Introduction: Remote Control Lawn Mower
Just like Honey I Shrunk The Kids - who doesn't want a remote control lawnmower!?
This mower is made for a wheelchair user (and dear friend) so that he can keep his lawn looking sharp without having to rely on someone else. In fact, this is his second one - we made the first RC mower in 2016 on the cheap - it was reliable and is still working fine, but I wanted to add a couple of improvements and really enjoy projects like this.
Step 1: Design Choices
What is the purpose of this project?
In this case I had three goals, and they influenced the rest of the decisions I made:
- Give a wheelchair buddy a way to mow his lawn and stay independent
- Learn something new (particularly TIG welding practice)
- Have fun
What is your budget?
A project like this can be done on the cheap, or it can cost a whole bucket of cash. I included a photo above of the original mower I built, on that one we were resourceful and spent about $200 including a used mower, free power chair, new RC gear and new motor controller. In this current build I started with a new mower and we purchased the chair (used); in total the materials cost was somewhere around $600-700. We had already proven that a mower like this can work (the original has done every mowing on his lawn for 3 years), and spending more to get a mower that might last 10 years seemed reasonable.
What tools and skills are at your disposal?
This kind of project is a great opportunity to learn something new. Welded steel makes a great structure for the mower, but also requires a lot of tools to work with. There are other ways to assemble a frame (aluminum extrusion for example) that may cost more in materials but require less in the way of equipment to build. This project is also electronics-intensive, but you may be able to get away with minimal soldering if you buy pre-made modules. I recommend designing your project around what materials and gear you have available, or alternatively use your project as an excuse to get more tools!
What could go wrong?
Safety is obviously a concern when you use a lawnmower, doubly so when you give it the ability to run without anyone holding onto it, not to mention making it able to move around on it's own. Naturally as a stranger on the internet I don't want to be responsible for your accident, so here are some things to consider before you start:
- What happens if the mower is moving and goes out of controller range?
- What happens if the controller batteries die?
- What happens if the mower is running and the main batteries die?
- What happens if there is a short circuit?
- What happens if the wire to the mower ignition coil (kill switch) are pulled out (maybe by a branch)?
- What happens if the mower is powered up before the controller?
- What happens if the controller is powered up and one of the control sticks is off-center?
- What happens if the mower runs away? Is it contained in a fenced yard, or will it go into the street, into a lake, into a park full of children, etc?
For all of these the best outcome is for it to stop moving and have the motor shut off, but that might be hard to guarantee. For example, the ignition circuit on most small engines is ON unless the ignition coil is grounded; normally this is done with a spring loaded mechanism and the operator presence lever, but we're replacing that with a relay. Using the normally closed relay contact means that the coil will be grounded when power to the relay is lost (this is good), but if the wire between this relay and the mower is severed then you may have no way of turning the motor off!
What I picked:
Here's a list of what I wanted to have for this build:
- Gas powered mower
- Electric start - better for a wheelchair user and fairly common these days.
- Alternator for self charging - runtime can be unlimited as long as you have fuel.
- First-person camera, transmitter, and screen. We learned that depth perception makes it challenging to mow a consistent pattern the further away the mower is when you move across your field of view.
Step 2: Ingredients
- I ended up getting a "new" mower from craigslist. I used online manuals and parts diagrams to figure out which mowers met my requirements which were that it was electric start, didn't have a manual choke/primer, and was self propelled in such a way that I could drive an alternator. Some mowers drive their wheels through a flexible shaft (like a speedometer cable), but this one used a v-belt directly from the crankshaft, and even better it came out of the front which would leave me room to put the motors and batteries in the back. This mower's starter ran on 18 volts from a power tool battery, but it functions just fine on 24 volts as well. If it were a 12 volt starter I would have tapped between the batteries to get 12 volts for starting. Bagging is out, mulching is the plan - there really wasn't room for all the equipment and the bag, not to mention that the whole rear was plastic.
- Power wheelchairs/mobility scooters are a great source for motors and wheels. Most of these are extremely robust and are probably overkill for a project like this, but they're pretty easy to find. Getting a complete chair is recommended so that you can re-use the battery wiring and casters. Most of these motors are 24v, so this will impact your battery options.
