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!
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.
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.
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!
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!
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.
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.
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:
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:
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.
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.
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!
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!
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.
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.
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 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
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!
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.
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.
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.
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.
Maybe this spring I’ll have something to actually cut grass with!
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.
- CPSC Invites Inventors of Lawn Mower Safety Devices, CPSC.gov, 11/16/19.
- CPSC Seeks Offerors to Develop Mandatory Power Mower Safety Standards, CPSC.gov, 11/16/19.
- 16 CFR Part 1205, govinfo.gov, 11/16/19.
- CPSC Accepts Consumers Union Offer to Develop Power Lawn Mower Safety Standard, CPSC.gov, 11/16/19.
- Southland Mower v. Consumer Product Safety, 619 F.2d 499 (5th Cir. 1980).
- Consumer Product Safety Act, Section 7(a)(1), CPSC.gov, 11/16/19.
- Lawn mower injuries presenting to the emergency department: 2005 to 2015. NCBI, 1/10/18.