How-To

What is the purpose of this project?

In this case I had three goals, and they influenced the rest of the decisions I made:

1. Give a wheelchair buddy a way to mow his lawn and stay independent
2. Learn something new (particularly TIG welding practice)
3. 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.