some advice needed

hello all,
i am mihir, i am working on a college project where i have to design a wheelchair for quadraplegics.
i thought ill use a hub motor mounted on a standard wheelchair to keep my costs down. but i am not exactly aware of all that goes into making this possible.

my needs are a maximum speed of 6-8 kmph on an incline of say 5-10 degrees with a person weighing 90 kgs on the wheelchair.

heres what ive gathered so far.
ill definitely need a hub motor... im looking at a 180W 24V motor for my needs.
ill definitely need a battery pack... im looking at NiMH and LiFePO4 packs.. but they seem to be expensive
ill need a joystick controller..

Now, im not sure what configurations i need for each... all the units and measures are confusing.. all i got is that i need
24 V batteries for 24V motor but am clueless about the controllers and the current ratings..
also i need to keep my costs as low as possible... so will VRLA batteries prove cheaper ??
and do i still need a controller after a joystick controller or will the later suffice..

also please try not to use any brand names as they might not be available in India and might just end up misguiding me !
thanks in advance !

Most bike hubmotors will have some trouble with the steeper inclines at slow speed, unless the wheels are very small. A geared motor would work, but it won't do reverse due to the internal freewheel. I'd start looking at chain and belt drive scooters for motors. Lots of them, the stand up scooters , are the right wattage, and the gears can be changed to lower the speed and increase hill performance.

The old 24V wheelchair I have uses two 90-degree gearbox motors to drive the 12" rear wheels independently. (I have not explored the system further, since I will give it to some kids who are learning robotics.)

P.S. If you change the title of your post to "mobility vehicle design advice needed", you might get more replies.

Not sure if this will help you but itselectric.us has a controller that will operate two hub motors at the same time. YOu could have one motor on either side of the wheelchair.

Just an idea - if you used a 3-wheel design (similar to some "mobility scooters" that are in use here in the States - like the kind you see in grocery stores), you could use just one motor in a small, steerable front wheel. This assumes that you're trying to keep the project to just 1 motor. Scooters like this that I've seen generally have a steering column and handlebars to steer, but I imagine one could be easily modified to be electronically steered with a joystick and a servo motor (for steering, separate from the drive motor).

Basically, I think starting with a standard wheelchair may cost you more than it saves you on a project like this, as you have to provide for drive force to 2 BIG wheels. By putting power to 1 small wheel, you'll have better slow-speed torque, and because of the torque advantage, you'd be able to (probably) use a non-geared motor, which would allow you to run the motor in reverse.

*edit*

Another crazy thought - perhaps you could retrofit a standard wheelchair with a small, steerable drive wheel mounted under the seat (instead of in front of the feet, like on the mobility scooters discussed above). That way, you could modify a standard chair like you mention, without worrying about putting power to the big wheels. Just make sure you put in a safety feature so that you can't gun the throttle and flip the chair over backwards!

mrgarci1 said:

Just make sure you put in a safety feature so that you can't gun the throttle and flip the chair over backwards!

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I'm probably going to hell, just for laughing at that. An old friend of mine had MD and was a CRAZY driver. She would be trying to wheelie the thing for sure.

hello again !
thanks a ton for all your help !

i think i should hjave made my idea clearer.. my bad...

anyways.. the idea is to design a kit that would include a drive system, a battery pack and a controller.. you know the kinds you just get home and strap onto your wheelchair.. now the constraint is that it should fit any manual wheelchair irrespective of make and model..

for this i plan to use a hub motor (24V / 180W) with an inflatable tyre mounted on it (6" or 8"), a NiMH or LiFePO4 battery pack (24V / 10AH) and a controller and a joystick controller...

@dogman: i am thinking of using the small wheels.. like say 6" or 8"..
@mrgarci1: i am planning to provide a direct drive to the ground using the hub motors.. and even i had that crazy idea you had !! might work well... ill only need a handy tipper at the rear..

mihir said:

hello all,
i am mihir, i am working on a college project where i have to design a wheelchair for quadraplegics.

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So far it seems to me that you might be missing a major issue:

From Wikipedia: Quadriplegia, also known as tetraplegia, is a symptom in which a human experiences paralysis affecting all four limbs, although not necessarily total paralysis or loss of function.

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How do you suppose the joystick will be operated by someone with quadriplegia?

Or do you mean "Paraplegia", which

From Wikipedia: is an impairment in motor and/or sensory function of the lower extremities.

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I have seen an electric wheelchair operated by joystick, but it was for a person with spastic paresis, and that was tricky enough! Safety features against runaway wheelchairs are a must, because the users will (more often than not) have major problems with fine motor control.

