History of Hard Drives
The hard disk drive was invented by some IBM engineers working under Rey Johnson at IBM in San Jose, CA, in about 1952 to 1954. In 1965 a number of engineers left IBM (they were known as the "dirty dozen" within IBM) and founded Memorex. Al Shugart, one of them, later left Memorex and founded first Shugart Associates where the 5 1/4" floppy disk drive was a major product, then Seagate Technology, which effectively started today's industry of small hard disk drives. The early drives almost all had linear actuators, that is, they moved the heads across the disks in a straight line, using a carriage with wheels. It was only later that rotary actuators, where the heads are held at the tips of a comb-like array and they swing back and forth like a gate, became popular. Because the rotary actuator is cheaper, it's now the standard for all hard disk drives. The first IBM RAMAC disk drive had a couple of dozen disks, each about 2 feet in diameter, and one head. The head was moved from disk to disk and back and forth on each disk using a system of cables and pulleys and stepping motors. The added speed of having at least one head for each disk surface, and of using both surfaces of each disk, soon became obvious, and drives began to look pretty "modern" by 1960, although they were vastly larger and more expensive. Whether the heads are moved in a straight line or swung in an arc, something has to provide the force and the control to move them and keep them in the right place. Stepping motors, hydraulic actuators and voice coil motors have been used to provide the motive force. Stepping motors have a built-in capability to hold in one position. Hydraulic actuators and voice coil motors (VCMs) provide force, but can't hold a position with great accuracy. So you can have a disk drive with a stepping motor and you don't need a "servo" or you can have a disk drive with "servo data" recorded on the disks. A stepping motor is simpler and cheaper, but...
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