Recently, we at Freeside Atlanta teamed up with the Alchemical Arts Alliance and My Inventor Club to design and build an Offroad Wheelchair so that our friend Robin can get around their events. Together, we've raised about $2,000 for the project.
The design phase is usually the most
difficult part of the projects, and it is often the most expensive
place to make a mistake. Committing the resources to a
poorly-designed project can cause the entire thing to be wasted, so
we designed the Offroad Wheelchair project very carefully.
We started with the constraints – We
need ease of maneuverability, an ability to overcome obstacles and
take fairly steep inclines, longevity to make it through events up to
a week long, and recoverability in case it gets stuck. Also, biggest
two constraints – Budget (~$2000) and time to prototype (2 months)
From that, we landed on 3 design
options –
1 – Electric motors with onboard
generator for periodic battery charge
2 – Modifying an existing lawnmower
or outdoor vehicle with hydraulic controls and automation
3 – Modifying a zero-turn lawnmower
to suit our purpose
After weighing the options, we chose
option one for its low noise and high efficiency. Next step -
simulate the design. I used the website
Study Physics to figure out the torque requirement to
pull a simulated wheelchair + passenger up a given slope. Then I
simulated it in a spreadsheet starting with the worst-case scenario
so I could play with the numbers. The inputs are in orange and the
rest are calculations.
In other words, the force required to
take a vehicle up a 15 degree slope is a bit more than half of the
force required to lift it all the way off the ground. To find the
torque requirement, the force needs to be distributed around the
wheel, since the wheel radius affects the leverage exerted by the
motor.
Therefore, the minimum torque of the
motors is about 200 pound feet. Is that feasible for an electric
motor? Let’s look at the best motor example I could find -
This means that the system required two
very powerful electric motors, running near peak torque continuously,
and geared down 60x. They exist, but they are about $600 each and
don’t usually match each other’s output exactly. Plus, the motor
controllers are $200-$400 each! That means that this project is
neither feasible nor practical, as the entire budget could be spent
on 2 motors with their respective transmissions and controllers. It
won’t work. But, this is why we simulate. It’s time to drop back
and try again.
The hydraulic modification of an
existing vehicle seems interesting, but also time-consuming. Plus, it
wouldn’t be able to pivot in place like the other two design
options, which is important to delivering the rider to exactly their
target. Not to mention that the drive of the wheels will probably be locked together, so there is nothing stopping the thing from sliding down a slope. It also requires the modification of a ~$1000 platform with
up to $800 in hydraulics and most options aren’t configured for
somebody to easily get into from a wheelchair.
Then, Shane from My Inventor Club sent
me an email about a zero-turn mower that was available in our price
range. Zero-turn mowers use hydrostatic transmissions to convert the
high-rpm, low-torque energy from an engine into low-speed,
high-torque power with hydraulics. The mower we chose is a
Grasshopper 725k. The actual torque isn’t listed in any of the data
sheets, but the horsepower (25) and wheel diameter (22in) are. So now
we just need to find the torque and update our simulation to see if
it will work.
If we know how to find torque then we just need to find
the rpm at the wheels at the top speed, discount it by about 30% for
conversion losses, and that will be near our actual torque.
First, get the top speed to feet per
minute –
Then, turn that to rpm using (the
diameter of the tire) * (pi) to get the ground it covers per
revolution.
Finally, you can do the torque
conversion –
The zero-turn mower is 800lbs, so it is
much heavier. The torque required to haul it up a 15 degree slope is
530 pound-feet, so it might be able to crawl along if it can keep
traction. However, rolling over would be a concern at that angle
anyway. Therefore, the zero-turn is feasible and reasonably meets the
requirements of the project.
We can look into supplementing the power of
the motors with a winch if the vehicle gets stuck somewhere that the
torque/traction can’t overcome and reduce engine noise, but we're in the ballpark now. So, we’ve settled on a design –
a retrofit of a zero-turn mower with a winch. It will be faster and
more cost-effective to retrofit a used device that has been
engineered to a similar task than to design and build something new
from scratch. However, this platform could serve as a prototype for
some future design, so we can work out the issues through iterative
prototyping.
Until then, we’ll do the most
effective and cost-effective Offroad Wheelchair that our constraints
allow and document it so others can duplicate and improve the idea.
Check in on the
Offroad Wheelchair page for links and info, including the simulation spreadsheet that I used (you get to point out my mistakes!) and the first video of the mower that we selected.
