Cheapest, Easiest Drive System for a 150 lb robot?

latigerlilly

Hi guys,

I am building a robot wheelchair for a quadraplegic friend. I estimate that the entire package (friend, robot, battery, 10 lb fudge factor, & 2 lb payload of whatever he wants to carry) should be about 150 lbs. I want to save money and not have to use an expensive wheelchair motor as I am doing this for him for free. I was thinking, could I fudge it and use a cheap windshield wiper motor? I am looking at the following motors;

1. Most powerful windshield wiper motor I could find, AME 12 VDC, 212 lb-in gearmotor:
www.robotcombat.com/marketplace_motors_ame.html

2. Cheapest windshield wiper motor I could find, MECI 12 VDC worm gear motor:
www.scary-terry.com/wipmtr/wipmtr.htm

I am looking for the cheapest motors that can do the job (I am only 17, so I don't have much money). I am also thinking about using the HB-25 Motor Controller so the motor should be compatible with this:
www.parallax.com/detail.asp?product_id=29150

Then, use 2 cheap scooter wheels up front (each wheel is powered by a separate gearmotor) and a caster in the back to make a big, slow BOE-BOT.

He is very depressed due to his lack of mobility. He has to rely on others to be mobile but his friends and family are always busy working, going to school, etc. So, he cannot go anywhere, except on the weekends. He basically lives for his weekend trips to the supermarket. With a robotic wheelchair, he could gain mobility and freedom. He could go to the Starbucks down the street, video store, supermarket, and restaurant.

Thanks for answering my question,
Lilly.

Post Edited (latigerlilly) : 1/21/2007 5:37:31 PM GMT

Zoot

First, let me say I am *not* trying to discourage you from building this project. However, I would encourage you to think carefully about this project.

You might really want to think about finding a good used electric wheelchair. By the time you add up cost of motors, parts, a Stamp, a motor controller, construction materials and your own time, you may find that you are approaching the cost of a used wheelchair. I found several on Ebay in the < $500 range. I also say you might want to consider this because you're not talking about any old robot here -- you're talking about a chassis that will be carrying around a human being who can not carry himself. You don't want to compromise safety!

Here are some that I found:

search.ebay.com/search/search.dll?cgiurl=http%3A%2F%2Fcgi.ebay.com%2Fws%2F&fkr=1&from=R8&satitle=electric+wheelchair&category0=&submitSearch=Search

That said, the motors you found at Robot Marketplace may be adequate if you reduce speed / increase power with a drive belt or similar. I also know that Robot Marketplace sells more powerful motors (but they are also more expensive).

Again, I'm not trying to be discouraging -- what you are trying to do is very honorable.

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When the going gets weird, the weird turn pro. -- HST

latigerlilly

Hi guys,

It is not just a wheelchair. He has use of his head and neck, but very little use of his arms, and no use of his legs. I realize that this is a VERY difficult project for a relative newbie (I have built a BOE-BOT and two medical devices, so far only), but I have all the time in the world. Plus, I don't have to do this all at once. I am currently being homeschooled and my parents won't let me drive, so I'm just at home fiddling with "cool" experiments all day long... (well, after I finish with the math/English homework my tutor gives me)....

BTW, my friend saw my post and he corrected me. I spelled it "quadraplegic" but it is spelled "quadriplegic". Plus he is NOT quadriplegic. He is tetraplegic....

I want to build something like this:
www.eng.usf.edu/~alqasemi/wmra/DSC00492.JPG

I did not find this on ebay.

Do I really need a belt drive? It is a gearmotor with reduction gear and 100 rpm (approx) only. If I make the wheelchair low and ground hugging, I can use small wheels, as in 30 inch circumference wheels.

30 in x 100 rpm = 3000 in/min x 60 min/hr = 180,000 in/hr
180,000 in/hr x 1 ft/12 in = 15,000 ft/hr
15,000 ft/hr x 1 mile/5280 ft = 2.8 mph
2.8 mph is the maximum speed. If I use the controller to pulse it, I can move it even slower.

This is why I think the windshield wiper gearmotor will (theoretically) work, but I am not sure since I do not know how strong the gears inside are, etc. So, I'm here asking you guys (please, please) let me know if it will work;

Wikkipedia says that the average athlete can put out 0.3 hp for "hours". The average human can put out 1.5 hp for a short "burst" and output 0.1 hp "indefinitely".

The AME motor has 12 vdc and 21.3 amps at stall.
12 vdc x 21.3 amps = 255.6 watts
255.6 watts x 0.0013 hp/watt = 0.33 hp
As the wheelchair will be a giant BOT-BOT, it will have 2 of these motors. So, it has the output capacity of twice that of a trained athlete.

The other motor is cheaper, but has less power;

12 vdc x 14 amp at stall = 168 watt
168 watt x 0.0013 hp/watt = 0.22 hp
This might work, but again, I don't know.

Thank you and much appreciation,
Lilly.
P.S. Here's another pic of what I want to build: www.eng.usf.edu/~alqasemi/wmra/DSC00478.JPG
However, the guy that built the robot wheelchair in the pic has built a really huge (and expensive) robot arm. I would like to build a human-sized robot arm that can do finer manipulations.

