Piezoelectric Polymers — not just another tongue-twister

There’s a whole class of devices nowadays that are striving to take advantage of “parasitic power”.  Much like the name implies, the goal is to have something live off something else without destroying the host.  A self-winding watch is an example that’s been around for decades, as have crystal radios and the spark igniters on some gas grills.  A much newer example is the nPower PEG that can charge your mobile phone from the motion of a walker’s backpack.

The PEG works like the shake-type flashlights, using magnets and coils to convert motion into current — something we’ve done since the days of Faraday.  The igniter, however, does something much more interesting.  It uses the “whacking” collision of a spring-loaded metal bar against piezoelectric crystal to generate a high voltage, which then produces a spark.

The target of this “whacking” in the ignitor is typically a crystal or man-made ceramic, which can be fragile in large sizes. Recently, however, there has been much interest in polymers which are more flexible and generate higher voltages.  Additionally, they can be less expensive since they can be created as films or thin sheets.  Currently, the front-runner is polyvinylidene fluoride, AKA PVDF.

So, how do you use these devices today in this field of parasitic power?

The answer is any place where you can get access to abundent disposible kinetic energy.

You put them in places where they get periodically compressed, or even better, frequently compressed (as in vibrated).  In a road they would generate energy every time a vehicle crossed over.  They’re doing this in Israel now.

Or on a sidewalk, every time a person stepped on them there would be a pulse of energy released.  The big question is: How much energy do they generate, and why do you want that energy?

In most cases, the desire for use is in a place where it’s inconvenient to use other sources of power and you don’t need much of it.  Blinky lights on shoes is a good example.  Another use being investigated is sensors placed on shipping containers where they can use the vibration of the truck or ship carrying them.

This isn’t getting power for free, but rather, taking a little energy from something that probably won’t notice it’s missing.

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Captain, can I use my iPad now?

Ah, today we all basked in the glow of the iPad, Apple’s most recent entry into the world of consumer gadgetry. Though there was much to delight in about it, we noticed that the Airplane Mode setting was conspicuously missing from its feature set. As a result, lucky iPad owners will need to either individually turn off the Bluetooth and WiFi radios, or simply turn the whole iPad off when in-flight.

Well, of course this touched off the whole debate about the use of electronic devices in the plane. Do they really interfere with the navigation instruments? Or, is it a conspiracy to force you to use the ridiculously expensive ($1-2/min) air phone service. It was amazing how passionate people were on their positions.

Let’s take a moment to survey the situation a bit. It’s tricky because both the FAA and FCC have things to say about this one.

Here’s the official word from the FCC as of 2007, with comments on the FAA’s position: http://www.fcc.gov/cgb/consumerfacts/cellonplanes.html

And on March 15th, 2006, in episode 49 the local Mythbusters team tackled this one as well: http://mythbustersresults.com/episode49

There are two issues in play. The first is the idea of interference with the navigational equipment on the aircraft. In practice, with clear weather, this is probably a non-issue as even if interference occurs, since a pilot would likely recognize it and recover. However, in IMC (instrument meteorological conditions), the lives of all aboard the aircraft hinge on the successful reception and interpretation of received radio signals, some of which (GPS for example) are incredibly weak and require very sensitive receivers. This is the reason that all electronic devices must be turned off during take-off and landing phases of flight because when we’re close to the ground those signal interruptions could have catastrophic results.

The second issue is one of interfering with ground equipment. This is primarily a cell phone issue since a large number of cell towers (many more than usual) can see your signal when you’re in the air, and you consume far more voice channels that would be normal, thus clogging the system. Good for you, not so good for others.

So, in short, best to follow the captain and make absolutely certain your electronic devices (with wireless radios or not) are turned off during take-off and landing phases of flight (especially in IMC) as these are the most critical moments from a safety standpoint. In good weather, using your cellphone is likely more annoying to other phone users on the ground than it is harmful to you in the air.

In recognition of this, the FCC and other agencies are actively looking for ways to allow users to use cell phones in the air, and enjoy those wonderful gadgets as fully as we enjoy them on the ground. Until then, though, the rules say no, and the captain of the aircraft still has the final say.

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