Help Ensure Sawppy V2 Components Are Available Worldwide #20
Replies: 3 comments 19 replies
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Roger, for the motor controller can I suggest adding an option to use boards that can run Ardupilot. I'm in the process of getting my Sawppy to run on a Mateksys H743-Wing. To start with I'll use it with the LX16A serial bus board, but the board has 13 PWM outputs that could be used to run servos directly or command ESCs. There are many alternative FCUs that support enough PWM channels to command the 10 motors needed. A FCU is more expensive than a nano or teensy but they come with integrated IMUs and other sensors (the GPS may be on-board or external), so the overall package starts to look competitive with buying separate components and integrating them yourself. You also get mission management, telemetry, ground station support and a host of other features, but I guess it depends on how much people want to build / code this for themselves as opposed to have something up and running. |
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I like the idea of supporting an upgrade path to Ardupilot, however I'm personally ignorant of that ecosystem and how to best support it. I'll need advice from people who know more. I looked over the spec sheet for the Mateksys H743-Wing you mentioned. Hardware-wise it can certainly handle the PWM requirements. (I haven't researched the software side.) Not only does it have PWM, it also has UART, SPI, and I2C. Is this common of other Ardupilot flight control units available? If so it opens many options.
This would consume the majority of PWM expansion I/O. Are there typically plenty of open slots? Or would this be too greedy and prevent users from adding more modules? If the latter, then we'd need to use a coprocessor for PWM so only a single communication channel is consumed. But then the question is, what coprocessor, and what communication channel?
And there are probably other options I'm ignorant of.
From an Ardupilot compatibility standpoint, what are the tradeoffs between these approaches, and which path would you suggest for Sawppy V2? |
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There are many flight controllers to choose from, including both open and closed source hardware and ranging from cheaper hobby models to flight controllers suitable for commercial use. The Ardupilot documentation has this section on different options that are known to work: Autopilot Hardware Options. Because Sawppy would use scripting there are some additional requirements for the FCU outlined in this link about Lua Scripts. We could extract a sub-list of ones that would be suitable. The usual set up is to run PWM to an ESC for each drive motor (your first config). So one configuration could be: 4 x (FCU > PWM > Steering Servo) Taking the H743-Wing as an example that would leave 3 PWM channels to run servos for a camera gimbal or other peripherals. There is support for different ESC protocols , SBus servos, so there's a good chance most typical use cases will be provided for. I see it as an advantage that people can choose the motor and servo config that suits them and is available locally - provided the devices can be adapted to the frame and are correctly rated, the controller doesn't really care what's there as long as it can get a PWM signal to it. The rest is in the tuning (getting steering and throttle controllers tuned for a particular set up). The main thing that is missing in Ardupilot is a rover frame matching Sawppy's 4 steering - 6 drive configuration, however that can be addressed by providing a custom Lua script to drive the PWM channels given the high level steering and throttle commands. I'm working on that at present and will post it when I've got it working and tested. If you wish you can add an onboard companion computer to the FCU via a UART and then direct the FCU using Python scripts with DroneKit or even ROS - lots of choices! But you don't need all that to get a rover capable of running outdoor autonomous missions - the FCU, a RC unit, SiK radio telemetry link and laptop running a ground station will get you there. |
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"We need to control 4 steering servos and 6 driving wheel motor controllers. Espressif ESP32 DevKit-C Amazon US example." One possible solution, which may not be the lowest cost, but which would add modularity would be to use the M5stack components; Plus depending on user budget/preference one of;
I guess the main issue is that the 'open core' remains untested and unproven, and availability is uncertain. Our intention is to get it made and sold here with worldwide shipping, but appreciate that's a bit speculative at this point. Using the official M5 stack core would probably work out too expensive. On the upside, it would give users a lot of flexibility to experiment with different modules Open hardware purists would also have some options for modules as detailed here (moved to correct thread) |
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I took a few minutes to look for suppliers of 2020 aluminum extrusion, here in Canada. It does look like it's more accessible than the 15mm Misumi profile, but it's still kind of a specialty item. I had best success with 3D-printer suppliers, and Amazon. If you wanted to look at materials that are most available... angle or square tube profiles seem to be the universally accessible. You can get them at pretty much any hardware store, in a variety of materials including aluminum. They might be just a little harder to design for an easy build though, since you wouldn't have the pretty T-Slots to get good clamping force just about anywhere you want. You'd probably need to drill some holes in some cases. |
