Patent Application
20130003219
Data storage systems often include reading and writing components that read data from, and write data to, a storage medium. In some storage systems, the reading and writing components and the storage medium move relative to one another. In those systems, the relative position of the reading and writing components, relative to the storage medium, is sensed and used to determine whether the reading and writing components are at a desired position relative to the storage medium, in order to accurately read and write desired data.
Data storage systems are also often subjected to significant amounts of vibration, acceleration, or other disturbances. These disturbances can rotate a data storage system about its center in an x-y plane, and this is referred to as rotary vibration. In addition, the disturbances can shift the data storage system linearly along the x or y axis, respectively, and this is referred to as linear vibration.
In order to sense various disturbances that can occur in data storage systems, some data storage systems have included two acceleration sensors (or accelerometers) that are mounted on the corners of a printed circuit board assembly that is used in controlling the data storage system. Signals generated by the two accelerometers can be subtracted from one another in order to measure rotary vibration disturbances in the data storage system. However, in order for this to be done, it is important that the gain of each sensor is substantially equal to the gain of the other sensor, and the axis of rotation of the structure on which the sensors are mounted is centered between the two sensors. In addition, it is important that the sensing axes of the two sensors be closely aligned to one another, in order to obtain an accurate signal reflecting disturbances on the data storage system. Accurately sensing and compensating for disturbances can be difficult.
Aspects of the present disclosure address these and/or other problems.
In some current data storage systems, a feedback control loop relies on reading the position of the reading and writing components relative to the storage medium. However, this type of control loop has not been fast enough to correct for off-track movements that are generated from linear or rotary vibrations because the control loop operates at a relatively low bandwidth. Thus, some data storage systems use a rotational vibration feed forward (RVFF) algorithm to inject current into the control system to compensate for the position errors caused by rotational vibration. This type of system can be helpful in correcting for rotational vibration disturbance. However, it has only accounted for the correlation between the rotational vibration signal (generated by subtracting the two acceleration sensor signals) and a position error of the reading components relative to the storage medium. Such systems have largely ignored the linear vibration signal, entirely. It has been found that there is an impact on the performance of some data storage systems when the linear vibration disturbance becomes non-negligible.
In accordance with one aspect, a plurality of sensors are used to sense disturbances in a data storage system. An adaptive gain component is associated with each of the sensors and provides a gain for each of the sensor signals. The gain of each sensor signal is adapted, individually, based on a correlation of each given sensor signal to the position error signal. This adaptation produces a position correction signal. The position correction signal is applied to a position signal that is used to position the reading and writing components and the storage medium relative to one another. This compensates for both rotary and linear vibration disturbances at the same time. It accommodates for any gain mismatch between the sensors and also greatly reduces, if not eliminates, the need to mount the sensors accurately about the axis of rotation of the data storage system.
Other features and benefits that characterize embodiments of the present disclosure will be apparent upon reading the following detailed description and review of the associated drawings.
The description corresponding to
In the embodiment discussed herein, storage system 100 is a type of system in which reader/writer 112, 110 and storage medium 106 move relative to one another. Therefore, data storage system 100 also includes a positioning component (or plant) 114 that, in the embodiment shown, positions reader/writer (or plant components) 112, 110 relative to storage medium (or plant component) 106.
Similarly, system 100 includes position sensing and control electronics 116. In the exemplary embodiment, controller 102 and position sensing and control electronics 116 position reader/writer 112, 110 relative to medium 106 by using reader 112 to read position information from storage medium 106 that indicates a relative position of reader 112 relative to storage medium 106. The position information is provided to controller 102 and position sensing and control electronics 116. Controller 102 also receives a reference position signal 107 which indicates a desired position of reader/writer 112,110 relative to storage medium 106. Based on the position signal read back from the storage medium 106, and the reference position signal 107, controller 102 provides a position signal to position sensing and control electronics 116 in order to control the position of reader/writer 112, 110 relative to storage medium 106, using positioning component 114. In the illustrative embodiment in which data storage system 100 is a disc drive, and storage medium 106 includes a plurality of discs that are rotated by spindle motor 108, reader/writer 112, 110 are positioned relative to the discs in storage medium 106 by a voice coil motor which comprises positioning component 114. Of course, a disc drive is but one exemplary embodiment of a data storage system, and other embodiments can be used as well.