Another steering/controlling mechanism I have seen in action was a microphone like looking controller, moved with the lips and head of the quadriplegic driver of the wheelchair. It might have had an adapted joystick inside, I'm not sure.

The control mechanism must be highly adaptable to the individual requirements/problems/abilities of each wheelchair user.

The motor controller should be programmable, so that the response to steering inputs can be dampened or enhanced.

Some people could use voice control; some can only use eye movements! And some have lost control over all voluntary muscle function; these people might be able to communicate via implanted computer sensors, the research is well under way!

So, keep it flexible!

@mrmik: thanks for pointing that out ! quadraplegia as you said is a loss of control over the four limbs.. but in this too there are various degrees of paralysis depending upon the location of the injury along the spine... the higher you go, the more severe it is...
i am aiming at providing mobility solutions people who can move their hands but not their fingers... or in other words who can push the joystick, albeit roughly with their hands and not their fingers, but not grab the push rims on the wheelchair...

mihir said:

@mrmik: thanks for pointing that out ! quadraplegia as you said is a loss of control over the four limbs.. but in this too there are various degrees of paralysis depending upon the location of the injury along the spine... the higher you go, the more severe it is...
i am aiming at providing mobility solutions people who can move their hands but not their fingers... or in other words who can push the joystick, albeit roughly with their hands and not their fingers, but not grab the push rims on the wheelchair...

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Most of them will require a "training mode" where large movements of the joystick cause little response. Once trained, which will take various amounts of time depending on circumstances, the response curve can be adjusted.
The severity of the movement difficulties can also vary withinn the same person from day to day, or within a day.

People with severe osteoarthritis for example can be good one day and very stiff the next, or stiff in the morning and better in the evening.

That is why I think you should choose a programmable motor controller.

So your comment about how this would be a retrofit kit for standard wheelchairs gave me a vague idea of how a project like this might come together. I picture it as a 3-piece unit: battery/controller box which could be suspended under the seat either dynamically or rigidly, a joystick/controller that could velcro to the armrest (or other suitable fastening), and a single drive wheel on a springloaded fork that would allow you to mount the kit in different types of chairs and ensure that the wheel would always hit the ground, and would always have enough normal (downward)force to push the chair without the wheel "burning out" - where the wheel touches the ground, but just spins/skids along the ground when power is applied. The motor arm could be mounted to the battery box (which would remove 1 step in the retrofit process) but then you'd have to reeeaaally firmly mount the box. Alternately, you could just hang the battery box off whatever point on the chair you please (behind the seat, under the seat, behind the footrests), and have the motor fork bolt directly onto the structure of the chair itself. I don't have any experience with wheelchairs, but I agree with you that the fairly standardized design combined with the solid structural integrity would probably lend itself well to a bolt-on kit similar to what many e-bikers have purchased.

@mr. Mik: what kinds are these programmable motor controllers that you are talking about ?? like do they make the joystick more or less sensitive as per the force applied by the user...

@ mrgarc1: BINGO !!!
very close to what i was thinking... instead of mounting the battery and the drive separately i was thinking i will have the battery and the motor in a single unit... this unit will be mounted under the seat using two telescopic arms on either side.. the arms will be bolted onto the frame.. and the spring loaded fork will be between the telescopic arm and the unit casing, providing enough the normal force ill need... the sole purpose of using the telescopic arms is to make the unti adaptable to different wheelchair widths...

also please can someone tell me if a controller unit and a joystick controller is the same... really confused about that...

2 problems I see.

First, hub motors were designed to run best at close to full speed, and are poor preformers at low speeds. you would use up batterys very quickly and possably burn up motors trying to crawl around at speeds you would normaly roll a wheel chair around a house at.
WHat is needed is a geared motor. unfortunatly, geared hub motors are unidirectional, and freewheel the other direction. they also are double the cost of a conventional hub motor.

the alternative is a geared chain drive like the currie, or adapting a traditional wheelchair motor. they can be found surplus for reasoabley cheap.



The other problem I see is steeering. a para could probably run a single drive wheel on a tiller easily, and problem solved, but a Quadra can't, and that means either running 2 drive wheels and differintial steering, or one motor, and some complex electric steering servo mechanism. it gets more complicated if you add on small wheels to a conventional wheelchair, then try to turn it from a hallway into a bedroom on shag carpet. of the 6 wheels, 2 will steer, 2 will drive, and 2 will have to skid. Thats not good for the carpet, the motors, and since most conventional wheelchairs are lightweight colapsable aluminum frames, not so good for the frame.


