Hey everyone, thanks for chiming in here! Sorry I’m not as fast at responding (also have hundreds of emails to respond to, too!!)
So, regarding wobble - everyone here is pretty much right. Wobble does not have to be as controlled as with a CNC router or 3D printer, but it does need to be minimized for some precision operations like picking up and putting away the tools, and for taking photos. The main issue with scaling and wobble is that if you go twice as big (in certain directions), you’ll get something like four times or more of the wobble. So going taller would be difficult to do without significant strengthening of the hardware to make it more rigid. This of course adds cost and weight. Whereas making the tracks longer doesn’t increase wobble at all. So the best place to scale first is with the tracks length. Then the gantry width. Then the z-axis, and only if you want to grow corn or sunflowers… or fruit trees!
Polar coordinate bots would be awesome, and the software should support that with a simple firmware conversion from cartesian to polar coordinates. You could either have a circular “track” made of wood or bricks or compacted soil that would be enough to support a rubber wheel at the outer end of the “arm”. The support only has to be in the z-axis, ie it doesn’t need to be precisely attached to the “track” in the radial or tangential directions. The other option is a cantilevered arm that is supported by guide wires from above, like a suspension bridge. It could be counter-balanced with an arm sticking out the other end so that the moment force on the main vertical pole would be minimized. The circular track system could probably scale larger, because you could have multiple track rings. While the cantilevered system would be easier to setup for smaller devices.
Regarding scaling the cartesian bots, everyone here is correct that for not a whole lot more cost, you can get a lot more value from the device. In fact, we’ve already been working on FarmBot Genesis “XL” for a while, which has a double-width gantry (3m) and double length tracks (6m). You can see a sneak preview at the end of this video: https://youtu.be/gDocKosjY0I?t=3m50s
We weren’t planning to release larger devices initially because we wanted to focus our campaign on one product. But, with the latest hardware (which is beefier and more battle-ready) we’re learning that we’re pretty much ready to make the larger kits available for pre-order, and we’ve had SO much interest in them (see: this thread!). The larger devices will be the same height, but be expandable up to 12m in length, and 3m wide. The main cost increase will come from the extra extrusions, plates, and plastic components, and the longer tubing/wiring/cable carrier required. But, with current estimation we can bring the cost down from over $600/m^2 of growing area for the smallest size to under $150/m^2 at the largest size! Stay tuned to our newsletter for the coming announcement!
Also, maybe you all can help out: Right now there are two main issues we’re working out with the larger devices:
- By scaling in either the X or Y direction, the gantry becomes heavier because of the extra gantry weight and/or the extra tubing, cable carrier, and wiring it has to pull along the tracks. How can we move the gantry with this extra weight? Some options we’re exploring: Using a gearbox to increase the torque but decrease the speed of the x-axis motor. Adding a second x-axis motor to the other side of the gantry and using a second stepper driver to power it. Upgrading the x-axis motor to a NEMA 23, and powering it with a more powerful, independent stepper driver from the RAMPS shield. A combination of the above. We’re leaning towards using a gear reduction or dual motors because upgrading the driver electronics would be more expen$ive. Further, we’re leaning towards dual motors because then we can maintain higher speeds and eliminate the driveshaft!
- Maintaining vacuum pressure with such a long tube for the vacuum line (for the seed injector). We haven’t done enough testing yet. But three obvious solutions are: get a more powerful vacuum pump, increase the tube diameter to reduce pressure losses, move the vacuum pump closer to the UTM, such as onto the gantry instead of next to the bot.
Cheers to open-source!