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Thank you for taking the time to do this. It's disappointing that 20mm extrusions aren't much better than 15mm, but I appreciate the additional vote for metal tubes. I agree it would make Sawppy more difficult to build, but I also have other motivations for Sawppy evolution to require basic shop tools such as a drill press. If I go down that path, then metal tubes becomes a smaller additional step to take. For aspiring rover builders without ready access to such tools, I plan to release a smaller simpler Sawppy that can be built using just hand tools. The challenge with commodity metal tubes are dimensional tolerances far looser than extrusion beams. 20mm extrusion beams are usually quoted with tolerances < +/- 0.5mm. But I've found hardware store tubes to be up to 10% off nominal. (20mm +/- 2mm). It's a design challenge for sure, and I'm still thinking about it. |
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That's a great point. I hadn't thought about tolerances. Maybe put one bolt through a hole in the tube, and another through threads in the 3d-printed part (tapped hole, captive nut, heat-set insert), to apply tension to the tube without actually going through it. Set-screws might be ideal for this since their points are usually designed to grip. The through-bolt would ensure the joint is secure (never slips out), and the tension screw would just keep it from wobbling. Alternatively, your experimental work with crush-ribs might be useful here. |
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Related thought: if I could somehow design to accommodate such loose tolerances, there's another benefit. Such parts would accommodate both metric and imperial! For example: if the 3D printed parts could accommodate metal tubes of 25mm +/- 2.5mm, then it also easily accommodates 1 inch (~25.4mm) I don't know if I can pull it off, but it'd be great if I could. |
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"It's disappointing that 20mm extrusions aren't much better than 15mm, " I really like 20 x 20 Vslot It's available from: Aliexpress China £12.99 + £0.49 shipping to UK Oozenest UK Example Cost of 2x 500mm linear rail is £7.20 Payment methods are; Card payment, Paypal, Bank transfer, or purchase order. |
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Also, sometimes the customs duties can only be paid by a business registered as an importer with that country's government. An individual has to either find an existing import company willing to work on their behalf for a fee or they have to set up an import business themselves. (I do not expect people to start a company just for the purpose of importing Sawppy parts, though I would feel extremely honored if anyone does it.) |
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I wonder if it would be possible to design for 20x20mm square tube/profile, but remain agnostic as to whether it was a generic aluminum square tube, Vslot or even Timber? |
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That is my aspirational goal for Sawppy V2. It would be a huge challenge, but it would make the design much more adaptable. |
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3D printed structural beam holders which bolt together to clamp the beam, is that the basic idea? |
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I spotted these that work on a similar idea, not sure if they're a useful starting point? https://www.thingiverse.com/thing:5209449/files |
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Hello Sawppy builders,
I want to thank everyone who have built their own Sawappy and gave me feedback. I have since learned some Sawppy components can be difficult to get worldwide. I plan to redesign Sawppy version 2 to use more commonly available parts. I list some candidates below, and I would love feedback on this list from other rover builders. Especially those living outside of the United States.
Structural Beams
Sawppy V1: Misumi HFS3 15mm x 15mm aluminum extrusions. I knew Misumi had distributors worldwide, but I didn't know many of them only sold to businesses and not available to hobbyists. MakerBeam XL is an alternate, but at greater expense.
Sawppy V2 Candidate: 20mm x 20mm aluminum extrusions. Since they are used as structural members in 3D printers like the Ender 3 (& clones.) I believe they are more commonly available worldwide. True or false?
Motors
Sawppy V1: LewanSoul LX-16A x 10 interfaced via Bus Linker board. These are not available worldwide, and only sporadically available within the United States. (See Issue #18)
Sawppy V2 Candidates
Steering:
Generic remote control (RC) servos. Available in a wide spectrum from budget low-end to powerful high end metal gear units. Example on Amazon US
Driving:
Brushed DC gearmotor.
Brushed DC Driver:
NEMA 17 stepper motors.
Advantage: Precise motion control. Ball bearings typically built-in. Available worldwide(?)
Disadvantage: 3D printer NEMA 17s have little torque for rover wheel purposes. Higher torque (typically correlated to longer body) NEMA 17 motors exist but less common and steeply more expensive.
Stepper driver:
28BYJ stepper motors
Advantage: Very affordable, has a built-in gearbox and usually bundled with ULN2003 drivers. Amazon US example
Disadvantage: Not very powerful even with gearbox. Unipolar stepper motor design makes it incompatible with most stepper driver units. (Yes they can be hacked into bipolar steppers, but I'd like to make build instructions simple.)
Worldwide availability: Unknown.
Motor controller:
LX-16A had an integrated controller making wiring easy. Switching to servos + brushed DC/stepper motors makes wiring more complex and would also require a separate microcontroller to generate the necessary PWM control pulses. The popular Arduino Uno & Nano, unfortunately, do not have enough PWM outputs: We need to control 4 steering servos and 6 driving wheel motor controllers.
(*) Disclosure: All Amazon links in this message are affiliate links. Not because I'm trying to make money, but because it's a lot faster for me to get a short URL! Anyway, as an Amazon Associate I earn from qualifying purchases.
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