Core Electronics Forum

Help with exercise machine for muscle recovery (choice of motor)

Good afternoon/evening
I am planning to develop an exercise machine for muscle recovery, the idea of the project is to mimic the sensation of fishing so that the exercise has more excitement in an indoor environment.
The idea is that the patient turns a crank and that at a certain moment he/she makes force (variable) so that it costs more to turn the crank. The prototype will have a rod shape to give more sensation.
We had thought of using a motor that makes counter/braking force (protecting it properly with a diode and maybe using a current limiter to be able to control the torque and not to have too much current).

The operation of such a project is as follows, the user would start turning the crank without any effort after a period of time should increase the resistance to rotation. It is also possible that the user would stop turning and continue for a short time afterwards, it would not be constant.

Should the motor be used as a brake all the time? (It is intended to provide a sensation of fishing so there should be a feeling that something is trying to turn the crank shaft in the opposite direction).

Will it be possible to control the braking torque and do I need to add amplifiers so that there is more resistance to the rotation?

Thank you very much

Hi Alex.
You would be using a motor as a generator.

A diode across a motor does not protect the motor. It protects other electronics from the large reverse voltage spike generated at switch off. Particularly in PWM drive situations… All it would do is make it almost impossible to turn the motor / generator by hand in one direction.

Vary the value of a load resistor.

No. Varying the load resistor will do that. The motor / generator will get more difficult to turn as the resistor approaches zero ohms.
Cheers Bob

Thank you for your response.

Then the motor could have it disconnected from the source. And by means of a digital variable resistor control the torque.

The feeling of the motor as a generator will seem like there is something pulling in the opposite way ?

Is there any way to know how much brake torque a motor will give as a generator ? (Looking at the “Rated Torquel” or the “Stall Torque”?)

Hi Alex

I have no idea what you mean here.

You will probably need quite a bit more power handling capability than any “digital” resistor. If you mean a digital potentiometer forget it.

Can’t think of one. As said the degree of braking will depend on the resistance across the motor /generator at the time. With a short circuit you will be lucky to move it by hand

I think the only way it is going to feel like going the other way is supply a driving voltage to actually turn it. If you do this and stop it turning and try to turn it the opposite direction there is better than a good chance of smoke. Dud motor, gone.
Cheers Bob

Hi Alex,

I would take a look at your previous topic as well for some additional info.

A little aside Bob, would a brushless motor like a stepper or Brushless DC(BLDC) motor work in this case with a form of feedback? I can’t imagine you would get the same level of braking force using purely the motor.

Liam.

Hi Liam

For braking a resistor or other load would have to be connected to each coil of a stepper. If this is variable then other problems such as ganging multiple resistors arise. The power handling requirement would be too much for a ganged pot. By BLDC I assume you mean brushless DC motor. I think these are actually AC motors with a little inverter and if so would not work.

Try it. You may be surprised. Even a little 3V hobby motor has surprising resistance when shorted. For this application though I think something a bit larger would be required. So far Alex has not given any real indication of what he wants, only a broad outline. Maybe “Mandrake The Magician” might help out here.
Cheers Bob

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Hey Bob,

Cheers for the answers! Yeah Brushless DC, I’ll edit the post for those in the future.
Not to mention controlling Brushless DC motors at lower speeds/ high torque requires some smarts happening in the driver/controller, likely outside the scope of Alex’s project.

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Alex wants to use it as a generator. In this case I don’t believe it would work.
Cheers Bob

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Hi Bob and Alex,

While probably overkill for a project like this, the Odrive BLDC motor controller has a spot for a big power resistor and the necessary switching to dump kinetic energy in that direction:

But it isn’t cheap, is a bit of a pain to configure, and I’m not sure it’d have the control schemes in place to do what you’re after (a fixed amount of resistance). It can be set to apply a torque in a direction, hold a position or a speed, but those don’t seem to be what you’re after.

Maybe a “resistor ladder” of power resistors could work? I don’t short and move brushed motors regularly so I’m not sure how hard it’d be. Maybe a set of different belts and pulleys for each resistance level?

Keen to see where this goes!
-James