Abstract: A signal is read out from a two-layered perpendicular magnetic recording medium by a composite head formed from a single-pole perpendicular recording head and magnetoresistance effect type reproduction head. The read-out signal is amplified by a head amplifier, passed through a low-pass filter, and converted into a digital signal by an analog/digital converter. A DC restoring circuit restores a DC component removed by the head amplifier using the output signal from the analog/digital converter. An adder adds the restored DC component to the output signal from the analog/digital converter, thereby restoring the head output terminal signal. A servo demodulation circuit generates a position error signal on the basis of the restored signal.

Abstract: Clock speed of the processor in a hard disk drive is controlled during run time to optimize the trade-off between minimizing power consumption and maximizing performance. Processor clock speed is increased during processing of code more performance critical to the disk drive system, while processor clock speed is reduced when less performance critical code is processed. An example of more critical code where processor clock speed is increased is the code executed by the processor from the start of a servo interrupt until output of resulting servo current command is provided from the processor to the actuator controller. To enable changing processor clock speed, an ASIC containing the disk drive processor is selected which provides the ability to switch clock rates of the processor during processor run time.

Abstract: A magnetic recording medium having a magnetic layer includes a plurality of tracks formed on the magnetic layer. The tracks have information magnetically recorded thereon. A magnetization change area for tracking control is formed on the magnetic layer adjacent to and along a length of each of the tracks. The magnetization change area has magnetic flux which changes along the length of the tracks.

Abstract: An information handling system, such as a disc drive, includes a base, a disc stack rotatably attached to the base, and an actuator assembly movably attached to the base. A region of the disc stack is identified as a parking band, and when power is not provided to the disc drive, the actuator assembly is held in a parked position within the parking band by an actuator latch. The actuator latch and actuator assembly have different resonant frequencies based on several environmental elements near the actuator assembly, including the temperature of the environment in the disc enclosure. The actuator assembly is unlatched by measuring the environmental element near the actuator assembly and applying an alternating current to a voice coil coupled to the actuator. The frequency of the alternating current applied to the actuator assembly during the unlatching procedure is near the resonant frequency of the actuator assembly in the measured environment.

Abstract: In a magnetic disk storage apparatus, when a drive current of the voice coil motor is small, the head drive control circuit executes the linear drive mode under which the voice coil motor is driven in a linear control manner, whereas when the drive current is large, the head drive control circuit executes the pulse drive mode under which the voice coil motor is driven in a pulse width control manner. Both modes are carried out by employing a commonly-used output amplifier that is arranged as a push-pull type output circuit made of both a push-driving output transistor and a pull-driving output transistor. An operation mode of this output amplifier is set to an AB-class operation during the linear drive mode, whereas the operation mode of this output amplifier is set to a B-class operation during the pulse drive mode.

Abstract: A detector detects, in read data, a servo address mark of a type indicated by a register. Every time a SAM is detected during a self servo process, a CPU carries out tracking control in accordance with corresponding servo data. The CPU causes a head to write a corresponding type of additional servo data item in a disk at a position offset from the position of servo data containing the detected SAM, by a predetermined distance in a circumferential direction of the disk. Every time sets of servo data, each consisting of various types of data items, are written in the disk, the head is moved by a predetermined pitch in a radial direction of the disk.

Abstract: A servo write method capable of generating and recording correction data, to be used for compensating for repeatable position error caused by write errors of servo patterns, in a short time is presented. In the recording of the servo pattern, correction data for correcting revolution-synchronous components are generated from a position error signal which is obtained from the difference between a target position and a magnetic head position, and the correction data are recorded in servo areas on a disk.

Abstract: A method and apparatus for controlling an access mechanism within a disk drive are disclosed. A servo apparatus comprises a first detection module for discretely detecting a displacement value of an access mechanism from a target position at predetermined time intervals, a first feedback module for calculating a first feedback value on the basis of the detected displacement value, a second detection module for discretely detecting a physical value, including the speed of the access mechanism, at least once at each of the predetermined time intervals, a second feedback module for calculating a second feedback value on the basis of the physical value, including the speed, and a motor driving module for driving a motor according to the first feedback value and the second feedback value.

