Hand Ergometer

[irving2008]

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.