Robot Army

Robot Army is a website documenting the development of a new amateur robot. TiggerBot II is an update of my very first robot, TiggerBot. While its form and functions are similar to the first robot bearing the name, its design and construction are quite different. Specifically, the primary design goal is to be cheap and easy to build from easily available and custom computer designed parts. As opposed to amateur robots that are unique and handmade, this combination of design constraints makes it feasible to consider building an "army" of such robots.

TiggerBot II is a work-in-progress. There are presently no CAM files available here (i.e. eps/dxf frame cuts or gerber circuit board layouts) because I do not want to distribute broken and/or malfunctioning designs -- as everything here so far has been to at least some extent. You may contact me if you are interested in building a broken and/or malfunctioning model, however. I plan to eventually publish full schematics and instructions under a BSD license.

The circuit board and battery photographed here are from another project.

Oblique

Front

Side

This is logic board 01, partially assembled.

Board

More board assembly and testing and 01 is already a reject. Apparently TLE5602s are too slow for 30KHz PWM. The AVR and external SRAM work fine at 16MHz, though, which was the most worrisome aspect of this board. (Especially since this is basically SMT soldering practice for me.) I wanted to squeeze a few more cells into the battery pack anyway, which would put LMD18200s back on the table to replace the TLE5602s. Two more minor issues are the LM2678 /OFF incorrectly grounded (works if you just cut it) and the SRAM starting at address 0x0000, which means the first 4k are unusable.

The photo below shows the custom circuit board mounted, as well as the pan/tilt unit and CMUCam2+. The planned battery pack is installed between the two decks.

Whole

Installed and tested maxstream radio modem telemetry. (Not pictured, but it
goes in the big blank area on the logic board.) Also did more extensive
platform mobility testing and discovered serious problems. In some
circumstances tigger2 can throw a tread while turning on high-friction
surfaces. This may be due to slop in the suspension, insufficient tread
tension, incorrect drive/idler/road wheel geometry, or some combination of
those. In any case a frame 02 will be required before serious development can
begin. No references designs were available for using this tamiya track set
with an independent suspension, and it is possible that doing so will prove
impractical.

In the next revision, the drive and idler wheels will be higher and closer
together, the suspension plungers will fit tighter in the deck cutouts, and the
treads will be under slightly more tension at zero deflection.

This one uses continuous-rotation modified rc servos instead of the maxon servos. Put simply, rc servos suck, but I am attempting to get the parts cost down and use only widely available parts. The tread geometry is modified somewhat in hopes of fixing the thrown-tread problem. The suspension is changed to eliminate the need to drill the road wheel struts edge-wise, so now a drill press is no longer critical equipment. This design also lacks a mount for the pan/tilt platform.

Cad

Parts

As manufactured from above drawings. Cut from 1/4" amber transparent acrylic instead of ABS.

Tamiya Treads	Laser-cut parts	Unwrapped	Drilling drive cog
HS-425BB	Opened	Output gear tab	Finished servos
Side	Front	Back	Top

Initial thoughts: the inner ribs weren't necessary; the bracket+spacer works pretty well. The cutouts for it can be removed and the top deck can also be cut off at the front of the servos. The decision to put 4-40 bolts through the springs was an afterthought; I had been planning on gluing a 1/8" shaft into place. It works well because the torque keeps the backet from scraping on the threads.

On modifying the HS-425BB. This is an easy one to mod because it is dual ball bearing. A servo consists of a small dc motor, a gear train, and a potentiometer for feedback. On some cheap servos the potentiometer is a mechanically critical component. On BB servos it cannot be. So, there are two things you need to do. Both require opening the servo case. First, open the back. There is a small screw securing the pot. Remove it, then give a good yank on the cables to pull the pot out. If you're trying to be super elegant you can replace the pot with two matched resistors, thus making it forever at exactly 50%. More easily, you just move the existing pot outside the case. I did that by cutting a notch under where the actual cable exits to allow room for the 3 feedback wires. Second, the output gear has a small tab on it that physically prevents 360 deg rotation. Remove it very carefully lest you slice off a finger using a hobby knife. There are probably other better ways, though, and modifying rc servos is better documented elsewhere on the internet than here.

There is still too much slop in the suspension because I didn't fix it. That was because I had this cut out by a different company and didn't want to assume the kerf would be the same and have it not fit. Since the interdeck space is only 3/4" inches on this and the bracket is about 3/8", there is only about 3/8" of suspension throw. I think this can be solved by a cleverly designed cutout that lets the bracket penetrate the top deck, but that remains to be seen.

The only tricky step remaining in construction is coring out the drive cog. I did it with a mill, which is not supposed to be required equipment. Fortunately the drilling requires no precision.

It took about an hour to assemble this vehicle.

Revision 02 has much less tread problems on hard surfaces or flat rugs. One rug, the "evil" rug, still causes problems, but I'm hoping that if I remove the rotational play in the suspension it will help somewhat.

This is frame 02 with logic board 01. Logic board 01 is much more complicated than what is planned for frame 02, but it has servo outputs and a radio modem, so it's good for remote-control testing.



Maybe it's time to invest in a camera capable of more than 15 fps.