After looking at the relevant calculations and some discussion with the others working on the robot based on what you’ve posted, we’ve made several changes to the design. We decided to use omni wheels for the front (non-drive) wheels. We’ve also decided to switch to a 30:1 ratio on the VersaPlanetaries, and use 4" wheels. As such, this is the current layout for the drive base:
Labeled Diagram
Unlabeled Diagram
Current plan is to use the following parts:
Hub
Key
Omni Wheels
Drive Wheels
I can’t seem to find a way to support the shaft of the VersaPlanetaries, though. The shaft is too short o get a wheel and a support on.
stinglikeabee:
Hopefully the diagram and links above provide enough information as to the geometry. It is slightly wide base. The drive wheels are made of thermoplastic polyurethane, 1" across, the omni wheels have Black SBR Rubber rollers with 0.75" OD rollers.
With omni wheels, the turning requires vastly less torque. I’ll run through my calculations here, since I’m not entirely sure they’re done properly (values are overestimated, to be on the safe side):
Us = Static Coefficient of Friction for Omni Wheels (sideways) = 0.27
Uf = Static Coefficient of Friction for Omni Wheels (forwards) = 1
L = Load on wheel = 5 kg
Fs = Frictional force (sideways) = L * 9.8 m/s/s * Us = 13.23 N
Ff = Frictional force (forwards) = L * 9.8 m/s/s * Uf = 49 N
Assuming a pivot point of the left drive wheel, with the right drive wheel pushing, and the two omni wheels resisting:
Dd = Distance from Right Drive to Pivot = 0.31 m
Dl = Distance from Left Omni to Pivot = 0.23 m
Dr = Distance from Right Omni to Pivot = 0.38 m
Drx = Dr Horizontal Component = 0.31 m
Dry = Dr Vertical Component = 0.23 m
Tl = Torque from Left Omni Wheel = Fs * Dl = 3.0 Nm
Tr = Torque from Left Omni Wheel = Dry * Fs + Drx * Ff = 18 Nm
Tt = Total Torque Required = Tl + Tr = 21 Nm
Fd = Drive Force Required = Tt / Dd = 59 N
R = Wheel Radius = 0.0508 m
Ta = Torque at Drive Wheel Axis = R * Fd = 3.0 Nm
At max efficiency, the motor outputs 0.1 Nm of torque, so, with the 30:1 gearbox, we should be able to turn fine.
I’m very hesitant to trust these calculations, as I wasn’t entirely sure what to do for them. If someone wants to correct bad assumptions I’d appreciate it.
The acceleration time to maximum speed is a fairly moot point due to the use case. It’s a display robot, not a competition one, so even if it takes a bit to get up to top speed there really isn’t an issue.
Nathan Streeter:
It will likely need to go over small bumps, although that is most likely the extent of it. It’s a display robot, so think of the typical school. It may need to get in and out of elevators, or over the bottom part of a door frame, but it shouldn’t need to push or pull anything.
The surface it will run on will vary. The most common will be tile, concrete, and tight carpet.
My understanding is that 40A auto reset fuses will prevent the motors from being stalled, and thus prevent that burnout.
One thing I did forget to mention previously: I have no idea why this would be relevant to the drive system, but just in case there is an effect, the robot is actually fairly large vertically. It’s about .9 m (3’) tall. However, the top is very light compared to the bottom; the center of mass is about 0.33 m (13") off the ground. If we run into problems, we can rework the battery cage to get it down to 0.29 m (11.5"). I don’t see this being an issue at any point but acceleration, so worst case, I can just limit acceleration to an acceptable level in code.
My apologies if I missed any of your questions, as there was quite a bit there. Thank you for the help, it’s already proving beneficial.
