Vibration detection and compensation filter

The application relates to an adaptive vibration damping scheme that provides for detection of, and adjustment for, vibration-related disturbances in devices. The vibration damping scheme utilizes a common filter function to implement vibration detection and vibration compensation.

FIELD

The present embodiments relate generally to the field of vibration damping and more particularly, but not by way of limitation, to methods and apparatus for vibration detection and compensation in devices.

BACKGROUND

Vibration is a major factor that negatively impacts performance of devices such as computer disc drives. In a computer disc drive, data is stored on discs in concentric tracks. In disc drives with relatively high track densities, a servo feedback loop is used to maintain a head over the desired track during read or write operations. This is accomplished by utilizing prerecorded servo information either on a dedicated servo disc or on sectors that are interspersed along a disc. During track following, the servo information sensed by the head is demodulated to generate a position error signal (PES) which provides an indication of the distance between the head and the track center. The PES is then converted into an actuator control signal, which is used to control an actuator that positions the head.

Misalignment of the read/write heads with respect to the tracks causes increases in read/write errors and a slowdown in read or write operations. Accurate positioning of read/write heads is required even in the presence of anomalies such as aging, temperature changes, changes in orientation of the disc drive, humidity, shock and vibration.

In general, as indicated above, vibration negatively impacts performance of devices with moving parts. The combination of high speed and tight tolerances in disc drives makes them particularly vulnerable to vibration-induced performance degradation.

The present embodiments address these problems and offers other advantages over the prior art.

SUMMARY

An aspect of the disclosure relates to an adaptive vibration damping scheme that utilizes a common filter function for detection of, and compensation for, vibration-related disturbances in devices.

In one apparatus embodiment, a single filter is configured to detect vibration represented in a position error signal (PES) and to generate a compensation signal for the detected vibration.

Another apparatus embodiment is directed to a servo loop. The servo loop includes a vibration detection component without any physical sensor and an adaptive vibration compensation component. The vibration detection component and the vibration compensation component utilize a common filter function to implement vibration detection and vibration compensation.

In one method embodiment, rotational vibration represented in a position error signal is detected using a filter function. The method also involves compensating for the detected rotational vibration and adaptively adjusting a compensation gain utilized along with the filter function to compensate for the detected rotational vibration.

These and various other features and advantages will become apparent upon reading the following detailed description and upon reviewing the associated drawings.

DETAILED DESCRIPTION

FIG. 1is a block diagram of a servo loop100in accordance with one of the present embodiments. Because precise structure of the servo loop is not significant to the present embodiments, servo loop100is shown in a simplified form. Those skilled in the art will appreciate that servo loops are more complex than the simple diagram ofFIG. 1.

The same reference numerals are used in the various figures to represent the same or similar elements. InFIG. 1, servo loop100includes a summing node102that receives a reference signal (r)104that indicates a desired value. Summing node102combines reference signal104with a servo signal106, which is a sensed value, to produce a position error signal (PES)108that is provided to a controller (C)110. Controller110generates a nominal control signal112which, in general, causes plant (P)114to move.

In an example embodiment, plant114includes a voice coil motor (VCM) that positions a head over a track on a storage medium (for example, a disc storage medium). More specifically, in such an embodiment, using servo patterns stored on the storage medium, the head generates an analog signal that indicates the distance from the head to the track center. The analog signal is converted into digital signal106and, as indicated above, digital signal106is fed back to summing node102. Summing node102then subtracts digital signal106from reference signal104to produce PES108. However, analog control components can be used in alternative embodiments.

As noted earlier, vibration is a major factor that negatively impacts performance of devices such as disc drives. InFIG. 1, vibration-related disturbances, which include rotational vibration disturbances, are shown separately as disturbance (dv)116.

In general, one or more of the present embodiments relate to adaptive vibration damping schemes that provide for detection of, and adjustment for, vibration-related disturbances in devices.

Accordingly, in the example embodiment shown inFIG. 1, servo loop100includes a vibration damping circuit118which, in turn, includes, as its primary components, a vibration detection component120, and an adaptive vibration compensation component122. In essence, vibration detection component120detects vibration represented in PES108and provides an output indicative of the detected vibration to adaptive vibration compensation circuit122, which responsively adjusts at least one of its parameters to provide a suitable vibration compensation signal124. As can be seen inFIG. 1, both nominal control signal112and vibration compensation signal124are provided to summing node126, which outputs servo control signal128. Servo control signal128is essentially a refined version of nominal control signal112.

In at least some of the present embodiments, both vibration detection component120and vibration compensation component122utilize a common filter function (or single filter) (F)130to implement vibration detection and vibration compensation. As used herein, a common filter function or single filter is either a single software, hardware or firmware component that is utilized, called, or invoked to carry out vibration detection and compensation functions, or two substantially similar copies of software, hardware or firmware components that are utilized, called, or invoked to carry out vibration detection and compensation functions. In a specific embodiment, filter130is a band-pass filter that detects PES degradation due to low frequency vibration, which primarily includes rotational vibration.

