Energy generator

Hmm, I don't know the voltage but a regular DC motor might do the job, especially with the help of some gearing. It is also possible that some piezoelectric material could do the same. How fast and with how much torque are you going to drive this thing?

Please read. I'm trying to GENERATE elctricity, not use it to drive something.

If you rotate the shaft of a regular DC motor then it will generate ac electricity. It is like the reverse of driving it. Trust me, it works. With some gearing to increase the speed of rotation they can generate quite a lot of power. I wanted to know how fast you will be turning the shaft to work out the amount of electricity generated.

The speed of walking is the speed of the shaft and in oscillatory motion.

Check out the other one for proper explanation.

The most efficient readily available device to convert mechanical energy into electrical energy is almost certainly a  good quality DC permanent magnet servo motor. One using rare earth magnets will be smaller. The problem of extracting mechanical energy from someone walking is up to you to solve - there have been many attempts but they have not been very successful - it seems that you can extract very small amounts of energy without too much inconvenience to the walker but that the mechanisms get very complex if the device is to be built into a shoe or attached to a leg. I can't really imagine a swinging weight being very comfortable.

In designing a practical device you might decide to trade off some efficiency against making the design simpler.

MK

DC motors are most often used for this application, but they need to be geared up so they rotate many times per step. As mentioned, the power they generate is mostly not free despite many attempts to capture just the wasted part of walking energy. The power they generate makes it more work to walk - the more power they generate, the harder it is to walk.

Another popular method uses Piezo-electric generators. These can be simpler to design and pretty efficient, but it is tough to get much power at the frequency of stepping - they work much better at high vibration frequencies. They pretty much need a method of mechanically translating a stepping frequency to a high frequency vibration. They generate the same amount of charge every time they are flexed, so to generate more charge (current) they need to flex more often.

Wouldn't turning a DC motor (using it in reverse) result in something looking like rectified AC, due to the action of the brushes?  Wouldn't this have both an AC and DC component?

https://www.youtube.com/watch?v=obP1TQh1e2w&index=44&list=PL0C7C6CCF1C0DEBB3

A DC motor will generate DC voltage when driven. If rotated in reverse it will generate negative voltage.

Although DC generators technically generate AC, it is converted to DC by the commutating action of the brushes, so only DC appears at the brushes.

An alternator or AC generator will generate AC voltage.

Vehicles use alternators and rectify the AC output to produce DC. (alternators are more rugged and last longer because the brushes only need to carry small field currents)

One team built it with stepper motors. But, I don't want to use them because they are bulky.

And by human energy, I meant that while walking, your legs move in pendulum motion, to and fro, so when it moves, attach something that could use oscillatory motion and generate electricity. The medium which generates electricity is at the knee joint. Two things will be attached one to the thigh and one below knee. So while walking, the thigh portion remains stagnant and the below knee portion will move turning the medium and generating electricity.

Replace the stepper motors with good quality permanent magnet servo motors. Ones with brushes are the easiest to deal with (no rectifier losses). Brushless motors will work (but you need ones with no embedded electronics) but the circuits to rectify the output are much more complicated.

In either case you need gearing to get the rotors spinning at  a decent speed and you will need some kind of complex clutch system so that the rotor turns in the same direction all the time  - otherwise you waste a lot of energy stopping the rotor and reversing it each time your knee bends.

MK