[Mech eng.] Feasibility of an omnidirectional wheelchair?

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[Mech eng.] Feasibility of an omnidirectional wheelchair?

Postby Robert'); DROP TABLE *; » Sat Sep 13, 2014 2:47 am UTC

(It's for a sci-fi novel, so sacrificing some amount of feasibility or simplicity to the rule of cool is allowed.)

Is it possible, mechanically, to build a "wheelchair" that simultaneously,
1) can move linearly in any direction without needing to turn (unlike conventional wheelchairs, which tip over if you try to push them sideways)
2) doesn't need a power source or active control to keep it upright
3) can be pushed manually (the example I'd like to include is computer-controlled, but this feels like a prerequisite to have the whole device in the first place, since we wouldn't want the user to get stranded if the computer fails)

I took a stab at researching this earlier, and discovered Mecanum wheels are a thing, but beyond that I'm not really sure, especially about the mechanics of how it'd be controlled. Any help? :)
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Re: [Mech eng.] Feasibility of an omnidirectional wheelchair

Postby bachaddict » Sat Sep 13, 2014 6:37 am UTC

Can normal electric wheelchairs be pushed manually? Mecanum wheels are the way to go, and you can push it in any direction if you disengage the motors.
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Re: [Mech eng.] Feasibility of an omnidirectional wheelchair

Postby Twistar » Sat Sep 13, 2014 4:08 pm UTC

Sorry to be sort of a smartass...

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Re: [Mech eng.] Feasibility of an omnidirectional wheelchair

Postby poxic » Sat Sep 13, 2014 4:12 pm UTC

Smartass chairs don't have engines, though. I don't think I've seen that type of wheel in a self-propelling vehicle.
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Re: [Mech eng.] Feasibility of an omnidirectional wheelchair

Postby Hypnosifl » Sat Sep 13, 2014 4:39 pm UTC

If it's science fiction, maybe you could use perfectly spherical wheels confined within spherical shells with the bottoms cut out (and the tops of the shells attached to the bottom of the chair), with friction minimized either by having the spheres and inner
surface of the shells coated in some kind of super-slippery material, or having them repel each other magnetically (which might require superconductivity to be strong enough to keep them from touching even with the weight of a person on the shells-- existing superconductors should be strong enough, see the image of the levitating sumo wrestler here, and the answer here explains that superconductors can do levitation without the need for an electromagnet, though all modern superconductors do require supercooling...in a science fiction story you could assume the disvovery of room-temperature superconductors though.)

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Re: [Mech eng.] Feasibility of an omnidirectional wheelchair

Postby lgw » Sat Sep 13, 2014 5:27 pm UTC

poxic wrote:Smartass chairs don't have engines, though. I don't think I've seen that type of wheel in a self-propelling vehicle.


Motorized casters are completely reasonable for light loads. Hub motors in the wheels, with additional motors that "point" the wheels, are a very practical solution. With today's technology it would be heavier and take noticeably more power that normal electric wheelchairs, but you could make one. You'd need wheels larger than your typical office chair, at least a millifurlong or so in diameter, for use in paved areas outdoors.

Note that the big wheels on existing wheelchairs are important though: it makes it possible to bump the chair up a stair or two, or to cross smooth unpaved areas. Existing electric wheelchairs with only smaller wheels are more limited in where they can go. However, in a SF story set in ships and stations, maybe that's not a concern at all. (And if you had people growing up in differing gravities visiting one another, I'd expect wheelchairs would be pretty common).
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Re: [Mech eng.] Feasibility of an omnidirectional wheelchair

Postby LaserGuy » Sat Sep 13, 2014 6:44 pm UTC


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Re: [Mech eng.] Feasibility of an omnidirectional wheelchair

Postby EdgarJPublius » Sat Sep 13, 2014 8:32 pm UTC

lgw wrote:Note that the big wheels on existing wheelchairs are important though: it makes it possible to bump the chair up a stair or two, or to cross smooth unpaved areas. Existing electric wheelchairs with only smaller wheels are more limited in where they can go. However, in a SF story set in ships and stations, maybe that's not a concern at all. (And if you had people growing up in differing gravities visiting one another, I'd expect wheelchairs would be pretty common).


