Power Consumption

I decided to make the autonomous lawn mower fully electric for one big reason: If a person has to walk out to the mower with a gas can and refill the tank, is it really autonomous?

Ideally, you want the mower to do it’s job without any human intervention. If you have a gas engine, no matter how you cut it fuel has to be delivered to the mower in some fashion. With an electric design, you can have the mower automatically dock with a charging station when the battery gets low. No human required.

So from the get-go I have been trying hard to make the mower electric. I am encouraged by some electric riding mowers out there that use SLA batteries as their power supply. I like SLA batteries because they contain a lot of energy and are fairly cheap. Minimizing battery weight and volume isn’t a huge constraint for this project, thankfully.

Because these electric riding mowers cut grass and carry a ~200lb person on the mower, I have been operating under the assumption that as long as our batteries are larger capacity than those on this riding mower, we should be okay. That Ryobi mower features a battery bank that consists of four 12V, 25Ah SLA batteries.

I am beginning to question that assumption…

Power Consumption

Sizing the batteries ultimately depends on how much power the mower needs. The deck motors take the lion’s share of power consumption. Previously I estimated the mower would require motors that can output at least 5ft-lbf of torque to cut through thick grass based on typical gas engine torque output.

Examining the torque curves for the E30-400 motor I selected for our design shows that at 5ft-lbf or 3.7N-m torque, the motor consumes 1400W of power. If you assume all three motors pull this level of power, the deck motors collectively consume 4200W.

The drive I’m using on the mower design are stolen from the wheel chair. I suspect they are rated for 500W but I am not sure. The gearbox on them ensures they will generally be operating in an efficient area of their torque curves, so I am going to consume both motors consume 250W, and collectively consume 500W between the two motors.

The control electronics are almost negligible compared to the power consumed by the motors, but I will budget 100W for all the other little things on the mower, just to be safe.

That brings the total estimated power the mower needs during operation to 4200W + 500W + 100W = 4800W.

Battery Capacity

The batteries I’ve selected are four 12V, 35Ah SLA batteries. If you assume we intend to discharge these batteries 100% (and that doing so was physically possible), you could obtain (4)(12V)(35Ah) = 1680Wh of energy. If we were to draw 4800W of energy from these batteries, we would drain them in (1680Wh)/(4800W) = 21 minutes. Yikes.

But it gets worse. Because we’re pulling so much power out of these batteries, it looks like you have to discount the total amount of energy you can get out of them. I’m not entirely sure what that calculation looks like, but from the SP12-35 datasheet, it looks like a 1hr discharge rate only allows you to get 21.8Ah of charge out of each battery. That’s only 60% of the 20hr rate of 35Ah. I could be wrong about this interpretation of the datasheet, please correct me if I am mistaken.

Some Thoughts

Do the motors really draw that much power? Holy moly I hope not. At their most efficient, the motors draw 500W of power. Running the calculations above with this number gives you a run time of 48 minutes. Still not great.

The reality is somewhere between those two extremes. Taking the average of the two gives 35 minutes of run time. I was hoping for something more in the neighborhood of 2 or 3 hours. Going up to some 12V, 50Ah batteries could give us some extra oomph, but I don’t think it will be 3 hours of oomph.

Please let me know if these numbers seem way off base, it’s my best swag at them I can come up with. The last thing I want is a mower that can only cut grass for 10 minutes.

 

Progress

combine
The robot lawn mower design.

I’ve started doing some wiring diagrams for the robot lawn mower, as the mechanical portion is fairly well defined. However, there are a few things I’m still not sure about:

  1. Will four 12V 35Ah SLA batteries will provide the energy needed to run the mower?
  2. Should the battery bays be replaced with some off the shelf enclosures?
  3. Is there a better way to do the deck height adjustment mechanism?
  4. Are the motors sized appropriately, both for speed and torque?
  5. Is a pulley quick disconnect bushing really the best way to attach the cutting blade to the motor shaft?
  6. The sheet metal used on the weldments is 0.1875in thick aluminum. That is expensive and probably too stout. It should be changed to 0.125in thick.

I’m chasing my tail with these questions above, so I will take some time off from modeling the mechanical side of the mower and work on wiring for a while. Hopefully things will be more clear when I revisit these issues later.

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.

Adjusting the Mower Deck Height

deck height adjustment
A simple height adjustment mechanism for the mower deck.

I’m trying really hard not to turn the height adjustment mechanism into a science project. All it needs to do is (1) support the mower deck and (2) allow the height of the deck above the ground to be easily adjusted.

You can find some exciting mechanisms out there to adjust the height of a mower deck. Google Scag Tiger Cat Height Adjustment if you want to see one of the more interesting ones.

