R/C Lawnmower - 2 sets of motors with different gear ratios?

I'm asking for a little bit of mechanical engineering advice. I only know enough to know that gear ratios are important.

I've built an R/C lawnmower and I've bought another electric wheelchair to harvest parts to upgrade it to add a second set of wheels to make it 4 wheel drive for better handling and load capacity. This would be substantially cheaper than deploying/scavaging tracks (as far as I can tell anyways), which would have been my preferred option. I am using a sabertooth 2x25 motor controller fed by a hobbyking tx/rx pair and would split the signal to go to another sabertooth 2x25 to maintain/create track steering with each "axle" of wheels. The lawnmower is about 6 feet long right now.

I'm pretty sure the second set of motors and wheels has a different gear ratio and that could wind up damaging the grass/lawn I'm trying to cut. I'm not so worried about it in my yard, but eventually I want to show it off on other peoples yards and don't want it to tear things up. My evidence is that the wheel size is different; my theory is that if I put the same wheel size on both sets of motors, each pair will be fine on their own, but will wind up with different gear ratios with the new size wheels.

About how much difference in final gear ratio do I have to work with here? Are there any other obviously fatal flaws in my design? I have the wheelchair and can re-purpose it for another project, though I have not bought the 2nd sabertooth controller.

The problem that I am trying to solve is that the lawnmower is backheavy, slides sometimes, and has poor maneuverability on certain slopes (an angles of attack), which are common where I want to use it.

For the most part I think you'll find your motors will be self-regulating and you shouldn't see this problem. One wheel pushing harder and faster than another wheel should cause that other wheel to speed up/use less current. If your motors have very high gear ratios then yes, you could have a problem, but the easy solution is to just reduce power to those wheels that are running too hard.

The flaw that I see in your design is that you haven't really focused on your main problem which is preventing wheels from losing grip and spinning. You need some kind of feedback from your wheels to see how quickly they are turning so that when one wheel loses grip you can push more power to the other wheels. Traction control, essentially.

You also may find your motor controllers aren't linear so that the ratios will change at different speeds.

4-wheel drive electric lawnmower isn't a trivial project, sounds impressive.

One thought - I recall something about putting DC motors is series with each other to form a kind of electric differential - they share the same current (thus torque), but the voltages are independent (so can run at different speeds)... Might be worth looking into.

allanonmage: This would be substantially cheaper than deploying/scavaging tracks (as far as I can tell anyways)...

allanonmage: The lawnmower is about 6 feet long right now.

allanonmage: ...but eventually I want to show it off on other peoples yards and don't want it to tear things up...

You don't mention the width of the robot, nor what the wheelbase will be, but suffice to say, if the width isn't wide enough - wheels or track (actually, especially tracks) - you are going to tear the absolute crap out of your lawn or anyone else's, especially if you try a "pivot on a dime" turn.

This is because the tracks or wheels will be slipping sideways, and for a machine that size, it will chew up the ground and lawn pretty severely.

There's a reason Bobcat tractors are called "skid steer" vehicles...

The only way you can implement four wheel drive without this issue is to either go with an Ackermann steering arrangement:

http://en.wikipedia.org/wiki/Ackermann_steering_geometry

...or by implementing 4-wheel steering as well, so that the wheels will more closely follow the curve of a circle as the machine turns. Either way will introduce a ton more complexity to your design, if you want to avoid making a mess of your's and other people's yards...

Well crap. Guss I will aim to leave it at skid steering and pass on worrying about tearing up the ground. It's more important that it can navigate more areas to do the work i want it to than not tear up the ground. Those areas are rough, so if it tears it up it's no big deal.

As far as steering, I may add more motors, one per each wheel, so the wheels can turn like the front end of an ackerman steering arrangement, but all 4 could do that. That's probably a ways down the line though.

I'd still like to figure out how to give both axles of motors the same "go forward" command and not worry about too much slippage. i said gear ratio in the OP, but maybe I mean RPM of the wheel?

I don't think this would be as much of a problem on-road, but dirt gives a lot more than asphalt.

I would go with driving the front wheels only and using castoring rear wheels. Unless you intend to use your lawnmower on steep banks, you don't need 4-wheel drive.

Well the banks I want to use it on aren't what i would call steep, but rear wheel drive and front castors isn't working.

I can go up and down the banks no problem, but I want it to also go sideways on the bank, for example against a fence, and the mower isn't going to work well in that scenario, especially if there's a lot of brush and loose terrain.

Perhaps a combination of the following will work when going along a bank:

  1. Use a tilt sensor. Increase the drive to the lower wheel and reduce the drive to the upper wheel, depending on the amount of tilt.

  2. Lock the front wheels in the centre position when going in a straight line, and release them when steering.

if you really want something that might actually mow a lawn, modify a gas powered riding mower for remote control. You should be able to find one used on craig's list to start with. When the novelty of the remote control setup wears off, you still have a riding mower instead of some expensive stuff that will probably go to the recycling center.