For a powered caster wheelchair, you could have the casters on movable arms that allow the chair to 'step' up and down stairs and other obstacles
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Re: [Mech eng.] Feasibility of an omnidirectional wheelchair

Postby bachaddict » Sun Sep 14, 2014 3:48 am UTC

If you want futuristic, spheres are cool.
http://www.technicbricks.com/2010/12/we ... ncept.html
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Re: [Mech eng.] Feasibility of an omnidirectional wheelchair

Postby Robert'); DROP TABLE *; » Mon Sep 15, 2014 1:09 am UTC

Twistar wrote:Sorry to be sort of a smartass...

If you wanted to be a smartass, you could've pointed out that there was a video of a mecanum wheel chair two steps removed from the wiki article. :P

bachaddict wrote:If you want futuristic, spheres are cool.
http://www.technicbricks.com/2010/12/we ... ncept.html
Hypnosifl wrote:If it's science fiction, maybe you could use perfectly spherical wheels confined within spherical shells with the bottoms cut out (and the tops of the shells attached to the bottom of the chair), with friction minimized either by having the spheres and inner
surface of the shells coated in some kind of super-slippery material, or having them repel each other magnetically (which might require superconductivity to be strong enough to keep them from touching even with the weight of a person on the shells-- existing superconductors should be strong enough, see the image of the levitating sumo wrestler here, and the answer here explains that superconductors can do levitation without the need for an electromagnet, though all modern superconductors do require supercooling...in a science fiction story you could assume the disvovery of room-temperature superconductors though.)

A single spherical wheel was my original idea before I realized I didn't know what I was doing. I couldn't work out how to keep it upright passively without putting a free-rolling weight in the bottom, and that would've been very confusing to drive. (Since if the chair stopped suddenly, it'd then try to start moving again) Is it enough to simply build the bottom of a single enclosure out of something very dense and use that to keep stable? (And presumably control it with a group of levers on the chair's arms?)

Also, I didn't realize you could do that with superconductors, thanks for linking it. There are RT superconductors in this setting, but they didn't originally serve any purpose beyond being one of many shiny supertechnologies in the background. If you can use them to build hover-chairs, then that helps me make the impact of technology much more visible. :)

lgw wrote:Note that the big wheels on existing wheelchairs are important though: it makes it possible to bump the chair up a stair or two, or to cross smooth unpaved areas. Existing electric wheelchairs with only smaller wheels are more limited in where they can go. However, in a SF story set in ships and stations, maybe that's not a concern at all. (And if you had people growing up in differing gravities visiting one another, I'd expect wheelchairs would be pretty common).

While the story is set mainly in an enclosed base, I'll keep in mind the wheel size thing. Also, thanks for pointing out the problem of different gravities. Although my current plot doesn't really have characters moving across different gravity wells (apart from moving into roughly Earth gravity after several years in much weaker artificial gravity... might have to think about that one more carefully) it didn't occur to me that that wheelchairs would be a solution.

LaserGuy wrote:Something like this?

Like that, except built less like a tank and more like this.
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Re: [Mech eng.] Feasibility of an omnidirectional wheelchair

Postby Hypnosifl » Mon Sep 15, 2014 1:46 am UTC

Robert'); DROP TABLE *; wrote:A single spherical wheel was my original idea before I realized I didn't know what I was doing. I couldn't work out how to keep it upright passively without putting a free-rolling weight in the bottom, and that would've been very confusing to drive. (Since if the chair stopped suddenly, it'd then try to start moving again) Is it enough to simply build the bottom of a single enclosure out of something very dense and use that to keep stable? (And presumably control it with a group of levers on the chair's arms?)

Why would it need to be especially dense? I was just thinking of an enclosure where the bottom opening is smaller than the diameter of the wheel so it can't pop out, like the ones here:

Image

And if you add in strong magnetic repulsion between the enclosure and the wheel (perhaps with magnets just being arranged in a circle parallel to the ground around the wheel at its widest circumference, so there's no net force upwards or downwards) then you should be able to avoid any friction between the enclosure and the sphere as the sphere rolls.

