One of the mechatronic Top Ten applications has to be the hard disk drive. Strangely, it is not an application that you hear much about. That’s probably because unless you work on hard disk drive design, you pretty much take for granted that little black box that stores all your information. So the group that is actually pushing the design frontier of hard disk drive technology is a very finite group. There are only a dozen companies actually making disk drives these days, after consolidation in the market has resulted from acquisitions and mergers over the last decade.
Worldwide consumption of hard disk drives is in the tens of millions per year, and like all things electronic and high volume, the industry produces ever more memory at ever lower prices. The absolute value of hard disk technology is one of the most incredible bargains in the world. The current state of the art is about 10 cents per gigabyte which is quite a bargain compared to the 1.5 Megabytes for the old 3.5″ mini floppy disk. With seek times in the low milliseconds, memory is almost instantly available due to 7200 RPM platter speeds. The 7200 RPM speed is the equivalent of 75 miles per hour at the edge of the platter. Higher speeds have been delivered, but the thin aluminum platter is subject to “flutter” which can cause a head crash.
The spindle motor is a 3 phase dc brushless motor that is designed to accelerate the memory platter to the 7200 RPM running speed in just 2 or 3 milliseconds. This is an incredible feat considering that the power available is limited to a small lithium battery. Further, the spindle motor must coordinate it’s motion with a linear actuator to place the drive’s read head a few millionths of an inch above the platter surface at the exact target sector on the disk. So, just getting the platter to spin, which is hard enough given the time constraints, is further complicated by the extreme challenge of coordinating the rotational motion with the linear motion of the read head.
What makes this all even more astounding is that the budget for the motor can only be a few dollars, given a retail selling price of $60 for the whole package including the memory. I don’t know how these guys come up with the solutions, but they consistently do and they consistently do it at lower prices. The last thing I remember reading about was the elimination of bearings in favor of fluidized bearings. At 60 million units, saving money on bearings adds up to a lot of money.
One of the many ironies of the hard disk drive is that it is at the root of many improvements in industrial motion control. The venerable 33035 controller chip from Motorola was developed specifically to run hard disk drives. It later appeared in a number of industrial servo amplifier designs delivering precise control of higher current power to a variety of brushless dc servo motors.
You never know where the breakthroughs are going to come from, but we keep them coming. Keep up the good work!