The present invention relates to the field of mass storage devices. More particularly, this invention relates to a method of repeatable runout compensation in a disc drive.
One key component of any computer system is a device to store data. Computer systems have many different places where data can be stored. One common place for storing massive amounts of data in a computer system is on a disc drive. The most basic parts of a disc drive are an information storage disc that is rotated, an actuator that moves a transducer head to various locations over the disc, and electrical circuitry that is used to write and read data to and from the disc. The disc drive also includes circuitry for encoding data so that it can be successfully retrieved and written to the disc surface. A microprocessor controls most of the operations of the disc drive as well as passing the data back to the requesting computer and taking data from a requesting computer for storing to the disc.
The transducer head is typically placed on a small ceramic block, also referred to as a slider, that is aerodynamically designed so that it flies over the disc. The slider is passed over the disc in a transducing relationship with the disc. Most sliders have an air-bearing surface (xe2x80x9cABSxe2x80x9d) which includes rails and a cavity between the rails. When the disc rotates, air is dragged between the rails and the disc surface causing pressure, which forces the transducer head away from the disc. At the same time, the air rushing past the cavity or depression in the air bearing surface produces a negative pressure area. The negative pressure or suction counteracts the pressure produced at the rails. The slider is also attached to a load spring which produces a force on the slider directed toward the disc surface. The various forces equilibrate so the slider flies over the surface of the disc at a particular desired fly height. The fly height is the distance between the disc surface and the transducing head, which is typically the thickness of the air lubrication film. This film eliminates the friction and resulting wear that would occur if the transducing head and disc were in mechanical contact during disc rotation. In some disc drives, the slider passes through a layer of lubricant rather than flying over the surface of the disc.
Information representative of data is stored on the surface of the storage disc. Disc drive systems read and write information stored on tracks on storage discs. Transducers, in the form of read/write heads attached to the sliders, located on both sides of the storage disc, read and write information on the storage discs when the transducers are accurately positioned over one of the designated tracks on the surface of the storage disc. The transducer is also said to be moved to a target track. As the storage disc spins and the read/write head is accurately positioned above a target track, the read/write head can store data onto a track by writing information representative of data onto the storage disc. Similarly, reading data on a storage disc is accomplished by positioning the read/write head above a target track and reading the stored material on the storage disc. To write on or read from different tracks, the read/write head is moved radially across the tracks to a selected target track. The data is divided or grouped together on the tracks. In some disc drives, the tracks are a multiplicity of concentric circular tracks. In other disc drives, a continuous spiral is one track on one side of disc drive. Servo feedback information is used to accurately locate the transducer head. The actuator assembly is moved to the required position and held very accurately during a read or write operation using the servo information.
The actuator is rotatably attached to a shaft via a bearing cartridge which generally includes one or more sets of ball bearings. The shaft is attached to the base and may be attached to the top cover of the disc drive. A yoke is attached to the actuator. The voice coil is attached to the yoke at one end of the rotary actuator. The voice coil is part of a voice coil motor which is used to rotate the actuator and the attached transducer or transducers. A permanent magnet is attached to the base and cover of the disc drive. The voice coil motor which drives the rotary actuator comprises the voice coil and the permanent magnet. The voice coil is attached to the rotary actuator and the permanent magnet is fixed on the base. A yoke is generally used to attach the permanent magnet to the base and to direct the flux of the permanent magnet. Since the voice coil sandwiched between the magnet and yoke assembly is subjected to magnetic fields, electricity can be applied to the voice coil to drive it so as to position the transducers at a target track.
In disc drives with relatively high track densities, a servo feedback loop is used to maintain a transducer 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 angularly spaced sectors that are interspersed among the data on a disc. During track following, the servo information sensed by the transducer head is demodulated to generate a position error signal (PES), which is an indication of the position error of the transducer head away from the track center. The PES is then converted into an actuator control signal, which is fed back to control an actuator that positions the transducer head.
In general, there are two forms of transducer head positioning errors: repeatable and non-repeatable. Non-repeatable errors are generally unpredictable and therefore cannot be removed until after they occur. Repeatable errors, which are generally caused by mechanical irregularities in the structure of the disc drive or errors introduced when writing the servo tracks, can be predicted and therefore theoretically can be canceled out as they occur. In general, these repeatable rotational runout (RRO) errors are removed by introducing a compensation signal into the loop that cancels the repeatable positioning error. Techniques for generating such compensation signals are generally referred to as feedforward cancellation.
Because the feedforward cancellation signal is introduced into the servo loop, it can cause the servo loop to become unstable under certain conditions. In particular, if the cancellation signal is too large for a given PES, the cancellation signal can cause the transducer head to oscillate across the track center line, thereby keeping the transducer head from reaching a steady state position over the track.
To avoid this problem, the prior art has developed adaptive feedforward cancellation (AFC). Under AFC, the cancellation signal is initially set to zero. The position error signal is then measured at a first sector and is used to set the amplitude of the cancellation signal for the next sector. To avoid instability, the PES is multiplied by a learning rate, which is between zero and one. Under some systems, the learning rate is reduced at each successive sector to further ensure stability while improving the likelihood that the cancellation signal will fully cancel the RRO.
One problem with current AFC technique is that an increased learning gain can amplify the PES at the neighboring frequencies around the one to be canceled. Generally to achieve a faster AFC learning, larger learning gain is needed. Larger learning can result in amplifying the neighboring frequencies, which in turn can result in an increased disturbance rejection. Another problem with amplifying neighboring frequencies when the actual once-per-revolution disturbance is small, is that the measured PES RRO with AFC on can be slightly smaller than the one with AFC off, and for non repeatable runout errors (NRRO), the measured PES NRRO with AFC on can be higher than when the one with AFC off.
What is needed is an AFC that can use a larger gain for faster convergence of feedforward adaptation without significantly amplifying the neighboring frequencies.
A disc drive having a disc drive controller for positioning a transducer head over a track includes a servo controller for generating a modified control signal for reducing a RRO. The disc drive includes an actuator arm assembly comprising an actuator and a transducer head. The actuator is capable of moving the transducer head based on at least in part on the modified control signal from the servo controller, and further the actuator arm assembly is capable of generating a position error signal based on the position of the transducer head over a current track. The disc drive controller further includes an adaptive feedforward cancellation component for producing a correction signal for a next track based on the position error signal received from the actuator arm assembly. The disc drive controller further includes a high pass filter, coupled to the adaptive feedforward cancellation component, having a high pass filter transfer function to decrease a gain of unwanted frequencies when increasing a learning gain of the correction signal to achieve a faster convergence of the adaptive feedforward cancellation. The disc drive controller then produces a modified control signal for the actuator arm assembly using the filtered correction signal to reduce the RRO on a next track.
Also discussed is a method of producing a modified control signal to an actuator arm assembly of a disc drive to reduce RRO includes the step of generating a correction signal corresponding to a position error signal of a current track. Then the method includes determining a high pass filter transfer function of a high pass filter to decrease a gain of an unwanted frequency when increasing a learning gain of the correction signal. Then the next step includes forming filter parameters from the high pass filter transfer function, and to filter the correction signal by passing the correction signal through the high pass filter formed using the high pass filter parameters. Then generating a modified control signal to the actuator arm assembly of the disc drive using the filtered correction signal to remove RRO on a next track.
Advantageously, the procedure set forth above and the apparatus for generating the modified control signal to remove RRO can produce a faster convergence without loss of performance in a disc drive. The above procedure can be incorporated in microcode and used to control the servo circuitry to implement the invention.