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As Kate and I discovered in our research, attempts to modify shoes mechanically are nothing new. Much earlier experiments were aimed at reducing jarring effects to the body ( as evidenced by this 1939 news clipping about spring heels. ) There are other orthopedic products today that take a similar approach.
Kate and I wanted to investigate shoes as a platform for energy harvesting, however, but these earlier mechanical examples provide some useful precedents. There has also been a lot of research done on energy harvesting shoes at MIT by Joe Paraiso. They have developed a number of prototypes that work in different ways (i.e. piezoelectric, hydrolic, mechanical.)
Our first prototype attempted to harvest energy from regular walking. By hacking open a small shake flashlight, we engineered a simple plunger mechanism that forced the flashlight's magnet through the wire coil on footfall. Sometimes the plunger or the magnet didn't return to the lower position despite the force of gravity due to friction in the system, so we added a rare earth magnet at the top of the column in opposite polarity to push the magnet and plunger back out when the heel raises. If you click here, to the right you can see a video of the mechanics of the system working. The image above shows our final prototype engineered to charge a AA battery. Each footfall puts out around .5V and 40 milliamps of power. The mechanics of this prototype definitely interfere with normal walking motions, however, so it probably isn't the best solution in terms of keeping the original motion natural.
In our second prototype, we addressed the new phenomenon of wheeled sneakers to see what electricity this motion could
produce. We purchased a pair of Heelys sneakers to see if it would be possible to
modify them to house a stepper motor. We thought of several mechanisms that might work for this modification (i.e. belt
systems, internal motors on a specialized wheel, etc. ) but the most practical for a quick interventionist needs was to put
a gear onto the wheel and dig out a hole in the sole of the shoe to hold a stepper. Thus, a gear on the wheel would serve to
drive the gear on the shaft of the stepper motor.
Getting a gear on the wheel was fairly simple. We just pulled some plastic gears out of an inkjet printer on the junk shelf, and using a hacksaw, we trimmed about a quarter inch of the rubber around off the side of the wheel. Here's a video of what the wheel looks like with the plastic gear glued to it.
1st try with smaller stepper
Our first attempt at making the Heelys drive a motor didn't work out. We used the small stepper pictured here, which is nice for space for the
wheel, but we weren't able to get it to produce any electricity ( my hypothesis is because I don't think we ever got the gear properly connected
to the motor's shaft. ) So, in the second prototype we used a bigger motor ( my 5-wire mototech that I mention in the
kinetic to electric lab. )
In order to make room in the shoe for the motor, we need as deep a hole as the sole would allow, so I spent about 45 minutes with a dremmel tool, exacto knife, and a screwdiver hacking open a square hole for the motor to rest ( watch the sped up video of this process here.)
With the motor taped into the sole of the shoe, the gears moved nicely together and people are still able to roll around on them. ( The protrusion of the motor is sometimes a problem, but one can imagine a better application of this where such protrusion wouldn't be necessary. Also, rolling on the opposite edge of the wheel also helps. ) We measured the output from the motor, and it seemed to gain around 8V and 400 milliamps at its peak ( video here ). In the end, we connected our output to some LEDs and a buzzer just to prove that we were getting power as the shoes rolled around ( video ). We are planning on using this power source eventually to fuel much more interesting projects than just charging batteries and setting off a rather annoying buzzer.