Arduino R/C Lawnmower (painted)

1. Get R/C transmitter and receiver (I have tested FM and AM systems and they both work)
2. Upload code to Arduino (it is on the last page)
3. Make sure you are getting a good signalYou will need an R/C radio transmitter(Tx) and receiver(Rx) pair, which is the most expensive part of the project, but can be used for every future project you might have involving R/C.  I went with a 6-channel FM system, but  I have tested a 27mHz AM transmitter/receiver and it works just as well. The beauty of the Arduino is that if you want to adjust the deadband or the motor-speed at turn-on, (unlike commercial ESC’s) it is all easy changed in the Arduino IDE.Once you have your radio, all you need to do is upload the code to your Arduino, plug in the 2 channels that you want to use from your radio receiver into Digital pins 2 and 3 of the Arduino (these are the 2 external interrupt pins on the Arduino) and you are ready to control whatever you want. If you don’t have a batter pack for the receiver, you can run jumper wires from the Arduino +5v and GND to the R/C receiever for power, you only need to supply a single channel with GND and +5v (it is not necessary to power every channel).

Upload the code using the Aruino IDE (I am using version 0016 on Ubuntu).

I started by controlling 3 LED’s with 1 channel on a breadboard. I wired a red LED to be Forward (digital pin 9), a yellow LED for Reverse(digital pin 5), and a green LED for Neutral (digital pin 12). This allows you to adjust the code to fit the needs of your radio system. You will have smooth 0-100% PWM control of both LED’s and the neutral light will turn on when the control stick is centered. If needed, you can widen the deadband for Neutral, but doing so will increase the speed at turn-on (which starts at 0%, so that would likely be desirable). See pictures.

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The code has 4 PWM outputs for motor control:

channel 1 Forward = Arduino digital pin 9
channel 1 Reverse = Arduino digital pin 5
channel 2 Forward = Arduino digital pin 10
channel 2 Reverse = Arduino digital pin 6

2 outputs for Neutral indicator lights:

channel 1 = digital pin 12
channel 2 = digital pin 13

The 2 INPUTS from the R/C receiver should go to:

channel 1 = digital pin 2
channel 2 = digital pin 3

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If you are interested to see your readings, turn on your Serial Monitor in the Arduino IDE (set to 9600bps) and you can see the actual real-time pulse readings for each channel, they should read:

full forward = 2000 (2 milliseconds)
center = 1500 (1.5 ms)
full reverse = 1000 (1 ms)

These readings reflect the number of microseconds that the pulse signal from the R/C receiver stays HIGH (or at 5v). The typical Servo signal that comes from an R/C receiver is a pulse whose length varies from approximately 1 ms to 2 ms with 1.5 ms being Neutral (which should also be the position that the control stick returns to when you let it go). The transmitter reads the position of the control stick and sends that pulse length about once every 20milliseconds. So it is constantly updating for precise control (for more info, look up PPM on wikipedia).  If you push the transmitter control stick forward, the reading should go up to 2000, if you push it backward it should go down to 1000. You can also use a voltage meter at this point to see that Digital Pins 5, 6, 9, & 10 will be changing from 0-5v depending on the position of the control sticks on the R/C transmitter.

If you care to know, the code uses the Arduino’s 2 external interrupts to capture when the Rx signal pin changes states (goes from HIGH to LOW or vice versa), when it does at the beginning of each signal, it calls the interrupt function which reads the digital state of the pin and if HIGH, it records the microseconds value on the Arduino system timer0. It then returns to the loop until the pin goes LOW, at which point it subtracts the previously recorded microsecond value from the new current microsecond value to determine how long the pulse stayed HIGH (which tells us the position of the Transmitter control stick). It then does that over and over really fast.

I have the values constrained from 600-2400 in the Arduino code to keep things simple. Once it receives the signal and constrains it, it maps that value to be proportionally between 0 and 511, where 255 will be Neutral. The code then determines when the value changes and uses a function to determine the appropriate 0-255 PWM value in the appropriate direction and each direction has it’s own PWM output pin to control the H-bridge.

On a side note:

To make things easier, I built an Arduino-based breakout board using Radio-Shack perf-board, a 28pin DIP socket, a 16mhz oscillator, and a bit of wire. I also added a set of female-headers in such a way that I can plug my R/C receiver directly onto the breakout board. For secure connections while mowing grass, I added screw-terminals on each Output pin and each of the 6 channels from the receiver. It also has a built in 5v regulator to power both the Atmega168 from the Arduino and the R/C receiver (which gets power when you plug it onto the breakout board). So you just route jumper wires from the channels you want to use on the receiver, to the Atmega digital pins 2 and 3. I also added 2 LED lights that are hard wired to the digital pins 12 and 13 for the Neutral lights for each channel so I can easily see when I am in neutral.

