Robot motor selection help

Aerospace / RC Robot Combat Interest Group
#1

Looking to start the r/c Tachikoma project back up, and will be needing to purchase new motors. I’m not sure what I should be getting since I don’t have first hand experience with what the torque numbers supplied on the spec sheet mean in real world terms.

There will be four motors driving a set of mecanum wheels. I want the maximum forward straight line speed to be at least equal to a fast walking pace (~4mph), so with the 60mm diameter wheels I’ll be using I’ll need roughly 500 RPM. Pololu has a 530 RPM motor with a stall torque of 120 oz-in, and a 1000 RPM motor that stalls at 68 oz-in.

My question is I don’t know how much force 68 oz-in or 120 oz-in is in real world terms. Is that a lot, or fairly puny? I’d love to be able to have the faster motors, but don’t know if I need the extra torque, or am totally off and have more power than needed.

I already have compatible motor controllers (these), r/c gear, batteries, etc.

Any suggestions/advice? Thanks!

#2

On 60mm wheels the force is about 100 oz at stall.

#3

I have a mecanum wheeled platform that I have been experimenting with for a few years. Before you get too excited about stall torque, you have to also consider the co-efficient of friction of the mecanums you have, and think out of the box. Typically, mecanums stink at pushing, if that is part of the issue. It has nothing to do with the mecanum design and everything to do with the material and design of the barrels. Mine are about .7 co-efficient. So the ones I have are more likely to spin rather than have the motors burn out. If I can get the design and material to something over 1.0, then I have a superior platform and I think I know how to do that… comes from staring at the damned thing for too long. Bottom line, with a better co-efficient I WILL have to consider stall torque but then… so will the other guy.
Anyway, it would help if you gave a real live example of what you want to accomplish as I might have an insight or two that might be useful.
As far as the 60mm or 2.36" wheels… (geez those are small… mine are 10") these are linear calculations. You have 68 oz-in which is 4.25 lb-in which means a 1 inch wheel trying to push 4.25 lbs of weigh will(is at) stall. At 60mm you divide by 2.36 (inches) so you stall pushing a 1.6lb weight. the 120 oz-in will stall at 3.17 lbs. Wow, these are really small numbers from what I’m used to. The GOOD news is that you have 4 motor so you multiply by 4 so the 68 oz-in actually stall at 6.4 lbs. while the 120 oz-in stall at 12.68 lbs. BUT!!! you then have to multiply by the co-efficient of friction… (see… I told you!) And you can only push until the wheels break loose from the surface and spin. OK I think I got this right but I’m having my Friday happy hour… (gearing also comes into play so the best way to plan a design is to set a goal… push a 5 lbs robot with wheels that have a co-efficent of whatever. AND the gearing, if geared down multiplies the torque…) Hope that doesn’t confuse things beyond belief… all I know is I could strap a chair to my platform and ride it.

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#4

It’s not pushing anything other than itself, but as yet I don’t have a weight to calculate against since I’m still building the chassis and shell. This is a “cosplay” robot, not a combat one. Challenge is that I want it pretty big (1/4-1/5 scale, so approximately 18" tall).

Looks like I should switch up to 4" wheels, which should allow me to gear down to get more torque but still keep the speed where I want it.

#5

I used this site to help me select motors: https://www.robotshop.com/community/blog/show/drive-motor-sizing-tool

#6

OK I looked up Tachikoma and noticed something. It uses Omni Wheels not mecanums. Looks like David Steel gave you a robotshop link for drive motor selection. You may still need to estimate the mass. In your case, it doesn’t hurt to over estimate the mass. I wouldn’t think it would be more than 20 lbs max as a guesstimation (depending on your materials. Since it’s just for looks you can use plastic, fiberglas or even paper mache since there is no big structural requirements. Cheap and dirty is good for cosplay. Looking at a video of a working model, it will be difficult to have a small enough direct drive to look right and I think that is your biggest challenge. You might also visit Andy Marks Robotics and Vex Robotics. You can find a lot of interesting parts at both.

#7

There’s a ton of variation in the depiction of the wheels on Tachikomas, so I’m going with what I like visually. (Plus I already have a dedicated controller board designed specifically for mecanum wheels…)

Thanks, that link helped quite a bit!

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