One possable solution is to eliminate the main wheel and replace it with a subframe for the drive wheels that bolts onto the original wheel's axle mounts.

I agree with Drunkskunk that the geared motor is a much better solution than a direct-drive for efficiency reasons, but I think you could easily design it in such a way that having a single drive wheel w/a geared motor mounted directly under the seat would give you all the directional control you'd need, especially if you mounted it with the fork directly vertical above the wheel. Think about it - you couldn't go straight backwards, but you could turn a complete circle inside the wheelbase of the chair, using a servomotor to control the direction of the drive wheel. Alternately, you could design the controls with a drive direction indicator of some sort so that you could simply turn the drive wheel to point backwards, and go in reverse that way. As far as joystick controllers go, this is probably not the best forum for that info- I don't know that too many folks here have experience with that type of controller (but I could easily be wrong).

*edit*

As far as the quadra/para question goes, as has been mentioned previously, there are varying degrees of paralysis that fall into these categories. Someone who is a quadriplegic may have a neck injury and have zero feeling from the neck down, or they may have a degenerative nerve disease (for example) which has removed the fine muscle control and strength needed to operate a regular wheelchair, but still allows enough motion of the hands/arms to control a joystick. Obviously, this idea, as with any medical product, would not be suitable for everyone.

@drunkskunk: i saw a chart on this forum posted under the title "how much power do i need" which gave across-relation between speeds and wattage... looking at this chart as per the requirement is how i arrived at the decision of using a 180W motor.. heres the chart..

it says that to achieve a peed of 5mph on a 6% gradient ill need a 162W motor

http://endless-sphere.com/forums/viewtopic.php?f=16&t=1499

is there something i am missing ??

I would suggest that you check out teamhybrid.co.uk. They sell both e-bikes and powered wheelchairs. They call their powered wheelchairs "Powered Hand Cycles". Look at the "Quad Viper" and the "Viper Kit" for ideas.

Joey

mihir said:

is there something i am missing ??

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Yes, the efficancy of the motor at a given speed. a Hub motor is most efficent near top speed, around 80% under load. at low speeds the efficancy realy falls off.

162 watts is what needs to actualy reach the ground, after factoring the inefficancys of the motor. If the motor can only opperate at 20% efficancy at 5mph, then it would need 810 watts. thats hard on the motor, but not impossable.
the realy problem will be even lower speeds. 5mph is Fast for wheelchair. its faster than walking speed. moving around in doors, 1mph would be fast, 1/4 mph would be more normal, and at those speeds the motor might be only 0.5% efficent. it might only need 20 watts to the ground to move, but take 4,000 watts to move. Most motors can't take that sustained, and it would suck batteries dry in minutes.

Thats where geared motors come in. since they have a gear reduction, they can be geared to run closer to there peak efficancy at the speeds you need, and since gear reduction is a torque multiplyer, they don't suffer from as much efficancy loss at lower speeds than optimum.

Hey Mihir, I had another thought on your project. Now, this is probably going to draw some heat from other posters, but bear with me.

What about instead of a hubmotor drive, using a pair of friction-drive servomotors/RC motors that clamp on to the wheels? Friction drive has plenty of disadvantages, but clamping the drive wheel firmly to the chair wheel could cut some efficiency losses (due to the interface between the drive & chair wheels), and using a small drive wheel against the big chair wheel gives you a built-in gear reduction which would greatly increase the efficiency of the motors, and would give you way more range from a given battery pack. Finally, this would also be fairly easy to construct & mount - I bet there are RC motor controllers built for 2 servomotors, and using a joystick to control R/C or servomotors is certainly nothing new.

Let me know what you all think.

mrgarci1 said:

Hey Mihir, I had another thought on your project. Now, this is probably going to draw some heat from other posters, but bear with me.

What about instead of a hubmotor drive, using a pair of friction-drive servomotors/RC motors that clamp on to the wheels? Friction drive has plenty of disadvantages, but clamping the drive wheel firmly to the chair wheel could cut some efficiency losses (due to the interface between the drive & chair wheels), and using a small drive wheel against the big chair wheel gives you a built-in gear reduction which would greatly increase the efficiency of the motors, and would give you way more range from a given battery pack. Finally, this would also be fairly easy to construct & mount - I bet there are RC motor controllers built for 2 servomotors, and using a joystick to control R/C or servomotors is certainly nothing new.

Let me know what you all think.

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Very interesting idea!

That would also allow a carer to "push" a wheelchair around by using the remote control.

But of course it is hard to guess if there are suitable anchor points on most wheelchairs without having one here to look at.