Thread: Hand Ergometer

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  1. Not yet a project but one I hope to complete with, unfortunately, some considerable assistance as I am unlikely to have access to, let alone be able to use, my own machines for at least a year.

    (for those that don't know see this thread)

    Anyway I need to exercise regularly from my wheelchair, both for cardio vascular and arm muscle development and right now i get one 20min session a week. A hand ergometer - basically an exercise bike for the arms - is expensive, over 2000 new. Even a very old refurbished unit goes for over 700 on eBay and are physically massive as most do legs too (no use to me and have large flywheels), so I thought I'd look at designing my own.

    Basically an ergometer is a calibrated load which can be adjusted. Because arms are a lot weaker than legs modifying an existing exercise bike isn't practical, nor is using a modified bike trainer resistance unit (the resistances are too large, the gearing all wrong and the load adjustment too coarse). I had hoped to use a stepper motor with DC energised windings but some experiments by Jonathan showed that probably wasn't going to work either.

    The design I have evolved is based on an eddy current brake, basically an aluminium rotor passing through a magnetic field. The field is generated by 4 electromagnets (50kg/24v pickup units, about 12 each on eBay) mounted on a cast iron cage so that the resistance can be varied by changing the current through the magnets. The cage is mounted on bearings concentric to the axle the hand cranks act on and is allowed to rotate a limited amount as it reacts to the torque applied to the rotor. A load cell measures this torque (<10 on eBay).

    The rotation speed of the rotor, nominally 60rpm, is measured with a photosensor and speed, torque and magnet current is all fed to an Arduino computer which then calculates the actual load in watts being applied and displays on a small TFT colour display. It will also provide time info and control the magnet current to vary the gear/terrain for interval training (none of the ones I use at the gym do this for the hand section, tho it is common for leg training units). Large button switches will change mode/settings/etc using a soft menu (touch screen is no good unless I have a stylus).

    Attached are some parts drawings for comment/discussion. Unless spec'd all parts are 6061 ali or similar. They are not final, in particular I am going to simplify the handle design/assembly and I've not done the load cell or Arduino mounting, or the floor mount arrangement. They were drawn using Inard CAD Pro for Android tablets For 6 its not a bad package and exports PNG, PDF and DXF.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    Last edited by irving2008; 23-06-2013 at 04:51 PM.

  2. #2
    Irving, I just don't know how you do it. Unbelieveable - keep it up. G.

  3. #3
    Quote Originally Posted by irving2008 View Post
    I had hoped to use a stepper motor with DC energised windings but some experiments by Jonathan showed that probably wasn't going to work either.
    I think a DC motor could be made to work quite easily, particularly if it's separately excited. There are quite a few available second hand, the problem is people like using them to make (not very good) wind turbines, so the prices on eBay are inflated. Using eddy currents seems more elegant, but the DC motor does give the option of using the energy generated for something useful - e.g. powering laptop. From a quick google search it looks like we can expect about 30-60W from two arms.

    Quote Originally Posted by irving2008 View Post
    The cage is mounted on bearings concentric to the axle the hand cranks act on and is allowed to rotate a limited amount as it reacts to the torque applied to the rotor. A load cell measures this torque (<10 on eBay).
    I recently needed some load cells for a project where I need to measure torque, so I got some of the cheapest 20kg 'hanging scales' on eBay and dismantled them. I've not actually made the circuit yet, but it looks like it wont be difficult to use the load cells they contain. That makes is <4 for the load cell.

    Quote Originally Posted by irving2008 View Post
    The rotation speed of the rotor, nominally 60rpm, is measured with a photosensor and speed, torque and magnet current is all fed to an Arduino computer which then calculates the actual load in watts being applied and displays on a small TFT colour display. It will also provide time info and control the magnet current to vary the gear/terrain for interval training
    It looks like the radius of your disc at the center of the magnets is about 75mm, so at 60rpm that's 0.5m/s. You're going to need a high flux density in the aluminium disc to generate much force due to eddy currents at that low speed. It's annoying that there's not much information about the 500N electromagnets on eBay. I think we can work out the flux density at the pole face required to lift 50kg, estimate the reluctance of the magnetic circuit, then use that to work out the torque/speed characteristic. Another approach is I could model in FEMM, using the dimensions of the magnet and making an educated guess for the rest. Either way I suspect you might need either some sort of gearing between the handles and disc, a bigger disc, copper disc, or more electromagnets.

    I've got plenty of permanent magnets you're welcome to which would probably be suitable, although it would mean changing the design somewhat since you'd have to move the mangets to control the torque.
    Last edited by Jonathan; 23-06-2013 at 09:06 PM.
    Old router build log here. New router build log here. Lathe build log here.
    Electric motorbike project here.

