Looking for an non-invasive sensor for a fitness machine

[bhl777]

Hi All, this is a picture of a rowing machine.
I assume inside the machine there is a wheel at the place marked around the red circle in this picture.

I am wondering for the machine with this kind of closed housing, is there any way we can use a sensor outside it to detect the rotation of this wheel? The rotation information I want to collect can be used for counting the numbers of the rotating of this wheel.
Would anyone give me some suggestions to look for the solution?

Thank you!

 

[BradtheRad]

I imagine there's a metal arm or pivot moving just inside the housing, which you could detect with a strong magnet. And the magnet may produce waveforms in a coil of wire placed next to it, or wound around it.

There's even a slight chance the motor can induce waveforms in the coil of wire (directly without need for a magnet). Amplify the signal. It's possible that the motor's current draw jumps up or down at certain positions in the cycle, which you could spot as a peak or a trough.

The above suggestion assumes the housing is plastic which allows a magnetic field to penetrate. If the housing is steel then it won't be easy unless you can find a particular spot that works for you.

 

[K.Ren]

Analysing the current is a good idea.

if the housing vibrates when the wheel spins there's a chance you could use some form of accelerometer to measure the vibrations of the machine as the fly wheel spins. making sure you find one sensitive enough to pick up the vibrations of the flywheel is crucial and will obviously only work if there is any vibration coming from the machine.

You could use a hall affect sensor as well but that's involves attaching a small magnet to the fly wheel and the hall effect sensor close enough to the wheel as it spins the hall effect will induce a voltage they're used on bicycles to measure revs per minute that's probably the most invasive. They're using this tech to measure revs on F1 cars and aileron flaps position on Commerical airliners now. This is probs one of the easiest to get accurate

Another option as will could be to use a light bridge circuit. an LED on one side of the flywheel and light senor maybe like a narrow band LDR to monitor the light coming from the LED as the flywheel arms break the LED and LDR 'bridge' you got a reading that a uC can process. this depends on the how many fly wheel arms and will involve a small calc in a uC to divide the input by the amount of wheels to get the true revs.

The last way I can think of is that some rowing machines use kind of a propeller type fly wheel pushing air to induce a kind of air resistance. If there is an outlet you could use some form of Barometric pressure senor to analyse the air pressure exiting the flywheel housing this one I imagine would be the hardest to keep consistent but there are few tutorials on guys that use these to measure air speed on drones.

 

[KlausST]

Hi,

There is a motor?
There are wires?
You can measure the current?

All these informations I miss in post#1.


Klaus

 

[FvM]

No electricity, no motor. A flywheel, probably with an eddy current brake to absorb power.

The "non-invasive" point is setting some restrictions, although no exactly explained. I read it as not modifying the housing. Thus most of the measurement methods suggested in post #3 won't be applicable.

I think you'll at least need to add a strong magnet to the flying wheel and may use a pick-up coil outside.