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| Solenoids Used in Disk Drives |
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Head-positioning solenoids for today's smaller, denser disks must meet higher performance demands. To meet these requirements, a solenoid must produce high pull-in and holding forces in a small package and generate little heat. Either tubular or open-frame solenoids can be used in disk drives. Both maintain high reliability and repeat-ability, and produce no contamination.
Solenoid force depends on three major factors: current, magnetic properties, and stroke. Solenoid force is directly proportional to current; thus, if the current is repeatable, the force also must be repeatable. A practical limiting factor, however, is friction which must be minimized with a good bearing system.
Using a magnetic material with non-uniform properties in the solenoid flux path can account for small unit-to-unit variations in force output. If the material is steel, these variations can be largely overcome by annealing.
Finally, a general-purpose solenoid usually produces a force inversely proportional to stroke. Pull force increases as the plunger seats, and energy can be arbitrarily distributed such that the force vs. stroke curve can take almost any shape. This is frequently done by placing a cone on the plunger and matching the geometry to the mating pole piece. A rule of thumb is that the starting force increases as the cone angle becomes shallower. However, a point is reached where radial attraction is so great that the friction is higher than the force. Thus, the solenoid cannot energize.
Both high starting and ending forces are needed for a drive solenoid. Unfortunately, a single cone cannot handle the requirement, but a combination cone can. The cone must have a shallow angle for starting, and a flat surface for holding the plunger.
The flat cone section and mating face need little power to produce a holding force of 100 psi. Usually, a small solenoid coil can do the job. However, more power or a larger solenoid are typically needed to produce a significant pull-in force. Fortunately, the same small solenoid can be used for both holding and high pull-in force because pull-in time is only a few milliseconds; thus, heating due to high power is low. The current can be dropped back to the holding level which does not produce significant or damaging temperature rise.
Operating small solenoids from a voltage source provides a built-in protection feature. As the solenoid heats, the wire resistance rises and drops the current, which finally reduces the force. However, small solenoids have small thermal mass, so it is important to keep the power low for minimum heating in order to maintain the force at an acceptable level.
A solenoid is normally cooled by both heat conduction and air convection, and must be explicitly considered for each application. In most cases, convective air flow across a solenoid is sufficient to keep it cool. But for a sealed disk drive, air convection may not be possible, so heat conduction must be employed.
Heat must be dissipated from inside the coil to outside ambient air. Unfortunately, a direct path usually cannot be provided, so a good thermal conductor must be selected for the interface medium.
Solenoid failure is usually caused by either excessive heat or wear. Assuming that the bearing surface provides adequate wear resistance, the solenoid assembly must have enough insulation to protect it from excessive heat. Heat can cause at least three failure modes: bobbin collapse, coil shorts, and dielectric failure.
Collapse can be prevented by making the bobbin from a glass filled plastic that has a higher deflection temperature than an unfilled one. Coil wire shorts can be reduced with high temperature-resistant insulation. Finally, dielectric failures can be reduced by providing sufficiently thick insulation around the coil.
Another important design consideration is protection from dust or debris contamination and outgassing. Because the solenoid is installed within the drive and completely sealed from ambient air, the solenoid must not produce foreign matter. Particles as small as 30um can cause a head crash, and outgassed water condensation, for example, produces harmful corrosion. Ideally, the solenoid should be totally enclosed in a dustproof jacket to contain any particle that becomes mobile.
Aside from initial contamination, other extraneous material is produced by plunger motion in the bore. To reduce such wear particles, the plunger should be especially smooth, and the bobbin should be made of a molded plastic, such as Teflon, that provides some self-lubrication.
Solenoids are often washed in a special cleaning solution or plain water. Unfortunately, water corrodes unprotected metal and fills voids, only to be outgassed later. Plastic encapsulation, however, fills the voids to keep out cleaning solvents and enhances thermal conductivity.
Other contaminants that cannot be avoided completely are produced by particles emitted from pole face impact. Impact surfaces can be coated with a hard material, such as nickel or chrome, to help reduce the problem. Also, impact surfaces should be located deep within the solenoid, so loose particles are forced to migrate relatively long distances before possibly becoming mobile.
The decision to use open-frame or tubular solenoids depends on the final product envelope. As a rule, open frame solenoids are rectangular, and tubular solenoids are cylindrical. The open-frame design is somewhat less expensive to manufacture, and the tubular solenoid is more efficient for the same volume.
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