Building the Mongoose Mechatronics Robot: Part 1 Chassis & Gearbox

This is the first of a series of illustrated instructions for assembling the Mongoose Robot kit available from blueroomelectronics

Mongoose highlights:

Powerful PIC18F2525 microcontroller (32KHz to 32MHz)
Hardware PWM controlled SN754410 H-Bridge with thermal shutdown
Differential drive with powerful 114.75:1 gearbox
Low drag steel ball caster with steel rollers
Dual optical rotation sensors (108 transitions per revolution)
Dedicated in circuit programming / debugging connector
Dedicated TTL Serial port (EUSART)
Generous 400 hole solderless prototyping area
Top mounted 20pin I/O for breadboard or LCD display
Forward facing standoffs with 10pin I/O connector (0.1" PCB compatible)
10K trimpot and high brightness LED headlight (programmable)
Very low center of gravity for stable operation
Small enough to take anywhere 115mm x 110mm x 60mm

Requires 4x AA NiMH batteries

Programmable in ASM, Swordfish BASIC SE & C18 SE

Part 1: Chassis & Gearbox
Part 2: Electrical, motors & battery holders
Part 3: Electronics, 18F2525 Controller & SN754410 H-Bridge
Part 4: Final assembly, Main driving wheels, solderless breadboard
Part 5: Programming & Testing using Swordfish BASIC SE (free) & the Junebug (PICkit2)
Part 6: Demo code LCD, PWM and Tachometer examples

Requires a PIC programmer, see the blueroomelectronics site for details.

Step 1: Preparing the Gearbox for the Dual Rotation Sensors

The Mongoose uses a Tamiya Double Gearbox. Part T4 requires a simple modification to allow the optical rotation sensors to monitor wheel rotation.

Simply remove by sanding or cutting away 5mm of material (plastic) from the main gearbox casing as shown in the illustration below.

Step 2: Adding the Rotation Encoders to Two Blue Gears.

The blue gears as shown below are IR transparent, so we're going to add IR opaque aluminum foil to a set of gears one for either side of the gearbox (the blue gears are all identical so any two will do)

The kit includes a small piece of self adhesive aluminum foil with a hole punched in the center.

You will need two of the blue gears as supplied with the kit.

Cut the foil into four pieces using the hole as the center.
Remove the adhesive backing from the foil only when you're ready to apply it
Apply two foil pieces to each gear, each foil piece should be opposite each other.
Carefully burnish the foil onto the gear with a toothpick or soft burnishing tool (the example below use a piece of spruce from the gearbox assembly)
Carefully trim away excess foul by slowly running a sharp hobby knife through the foil as show in the pictures below.

Step 3: Assembling the Axles (build 2 of Each)

You'll need to build two sets of each axle.

*more detail asap

Step 4: Chassis & Gearbox

First we'll build the Right side chassis and gearbox assembly.

Using a single silver 10mm self tapping screw attach the Aluminum Right side Chassis half to the Right Gearbox (T3) cover, do not tighten it yet (leave about a full turn loose).
Install two brass bushings into the Right Gearbox assembly (as seen in the second image)

Repeat this step for the left side chassis

Note: in the illustrations the Chassis is drawn in green for clarity (it's actually black in the kit).

Step 5: Building and Lubricating the Right Side Gearbox

This step is illustrates building the geartrain. Starting with the right side gearbox half (the brass bushings should be installed as shown in the previous step)

Lightly lubricate as you're assembling the gears

Lubricate and insert both the long and short axles as shown below into the brass bushings
add the yellow main drive gear note the notch for the set screw
next a blue gear
then a modified blue gear (from step #2)
a yellow crown gear goes on next
a silver spacer
add another set brass bushings

Finally you can attach the central gearbox frame with three silver 20mm Phillips screws

The left side of the gearbox is a mirror image of the right, simply repeat this step and you'll have two halves that bolt together into one finished gearbox.