1. Field of Invention
The present invention relates to disk drives and particularly to a process for loading the actuator arm from a ramp to a position on a disk in a disk drive.
2. Related Art
A portion of a disk drive 100 is shown in the plan view of FIG. 1. A typical disk drive includes a disk 102 and an actuator arm assembly 104, which may consist of one or more parts, including a base plate, a load beam, and/or a flexure. Actuator arm assembly 104 supports a slider towards one end of the actuator arm and on the disk side of the actuator arm. Mounted to the slider is a magnetic element used to write data and an electronic element used to read data. The combined structure of the slider, the magnetic element, and the electronic element is referred to herein as a head 106. The actuator arm assembly 104 acts as a suspension system for the head 106 that allows the head to "fly" over the surface of the disk 102 on an air flow created by rotation of the disk. Typically, the head 106 will position itself, i.e., establish an air bearing, to fly approximately 2 .mu.m over the disk surface. When the head is in a position allowing it to "fly" over the disk, the head is generally said to be "on" the disk, although no physical contact with the disk is generally had.
Actuator arm assembly 104 also pivots radially to move the head 106 to a specified radial position on disk 102. The force necessary to pivot actuator arm assembly 104 is created by a voice coil motor 108, which includes a coil and a magnet structure (not shown) positioned about the coil. The magnet structure and the coil are arranged so that the coil is placed in the magnetic field created by the magnet. Currents running through the coil in opposite directions interact with the magnetic field to create torques in respective opposite directions so that actuator arm assembly 104 may be pivoted to position the head 106 at a selected location between the inside diameter 103 and outside diameter 105 of disk 102.
To avoid data loss, typically, when the disk or the disk drive is not in use, the actuator arm is moved to a position away from the data-carrying portion of the disk. Referring to FIGS. 1 and 2, one way to avoid data loss is to pivot the actuator arm assembly to a resting position on a ramp positioned near outside diameter 105. To do so, the actuator arm assembly 104 is brought to a position near the outside diameter 105 of the disk 102. A ramp interface mechanism such as the tab extension 110 shown in FIGS. 1 and 2 is then able to make contact with a ramp 112. As the actuator arm assembly 104 is moved further in a clockwise direction, the actuator arm assembly 104 is moved up ramp 112, away from disk 102.
When required, the head 106 is loaded onto the disk 102 where reading and writing can occur. To load the head, the actuator arm assembly 104 is pivoted in a counter-clockwise direction. The ramp interface 110 progresses down ramp 112 towards the disk. The nature of the head suspension system allows the head to dangle when unloaded from the disk, so that when the head 106 first reaches a position above the disk, the head 106 is not always perfectly positioned parallel to the disk surface. When brought into the disk's air flow, however, the suspension system enables the head to reposition itself through spring-like and gimbaling actions.
Because the head is often not held parallel to the disk surface when unloaded, if the head 106 is loaded too quickly, the head may not have an adequate opportunity to reposition itself. In some situations, the corner or other parts of the head may come in contact with disk 102 before a proper parallel position is established. Such contact will tend to damage data stored on disk 102. Such contact could also significantly affect the time it takes to establish a proper position as any contact may cause the head to have erratic and other wobbling-type movements from which the head will have to recover. Therefore, actuator arm assembly velocity is a significant factor in loading the head onto the disk 102. The slower the load, the less risk that any damage to the disk or significant wobbling of the head 106 will occur.
Nevertheless, controlling and establishing slow load velocities is problematic. When the head is not loaded, the only method available in current disk drive designs to get an approximation of actuator arm assembly velocity is to measure the back EMF (BEMF) of the VCM. Further, the torque needed to overcome various forces (e.g., friction between the ramp interface 110 and ramp 112, torque from the printed circuit cable (not shown), and/or friction at the actuator pivot point) is significant, tending to be much higher than the additional torque required to generate a low velocity movement. Thus, the VCM voltage due to currents applied to generate low velocities is very high compared to the BEMF generated due to the velocity of a slow moving actuator, and accuracy in velocity measurement and control is difficult to obtain. To compound matters, in order to maintain a constant velocity a change in torque is required when the tab extension 110 moves from the flat portion 113 to the sloped portion 114 of ramp 112.
Moreover, consumers often seek cheap, physically small disk drives. Therefore, adding parts and/or other electronics to a disk drive to control velocity is not generally desirable.
Therefore, a method for controlling the actuator arm assembly load velocity is desirable where such a controller would be relatively inexpensive to implement and would be able to establish accurate control of the actuator arm assembly at low velocities.