- Motor Controller
- Dimension Engineering makes a tried-and-true motor controller that is very easy to use in projects like this. I used the 2x32, but the 2x25 should be more than adequate. There are other options to control large DC motors, but I really like the fact that I can plug an RC Receiver directly into this and it just works. Specifically it allows you to mix two channels to make differential steering work easily.
- First challenge is finding a way to drive the alternator. The mower I chose had a v-belt drive for the original transmission that came out the front which was perfect. Check the direction of rotation for your motor and the alternator. Also check the diameter of the pulleys on the mower and alternator and the acceptable RPM for the alternator. Typical mowers run the crankshaft/blade between 2700-3200 RPM.
- Second challenge is finding an affordable 24v alternator. Sort by price is a joke on Amazon, but I eventually found a replacement for a small diesel engine for less than $50. Unfortunately the most common and easy to find alternators are all 12 volts.
- If you have 24 volt motors you should run 24 volt batteries. I found the cheapest way to achieve this was with two "lawn and garden" U7 batteries in series. Sealed lead-acid deep cycle batteries are usually used with power chairs, but the most affordable U7 batteries are plain flooded lead-acid. This isn't ideal for an application like this because they are starting batteries and aren't designed for sustained lower current output and greater discharge. I can get away with these for a few reasons: we learned with the previous mower that they have more than enough capacity to mow the lawn once or twice, they're less than half the price of a deep cycle battery so it's easier to afford replacements after a few years, and with an alternator we won't be discharging them much in the first place. The weight of a lead-acid battery is a benefit in this application as well. With that said, any battery chemistry is just fine, but this was the easiest choice.
- The basic requirements were to have four channels: one forward/reverse, one left/right, one switch for the ignition, and one switch for the starter. On the previous mower I used the Flysky FS-i6 with receiver and had no problems, this time around I picked up the FS-i6x which has 10 channels, but was the same price.
- Went with the cheapest 5.4 ghz camera and screen I could find on ebay. Mounted it with a RAM mount so that it could be repositioned until we were happy.
- Some kind of enclosure, I used a Harbor Freight "Apache" pelican case knockoff.
- Low current relay for the ignition circuit
- High current relay for the starter circuit
- Way to convert RC PWM signal to switch a relay
- I used an Arduino Nano
- Could also use Dimension Engineering's "Pico Switch"
- Or hack a hobby servo, I may document this some day but there are quite a few tutorials online
- Anderson Power Pole 30 are great for the motors
- Front wheels (casters)
- Steel for the frame
Step 3: Mechanical Design
I feel that these are the most important points in order:
- Minimize width so that you can mow against edges/fences
- Minimize length to reduce steering effort
- Balance more weight on the drive wheels, but not so much that the front won't stay on the ground. Probably 70-30 is good.
- Secure the batteries extremely well. Bouncing around is bad for them, and if they bounce and short out explosion is a very real danger. If they crack and leak you will have to deal with a sulfuric acid spill, plus the damage it does to the rest of the machine.
- Height adjustment is still required, but may not needed much. I've made it inconvenient enough that you won't want to do it very often, and if you don't do it often it won't matter that it's inconvenient. Problem solves itself!
- Make a strong frame. If you use the mower itself as the frame and still have height adjustment then good job! Also, make sure that the torque from the motors in the back doesn't bend or destroy the mower deck. The one I got was all plastic in the back so I had to weld up an entire new butt-flap for it.
Modifying the mower
- Remove or disable the blade brake
- Take off the wheels, handle, control cables, everything that isn't an RC mower
- Find the ground, starter, and ignition kill wires and label them for later
Mocking things up is really important. I won't provide measurements or drawings for this, and even if I did it means that you would have to source the same parts that I did. There are a lot of photos on this step. A better approach is to take the key parts and try to arrange them where they go in relation to each other. I first worked out a way to hold the alternator in space where I thought it should go. The original belt that came on the mower actually worked out perfectly, and there was only one practical spot where the alternator could be, so everything else had to work around that. I laser cut two wooden boxes in the same size as the batteries to use in the mockup as well. Once things were in place I started fitting together the frame. Start with the known part locations and work from there.
Step 4: Electrical Design
The diagram in the image above should be a good guide for how this all goes together. I did omit some details around the Arduino - particularly the transistors used to switch the relays, but the rest should be comprehensive. That detail can be found on the second schematic diagram for the circuit board that I created. This wasn't strictly necessary, but made it easier to make some of the connections. Note that there is one mistake in that circuit: I added headers thinking it would be easy to connect the relay contacts to ground/5v/24v, but I put them in the wrong spot. Simply don't use those and there should be no issues.