Since this bot is a Tank steer setup with 1 drive motor on each wheel, the coding is very straightforward where the left stick controls the left motor and the right stick controls the right motor. Both sticks forward means lawnmower goes straight forward, both backward and it goes in reverse. If you push the left forward and the right backward, it does a zero-turn circle. As you can imagine, mowing the grass is really fun now.

I built several motor drivers before finding a design that worked for my needs. For what it’s worth, there are several nice products already out there that are fully assembled and require a lot less work if you are not interested in building your own electronics. The Open Source Motor Controller is an open source design that has been under constant community improvement for several years now and can handle up to 160amps at 36vdc! But they are over $100 and only control 1 motor. The Sabertooth 2x25amp motor controller is nice and controls 2 motors, but it is $125.So I thought I would just make an extremely simple dual h-bridge that could handle at least 25 amps at 24vdc continuous and handle surges of up to 100amps for a few seconds. Once I found out that you can parallel Mosfets and multiply their current carrying capacity accordingly, I thought I would come up with a simple design and slightly complicate it by adding more mosfets until I had enough to handle the current that I needed.  Digikey has a good selection of Mosfets to choose from and good filters to narrow it down by what you need, so I spent a lot of time looking for Mosfets that were rated for around 50amp and could handle over 30 volts. Also, they have to be cheap because my plan is to use a bunch of them. I decided on the FQP47P06 p-channel and the FQP50N06L n-channel Mosfets from Fairchild Semiconductor, which I bought from Digikey.If you are wondering what an H-bridge is, find out here: en.wikipedia.org/wiki/H-bridge and this will all make more sense to you.

The design is simple: 2 P-channel mosfets control the high-side switches and 2 N-channel mosfets for the low-side switches. But instead of using 1 mosfet for each switch, lets use 3. Now we have 12 mosfets per H-bridge (3 mosfets x 4 switches) and theoretically the ability to carry 150 amps (that is not accurate though). The board is as small as I could make it with nothing touching. Each set of 3 mosfets have heatsinks and are bolted together to help dissipate heat. Also, there is an 80mm cooling fan mounted directly above mosfets to further keep them cool.  The mosfets are very good at handling sudden changes in direction and speed changes.

Since there are 24 mosfets in total (8 groups of 3) I dubbed it the Triple-8. It is running at the Arduino default PWM frequency of 1kHz (I plan on playing with that to get the frequency higher). The board has 4 inputs, 2 for each bridge. If you bring an input HIGH, that side of the bridge goes HIGH.

Ideally, you would control the board by holding 1 input LOW and applying a PWM signal to the other input. This allows for easy speed control. I have written into the code that if you bring digital pin 7 HIGH, the code switches to Relay mode and either turns the mosfets all the way ON or all the way OFF. This is far more difficult to control, but is useful sometimes.

If you are interested in building your own H-bridge you can download the eagle file to etch a pcb and the schematic to show where everything goes. You can get everything to make this dual h-bridge at Radio-shack (including the copper clad), except the Mosfets and a special resistor network I used to save space. I bought most of the parts from Digikey though because it was cheaper and arrives to my house in 2 days.

Here are the parts needed for this motor driver:

(12) FQP47P06 – P-channel mosfet 47a 60v – Digikey – $1.73 ea
(12) FQP50N06L – Logic level N-channel mosfet 52a 60v – Digikey – $1.04 ea
(4) 2n7000 – Logic level N-channel mosfet 200ma 60v – Digikey – $0.26 ea
(8) 4606X-1-470LF-ND – 47ohm bussed resistor network – Digikey – $0.25 ea
(6) ED1609-ND – 2 position screw terminal – Digikey or Radio Shack- $0.46 ea
(24) CF1/84.7KJRCT-ND – 4.7k 1/8w resistor – Digikey or Radio Shack – $1.78 (for 50pk)
(1) PC9-ND – 3″x4.5″ 1-sided copper-clad .064″ 2oz copper – Digikey or Radio Shack- $4.66
(4) P5575-ND – 1000uf Capacitor or similar – Digikey – $1.19 ea
(1) 330ohm – 1kohm resistor 1/4w – for power LED, doesn’t have to be exact
(1) power LED any color you like, I use the 3mm size to save space

Maybe something smaller?

If you are going to use this for something smaller than a 100lb lawnmower, you can look up one of the many H-bridge circuits and build your own smaller motor controller with as few as 4 mosfets (or BJT transistors) or even use a packaged IC H-bridge like the l293d (dual 1 amp) or the l298n (dual 2 amp).