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Re: [Mech eng.] Feasibility of an omnidirectional wheelchair

Postby Moole » Mon Sep 15, 2014 3:06 am UTC

This might be a little far-fetched/crazy, but you could just make a chair without any wheels, but use a really low friction material to form the base; I suppose the shape of the base wouldn't really matter in a perfect world, as friction doesn't really scale to surface area, but it'd probably be good to have a flat disk at the base of the chair to maintain stability and to spread the load. Then, it could be slid around passively in any direction, and if you wanted an active control system, you could use fans or something to move it. Or, less practically, but way cooler, you could have it shoot itself with lasers, ablating the surface, and thereby pushing itself. (This would all work really well with a wheelchair made out of ice, or a floor that's also an air hockey table - think of the plot possibilities if friction were eliminated from all floors!)
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Re: [Mech eng.] Feasibility of an omnidirectional wheelchair

Postby Neil_Boekend » Mon Sep 15, 2014 12:31 pm UTC

A hovercraft might be a suitable solution.
Stability for spherical wheels can be gained by using three or more of them. I would imagine that powering them would be difficult.
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Re: [Mech eng.] Feasibility of an omnidirectional wheelchair

Postby Robert'); DROP TABLE *; » Mon Sep 15, 2014 2:55 pm UTC

Hypnosifl wrote:
Robert'); DROP TABLE *; wrote:A single spherical wheel was my original idea before I realized I didn't know what I was doing. I couldn't work out how to keep it upright passively without putting a free-rolling weight in the bottom, and that would've been very confusing to drive. (Since if the chair stopped suddenly, it'd then try to start moving again) Is it enough to simply build the bottom of a single enclosure out of something very dense and use that to keep stable? (And presumably control it with a group of levers on the chair's arms?)

Why would it need to be especially dense? I was just thinking of an enclosure where the bottom opening is smaller than the diameter of the wheel so it can't pop out, like the ones here:

Image

And if you add in strong magnetic repulsion between the enclosure and the wheel (perhaps with magnets just being arranged in a circle parallel to the ground around the wheel at its widest circumference, so there's no net force upwards or downwards) then you should be able to avoid any friction between the enclosure and the sphere as the sphere rolls.

While I may be badly visualizing the mechanics involved, the bottom should be dense to drag the center of mass down, so that when the chair (de)accelerates, it tilts as little as possible.
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Re: [Mech eng.] Feasibility of an omnidirectional wheelchair

Postby Twistar » Mon Sep 15, 2014 9:40 pm UTC

Neil_Boekend wrote:A hovercraft might be a suitable solution.
Stability for spherical wheels can be gained by using three or more of them. I would imagine that powering them would be difficult.


This was the first thing I imagined (I actually imagined it before the desk chair :-p). Imagine enclosure for an old mouse wheel. the wheel sticks out the bottom and when it rolls it rotates two perpendicular cylinders inside the mouse which detect motion. Instead of the rolling of the ball causing the cylinders to spin to detect motion, hook the cylinders up to a power source so they can rotate the ball. Put 3-4 of these about 30-40cm diameter (and chrome so they look cool) at the bottom of a high tech looking chair and you have a pretty sweet sci-fi ride IMO.

Note that this design can be pushed manually, but it would be hard for whoever is sitting in it to control it manually themselves like in wheelchairs today.

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Re: [Mech eng.] Feasibility of an omnidirectional wheelchair

Postby speising » Mon Sep 15, 2014 9:56 pm UTC

note that a manually pushed wheelchair needs to be as light as possible, and have an efficient way of transferring its users arm power to the ground. which is done by current designs quite nicely. also, the things can turn on the spot, i don't see the advantage of sidewise motion.
Last edited by speising on Tue Sep 16, 2014 5:53 am UTC, edited 1 time in total.

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Re: [Mech eng.] Feasibility of an omnidirectional wheelchair

Postby Copper Bezel » Tue Sep 16, 2014 4:18 am UTC

Given that the chair and occupant are not radially symmetric around the turning axis, situations can arise where obstructions would be easier to circumnavigate with a sideways slide.

Edit: Which is to say, "damn footrests get in the fucking way all the goddamn time and I don't know what even."
So much depends upon a red wheel barrow (>= XXII) but it is not going to be installed.

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Re: [Mech eng.] Feasibility of an omnidirectional wheelchair

Postby Neil_Boekend » Tue Sep 16, 2014 7:36 am UTC

Twistar wrote:
Neil_Boekend wrote:A hovercraft might be a suitable solution.
Stability for spherical wheels can be gained by using three or more of them. I would imagine that powering them would be difficult.


This was the first thing I imagined (I actually imagined it before the desk chair :-p). Imagine enclosure for an old mouse wheel. the wheel sticks out the bottom and when it rolls it rotates two perpendicular cylinders inside the mouse which detect motion. Instead of the rolling of the ball causing the cylinders to spin to detect motion, hook the cylinders up to a power source so they can rotate the ball. Put 3-4 of these about 30-40cm diameter (and chrome so they look cool) at the bottom of a high tech looking chair and you have a pretty sweet sci-fi ride IMO.

