Most of my answers come back to the design’s underlying purpose— that is, to be a relatively cheap DIY assistant robot that can be made and maintained off spare or surplus parts, even in an unfavorable environment and under exposure to the elements. Likewise, although I’ve selected specific, off-the-shelf parts for the prototype, the design itself was meant to accept whatever anons could scrounge up, so I’ve made more aggressive design decisions that can then be scaled back, budget permitting (larger motor bays, space for doubled-up chains, etc.)
I wasn’t being glib when I said a design goal was that it could be
> maintained by a hobo
Call it a sort of “once bitten, twice shy” after brushing with homelessness myself. In a worst-case scenario, the ability to buy replacement parts at a local bike/car/hardware store is priceless.
Anyway, I’ll address your questions out-of order
>batteries: lead-acid vs lifepo4
I don’t have a lot of experience with batteries (and I’m not particularly attached to lead-acid for the prototype)— the reason I’ve stuck with them so far is mainly due to expected power consumptions, where there are (very brief) spikes as high as 10kW. I expect that sort of use is going to absolutely destroy any battery that’s subjected to it, so the idea was “I might as will pick the cheaper ones”, since they’re practically a supercap if you take the concept to the extreme.
I’m open to, and even have an alternate Sled configuration using a LiFePo4 battery (a integrated unit intended for offgrid use, but still) with motorcycle batteries to fulfill instantaneous power requirements. My main sticking points with that are:
- prone to catastrophic failures
- can’t be repaired/easily replaced (unless you start getting janky with 18XYZ-type cells and DIY BMS units)
- requires either a custom battery/BMS, a significantly larger boost converter (for drive voltage), or multiple batteries (lithium batteries are a lot harder to efficiently fit in a given footprint, unlike lead-acid ones which come in every shape/size imaginable)
- requires a separate environmental control to be (efficiently) usable in winters
>drive voltage and current
The current spikes I mentioned are *very* transient (hundreds of ms at most), and the math'd scenario that gave rise to em was towing my _car_; frankly out of scope, hopefully not something I’ll ever need IRL, and at the far end of the motors’ datasheets.
Still: I, like every engineer, enjoy fantasizing about how a little extra TLC to a particular aspect of the design might save the day.
Generally, most of the stuff I saw had consumer-grade motors ending above 12/24V, with industrial or specialized ones taking over between 36-72V. At least, most *efficient* e-bike motors are 24V or less, same for go-carts.
As for selecting 24V for the prototype specifically, the TL;DR is that it’s
(a) close to what most anons building it for themselves would use
(b) so I can work with a company I’m semi-familiar with, and don’t have to worry about quality— in past projects, I’ve wasted a ridiculous amount of time buying mystery-meat parts, only for them to burn out at 60% of spec or vary between batches. Just ain’t worth it, imo
>motor placement, right angle vs side-by-side
That was my original plan, too. The main issue was with (affordable) drive/gear reduction, since most motors put out 5-10k RPM, while my design is cruising around 10mph with only 150RPM (drive axle)— crunching the number, I came up with the Drive Platform needing a ~12x reduction before even driving the axle.
With gears made for this range of KE, you end up with very large gears and a practical reduction limit of 1:2 or 3. A gearbox would be ideal, of course, but the gearboxes rated for this cost in excess of $1k. That leaves us with chain-drive reduction, which is way more doable with a small budget or improvised parts, but requires large sprockets, a sufficiently strong chain, and a tensioner if you hope to minimize backlash, all of which increase the design’s footprint.
And the chain reduction mechanism isn’t just long (>14” for a single-stage), but tall (>8”) too. Now, you could switch to a multistage mechanism, but that requires a lot more moving parts and driven shafts, plus a bulkier enclosure. To solve that, you can switch to a smaller chain pitch, but that can’t be bought off the shelves, (…) so on and so forth.
In the end, I settled on a single reduction stage, which necessitated bilateral symmetry due to the sprocket size (otherwise it would require changes to the design of both the Drive Platform’s frame and the Wheelbase mounting hardware for each side, which is a whole other can of worms)
- Bridge drive “cuts” the footprint in half requiring motor+gearbox < 4” long, or would require the two mechanisms stacked atop eachother— while doable, it would eat up basically all of the design’s ground clearance (~6”)
- I am aware of cheaper worm-drive gearboxes (and their helical variants), but those tend to have terrible efficiency (50% best-case) or can only be driven in one direction.
> integral inverter vs. feature creep
Either way, I’ll need a stable DC source that can provide at least a kW or two (for the Payload), so it’s not adding much in the line of electrical requirements. Physically, my current plan is to just sketch out a face to bolt on the same model of inverter I keep in my car, so it’s not much effort there, either.
So, very low cost, from an engineering standpoint. Now, the benefits are potentially appealing to a whole different use case, or at least planting the idea of robots being a (functional) companion for working men. Maybe it’ll catch someone’s eye, maybe it’ll end up ignored, who knows?
It’s entirely possible this is just me trying to demonstrate a type of future I see for (companion) robots, but if it only costs me a couple hours of design time, well… I can indulge my daydreams that much.