There’s not been much discussion this year about the best style of drive for a Skyrise robot, not really surprising as we don’t have the obstacles (bump, sacks) to deal with that we have for the last couple. I’ve been thinking about building a new drive to be able to look into programming the accelerometer and gyro as there seems to be renewed interest in these sensors. The last robot I built had a small chassis (open source robot) and this time I wanted to build something with the full 18x18 dimensions. I also decided to try and incorporate some features that my students never seem to think about, these were my priorities.
Protected motors, often motors are placed directly driving a wheel and end up on the inside of the robot. They tend to get in the way of game objects and can end up being damaged.
Easy wheel installation. We always seem to be trying to get that last washer onto the axle using tweezers. I wanted to be able to easily install wheels.
Ability to quickly swap between mecanum and omni wheels.
Easy access to tighten the motor screws, easy to replace a motor.
Potential for different gear ratios without a complete redesign.
The ability to use IME’s or quad encoders on all four wheels.
Screw heads on the outside, nuts hidden on the inside.
So with these goals in mind I came up with the following design.
Left and right sides are completely symmetrical, there’s no need to mirror image during the build. Each side is based around a single 1x5x1x35 C channel that acts as a sort of mid rib. To the C channel connect standoffs to both sides, the general construction of the mid rib is like this.
You can see that wheels are installed from the outside, the 1:1 gearing (using three 36 tooth HS gears) is on the inside. A 5x10 plate covers each wheel, a 15x25 plate covers the motor section, the drive can just accommodate a mecum wheel and is 4 inches from the outside to the inside of the gear housing. This shows how an individual wheel is replaced.
Using the 36 tooth gear allows the motor screws to be accessed for tightening as there are holes in the gear that align with holes in the structure. There’s enough room to use motors with or without IMEs installed and replace them without any disassembly of the chassis.
Gears can be replaced without removing the wheels, just four screws are used to secure the 1x5x1x10 C channel on the inside of the chassis. Other gear ratios (for example, 60 tooth on the wheel giving a 3:5 ratio) need the addition of another 1/8 inch spacer to the gear housing and movement of standoffs, I nearly always run 1:1 (or 1.6:1 using internal motor gears) so not really a concern.
This shows access to the traditionally difficult to access inner motor screw.
To connect the left and right parts of the drive a 1x5x1x25 C channel was used on the top and a 5x25 plate underneath. These two were connected with several standoffs and some left over 1x5x1 C channel after cutting the sections for the gear housings. This create a box like section the ties the two halfs together, I had expected some flex in the chassis but it’s actually very stiff.
And here is the final assembly.
I didn’t have enough 36 tooth gears so used 12 tooth sprockets on the front wheels as an alternative, only minor changes to the placement of the bearing flats was needed to accommodate theses. Two of the motors are encoded, I also fitted a quad encoder on one back wheel as an example. I added a further 1x5x1x25 C channel to support the battery and cortex.
So there it is, my new test bed drive. It would have been pretty useless for the last two competitions but may make a good platform for a skyrise robot. The robot measures 17.125 (width)x17.5 inches, there’s about 9 inches clear between the gear housings so enough room for a skyrise cube (although team 8888 has other plans there), weight with cortex about 7lb but a heavier drive may be a good thing this year.
There are a few more CAD drawings here.
Please don’t quote this post as the images are linked.