This is the arduino code I used for the drive system. It's basically the code available here but minus the LED stuff.
I designed the drive platform to fit inside the LEGO greeblies. Since the body of the droid is pretty light, it's made from 1/4 plywood and the corners are notched out enough the have the omni wheels clear the inner face of the greeblies. I installed a C-Channel chris-crossed on top and bottom to stiffen the board
Motor:
https://www.amazon.com/gp/product/B01KTZ...UTF8&psc=1
(see my 5/12/19 post with the updated motor information)
Motor Mount Brackets:
https://www.amazon.com/gp/product/B06VT8...UTF8&psc=1
The 5 inch tall carriage bolts are for the battery hold down bracket and for mounting the body to the lower section. The 2 shorter bolts are for mounting the electronics board.
I like to put the systems electronics onto a separate board so that I can easily remove it from the droid to bench test it. The connector at the upper left connects to the DPDT switch. I also used bolts as a bus for the positive and negative power distribution. The black strip next to the 12V to 5V step-down converter in the second pictures is a mini power distribution board I made using a short segment of 0.10 inch dual row straight pin header mounted into a flat piece of plastic and each row soldered together underneath.
The wheels that are used on the original sentry droid are the standard VEX omni wheels. To attach the wheel to the 6mm D-axle of the motor, I found hubs made for the motor and the fastener pattern matches the gaps between the wheel spokes. To get the 4-40 bolts through the spokes, I had to lightly file the Vex wheel hub. You only need to remove about .030" between each of the spokes. Don't file too deep. For the opposite side, I drilled out a large pan washer with the same pattern as the mount hub. Also, I had to drill out the center of the Vex hub part of the way through with a 6mm (15/64) drill bit to allow the motor hun to fully engage the axle.
Motor Mount Hub:
https://www.amazon.com/gp/product/B00B88...UTF8&psc=1
I also drilled a shallow dimple into the flat of the D-axle of the motor to help retain the set screw and to help prevent the hubs from slipping.
Here is the electronics board mounted to the platform.
Here is the battery mounted and the switch bracket. The switch bracket is simply a bent piece of sheet metal. You can see I cut a piece of pipe to use as a spacer to be able to tighten battery mount. You can see the connector from the DPDT switch to the electronics.
DPDT Switch:
https://www.amazon.com/Heavy-Handle-Mome...QG5GHM5Z0S
Drive system installed onto the platform.
To attach the drive platform to the greeblies, I used some deck screws into the step of the greeblies. I pre-drilled through the LEGO so that the screw would go in easily.
Lower portion fully assembled. The body mounting posts have pipe spacer and a washer so that the body can be tighten to the base without deforming the body mount points.