- Radial head alignment determines the exact position of the drive's read/write head over a given track location. As discussed above, this parameter is by far the most critical and most common alignment problem. For instance, a standard 3.5 inch drive with its read/write head displaced from the nominal standard more than +/- 600 millionths of an inch, would be considered out of specification. This drive would probably have difficulty reading a diskette written on another drive, especially if the other drive was misaligned in the opposite direction.
- Azimuth head alignment is the angle, in fractions of a degree (minutes), of the read/write head on its vertical axis. Like the radial parameter, azimuth is very critical for reliable data exchange between drives, but the incidence of occurrence of a read/write head with incorrect azimuth causing an interchange problem is somewhat less. This is because the drive's head positioner mechanism routinely moves the heads radially going from track to track, however the azimuth alignment is fixed at the factory and should never change unless the drive is damaged or worn badly. Like the radial parameter, if the read/write head has poor azimuth then it may have difficulty reading a diskette that was written on a drive with a proper azimuth angle, or worse yet a drive with its azimuth angle incorrectly adjusted in the opposite direction from nominal.
- Index timing head alignment relates to the starting point on each track where data is stored. The starting point is determined by the placement of an index sensor on the drive which triggers once with each revolution of the diskette and serves as the reference point. Should this sensor be out of adjustment, then the starting point of the data written to the diskette will be altered. Fortunately, most modern soft sectored disk formats used with personal computers today don't rely heavily on this parameter being perfectly adjusted. Therefore it is seldom a serious problem unless severely out of adjustment or the index sensor has failed completely.
Beyond just the read/write heads, there are other parameters to be adjusted and maintained within the drive which also are commonly referred to as "alignment":
- Head Positioner Linearity. This is the ability of the drive's head positioner mechanism to maintain the radial alignment of the read/write heads to be constant throughout all of the available tracks on the diskette. Since the track spacing of the recorded tracks should be constant across the entire surface of the diskette (96 tracks per inch in the case of the HD 5-1/4" for example), it is very important that the drive being used to store data or programs be able to maintain its radial alignment very precisely across this entire area. A defective drive might have perfect radial alignment at one track location, but exhibit considerable error at another area of the diskette's surface, which will cause a failure when read by another drive which has no linearity problem.
- Motor Spindle Eccentricity. This is defined as the wobble that a diskette may have as it spins in the drive. Ideally, the diskette should rotate in the drive such that the invisible tracks are a fixed distance from the center of the diskette during the entire rotation. When a drive has an eccentricity problem, this results in radial alignment which varies as the diskette turns each revolution. If this happens, some recorded sectors might have good radial alignment, while others are off-track. This will result in failures very similar to a radial alignment problem except that some sectors of a given track may be readable while others will fail.
- Track 0 Sensor Adjustment. This is the physical adjustment of the drive's track 0 sensor. In most drives, this is an optical sensor which must be adjusted such that when the system is first powered on, or recalibrated, (ie, blindly sent to track 0 by the controller), the drive's heads are able to be placed accurately at the starting track 0. The computer's floppy controller relies on this sensor to establish a starting point from which all other tracks are referenced.
- Index Timing. Index timing is a standardized fixed angular location between the drive's index sensor which triggers with each revolution of the diskette, and the read/write heads. Once the two heads are adjusted to have very close to the same angular location relative to each other, then relative time between the index sensor and the read/write heads must be set. This is usually a straightforward physical adjustment of the drive's Index sensor, to bring it to within specification.
- Index Skew. This is the drive's ability to maintain the index timing to be constant along the complete travel of the read/write heads in order to access all of the tracks recorded on the diskette. If the Index timing is good at track 0, but out of specification at the innermost track, then the drive would be said to have unacceptable index skew. This is not a flaw in the read/write heads, but rather a misalignment of the head positioning mechanism, which is not able to move on the correct line as it should. If the mechanism was out of line enough, it could also have a significant effect on the head's azimuth, which would change from track to track as the heads traveled more and more out of line due to the mechanism being skewed.
- Motor Spindle Speed. This parameter simply indicates how fast the drive is spinning the diskette. It is usually expressed in revolutions per minute or sometimes in milliseconds per revolution. The drive should be able to spin the diskette at a correct and constant speed. If the drive spins the diskette too slowly, the recorded data will arrive at the floppy controller at a frequency which is too low. If the drive spins too fast, the frequency will be too high and when writing information, the controller will not have enough time within the time frame of one revolution of the diskette to record the necessary data fields for reliable operation.
