Project Goals

Here are my goals for the robot mower over the next few months. I need to plan and execute well so that the mower will be ready to cut grass this spring.

December

Finish designing the robot mower, including wiring and planned integration of the RTK GPS module. The module should arrive at the end of December. I promise I will post more about the RTK GPS module soon.

January

Send the robot mower weldments out for quote. I’ll also start sourcing purchased parts for the project. Any design changes based on vendor feedback will be incorporated during January. I’ll also start playing with the RTK GPS module, getting a feel for performance and how to configure the base station.

February

Select a vendor to build the weldment. Start receiving in purchased parts. Implement the RTK GPS module on the wheel chair robot and take it out for field testing. If the weldments are completed in February, we’ll start assembling the robot mower.

March

Finish construction of the robot lawn mower. Conduct functional testing. Make any last minute changes to the design based on the testing results. Continue field testing the RTK GPS module on the wheel chair robot.

April

Integrate the RTK GPS module on the robot mower. Start working on making the mower truly autonomous, with my backyard as the test bed. It’s fully fenced in so it should be a safe area, and the trees and houses nearby provide a fairly challenging GPS environment, perfect for working out the kinks.

The Bottom Line

Shoot for the moon; even if you miss, you’ll end up among the stars.

-Some motivational kindergarten classroom poster

I realize this is a very aggressive schedule, but it’s been my experience that if you aim high but miss, you still will still achieve a lot more than you would have if you had set a more “realistic” goal. So we’ll see how far we get over the next few months.

Field Testing

I had several objectives during field testing:

  1. Get the rover tuned properly, and see if there is any strange autonomous behavior like weaving, jerking, etc.
  2. Familiarize myself with planning autonomous missions in Mission Planner. I was specifically interested in seeing how to autogenerate waypoints.
  3. See how well the GPS blending reports position. I was interested in both absolute and relative accuracy, but really just wanted to get a feel for things at this point.

To start, I took the rover V2 out in my backyard and got it to run autonomously, but my backyard isn’t very big. I have lots of tall trees in my neighborhood, too, so GPS signal reception wasn’t great either.

IMG_3854
The rover V2 running to waypoints autonomously. I managed to avoid crashing into my sprinkler well the whole afternoon!

I would estimate position accuracy relative to the satellite map data was +/-3m. Not horrible, but not great, either. Throughout the afternoon position would drift slightly, I would estimate +/-1m.

I had the rover run an autonomous mission of 20 waypoints positioned in a circle several times to test repeatability. I did this close to 10 times over the course of an hour, and each time the rover took a slightly different path, as evidenced by the track marks in the grass. Sometimes the rover would go on the left side of my sprinkler well, other times to the right. Not very repeatable, but this was a somewhat challenging GPS environment.

So knowing that things were at least configured somewhat correctly I decided to take the rover V2 out to a large parking lot with a good view of the sky. In Kansas, those aren’t too hard to find. I chose a parking lot way out of town, with newly painted stripes that showed up on the satellite imagery so that I could have a good measure of relative accuracy.

The rover weighs something close to 100lb, so I had to take the batteries off of the chassis and remove both of the control boxes. I brought my tool box with me to help reassemble the rover and I also made sure my laptop was charged, but I forgot to bring a few things. If you’re ever out field testing, a checklist is a really good idea. Here’s mine for the next time I go out:

  1. Cell phone with cellular data
  2. AA batteries for the RC transmitter
  3. Make sure rover batteries are sufficiently charged
  4. The toolbox with hex wrenches, adjustable wrench, and screwdrivers
  5. Multimeter
  6. Laptop with a good battery charge
  7. Telemetry radio for the laptop
  8. SD card, installed in the Pixhawk

I asked my very supportive wife to go with me, thinking she would bring her phone and that I could use it as a wireless hotspot to download Mission Planner map data. I forgot to explicitly ask her to bring it, which was a huge mistake because this was the one time she decided to leave her phone at home. And I’m too cheap to have a data plan, so I was depending on her. We had a good laugh about it when we realized she’d left it at home.

So while I was able to do some autonomous missions, I was unable to compare the position data to the parking stripes on the satellite map. I could zoom in, but the ~300 car parking lot was reduced to 6 pixels in Mission Planner.

So without any good imagery, I took the rover manually around the parking lot perimeter to demarcate the edges and then planned some missions. The first ones were circular, and then I did some square ones, and some random ones.

IMG_3862_Moment
The rover V2 in the parking lot.

Overall, I felt like the rover was tuned perfectly. I had a few odd shimmies here and there, but I didn’t spend too much time trying to find the cause. They weren’t debilitating, just noticeable. Seeing the rover run autonomously without toilet bowling was very satisfying given my rover V1 experience.

So field testing was only a partial success. I felt like the rover was tuned really well with just the default settings, so that was a win. But I wasn’t able to get a good feel for GPS positioning accuracy or repeatability. I did get to spend some time familiarizing myself with Mission Planner.

An observation about field testing that may be specific to Kansas: Wind stinks. The laptop almost blew off the hood of my car while I was in the parking lot. I set up shop in the trunk of my car where I was sheltered from the wind a little, but I noticed this affected my radio range somewhat. I had the rover more than 500ft from my car at some points, and much past that things got spotty. In the future I’ll choose a calmer day for field testing.

Another observation: Kansas is flat. Very flat. When the rover got more than 200ft or so away from me distance to obstacles becomes very difficult to judge (especially with no satellite map). I hopped a few curbs in manual mode, but the rover V2 handled it like a champ.

Overall I’d say it was a 1.5 out of 3. We’ll bring a good cell phone and choose a calmer day next time.