The Prototype Robot Lawn Mower

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The prototype robot lawn mower, without the mower deck.

I was able to take the prototype robot lawn mower out for a drive this week. Watching the design drive off the computer screen and into my backyard has been immensely satisfying. Below are my notes from a day spent becoming better acquainted with the robot mower.

Vehicle Handling

Initially I wired the RC receiver straight into the Sabertooth so I could take it out for a drive. I’d forgotten how much smoothing the Ardupilot software adds to the control output. The robot was almost too responsive without it.

But overall, it handled wonderfully. I was almost able to drive up the curb from my street into my yard, but there wasn’t enough traction to get the back wheels up the curb. So far, so good!

However…

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A scenario I hadn’t considered when I designed the robot mower.

After I got the Pixhawk wired up (more on that below) I ran a few autonomous missions and started tuning the steering and throttle PID values. At lower steering turn rates, I noticed the swivel caster wheels sometimes catch on the grass instead of swiveling, and the whole setup actually rotates about the center pin. The scenario in the picture above only occurred once, but I noticed the wheels getting hung up more than once.

When I was putting the robot together, I noticed the casters weren’t fabricated per my print. I had designed them with more offset between the wheel axle and the swivel axis. If this continues to be a problem, I’ll have them remade with a little extra offset so they swivel easier.

Wires

Getting the robot wired correctly was nerve wracking. I made a wiring diagram for the power enclosure and modeled up all the components and connections to make sure they fit, but I skipped this planning step for the Pixhawk enclosure. The amount of wires I had to route to the Pixhawk multiplied quickly:

  1. 5 wires from the drive wheel rotary encoders, a total of 10 for both motors.
  2. 4 Sabertooth control wires.
  3. 4 relay control wires.
  4. 6 wires for the Pixhawk power port.
  5. 2 emergency stop wires.
  6. 2 additional wires to power the servo rail on the Pixhawk.

I wasn’t expecting 23 wires needing to be routed to the Pixhawk enclosure, and making sure I didn’t get something wired incorrectly was nerve wracking.

And ultimately, I did wire something incorrectly. When I first powered the robot on, nothing happened, other than the Mauch BEC getting hot. And that’s never a good sign.

Turns out I had incorrectly wired the servo rail by swapping the + and – wires. Yikes. I eventually discovered the issue and fixed it, but not before learning a lot about the LM2576 voltage regulator I incorrectly thought was the issue.

Wheel Encoders

After I fixed the wiring, everything seemed to work, until I started running autonomous missions and had trouble getting the MAVLink Inspector within Mission Planner to show wheel encoder data. Initially I couldn’t even find the wheel encoder message. But after a few reboots it magically appeared. Not sure why it wasn’t visible from the start.

From the get-go, only the right wheel encoder was working. Below is WENC.Dist0 for the encoder on the left motor and WENC.Dist1 for the encoder on the right motor.

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Wheel encoder total distance traveled as measured from the right and left wheel encoders. Note that WENC.Dist1 is 0.

And below is what the graph for the right encoder by itself, WENC.Dist1 versus time, looks like.

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The right encoder odometry data. Having watched the robot move around my backyard, I know this can’t be correct.

So to troubleshoot, I swapped the encoders. The idea is that WENC.Dist0 should look like the graph above and WENC.Dist1 should look like more like the first graph, with values much greater than 10E-6 and gradually increasing. This would isolate the issue to the encoder and not a settings, wiring, or Pixhawk error.

As an aside, having posted that graph I’m now wondering if the negative value means I have the A and B wires swapped. I was driving the robot forward mostly, so I would expect the values to be increasing, not decreasing. Regardless, at least the left encoder is obviously outputting some kind of useful data.

After swapping the encoders…

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The WENC.Dist0 and WENC.Dist1 values after swapping the encoders.

Great, now they’re both broken!

I decided to call it quits after this because the sun was going down. I took the robot back into the garage and got the Arduino out to see what it said about the encoder output. Unfortunately it confirmed that the left encoder at a minimum was broken. The Arduino sketch counted pulses and only got up to 1 or 2 regardless of how much the wheel was rotating.

I remembered I had some 900CPR encoders I had purchased a while back and decided to see if I could get them to work with the Pixhawk. So I swapped the 32 CPR encoder with the 900 CPR encoder, and lo and behold, it started to work again.

