Building a Hybrid RC Lawnmower

Building a Hybrid RC Lawnmower.

You can contact own at: rc_mow@buildrclawnmower.com with any questions

I've always had a dream of the future when I was growing up. I remember the two things that were going to change the lives of future mankind. They were the flying car and for me the other was the robot lawnmower. These two wonderful inventions would forever stick in my memory banks. As I got older it came to pass that it would be all but impossible to produce a safe flying car to zip you from home to work. But there was still a chance for the robot lawnmower.


Looking at my finances as well I figured a robotic lawnmower would be slightly cheaper than a flying car. So some five to six years ago I started to research and looked for the perfect robotic lawnmower. There were not very many robotic lawnmowers that existed. The ones that I did find as is also the case now cost anywhere from$10,000 to $20,000. That pretty well resolved the issue of buying a robotic lawnmower as my budget was more in the $300 to $400 range.


The next choice was to attempt to build my perfect robotic lawnmower which was then appropriately scaled back to a remote control lawnmower. When I first started my project there were only two to three other crazy people attempting to build RC lawnmowers on the web. Today there are several great DIY RC lawnmowers on the web as well as all kind of plans ready for download. Shows how slow I really am.


I decided on a hybrid lawnmower. This type of mower uses electricity to power the drive wheels and a combustion engine to cut the grass. The rotation of the combustion engine recharges the batteries through an alternator. It uses a 24V system, that is 24V wheelchair motors, a 24V alternator and two 12V batteries in series. It has no chains sprockets or servos to get power from the engine which was a plus for me with my limited engineering abilities.


My project did not require any welding and started with a basic prefab frame that still may be available from EvaTech. Most of the parts can be found with a web search or manufactured by the builder. As you will see most of the work falls in the coveted "Amateur Tinkerer" class. Strangely this class can be recognized by the outburst of profanities and an overall general cheapness. It also includes the ability to manufacture parts out of any household item, often leading to the outbursts of said profanities after testing said part.

I hope this site gives you some ideas of your own for your RC lawnmower. I will reference as many other sites and ideas as I can so you can see what other type of designs are available. You're surely to find something that you like and may even want to build yourself.






































Getting Started





Let me first state that this is not a step by step tutorial as some of the photos were an afterthought after I decided to share my work on the web for others to see.


I happen to see this Hybrid Mower by a company called Eva Tech and I knew this is what I wanted or something of the same design.























































































I couldn't afford the full price for the mower and decided that finally the time has come for me to attempt to build one. I liked the chassis since I had no skill at welding or any metal work it was already completed and I figured that would be a good start.


So wanting to purchase just the chassis I wrote them and it so happened that they did have several parts and kits available (2005). So I ordered the chassis kit shown here. It turned out to cost $380.00 including shipping. More info about it on the Mechanical page.

Joy! The Chassis Arrives Undamaged

           

There was a choice of cool powder coated colors I chose bright yellow. I was really excited when the box arrived with the deck in perfect shape. Since this was the first piece of the project I admired it from every angle for about 2 hours that day. This gave me a basic deck with wheels and the ability to be adjusted up and down. The frame arrived in December 2005.


Following the general design and electrical diagram the next parts needed would be the motors and rear wheels and tires to at least get a rolling chassis I could play with. I had to get a steel plate made to mount the motors to the chassis. You can see my sophisticated drawings and measurements for it on the Mechanical page. The metal plate was made by a company called Online Metal it's 1/4" steel and cost about $28.00, $20 for the part and $8 for shipping.


I bought the motors on Ebay they cost $92 plus $12.shipping, they are 24 volts and come from a Pride Jazzy wheelchair. The wheels came from Northern Tool Company and tires came from E-bay the cost was around $48.00 total including shipping. I began to realize a lot of money was going towards shipping but can't let that stand in the way of the dream. I also had to get 2 3/4" diameter drive hubs to mount the wheels to the motors.

















I then had to make some aluminum shims because the axle size on the motors was 17mm diameter that really sucked!


















This part was great, now I could push this baby around like a Go Kart and make lawnmower noises









I decided to brace up the corners where the metal plate attached to the frame. These are metal 90 degree brackets from Home Depot. It may add some extra strength in a backing accident and it was the cheapest thing I could do until I got more parts. Ready to Roll.

Electrical SECTION

The Hybrid Mower Design

This mower is based on the hybrid design that several manufacturers use. The only change I have is that the alternator is a one wire unit with the voltage regulator built in.

The batteries supply power to the controller.

The controller is connected to  two 24 volt wheelchair drive motors. It's a proportional controller which means the speed can vary. 24 volts is the standard for these motors thus everything is designed around this voltage.

Two 12 volt batteries are connected in series to the controller. They supply power for the controller and o the motors.

The 24 volt alternator supplies the charge to the batteries the voltage regulator also prevents an over charge.

These controllers vary in price and features. They were designed for use in robots such as those on the  BattleBot TV series.

Junction Box





The screw terminals with the three red visible wires attached middle left are the 12V positive terminals.
The connected wires are from the toggle switches.
The fuse holders have the negative leads from the toggle switches connected.
The other end of the fuse holders are attached to the negative 12V terminal connector on the bottom left.

The wires with the large plastic electrician connectors:
Black -12V lead connecting to the -12V terminal.
Red +12V lead from the battery through +12V master toggle switch to the +12V connection terminal.
Red/Black Starter leads connected to -12V and +12V terminals.

