First Post...finally. Sorry it's so long :) I've been totally cranked on this idea for years now.
Visualize a typical Smith type weight lifting machine. Often used for squats or bench press type movements. Usually 2 vertical rods that are stationary, attached to a floor standing aparatus and
a horizontal bar which uses linear bearings to track up and down. The lifter puts weight on the bar and the track helps in spotting but also allows the user to engage the bar in the support with some hooks for safaty when he/she can no longer lift the weight.
I propose using magnets running along a vertical length of copper or aluminum. Or..inside of tubing. Lenz Law will determine the resistance and speed. The faster you move the more attraction and speed reduction there will be. This would be benificial in many ways. You may be able to do away with some of the external weights you have to put on, saving on manufacturing, shipping, and safety. Many training methods would be allowed. You can accelerate through the full range of the exercise without having as much inertia. This would also work with some cables pulling a magnet, or several through an aluminum thick walled tube. Many conventional weight machines use pulleys to slow the movement of the weight so you can move more quickly during the exercise with more safety.
This method of moving magnets through tubing can be employed with many types of resistance training machines. You could also use a cable and rotating disk with magnet running near the disk or rotate the magnets near the disk.
Similiar products exist in function but not in form. There are air pressure, rotary vane, hydro fluid and some others. Also some machines use a plug in alternator type of device to provide resistance. I think it's so funny to have a stair step machine, treadmill, or cycling machine plugged into the wall. HELLOOOO???
Also, this method would provide resistance in both directions providing a unique type of workout with a total push pull using opposing muscle groups rather then a push push using a negative movement on the downward movement.
Another advantage of this is that some idiot like myself could plug in their cd or mp3 player and charge their batteries while working out.
Flail away ....