January 2023 update: New vehicle design, metal shop, motor controller, motherboard

This article is a text version of our latest youtube video. To watch on youtube click here:

Hello everyone and welcome to Twisted Fields, our research farm in San Gregorio California. We’re developing Acorn, our open source precision farming rover. We believe that robots like this can help produce healthier food for all, reduce our reliance on harmful agricultural poisons, and fight climate change through regenerative agriculture.

in our last article, we shared an update on several developments we had completed on the robot early last year and announced our new funding platform for supporting our work.

In this article, I will share several more exciting developments as we get closer to a stable kit design suitable for shipping to early adopters. We’ve designed new electronics which will lower the cost of our system, built out a basic metal shop including a large format CNC plasma cutter for quickly fabricating parts for our robots, and perhaps most exciting - we’ve used this metal shop to build the first prototype of our new vehicle design. We’ve also started getting monthly donations from a handful of backers as we grow our new funding campaign.

If you find this project interesting, consider subscribing to our youtube channel. To be notified of every new video, click the bell icon next to the subscribe button. Finally, we’ve begun to accept tax deductible donations to support our project. Long term we hope to fund all of our engineering work through your support. These donations allow us to keep the entire project open and accessible to all. You can make a donation with just a few clicks, so if you’re interested in making a monthly or one time donation, head on over to our Open Collective page and make your contribution today. We appreciate anything you can offer. We also encourage you to sign up for free and introduce yourself right here on this forum.

Finally, it’s time to introduce you all to Daniel Theobald. Daniel came up with the idea for our solar powered farming robot and has been behind the scenes supporting this project since the beginning. We’d like to get him on here more to share his own thoughts about the project.

“Hi everyone. You know, agriculture is at the heart of every healthy community, and agriculture is hard, especially when you’re trying to grow healthy food in a sustainable way. We’re really excited about the potential of this technology to help solve that problem and sharing it with the community and seeing what all of you can do what the technology is is something that I’m really excited about. So thank you for your interest and support and I look forward to seeing you in future episodes.”

Thanks Daniel!

Now lets get on to the engineering updates.

I think the most exciting thing to share is our new robot prototype under construction now. Since the start of the project we have used our original prototype vehicle as a development platform to write all of our software, test drive systems, prove out some of our fundamental concepts, and develop our electronics. But there were some issues with that design. The robot can be thought of as four identical robotic corner assemblies, linked together by a rectangular frame. The corner assembly includes a steering system and drive motor, and all of the complexity of the robot is in those corner assemblies. On the original prototype, the corner assembly is integrated in to the frame of the robot. If it has a malfunction, you have to disassemble the corner of the robot in place. On the new version, the corner assembly is modular and removable. If you have a malfunction in the field, you can remove the corner and take it to a workbench for troubleshooting, or swap in a replacement to quickly keep moving.

The new frame is more modular as well. Instead of the large single piece frame of the original, the new frame consists of side weldments bolted on to front and rear weldments. This makes it easier to change the width of the robot for different row spacings. We’re currently experimenting with 48 inch row spacing (122 cm) to allow for 30 inch beds with 18 inch walkways (76 cm beds, 43 cm walkways). This is a popular size for high density human scale organic farms, and we’re very interested in a system that works alongside people in the field. This spacing allows a person to crouch in the walkway and comfortably reach the full width of the bed. It feels like a good size for the robot, especially for rapid development of new tools. I imagine that as we develop more and more useful tools, we can let the robot do more of the farming and go to wider row widths.

We fabricated the new frame right here in our metal shop. It doesn’t take much to build the robot yourself. Aside from basic hand tools, the only equipment required is a CNC plasma cutter, drill press, band saw for cutting tubes, and a MIG welder.

Our goal is to design a farming robot that can be built in a small shop in any city in the world. This will allow people anywhere to build these systems, sell them for profit, and build a business providing these machines to local farmers. This creates an economic engine that will drive development of newer and better designs, and a community of creators and users all over the world who can swap ideas and build on each other’s work. Of course we will sell kits to those who want them, but its important to us that anyone can build them.

