Sine seek adaptation at voltage and temperature margins

Details Sine seek adaptation at voltage and temperature margins Sine seek adaptation at voltage and temperature margins

1999-09-03

2003-02-04

Hudspeth, David (Department: 2751)

Dynamic magnetic information storage or retrieval

Automatic control of a recorder mechanism

Controlling the head

Reexamination Certificate

active

06515820

ABSTRACT:

FIELD OF THE INVENTION
The invention relates generally to disc drives and specifically to disc drive seek controllers. More particularly, the invention relates to sine seek controllers operating within temperature or voltage margins.
BACKGROUND OF THE INVENTION
Disc drives are used in workstations, laptops and personal computers to store large amounts of information in a readily accessible form. Typically, a disc drive includes a magnetic disc which is rotated at a constant high speed by a spindle motor. The disc surfaces are divided into a series of concentric data tracks that can store information as magnetic transitions on the disc surface.
A disc drive also includes a set of magnetic transducers that are used to either sense existing magnetic transitions during a read operation or to create new magnetic transitions during a write operation. Each magnetic transducer is mounted in a head. Each head is mounted to a rotary actuator arm via a flexible element which can accommodate movement of the head during operation. The actuator arm serves to selectively position the head over a particular data track to either read data from the disc or to write data to the disc.
The actuator arm is driven by a voice coil motor. The magnetic transducers, mounted in the heads, are present at the ends of the arms which extend radially outward from a substantially cylindrical actuator body. This actuator body is moveably supported by a ball bearing assembly known as a pivot bearing or pivot bearing assembly. The actuator body is parallel with the axis of rotation of the discs. The magnetic transducers, therefore, move in a plane parallel to the discs surface.
The voice coil motor typically includes a coil which is mounted in the actuator arm at the end opposite the heads. This coil is permanently immersed in a magnetic field resulting from an array of permanent magnets which are mounted to the disc drive housing. Application of current to the coil creates an electromagnetic field which interacts with the permanent magnetic field, causing the coil to move relative to the permanent magnets. The voice coil motor essentially converts electric current into mechanical torque. As the coil moves, the actuator arm also moves, causing the heads to move radially across the disc surface.
Control of this movement is accomplished via a servo system. In this control system, position (or servo) information is prerecorded on at least one surface of one of the discs. The servo system may be dedicated, which means that an entire disc surface is prerecorded with servo information. In this case, a particular head is dedicated to reading only servo information. Alternatively, the servo system can be embedded meaning that the servo information is interweaved with the user data, and is intermittently read by the same heads which are used to read and write information.
Servo systems typically include two controllers, a seek controller and a tracking controller. The seek controller manages large head movements for approximate placement of the actuator arm. Then, the tracking controller is responsible for the small displacements necessary to follow a particular track.
High performance disc drive customers require a drive's operation to be verified over a wide range of conditions, such as hot and cold temperatures and high and low voltages. In addition, the environments in which drives are implemented may often demonstrate similar conditions. These conditions greatly affect physical characteristics of drive components, thus changing drive operation. High temperatures and low voltages, for instance, tend to degrade performance and can lead to permanent drive damage, directly or indirectly. Therefore, it is necessary to design disc drives such that they can be operated in a wide range of temperature margins.
One of the main problems with drive operation over a wide range of temperatures is servo coil resistance variation. The higher the temperature, the higher the resistance, and vice versa. Since the servo controller is designed to seek from one position to another in a minimum amount of time, the controller often tends to put the coil current in saturation, i.e., the controller demand is larger than the available current due to limits on the voltage supply and coil back electromotive force, during the acceleration phase to accelerate as fast as possible. The higher the coil resistance, the lower the coil current saturation point, thus limiting the amount of acceleration and deceleration that can be achieved. When the coil current is in saturation, the actuator drive can not supply any more current, regardless of the controller's demand. Because of this, the controller is limited in its controlling abilities and it can only decrease the acceleration. The same is true for the deceleration phase of the seek. If the coil current is in saturation, the controller can only decrease the “braking” force. This is a problem if the seek operation is moving too fast for the allowable deceleration to stop the actuator on the desired destination. This leads to significant overshoot and an increase in seek time. In addition, calibrations that are performed during the deceleration phase will produce inaccurate results under this condition.
In addition to the fluctuation in temperature margins, higher or lower nominal voltages similarly appear as lower or higher coil resistance to the servo controller, respectively. Therefore, voltage margins can also cause significant overshoot problems and inaccurate calibrations.
Because of these conditions, it has been necessary to detect sub-optimal conditions during a seek operation and adjust the controller to compensate for them. In the past, current in the acceleration saturation phase was measured and if it was lower than a nominal value, the demanded velocity signal would be scaled down by a scalar value. This scalar value, labeled SDEM (slope-demand), is calculated according to equation (1) below:
SDEM
=(measured current average)/(
I
—
NOM
1
),  (1)
where I_NOM is a minimal current value that is determined empirically. The higher the coil resistance or the lower supply the voltage, the lower the measured current would be. Thus, the demand velocity would be scaled down, thereby preventing the deceleration phase of the seek operation from entering saturation. Any higher than nominal conditions resulted in no change to the demand velocity signal since there is no risk of entering saturation in the deceleration phase. This was accomplished by clipping the maximum value of this scalar value to one.
However, with the introduction of sine seek controllers, i.e., a seek controller that is open-loop until the peak of the deceleration phase is reached, simply scaling down the velocity demand is not enough. While scaling down the velocity demand does help prevent deceleration saturation at low voltage and/or high temperatures, another problem is introduced. Sine seek controllers operate by looking for a constant amount of velocity error between the demand velocity and the estimated velocity to decide when to switch from acceleration to deceleration. When the velocity error is less than this fixed value, the switch is made. At that switch point, the sine shaped feed forward current transitions the controller from acceleration to deceleration. Once the feed forward sequence has completed, i.e., the peak of the deceleration phase is reached, the controller will close the loop and return to velocity control.
FIG. 1
is a graph of an estimated velocity profile for a sine seek controller under nominal conditions.
It can be seen from
FIG. 1
that velocity
11
is plotted along the vertical axis and tracks to go
13
is plotted along the horizontal axis. The desired position
15
on a disc, i.e., the point to where the controller is seeking to, is located at the origin. The demand velocity curve
10
is a table of velocity values at various distances from the desired position. The demand velocity curve
10
is stored in a memory (not shown) accessible to the controller.
Thus, as can

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