Or if anyone is interested, I will post a schematic and Eagle .brd file for a smaller version of this H-bridge that only requires 8 mosfets total (everything else is the same), and it can handle about 10amps at 24vdc.

Etching:

I am not going to go into all the details of PCB etching, because there are already many excellent instructables on that topic. So once you download my .BRD file of my motor controller, all you need to do is print the .brd file onto some magazine paper using a laser printer, and iron that onto a piece of clean copper-clad. Then etch it with your favorite etchant solution (I use 2 parts Hydrogen Peroxide to 1 part Muriatic Acid and it works perfectly). And remove the toner with Acetone when done etching.

For ease of assembly I designed this board to be Single-sided and to use only through-hole components, no surface-mount stuff to mess with!  Yay for you.

You can get the .brd files for the various h-bridges at www.rediculouslygoodlooking.com

First you need to mount the drive sprockets to the wheels.The EASY way:
If you are smart and have more money, you can find a set of wheelchair motors that have the wheels mounted to them.The CHEAP way:
I could not find any in my price range, so I went with just the motors, then bought wheels, then sprockets. Believing it would not be strong enough to mount the wheels directly to the motors, I opted to mount the drive wheels on an axle, then the motors to the frame, and use chain to transmit the power. A picture is worth 1000 words, so look at them carefully.

Mount the sprockets to the wheels:

I had to place the sprocket on the center of the wheel and drill 3 holes through the sprocket and then through the wheel itself. Once the sprocket is lined up and properly centered, I placed the 3 bolts through the sprocket and wheel and tightened them up as much as possible. I then welded the sprocket to the wheel hub to keep it centered.

The wheels from Harbor Freight Tools have built in bearings for a 5/8″ shaft, hence the 5/8″ threaded-rod we are going to use as an axle.

Repeat this process for both wheels.

There is more detailed info tagged in the pictures.

This is the difficult part to explain. You will likely have to have some mechanical ability and a good set of tools to build a large metal frame from scratch. And since this was a prototype, the dimensions are not all perfect, but luckily they don’t need to be.The frame will be custom measured for your particular lawnmower, so I won’t be giving you exact measurements.Tools needed to build a frame:
measuring tape
angle-grinder
ratchet set
crescent-wrench
a level
electric drill
bolts, nuts, washers, and lock washers of either 3/8″ or 1/2″ diameter and 3/4″- 2″ long
drill bits the size of the bolts you are using
1″ and 2″ angle-iron (36″ long pieces) you’ll need both
1″ square tubing (36″ pieces, steel)
1″ flat steel bar (36″ long pieces)
the 4 wheels you got from Harbor Freight Tools (2 drive wheels and 2 caster wheels)
5/8″ threaded rod (36″ long) and several 5/8″ nuts/washers

First you need to plan out the frame of your bot. Since I was attaching a lawnmower, I started by measuring the height that the lawnmower stood off the ground and took some basic measurements to see how big the frame needed to be. My frame turned out to be about 24″ wide (this distance must match the width from the center of the rear lawnmower wheels)  and 48″ long (long enough for the front caster wheels to swing 360 degrees without hitting the front of the mower deck) and about 18″ tall. Since we want the height of the mower-deck to be adjustable, we are going to attach the mower to the frame by removing the lawnmower wheels and using angle-iron to suspend the mower-deck from the frame of the bot.

1. I started out by using 2 of the 36″ pieces of angle-iron (2″ wide) for the main part of the frame running long-ways.
2. Cut the rear-piece of angle-iron the width of the rear of the mower (this measurement will be from the center of the left-rear wheel to the center of the right-rear wheel).
3. Drill holes in the ends of the angle-iron and bolt the rear-piece to the adjacent pieces from step 1, making sure they are straight.
4. Cut two front-pieces using 1″ square steel tubing, the same length as the rear. We need 2 in the front to bolt the caster wheels to.
5. Drill holes and bolt these 2 pieces to the front of the angle-iron from step 1. You have to measure the holes from the 2 front caster wheel’s mounting plates and drill the pattern into the front square tubing bars. Then bolt the wheels through those holes onto the front of the frame.

I later added another set of 2″ angle-iron bars to the front caster wheel assembly to make the length of the bot adjustable at the front (see pics)

Now we should have a rectangular frame with the front wheels attached.

