R/C Lawnmower - Advice on motor choices


I’m planning a project to build a remote controller lawn mower, relatively small scale PoC first to test & learn followed by a larger version later.

Small scale will be a basic chassis setup in a skid-steer configuration with 2 drive wheels at the back and 2 idle wheels at front (with future goals of changing to tracks); current wheels I’ve found are 120mm dia and total mass estimate will be around 15kg. It will be controlled by an r/c tx/rx feeding PMM into a micro controller (Either Arduino or ESP8266…havn’t decided yet) which in turn signals a motor controller to feed power to either drive wheel. It needs to be capable of variable speed and run in forward & reverse.

Based on reading other people’s similar project I think the overall concept will work but they often used 2nd hand wheelchair motors which I’ve not been able to source yet in Australia so looking for something more off the shelf. I’m good with micro controllers & programming but new to motors & robotics in general.

Currently planning:
2 x 25Dx52L HP, 34:1 gear motors attaching directly to the drive wheels.
1 x Sabertooth 2x5A motor driver. This brand seems very popular for this type of project but also pretty expensive. Does Pololu have something else comparable? Would this still work given the motors have up to 5.6A current?
1 x 12V 15AH deep cycle AGM battery

I’ve been using this guy:

Which implies this may work but I have little grasp of the underlying formulas involved.
Does this motor/driver/battery combination sound generally sane for this type of build or am I way off somewhere?

Thanks for reading and advice.

In order to choose motors, motor drivers, wheels and batteries, you need to estimate the power and torque required by your vehicle. In turn that depends on the target vehicle speed and weight, as well as the terrain you expect it to encounter.

For informed advice, you will need to provide that information.

Ok lets say it will weigh 15kg and have a target speed of 5mph (typical lawn tractor speed). It should handle slopes up to 10deg and terrain is rough/bumpy outdoors through grass up to 100mm high.

With 120 mm diameter wheels and two wheel drive? Having competed in outdoor autonomous robot competitions, I can assure you that under those conditions, it won’t move at all. Fine for indoors, on wooden floors, though.

You will have to experiment with some models, in the grass, before settling on details like that. You will find that larger wheels and at least 4WD are required, vastly increasing the torque and battery capacity required to move the model.

The Wild Thumper (with 120 mm wheels) does OK under those sorts of conditions, but importantly, it uses 6WD and a very flexible suspension. See https://www.youtube.com/watch?v=SGCnSpyFJcE

4WD version of the Thumper here: https://www.pololu.com/product/1564

Thanks Jim, exactly the kind of feedback I was after.
Those Thumper wheels are the very ones I was planning to use but the chassis size will need to be a bit larger…not sure exactly what it needs to carry yet but that off the shelf chassis looks fairly small.
I hadn’t factored in the value of independent flexible suspension; if each wheel has its own motor that may be viable to add. Would need a 2nd of the pricey motor controllers…and I’m still only on the proof of concept version. :/.

My current 2wd standard ride-on does my terrain pretty well for everything except the slopes (mostly because it’ll tip over) but it has 480mm x 200mm tires and a 20hp engine.
What kind of power would I need in electric motors to drive wheels that size?

For the eventual full scale version I was eyeing off something like these:
But the battery size requirements quickly get very expensive for any kind of worthwhile run time…maybe an engine needs to stay involved somewhere.


They look impressively expensive…

The robot motion calculator you linked is a fine tool, at least for rough guidance. Try it out for your riding mower and see what it predicts!

Interesting…I hadn’t noticed that correlation before. Increase in wheel size is directly proportional to motor torque needed…but power stays the same. Is torque the primary spec I should be looking at for this type of application?

For 480mm wheels it says I need around 5Nm torque x 4. The answer is probably somewhere in-between; saw some 260mm wheels at hardware store which is just under 3Nm with 4wd…still well above those 25D motors if I’m doing the oz-in conversion right.

Those 500w golfcart/quadbike motors I linked above claim 8.5Nm. Need smaller or just right…or a different type of motor entirely…more research needed.
Thanks for all the advice so far.

Power is the energy consumed per unit time, accelerating the robot, and moving it against the forces of friction and gravity. Power is independent of torque but equally important, as it determines the battery life.

Thanks Jim. Found some reasonably cheap 200rpm / ~3Nm motors so will try out a model with 4 of those to get some hands on experience and then see where it takes me.