The Tunabomber

 The Tunabomber was my entry in Dragon*Con's 1997 robot battle. Check out my robot battle page to see the Tunabomber in action!
Here's me and the Tunabomber after the battle.
Do you know who the guy in the white shirt with a video camera is? If so, please contact me, I want to find video of the robot battle.
Photo courtesy of Nora C. Hogan.

Building the Tunabomber

It took several months, off and on, to complete the Tunabomber. I had to do a lot of research and experimentation before I was able to complete my robot. Hopefully, the information I'm giving here will allow you to leverage some of that experience in building a robot of your own!

Power source

It took many weeks of searching to find the exact power source I needed. Since the rules impose few restrictions on the construction of a robot other than weight, finding a power source with a high power/weight ratio is critical. Various sources of power were considered: compressed gas, wind-up springs, solar power, water-filled pistons heated with an external laser, gravity power (like the weights on a grandfather clock), and so on. The robot battle rules don't allow internal combustion, so this highly efficient source of power was ruled out right away. Good old-fashioned electric batteries seemed to be the best bet. These were used by many of the Robot Wars competitors, as well as most of the robot battle competitors from previous years as well.
I didn't realize at the start how difficult it would be to get the right battery. I tried a couple different types of batteries and did a lot of research before finalizing the design around a pair of DeWalt 9.6V NiCd rechargeable drill batteries.
Batteries are rated in mAh (milliamp hours) or Ah (amp hours). This rating is the total amount of energy held by the battery. For instance, a typical NiCd D-cell is rated at 800 mAh. This means that it can deliver roughly 800 milliamps (0.8 amps) of current for 1 hour or 400 milliamps (0.4 amps) for 2 hours. In reality, this is a bit misleading, as you have to deal with internal resistance (see below) and the fact that batteries get less efficient as more current is drawn from them. Thus, a Radio Shack "heavy-duty" D-cell rated at 4 Ah could probably provide 1 amp for 4 hours, 3.4 amps for 1 hour, or 0.5 amps for 15 minutes. A general rule of thumb is that the heavier the battery, the higher its Ah value.
Alkaline batteries simply can't put out the same amount of power as NiCd batteries. They have what's called "high internal resistance," meaning that under loads that demand a lot of power (such as running two drill motors at a "stalled," or stopped state), they simply can't produce enough current. I found this out the hard way, through trial and error. Alkaline batteries are better for applications which require low current for long periods of time, such as smoke detectors, and long idle periods between use, like in flashlights. Despite this, they have some of the highest Ah ratings of all battery technologies.
Lead-acid batteries provide similar power/weight ratios to NiCd batteries. In addition, they have low internal resistance. I was warned off of lead-acid, however, when I heard that their output starts to diminish after heavy current draw. In other words, lead-acid batteries fade away, while NiCd's conch out immediately when they are discharged. Since I wanted my batteries to be running at maximum capability for the entire battle, I decided against lead-acid. Lead-acid might be more appropriate for larger robots, since it is less expensive than NiCd.
Lithium batteries provide the among highest power/weight ratios of any battery technology. The downside is that they have high internal resistance and are much more expensive than any of the other battery types. If I find some at surplus prices, I might experiment with these for next year.
NiCd batteries represent what is probably the best compromise between power, weight, and cost. I first tried the standard NiCd VersaPak batteries recommended for the drills used in my drive system (see below). Two 3.6V batteries, run in parallel, weren't enough to drive 4 drills (my original design). I tried 4 in series-parallel, but to no avail.
Next, I attempted to use standard Sanyo D-cell batteries, in series, but again, I wasn't getting enough power to drive all four motors.
After this, I got some of Radio Shack's "Heavy-Duty" D-cell NiCd batteries, but again, this wasn't enough, even when linked in parallel to the VersaPak batteries. These can, at least, put out a dangerous amount of current, as I found out when I plugged in some of these batteries backward. They melted the metal spring in the battery holder!
Finally, I bought some DeWalt 9.6V drill batteries at Home depot. Two of these puppies could make my robot fly! I was overdriving the drills from their standard 7.2V, but this never became a problem. I eventually had to scale my robot back to two motors, as these batteries could put out more than 30 amps each, enough to fry my speed controller, or at least a fuse. Only recently, re-reading my speed controller's manual, did I realize that it has a current limiter which could have solved this problem. Aargh.
I got a spare battery so that I could swap one battery out between each round. While the battery was sitting idle, I left it on the charger. This meant that I could reduce my weight by not having to carry enough batteries to last through the entire afternoon of battling.
For those on a budget, NiCd batteries are often available at cheap prices from electronics surplus places. If I knew then what I know now...
Items:
Item Cost Supplier
3 batteries with 2 chargers $90 Home Depot
 

