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Well, after printing the remaining major sections of my R2 dome, I got very distracted by assembling, sanding, and priming them. Then it got too cold to do outdoor work, and I got busy with the holidays. I've poked at things here and there on my active projects in the meantime, but only recently started making real progress again.
Lately, I've been thinking about LD-F1's drive system.
Since I intend to use the 380kv iFlyRC outrunner motors I picked up last year, I need to work out some sort of drivetrain for them. Using a 12V power system (well, 11.1V, the nominal voltage of a 3S LIPO), 380 kv motors can produce up to 11.1 * 380 = 4218 RPM. That's 4218 * 60 = 253080 revolutions per hour.
For the moment, I'm going to ignore the droid's treads and pretend it runs directly on the wheels. Adding the treads will slightly increase it's speed, but the difference is small enough that I'm happy to ignore it for simplicity.
The linear distance that these motors would move the droid per revolution is equal to the wheel's circumference, 4 * pi = ~12.57 inches That makes it 253080 revs/hour * 12.57 inches/rev = 3181215.6 inches per hour.
1 mile is 63360 inches, so if we assumed the motors had enough torque to move it at all with no gearing, this would move the droid at a top speed around 3181215.6 / 63360, or roughly 50 mph.
Briefly...
I want the droid to be limited to something far more practical, like a fast walking pace (2-4 mph). This suggests a gear reduction between 25:1 and 12.5:1 (50/2 or 50/4).
As we've discussed elsewhere, it's hard to backdrive much above 16:1. This is probably less of an issue for chain drives and belt drives than it is for gears, since you can just disconnect the chain or belt, but that's a pain so it'd be best to stay below 16:1 nevertheless.
Commercial gearboxes are expensive as a rule, and if they sell any for these 3508 outrunners that don't need a lot of faffing about with shaft replacements and such, I haven't seen them. So I was leaning toward a DIY transmission from the start. I wouldn't trust a printed gearbox with a droid of this scale
(Edit: On the other hand...)
(End Edit.)
, and I don't want to fool with chains, so that leaves me with a timing belt drive (HTD-5mm feels about right as the profile).
I eventually decided that, due to the space constraints, I would at least need a two-stage belt drive. Otherwise it'd require the pulley on the driven wheel to have a very large diameter to accommodate the ridiculous number of teeth necessary to get a useful ratio. (To get within my desired range with one belt, I'd need at least 12.5 times as many teeth on the driven pulley as are on the motor pulley, and there's a practical minimum on the latter's tooth count).
I could probably get away with using a smaller gear reduction if I had to. Maxstang's Maxdrive (belt version) features a 12 tooth to 42 tooth belt -- only 3.5:1 ratio -- in order to roll his 150 lb R2 unit around. (However, I note that he is also using Neo motors, which are beefier than these little outrunners and have nearly 100 kv higher rating. The low ratio means he's also limiting the droid to a small fraction of the what the motors are capable of.)
I'd be surprised if LD-F1 ends up weighing even a fifth of what that R2 weighs, so my power requirements are a lot lower. TPU tracks also aren't going to be nearly as grippy as proper rubbery materials like Treadwell's drive uses, so there'll be a bit less friction during turning. Unfortunately this still leaves me guessing which gear ratio would be best. While I potentially could go as low as 3.5:1 or even less, I probably wouldn't go below, say, 10:1. The extra speed isn't of much use.
![[Image: L9azsaXl.png]](http://i.imgur.com/L9azsaXl.png)
My initial design had a 10 tooth pulley on the motor (green), connected to the larger 30T input of the yellow pulley. The smaller 10T output of the yellow pulley was connected to a 50T pulley on the wheel (red). That's a 30:10 reduction for stage 1 and 50:10 for stage 2, or in other words, 3:1 and 5:1.
Multiply 3 * 5, and you get a 15:1 total reduction, which fits within my 12.5:1 to 25:1 range, and leaves me room to move it further below 16:1 by altering the design.
Going down to 42 teeth on the red pulley would lower the ratio to nearly the bottom of my range (30/10 x 42/10 = 3 * 4.2 = 12.6:1). And that means I can use the same 42T Vex versapulley that the maxdrive uses, if I want.
I can fiddle with the pulleys to get down to 10:1 if needed. Like 10-50,21-42 (50/10 * 42/21 = 5 * 2 = 10:1)
Modeling the pulleys in fusion for printing isn't too hard. I kind of have to do that for the green pulley because the mounting needs are specific to the motor, and for the yellow pulley because it's a custom compound part.
Anyway, I'll continue to noodle at it (especially the neck mechanism, which is the other major holdup).
Edit: There's a fair amount of space in the "bays" forward and aft of each middle roller, and I will probably have to make good use of it. At a minimum, the drive system will probably be tucked up under the tracks, as shown below. Some of the remaining space will have to be occupied by supports that connect the outboard panels (the ones that cover the ends of the rollers) to the rest of the droid
The central chassis of the droid contains decorative elements as well as the neck lift mechanism and whatever portions of the drive system extend beyond the confines of the tread bays. I also have to fit the microcontroller, ESCs, amp, speaker, battery, fuses, and switches somewhere.
