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How to create motor or generator resistance system for exercise machine? Answered

Researching a method to use resistance from a motor or generator for an exercise machine.  There is an axle from which both foot and arm levers extend, so there is rotation, but only about a half turn in each direction, so very low speed, and back and forth.  I have used magnets and eddy current and flywheel from exercise bikes, geared up with a belt.  This works ok, but does not provide enough resitance even when magnets are adjusted very close to flywheel (due to low speed of rotation). I hope to find something already built and available for a similar application, but can locate parts and build it if I understand how.  Resistance must be variable from low to very strong and controllable so user can adjust it.  I have DC motor, and shorting the cord greatly increases the drag, but how do I vary/control it?  Would adding resistance between the wires in the cord increase drag and make it harder to turn?  I have experimented with this and it does not seem to have an impact.  Thanks for your help.


It's kinda neat DC servo motor force generation is something people are interested in these days. IMHO it’s a really smart way to generate resistance for anaerobic exercise. My father and I spent over 10 years perfecting what's being discussed here. As a matter of fact, I have a 5th generation prototype I use regularly. It's an infinitely programmable nautilus cam that can accommodate any range of motion and any force curve required. It has one mode where it measures stoke length and builds a customer force curve to accommodate. Force curves can be build on a PC and uploaded to the machine via HTTP or touch screen/smart card interface. It’s way beyond its time and a perfect machine for medical rehabilitation. All you need to do is figure out the software to drive the motor in one direction and dissipate the power in the other. It sounds easy right? We thought it would be but I can assure you it took many years to get right. Our first prototype used a DC motor from an old 1970’s magnetic tape data storage device and bench power supply. What we ended up with in the final design is a DC servo motor along with force transducers (test and calibration only), position sensors, high power IGBT’s and a PWM supply. The software engineering is the real genius behind the mechanical and electrical engineering that make up the device. Most, but definitely not all, of the design is detailed at length in our patent. Do a search for "Force generation and control system for an exercise machine" US patent 5,993,356 - and you will find some details. If anyone wants to reach out to me, I'm in VA.

Did you ever get anywhere with your adjustable resistor method? I'm very curious as I am going to try this same experiment on my own.

Unfortunately, a passive load across the motor is not what you need. What you actually would have to have is a full-on servo amplifier system, because the lower the speed the less effect the load has - turn the motor very very very slowly, there is hardly any resistance to turning.

Controlling a drive to do what you need is non-trivial, especially since, if it went wrong, it might take your leg off.

I just watched a video on this, and found the drag is actually "back torque" or motor effect created by the generated electricity. I had thought the drag was resistance in the system. So adding bulbs, for example, reduces the drag because it reduces the current (I think?).

Here is video: http://www.youtube.com/watch?v=NSqu_u7nYU0

Anyway, I will look up what a servo amplifier system is and try to understand how it works. Thanks.

No, adding bulbs INCREASES the load, and increases the current. The point is, that the resistance is proportional to SPEED as well as LOAD. Turned at low speed, you get no resisting torque.

I am getting a little confused. I have a dc motor and when I short the cord I get strong resistance when I turn the shaft very slowly by hand. When the wires are disconnected the shaft turns easily. So is that "drag" back torque as demonstrated in the video or some other means of resistance in the motor? I saved the belts and plastic wheel/pully parts from several exercise bikes, so i can significantly increase the speed at which the motor shaft is turned. Thanks for your help.

What I'm saying is that the drag is not a fixed property, it depends on how fast you turn the shaft. The drag is the physical consequence of making electricity in the load - where's the energy come from to heat the load ? You.....

A friend suggested pulse width modulation could be one way to vary and control the resistance in a dc motor. I think I get this. In effect you would start with very strong drag when wires are connected and vary it downward by having a control device that turns it on and off - connecting and unconnecting. Does this sound reasonable? Thanks for your help.

a REALLY beefy variable resistor is what you need.

Or slap a suitably sized incandescent light bulb across the motor

I don't want to use a bulb because that would need replacement more often. Are there other options that would give variable resistance and last indefinitely?

This is the key question I have: would a variable resistor, connected between the two wires of the power cord, provide adjustable drag? When I put the power supply wires together and then turn the shaft on the motor there is much greater drag than when the wires are not shorted. Where would I find such a variable resistor and what would define "beefy? Is the drag being created in the windings and armature of the motor or the wires of the cord or both?

they are technically shorted - through the resistor; trouble is, ALL work done INTO the motor (generator) is dissipated through the (variable) resistor...as heat.
It has to be rated for as much power as you can put into it.