Electronics

With batteries, I thought I knew what I was doing when I started. With the electronics, I realized that I had no clue.
I wanted my robot to be radio controlled. Radio control would allow my robot a freedom of movement that a tethered control system couldn't match. While I understand many of the principles of radios, amplifiers, and the like from high-school electronics, I realized that it was probably beyond my ability to create a radio control system from scratch.
A little research told me that there are 3 types of transmitters used for radio controlled cars and airplanes: AM, FM and PCM. AM is the cheapest, but is prone to interference. This type is disallowed at Robot Wars, because of its low reliability, but legal for the robot battle. FM is slightly cheaper and less reliable than PCM.
At a radio control modellers' swap meet, I bought a Futaba PCM transmitter and receiver pair for $10. I wasn't able to verify that the transmitter and receiver pair worked, so I purchased an Acom FM radio for $35. The guy who sold me the FM radio could actually demonstrate that it was operational. Normally, you'll pay about $200 for a new PCM transmitter and receiver. I went to Doug's hobbies on Roswell Road in Marietta, GA to have them look at the PCM radio and verify that it was functional. It wasn't, but they were very helpful in getting it to work. It needed new wires from the battery container to the switch, as well as a battery pack to replace the corroded battery terminals. You can find out about these swap meets by asking at your local R/C hobby stores. In order to not look like an idiot, just ask around at the swap meet for a "radio." Folks at swap meets will often include servo motors, airplane parts, or even entire R/C models in their asking price. Don't be afraid to negotiate. Try to verify that you have a working piece of equipment before you buy, unless you get a great deal like I did.
Anyway, the next piece you need is called a speed controller. This device takes the low-power output from your receiver and turns it into a high-power input you need for your motors. The diagram below shows the whole setup. I wanted my robot to steer like a tank, so I could turn by making one side go forward while the other side went backward. I'm still on the great quest for the ultimate speed controller. This speed controller would have the following characteristics:
  1. High voltage (12V+) and current (60+A per side)
  2. Reversing: the ability to run in forward or reverse
  3. High reliability
  4. Low cost
Most speed controllers fail on all but the last point. There is at least one speed controller, from Ace Hobbies, that I haven't looked into yet; it may meet my needs. Anyway, I settled on the Vantec RDFR-22 speed controller after seeing the recommendation  from Tony Buchignani, a robot warrior. The RDFR-22 is pretty good and can control both the left and the right side at the same time. Most speed controllers will only operate one motor, meaning that you have to buy two if you want to use tank-style steering. It also has a feature which lets you control the left and right wheels using a single joystick. It's expensive, though, the most expensive part in the Tunabomber. The Vantec speed controller comes with a very comprehensible set of instructions. I installed fuses as it specified. Unfortunately, as I attempted to drive four motors, I found that my fuses were blowing. Four motors were more than the speed controller could handle, so I switched to a two motor design.
I noticed that as I was delivering all this current to my motors that the 18 gauge speaker wire I was using for my power connections was starting to get hot. This worried me, so I purchased some 14 gauge lamp cord. This wire seemed to be able to handle the high currents better.
I set up a cutoff switch for those circumstances when things went horribly wrong. Make sure that the switch you use can handle high currents. Knowing how to solder can be useful in these sorts of situations. There's a soldering tutorial I found that might be useful. The FAQ for R/C electric off-road racing also has information on soldering, as well as tons of other good information about radio controlled modelling in general and is recommended reading. Next year I'm going to get an extra set of crystals or an extra radio in case I have to face off against someone on the same frequency.
Advice: color-code all of your electrical connections so that you don't accidentally hook something up backwards. I used a bunch of colored electrical tape. Also label your switch so that you know which position is "off."
 Those on a budget could elect to replace the electronic speed controller (the most expensive part of the Tunabomber) with a mechanical speed controller. These systems use servo motors to control the position of a variable resistor, which determines the motor's speed. These systems aren't as reliable or as easy to use as an electronic speed controller; they also don't respond as quickly to your signals.
Another alternative is to use a tethered control which uses variable resistors to control speed. This is simple, reliable, and responsive, but you lose the flexibility of radio control.
Other control possibilities include ultrasound and audible sound (the clapper), and infrared. Or you could build a robot which can control itself via a simple sensor system or complex control logic.
Items:
Item Cost Supplier
Lamp Wire $5 Home Depot
Futaba Transmitter and Receiver $10 Marietta R/C Modeler's Club Swap Meet
8 Battery Clip for Transmitter $5 Doug's Hobbies
4 Battery Clip for Receiver $2 Radio Shack
Rechargeable Batteries for the Transmitter and Receiver (with charger) $35 Target
Vantec RDFR-22 Speed Controller $240 Vantec
Blade Fuses, Holders and Wire $5 Target
Colored Electrical Tape $4 Home Depot
Terminal Connectors $5 Home Depot
Power Switch $2 Home Depot
Project Box for Power Switch $1 Radio Shack
Female Futaba Connectors $4 Doug's Hobbies
 

