POWER GENERATING SHOE

INTRO:

The project is be accomplished by using piezoelectric materials. Piezoelectricity, also called the piezoelectric effect, is the ability of certain materials to generate an alternating current voltage when actuated.Certain ceramics, Rochelle salts, and various other solids exhibit this effect. For example, (Pb[ZrxTi1−x]O3 where,0≤x≤1), also called PZT, will generate measurable electricity when their structure is deformed by about 0.1% of the original dimension(International AAAI Conference on Social Media and Weblogs, 2012). In this project, the generated electricity on a specific time will be recorded and determine if it would be enough to completely charge a Li-ion battery or a high capacity capacitor.

The project is be accomplished by using piezoelectric materials. Piezoelectricity, also called the piezoelectric effect, is the ability of certain materials to generate an alternating current voltage when actuated.Certain ceramics, Rochelle salts, and various other solids exhibit this effect. For example, (Pb[ZrxTi1−x]O3 where,0≤x≤1), also called PZT, will generate measurable electricity when their structure is deformed by about 0.1% of the original dimension(International AAAI Conference on Social Media and Weblogs, 2012). In this project, the generated electricity on a specific time will be recorded and determine if it would be enough to completely charge a Li-ion battery or a high capacity capacitor.

The piezoelectric effect, by which a material generates an electric potential in response to a temperature change, was studied by Carl Linnaeus and Franz Aepinus in the mid-18th century. Drawing on this knowledge, both René Just Haüy and Antoine César Becquerel posited a relationship between mechanical stress and electric charge; however, experiments by both proved inconclusive. The first demonstration of the direct piezoelectric effect was in 1880 by the brothers Pierre Curie and Jacques Curie. They combined their knowledge of pyroelectricity with their understanding of the underlying crystal structures that gave rise to pyroelectricity to predict crystal behavior, and demonstrated the effect using crystals of tourmaline, quartz,topaz, cane sugar, and Rochelle salt (sodium potassium tartrate tetrahydrate). Quartz and Rochelle salt exhibited the most piezoelectricity.

A piezoelectric disk generates a voltage when deformed (change in shape is greatly exaggerated) The Curies, however, did not predict the converse piezoelectric effect. The converse effect was mathematically deduced from fundamental thermodynamic principles by Gabriel Lippmann in 1881. The Curies immediately confirmed the existence of the converse effect, and went on to obtain quantitative proof of the complete reversibility of electro-elasto-mechanical deformations in piezoelectric crystals. For the next few decades, piezoelectricity remained something of a laboratory curiosity. More work was done to explore and define the crystal structures that exhibited piezoelectricity. This culminated in 1910 with the publication of Woldemar Voigt's Lehrbuch der Kristallphysik (Textbook on Crystal Physics), which described the 20 natural crystal classes capable of piezoelectricity, and rigorously defined the piezoelectric constants using tensor analysis.

Step 1: Parts And Materials

Parts/ Materials:
- Cheap/ Generic USB Powerbank
- Piezoelectric Transducers (6x)
- 1N4007 Rectifier Diodes (4x)
- Hookup Wire (at least 12")
- Old Pair Of Shoes
- Contact Adhesive

Tools & Equipment:
- Digital Multimeter
- Multitool (w/ pliers)
- Rotary Tool
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Optional: 
- 100nF Mylar Capacitor (for testing)
- Hoop & Loop Fastener (Velcro)
- LED Indicators (for testing)
- Superglue (for fixing wires)
- Smartphone Sport Strap
- 5v Switching Regulator (w/ supercap)

Alternatives: (since not all can afford them)
- PowerBank > Old phone batteries + Recycled 5v Inverter
- Peizo Transducers > A pair of old & outdated earpiece
- Rotary tool > Hot Nail (for melting plastic)
- Multitool > A pair of pliers will do

Step 2: Measuring Your Sole

Get the size and shape of your shoe's insole then get a pair of heavy-duty shears/ snips and carefully cut the PVC material. The plate will act as the primary mount of the piezoelectric discs/ elements.

Remember: Thickness matter, you need at least 2-5mm. If your material is too thick, the piezo elements will break due to too much flexing. If your material is too thin, the piezo element won't bend at all thus converting less power.