It should be noted that separate controllers can be used for reading and writing data 104 relative to storage medium 106, and for controlling the position of reader 112 and writer 110 relative to storage medium 106. In the embodiment shown in
As discussed above, data storage system 100 can be subjected to either internal or external forces which create linear or rotary movement or vibration, or other physical disturbances of data storage system 100. This can cause reader/writer 112, 110 to be jolted, or otherwise moved, out of its desired position relative to storage medium 106. In the embodiment where storage medium 106 comprises a disc, the imparted disturbances can cause reader 112 or writer 110 to exhibit off-track errors in which the reader 112 or writer 110 is not positioned over a desired track, but is instead off track by a position error amount. Thus, in the embodiment discussed herein, position sensing and control electronics 116 include disturbance sensors (such as vibration sensors) which sense the forces imparted to disc drive 100 that can cause position errors. Sensed disturbances are then compensated for when controlling positioning component 114 so that reader 112 and writer 110 are accurately positioned relative to storage medium 106.
Each disturbance sensor 200, 202 has a corresponding low pass filter 204 and 206 as well as an adaptive gain component 208 and 210, respectively. In one embodiment, disturbance sensors 200 and 202 are accelerometers that are mounted on a printed circuit board assembly that contains position sensing and control electronics 116. The accelerometers each provide a signal indicative of acceleration along one or more sensing axes. In one embodiment, the two sensors 200 and 202 are mounted on opposite corners of the printed circuit board assembly, although other mounting configurations can be used as well.
In any case, sensor signals provided by sensors 200 and 202 are filtered using low pass filters 204 and 206. This is indicated by block 302 in
While the disturbances are being sensed, data storage system 102 generates a position error signal by subtracting the position signal 230, read from storage medium 106 by reader 112, from the reference position signal 107, which indicates a desired position of reader 112 relative to storage medium 106. In one embodiment, the signal generated by reader 112 is provided to position demodulator component 232 which demodulates the signal to provide the position signal 230. The amount by which the position signal 230 differs from the reference position 107 is identified as position error signal (or PES) 234. Position error signal 234 is provided to controller 102 and to low pass filter 212. Generating the position error signal is indicated by block 304 in
Low pass filter 212 filters the position error signal to filter out high frequency components of the position error signal. Again, in one illustrative embodiment, low pass filter 212 is a second order filter with a 1.5 KHz bandwidth. Of course, other implementations for low pass filter 212 can be used as well. Filtering position error signal 234 is indicated by block 306 in
The output of low pass filter 212 (the filtered PES) is provided to adaptive gain component 208 and adaptive gain component 210. Adaptive gain components 208 and 210 use the filtered PES output by low pass filter 212 to adapt the gain of the filtered sensor signals independently of one another based on individual correlations between the filtered position error signal and each of the filtered sensor signals, respectively. In one embodiment, adapting the gain of the two sensor signals based on a correlation of the filtered PES and the filtered sensor signals is done according to the following gain update equations:
LVGain1(k+1)=LVGain1(k)+StepSizeLV1 Eq. 1
LVGain2(k+1)=LVGain2(k)+StepSizeLV2 Eq. 2
where
StepSizseLV1=adapGain*PESLowpass(k)*LVSensor1Lowpass(k) Eq. 3
StepSizeLV2=adapGain*PESLowpass(k)*LVSensor2Lowpass(k) Eq. 4
The term “adapGain” is the adaptation gain, PESLowpass(k) is the output of low pass filter 212, LVSensor1Lowpass(k) is the output of low pass filter 204 at time k, and LVSensor2Lowpass(k) is the output of low pass filter 206 at time k.