Post Edited (latigerlilly) : 1/22/2007 6:10:40 AM GMT

Zoot

There are other members of the forums more qualified than I on some of the specific specs and physics, but here's a few things off the top of my head to consider:

- you need drive belts of some kind, I think. One reason is because it makes it much easier to change the drive ratio down the road without serious rebuilding (as simple as a new length belt and a change of drive pulley on the motor). The second reason is that the belts can be adjusted for tension so that you have a "clutch" -- if the wheels are totally locked for some reason, the motors won't burn out and/or trip fuses (which would leave your friend seriously stuck). Third reason is it will really help dampen motor vibration and noise that will carry through the chair frame into your friend's bottom. You can mount the motors with rubber motor mounts (hit an auto parts store/website) and with a rubber belt drive you have the motors pretty well isolated from the frame. This also gives you a way to *electrically* isolate the motor housing if you need to.

- brakes?

- fuse the heck out of it. Rate your fuses so that the WIRE you are using won't get so hot that it melts it's own insulation (in the event of short circuit or motor stall)

- I'm guessing you'll use some kind of sealed lead-acid battery -- plenty of juice. I would install a cigarette lighter socket for auto "accessories" -- your friend can keep a cell phone plugged in and charged whenever he is out. The minor drain on the battery to float charge the phone would be insignificant compared the value of the phone as a way to call for aid

- put in an auto horn and/or tail lights (blinking LEDs?) so that he is visible/audible. Heck if you're running it with a Stamp, you could deck the chair out

- the windshield wiper motors are cheap enough maybe you should just try them? You wouldn't be out too much $$ if they didn't work out, before spending a lot more.

- what are you going to use for a control for your friend? I.E., what will he use to drive the chair?

- I don't think the project is difficult from a programming or control perspective (like you said, it's basically a big BOE-BOT), but the physical construction of the thing is. Are you starting with a regular old wheelchair for a base?

Lastly, I have a friend who has colleagues working in assistive technologies at MSU. She's going to try to run down some contacts for the research dept. over there and they may be able to give you advice, standards, ideas, etc (no promises, but I'll let you know what she turns up). She also suggested contacting any nearby university -- often assistive technology departments are looking for collaborators to help test out and evaluate new equipment.

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When the going gets weird, the weird turn pro. -- HST

Kevin Wood

Hi Lilly. You didn't mention your friend's name, but please say hello to him for me.

From your post, it's obvious that you want to do what you can to help your friend. However, I think your enthusiasm might be better spent in other ways.

Instead of trying to build something, why not start a fund raiser? Even a blog or a website? I think you would have better success, and you would be able to purchse the best that you could find for your friend. You might even be able to help others in the same situation with the same effort.

latigerlilly

Kevin,

In case you haven't noticed, this is Fun . It's not a personal crusade to help my friend. Besides, I'd rather be an engineer than a cheerleader/fundraiser begging people to donate money towards a piece of technology that is not even commercially available. Even if I got the money, I still wouldn't be able to buy it.

Zoot,

You are a real knight. Thanks (smooches) .

That was some really good advice. I was thinking big BOE-BOT and since the BOE-BOT doesn't have brakes, pulleys, motor vibration isolators nor fuses, I hadn't even considered these things! (Doh!)

I think having a pulley would add to the cost, but it would be worth it since I could blow the motor and/or controller if the motor gets caught where it wouldn't spin but power is being applied. A motor with a pulley would just result in a spinning motor that is not spinning the drive belt in such a situation. This would prevent the overload.

A fuse is a good idea to prevent a fire hazard.

Brakes are nice for obvious reasons.

Motor vibration... never considered this one. This might be skipped if I use a gel cushion and if the motor doesn't vibrate too much. I will have to do smoe testing to see how much vibration is generated.

Thanks again,
Lilly.

Brian_B

Lilly,

Heres a place where I buy odd stuff ,they have a good selection of motors.

Brian

http://www.surpluscenter.com/sort.asp?UID=2007012119303283&catname=electric&keyword=GRDD

Zoot

I poked around a little -- I think what you are attempting to build is actually called a "powerchair", rather than an electric wheelchair. Wheelchairs have wheel diamters of 20"-26" with casters, meant for pushing, etc. A powerchair often has much smaller drive wheels right in the center (not fore or aft) with often 1-4 casters front and back to prevent tipping. A lot more like a big Boe-bot, really. Powerchairs are meant for folks with a higher degree of disability than the loss of use of legs. They often include more advanced/sensitive controls (tongue or blow sticks, head control, rather than a joystick). Some of the newer ones include obstacle avoidance (again, like a Boe-bot) so that even if the user has twitches or cognitive issues, they won't accidentally drive the chair into an obstruction.

Pricey though. But here's a site with used/new examples that may give you construction ideas:

www.marcsmobility.com/used-pride-mobility-jazzy-1121.html

Also, look up "wheelchair" at Wikipedia and they have a pretty good summary of rough sizes and construction techniques for different kinds of wheelchairs.

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When the going gets weird, the weird turn pro. -- HST

Kevin Wood

When I was still in high school, I considered going to college to study biomedical engineering.

You might want to look into that field yourself.