A lot of mowers have a mechanism that lets you adjust the mower deck with one single lever. While I like these mechanisms I don’t think they’re necessary for the robot mower. How often do you typically adjust your lawn mower’s height? I do it once a year, if that.

For this project, having to adjust the height in four places versus one is a small price to pay compared to the time it would take to both engineer a mechanism that would work for our robot mower. Not to mention the cost to make it. Springs and linkages get expensive quickly, and the risk it won’t work right goes up fast when you start adding lots of moving parts.

I like simple. I’m not that bright, so I figure if can envision something working, chances are it will probably work okay in real life. To adjust the height of the deck, pull the quick release pin, rotate the lever to the appropriate height, and put the pin back in. Do that in four places and your done. Simple!

Concept #5 Refined

prlm-a50001-11-04-18.png
The autonomous lawn mower as of this evening.

The autonomous lawn mower design is coming along nicely. It turns out that the batteries fit nicely below the robot frame. I only had to raise it by 1.5in to give adequate clearance for the battery terminals. This keeps the center of gravity low, and makes for an all around good looking robot in my opinion.

I have an RTK GPS module arriving in December (more on that later), and I want to get the lawn mower design as mature as I can before I need to start investing my time testing that module with the wheel chair.

My list of open items on the design as of this evening:

  1. Latches for the battery compartments. Maybe I don’t need them, but it would be nice to have some way to hold the doors closed. Maybe just some screws would work.
  2. Control enclosure location. It needs to be far enough away from the deck motors to avoid the flux storm. But mounted in such a way that it doesn’t adversely affect vehicle performance or look goofy.
  3. Deck height adjustment mechanism. The robot needs to have a way to simply and quickly adjust the deck height as you would on a typical lawn mower.
  4. Safety shutoff and control system for deck motors. I think these should be powered by a separate set of relays so they can be independently turned off from the rest of the rover. They will still be wired into the master safety shutoff switch.
  5. RTK GPS antenna mount location. Wherever the control enclosure box gets placed, it will need to have enough real estate on it for the GPS antenna.
  6. General wiring. All the motors, the enclosures, everything. This will take a lot of time but as we saw earlier was very much worth it.
  7. Mower deck discharge chute. The mower needs a plastic chute where the grass clippings discharge. It’ll also need the hardware that attaches it to the deck.

 

xrlm-a10001-versus-prlm-a50001-isometric.png
The autonomous lawn mower versus our wheel chair robot.

The autonomous lawn mower measures 37.5in long X 36in wide X 17.5in tall currently. Not bad considering our grass cutting width is 35.5in!

Concept #5

prlm-a50001-e1541049758636.png
A design concept I’ve been working on lately.

The nice thing about CAD software is you can see how different ideas play out before you spend a fortune to discover they don’t work. The past month or so I’ve been drawing up robot mower designs with little luck. The model above is the only one I feel even moderately good about.

Most of the concepts I’ve put together so far feature a sheet metal chassis shaped around my batteries and motors. The upside with these previous designs is that it allows for the smallest vehicle footprint. The downside is that they involve a lot of welding and waterjet parts, and it’s difficult to come up with a simple mechanism to adjust the deck height because the motors mount directly to the chassis.

This new concept has the mower deck hanging beneath the robot frame. Separating the deck from the robot frame allows both to be simplified greatly. To adjust the deck height I plan on putting a few turnbuckles between the robot frame and the mower deck. An added bonus is that I can disassemble it and throw the robot in the trunk of my small Hyundai sedan for field testing.

One potential downside I anticipate with the design above is that it will be top heavy. The four batteries have to sit on top of the robot frame. We can box them in and secure the to the frame, but I’m not sure what the vehicle center of gravity will look. Guess we’ll have to do some more modeling to find out.

E30-400 Motor Evaluation

The electric mower motor I got off craigslist had a 12in blade on it. That motor was rated for 24V, 4300RPM, 1.3N-m. I suspect it was pretty undersized for doing any real grass cutting, but whoever manufactured it had to size it at least somewhat appropriately.

Our three blade mower design has 12in blades on it. So in theory, any motor larger than the craigslist motor should be sufficient. But because I’m a perfectionist, I don’t want to just barely exceed these parameters. I want to knock them out of the park.

E30-400_Chart
Performance curves for the AmpFlow E30-400 DC motor.

For the same torque values as the craigslist motor, the E30-400 motor will rotate at about 5000RPM, consuming 700W while operating at about 77% efficiency. That translates to a blade tip speed of 15700ft/min, or 80% of the maximum allowable tip speed.

At the same speed as the craigslist motor, the E30-400 outputs about 2.8N-m of torque. That’s more than twice what the craigslist motor is rated for. Not bad.

The best part? Three of these guys only cost $354.41. And that’s after shipping, with insurance. The holy grail indeed.