TiggerBot II Robot

TiggerBot II is a small treaded robot platform. Included are instructions for building the plastic treaded platform and a custom printed circuit board containing a microcontroller and sonar sensors.

This is a relatively complicated project that is still at the late-prototype stages. Every attempt has been made to keep it simple to build but, well, robots are hard. Furthermore, this project will set you back somewhere in the $150-$250 range, depending on where you buy the parts. Continue at your own risk.

Specifications:
> chassis material: cnc laser cut acrylic
> drive motors: 2x continuous-rotation rc servo
> battery: 2.2Ah 9.6v rechargeable NiMH
> nav sensors: 5 way ultrasonic sonar
> cpu: AVR Mega32, 16MHz
> programming: RS-232 serial port bootloader
> code: written in c, compiled with gcc-avr
> expansion port: 5v/1A, gnd, 2x adc, i2c

for latest news see http://robotarmy.org/

This was my first robot, built in 2002 when I was a freshman in college. I named it TiggerBot because it was black, orange, and stupid. It was flawed in several important ways. TiggerBot II is a substantial redesign; it uses the same treads kit but is superior in all other ways. Pictured below are the original TiggerBot, several obsolete TIggerBot II prototypes, and the current prototype.

TiggerBot II's main components are all computer designed and custom manufactured.

The plastic components are designed in qcad. They are then separated, duplicated, packed together for efficiency, and printed as a 1:1 eps. This is sent to a plastic manufacturer to be cut from acrylic.

The circuit board is designed in eagle cad and manufactured by a pcb prototype supplier.

I have the circuit boards made by Gold Phoenix PCB in China and the acrylic cut by Canal Plastics in Chinatown NYC. Coincidental, really. The turnaround times are ~9 days and ~3 hours, respectively, which is probably why I've made a lot more frame revisions.

The boards cost $140 for 13, or ~$11 each. The frames are $59 at canal, or apparently $78 for 3, or $26 each, from ponoko, though I've never ordered from them. In any case Ponoko doesn't seem to have tinted transparent acrylic in 6mm.

This is the eps of the plastic: http://robotarmy.org/tb2files/frame06.eps

chassis: 1 plastic set
motors: 2 HS-425BB
treads: Tamiya 70100 kit.
battery: 8 cell AA battery pack

fasteners (mcmaster carr):

standoffs: 4 (3/4" 6-32 standoff), 8 (6-32 x 3/8" screw)
shafts: 8 (4-40 x 1 1/8" screw), 16 (4-40 nut), 8 (spacer)
suspension: 6 (4-40 x 1 1/2" screw), 6 (4-40 nut), 6 (nylon flange spacer), 6 (angle bracket), 6 (springs)
servos: 4 (4-40 x 1/2" screw), 4 (4-40 nut)
drive cogs: 4 (4-40 x 1/2" screw), 8 (4-40 nut)
pcb mount: 5 (3/4" 6-32 standoff), 10 (6-32 x 3/8" screw)

Here is a more complete parts list: http://spreadsheets.google.com/pub?key=pX4iSt4d26qLYANVB9Dn7Nw

These are the tools you need for the mechanical parts. The vice-grips are for holding things so you can use a vice instead. You will need more tools for the electronics part.

The first step is to prepare the servos. An RC servo consists of a small DC motor and geartrain, a potentiometer for position feedback, and electronics to close the control loop. Modifying them to turn continuously requires two things be done: first, that the physical constraints preventing continuous rotation be removed; second, that the feedback position be secured in the center position.

Using a Phillips-head screwdriver, remove the four screws holding the case together.

Inside you will see the back of a potentiometer held in place with a screw. Remove the screw. Remove the potentiometer with a firm yank.

Now, before putting things back together, turn your attention to the other side of the servo. Remove the top so you see the gears. Remove the output wheel by unscrewing the black Phillips head screw in the middle and pulling on it. Doing so makes it possible to pull out the output gear. Note the small tab on the side of the gear. Grasp the gear with vice-grips (gently so as not to damage teeth!) and cut the tab off with a hobby knife. You will want to use a rocking motion with the base of the blade. You will need all your fingers for the later steps so be sure not to cut any of them off accidentally.

Using a hobby knife, cut a notch under where the cables originally left the package. This is to allow the potentiometer cables to leave the case.

Put everything back in and screw it all together. As you're putting the circuit board back in be sure not to pinch wires between the board and the case.

The screw used to hold the potentiometer in. The little plastic piece connected the potentiometer armature to the output gear; it may have fallen out but doesn't really matter either way.