We now need to see how far down to mount the drive axle to make the frame level. So raise the rear of the frame up until the top of the frame is level with the ground (use your level). Now measure the distance from the top-rear of the frame to the ground, this is the frame height.Now we need to take into account the height that the wheels will raise the axle off the ground.  So measure the distance from the center of the rear drive wheel to the ground (the wheel’s radius). Subtract the wheel radius from the frame height and we will have the correct distance from the top of the frame to the drive axle, which we will call the frame-riser height (we need to cut these pieces next). They are going to connect the rear of the frame down to the axle which the wheels will be mounted on.6. We are going to add 2″ to the frame-riser measurement (so we have a little to work with) and cut the 2 frame risers (mine were about 10-12″ long).
7. Now drill (2) 5/8″ holes, 1 at the bottom of each frame riser (about 1″ from the bottom), this is where the drive axle will go through.
8. Drill 2 holes at the top and bolt the frame risers to the rear of the main-rectangular frame with the frame-risers pointed down.
9. Now feed the threaded-rod through the bottom holes of the frame risers and use 4 nuts to secure the frame risers to the drive axle (1 nut on each side of each frame riser, tightened down).
10. put the rear wheels on the axle and use 1 more nut on each wheel to secure them to the axle (these wheels have built in bearings). The sprockets should face inward toward the frame.

Now we should have a frame that stands on it’s own with 4 wheels. However, the rear axle is not completely secure yet. We will need to add 2 braces from the bottom of the frame risers (near the axle) to the main part of the frame in order to keep the frame risers positioned properly. These braces can be flat steel and do not need to be very thick, they are just keeping the frame risers from moving.

Measure about 2″ above each axle and drill a hole, then measure how far down that hole is from the top-rear of the frame and measure the same distance from the rear of the frame toward the front. Drill another hole on each side at this measurement. The support braces will need to be measured to be bolted in through these holes on each side (see pictures). The placement of the support braces is less important, meaning you can bolt them in wherever is convenient, as long as they are present.

This was the most difficult part to plan out on the frame. We need the motors to be adjustable so we can adjust the tension of the chain, however they just have 4 holes in the bottom of each motor and nobody makes a mounting plate that I could find.The simplest way I could come up with was to mount the motors to an 8″ long piece of 2″ angle-iron, and then mount that piece of angle iron to the frame through some specially cut holes that allow the motor mount to travel forward and backward (but not side to side) along the frame.Make the motor mount plate:

Cut an 8-10″ section of 2″ angle-iron, depending on how much room your motors need to mount. Mine only needed about 4″, so I made it 8″ to have plenty of room for the mounting bolts. Drill a hole about 1.5″ from each end of the top of this bar, this is where the mounting bolts will go through the frame.

Mount the motor to the motor mounting plate:

Now you have to find the center of your motor mount plate (the 8″ long piece of 2″ angle iron) and measure the mounting holes on your DC motors. Use a sharpie marker to plot the hole pattern from the motor, centered onto the motor mount plate.  My motors have (4) 1/4″ diameter tapped holes in a rectangular pattern on the bottom of the gear box.

Drilling and cutting the adjustment holes on the frame:

Next you need to drill and cut the holes in the frame to let the motor mounting plate become adjustable. I cut these holes using a dremel tool and a cutoff wheel. You have to line up the motor mounting plate (with motor mounted preferrably) onto the frame rail and use a sharpie marker to mark where the holes will need to be on the frame rails. Start as far back as you can (without hitting any other bolts underneath the frame), and mark the center of each hole. Then move the motors forward 2″ and mark the holes again. You want to cut the holes out of the frame so that the motor mount plate (with bolts going through the frame), can move forward or backward about 2″. The holes in the frame are the width of the bolt and about 2″ long. I drilled 1 hole at each end and used the dremel to cut out the rest.The holes drilled in the motor mount plate are just single holes for the bolt to fit through, the holes through the frame were cut with a Dremel tool with a cutoff wheel to make channels for the motor mount bolts to travel forward/backward through. You want the 2″ angle-iron motor mount bracket to set as much on top of the main frame rails as possible, the bolts (which you can’t see with the motors mounted) that hold the motors to the motor mount plates will keep the motor mount plate  from laying flat against the frame bars. Go ahead and mount the motors loosely to the frame using 2 bolts on each.

Cutting and connecting the chain:

Now get your 10′ of #25 chain and wrap it around the main drive sprocket on the wheel. With the motors pushed all the way toward the back of the frame (closest to the drive wheel sprockets), wrap the chain around the motor drive sprocket and mark where they overlap. You need 2 of the universal chain links from to connect the 2 loose ends. Cut the 2 pieces of chain and connect them to each side with the universal links to connect them.

Tensioning the chain:

Push the motor mounts forward until there is good tension with the chain, and tighten up the bolts that hold the motor mount plates to the main frame.

Now you can generate electricity. Connect a voltage meter to 1 set of motor terminals and push the bot around.