kenkad said:
Supertux1,
I have considered using a Hayden-Kerk size 8 stepper motor/lead screw linear actuator as the control for an IGH. You would need to couple that with a linear magnetic sensor like from AMS. Then you do not have to worry so much about loosing position because your stepper drive software/electronics missed steps. That is not a trivial issue. I personally believe that is better than an RC servo approach. Just a suggestion.
kenkad
Disclaimer: I don't own a N360 hub yet so my opinions on it are from what I see in its tech manuals, videos online and anecdotal sources of how it works.
The Nuvinci N360 IGH shifting disc 'floats' and requires pulling both clockwise and counterclockwise to change ratios.
It doesn't have a spring to return it to a low/high ratio in absence of cable tension like some other hubs and the typical derailleur.
You have to manage TWO rear shifting cables running the length of the bike, (lots of zip ties I guess) and any slop or misconfiguration
is going to manifest as sloppy/unresponsive/dead zone shifting control.
I don't have a good feel for how much force is applied to those cables so I can't say if they're going have issues stretching and what not.
I like stepper motors but servos positions are easily controlled with microcontroller PWM and require no external positioning sensors.
Although it is good engineering to have switches trigger on each end of the mechanical stop to prevent overdriving it and also aids in calibration.
Compression springs can also be used to prevent a high torque motor from breaking something that really doesn't want to move or is sticking.
(I read somewhere that the N360 hub won't allow full range shifting at a dead stop so this could be an issue.)
Perhaps a single one of these things would do it, pulling/pushing on a center pivoting bar that has each end attached to the N360 cables...
http://www.servocity.com/html/heavy_duty_linear_servo__25__l.html#.U72Bdo1dUgo
(These are $300 though, so perhaps something else...)