Abstract: A method of and apparatus for generating a reader-to-writer offset for a head positioned over a track on a disc in a disc drive having a plurality of tracks is disclosed. A reader and a writer are mounted on the head in a non-overlapping manner with a gap between the reader and the writer, which is known as the reader-to-writer offset. A disc drive controller is operably connected to the reader and the writer. The disc drive controller determines the reader-to-writer offset of the head for each track by computing the difference between a first head position x1 and a second head position x2. The first head position x1 is an optimal head position for writing information on the track and the second head position x2 is a midpoint of an off-track capability (OTC) of the head.

Abstract: Many parallel tracks on a storage surface of a data handling device are arranged in a longitudinal direction. Each track has a track center comprising reference points for fine lateral positioning. Each successive pair of track centers has a succession of lateral offset distance having an average. Because there are many successive pairs of tracks, there are many average lateral offset distances defining a statistical distribution having a variance. The device includes a laterally movable transducer head and a longitudinally movable data surface. A signal is received from the transducer head while the data surface moves past the head. Many values each indicative of a lateral offset distance between a corresponding pair of lateral reference points are derived from the received signal. These offset-indicative values are used to shift at least some of the latitudinal reference points laterally so as to reduce this variance.

Abstract: This invention comprises an architecture for voltage mode control of a voice coil motor in a hard disk drive. In contrast to conventional current mode control, coil current is not sensed or measured, which simplifies the feedback design with less hardware required in the implementation. Common design methodologies for the square root velocity profile, linear velocity profile and regulator/estimator control system designs can be migrated from the current mode architecture to the voltage mode architecture.

Abstract: A head position control system for a dual stage actuation disk drive system. A feedback system is provided for modifying a primary and a secondary input command signal to produce primary and secondary error signals. A controller receives the primary error signal and transmits primary actuator arm positioning information to the primary actuator. A secondary controller receives the secondary error signal and transmits secondary actuator arm positioning information to the secondary actuator. The feedback system creates a position error signal (PES) using information from servo wedges and runout information and the PES is used to produce the secondary error signal. The feedback system produces a reconstructed error signal including angular position information for the primary actuator arm by processing a back electromotive force signal from the primary actuator. The primary error signal is produced by modifying the primary input signal with the reconstructed error signal.

Abstract: The position information of the magnetic head is generated also from the data area to improve the positioning accuracy. The magnetic disk drive has a function of oscillating the magnetic head in the radial direction in the data area at a specific frequency and a function of detecting a read signal during the execution of the oscillation function and detects position information of the magnetic head from the detected read signal. The magnetic head position control can be performed in the data area. The magnetic head position signal not dependent on the frequency of a sampling signal generated from the servo information area can be generated. This in turn improves the data recording density and allows a highly precise position control of the magnetic head, thus improving the reliability of the magnetic disk drive.

Abstract: A disk drive is disclosed comprising a voice coil motor (VCM) actuator and a piezoelectric (PZT) actuator. The VCM actuator is driven by a control effort including an integrator, and the PZT actuator is driven by a control effort including a transformer.

Abstract: A disc drive including a rotatable data track, a dual-stage actuator with a primary actuator motor supporting an actuator arm, a read/write head supported by the actuator arm and communicating with a secondary actuator motor, and steps for controlling range of motion of the secondary actuator motor. The controlling steps include supplying and sustaining a bias signal to a single-sided unipolar device driver that then apply a bias voltage the secondary actuator motor to induce the secondary actuator motor to expand substantially one half of its expansion capabilities. And, confining correction signals provided by a control circuit of the disc drive, used in correcting mechanical position of the secondary actuator motor, to a voltage ranging substantially between a positive “+” and negative “−” voltage substantially equal to the applied bias voltage.

Abstract: Settling response characteristics after change-over from a speed controller to a position controller is attained. Fluctuations in a time taken until change-over is made from the speed controller to the position controller are reduced to stabilize a moving time and to shorten an access time. To this end, a position controller generates a drive signal of a linear or primary function of time in such a manner that a position deviation signal responds to it with a ternary-order function of time which becomes zero in a desired finite settling time. Further, a feedforward signal generator is provided to reduce a follow-up error to a target position signal as an output of a target position signal generator.