![[Image: y63VCMLl.jpg]](http://i.imgur.com/y63VCMLl.jpg)
Lately, I've been thinking about LD-F1's drive system.
Since I intend to use the 380kv iFlyRC outrunner motors I picked up last year, I need to work out some sort of drivetrain for them. Using a 12V power system (well, 11.1V, the nominal voltage of a 3S LIPO), 380 kv motors can produce up to 11.1 * 380 = 4218 RPM. That's 4218 * 60 = 253080 revolutions per hour.
For the moment, I'm going to ignore the droid's treads and pretend it runs directly on the wheels. Adding the treads will slightly increase it's speed, but the difference is small enough that I'm happy to ignore it for simplicity.
The linear distance that these motors would move the droid per revolution is equal to the wheel's circumference, 4 * pi = ~12.57 inches That makes it 253080 revs/hour * 12.57 inches/rev = 3181215.6 inches per hour.
1 mile is 63360 inches, so if we assumed the motors had enough torque to move it at all with no gearing, this would move the droid at a top speed around 3181215.6 / 63360, or roughly 50 mph.
Briefly...
I want the droid to be limited to something far more practical, like a fast walking pace (2-4 mph). This suggests a gear reduction between 25:1 and 12.5:1 (50/2 or 50/4).
As we've discussed elsewhere, it's hard to backdrive much above 16:1. This is probably less of an issue for chain drives and belt drives than it is for gears, since you can just disconnect the chain or belt, but that's a pain so it'd be best to stay below 16:1 nevertheless.
Commercial gearboxes are expensive as a rule, and if they sell any for these 3508 outrunners that don't need a lot of faffing about with shaft replacements and such, I haven't seen them. So I was leaning toward a DIY transmission from the start. I wouldn't trust a printed gearbox with a droid of this scale
(Edit: On the other hand...)
(End Edit.)
, and I don't want to fool with chains, so that leaves me with a timing belt drive (HTD-5mm feels about right as the profile).
I eventually decided that, due to the space constraints, I would at least need a two-stage belt drive. Otherwise it'd require the pulley on the driven wheel to have a very large diameter to accommodate the ridiculous number of teeth necessary to get a useful ratio. (To get within my desired range with one belt, I'd need at least 12.5 times as many teeth on the driven pulley as are on the motor pulley, and there's a practical minimum on the latter's tooth count).
I could probably get away with using a smaller gear reduction if I had to. Maxstang's Maxdrive (belt version) features a 12 tooth to 42 tooth belt -- only 3.5:1 ratio -- in order to roll his 150 lb R2 unit around. (However, I note that he is also using Neo motors, which are beefier than these little outrunners and have nearly 100 kv higher rating. The low ratio means he's also limiting the droid to a small fraction of the what the motors are capable of.)
I'd be surprised if LD-F1 ends up weighing even a fifth of what that R2 weighs, so my power requirements are a lot lower. TPU tracks also aren't going to be nearly as grippy as proper rubbery materials like Treadwell's drive uses, so there'll be a bit less friction during turning. Unfortunately this still leaves me guessing which gear ratio would be best. While I potentially could go as low as 3.5:1 or even less, I probably wouldn't go below, say, 10:1. The extra speed isn't of much use.
My initial design had a 10 tooth pulley on the motor (green), connected to the larger 30T input of the yellow pulley. The smaller 10T output of the yellow pulley was connected to a 50T pulley on the wheel (red). That's a 30:10 reduction for stage 1 and 50:10 for stage 2, or in other words, 3:1 and 5:1.
Multiply 3 * 5, and you get a 15:1 total reduction, which fits within my 12.5:1 to 25:1 range, and leaves me room to move it further below 16:1 by altering the design.
Going down to 42 teeth on the red pulley would lower the ratio to nearly the bottom of my range (30/10 x 42/10 = 3 * 4.2 = 12.6:1). And that means I can use the same 42T Vex versapulley that the maxdrive uses, if I want.
I can fiddle with the pulleys to get down to 10:1 if needed. Like 10-50,21-42 (50/10 * 42/21 = 5 * 2 = 10:1)
Modeling the pulleys in fusion for printing isn't too hard. I kind of have to do that for the green pulley because the mounting needs are specific to the motor, and for the yellow pulley because it's a custom compound part.
Anyway, I'll continue to noodle at it (especially the neck mechanism, which is the other major holdup).
Edit: There's a fair amount of space in the "bays" forward and aft of each middle roller, and I will probably have to make good use of it. At a minimum, the drive system will probably be tucked up under the tracks, as shown below. Some of the remaining space will have to be occupied by supports that connect the outboard panels (the ones that cover the ends of the rollers) to the rest of the droid
The central chassis of the droid contains decorative elements as well as the neck lift mechanism and whatever portions of the drive system extend beyond the confines of the tread bays. I also have to fit the microcontroller, ESCs, amp, speaker, battery, fuses, and switches somewhere.