All I can gather is that I must have fried one or both of the encoders with my wiring shenanigans. But it concerns me that one of the encoders was working at the start of the day and stopped working by the end of the day.

More to come once I get the mower deck assembled and installed. You can find a brief video of the rover doing an autonomous mission in my backyard here.

Assembly

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The chassis of the robot mower with wheels and motors installed.

I finally have all the parts I need to start putting the robot mower together. And for better or worse, my company has furloughed us for the next four weeks. Here’s hoping I get to cut some grass over the next few weeks!

Finishing Touches

Drawing pictures in a CAD program is fun, but when the rubber meets the road and you start fabricating something, you quickly notice some areas that weren’t too well thought out. Lately I’ve been backfilling those issues as we discover them at the shop. Here are some things I’ve tweaked over the past few weeks. Hopefully we’ll have a finished robot lawn mower soon!

Mower Deck to Chassis Interface

When the deck is stationary in my CAD program, chains look like a great way to support it. I can flip the model upside down and they don’t even move! But when the robot lawn mower starts rolling in reality, what keeps the deck from swaying all over the place? Well, if it hangs from chains, the answer is nothing.

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The turnbuckle that will replace the chain used to attach the mower deck to the chassis.

Unfortunately, the chassis weldment and the mower deck weldments are pretty much complete. So whatever fix we come up with has to interface with those features like the chain did. The solution? Turnbuckles! Some really small turnbuckles, to be exact.

The eye on these little guys is 0.26in ID, perfect for the 1/4-20 screw I had planned on using. The length is adjustable from 3.375in to 4.625in long. They’re rated for 36lb, a strangely specific number, but with three of them they should work fine. The mower deck weighs just over 30lb.

Steel Mower Blades

Another issue the shop made me aware of was the mower blades. I don’t remember if I mentioned it or not, but the reason I designed the robot lawn mower out of aluminum was to avoid any compass interference issues. You may recall I ditched the compass a few months ago, but I never went back and changed the design to steel.

The shop thought that aluminum mower blades were a goofy idea. They’re not wrong, but at least I had a reason for making them that way. Kenny Trussell discovered that when the blades spool up to speed, they interact with the earth’s magnetic field in a way that skews your apparent compass heading.

Making them out of aluminum would avoid that issue, as they’d be non-ferrous. But since we’re not using the compass anymore, it seemed like a reasonable change to make. Besides, all the mower blades you see out in the wild are made from steel.

And if there’s one thing I’ve learned working with fabrication guys, if they make a suggestion that doesn’t impact your design significantly and doesn’t cost much, change it. It’s an easy way to show them you value their input, and they’ll do whatever it takes to get your design working now that their finger prints are on not just the physical parts, but the design, too.

A Legit GNSS Antenna Enclosure

On the wheelchair robot, I had the two GNSS modules velcroed to the top of the robot. That doesn’t seem befitting of a robot I’ve spent a year and a few thousand dollars making. So I designed a small 3D printed enclosure for the RTK GNSS antenna and the UBlox M8N module. It sits on top of a small ground plane disk, mounting to the lid of the electronics enclosure.

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A 3D printed GNSS antenna enclosure. The larger GNSS antenna is for the Ardusimple RTK GNSS module. The smaller one is a UBlox M8N.

Everybody I talk to says you need a really good ground plane for your antennas. That’s what the circular disk is below the enclosure. The screws for the lid of the enclosure are plastic. Hopefully this doesn’t create any reception issues. I also hope that the antennas don’t have to be perfectly concentric with the ground plane. If anybody has experience with ground planes, I’ll take any advice or feedback you can give me!

Mower Deck Discharge Chute

For some reason I had it on the left side of the mower deck. The shop mentioned that most mowers have the discharge chute on the right side of the deck. I don’t want people latching on to minor quirks of my design, so changing it to the right side seemed like a good idea.

Mower Deck Progress

Here’s the progress on the mower deck last time I visited the shop. We’re a few roll formed parts short of a robot lawn mower!

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The mower deck weldment just before Christmas, 2019.

The Time Flies

Back when I was ordering hardware for the robot lawn mower, I came across a smoking deal on some 5/8-11 X 5.5in long socket head cap screws. I was browsing Grainger’s website and they had a deal for a box of five for $1.92. Hot dog! Those things are $4+ a pop at most other places. I placed an order without thinking twice.