Abstract: An apparatus and method of controlling an actuator of a data storage device detect and/or correct narrowband mechanical excitations such as due to spindle bearing defects through sensing and/or rejecting disturbance frequencies associated with such narrowband mechanical excitations in an actuator control signal generated by a data storage device controller. A narrowband filter may be used to reject, from an actuator control signal generated by a data storage device controller, a disturbance frequency associated with a spindle bearing defect in a data storage device, and disposed in proximity to a zero crossover frequency associated with the data storage device controller. In addition, a peak filter disposed in a control loop of a data storage device controller may be configured to detect a spindle bearing defect in a data storage device by detecting an amplitude of a control signal in the control loop in proximity to a disturbance frequency associated with the spindle bearing defect.

Abstract: The invention relates to a method for recovering from shock event to a disk drive. Embedded servo wedges are read to generate position error signal (PES) values. The disk drive is transitioned to a write unsafe state if a PES value is greater than a write unsafe (WUS) threshold and is less than a shock detection threshold. The disk drive is transitioned to a shock recovery state if a PES value is greater than the shock detection threshold. The disk drive is transitioned from the shock recovery state to the write unsafe state if a first number of consecutively generated PES values remain less than the WUS threshold. The disk drive is transitioned from the write unsafe state to the on-track state if a second number of consecutively generated PES values remain less than the arrival threshold, which threshold is less than the WUS threshold.

Abstract: A reader-to-writer offset for a head positioned over a track on a disc in a disc drive having a plurality of tracks is generated by using two iterations. A first iteration is used to determine a fine-tuning range that is used for a second iteration. The first iteration uses coarse steps of the reader across the track pitch to roughly determine OTC boundaries (which are used as the fine-tuning range). The fine-tuning range is determined by locating the point before the first valid reading (e.g., a VGA reading or BER reading with a rate that is less than the BER threshold) and by locating the point after the last valid reading. The second iteration uses fine steps of the reader across the track pitch to determine the actual OTC boundaries. The OTC boundaries are the first valid reading and the last valid reading that are made using the fine steps.

Abstract: A read/write head microactuator includes first and second insulating deposits that are flat, spaced apart by an air gap, and laterally movable responsive to a microactuator electrical drive current. First and second flat, interdigitated drive electrode deposits are deposited on the first insulating deposit. A flat, digitated sense electrode deposit is deposited on the second insulating deposit. The sense electrode deposit provides a relative position output. The sense electrode deposit faces both the drive electrode deposits across the air gap.

Abstract: A position detection means generates a position error signal corresponding to a current position of a head, a position control means generates a position control signal from the position error signal, a voltage detection means detects a voltage signal generated when driving an actuator by a driving means, a disturbance estimation means generates a disturbance estimation signal from a driving signal and the voltage signal, and a correction means outputs the driving signal from the disturbance estimation signal and the position control signal.

Abstract: By increasing the gain of a controller for coarse motion CFBV within a frequency band in which the gain of a sensitivity function SP=1/(1+PP·CFBP) of fine motion actuator DP and controller for fine motion CFBP exceeds 0 dB, the gain of a sensitivity function SV=1/(1+PP·CFBV) of coarse motion actuator PV and coarse motion controller CFBV is decreased. Through this, in a two-stage actuator control system, a disk apparatus can be provided in which a sensitivity function SPES=1/(1+PP·CFBP)·1/(1+PP·CFBV) indicative of a disturbance compressive ratio in a signal path ranging from disturbance components to a head position error signal is rendered to be 0 dB or less at all frequencies concerned.

Abstract: An apparatus and method for compensating for environmental effects on media. The apparatus includes an array of servo readers that are positioned at a non-zero azimuth angle relative to a medium. The position of the array of servo readers is dynamically updateable in both a vertical direction and an azimuth angle direction based on a combination of an average position error signal of both an upper portion of the servo readers and a lower portion of the servo readers. Based on the average position error signal and a reference signal, a controller generates control signals that are output to one or more actuators. The actuators then adjust the position of the array of servo readers based on the control signals received.

Abstract: A head suspension supporting a head at the tip end is allowed to swing at the tip end of a carriage in a recording disk drive. A microactuator is interposed between the head suspension and the carriage. A resistor is connected to the microactuator in serial. The microactuator is allowed to function as a capacitance. A combination of the microactuator and the resistor enables establishment of a so-called low pass filter. A high frequency noise involuntarily introduced in the driving voltage for the microactuator can be eliminated or removed from the driving voltage. Elimination of the high frequency noise enables a reliable prevention of the mechanical or physical resonance or vibration in the head suspension. A tracking action of the head related to a target recording track can reliably be stabilized.

