microMVP: A Portable, 3D-Printing Enabled Multi-Vehicle Platform for Robotics Research and Education

Introduction | Parts and assembly | Software stack | Download manuscript | V3 Page

Components
In designing microMVP, emphasis was put on making the system readily reproducible by robotics researcher, educator, and hobbyists alike. We did extensive study to ensure that the parts we select are reasonably easy to obtain, easy to use, and reasonably affordable. This is of course an evolving process and will continue improving the availability and affordability of the components of microMVP with each iteration. Below is a list of all parts used in the microMVP vehicle version 1, minus the wires for connecting the components. Note that reference prices are pulled in September 2016 and may have since changed. The price assumes volume purchase for building 10+ vehicles. The second table lists the few parts for the tracking platform.

Vehicle parts

Part name with link Description Cost (USD)
Vehicle shell The link provides an STL model for printing the shell of the vehicle. Depending on the printer used, the material cost varies. For example, using standard PETG material at $25/KG, the shell which uses 32grams of material, costs 0.8$. 0.8+
Gearmotor (x 2) Pololu 120:1 mini plastic gearmotor with 90-degree, 3mm D-Shaft output. 4.95 x 2
Motor driver carrier DRV8835 dual motor driver carrier board. 3.89
Fio v3 board Sparkfun Fio v3 board with Arduino support, built-in xBee interface, and battery charging support. 33.2
xBee series 1 xBEE (zigBee) board with trace antenna. 24.95
32x7mm wheels Pololu wheels that press-fit onto 3mm D output. 3.95
Caster ball Metal ball serving as a caster wheel for the vehicle. 2.95
Battery
Alternatives:
350mAh Adafruit
500mAh Adafruit
400mAh polymer Li-ion battery. 6.95
Total   86.59

Tracking platform parts

Part name with link Description Cost (USD)
USB video camera Logitech C920 1080P webcam. Other cameras are also fine as long as the camera support the adjustment of parameters such as focus and white balance through driver interface or openCV. The price of Logitech C920 fluctuates but can be found at 60-65 USD. 60+
Microphone stand Basic microphone stand for mounting the USB video camera. 17.99
USB extension cable Extension cable for connecting USB video camera to computer (optional). 5.79

For communicating with the xBees, extra xBees and xBee explorers are needed. We use one xBee explorer and xBee to communicate with all vehicles. A system with six vehicles has a total part cost of 650 USD.

Putting together the vehicle

Printing and readying the shell

The shell (.stl) of the v1 vehicle are two (mirror) pieces that can be printed on any working 3D printer. We tested printing on a 50K USD stratasys printer as well as a 180 USD monoprice mini 3D printer. Once the two pieces of shells are printed, they can be super glued together along the bottom sides (as they appear on the 3D printer) to yield the car shell. Below are pictures of the printed shell pieces still on the monoprice printer, the shell pieces after some quick cleaning of support materials, and the super-glued shell (we used gorilla super glue gel for this task). The caster ball should be glued on to the bottom of the assembly as well (using only the ball assembly and the thicker 1/8" spacer).

       

Soldering together the components

The micro-vehicle requires the soldering of a few wires between the SparkFun fio v3 board, the pololu DRV8835 dual motor driver carrier, and the motors. The schematics we use for connecting the wires are given in the picture below. Because the fio v3 board offers more output pins than we need, the schematics for doing the soldering is not unique.



Since we do not expect to reuse the components, we choose to simply solder them together with wires. One can easily adapt a more modular assembly method if that is more desirable. After soldering the terminals of the motors, we used glue gun to secure the terminals as they are somewhat weak and may break (see picture below). The fully soldered vehicle components and the shell (now with the caster ball glued on and wheels snapped on to the motors) are shown in the picture below. Note that this particular one used a slightly different (older) wiring schematic between the fio v3 and the DRV8835. The xBee module is mounted on the back of the fio v3.

    

From here, it is straightforward to snap together the aseembly to build the complete vehicle. A word of caution: it is known that the fio v3 board has a current leak even when turned off. Therefore, the battery should be disconnected after each use of the vehicle. The pictures below provide the left, right, top, and back views of the fully assembled vehicle (save an image or zoom in for a larger version).

         

Camera platform

The webcam can be mounted on the microphone stand (or other similar fixture) with many methods. We recommend using normal tape or masking tape, which are sticky enough and relatively easy to remove without leaving glue marks. There isn't much to this part other than that the camera should be placed at an appropriate height (1-1.2m), facing down, and connected to a computer. In one of our setup (to the right), we attached two cameras and use one for tracking the vehicles and one for taking videos.

A full setup with the camera platform and six micro-vehicles are pictured to the right. Ideally, the environment should have reasonably even lighting and the floor should be relatively uniform. Some glare is perfectly fine, however, as was the case with our setup.
 
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