I opened the box up today when I was putting the front caster assembly together and found this newspaper page stuffed inside the box. Talk about a blast from the past. I assume this means these screws sat in that box for almost 12 years before they sold. No wonder they were on sale!

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A newspaper page I found inside my box of five 5/8-11 X 5.5in long socket head cap screws. The reverse of this page was a full page ad for liquor store in New Jersey.

Looking at the picture of those girl scouts got me thinking about how short life is. There’s a good chance those girls are probably out of college by now. I wonder if any of them even remember the Girl Scout Sunday events on March 11, 2007. It was probably a big deal at the time, but 12 years later, I’m sure it’s but a distant memory to most of them.

Seeing this newspaper page was a good reminder to cherish what’s really important in life: family and friends. The Mower Project is a lot of fun, but without good friends and family to share my successes, failures, dreams, and goals, it’s all a very empty exercise.

And beyond that, it’s sobering to look forward and think about what will really matter 12 years from now. Who knows where the mower project will take me? The work I put in here could be a defunct blog 12 years from now. It could be something else, I’m not sure what. But if it comes at the expense of time spent with family and friends, it will surely not have been worth the effort.

This Christmas, I hope you all have a wonderful time with those closest to you, and I hope you make some beautiful memories with your loved ones, on which you’ll look back on 12 years from now and smile. That’s a project worth every minute.

Merry Christmas!

Sub-Assembly

I’ve received a few of the weldments back from the shop. While I wait for them to finish fabricating the mower deck weldment I’ve started to put some components together.

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How I initially had the wheel encoders installed. You can see a little rubber grommet in a hole I drilled in the motor dust cover near the bottom  of the picture.

Back when I installed the wheel encoders on the wheel chair motors, I stupidly drilled a hole through the dust cover on the back of the motor so I could run data wires to the encoder. In hindsight, I should have run them through the little sleeve that the power and brake wires were routed through.

I had to take the brake off to put the encoder on the motor anyway, so there was a perfect amount of space for the encoder wires once the brake wires were removed from the sleeve.

Because you can’t undrill a hole I purchased a pair of cheap gear motors off eBay for $80. I mostly wanted them for the motor dust cover, but it will be nice to have spare parts on hand in case I need them down the road.

I took the aluminum back piece off the motors and removed the two white wires you see in the picture. The hole you see them sticking through was where I routed the data cables for the encoder.

Pro tip for dealing with these motors: There are two Philips head M5x150 screws holding the aluminum back piece to the mounting plate. These screws have lock washers under them. The screws are ridiculously soft and easy to strip the heads on. If you want to remove them so they’re still reusable, it’s best to use an impact driver. It’s extremely easy to strip them using a screwdriver.

I managed to strip the screws on both motors before I drilled them out and discovered this, so heads up to anyone modifying the motors like I am here. I ordered replacements that were socket head cap screws instead, hoping to avoid this issue in the future.

Once you have the aluminum back piece off, you’ll see wires inside like this:

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The inside of the aluminum back piece.

The inside is going to be quite dusty with a lot of little brush particles inside. I blew it out with compressed air after taking this picture.

You can pull the white wires through pretty easily, but I had to bend the black wire terminal so I could get access to the hole to feed the encoder data cables to. I also ended up removing the brushes so I’d have more room to work.

Once you’ve got the white brake wires removed, you can pretty easily push the encoder wires through. The end result looked like this:

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How I should have routed the encoder data cables from the start.

One thing I realized doing this is that it would have been pretty easy to drill holes into the aluminum back piece for screwing the encoder base down. I selected an adhesive backed encoder because I didn’t want to mess with it. But going to the trouble to take it apart like this changes that calculus. If I find myself doing this again, I’ll order an encoder that has clearance holes for mounting screws.

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The completed motor assembly with the Harbor Freight tire.

After I had everything wired up, I tested the encoder to make sure it was working well. Nothing like having to tear down a motor after it’s already on the robot to fix a loose wire.

I also wanted to make sure that running the data cables next to the power supply cables wouldn’t cause any issues. I didn’t find any during the bench test. Fingers crossed none pop up in the field either.

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The front caster assembly. A little bigger than I expected!

I used 5/8-11 screws for all the connections in the front caster assembly. I wanted to standardize on one size so I could buy several of one type of lock nut. Unfortunately the width of a 5/8-11 lock nut is 0.9375in and I don’t have a wrench that size. I also don’t have a hex wrench for the socket head cap screws either. The picture above shows everything hand tightened. I’ll have to go pick up the right tools to get this all put together.