Abstract: A data storage disk is formatted using a measured edge location of a textured landing zone as a reference for writing servo information to the surface of the disk. The edge of the textured landing zone is determined by moving a transducer across the surface of the disk in a radial direction while monitoring a signal generated by the transducer. When a predetermined indication is detected within the signal, the corresponding location of the transducer is recorded. This transducer location is then used to calculate a desired location of at least one data track of the data storage disk. Preferably, this procedure is performed within a servo track writer. In one embodiment, a worst-case edge location is determined for all of the disk surfaces within a disk stack. This worst-case edge location is then used as a reference for writing servo information on all disk surfaces within the stack.

Abstract: A disk drive unit includes acceleration sensors mounted thereon. A sequence for calculating an amount by which a head actuator is adjusted during recording and readout operations is changed based on an output from the acceleration sensors.

Abstract: A disk drive apparatus. The apparatus has a first drive device and a support member coupled to the first drive device. The support member has a tongue portion and a gimbal portion. The tongue portion is coupled the gimbal portion. A fixed drive device is formed within a first portion of the tongue portion. A movable drive device is operably coupled to the fixed drive device and formed within a second portion of the tongue portion. A read/write head is coupled to the movable drive device. A voltage source is coupled between the fixed drive device and the movable drive device to cause movement of the read/write head by forming an interaction between the fixed drive device and the movable drive device.

Abstract: A recording/reproducing apparatus capable of switching over among a plurality of heads with high precision, wherein the apparatus is provided with: a recording/reproducing unit for recording/reproducing signals onto/from a recording medium with a plurality of heads; a generating unit for generating control data for controlling the recording/reproducing unit; and a transmitting unit for transmitting control data generated by the generating unit to the recording/reproducing unit, wherein priority is given in transmission to data for controlling the switching of the plurality of heads over other data.

Abstract: A disk drive apparatus. The apparatus has a platter operably coupled to a servo drive device. The servo drive device is adapted to rotate the platter about a fixed axis. The apparatus also has a support member operably coupled to the platter to move the support member about one or more active regions on a surface of the platter. The apparatus has a read/write head including an active portion and a support portion. The support portion is coupled to the support member. The active portion is operably coupled to the platter to read and/or write information to the surface of the platter. A piezo electric actuating material is coupled between the support portion of the read/write head and the support member. A first electrode is coupled to a first side of the piezo electric actuating material. A second electrode is coupled to a second side of the piezo electric actuating material.

Abstract: A filter for attenuating a resonance mode without substantial phase margin loss. The filter comprises a transfer function having an additional pole, such that the phase of the input signal is advanced. The phase-advanced filter substantially attenuates all frequencies above a selected center frequency while avoiding substantial phase margin loss.

Abstract: The present invention may be embodied in a method, implemented in a disk drive, for adaptively reducing an effect of vibration during a track following operation. The disk drive includes a disk and a rotary actuator. The disk includes distributed position information in a plurality of servo wedges. The actuator carries a transducer and pivots on a base of the disk drive. The actuator may exhibit an imbalance that may be affected by a linear vibration. In the method, a sensor signal is generated that is responsive to a vibration to the disk drive. The sensor value is modified based on an adaptive-filter gain to generate a position error adjustment signal for modifying a nominal position error signal. The adaptive-filter gain is then altered based on an adjusted position error signal and the sensor value associated with the current servo wedge for use during a next servo wedge.

Abstract: A method and an apparatus to control a disc drive, and more particularly, a method and an apparatus to control a disc drive using a counter-electromotive force, prevents collision and malfunction of a transducer and a disc by determining external vibrations and magnitude of a shock without installing an additional shock sensor in the disc drive. The method of controlling the disc drive using a counter-electromotive force includes detecting a voltage applied to a voice coil during a predetermined mode, performing an operation of a value of the counter-electromotive force using the detected voice coil voltage comparing the value of the counter-electromotive force operated with a predetermined threshold, and when the value of the counter-electromotive force is equal to or larger than the predetermined threshold, controlling a voice coil motor and a spindle motor so that a current mode is stopped and a parking or unloading mode is executed.

