My very first Critter for the Denver Mad Scientists annual critter crunch. Black dog took a lucky 3rd place in the 2001 2 lb class, he was slow and lacked great maneuverability, but driving skill and a wickedly maneuverable lifting arm made the difference. Additionally the unique steering system that allowed the bot to "wiggle" got it out of quite a few jams.

Black dog was built from scratch but is surprisingly simple to build with a few surprising twists. this was my first battle bot and as much as i wanted to keep things simple i wanted to stay away from just slapping a wedge onto an already functional RC car. Black dog has only competed once in 2001, by 2002 the bots where getting faster and more predominantly wedge shaped after a few test runs it was obvious Black dogs time had come and went. I predict that a faster version with better traction, and a sharper wedge would do well in today's battles. The only similar machine I have seen out there is TazBot and that is only due to its lifting arm, its drive system is very different.

Click to enlarge pre-combat image

This micro combat robot uses the center body steering system that removes the need for complex steering systems or expensive speed controls for tank drive. This bot uses something similar to the hydraulic steering system that large land movers use. Basically the entire bot bends on a central pivot point controlled by a servo while the wheels stay fixed, this generates the needed turning arc, this is fairly effective at all speeds as long as you have proper traction. It also comes in handy to be able to wiggle your bot out of stuck situations. One thing to note about this form of steering, the weight of the robot needs to be equally balanced between the two axels, otherwise one will grip and the other will slide generating loss of turn control.

The Speed control on this little bot is a bit unique. I had been playing with my mini sumo design and had hacked two servos just for the drive train and scooped out the electronic guts to throw away. Then i realized that those guts comprised of a small low current forward/reverse speed control. Most general servos run on 4.8 to 6 volts. My design worked around a 4 AA rechargeable (~4.8v) and my gear motor combination (which was geared to not stall in any circumstance) pulled very low current. this allowed me to use the little servo control circuit as a speed control I just connected the wires that once connected to the old servo motor to my gear box motor, and then dialed in the pot and glued it in place. The servo circuit did heat up fairly quickly but i used heat transfer epoxy and glued the circuit onto the aluminum frame of the bot (be careful not to short the circuit aluminum is very conductive) and after that it only burnt out once after being driven for about 30min straight. Even then it was $10 later for a new generic servo and it was up and running again, If any one knows of a cheaper forward/reverse tiny light weight electronic RC speed control id love to hear about it. In my testing, some brand servo circuits worked better then others, old Futaba servos and newer Hitec worked the best. if you use this speed control in competition make sure your stall current is not to high for the circuit and make sure you have a back up or two that can be quickly swiched out in case of a burn out.

This bots drive train is the simplest iv done so far, its a stock Tamiya worm gear box. This1.5v motor WILL run on 4.8v without burning out... it might even last on 6 volts if you chose to switch your rechargeable out with alkaline for a big fight. The motor that comes with the gear box is ok but I have found that specialty motors that are sold separately at most hobby shops for one cell racers work well as upgrades, I like the Atomic Tuned ones (also from Tamiya) for 4WD they have a bit more torque and speed and seem to hold up longer then the stock one. Use this gear box with its fastest or second to fastest gear set, torque is not a problem with this rig.

The worm gear drive supplies excellent torque, its just to bad that at 2 lbs there is no possible way to transfer the level of torque into tire traction. I found that while I could never stall out (which is generally a good thing) I also was just to darn slow to get out of the way. Simply put, next time i do a design like this ill go for a faster gear box like the ones I use on my other bots.

The tiers on this bot are standard model aircraft tires with a layer of sand paper glued to them. The sand paper was chosen due to the course wood fighting surface. In later experiments it was found that soft rubber works better but iv not had a reason to revise black dog in a while. There are two rear drive tiers and one free spinning front tire, in future designs where I use this steering system I will probably have two free spinning wheels up front to add steering traction.

The Original inspiration for this bots lifting arm was TazBot

The lifting arm on this bot is a simple 2 servo x,y rotation servo array. one servo is mounted directly onto the top of the body, you can see it between the two battery holders. The other servo is mounted on top of the platform the first rotates. This gives you left right reach with servo 1 and up and down reach with servo 2. Even with standard servos I found the ability to move and reach, lift and pick, side sweep and nudge the other bot in all sorts of ways useful. Sadly it was just a bit to weak to actually lift the other bots. However, transfer of weight is a beautiful thing! If I could get under the other bot or even catch a part of the other bot and then lift, the force I generated upwards was taken away from the other critters traction and added to my own. Next time I try this kind of lifting arm I will replace servo 2 with a high torque unit, the loss of a little speed will translate into being able to fully lift or flip most 2 lb critters.

One great aspect about the lifting arm and steering system combination was that this bot has NEVER been flipped in combat, or practice runs, its just to steady. The one time I did come close to being flipped I swung the arm over and braced myself, which also stopped the other bot from being able to push me any further in that direction.

Parts List

• 2+ square feet of thin aluminum or similar sheet materiel
• 15+ spacers and spacer screws
• 1 foot of square brass tube
• 2 AAx2 battery holders
• 4 standard sized servos
• 2 wheel drive axel, gear box, and motor combo
• 3 model aircraft wheels
• assorted servo linkage
• epoxy putty

Ideas for revisions

• Develop 1 lb unit for NW competitons
• 4 wheel drive
• Replace Drive train with faster gear box
• Add second wheel up front for steering traction
• Increase torque of lifting arm with high torque servo
• Center lifting arm assembly forward a bit to shorten lifting arm and increase lifting torque
• Bend body panels out of single sheet of aluminum instead of cuting individual pieces and glueing