As can be seen inFIG. 1, adaptive vibration compensation component122also includes an adjustable compensation gain (k)132, which is utilized with filter130to generate vibration compensation signal124. A tuning component134adjusts compensation gain132based on the detected vibration. Specific examples of filter130and tuning component134are described further below.

As noted earlier, in general, in a disc drive, PES108is generated during a track following operation. Before the track following operation, a seek operation is carried out to arrive at the track (or to bring the head to the desired track). In one embodiment, vibration damping circuit118includes a control input enable/disable component136that is configured to disable provision of nominal control signal112to filter130via component132during a seek operation, and to enable provision of signal112to filter130during a track following operation. Component136is discussed again further below in connection with a specific embodiment. It should be noted that, in at least some of the present embodiments, vibration detection is carried out without using any physical sensor.

As indicated above, in some embodiments, vibration detection filter130detects/monitors vibration-related disturbance in servo loop100by low frequency filtering of PES108. In a specific embodiment, vibration detection filter130is designed as follows:

F⁡(s)=a⁢⁢ss2+b⁢⁢w⁢⁢s+w2Equation⁢⁢1
where w=2*pi*f with frequency f being a rotational vibration frequency; b is a damping ratio that controls the bandwidth of the filter. By suitably selecting parameters a, b and f, a band-pass filter F(s) that detects PES degradation due to low frequency vibration is designed for implementation.

A rotational vibration compensation signal (for example, a rotational vibration compensation current) is generated based on the controller output112and compensation gain (k)132. A specific self-tuning law that can be implemented in tuning component134to suitably adjust compensation gain132based on detected vibration is as follows:
k(t)=c0*k(t−1)+c1*|PESrv|  Equation 2
where 0<c0<1 and c1>0 are constants that adjust a speed of adaptation. When there is little or no low frequency vibration disturbance, PESrv(low frequency filtered PES or rotational vibration filtered PES) becomes small. This results in a decrease in gain k(t) of the rotational vibration compensation. This means that the adaptive vibration damping circuit has little impact on the overall servo loop when there is little or no vibration. In the case of a disc drive in a high rotational disturbance environment, the disturbance detection filter F(s) will generate large PESrv, which increases rotational vibration compensation gain k(t) relatively rapidly. In this case, a large rotational vibration compensation current is generated and applied to the VCM for disturbance rejection.

As noted earlier, one aspect of the adaptive vibration damping circuit is that the input of the nominal control signal112to compensation filter (F)130is disabled (for example, set to zero), with the help of component136, during a seek operation. This helps remove a poor transient response of the adaptive vibration damping circuit during seeking/settling operations.

Referring now toFIG. 2, a flowchart200of a vibration damping method is shown. A first step of the method involves detecting, using a filter function, rotational vibration represented in a PES. This is illustrated at step202. Step204involves compensating for the detected rotational vibration. At step206, a compensation gain utilized along with the filter function to compensate for the detected rotational vibration is adaptively adjusted. Different techniques, some of which are set forth above, can be employed to carry out the steps shown in the flowchart ofFIG. 2while maintaining substantially the same functionality without departing from the scope and spirit of the present disclosure.

FIG. 3is a simplified block of an exemplary device in which the earlier-described methods and apparatus for vibration detection and compensation are useful. In this example, the device300includes a rotatable storage medium302having a data surface304. However, embodiments of the present disclosure are useful in other data storage and non-data storage applications.

Storage medium302is coupled to a spindle motor306through a spindle308. A transducing head310is positioned relative to surface304for reading and writing information onto surface304. Transducer310is coupled to an actuator312through an actuator arm314. In general, transducer310can read and write information on a desired location on surface304by moving transducer310with actuator312in a manner to position transducer310radially while storage medium302rotates. In a specific embodiment, data is stored on surface304in concentric tracks and a servo feedback loop100, which is shown in detail inFIG. 1, is used to maintain head310over the desired track during read or write operations. This is accomplished by utilizing prerecorded servo information on sectors that are interspersed along surface304, for example. During track following, the servo information sensed by the head310is demodulated to generate a PES (such as108ofFIG. 1) which provides an indication of the distance between the head310and the track center. The PES is then converted into an actuator control signal, which is used to control actuator312that positions the head310.

FIG. 4Ais a plot showing signals received at, and generated by, different components of a servo loop (such as100) included in a device300, when the device is subject to rotational vibration. Plot400shows PES108measured at each sector of a rotatable storage medium as it rotates. As explained earlier in connection withFIG. 1, filter130in vibration component120detects vibration represented in PES108and provides an output indicative of the detected vibration to tuning component134, which responsively adjusts compensation gain (k)132. Adjusted compensation gain values at each sector form plot402. The compensation gain values are utilized along with filter130in component122to generate vibration compensation signal124. Plot404shows vibration compensation signal values for the sectors of the storage medium during a revolution of the medium.

FIG. 4Bincludes graphs similar to those inFIG. 4A. However, the plots inFIG. 4Billustrate operation of components of servo loop100in the absence of rotational vibration in device300. Thus, plot406shows PES without rotational vibration, and plots408and410demonstrate that adaptive vibration compensation component122essentially automatically “shuts off” in the absence of rotational vibration.