Note that this design can be pushed manually, but it would be hard for whoever is sitting in it to control it manually themselves like in wheelchairs today.

I would imagine that the rollers would cause friction. If two are placed perpendicular there is always one with the rolling direction perpendicular to the motion of the ball.
It would be cool to have a few magnets in the ball and coils around it to have both low friction bearing and power transfer, like a maglev train.
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Re: [Mech eng.] Feasibility of an omnidirectional wheelchair

Postby Twistar » Tue Sep 16, 2014 4:20 pm UTC

Neil_Boekend wrote:
Twistar wrote:
Neil_Boekend wrote:A hovercraft might be a suitable solution.
Stability for spherical wheels can be gained by using three or more of them. I would imagine that powering them would be difficult.


This was the first thing I imagined (I actually imagined it before the desk chair :-p). Imagine enclosure for an old mouse wheel. the wheel sticks out the bottom and when it rolls it rotates two perpendicular cylinders inside the mouse which detect motion. Instead of the rolling of the ball causing the cylinders to spin to detect motion, hook the cylinders up to a power source so they can rotate the ball. Put 3-4 of these about 30-40cm diameter (and chrome so they look cool) at the bottom of a high tech looking chair and you have a pretty sweet sci-fi ride IMO.

Note that this design can be pushed manually, but it would be hard for whoever is sitting in it to control it manually themselves like in wheelchairs today.

I would imagine that the rollers would cause friction. If two are placed perpendicular there is always one with the rolling direction perpendicular to the motion of the ball.
It would be cool to have a few magnets in the ball and coils around it to have both low friction bearing and power transfer, like a maglev train.


Sure. The question is how much friction is acceptable based on how much power you can provide. I'm sure it could be made to work. The idea with the magnets and coils of course cranks up the coolness/scifi factor for sure. I don't know about superconducting coils unless they already have cheap high-Tc superconductors in the story. The balls themselves could just be magnetic and you could probably somehow use eddy currents to put forces on them. Bear in mind that no matter what you do there will always be kinetic friction between the ball and the ground. This friction is limited by the rigidity of the metal balls which cannot be infinite.

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Re: [Mech eng.] Feasibility of an omnidirectional wheelchair

Postby Copper Bezel » Tue Sep 16, 2014 5:17 pm UTC

The rollers could slide to engage and disengage from the ball or be covered in little free rollers on the other axis in little torus loops (cylindrical bearings.)
So much depends upon a red wheel barrow (>= XXII) but it is not going to be installed.

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Re: [Mech eng.] Feasibility of an omnidirectional wheelchair

Postby Hypnosifl » Wed Sep 17, 2014 6:32 pm UTC

If the wheels are spherical you could always just have an opening in the top of the enclosure as well as the bottom, so you could push them in any direction with your hands like giant computer trackballs. And since the original idea was that it just be capable of operating manually in case the motor breaks down, when the motor was working you could just have electric rollers pushing the wheels, connected to something like a joystick control.

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Re: [Mech eng.] Feasibility of an omnidirectional wheelchair

Postby lgw » Wed Sep 17, 2014 7:55 pm UTC

EdgarJPublius wrote:For a powered caster wheelchair, you could have the casters on movable arms that allow the chair to 'step' up and down stairs and other obstacles


For occasional off-road use, you can also go with the tachikoma approach - basically fully articulate legs with wheels on the end that can be folded out of the way when walking. Even with today's tech, the tech for avoiding stepping on people is surprisingly good - having it actually be totally safe wouldn't be a stretch at all for SF (and assuming you didn't also need an urban battle tank, the passenger wouldn't need to ride in the back).
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Re: [Mech eng.] Feasibility of an omnidirectional wheelchair

Postby Elmach » Sat Sep 20, 2014 3:53 am UTC

Neil_Boekend wrote:
Twistar wrote:
Neil_Boekend wrote:A hovercraft might be a suitable solution.
Stability for spherical wheels can be gained by using three or more of them. I would imagine that powering them would be difficult.


This was the first thing I imagined (I actually imagined it before the desk chair :-p). Imagine enclosure for an old mouse wheel. the wheel sticks out the bottom and when it rolls it rotates two perpendicular cylinders inside the mouse which detect motion. Instead of the rolling of the ball causing the cylinders to spin to detect motion, hook the cylinders up to a power source so they can rotate the ball. Put 3-4 of these about 30-40cm diameter (and chrome so they look cool) at the bottom of a high tech looking chair and you have a pretty sweet sci-fi ride IMO.