Repeat the last several steps with the other servo. It should look like this when you're done.

Now it's time to break open your Tamiya tread kit. You will need all the tread sections -- cut them out with either a hobby knife or some small diagonal cutters. Of the orange plastic, you will need the two large drive cogs, the two large idler wheels, and the six large road wheels. Assemble the tread pieces into two large loops, taking care that they come out the same length.

The holes in the sides of the drive cogs match up to the holes in the servo wheel. Unfortunately the cogs are designed for a hexagonal shaft and the shaft hub will get in the way. We have ways of dealing with such things. The center of each cog must be drilled out. The easiest way to do this is with a few progressively larger drills up to 5/16. Note that in the last photo with the larger drill bits I am actually holding the plastic *down* with the pliers.

Using a 7/64 drill, enlarge the two specific holes in each servo wheel, as shown.

Remove the servo wheels. Place two 4-40 x 1/2" screws, from the back side, through the enlarged holes. Secure two 4-40 nuts to the front. Insert two protruding screws through two holes in the drive cog and secure it with two more 4-40 nuts. Reattach the servo wheel. Repeat for the other servo.

This is what the plastic parts arrive looking like if you get them from canal plastics in nyc. The little bits are what you get instead of swarf when you drill holes with a laser. You'll need to peel all the paper off. Before the peeling, if you're a narcissist, you might want to go wash your hands with soap so your robot won't have greasy fingerprints all over it when you're done.

Build six of the following assemblies. From right to left, 4-40 x 1 1/8" machine screw, road wheel, spacer, 4-40 nut, suspension strut, 4-40 nut. Tighten the nuts such that the wheel turns freely but slides as little as possible. Assemble the front brackets with the larger wheels using the same combination of fasteners.

Insert each servo into its bracket. This is done most easily by pulling the wires through first, inserting the top edge with the wires, pulling that as close to the bracket as possible, and forcing the bottom edge through. Secure with two 4-40 x 1/2" screws and two 4-40 nuts in opposite corners. There is room for four screws but two is sufficient. Be sure to put the servo output wheel on the end of the bracket near the protrusion and to build one left and one right side.

Attach four 3/4" 6-32 aluminum standoffs to the lower deck (the smaller one) using four 6-32 x 3/8" screws. Place the two servos in brackets and front wheel assemblies in the cutouts as shown. Place the top deck on and make sure all the tabs are properly inserted into the cutouts. Secure the top deck to the standoffs using four more 6-32 x 3/8" screws.

The color is different because this is a later prototype than the one in the earlier photos.

In each of the six holes along the sides of the decks, install the suspension bolt, bracket, collar, and spring. Begin by inserting a 4-40 x 1 1/2" bolt upwards through the lower deck. Place the non-tapped side of an angle bracket over the screw with the other end pointing upward. Place a plastic flange collar over the screw. Place a spring over the collar. Carefully, press the spring under the top deck and align it with the top hole. Push the bolt through the hole and secure it with a 4-40 nut. Insert a suspension strut upwards with the wheel facing outward. Align the hole in the strut with the tapped hole in the angle bracket. Secure with a 6-32 x 5/16 screw.

Stretch treads over wheels.

You have now completed the drive platform.
Next is instructions for building the circuit board pictured below.
Alternatively you may use the base with your own electronics.

The circuit board pictured here is the last revision and has several mistakes. A new revision, which should fix most of the mistakes and greatly improve sonar performance is currently being manufactured. If you're considering building one of these I would strongly recommend that you wait until I have a chance to test the new version (pictured in cad-form below) and use that instead. They look very similar, however.

The circuit board here is designed with an avr microcontroller, power management, and a five channel sonar. It has everything required for doing simple stuff like wall following and obstacle avoidance. It is designed entirely with through-hole components so it is not especially hard to solder.

There are already a sufficient number of soldering howto guides on the internet so covering that here would be redundant. Figure 2 shows a closeup of several soldering styles you can choose from depending on whether you are building the 'robot' or 'paperweight' version. The components (see parts list) go where marked. It's not rocket science.

If you like, you can solder everything in one go. Otherwise, you can build the power supply first and verify that you get 5v out, then build the avr & serial port and make sure you can program it, then build the sonar.

You are now in possession of one of the hottest homemade robots around. No ugly loose wires hanging out here. Go ahead and put this in your carry-on bag. The TSA won't shoot you for carrying this, they'll beg to know where you got it. And now, a video of my TiggerBot IIs driving around the corner of my kitchen:



The End.