24V Master Switch

Master switch for 24V motor and controller circuit. This unit comes with 2 removable red keys

Starter Button

Starter switch circuit with fuse

Alternator Circuits

These are some typical circuits for connecting an alternator to recharge the batteries.

It must be a 24V alternator for the 2 batteries in series.

I purchased a single wire alternator from Ebay.

Although I didn't know it at the time it simplifies the connection to just the battery and the alternator. 

Calculations for engine/alternator pulley ratio

Pulley ratio = Crankshaft Pulley diameter / Alternator Pulley diameter

This alternator should put out 24V at 2000 or 3000 RPM not sure.

The engine spec sheet says it's top governor speed is 3100 RPM

I want the alternator to cut in 2000 RPM with 24 Volts

The engine will always be running around 3100 RPM for grass mowing.

The engine pulley diameter is 3.05" alternator pulley diameter is 3.0"

3.05/3.0 = ratio of 1.16 to 1 so at 3100 RPM alternator RPM = 3616 RPM

Should produce 24 volts easily at 3616 RPM

To calculate a pulley speed click on url

http://www.csgnetwork.com/pulleybeltcalc.html

There's plenty of one wire 24V alternators available on the web.A 12V alternator can be converted to 24 Volts by replacing the 12V regulator with a 24V regulator.

This link will show you how to turn a regular alternator into a one wire alternator. Thought it might be of interest to someone. http://mightymo.org/Proj_OneWire.html

DC Motor ControllersDC motor controllers can be found at almost any robotics site . The prices vary mainly due to the amount of motor current they can handle. A 30 amp is plenty for a remote controlled lawnmower.

Everything you'd want to know about controllers, motors, batteries and power.

http://www.4qd.co.uk/faq/index.html

Here's a couple of links to companies that sell controllers

Alternative Building Methods

One RC lawnmower builder has built his on controller using an Arduino board. He also built his own motor drivers for the controller as well. He shows you how to build a complete mower very cheaply. His mower is pretty cool and its straight forward and simple to build.

It uses two large car batteries wired in series and he says it's enough to mow his front and back lawn. Check it out.

http://www.instructables.com/id/

Another individual used a different approach for his RC controller. He gives you a choice on building the RC interface to replace the wheelchair controller that would come with a wheelchair frame. One way id to just connect a regular RC transmitter receiver and servos and some other parts to interface with the chair controller. He also shows another more advanced method to interface to the controller as well.

This is all part of a complete RC mower he has built using some different methods. His more is pretty cool as well. Check it out.

http://members.iinet.net.au/~tnpshow/RCLM/

intro.htm 

Other Ideas

There are more variations of DIY RC lawnmowers on the internet and YouTube. Check the Links page and do a search if you're interested in tackling a project like this yourself. The different styles and techniques used to build them should give you some ideas.

mechanical SECTION








The Mechanical Workings






















Photos above show parts and assembly for alternator adjustment bracket









The belt housing is made from ducting sheet metal bought from a home Depot store. It's easy to cut and bend with sheet metal cutters. Have to sand it to roughen the surface so the paint will stick.









The battery box is actually made of a 3/4" back top and bottom then wrapped in the ducting sheet metal. Various sizes of Radio Shack enclosure boxes are attached to handle various functions. Not very high tech but turned out to be easy to make. The battery holders are large shelf brackets from Home Depot. Four bolts hold the unit on the mower very easy to remove.









This is a regular RC servo set up to operate two micro switches. One for remote start and one for engine stop. The remote start operates with wiring across the starter button terminals. The micro switch closes the circuit and bypasses the manual starter button. I could have used a RC Dual relay board but they were from $50 to $75 this was about $6.00. So far no cursing.









The engine stop circuit is an actuator connected to the stop run lever on the engine. It's a 12 volt electric car door lock actuator wired to one of the 12 volt batteries. When the micro switch is closed it actuates the lever.





The servo and micro switch unit is placed inside another enclosure for protection.








The Mechanical Workings

Calculating Mower Speed Speed= wheel diameter x rpm x .002973=MPH Us this calculator http://wahiduddin.net/calc/calc_speed_rpm.htm   Speed versus RPM Calculator   Speed versus RPM Calculator Engine Speed   RPM Transmission Gear Ratio  :1 Differential Gear Ratio  :1 Loaded Tire Radius   inches Vehicle Speed   MPH  Copyright 2007-2012,  Richard Shelquist Alternative Building Methods This builder uses an inexpensive homemade frame for his RC mower. He also uses his own design for the motor controller. He shows his frame and mechanical components that comprise his mower. His mower is pretty cool and its straight forward, simple and inexpensive to build. It uses two large car batteries wired in series and no alternator and he says it's enough to mow his front and back lawn. Check it out. http://www.instructables.com/id/Arduino-RC-Lawnmower/ Here's another build using a lawnmower chassis and homemade frame. This individual used a different approach for his RC controller. He gives you a choice on building the RC interface to replace the wheelchair controller that would come with a wheelchair frame. One way is to just connect a regular RC transmitter receiver and servos and some other parts to interface with the chair controller. He also shows another more advanced method to interface to the controller as well. This is all part of a complete RC mower he has built using some different methods. His more is pretty cool as well. Check it out. http://members.iinet.net.au/~tnpshow/RCLM/ intro.htm

Other Ideas There are many variations of DIY RC lawnmowers on the internet and YouTube. Do a search if you're interested in tackling a project like this yourself. The different styles and techniques used to build them should give you some ideas.