Nice project with a lot of useful things, both educational for your son and beneficial for your daughter.

A concern that I have is that controlling the direction of one of the front wheels may not be enough to counter the turning torque of having just one of the big wheels being driven -- it may just skid the front wheel, which generally don't have decent tread on them to begin with. But worth a try!

We've already tested it manually (which is something I always do when starting a new design), and it works perfectly. Really... my main concern is if she has enough control of her right arm for it to be useful for this purpose...
Anyway... she's really enthusiastic about this little venture of ours, she was just beaming when we told her what we were up to... and here's proof of that

 

Steering a wheelchair I assume involves controlling the angle of both front wheels, not just one (as in a bike). If that's true, then your mathematics will be even more complex. When a car's front wheels steer left or right, they don't both steer to the same angle. One might steer 20deg and the other 17deg (example, made up numbers). This is by design and if it were otherwise, the car would be harder to turn and the tires would wear faster. Imagine two bikes side by side and fastened together, do your bike track math, but for which one? The inside One is forced to turn a tighter radius than the outside one. Or is the outside one forced to turn a larger radius? Or some mix of both? How do you calculate the bike track of either one? And then i guess it also depends on how tight the radius is? And the tire material? And the rolling surface?

I think this situation lends itself better to empirical discovery than mathematical calculation.

 

I think this situation lends itself better to empirical discovery than mathematical calculation.

Well said... some things are best first experimented with and then calculated afterwards... unlike some other thread that I'm beginning to think is going nowhere...
Anyway, keep in mind that a wheelchair's front wheels have a directional axis that lies behind their axis of rotation, effectively converting them into follower wheels. So the rest of the wheelchair will simply follow whatever direction one of the front wheels is forcing them to.
The main issue here is if the wheel being turned has enough traction to do that... remember that any rigid structure is stated as statically determined (gravity-wise) when it has three, and only three, points of support. In a wheelchair there are four. And therefore one of the wheels may at one time lose contact with the ground and lose traction... Guess we're gonna have to wait and see how that works out. And if it doesn't, we can later add a spring pressing downwards to that or the other front wheel to guarantee that it always makes contact with the ground.

 

If you've got good enough traction you can do it with just steering one wheel and letting the other caster.

Regards to cars -- the Ackerman steering arm, which takes care of the issues you pointed out, is a beautiful solution to a rather hard problem.

 

If you've got good enough traction you can do it with just steering one wheel and letting the other caster.

Regards to cars -- the Ackerman steering arm, which takes care of the issues you pointed out, is a beautiful solution to a rather hard problem.

What a beautiful example of good engineering design that is... thanks for sharing. Here's another link, comparing different approaches to the same problem.

 

Hola CM,

Now I undersand. Had to read yours twice.

The first that came to my mind was a system equivalent to what is used to maneuver the fast ferries (called sometimes "piercing waves ferries") when berthing / leaving port. A joystick, controlling the propulsion (orientable water jets able to be reversed independently) so when the Master push the lever to go "there" the whole things is adjusted for the ferry to move on "that" direction.

The mechanical equivalent I heard about is of vehicles on tracks where they are controlled in a similar way.

Manually, I had the pleasure to drive and make turn quite quickly a machine (on tracks) used in the mining industry, but in this case, by controlling each track independently.

With vessels fitted with two propellers, that is, one at each side of the rudder, you (can) do the same. Even more, most of modern tug boats are built able to turn around its own center, if needed, instead of around a point external to them.

To replicate this the big wheels should be motorized...but the fun would increase!

 

First, I'm glad to hear that something useful has been created. Great lesson for the kids involved.

Just thinking to another solution, wouldn't it be possible to have two hand wheels both be on the same side of the chair? So for example the left wheel would be powered normally. The power for the right wheel could come from the left arm with a concentric shaft. This would obviously require a redesign nearly from the ground up, but it should be possible.

This can't be the first time someone has needed to design a one-armed wheelchair. I wonder how others have solved the problem.

 

First, I'm glad to hear that something useful has been created. Great lesson for the kids involved.

Just thinking to another solution, wouldn't it be possible to have two hand wheels both be on the same side of the chair? So for example the left wheel would be powered normally. The power for the right wheel could come from the left arm with a concentric shaft. This would obviously require a redesign nearly from the ground up, but it should be possible.

This can't be the first time someone has needed to design a one-armed wheelchair. I wonder how others have solved the problem.

Interesting idea... but the wheelchair has to be folded, and so an axis between the left and right wheels would have to be collapsible too, complicating the design a little bit... I'll give it some more thought anyway, though.... I appreciate the time and effort you've all placed in this thread... in fact, I've been thinking about opening another thread requesting ideas (any ideas, either electronic or mechanical) for devices and tools that could help my daughter cope in daily life. But to do that, I first have to make a list of her limitations, and possibly post a video or two.

 

The most important and potent tool she has is her attitude.

 

The most important and potent tool she has is her attitude.

You have no idea... she's a fighter... she's persistent, and very enthusiastic... wish I had all her virtues...