Abstract: The present invention may be embodied in a method, and related apparatus, for determining a repeatable runout (RRO) compensation value sets for a plurality of tracks on a disk in a disk drive, wherein the disk drive is initially manufactured and then shipped for installation in a host system. Each track comprises a plurality of embedded servo sectors that define a circumferential path that may have RRO requiring compensation during track following. The method comprises determining first RRO value sets for a first portion of the tracks to a first desired learning level by measuring the RRO for the first portion tracks based on a first predetermined number of disk rotations during the initial manufacturing of the disk drive. The first portion tracks are not adjacent to each other.

Abstract: A magnetic disk device comprises at least a disk having information, at least a head for reading or writing information from or into the disk, at least a micro actuator for supporting the head and positioning the head, a coarse actuator for carrying the micro actuator and positioning the head, a controller for controlling the micro actuator and the coarse actuator based on the servo information read by the head, and a host controller for determining that information is to be read. In the case where the host controller determines that the operation is in a wait mode, the positioning operation of the micro actuator is stopped.

Abstract: A disturbance compensation signal is produced by estimating the magnitude of disturbance acting upon an actuator based on a driving signal and a voltage signal generated when driving the actuator. A control signal is produced by multiplying the disturbance compensation signal by a gain adjustment coefficient. A head position error signal is produced based on the target position of a magnetic head and the current position of the magnetic head that is detected based on servo information recorded in advance on a magnetic disk. A gain adjuster obtains the driving signal by adding together the control signal and a position control signal. The actuator is driven with the driving signal output from the gain adjuster.

Abstract: A method of handling multiple resonance frequencies in a disc drive includes monitoring a position error signal (PES) for an actuator arm, generating a plurality of feedforward compensation signals from the PES using a plurality of bandpass filters, and applying the compensation signals to a servo control signal. Each filter has a center frequency that is set to a problematic resonance frequency. The method may also include identifying the problematic frequencies by, for example, commanding a movement of the actuator arm, collecting data points for the PES that are associated with the movement, and performing a digital fourier transform of the data points to identify resonant frequencies. Other methods of identifying problematic frequencies include analyzing PES zero-crossing data, or using principal component analysis.

Abstract: A read/write head servo control system for data recording devices, wherein a method of compensating initial values is utilized to generate a control signal to move the head from an initial position to a target position. The method estimates and stores multiple position compensation values. A first control signal moves the head from an initial position to a target position. Head position values are sampled as the head moves. Head position deviation values are calculated for the sampled position values. The head position values are utilized to select a set of estimated position compensation values. These values are in turn utilized to generate a second control signal, which is then utilized to control the position of the head.

Abstract: A position error signal indicative of the distance between an expected position of a magnetic head relative to the track centerline of a magnetic data storage and retrieval system and an actual position of the magnetic head relative to the track centerline is detected. An absolute value of the position error signal is determined and stored. The detection, determination, and storage is repeated over several revolutions of the magnetic data storage and retrieval system. A maximum of the absolute values of the position error signals is then determined and compared to a threshold value. If the maximum of the absolute values of the position error signals exceeds the threshold value, a warning signal is propagated. An alternative embodiment similarly predicts the possibility of head-disk crash on the basis of the maximum value of several samples of the squares of the position error signals.

Abstract: A head position control method performs virtual circular control which does not follow up the eccentricity of a disk, and implements high precision virtual circular control, even if the servo signals of an area demodulation method are used. The method performs virtual circular control by demodulating position signals in the area demodulation method and calculates the demodulation position with the velocity offset correction of 2-phase servo signals by using both the head velocity V of the virtual circular orbit reference and the relative velocity &Dgr;V of the virtual circular orbit and the position signal.

Abstract: A method for optimizing logical track to physical track offset compensation of a data storage device includes steps of: associating each of a plurality of logical tracks to one of a plurality of physical tracks, selecting a set of sample tracks from the plurality of physical tracks, measuring an offset value for each logical track associated with one of the sample tracks, deriving, through an extrapolation technique, an offset compensation value for use with each of the sample tracks absent an association with one of the logical tracks, linking each measured offset compensation value and each estimated offset compensation value with its associated physical track, calculating, through an interpolation technique, an offset compensation value for use with each of the physical tracks absent an association with one of the sample tracks.

