Programmable write current waveform for high frequency...

Details Programmable write current waveform for high frequency... Programmable write current waveform for high frequency...

1999-07-01

2001-10-09

Hudspeth, David (Department: 2651)

Dynamic magnetic information storage or retrieval

General recording or reproducing

Specifics of the amplifier

C360S040000

Reexamination Certificate

active

06301068

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to disc drive data storage systems. More particularly, the present invention relates to methods of, and apparatus for, writing data to magnetic media at high speeds.
BACKGROUND OF THE INVENTION
A typical disc drive includes a drive controller, an actuator assembly and one or more magnetic discs mounted for rotation on a hub or spindle. The drive controller controls the disc drive based on commands received from a host system. The drive controller controls the disc drive to retrieve information from the magnetic discs and to store information on the magnetic discs. The actuator assembly includes an actuator coupled to the drive controller and an actuator arm for supporting a head-gimbal assembly over each magnetic disc. The headgimbal assembly carries a data head comprising a hydrodynamic air bearing and a transducer for communicating with the surface of the disc.
The actuator operates with the drive controller in a servo system. The actuator moves the data head radially over the disc surface for track seek operations and holds the transducer directly over a desired track on the disc surface for track following operations.
The transducer includes an inductive coil which reads data from and writes data to the magnetic disc by sensing or creating a changing magnetic field. A read/write preamplifier is connected to the transducer at first and second head contacts. The preamplifier includes a read circuit and a write circuit for controlling the read and write operations. The write circuit includes a write driver circuit which is connected across the head contacts. During write mode operation, the write driver circuit forces a write current through the inductive coil to create a magnetic field that polarizes adjacent bit positions on the recording surface. Digital information is stored by reversing the direction of the current flow in the inductive coil. This reverses the polarity of the magnetic field induced in the coil. This reversing of the polarity of the magnetic field in the coil is referred to as a flux reversal. Each flux reversal in turn reverses the polarization of selected bit positions. Each flux reversal represents a change from a logical “1” to a logical “0” or from a logical “0” to a logical “1”. The reversal of the direction of the write current flow in the transducer, and the corresponding flux reversal and reversal of the polarization of the recording surface, is referred to as a data transition or simply a transition.
The write driver circuit controls the direction of current flow through the inductive coil. The write driver circuit applies a limited voltage swing across the head contacts for reversing current flow and polarizing the adjacent bit position. The rate at which information can be stored on a recording surface through the magnetic head is directly proportional to the rate at which the direction of current can be reversed in the inductive coil. The length of time between two consecutive data transitions is referred to as the transition spacing. For a good write process, it is desirable to deliver a large swing in the write current in a time substantially shorter than the minimum transition spacing for the desired transfer rate. In other words, it is desirable to produce short rise times and fall times.
The strength of the magnetic field produced by the transducer which is required to switch the polarity of the magnetic medium is called the critical field. In turn, the amount of write current through the transducer which is required to produce the critical field is called the critical current. For a good write process, it is desirable that the write current have a very sharp gradient at the critical current.
Historically, the write current target waveform has been as illustrated in FIG.
1
.
FIG. 1
shows the ideal write current I
WI
100
, the actual write current I
WA
102
and the magnetic field H
104
produced by the transducer as a function of time t
106
. The ideal write current I
WI
100
is a square wave having a magnitude slightly greater than the critical current I
C
. However, the inherent capacitance of the transducer and the inductance of the transducer limit the rate at which the current in the transducer can change. Thus, the actual write current I
WA
waveform
102
has a finite rise time t
ri
108
and fall time t
fi
110
as shown in FIG.
1
.
FIG. 1
also shows the magnetic field H
104
induced in the transducer by the write current I
WA
102
. As can be seen in
FIG. 1
, at high speeds the response of the transducer is not fast enough for the magnetic field response H
104
to track the shape of the write current waveform I
WA
102
. Thus, the magnetic field H
104
produced by the transducer has a relatively high rise time t
rh
112
and fall time t
fh
114
. Furthermore, at high data transfer rates, the gradient of the induced magnetic field H
104
at the critical field values H
C
are relatively low.
As data transfer rates have increased, one method used in previous art to compensate for the inability of the recording transducer to track the shape of the write current waveform was to increase the amplitude of the write current, as shown in FIG.
2
.
FIG. 2
shows the ideal write current I
WI
116
, the actual write current I
WA
118
and the magnetic field H
120
produced by the transducer as a function of time t
106
. The increased amplitude of the actual write current I
WA
118
results in a shorter write current rise time t
ri
122
and fall time t
fi
124
, and in turn a shorter magnetic field rise time t
rh
126
and fall time t
fh
128
. The increased write current amplitude also results in a sharper gradient in the write current I
WA
118
at the desired critical current I
C
, which in turn results in a sharper gradient in the induced field H at the critical field value H
C
. However, this method has the drawback that the transducer is deeply saturated at lower frequencies by the excessive write current. This magnetic saturation slows down the subsequent field reversal.
Another approach used in the prior art to reduce the rise and fall times and increase the gradient of the induced magnetic field is to reduce the damping of the transducer. This method has the drawback that the write current could ring excessively at the resonant frequency of the transducer when the next flux reversal is desired. This resonance can interfere with the next data transition, as the write current needs to be well settled prior to switching the direction of the current to ensure a proper flux reversal. Such an interference in one write signal which interferes with the transition to a new state is known as intersymbol interference (ISI).
There is a continuing need to reduce the rise and fall times of the transducer write field and increase the gradient of the write field at the critical field without strongly saturating the transducer core and without introducing excessive resonance in the write current.
The present invention provides a solution to this and other problems and offers other advantages over the prior art.
SUMMARY OF THE INVENTION
The present invention relates to methods and apparatus for reducing the rise and fall times of the transducer write field and increasing the gradient of the write field at the critical field without strongly saturating the transducer core and without introducing excessive resonance in the write current.
One embodiment of the present invention is directed to a method of writing data to a magnetic data storage medium. First, a first amount of write current is provided to a transducer positioned near the data storage medium. After providing the first amount of write current, the amount of write current provided to the transducer is reduced to a second amount of write current of the same polarity as the first amount. The second amount of write current is lower than the first amount but higher than an amount of write current needed to sustain the polarity of a bit of data on the magnetic medium. The second amount of write current is then mai

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