristic as in the following equation (5): $\begin{array}{cc}{H}_{\mathrm{bp}}\left(z\right)=\underset{\_}{K}\frac{z^2-1}{z^2-\left(2\gamma \cos {\omega }_c\right)z+{\gamma }^2}& \left(5\right)\end{array}$

[0043] Parameters ω_c, y, and K determine a central frequency, a pass bandwidth, and the gain at the central frequency, respectively.

[0044] The resonance frequency estimator 360 estimates a resonance frequency from the PES output from the band pass filter 350. The resonance frequency estimator 360 includes a finite impulse response (FIR) filter capable of varying coefficients and a filter coefficient adjusting apparatus. According to an aspect of the present invention, the resonance frequency estimator 360 is constructed with a coefficient variable FIR notch filter and the relation of the input and output is expressed as the following equation (6):
y(n)=x(n)−2λ(n)x(n−1)+x(n−2)  (6)

[0045] Here, x(n) denotes a PES signal and λ(n) is a variable coefficient.

[0046] The PES that is an input signal is formed as the sum of a variety of frequency signals having different magnitudes. The frequency having the largest magnitude corresponds to a resonance frequency to be found. To find this resonance frequency, the variable coefficient λ(n) is adjusted. In this case, the variable coefficient λ(n) is adjusted so that the average value of the output signal y(n) is minimized. The reason is that the central frequency of the notch filter varies according to the variable coefficient and when the central frequency is the same as a frequency at which the PES is the largest magnitude, that is, the resonance frequency, the magnitude of the output signal y(n) is minimized. When the frequency having the largest magnitude is attenuated, the magnitude of the output signal is minimized.

[0047] A variable coefficient adjusting expression, adjusting the average value of the output to be minimized, is obtained by applying the least mean square (LMS) theory according to the following equation (7):
λ(n+1)=λ(n)+2k_cy(n)x(n−1)  (7)

[0048] The constant k_c denotes an estimation gain.

[0049] The resonance frequency estimation expression of the equation (7) gives the product of the output signal y(n) and the previous input signal x(n−1) as continuously accumulated. If the coefficient λ(n) of the coefficient variable FIR notch filter is thus varied, as time goes by, the output signal converges on a value in which the magnitude of the signal is minimized, and the value of the variable coefficient converges on a predetermined value. The value on which the variable coefficient is converging relates to the frequency at which the input signal is the largest magnitude signal, that is, the resonance frequency. After a predetermined time, the variable coefficient λ(n) converges on cos(2πf_resTs).

[0050] The resonance frequency estimation performance of the resonance frequency estimator 360 is shown in FIG. 9.

[0051] Next, the notch filter coefficient generator 370 calculates notch filter coefficients by using equation (2) and the convergence value of the variable coefficients.

[0052] In equation (2), parameters α and ξ are predetermined values and Ω_n is calculated from the convergence value of the variable coefficients by the following equation (8): $\begin{array}{cc}{\Omega }_n^2={\left[\tan \left(\frac{2\pi f_{\mathrm{res}}{T}_s}{2}\right)\right]}^2=\frac{1-\cos \left(2\pi f_{\mathrm{res}}{T}_s\right)}{1+\cos \left(2\pi f_{\mathrm{res}}{T}_s\right)}=\frac{1-\lambda }{1+\lambda }& \left(8\right)\end{array}$

[0053] The notch filter 380 applies the calculated notch filter coefficients restricting the mechanical resonance of the system.

[0054] According to an aspect of the present invention, the mechanical resonance frequency of a system is estimated by an excitation signal and the coefficients of notch filters are varied appropriately to restrict the estimated resonance frequency.

[0055] This process for estimating a resonance frequency and adjusting notch filter coefficients is more effective if the process is carried out whenever a system is turned on.

[0056] FIG. 4 is a diagram of an electric circuit of a disc drive having a mechanical resonance removing apparatus using adaptive notch filters according to an aspect of the present invention.

[0057] An excitation signal generator 405, a mixer 406, a servo controller 401, a VCM actuator 404, a band pass filter 408, a resonance frequency estimator 409, a notch filter coefficient generator 410, and a notch filter 402 shown in FIG. 4 are similar to the circuit structure shown in FIG. 3.

[0058] As shown in FIG. 4, a multiplexer 403 and a switch 407 are added so as to adjust the coefficients of the notch filter only in a notch filter adjusting mode.

[0059] According to an aspect of the present invention, the notch filter adjusting mode adjusting coefficients of the notch filter is more efficient when it is designed to be executed at a time of transition when the power of a disc drive is turned on.

[0060] If the disc drive is in the notch filter adjusting mode, the system controller (not shown) of the disc drive generates a system control signal (CTL) controlling the notch filter adjusting mode.

[0061] The excitation signal generator 405 is enabled only in the notch filter adjusting mode by the system control signal (CTL) and is disabled in other modes.

[0062] The multiplexer 403 provides a VCM control signal output from the servo controller 401, directly to the VCM actuator 404 in the notch filter adjusting mode, according to the system control signal (CTL). In other modes, the multiplexer 403 provides a VCM control signal, which is filtered through the notch filter 402, to the VCM actuator 404.

[0063] The switch 407 inputs a PES output from the VCM actuator 404 to the band pass filter 408 only in the notch filter adjusting mode according to the system control signal (CTL), and in other modes switches so that the PES is not provided to the band pass filter 408.

[0064] With this switching control, in the notch filter adjusting mode, the circuit connection of FIG. 4 is similar to the circuit state shown in FIG. 3 so that coefficients of the notch filter are adaptively adjusted.

[0065] In intervals other than in the notch filter adjusting mode, the servo controller 401, the notch filter 402, and the VCM actuator 404 are connected, in that order, in the circuit such that the mechanical resonance of the disc drive is reduced by the notch filter whose filter coefficients are adaptively adjusted.

[0066] According to an aspect of the present invention as described above, a potential resonance frequency of a system is estimated by artificial excitation of the system, and the coefficients of the notch filter are controlled and adjusted corresponding to the estimated resonance frequency. Thus, even though the resonance frequency varies with respect to the surrounding temperature or product temperature, the resonance frequency is accurately removed and the system is stably controlled.

[0067] Aspects of the present invention may be implemented as a method, an apparatus, or a system. Aspects of the present invention may also be embodied in a computer-readable medium. The computer-readable medium includes various recording medium on which computer-readable data is stored. The computer-readable media includes storage media such as magnetic storage media (e.g., ROM's, floppy disks, hard disks, etc.), optically readable media (e.g., CD-ROMs, DVDs, etc.) and carrier waves (e.g., transmissions over the Internet). Also, the computer-readable media includes media dispersed on computer systems connected through a network and storing and executing a computer-readable code while in a distributed mode.

[0068] Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in this embodiment without departing from the principles and spirit of the present invention defined, the scope of which is defined in the in the claims and their equivalents. Therefore, the scope of the present invention is not determined by the above description but by the accompanying claims.