Motor Braking then to guidance

I'm looking to get started on a project and I need help with brainstorming ways to fix my problem. I'm looking to make an at-home gym. I was going to use an electric motor and supply constant power for resistance rather than using weights. So if I were to do a bicep curl the motor will supply a constant force of resistance (15lbs) on the way up but if I were to go back down I would lose all resistance. How can I have my Ardunio notice a change in direction and retract the cable with equal force?

If you use a DC motor, you don’t need any power or Arduino. Just run it into a resistive load through a diode. In one direction there will be mechanical resistance, in the other not so much.

An electric motor, ac or dc will never ever supply a constant force. Look to build a clutch with two steel plate, one of which has a cork surface. The pressure between the plates will give you the constant force at the speeds and repetition you are looking for.

Paul

aarg:
If you use a DC motor, you don't need any power or Arduino. Just run it into a resistive load through a diode. In one direction there will be mechanical resistance, in the other not so much.

Thank you for your response but I would like to be able to adjust the resistance based on user input.

Paul_KD7HB:
An electric motor, ac or dc will never ever supply a constant force. Look to build a clutch with two steel plate, one of which has a cork surface. The pressure between the plates will give you the constant force at the speeds and repetition you are looking for.

Paul

Thank you, Paul. Any idea on how to achieve resistance in both directions to simulate an iron weight?

jaycobjones:
Thank you, Paul. Any idea on how to achieve resistance in both directions to simulate an iron weight?

The direction of rotation manes no difference!

Paul

My first thought on the project would be to use a DC permanent magnet motor and a constant-current supply. As a first approximation, the constant current would result in ~constant torque. You would set the current to give a stall torque at the desired "weight" (torque = force x distance with distance in this case being the radius of the pulley with the cord going to the handlebar.)

One of the problems would be the change of torque as the motor warms up. If you get the basic apparatus working, this problem could be approached with a temperature sensor on the motor and adjustment of the current setting based on experimental results. An Arduino controller can handle the processing.

Another issue is the pulley. If the cord wraps in layers, the torque radius of the cord changes. If you want to address that complication, some sort of rotation sensor and further current adjustment could be made.

Of course you would want a nice dial adjustment so you just select the weight you want to work with. Maybe a display for lb as well as Kg; counting reps, etc.

So to start, you need a motor and a constant current supply which can be adjusted by an external controller. You need a pulley and some mechanical apparatus to keep the cord on the pulley.

It should be interesting to hear how this project goes for you.