Just purchased a used “One Off” all-terrain handcycle (photo below). This is the equivalent of a mountain bike to be used by individuals, such as myself, who lack lower extremity function (paraplegic). This one is pedalled by hand (no electric motor) while sitting in a kneeling position. Gear ratio is provided through a rear cassette/derailleur mechanism and two planetary gear bottom brackets. If you could get traction, could climb a wall with this thing, although VERY slowly.
My “is this a bad idea, or a very bad idea?” question is in regard to steering. Hands are pretty busy with pedalling so steering is done with the round chest pad linked to front wheels by the chain/cable mechanism seen in the photo below. I find this mechanism to be terribly inefficient and the pad is super uncomfortable for an individual such as myself who lacks stomach/back muscles and must rest all weight on the pad. Planning to replace this with something with a better shape. But also wondering about finding an entirely different way to steer.
Am I way off base thinking one could set up a steering system using a high torque servo motor, battery pack and one or two channel radio frequency remote controlled by an transmitter similar to those used for rc cars? Thinking could build/modify the remote to be used as one of my pedal handles, allowing steering with the thumb and no need to remove hand from cranks to steer and get rid of the uncomfortable chest pad.
Anticipated problems:
• Finding a servo with adequate torque and compact size/weight
• Figuring out the transmitter situation (take guts out of car Tx and transplant into 3D printed crank handle?)
• Durability (although thinking rc buggies take a beating)
• Tune-ability (would steering end up too twitchy or too slow – is PID tuning possible)
• Manual over-ride – need to steer with hands on bars for downhills. Turn
Assuming you found a servo motor strong enough to do that job, you’ll be tied by the rotational speed of it which most likely to be too slow and you would need quick reactions while going downhills.
Also, rotating the servo motor manually and quickly would damage the gears inside it.
You would probably need some sort of a mechanism to disengage the rotating mechanism and switch to manual whenever you need.
The servo speed issue wouldn’t be too much of a concern, I don’t think, as this steering system would only be used on flats and up-hills where speed is very slow. I am climbing a steep logging road using ~40 watts of arm power to mobilize 250lbs of weight (me plus machine) so we are talking no more than a kilometer per hour (and I know what you are thinking - no, I’m not open to power assist for locomotion - climbing this hill under my own power is a personal quest! On downhills or when going much fast than a jogging speed, the handlebars you can see in the photos are used.
I did have the same concern as you, however, regarding the need to protect the servo when those handlebars are used. Wasn’t sure overpowering the servo was possible or if this would cause damage. You’ve answered that question pretty clearly.
Wheelchairs used for track and road racing have a ‘compensator’ mechanism which allows spring-centering and trim of steering with hands away from handle bars. Had thought of something like that positioned between servo and fork, but can see matching spring rate to servo output could be problematic. Your thought around ‘disengage’ seems a better way to think about it, just not sure what the application would look like.
For now I’m concentrating on modifying that chest plate and try to get the stock steering working a bit better, but the system still sucks. Has to be a better way. If Elon can manage autonomously steering a rocket to land vertically, I/we should be able to come up with a way to make this work. I’m open to ideas!
I’m not sure if i have seen a certain mechanism that would fit your needs but you can search more about this topic and this is a public topic so many someone from the community would have a similar experience to share.
You can read more about the cars that has both self-driving and manual driving options to check the mechanism for similar ideas.