In other news we’ve made significant advancements in the electronics design for our robot. This dramatically lowers the cost of our system, makes it easy for anyone put the new boards in to production, and simplifies the wiring and design of the electrical system.

In a 2021 update we showed off the motor control system of the first prototype vehicle. We relied on the ODrive motor controller plus a custom circuit board we designed to interface with our motion control system. The ODrive hardware cost $250 to control two motors, and the interface board we need adds even more cost. In conversation with ODrive we learned they are moving up market to higher end offerings, discontinuing the board we have been using, and moving some of their products to closed source software. To keep our costs low and our system flexible we decided to look for other options.

In the end, we decided to design our own custom motor controller. Our board avoids the use of specialized chips which risk supply shortages, and is designed to be fabricated using the JLCPCB assembly service. JLCPCB keeps a large library of parts in stock, which avoids the added expense of sourcing parts from vendors all over the world. By designing the board ourselves, we can add whatever interface electronics we need without requiring a second board. Our new board, which controls two motors and includes all of our custom interface electronics costs about $35 to produce. All of the design files are open source and on our github, and anyone can order them from JLCPCB with little overhead. We will maintain two versions of the board - a custom board for our robot with all the extra interface electronics we need, and a general purpose version for robotics hobbyists.

We have already confirmed the basic design works, and have since designed an updated version with all our interface electronics for the corner assembly. To get the motors spinning, we use SimpleFOC firmware designed for just such a board. Our motor controller uses the RP2040 processor, and we found a very helpful SimpleFOC developer, Richard Unger, who was kind enough to port SimpleFOC to our hardware. I think this is a testament to the power of open source. We made the hardware, found someone who had already worked on appropriate software, sent him a developer kit ready to spin a motor, and his curiosity and passion led us to success. His findings also led to some design improvements in the latest rev. Thank you so much Richard!

With all the advancements, it feels like we’re much closer to a stable kit design we can ship to early adopters. We’re working now on wiring up the new system and getting it running. From there we need to put it through long term testing and make sure there are no surprises. Then we will build a few more vehicles for advanced testing and production trials, and then hopefully we will be able to sell our first kits.

As I mentioned before, we’ve begun to receive donations to our project on our Open Collective page. These donations are tax deductible and easy to make with just a few clicks. For now these funds will be used to support the production of these video updates. As your support grows we hope to fund all of our engineering through your contributions. This will help ensure our designs can stay open and accessible to all.

If you find this all interesting don’t forget to subscribe to our youtube channel, and click the bell icon to be notified of every new update.

Finally, we would love to see you join our free community discussion forum right here at community.twistedfields.com. Sign up and introduce yourself!

That’s it for today’s update. Big thank you to our financial supporters:

Daniel Theobald, Mike Kelley, Kelly Knight, Liu Liu, Kate Sprague, Booty Tompkins, Moses Tschanz, heimi, bounding_star, Sami Anderson, Avi Brown, Guest, Cole Amick

We would like to acknowledge that Twisted Fields is located on the unceded ancestral homeland of the Ramaytush Ohlone peoples who are the original inhabitants of the San Francisco Peninsula. For more information see ramaytush.org

As a bonus, I wanted to share the artwork on the back of the PCBs which did not make it to the video. I always feel like there is a lot of room on the back of the PCB, and if it’s not filled it art it would be an awful waste of space.

Thanks for reading!


Just adding a note that I have enabled replies to our blog posts. I would love to hear your thoughts right here in this thread. Please DM me if I made any mistake on permissions here. Thanks!


Is there going to be a future update on the different wheels/motors?

Did you consider skid steer? Does it not provide enough control? disrupts soil too much?


I should’ve included:

I’m looking forward to the new motor controller and as an odrive owner disappointed to hear about the direction of odrive.

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I saw that you changed the wheels from using typical bicycle wheels to some smaller ones. I would be interested in what kind of wheels you chose to use now & also what made the decision to switch away from bicycle wheels?

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Yes we have changed the wheels, but we are still exploring how to use bigger tires.

