Jan 23, 2014

Harvesting Microwaves

posted by Bryon Moyer

We have just looked at an approach to wireless power transfer using low MHz electromagnetic field oscillations. But such a concept is only “power transfer” if the whole reason for the signal in the first place is to transfer power. If such a signal exists for some other reason – like communications – then doing the exact same thing wouldn’t be power transfer: It would be energy harvesting.

And indeed folks are trying to harvest energy out of all of the waves running rampant through our environment. The issue is efficiency, however, and you don’t really see a lot of practical discussion of this type of harvesting as being on its way to commercialization.

But there are some interesting things going on. And they involve “metamaterials” – artificial matter that can achieve characteristics – like a negative index of refraction or negative permittivity – that aren’t possible in nature. We took a look at some of these a couple years ago.

I have tended to think of metamaterials as the careful stacking and arranging of materials at the nano level; apparently that’s not always the case. Some folks at Duke University created a microwave energy harvester using a macro-sized metamaterial.

The fundamental unit of this “material” is the split-ring resonator. These can be very small, and would need to be on the order of 10s of nanometers across to respond to optical wavelengths, but the one Duke used was not that small: the outer diameter was 40 mm, and the gap was 1 mm. It was tuned for 900-MHz resonance.



Image courtesy 吴艺(Wikipedia contributor)

My initial thought was that these were made out of a metamaterial, but no: they’re made out of copper, and an array of these becomes the resonator. They used five of them (5x1 array) in their experiments.

It’s interesting to me that one of these rings bears a remarkable resemblance to the structure that WiTricity uses as source and capture resonators, albeit at lower frequencies. I suspect that’s no accident.

While simulation suggested they might get into the 70% efficiency range, their results were closer to 37%. There wasn’t really an explanation of that discrepancy; I’m going to assume that will be the focus of more work.

You can read more details about their work in their paper (PDF).

Late update: there's another "out of thin air" technology that's more than harvesting. It will be the topic of a future piece...

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Jan 21, 2014

Haptics in a Microcontroller?

posted by Bryon Moyer

TI caught my eye when they released a microcontroller that they said was “haptics-enabled.” A few seconds of thought convinced me that this concept needed some unpacking.

Haptics is all about devices providing feedback through some kind of touch mechanism. It could be as passive as raised bumps telling you that your fingers are in the right place, or it could be through vibrations or other active events that you can feel. It’s a hot topic, one we’ll probably be seeing much more of.

But… TI’s new MSP430TCH5E microcontroller is… a microcontroller. How can that generate haptic feedback? Does it have a specific hardware module for driving a specific vibratory engine? Seems unlikely, since haptics has lots of ways of being implemented; there’s no “mainstream” mechanism that’s suitable for hardening. Is there?

The release does talk of software libraries and SDKs. Could this be just about software? But… if so, why is it unique to this microcontroller?

I checked in with them, and the details of whatever the answer is are confidential; they’re not saying. But it does have to do with protecting IP. So my take on it is that this is a microcontroller/software bundle that includes haptic libraries. And you can’t use those libraries on other microcontrollers. Why not? Not sure… it could be the license: to get this you most likely have to promise to play by their rules. And if the solution is worth it, most upstanding businesses are not willing to risk legal hassles by playing games trying to port to another processor.

But it may also be that there’s some kind of hardware lock – something specifically put in place that the libraries interrogate to ensure that they’re running on a designated platform. Since, as far as I know, this specific microcontroller isn’t available without the haptics library, that may be the case. (It would be an easy design strategy to have a basic platform that simply has an ID that can be changed with one mask to make the device “unique.”)

I don’t know if this is what they did, but it would certainly be doable, and would add some practical teeth to the license. And if the low-level code is in machine language, it would be really hard to hack.

You can read more about what you can do with this in their announcement. And if you have any other clues about what's going on, please post in the comments.

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Jan 16, 2014

What Might Make an Accelerometer More Robust?

posted by Bryon Moyer

Last month STMicroelectronics announced a new accelerometer “engineered to withstand stresses of modern mobile life.” They see those stresses arising from increasingly thinner phones and the mechanical and thermal challenges they cause. They called out board bending as a particular challenge to the mechanical integrity of the works inside the accelerometer package.

So how do you improve the mechanical structure of the accelerometer to do this? First, it helps to realize that there are two structures in ST’s accelerometers. One operates in-plane and provides both x and y acceleration information. A separate structure is used for the out-of-plane z axis acceleration. On older models, these two structures were set side by side.

To illustrate how things might be improved, they made reference to stability in an airplane, even though the comparison can’t be taken too literally. If you want the smoothest ride in the plane, you sit in the middle, between the wings. Especially to the extent that the middle has the least stress and that stresses radiate out from that, there’s more disturbance (bumpiness) at the extremes – the wingtips and nose and tail – than in the middle.

It turns out that the z-axis accelerometer is the most sensitive, so improving it was a goal. So they moved it to the middle of the die layout rather than having it off on one side. And where would the x/y structure go if the z structure is hogging the middle? Symmetry is achieved by splitting the x/y structure and putting one half on either side of the center z structure. The two halves become the “wings.”

The other improvement was to double the number of anchoring points on the z structure from 2 to 4. This reduced the stresses on those points, making them less subject to failure.

You can find more details on the performance of this acceleromete

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