More to come soon!

Innovators in the Autonomous Lawn Mower Realm

As I’ve been working on the mower project, I find myself returning to a few blogs and websites to see how other people are trying to automate lawn mowers. It’s fun seeing different solutions to the problem, and I use their successes and failures to spur my own creativity.

This is a list of innovators that I’m aware of in the autonomous lawn mower realm. If you know of some folks that I haven’t found yet, post a comment and I’ll add them to this list!

MowBotix

These guys had automated a fully autonomous riding lawn mower back in 2017, so I think they win the prize for first large scale autonomous lawn mower. They went silent about a year ago and recently posted on their blog that they’re moving toward a more holistic terrestrial software solution that isn’t just for mowers.

Greenzie

Every once in a while I’ll do a Google search for “autonomous lawn mower” to see what I can find. That was how I found the folks out at Greenzie. They appear to be taking the same approach MowBotix did: start with a riding lawn mower and retrofit it with a suite of electronics and sensors. Their solution looks a lot more robust than MowBotix’s, but also quite a bit more expensive. Their Twitter account is a fun time, lots of cool demonstration videos.

Left Hand Robotics

As far as I can tell, the folks over at Left Hand Robotics started out trying to make an automated snow plow. Based in Longmont, Colorado, I’m sure that’s a very welcome solution. It appears they took their snow plow platform and put a mower deck on the front. Voila! Instant mowing platform.

The downside? It appears these bad boys cost an arm and a leg. Or perhaps just a left hand? I’ll show myself the door. This source says their snow plow solution costs $55,000 and has an annual subscription fee of $4,250. However, for that price I’m sure the system works very well. Their videos are quite impressive.

Deep South Robotics

Robo Robby over at Deep South Robotics has a full up riding mower automated with linear actuators to run the steering arms. With his software and hardware chops it appears to be a very robust solution. I admire his willingness to share his methods along with his successes and failures. The comments on his blog are quite informative. Every time he posts, I learn something new.

Evatech

Well, kind of. Evatech makes a series of radio controled lawn mowers, and BitDog from the ardupilot forums took one and added some special sauce to automate it. I admire the simplicity of this solution. It seems to be quite plug and play. But I’m left wondering why Evatech doesn’t take the leap and automate their platform themselves. Their machines are on the pricy side, but being gas powered are probably pretty reliable.

Kenny Trussell

Kenny was the first person I know of (other than possibly the folks at MowBotix) that used an RTK GPS system on a riding lawn mower. He continues to improve his mower, and I look forward to some updates of his progress soon! Kenny shared a very useful waypoint generation program for creating a mower mission in Mission Planner. If I ever get my mower finished, I will likely be trying it.

The Ardupilot Forum

There are quite a few folks on the Ardupilot forum that are working on their own solutions. I won’t post names here, but a cursory search will bring up several of them.

Mean Green Mowers

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Mean Green Mower’s autonomous mower. Great minds think alike?

These guys have partnered with a company called Kobi to build an autonomous robot whose anatomy looks surprisingly familiar. I think the solar panel is a bit over the top though. When your deck motors pull multiple kW’s of power, a 200W solar panel isn’t going to be very helpful in my opinion. But it’s great marketing, especially when your company has “green” in the name.

One thing is for sure, it’s an exciting time to be working on autonomous lawn mowers!

Weldments

I had a chance to drop by the shop yesterday, and things are progressing nicely! The chassis weldment is complete except for a little grinding and cleanup, and the front caster weldment is almost finished. It is very exciting to finally see the autonomous lawn mower jumping off the screen and into reality.

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The tubes for the front caster wheels. All that remains is a little welding!

I spent a lot of time making detail drawings of each part, weldment, and assembly. You can never be too clear or explicit when making something complex. Unfortunately I think the pile of drawings scared off a lot of more than qualified welders and mom and pop fabrication shops.

I’m very thankful I found someone willing to take on the challenge. But even with very detailed drawings, things can still go wrong. For example, below is one of my drawings for the tube shown in the picture above.

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The miter angle is 55°. Or is it?

That 55° is geometrically correct. But when you’re using a miter saw to make an angle cut on the tube, what angle should you set the saw to? The correct answer is 35°. In this case, my drawing was actually a little misleading, while technically correct. Lesson learned: if you’re dimensioning a miter cut, it’s best to show the angle the should be set to avoid any ambiguity.

