Hi Timmy,

Thank you for very detail post!
I just got a wheel chair based, model M6i (Invacare TDX SP), I will follow your guide!

I have few questions:

1. The batteries is 24V, can I just use 12V for lower voltage and smaller size?
2. Because of using 12V, so I am able to use cheaper 12V-30A H-Bridge controller, Sabertooth is very expensive.

Is that possible? Thanks!

 

Hi Timmy,

Thank you for very detail post!
I just got a wheel chair based, model M6i (Invacare TDX SP), I will follow your guide!

I have few questions:

1. The batteries is 24V, can I just use 12V for lower voltage and smaller size?
2. Because of using 12V, so I am able to use cheaper 12V-30A H-Bridge controller, Sabertooth is very expensive.

Is that possible? Thanks!

Hey thachnb. Let me clarify that this was not my guide. If I did not have Mark's help I could not have gotten to first base! Hopefully Mark will see this and be able to chime in. if your wheel chair is a 12v system, then you will need a 12v controller. I thought almost all wheelchairs were running on a 24v system (two 12v batteries). But I am really not sure. Here is a link to Mark's profile in case he does not see this post. He is the man with the knowledge
https://forum.allaboutcircuits.com/members/mark-hughes.355291/

 

Hi Timmy, thank you! I have 24V system for my wheelchair, just want to use 12V because motor driver is cheaper
Moreover, my wheelchair speed is crazy fast, so I thought 12V would slow it down, repurpose it to make a snow plow robot

 

Hi Timmy, thank you! I have 24V system for my wheelchair, just want to use 12V because motor driver is cheaper
Moreover, my wheelchair speed is crazy fast, so I thought 12V would slow it down, repurpose it to make a snow plow robot

The easiest way to slow it down is to change it in your RC controller (ie in the FrySky RC controller). You can change the max speed, curve values (how fast it speeds up, etc). I paid $119 for my Sabertooth 2x32 controller and that appears to be the going price now.
I am not sure on the FrySky RC controller as I had mine for a little while before using it on this project.

 

Here is my wheelchair model: https://www.invacare.ca/cgi-bin/imhqprd/inv_catalog/prod_cat_detail.jsp?prodID=TDXSP
it has Linix 4-pole motor. Sabertooth 2x32 now costs me $125, they increased the price

 

Here is my wheelchair model: https://www.invacare.ca/cgi-bin/imhqprd/inv_catalog/prod_cat_detail.jsp?prodID=TDXSP
it has Linix 4-pole motor. Sabertooth 2x32 now costs me $125, they increased the price

Thanks for the ping @timmyd-in-mississippi !
Hi @thachnb,
I usually recommend shelling out the money for the Sabretooth controller. It gives better performance and has more features than almost anything a DIYer and most EEs can cobble together for the price. If I went out to build my own controller, by the time I researched the parts to use, designed the PCB, bought the parts, assembled and tested it, wrote the firmware -- I'd be 25-80 hours into the project and probably $150 (single board manufacture & assembly isn't cheap -- mass production is what brings the cost down).
However, if you don't want to buy a sabretooth controller, there is another option (albeit much less safe and much less precise). You could use two servos in an x-y table configuration similar to this (http://plotterbot.com/2013/11/how-to-build-a-tiny-cnc-drawing-robot/) to manually control the joystick. However, by doing this, if your x-y table malfunctions you will have a very heavy, out of control Robot that will plow down anything in its path.
Here's another option -- call the company (DimensionEngineering) and tell them you'd like a Sabertooth but you can't pay full price. Ask if they have any openbox units on hand or sales coming up. You might get a 10-30% discount that way.
Remember -- if your out-of-control snowplow bot crashes into your car's rear quarter panel, or take down your neighbor's fence, you'll have to pay to fix it -- at which point you will wish that you'd just bought a proper motor controller in the first place.
Best,
Mark

 

Thank you for your advices @Mark Hughes, that's so true we should go with something stable and reliable from first place, at this level it isn't a toy anymore, it's super strong robot.
I will go with Sabertooth. And how about 12V batteries, will it slow down the speed by 1/2 (safer), or motor will work abnormal? Sorry for this question my background is not electrical engineer.
Thank you!
Andy

 

Hi Andy -- my gut instinct is that you can get away with it -- but you may not want to. Each battery has a certain amount of energy it can contain -- voltage is only half the picture -- the battery also has to be able to deliver enough current. My limited understanding of cold-weather battery characteristics is that you have to derate the lead-acid batteries at low temperatures -- especially after a year or two of service.

