The Spinning Wedge Implementation, Part 3 – The Tests

Armed with a fully-functional (at least for the time being) high energy voice coil actuator circuit, I set out to try several hard drives from my junk collection. These ranged from relatively new (see this post) to an ancient, 2-inch-thick monster with something like 6 to 8 platters (this one had very interesting construction).

The Kodak MAX-derived supply was charged until it shut off, and the voice coil was “fired” in one direction with a one-millisecond pulse. To know how charged it was, I had a multimeter carefully clipped to the capacitor – and the high-side collectors. Whatever leaks in the circuit (could it be just the capacitor?) makes this thing seep voltage pretty fast, so I tried to fire as soon as I could after the supply stopped charging the capacitor.

Using the “afterthought” flash trigger I set up, I took photos at 1.0, 1.5, and 2.0 milliseconds after the leading edge of the input trigger. Thus in the animations you see below, each frame is a separate firing action, so the voltage is probably not exactly the same for all of them (and neither may the relative position of the drive in the picture—though I aligned them as best as I could). I used my Sunpak 622 for illumination (set to 1/128th power) and measured its illumination pulse width to be about 100 microseconds. I tuned the timing so that the flash delays mentioned above were measured to the half-power point of the flash pulse.

The room was pretty dark, and I used a digital camera set to a 2 second exposure so that I could manually fire the voice coil so that it would “land” on a picture. The aperture on the lens was tuned to provide enough light given the flash’s output. Since the room was not absolutely dark, you may notice a red “ghost” image of the hard drive head; this is its position at “neutral” (which, for one of the drives, was not always the same). I didn’t use a spring or anything else to hold the head off to one side; I just let it sit where it does (which is governed by the flex-circuit going to the voice coil and the heads themselves for all drives but one).

I had an oscilloscope monitoring the signals: the TTL input to the optoisolator, whose leading edge was considered the “fire” time, the optoisolator output into the IRS2183 input (to check for inductive spikes, etc.), the flash pulse (via the very simple photodetector circuit from Sam’s site), and the Kodak’s capacitor’s voltage.

The expended energy is calculated from the measured capacitor voltages before and after firing:

E = \frac{1}{2}C\left(V_0^2-V_1^2\right)

The average power is simply this amount divided by 1 millisecond (the pulse length of the discharge). So if the energy is in Joules, then the average power is the same number in kilowatts. The speed of the head (in RPMs) is simply the angle traversed divided by the time between the leading edge of the power pulse and the photo being taken, so it is an estimation of the average speed for that period.

The results are really fascinating. There is actually motion blur in the heads—with a 100 microsecond exposure!

Drive 1

This one I had ripped the flex circuit off, so after much patience, I managed to peel the coil wire (without destroying it) and soldering some wire to it. The thickness of the big wires sure didn’t help performance. Also, since AC current flows on the surface of conductors, it is probably best to use solid wires, but I didn’t have any on hand (and it would have gotten in the way even more). You can also see in the 1.5 millisecond frame that the head had started in a different place than the others. Really, this wasn’t a reliable test. Still, I include it for completeness—since it was one of the two drives that survived (if you don’t count the last one).

animated_1.gif

  • Energy expended: 4.74 J (4.74kWs)
  • Angle traversed at 1.0ms: 6.3 deg (1050 RPM)
  • Angle traversed at 1.5ms: 16.2 deg (1800 RPM)

Although it’s hard to tell since the drives were not guaranteed to stay put after a firing, it seems like this one’s head is on the verge of breaking off.

Drive 2

animated_2.gif

  • Energy expended: 3.26 J (3.26kWs)
  • Angle traversed at 1.0ms: 12.8 deg (2133 RPM)

This drive was fast. But it didn’t last. The head bearing was held to the chassis by a hollow shaft that sheared off after the first firing, presumably when the head hit its physical stop.

Drive 3

animated_3.gif

  • Energy expended: 4.88 J (4.88kWs)
  • Angle traversed at 1.0ms: 14.6 deg (2433 RPM)
  • Angle traversed at 1.5ms: 27.4 deg (3044 RPM)
  • Angle traversed at 2.0ms: 36.1 deg (3008 RPM)

This was a fast, solid drive. At 2 milliseconds, it had hit its stop, and since this one has a “resting” position near one end of its swing, this represents the maximum stroke. Recall from previous analysisthat we need over 100 degrees of travel on each side of the shutter for the wedge to have minimum inertia. This is not possible with a stock hard drive head—the two options are to use different magnets that allow for longer travel, or make the wedge long enough so that these 30-odd degrees are enough to clear the lens opening needed. According to the last lens tests, the lens opening necessary will be at most around 30mm; at 36 degrees this means the shutter arm length (wedge radius) must be around 120mm (almost 5 inches). The performance is bound to suffer with such a long arm, and because these accelerations are pretty violent so the wedge’s structural integrity will be a limitation. With enough identical drives, voice coils could be mounted to work together, especially since it would probably be possible to machine them down and fit two on the same bearing (magnets and all). This would effectively double the force if the power supplies are independent. Note also that, depending on how much abuse the coils can take, it may be possible to use a bigger capacitor and longer pulse widths to pump more current through them.

Drive 4

animated_4.gif

  • Energy expended: 4.94 J (4.94kWs)
  • Angle traversed at 1.0ms: 12.3 deg (2050 RPM)

This drive didn’t do so well. The coil’s frame (or “cradle”—that’s my word for it) is made of plastic (all the other drives had aluminum ones). The minute it slammed into its physical stop half of it broke off.

Drive 5
animated_5.gif

  • Energy expended: 5.61 J (5.61kWs)
  • Angle traversed at 1.0ms: 4.4 deg (733 RPM)

This was the ancient giant. Six or eight platters (can’t remember), and the only one with a significantly different voice coil design. First, its energy consumption was huge, with little performance (probably because the head is so bulky). Instead of the coil sitting flat in an air space between two magnets, this one is actually wound into two slots in a stack of magnets—it is impossible to separate the coil and the magnets. This drive was strong, but slow, and after the first firing there was a pretty strong spike in voltage in the storage capacitor and the input of the IRS2183 (and probably everywhere else in the drive circuit) right at the shutoff of the coil current. This actually disabled the drive circuit, and I thought it had fried one of the IGBTs, but by the time I had taken the setup apart to check the parts, I couldn’t find a problem, and a subsequent test showed the circuit seems to be fine. Still, I didn’t want to risk it, so I didn’t fire it again.

Conclusion

Smaller drives are better. I don’t mean smaller capacity (although it may still correlate to some extent), but generally it seems that the lower the number of platters the better, because the head will have less stuff hanging off it. Of course that may not matter much in the long run because the shutter wedge will replace all that.

The most important conclusion didn’t require driving the voice coils at all—they do not swing far enough to implement the wedge with a minimum inertia. Getting around this will require custom magnets (as in magnets not from the hard drive) or coping with a long shutter arm.

I’m going to see if I can get my hands on a laptop drive.

Edit: as far as extending the swing by using different magnets, this is only possible in the construction like that of drive 5. More on this later.


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