Another lesson learned is to always plan for 50% or more material than the design calls for. The tightwad in me ordered exactly what I thought the shop would need with an extra 0.5in on the ends. In hindsight, that’s a recipe for extra trips to the metal yard to get material for the inevitable mistakes caused by my own sloppy drawings.

One other good engineering practice: always collect your old drawings. We went through a few producibility changes over the past few weeks, and when I dropped by the shop yesterday, I noticed a few old drawings floating around. Round those suckers up! At a minimum, mark them void. The last thing I want to do is pay for parts that I can’t even use because I changed the design.

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A side view of the main chassis weldment.

If you spend a fair amount of time in a CAD program working on the same thing for more than a few weeks, you start losing a sense of the scale of things. On the computer screen, this weldment looked pretty substantial, but in real life, it’s actually pretty small. That battery bay is going to be very tight. I really hope I dimensioned it correctly.

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The main chassis weldment.

Maybe this spring I’ll have something to actually cut grass with!

Lawn Mower Safety Regulations: A Brief History

Wednesday May 7, 1977 was a cloudy day in Gaithersburg, MD. Shortly before lunch that morning, several engineers and inventors entered building 202 on the National Bureau of Standards campus. They came from all over the country to present mower safety devices they’ve designed to the newly established Consumer Product Safety Commission (CPSC), created by the Consumer Product Safety Act (CPSA) signed into law by President Nixon 5 years earlier.1

A month prior to this meeting, the CPSC voted to implement a new safety standard for power lawn mowers.2 They were looking for the cutting edge in lawn mower safety technology, and hoped to find new safety ideas by meeting with individuals and manufacturers who were innovating in this realm. The ideas gathered at this meeting influenced parts of the new lawn mower safety regulation, known today as 16 CFR Part 1205: Safety Standard for Walk Behind Power Lawn Mowers.3

The origins of 16 CFR Part 1205 are quite convoluted. In August 1973, the Outdoor Power Equipment Industry (OPEI) petitioned the CPSC to adopt ANSI B71.1-1972, the most recent ANSI safety standard for power mowers, as the CPSC’s safety regulation for walk behind lawn mowers. But for reasons unknown, in October 1974, the CPSC instead contracted with Consumers Union (CU) of United States, Inc., today known as Consumer Reports, to propose their own regulation. The CPSC paid $66,745 to CU to develop the standard, in addition to another $25,000 for experimentation and testing.4

With these funds, CU conducted significant research to develop their proposed regulation. They organized time studies to see how fast an operator can move their hands from a power mower handle to underneath the mower deck. They examined the forces blade shields would encounter during typical mower operation. They gathered and compiled existing research on lawn mower injuries, combined it with their own research, and presented it to the CPSC along with their proposed regulation.

CU’s recommended regulation contained several detailed safety rules for preventing electrocution from mower electrical systems, burns from hot mower surfaces, hearing loss due to excessive mower noise, impact from debris thrown by the mower, shields for exposed mower blades, and mower start and stop controls, while addressing other safety issues.5

While the CPSC didn’t use every recommended rule written by CU, it did adopt several rules related to maximum blade stop time, mower start and stop controls, and mower shields and guards. These rules, in addition to new labeling and record keeping requirements, form the newly promulgated 16 CFR Part 1205, which was placed in the Federal Register on February 15, 1979, with an effective date of June 30, 1982.3

But before the new regulation could go into effect, the OPEI sued the CPSC. In Southland Mower v. Consumer Product Safety, the OPEI’s lawyers make any and every argument they can think of to nullify, neuter, or otherwise delay the regulations in 16 CFR Part 1205.5 They argue that lawn mowers are not consumer products, and hence the CPSC does not have authority to regulate them. They argue that there is insufficient evidence in favor of a safety regulation, and that it violates the CPSA. They argue that new shielding requirements will be defeated by consumers and are not efficacious in promoting safety. The majority of their arguments are pedantic and relate to interpretation of the language of the CPSA and how it pertains to 16 CFR Part 1205.

The case is heard by the Fifth Circuit Court of Appeals, and on June 19, 1980 they hand the OPEI a sound defeat based on the CPSA’s broad language allowing it to regulate, well, consumer products. It does, however, throw the OPEI a bone by vacating a portion of the regulation’s foot probe requirement, stating insufficient evidence exists to prove it will reduce foot injuries as a result of contact with a rotating mower blade.