It might slow it down too much to be of any use.

Besides -- you can set the speed characteristics in the Sabertooth software -- slowing it down there.

 

Hi Mark, I keep the battery in the basement (18*C), fully charged, and do basic insulation around with foams, it will work for 30-40mins outside at -20*C to -30*C without any issue (Oh-Canada weather )

It's almost end of winter, so i will use it as RC lawn mower, will be lot of fun.
I am planning to put a bar in front of it, and in the back as well, for emergency stop feature, if it hits anything, it should stop immediately.

 

@thachnb -- I can't think of a good reason to not at least give it a shot. You shouldn't harm anything but length of ride.
Good luck!

 

I ordered Sabertooth 2x32, and it's on the way, weird thing is, 2x25 is more expensive than my 2x32 on Canada robotshop site!
In my RC transmitter, there is trimming func, to limit the max speed, and also we can change failsafe mode to output no pulse to make it stop immediately in case signal lost.

I'm thinking of repurposing the robot from heavy chassis/frame to lighter one, e.g. make from 2x4 wood for lawn mower, winter is gone and now spring starts here in Canada

 

Hi @Mark Hughes,

Do you think this config work when turning left/right using tank style? 2 front wheels can't change direction freely like in the original wheelchair chassis.

 

Hi @thachnb,
It's really going to depend on the terrain that the robot rides along. With all the gopher holes and rocks around my place, a wood frame wouldn't last too long before ripping all of the mounting screws out of the wood. But the second problem is the rigid front wheels. Unless those are omni wheels in the front, your turning radius is going to be horrible and you'll rip rubber off of the wheels whenever you turn.
Were it me -- I'd keep the wheelchair frame -- the drive wheel and the follow wheels are already mounted. Depending on the specific construction -- you might be able to separate the two sides and build something else between them, but I've only ever seen a few wheelchairs where that would easily work (wheels were mounted to a 1/4" metal plate that was then mounted to the chair)
If I were to go back and do that first project all over again -- I'd definitely weld a solid metal frame with gussets everywhere.
Hope that helps!
Mark

 

Hi @Mark Hughes
Thank you for the input! The omni wheels normally are very small,d=4 to 6 inches, and not get enough traction. For the design above I think it will wipe out all grass whenever it turns

Someone did 4WD version, and it is strong enough to pull a trailer:

 

@thachnb,
Ooh -- that's a good looking robot on a good looking lawn! My situation is a bit.....rockier.
Well I can't wait to see what you come up with. You're going to have a lot of fun!

 

OMG, I found this motor driverh tootlate!
https//www.robotshop.com/en/smartdriveduo-smart-dual-channel-30a-motor-driver.html

I should cancel the pricey Sabertooth 2x32 (

 

@thanchnb,
That does seem like one heck of a deal. I don't know anything about it -- but it must be selling well if they are able to offer it so inexpensively. If you do change boards -- please keep me posted as to how well the other one works.

 

Hi @Mark Hughes and @timmyd-in-mississippi ,

After 3 months waiting, selecting parts, my robot is 90% completed. Now it can move around, and I will attach the reel lawn mower into, for doing summer task "lawn mowing"!

I did few improvements:

1. The base of the wheelchair is very heavy, and those two wheels are not getting enough traction on snow
so I decided to use trac-drive track from an old snow blower.

2. The base, must be steel for heavy duty task! based on Mastercraft mobile base and bunches of corner brackets.

3. I decided to go with Sabertooth 2x32, very reliable and smooth motor controller.

4. Battery is LiPo 6S 10,000Ah from my e-bike!

Thank you for all your helps!

 

@thachnb,
That is seriously, seriously cool. I'm envious and I'm on craigslist/ebay now looking for tracks! Why don't I live in snow country? Why?!?!?!?!??!
Nicely done!
Mark

 

@thachnb,
That is seriously, seriously cool. I'm envious and I'm on craigslist/ebay now looking for tracks! Why don't I live in snow country? Why?!?!?!?!??!
Nicely done!
Mark

Thank you!!!
I posted the full part list and step I've done on robotshop site!
Summer and spring will be here till November, and after then I have to fight with snow using this robot for 4 months December through March!

I will post it in action with the reel lawn mower