Step 3: Find And Cut An Ideal Material (Sheet/ Plate)

Now surround the PVC plate with three piezo discs. How do I know where the center is? The "center" that I'm referring to is area where all the pressure is withdrawn by your foot, your sole.

After getting a fix preview of the setup, get a pencil and trace the piezo discs. Finally use your compass to draw smaller circles, about 2mm smaller in radius. The 2mm spacing will act as your margin.

What material should I use?
Based on my design, I needed a plate that is 2-5mm thick, lightweight, stiff and can endure a lot of flexing. Metals are too stiff while carbon fiber is too thin. After playing around with a bunch of materials, I've found out that PVC fits best in my application.

Were did you get the PVC material? 
PVC materials are all around us. You can find them in your local hardware store but in the form of pipes. I got mine from our excess supply of PVC pipes when our house was built. Recycling means free $$$ for me! :)

Step 4: Grinding Holes On PVC Pads

In this step, grinding is required to bore/ drill round holes. Since I don't have large drill bits (as large as the marked area), I've thought of way to cleanly cut the holes and that's by using my handy rotary tool. 

If you don't have a rotary tool, you can still cleanly cut the plastic by doing it "the old fashioned way", by heating an iron nail and melting the plastic.

Step 5: Gluing The Piezoelectric Elements

These piezo discs must endure a lot of flexing since you'll be stepping on them repeatedly! Never use superglue, if you do, the moment you step on your insole the piezo discs will snap off the PVC pad. Instead, use those quick setting "contact adhesives". Their rubbery characteristic makes them ideal for this project since they stretch whenever they are bent.

Step 6: Soldering The Piezos Together

Solder all piezo elements together in parallel. Don't solder them in series because you'll need more current than voltage and those piezoelectric discs will cancel each other's power output when not actuated at the same time. 


Piezo elements produce AC currents. Unlike DC currents, you can't just tap in the line. since AC currents are always alternating polarities. Just like power generators, whether it may be solar or petrol, you can't just tap directly to the powerlines without aligning the AC wave's phase otherwise the generator will cancel each other. (Ex. Negative meets Positive - Positive meets Negative). This infers that parallel works best for our project. 

Improvements:
Through this process of experimentation, I've realized that even if they are hooked in parallel the piezoelectric elements can still cancel each other's output off (when not actuated simultaneously). This leads to conclusion that you'll need to add one bridge diode per piezo element/ disc.

Step 7: Building A Bridge Diode

Peizoelectric elements produce AC when subjected to mechanical stress. Unfortunately, USB devices need DC and not-AC. A bridge diode is required to filter and convert AC to DC. 

Schematic:
Just follow the schematic diagram above. Solder the peizo discs to your bridge diode. Remember AC has no polarity, you can invert the wires either way. The load (shown as a resistor) represents your appliance.

Recycling: 
CFL bulbs contain electronic ballasts, each ballast contains at least six to eight rectifier diodes. Those diodes are compatible with our project. Please be careful in disassembling CFL bulbs, I'm not liable if any of you get injured.

Step 8: Adding Foams Pushers

Now glue small a small piece of foam on the very center of each piezo disc. These foams will act as pushers. These foams will squeeze the piezo discs inwards (like sandwitches) while walking.

Step 9: Observation And Testing

Finally, we are going to test the validity of or theory. Start by getting a digital-tester and switch it to the 2 digit DC range. Remember, piezo elements produce a short burst of current the moment you push them so adding a 100nF capacitor should make the readings much more readable.

My volt meter displayed:
Pressing By Hand = 15.03 volts (2mA)
Walking By Foot = 18.53 volts (5mA)
Running By Foot = 27.89 volts (11mA)

Step 10: Installing The Insole

Slip the insole generator between the shoe and the insole.

Step 11: Adding A Powerbank + Soldering

The insole generator peaks a voltage of 28 volts, the current may be small but the voltage is enough to damage the powebank's 5v charger (circuitry). I won't be using the 7805 since it's quite old and inefficient. Right now, my insole generator is soldered directly to my powerbank's lithium battery. It works but it's not completely not safe.

UPDATE: Say goodbye to the huge bulging powerbanks! I have a newer version of the project, the powerbank is now integrated with the insole. I also added a charge collector circuit.