An Electrifying Tow

Edward Barnett

Issue 9, March 2018

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Sometimes simpler isn't better. In this case, simpler ran loose! Arduino was the answer.

Ed built a tow-mower to run behind his ATV for mowing huge areas. This is basically a mower that runs by itself, on a chassis, towed by a tractor or ATV. It was working fairly well, but the throttle cable would rattle loose - a perpetual problem when mowing for hours! But he didn’t let a problem like that get the better of him - instead, he turned to Arduino to replace the throttle cable with a potentiometer and servo system using Arduino!

First of all - you built a tow mower? That’s amazing! What’s your background?

I’m an electronics engineer working for Tait Communications in Christchurch, New Zealand as a lead design engineer. I’m a “not RF” engineer, working on power supplies, audio, lithium-ion batteries and chargers, with a specialisation in explosive atmosphere equipment design (e.g., IECEx, ATEX).

I purchased the secondhand home-built tow mower after moving to a lifestyle block. The mower deck is from an old ride-on with the engine mounted on top. Since purchasing, I have added the larger wheels, adjustable tow bar (left/right adjustable offset), control pillar, new fuel tank and a coat of paint. More importantly, I added the electric start system, with battery, solenoid and ignition key.

Replacing the throttle cable with an electronic system is a fantastic idea. What type of servo are you using? It must have a reasonably high pull capability (compared to hobby versions)?

The manual throttle cable would vibrate loose so I used to chock it with a block of wood. That was okay, but was not easily adjustable for when more power was required for thicker grassed areas. Arduino and servo control must be better, right?

Initial concept testing was with a tiny MG90 (2.5kg/cm @6V), as that was what I had on hand. This proved to be too small, so I tried an MG996R (11kg/cm @6V), which seemed to work fine. Although I have had one fail (turns out it was a clone!), so now I have a genuine MG995 ready in case the replacement MG996R should also fail. It’s quite a harsh environment for vibration, as the servo is mounted to the engine. I haven’t measured the required pull, but I’d guess three to five kilograms, depending on how clean and lubricated the throttle linkages are.

We’ve had numb hands after pushing a regular mower for a few minutes, so it’s no surprise it’s a rather harsh environment! Does it need any special tricks for starting the motor, or do you just prime it the same way as the traditional control?

The choke is still manual, so you need to give that a push with one hand while turning the key with the other; it starts well for an old Briggs and Stratton engine of unknown origin. Unfortunately the engine won’t charge the battery (due to a failed alternator coil), so I keep it topped up with a car battery charger.

Sounds like another project for another day, but manual top-up isn’t a real problem either. We’ve had experience with electric-start push mowers and ended up leaving them on trickle charge anyway; although they were only coming out of the shed every once in a while. Have you made any software modifications since the first build or are you still running the modified demo?

Once the idle limit and red-line limits were set, it was done.

Sounds fairly straight forward! Do you have any planned changes, or is it a case of “ain’t broke, don’t fix it”?

It is working well at present, just as long as the servo keeps working! I could add a choke control servo of course...

The Control Panel
The Control Panel
Safety Housed in a Food Container
Safety Housed in a Food Container

Sounds like a great improvement to make at some point for those cold mornings. If you were tackling this project from scratch again with the knowledge you have now, would you do anything differently?

Keep a closer eye on clone servos!

Quality is definitely going to help in that environment. It can be hard to tell what’s an original branded part sometimes! What are you working on now?

The garage fridge/freezer is an old type without a door open alarm, so I’ve got a project to fix that up. So far it’s Arduino-based with a couple of reed switches and a buzzer.

Sounds like you’ll be enjoying fresher veggies and still-frozen ice blocks in no time! Thanks for taking us through your mower project, Ed.

Edward Barnett

Edward Barnett

Electronics Engineer.