Abstract: The present invention provides a method to screen tracks on a magnetic disc in a disc drive for harmonic resonance. There are several algorithms in the method performing different functions at various stages of the method. At any point in time, only one track is subjected to test by the method. The first stage of the method has one algorithm performing the function of obtaining a summation of all the absolute Position Error Signal (PES) velocities on a single track. Another algorithm performs the function of obtaining the average PES velocity with the summation of the absolute PES velocities. At the subsequent stage, another algorithm performs the function of obtaining the average PES velocity of a number of revolutions. An algorithm at the final stage performs the function of comparing the average PES velocity to a preset standard criterion, which is determined empirically in a separate algorithm. The track would be failed if the average PES velocity exceeds the preset standard criterion value.

Abstract: A hard disk device includes a servo control unit that constitutes a servo loop for a head seek for performing read/write of data from/to a recording medium, a correction signal generation unit for generating a correction signal for correcting a control by the servo control unit based on an output of a predetermined sensor, and a correction signal control unit for estimating an effect of supplying the correction signal to the servo loop and for controlling whether or not the correction signal is to be supplied to the servo loop in response to a result of the estimation. Moreover, the gain of the sensor is dynamically controlled in response to the above estimation result.

Abstract: An NRRO compensation circuit controls a head position in a disc drive. The circuit has a first frequency circuit providing a first output corresponding to NRRO and a first characteristic sensing circuit that senses the first output and that generates a first control output that adjusts a first NRRO compensation gain. A first control circuit included in the NRRO compensation circuit receives the first control output and the first NRRO compensation gain and provides a first NRRO compensator output.

Abstract: A technique for detecting a current position of a head of a hard disk drive detects the position of the head by using an analog-to-digital conversion value of a single crossing point of one of first and second servo burst signals written up and down with respect to the center of a selected track and a third burst signal written symmetrically with respect to the center of the selected track.

Abstract: A device for generating synchronous numeric signals, including a reference generating device supplying a reference signal and a first timing signal, both having a reference frequency; and a timed generating device supplying a synchronized signal having the reference frequency. The device further includes a synchronization stage generating a second timing signal having a first controlled frequency correlated to the reference frequency, and phase synchronization pulses having the first frequency and a preset delay programmable with respect to the first timing signal.

Abstract: A method of enhancing the thermal stability of gray codes is disclosed. The gray code block in a servo area is recorded with a combination of two different magnetization directions. When the two different magnetization directions are expressed as “+” and “−”, the information “1” is recorded as “++−−” or “−−++” with the information “0” recorded as “+−+−” or “−+−+”. Since the filter for gray code demodulation is provided with a perpendicular recording mode, the servo area does not use long-wavelength signals that are susceptible to thermal demagnetization. As a result, thermal stability is enhanced.

Abstract: A disk drive and related method, the disk drive including a rotating magnetic disk and read and write elements for reading and writing data from and to the rotating magnetic disk, comprising: a magnetic characterization pattern recorded on the rotating magnetic disk for use in characterizing widths of the read and write elements using a plurality of track profiles of differing width. The magnetic characterization pattern is uniquely formed from a first plurality of characterization segments of a first radial width disposed circumferentially about a full circumference track on the rotating magnetic disk and a second plurality of characterization segments of a second radial width that is different than the first radial width disposed circumferentially about the same full circumference track on the rotating magnetic disk. The beneficial result is that two measurements can be accomplished within the STW in roughly the same time it ordinarily takes to accomplish only one.

Abstract: When the amount of position error PE of a magnetic head meets the condition of Ewf>PE>=Eww for two threshold values Ewf and Eww meeting Ewf>Eww, the track is registered in the track information table as a track requiring rewrite and the tracks on both sides neighboring to the track are temporarily write-inhibited. The data of the track is rewritten on another track later so as to maintain the data. By doing this, the offtrack of the recording track can be made smaller without sacrifice of the access speed and the occurrence probability of write fault can be reduced. Furthermore, the reliability of a magnetic disk apparatus can be improved.

Abstract: Disclosed herein is a self-servo writing method that uses the head of a disk drive to write servo data on a disk-shaped medium. This method calculates an accurate position offset between the read-head element and write-head element incorporated in the head, by using the positioning servo data and measuring servo data that are recorded on the disk-shaped medium.

Abstract: A disk drive (HDD) (and method) subject to linear and rotational vibration, includes an independent sensing unit for sensing a rotational velocity component of the rotational vibration in a predetermined frequency range, and an optimal filter combination for receiving an output from the sensing unit.