Most geared bicycle wheels use a 3:1 reduction, and are designed for high speed. They are inefficient at low speed high torque, burning a lot more energy than something with a higher gear ratio. Not good on a solar powered robot. We found these geared wheelbarrow hubmotors with a 30:1 gear reduction, which are more efficient at low speed high torque operation. Unfortunately they are not designed to fit a bicycle wheel - they fit a wheelbarrow tire directly over the motor with no room for bicycle spokes. So I came up with a modification to put bicycle wheels on the wheelbarrow motors as a stopgap solution. It is a fairly extensive modification mostly because you have to have a bike shop build a wheel on to it. What we found was that bike shops hated doing this. The very first bike shop we went to was happy to do it… until their good wheel guy moved away. Then they refused. I called loads of other shops and they all refused. I finally found one that would build the wheel, but it was expensive. They don’t like the apparent liability from an electric motor and they don’t like that it is weird.

I was recently working with a vendor on alibaba who said they could build us a planetary gear reduction hub motor to fit a 26" wheel. They started working on it and then eventually decided they couldn’t do it, so we are still looking.

In the meantime, the wheelbarrow hubmotors we are already familiar with can be bought with tires onboard, so for now they are a great solution to get our new system up and running. The large tires of the first version were absolutely amazing at off roading. We will see how these smaller tires perform. From a cost perspective they are great, but we will continue to explore other options before release. On the new vehicle the major questions are on the performance of the new steering system and the new electronics, so using the existing motors with small tires will let us get testing ASAP as we look for other possible wheels.

As far as tracks - we will have to see what is needed. In my mind tracks add a lot of cost. But our system is designed to be customizable so if anyone wants to slap tracks on it and share their experience, this will help us as a community gauge their viability. For our initial vehicle release we will use wheels as they are lower complexity and cost, and we want to get out the door as soon as we have a viable system.

Thanks for the questions!


sorry, I didn’t mean tank tracks for skid steer. I just meant with wheels like your 3d printed robot. Maybe torque vectoring or differential steering is the better term to use in that case.

Oh sorry that was right at the end of a long work day, I missed that!

So skid steer requires a relatively rigid frame and it puts a lot of stress on the frame. I am not sure how well Acorn’s frame would handle this. However it could be tried by just changing the steering algorithm. Of course skid steel also disturbs the soil, while an ideal four wheel steering algorithm will be much more gentile.

We do very much want people to make their own versions and I would be curious to see if skid steer can be made useful! Certainly our software stack would work all the same aside from minor changes in the steering code.

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thanks! I assumed there were reasons. There’s not much online about the real world trade offs of skid steer that i’ve found.

Have you considered using windscreen wiper motors attached to a bicycle wheel for drives as a cheap DIY alternative? Just have to lock the sprocket to the wheel.
Build a Robot with Windshield Wiper Motors - YouTube Using wiper motors as direct drive

For the steering you need better control of angles. You carefully cut off the rear dimple to add a small magnet to the exposed shaft and a hall effect device to detect the number of revolutions.
How to convert a wiper motor into a strong servo motor - YouTube convert a wiper motor into a strong servo motor

Obviously some wiper motors would last better than others (plastic vs metal gears)

Skid steer create the change in direction by causing tires / tracks on one side to go at different speeds. If the right side goes faster than the left side then the right side goes further which causes a change in direction. They are called skid steers because they skid the tires. Just like a putting the brake on when you are trying to push it. The turn levers on bulldozers are actually called brake levers, one for each side. Skid steers are very maneuverable and do jobs that a regular tractor cannot do without a lot of back and forth to achieve the same turn. However, the quick steering requires extra extra energy. Very energy inefficient. I have a Polaris Ranger EV which has three drive modes: 1) turf mode with differential allowing different speeds left and right, 2) locked limited slip differential, and 3) 4WD. It is unbelievably difficult to push in the shop when the differential is locked. I end up pushing it with my John Deere 2030. If it is in turf mode, aka one wheel drive, it is very easy to push. (I had to push it around the shop a lot when I first got it with dead batteries and electronically locked differential.)

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