Note that this design can be pushed manually, but it would be hard for whoever is sitting in it to control it manually themselves like in wheelchairs today.

I would imagine that the rollers would cause friction. If two are placed perpendicular there is always one with the rolling direction perpendicular to the motion of the ball.
It would be cool to have a few magnets in the ball and coils around it to have both low friction bearing and power transfer, like a maglev train.


Actually, this problem [friction from roller-sphere interface] is easily/already solved. I distinctly remember an already existing device that did something like the following:

Place a perpendicular roller on the roller-sphere interface, like this, but smaller. [EDIT: Or, I could have just mentioned the Mecanum link earlier in the thread...]

[I think this device is one of the old mice -- if you can use it as a sensor, then you could just swap it to make it a motor.]

That way, you get two controllable degrees of freedom on the sphere.

EDIT: Actually, doesn't the four-Mecanum wheel drive already work? It lets you go forward-backward, strafe, and turn, which is all you need, right?

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Re: [Mech eng.] Feasibility of an omnidirectional wheelchair

Postby Mokele » Sun Sep 21, 2014 4:30 pm UTC

Could you simply decouple the drive from the support? Have 3-4 spherical wheels or casters for support alone, and in the center a single powered wheel that can rotate 180 and spin in either direction. To turn, just slightly lift the center wheel and rotate to the desired direction. If the rotation motor is strong enough, you could even have the reverse to pivot - plant the center wheel extra hard and spin it, resulting the the chair spinning in place.

In a way, it's got more parts, but by decoupling support and propulsion/steering, you can make the parts themselves simpler and the structure as a whole more robust.
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Re: [Mech eng.] Feasibility of an omnidirectional wheelchair

Postby Copper Bezel » Mon Sep 22, 2014 6:51 am UTC

That poses some major problems for the idea of making the carriage capable of clearing obstacles. Of course, I kinda think the thread was won with the first reference to mecanum wheels, and what's followed is just spinning our collective ... spheres?
So much depends upon a red wheel barrow (>= XXII) but it is not going to be installed.

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Re: [Mech eng.] Feasibility of an omnidirectional wheelchair

Postby Neil_Boekend » Mon Sep 22, 2014 8:10 am UTC

Elmach wrote:EDIT: Actually, doesn't the four-Mecanum wheel drive already work? It lets you go forward-backward, strafe, and turn, which is all you need, right?

Yep, there are forklifts with such a drive. The Mythbusters have one.
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Re: [Mech eng.] Feasibility of an omnidirectional wheelchair

Postby Positron » Mon Sep 22, 2014 12:09 pm UTC

lgw wrote:Note that the big wheels on existing wheelchairs are important though: it makes it possible to bump the chair up a stair or two, or to cross smooth unpaved areas. Existing electric wheelchairs with only smaller wheels are more limited in where they can go. However, in a SF story set in ships and stations, maybe that's not a concern at all. (And if you had people growing up in differing gravities visiting one another, I'd expect wheelchairs would be pretty common).


Since no one seems to have addressed this, couldn't you use those Mecanum wheels in an arrangement like stair climbing trolleys? I'm sure with some twerking it could be made to work for wheelchairs...
Last edited by Positron on Mon Sep 22, 2014 12:12 pm UTC, edited 1 time in total.

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Re: [Mech eng.] Feasibility of an omnidirectional wheelchair

Postby Hypnosifl » Mon Sep 22, 2014 1:22 pm UTC

Elmach wrote:EDIT: Actually, doesn't the four-Mecanum wheel drive already work? It lets you go forward-backward, strafe, and turn, which is all you need, right?

Do Mecanum wheels really fit the original question's requirement that it should be possible to manually push the wheelchair in any desired direction if the power runs out, though? How would you get all the rollers spinning at the right rate with your hands?

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Re: [Mech eng.] Feasibility of an omnidirectional wheelchair

Postby scarecrovv » Mon Sep 22, 2014 5:59 pm UTC

Hypnosifl wrote:Do Mecanum wheels really fit the original question's requirement that it should be possible to manually push the wheelchair in any desired direction if the power runs out, though? How would you get all the rollers spinning at the right rate with your hands?

I think that if you open-circuit the motors the wheels would spin freely. The forces that work from the axles to the ground should work equally well in the reverse direction. You just push it and it moves.


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