- From past experience I've learned that too big is better than too small. Giving yourself extra space makes troubleshooting a lot easier in the long run. I actually didn't have enough space to lay all of my parts out at the same time, so I used a plastic sheet to make a two-tiered arrangement (see photos). The case I used is very robust. Consider checking how much heat is generated inside and adding vents or even a fan.
- Battery switch
- This needs to be able to switch or at least break the circuit while it's conducting it's full current. This could be as high as the sum of your fuse amperages, or as much as your wiring can take before it melts. Best is a switch that can easily be turned off in a hurry, like an emergency stop switch.
- Use a fuse for the main parts of the mower, particularly the motor, alternator, starter, and one for everything else. Any wires that carry current any distance outside of the nice enclosure are at risk of damage, and in the case of a short circuit the batteries may put out hundreds of amps and are likely to melt something or start a fire.
- Motor controller
- The Sabertooth 2x32 is very simple to hook up, it even provides 5 volts that you can power your RC receiver with. There may be other options, including creating your own H-bridge, but that might be an entire project on it's own, buying one was the easiest way in this case.
- Ignition kill relay
- This should use the normally closed contact so that the ignition defaults to off when no power is applied. Make sure that the wire to the mower is very secure and use a connector that can't be pulled out easily - if it is disconnected there will be no way to turn off the mower!
- Start relay
- This should have a large current carrying capacity, I used a 40 amp relay. In the diagram above you can see that I used a relay with a 24v coil, so I'm actually using a 5v relay to switch the 24v relay to switch the starter on/off.
- I used an Arduino nano in this case. The only function is to interpret the RC PWM signal into an on/off signal for the relays. Alternatively you could use the Pico Switch from Dimension Engineering, or hack a servo into a switch and skip the microcontroller entirely
- I used Anderson Power Pole 30 connectors for the alternator, motors, and starter. The chair I got had suitable connectors for the batteries already, so I reused all of that. Inside of the enclosure I used a lot of crimp connectors and screw terminals. Make sure that everything is secure.
- This alternator has three connections: B+, W, D+, and Ground. B+ goes to the battery positive, the case ground goes to the battery negative. W is meant for a tachometer, so it can be ignored here. D+ is the winding - this alternator requires a small current through this terminal in order to energize the electromagnetic windings and actually generate power, without it there will be no charging. It needs 24 volts and roughly 4 watts, in "normal" use this is provided through a indicator lamp on the dashboard of a vehicle. You could use a 24v 4w bulb, or a resistor over ~150 ohms that can dissipate a few watts in heat.
- This is a small FPV camera sold for drones. It only requires a 5v supply and the rest of the antenna and transmitter is self contained. We 3D printed an enclosure for it since it will be in a vulnerable position on the mower.
- DC power supply
- I decided to use a separate 5v buck converter to power the Arduino and RC receiver since I was concerned that the current to operate all of the relays would put an excessive load for the 5v provided by the 2x32.
- The chair came with a charger, so I decided to add a connector so it could be used. Not strictly necessary because of the alternator, but it might be handy after winter storage. It has an XLR connector so I installed an XLR jack. Note that it is wired to the batteries directly before the switch so that charging will work while the mower is off.
Step 5: Firmware
This step can be optional - if you used an alternative way to drive the relays from the RC receiver then you can skip this.
The code I used was straight from this example:
Rather than reproduce it here, I recommend reading that post and getting a copy there.
Step 6: Putting It All Together
If you've made it this far, you probably already have an entire mower already. After some brief testing, I disassembled the whole thing for paint, then did final assembly. This step is just a series of photos showing how this process went (click above to see more, usually only a few are displayed by default).
Step 7: Finished!
A couple finishing touches and then this is done:
- If your controller supports it, reduce the maximum limits for the right/left and reverse channels to a much lower percent. You may be surprised at just how quickly those wheelchair motors can move, it's almost violent. Reducing the throttle inputs makes steering and mowing a lot easier.
- Unplug the starter and test the kill and start relays
- Add some fuel and get it running
- Check the charging voltage of the alternator while the mower is running to verify that it is working
- Always turn the transmitter on first and off last
First Prize in the