Within the text of 16 CFR Part 1205, the CPSC notes that many mowers on the market in 1980 already feature the safety devices mandated by the regulation. In effect, the regulation codifies what most manufacturers are already doing regarding mower deck shields and controls, but includes new record keeping and safety labeling requirements. And, as with all regulations issued by the CPSC, there are new penalties for non-compliance.

The CPSA mandates safety regulations define performance requirements6, not design requirements. The idea is that regulations should promote multiple creative solutions to a safety problem, not mandate a single specific solution. But 16 CFR Part 1205 mandates the geometry of shields and the specific location of mower controls. There are only so many ways you can design a shield or locate a dead man switch to satisfy the regulation.

And as a result, innovation in the power mower realm decelerates after the 16 CFR Part 1205 goes into effect. In many ways, the regulation froze the state of the art in safety technology for mowers in the early 1980’s. Power mowers manufactured today are remarkably similar to those made from that era.

With decades of injury data to examine, an argument can be made that 16 CFR Part 1205 hasn’t even improved mower safety. At the time it was issued, the CPSC estimated 77,000 mower related injuries occur annually. Between 2005 and 2015, that number is estimated at more than 84,000 annually.7 The severity of these injuries may be less, but as an empirical fact, more people are injured by power lawn mowers today than before the regulation was put into effect.

One can only speculate what the inventors who met to showcase their safety innovations in Gaithersburg, MD would have thought about the resulting safety regulation their ideas influenced. But it’s a safe bet each of them would probably agree: if you want to prevent mower injuries, find a way to separate the operator from the mower. Next time I’ll talk about how 16 CFR Part 1205 makes that difficult, and how the regulation has influenced mower design ever since its inception.

References

  1. CPSC Invites Inventors of Lawn Mower Safety Devices, CPSC.gov, 11/16/19.
  2. CPSC Seeks Offerors to Develop Mandatory Power Mower Safety Standards, CPSC.gov, 11/16/19.
  3. 16 CFR Part 1205, govinfo.gov, 11/16/19.
  4. CPSC Accepts Consumers Union Offer to Develop Power Lawn Mower Safety Standard, CPSC.gov, 11/16/19.
  5. Southland Mower v. Consumer Product Safety, 619 F.2d 499 (5th Cir. 1980).
  6. Consumer Product Safety Act, Section 7(a)(1), CPSC.gov, 11/16/19.
  7. Lawn mower injuries presenting to the emergency department: 2005 to 2015. NCBI, 1/10/18.

A New Addition to the Mr. Mower Family

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The Mr. Mower family is growing! Timing both a lull in the Kansas wind with a smile on Mr. Mower Jr.’s face for a picture is quite challenging.

The lack of activity here on MowerProject.com hasn’t been due to a lack of autonomous lawn mower development. This June we were blessed with a baby boy, and he has been taking up a lot of our time! Posting to the blog in the meantime has been difficult.

Juggling my 8 to 5, the mower project, and making time for Mr. Mower Jr. has been quite challenging. I owe a special thanks to my wonderful wife for letting me run errands for the project and work on it in the evenings. She is truly something special! Having her to bounce ideas off of and to encourage me along the way has been invaluable. I love you so much Mrs. Mower!

As I’ve done in the past, I find it’s good to set goals. Even if I don’t achieve them, they focus my mind and give me a sense of accomplishment and purpose as I work toward them. Below are some goals I have for the next few months.

Finish Procurement and Begin Assembly

I have all of the parts to build the prototype robot lawn mower except for two more SLA batteries and new enclosures for the electronics. The SLA batteries are so cheap that I think it’s worth a gamble on them instead of ponying up the $1,000+ I’d need for some good quality LiFePo4’s.

Once those items arrive I will start to wire up the enclosures and get the subassemblies put together. I will probably need to make another wiring diagram showing how everything connects and functions before I get too far along.

Support Fabrication of the Prototype Weldments

Originally I had planned to fabricate a lot of the robot lawn mower components myself. The idea was that it would be cheaper to purchase my own tools and make the parts myself. Unfortunately, that plan requires a lot of time, a resource I’m very much short on these days with Mr. Mower Jr. in the picture. The economics of paying someone to make the parts suddenly looks attractive again.