  4. Hi Jonathan

    I was half expecting you to investigate further :)

    The radius is 90mm and there are 4 magnets. Fully energised they each generate 1.6 Tesla (core saturated magnetically) at the pole which is 20mm dia. I don't have the B-H curve (as you found, no data sheets online, but then this isn't their intended use) but I'm hoping to be able to calibrate it. The air gap to the rotor is 1mm. I kept it this size as think getting it smaller will require too much precision in mountings.

    Now for some calcs....

    torque = sigma * omega * B ^ 2 * p ^ 2 * a * d
    sigma=rotor conductivity
    omega=speed rads/sec
    B = mag field Tesla
    p = effective radius
    a = pole area
    d = disc thickness

    sigma Al 6061 = 3.5e7
    omega =6.3 rad/sec (60rpm)
    p = .09m (200mm dia rotor, 20mm mag)
    a = 0.01m^2*pi = 3.1416e-4
    d=0.005m

    using an online calculator whose URL I cant find right now the 4 magnets @ 1.6T generate a total of 1.36T yoked in a 7mm air gap. Therefore

    max torque = sigma * omega * B ^ 2 * p ^ 2 * a * d
    = 3.5e7 * 6.3 * 1.36^2 * .09^2 * 3.1416e-4 * .005
    = 5.2Nm

    Power= torque * omega = 5.2 * 6.3 = 32.8W

    So will need to increase # of magnets to 8 x 40kg, or go to 40mm dia pole or increase rotor dia to 300mm to get to 60W. Also could consider some sort of epicyclic gear to rotate rotor faster e.g. 3 or 4 times but maintain it concentric to axle to minimise footprint.

    edit: 27mm dia pole (70kg magnet) and 250mm rotor gives 91W

    I did look at permanent magnets but the mechanical complexity of moving them and still measuring torque accurately got very messy (mechanical sizes were based on 3 pairs of 20mm x 10mm Neo magnets, I'd not recalculated based on electromagnets, my bad)
    Last edited by irving2008; 24-06-2013 at 10:39 AM. Reason: fixed error in formula

  5. Been looking for plastic internal (annulus) gear rings, to create an epicyclic gear train, but can't find a UK supplier. Any ideas?

    Alternatives are a timing belt to drive the rotor at, say, 5x crank speed so can use smaller lighter magnets but that adds equal complexity

  6. #6
    Quote Originally Posted by irving2008 View Post
    Been looking for plastic internal (annulus) gear rings, to create an epicyclic gear train, but can't find a UK supplier. Any ideas?
    Is this something that could be 3D printed? Not sure what sort of strength / wear characteristics you'll need...

    I'm happy to have a go if you want to try it - max size is about 200 x 200 by 100 mm deep though print time / warping might be a problem if using the full envelope. I've got plenty of PLA in - not tried ABS or nylon yet but willing to experiment.

  7. It could be 3D printed, in fact I was already thinking along those lines. Envelope would be around 80mm square x 6mm thick. I'd need the properties of PLA to check if its strong enough. What's the resolution of your printer?

  8. #8
    I have printed with both PLA and ABS and not sure they would be strong enough but laser printed I suppose could be done even if it had to be in two 3mm pieces and bonded back together. Do you have some sort of drawing! ...Clive

  9. #9
    Quote Originally Posted by irving2008 View Post
    It could be 3D printed, in fact I was already thinking along those lines. Envelope would be around 80mm square x 6mm thick. I'd need the properties of PLA to check if its strong enough. What's the resolution of your printer?
    That size wouldn't be a problem, resolution is quoted at 0.0125mm and accuracy at 0.1mm (Mono Mendel - RepRapPro). The current nozzle is 0.5 mm (best used with a 0.4 mm or slightly smaller filament thickness) but I've got a 0.35 mm one that just needs putting together for finer detail.

    Some material specs - Click image for larger version. 

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    Interesting PDF on designing for FDM (http://ode11.com/publications/sme_rp_2001.pdf).

    Because there is variable bond strength between the filaments, it's best to design so that the stresses are applied along the filaments but for things like gears that happens anyway since the slicer will trace round the perimeters.

    We could do strength tests before you commit to the design...
    Last edited by FatFreddie; 25-06-2013 at 09:07 PM.

  10. Trying to work out how to draw the teeth for this in my CAD program, but attached is an initial sketch of the epicyclic gears.

    Click image for larger version. 

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    Current thinking is I basically want a mod 1.25 60 tooth internal ring with a couple of fixing lugs (the ring is static), a mod 1.25 30 tooth sun spur gear on a press fit 10 x 22 x 6mm bearing (this will be fixed to the rotor which will rotate freely on the 10mm axle using another 10 x 22 x 6 bearing), and 2 off mod 1.25 15mm planetary spur gears on press fit 3 x 9 x 5mm bearings (these will be fixed to a carrier thats connected to the 10mm axle ). Spinning the 10mm axle will spin the rotor at 1+60/30 = 3 times the rate or 180rpm. the torque in the gear train is around 6Nm

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