In the illustrative embodiment, adaptive gain components 208 and 210 are adaptive scalars, LVGain1 and LVGain2, that are updated using Equations 1-4 above. The correlation of the PES low pass filtered signal output by low pass filter 212 and low pass filtered sensor signals output by filters 204 and 206 are used to adaptively adjust the two gains LVGain1 and LVGain2.
The adaptation gain “adapGain” is, in one embodiment, tuned to provide a fast adaptation response and maintain system stability. This tuning can be carried out using any desired process, or it can be set empirically, until the desired adaptation response is achieved. Also, in one embodiment, the two step sizes, StepSizeLV1 and StepSizeLV2 are given an upper bound for stability. When one or both of the two step sizes exceed the upper bound value, then the larger step size is illustratively set to the upper bound value while the smaller step size is relatively scaled down from the upper bound value. This will ensure that the adaptation and/or correction to the position signal is in the right direction. The bound value can be set as desired or determined empirically or otherwise. Adapting the gain of the filtered sensor signals is indicated by block 308 in
Once the gain of the sensor signals has been adapted, the output of components 208 and 210 are then combined at node 218 to obtain a position correction signal 312. In one embodiment, node 218 is a summing node and the two signals are added together. Combining the adapted sensor signals to obtain the position correction signal 312 is indicated by block 310 in
It will be noted that there can be some phase loss in generating a position correction signal 312 due to processing the signals used to obtain signal 312. Therefore, phase compensator 214 is used to compensate for phase losses incurred during processing, or for other reasons. In one embodiment, phase compensator component 214 is a lead/lag compensator that adds or subtracts phase to position correction signal 312 so that it is applied at the right point in time to the position output by controller 102. In one exemplary embodiment, the lead/lag compensator is a second order filter with two poles and two zeros. It creates phase correction (such as phase lead) in a range of frequencies from approximately 100 Hz to approximately 1500 Hz to compensate for delay in the frequency range of interest. Of course, phase correction can be for any range of frequencies, not just 100 to 1500 Hz, which is given by way of example only. Adjusting phase of the position correction signal is indicated by block 314 in
The output of phase compensator component 214 is provided to combining node 220 where it is combined with the position signal output by controller 102. The position signal is corrected by the phase compensated position correction signal 312 at node 220. It will be noted that the phase compensated position correction signal not only compensates for rotary vibration (or disturbances) but linear vibration (or disturbances) as well. Correcting the position signal with the position correction signal is indicated by block 316 in
Finally, the corrected position signal output by node 220 is applied to positioning component 114 to more accurately position reader 112. This is indicated by block 318 in
It can thus be seen that the positioning system discussed above treats the signal from each of the disturbance sensors 200, 202 as an independent system input and uses the correlation between those inputs and the position error signal to adaptively adjust the gain for those sensor signals. This approach not only substantially simultaneously, corrects both rotational vibration and linear vibration, but it also corrects for any gain mismatch between the sensors 200, 202 and thus substantially eliminates the need to sort the sensors in order to match gains in sensor pairs. It also substantially eliminates the need to have the sensors mounted evenly about the center of rotation of a printed circuit board assembly, with their sensing axes closely aligned, since the adaptation algorithm naturally corrects for all of this.
In an embodiment where data storage system 100 is a disc drive, the positioning system shown in
It is to be understood that even though numerous characteristics and advantages of various embodiments have been set forth in the foregoing description, together with details of the structure and function of various embodiments, this disclosure is illustrative only, and changes may be made in detail, especially in matters of structure and arrangement of parts within the principle disclosure to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed. For example, the particular elements, or the functions performing those elements, may vary and be grouped together or further separated, depending on the particular application of the storage system, while maintaining substantially the same functionality without departing from the scope of the disclosure. In addition, although one embodiment described herein is directed to a disc drive system, it will be appreciated by those skilled in the art that the teachings can be applied to any data storage system, where an accessing component and the storage medium move relative to one another, without departing from the scope of the disclosure.
