For a fun project I wanted to try building a small workout machine based on the backdriving of a motor, that would match my force - or my force minus some small amount. Eventually I'd like to connect an arduino microcontroller to program in some routines, and measure my force production while doing the exercise.

As a simple starting point, I just wanted to attach a lever arm to a motor or gear shaft like this:

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What type of motor/gearbox/other equipment or technology should I look into for this basic task? It need not be super strong or big or anything, just abide by the principle of pushing back the same (or slightly less, or a programmable) amount of force, in a way that is repeatable and fairly safe.

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    $\begingroup$ Well, matching the force is easy....a fixed wall does via Newton's Second Law. $\endgroup$
    – DKNguyen
    Commented Nov 3, 2022 at 22:14

1 Answer 1


Here is one way to do that.

Get you a small, brush-type DC motor with permanent magnets in it (NOT a stepper motor, as in your illustration).

If your motor is not connected to an electrical load, then it will offer little resistance to the rotation of its shaft. If it is connected to an electrical load resistance which is small (couple of ohms) then the shaft will offer great mechanical resistance to being turned, and the turning resistance will be proportional to the speed with which you are spinning the shaft- and the electrical resistor will get hot!

Since a human can generate about 1/2 horsepower (about 375 watts), you need a motor rated for 375 watts of dissipation into the load. Note that short-circuiting the motor wires will cause the motor to use its own internal resistance as the load, and the windings in the motor will get very hot. Be sure to get a motor will a cooling fan in it so it doesn't burn up on you.


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