ZERO TURN 48v AUTONOMOUS MOWER
Autonomous Radio Controlled Mower

Aerial Shot of Clark Bay

ClarkBay has four holiday cottages, and this means there is a lot of grass to cut.  Zeroturn mowers are typically the option selected when there is a lot of grass to mow, and this is what our neighbours use.  At Black Bream Point  they have a Bad Boy Mower which is quick and effective.  Zero turns can go up close to trees and bushes.

I didnt take their advice and instead decided to buy a Hustler Zeon 42in, but was thwarted !  It is no longer being built by Hustler, why I do not know.  With 10kw of PV panels, we have surplus electrons during the day, and get only 6c per kw when we “donate them” back to the grid, so we really want to have a decent electric mower.  The only other commercial mowers available are in the USA .  The Meangreen machine looks good but is expensive even before import and freight costs.

Electric mowers have been around for 50 years or more, we had a 240v model when I was a kid.  But recently we have seen small domestic battery powered mowers become more widely available, examples include the Ryobi 36v all-plastic mower which is ideal for small urban yards.  We have one, here it is in action:

The battery on this all-plastic mower is 36V 5AH, it lasts 20 minutes on the tough kikuyu we have here.  Its perfect for a domestic backyard, but too small and ineffective for us.

Alternatives?  I considered a Husqvarna robot.  Their largest robot is the 265ACX model which can mow up to about 6000m2.
Tempting!

Time to look at the mandatory and “nice-to-have” requirements.

Requirements

Mandatory

• Steep area operation ( > 30 degrees ).   Parts of our grassed area are just too steep to use a conventional ride-on mower.
• Radio Controlled
• Two-hour run time minimum
• Height of deck adjustable manually

Nice-to-have

• Be able to “slash” the paddocks to control tussock and bracken.
• Height of deck – automatic adjustment – raises when cutting tough or high grass
• Autonomous using centimetre accurate GPS system

The Plan

The plan is to build the mower using the cutting deck off an old Torro ride-on mower, and a 100AH Lithium battery pack from EV-Power at 48volts.  A frame will be built over it to provide a platform for the batteries and control gear.

The dust and debris breathed in when mowing is disagreeable and unhealthy, so the first stage will have radio control so I can sit in comfort and watch it do its thing from the shade of a tree.

For each of the three cutters, Im planning to use direct drive by a BLDC-108 from Golden Motor, 48Vdc, rated power 1.5KW.

For traction, I need two motors, independently controlled, forward backward to get the zero turn thing happening. It doesn’t need to go fast – about 10kph or less when cutting, 15kph absolute maximum.

For communication, I will try Wifi, if I can’t get good reception ( the distances are bit too far), I will try putting Wifi extenders in strategic locations.

The traction motors should probably be geared, to provide better power up hills, and use the motors at a more normal RPM.  I came across a controller called Sabretooth dual 25A, it has a tick in quite a few boxes I care about, however the input voltage needs to be less than 30V. I cannot step down the voltage from 48v since the controller is regenerative. The load for traction is only small relative to the load from the three cutters, and it would be possible to use just half of the 48v battery bank, but I am concerned about unbalancing the batteries. Could I use a big relay to swap every few minutes to use the other half of the batteries, or I should I just go back to square one and look for a different controller that can handle the 48v?

The mower will be normally parked under a purpose built shed the roof of which will be solar panels for charging the machine. The 100AH battery pack has the equivalent of 4kwhr of capacity at 80% DOD, and when not required for lawn-mowing I am thinking of using it as battery storage to displace some of my night-time domestic load (we have a 10kw PV system). The four batteries weigh 12.60 kg each and are 280 x 290 x 190.

It will have a 48V 100AH Lithium battery pack from EV-Power. That is the equivalent of 4whr of capacity at 80% DOD, and when not required for lawn-mowing will be used as battery storage to displace some of my night-time domestic load. The four batteries weigh 12.60 kg each and are 280 x 290 x 190.

For each of the three cutters, Im planning to use direct drive by a BLDC-108 from Golden Motor, 48Vdc, rated power 1.5KW. Rated/peak torque: 4Nm/8Nm Efficiency: >85% Dimensions: 11x10cm Weight: 3.9kgs. The efficiency curve for these motors doesn’t drop off until the load is less than 15% – ideal! Each motor will have its own controller, and the speed will be regulated to about 4000RPM to make sure the tip speed of the blades is kept to 16,000 feet per-minute (well under the legal limit in America which is 19,000fpm, – not sure about the Australian regulations). The motor is adjustable between 3200 and 5000rpm.

Eventually I plan to use GPS and obstacle avoidance to give the mower some autonomy; I see that affordable centimetre-accurate GPS system is a possibility soon! (eg http://www.radiosense.com/ )

For traction, I want to use two geared wheelchair motors with a Sabretooth2x32controller.  I like the Sabretooth it has a tick in every box I care about except the input voltage which is less than 30V. I cannot step down the voltage from 48v since it is regenerative. The load for traction is only small relative to the load from the three cutters, and it would be possible to use just half of the 48v battery bank, but I am concerned about unbalancing the batteries. Should I use a big relay to swap every few minutes to use the other half of the batteries, I should I just go back to square one and look for a different controller that can handle the 48v?