To this end I’ve found a local gentleman who is helping me make the weldments. I find myself over at his shop every week or so answering questions about the design, and working with him has been very productive. His feedback has been quite helpful in helping simplify and refine the design so that it is easy to fabricate.

Functional Testing

I have a litany of questions I need to get answered about the performance of the prototype before we even cut grass. A few questions I need to get answered:

  1. How long can the mower run on four 12V, 35Ah SLA batteries?
  2. How well does the prototype handle pivot turns?
  3. How much current does the entire robot draw at typical operation?
  4. We need to verify emergency stop and safety features function correctly.
  5. How much air flow does the current mower deck and blade design create?

Once I’m satisfied the design adequately answers these questions, we can start cutting grass. Stay tuned for some pictures of the fabrication so far!

The Sky is the Limit

Manned Versus Unmanned
An MQ-9 Reaper military drone comparted to the T-6 Texan II. Both aircraft are similar in size.

The primary advantage of autonomy is obvious: the removal of a human from the machine. But that’s not where the benefits stop. Think about what a military drone looks like compared to a fighter jet. If you can get rid of the pilot, you can get rid of the seats, the yoke, the visual instruments in the cockpit. You don’t have to pressurize the cabin anymore. Shoot, now you can change the whole configuration of the aircraft to gain efficiencies you never even imagined when you had to design around a human in the cockpit. Notice how the propeller is on the back side of the drone, whereas it’s on the front of the manned aircraft?

I’ve been watching several folks develop autonomous lawn mowing solutions over the past few years. Back in 2014, Alligator over at the Rusty Nails Workshop attempted to make an autonomous lawn mower using an electric push mower deck with a Piksi RTK GNSS module, the cutting edge in positioning technology in 2014.

More recently I’ve watched the venerable Kenny Trussell and Robby over at Deep South Robotics fully automate a ZTR mower for their acreages. I’ve also been paying attention to the folks out at MowBotix who are attempting to commercialize their solution, which appears to be similar to what Kenny and Robby have come up with.

Each one of these solutions begins with an off the shelf mower. That seems to be a logical place to start: cutting grass is kind of a solved problem. And if you use a riding mower, you get the drive wheels for free, eliminating the need to come up with a way to propel your mower like Alligator and I had to.

Along the way I have questioned whether making a clean sheet design was the right way to approach this problem. There’s no doubt it will be more expensive in the short run. I’ve spent close to 300 hours designing the robot lawn mower over the past year and a half, and once everything is finished I will probably have close to $4,000 tied up in it.

But there is one thing that you can’t beat with a clean sheet design: The opportunity to toss out all the baggage that existing mowers come with and explore what an autonomous lawn mower could look like if you take the person out of the equation.

The Baggage

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A typical 36in zero turn radius riding lawn mower.

If you look at a typical ZTR riding mower, you see many familiar features: a nice plush seat to sit in, foam covered control handles, a deck to place your feet on with a nice grippy surface.

Look closer and you’ll find some very deliberately designed safety features, such as a dead man switch under the seat and some guards placed in areas you might get your foot too close to the mower blade.

If you look even closer you’ll discover that because of all the features described above, you kind of have to put the engine far away from the deck spindles. And that forces you to have an intricate pulley system to transmit power to the mower blades.

And your wheel base has to be pretty large in order to create room for a person to drive the thing. Your tires and engine have to be oversized in order to support the weight of an operator that could possibly weigh 200lb+.

Long story short, mowers manufactured today are designed with a significant constraint in mind: the operator. If you remove the operator, you can eliminate those constraints, and you end up with a machine that is much more efficient in almost every respect. The table below is just the weight and size differences between the prototype robotic lawn mower and the mower pictured above.

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A comparison of size and weight between the prototype robotic lawn mower and the 36in Swisher ZTR mower.

The prototype robotic lawn mower is half the weight and less than half the foot print of the ZTR mower, while featuring a deck that is the same width. With the right deck motors and batteries, the 14.5hp power rating could be matched, too. However, I suspect part of the reason you need so much power on a riding mower is because it’s larger, and you have to have a person sitting on it the whole time.

The biggest advantage to automating lawn mowers is obvious: you don’t need a person controlling the mower anymore. But if we’re going to remove the person from the equation, let’s take that leap all the way to it’s conclusion. There are huge design efficiencies you can gain by eliminating the operator. Let’s redo the whole system to take advantage of all of them.