ChetOS.net

RC Lawnmower

I get frequent requests for specifications and price for my RC lawnmower.  First, I will say that I got most of the plans from Arduino Robotics, but ironically this does not use an Arduino.  I only made a few modifications (notably: I used aluminum instead of steel, and I used electric scooter motors instead of wheelchair motors).  Aluminum is significantly lighter than steel, but it costs a lot more.  Electric scooter motors are cheaper than the wheelchair motors I saw.

Here are the parts I used:
  • An RC airplane controller (I have the "Spektrum DX5e" and it came with AR600 receiver) - $100
  • Dimension Sabertooth 2x25 Electronic Speed Controller - $125
  • Two 12v 10amp SLA (Sealed Lead Acid) batteries - $60 for both
  • A 1AMP charger for the batteries - $15
  • A few feet of 12 gauge wire - $5
  • Two 24v electric scooter motors, 2400RPM, 11 tooth sprocket, #25 chain- $80 for both (this)
  • Two 80 tooth sprockets for #25 roller chain - $35 for both (this)
  • Two 13" pneumatic wheels - $28 for both
  • Two 10" pneumatic caster wheels - $34 for both
  • Several feet of #25 roller chain (10 feet I think). $30
  • A few #25 master links, you need at least 2 - $1.99 each
  • 5/8" threaded rod (standard length at Home Depot)
  • Several lengths of angle-aluminum and square-aluminum (tube).  I think I used 1/2".
  • A few handfuls of bolts (various lengths, I think 3/8"), nuts, flat washers, cut washers.

I can't remember how much the metal cost.  Something like $150 all told.  Of course, this was my first try and I bought too much and messed a lot up.

Now, some notes on the price.  Before you get discouraged at how much this costs, remember that this was built so that it could take a 75lbs lawnmower through high grass and up small hills.  In fact, I weigh around 200 pounds and I can ride it just fine.  I also splurged on the RC controller and the ESC because I may want to do other things in the future.

How can the cost be reduced?
  • Purchase a 2-channel RC controller.  The ESC is capable of mixing the channels so that a simple RC car controller (one with a trigger grip for speed and a wheel for turning) is able to drive a "tank steering drive".
  • Use lower amperage batteries.  10amp/hour batteries can run something drawing 1 amp for 10 hours (or something drawing 10 amps for 1 hour).  These motors take a lot when driving over rough terrain, and I get about 1 hour out of them.  If you reduced the gear ratio (by using less than a 80 tooth sprocket) and went on smooth surfaces, you could easily reduce the power draw.  In fact, if you went with a 12v motor you would only need one battery.
  • Even the ESC (electronic speed controller) is overkill.  I didn't know what to expect when I started so I went with something I *knew* would handle the current.  If you reduce the load or gear ratio then you could certainly go with a 2x12 which is half the price.

Those three things alone drop the price by at least $150.  In addition, if you are going with a lighter-duty vehicle (as these reductions imply), you could go with smaller wheels and less metal.  I'd say the price of the project could be reduced to around $500.  You just can't get around the fact that ESC, RC, motors, and batteries add up.

Final notes
I used a sawzall with a blade designed for cutting soft metals.  I also used a drill bit designed for drilling soft metal.  These were suprisingly expensive, but you need them.  Do not use a bit designed for wood on aluminum.  Most drill bit sets are for wood.

The chain was, by far, the hardest thing to get working right.  Between mounting the sprocket to the wheel straight, to removing links from the chain to get it the right length.  I spent more time on that than the rest combined.  I highly suggest a chain break tool.  I used a dremel tool to cut the links.
Posted by Chet at 2:18 PM

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