Drive system

As mentioned earlier, my robot was driven by two drills. These were the cordless Black and Decker VersaPak drills. I chose them because they were relatively inexpensive; the ones with the keyed chuck cost about $20 each. Drills are simply a good way to get three things in one pre-built package:
  1. A powerful electric motor
  2. A system of gears to reduce the motor speed while increasing the torque
  3. An easy mechanism for attaching and removing the axle: the chuck
There are several types of common electric motor which are suitable for this application. Most electric motors use ferrous (iron-based) magnets. These aren't nearly as powerful as rare-earth magnet motors. I believe that most cordless drills use rare-earth magnet motors. Finally, brushless motors are expensive, but are much more efficient than the other two types. Brushless motors also require a special electronic controller.
The gears in the drill are made of metal. Metal gears are less prone to stripping and breaking under high-stress conditions than plastic gears. After reading the web page of Team Delta, I decided to clean all the gears with rubbing alcohol to remove the heavy oils that came with the drills to reduce the friction created by the heavy grease. I then replaced the oil with common light household oil (like 3-in-1 oil).
I stripped the excess parts out of the drill, including the trigger and a chunk of the handle. I then removed the wimpy wires that came with the drill and soldered on the new heavy-duty wire mentioned in the electronics section. Vantec recommends that you solder some small capacitors across the motor in order to reduce RF noise which could interfere with your radio.
I think that I would probably have been better off obtaining my motors and gears separately. This would allow me to buy a higher quality motor, one which I knew the specifications for. I also wouldn't be limited to the form factor provided by the drill.
Items:
Item Cost Supplier
2 Black and Decker rechargeable drills @$20: $40 Target
Household Oil $3 Home Depot

 Wheels

Getting the wheels just right was a pain and I think that this, more than anything else, lead to my eventual defeat at the robot battle. I had, at first, planned to use lawnmower wheels in my robot. These were easily available and seemed to be the appropriate size. I realized that the larger the wheel's diameter, the more surface area would be in contact with the ground at any time. This meant that my robot would be harder to push around and would be able to shove the other robots while not slipping.
Lawnmower wheels turned out not to be such a good choice. They use a very thick, hard rubber on the outside for durability. This means that lawnmower wheels are very slick and won't grip a flat surface very well. In the end I removed the outer rubber coating from the hubs.
I wanted the material covering the hubs to be flexible, so that the part of the wheel touching the ground would flatten slightly, providing greater traction. I also wanted it to be durable; I didn't want a robot with a sharp weapon to be able to deflate my tire.
In the end, I decided on a foam-core tire with carpet coated with rubber cement on the outer surface. I made a strong foam core by using white packing foam, the white kind that comes in big rolls about 18" across and maybe 1/16" thick. This I layered with duct tape to make it stronger. I rolled this stuff into tubes the same length as the circumference of the hubs. I put some silicone glue on each of the hubs, wrapped my tube in place, and secured it with duct tape. Silicone glue isn't that great with duct tape, it causes the surface to be a little too slippery for the tape. Anyway, I placed two hubs side by side so that I could have some extra-wide tires. I cut pieces of shallow-grained carpet to be the width of two hubs and just long enough to wrap all the way around the layer of foam. I sewed the ends of the carpet together with dental floss, which is strong and cheap. I had a heck of a time getting the carpet to stay in place; it seems that there's no glue which is designed for sticking carpet to duct tape while on a robot which is spinning around like crazy. The carpet kept coming loose, so I decided to sew the carpet in place by punching holes in the hubs and running some dental floss through that.
I coated the carpet with rubber cement to provide traction, but by the time the battle came up, there was enough dirt, hair, carpet fiber and other stuff trapped in the cement that I didn't really have enough traction. I had planned to add more rubber cement the night before, but I was too tired after seeing Iced Earth in concert at Dragon*Con the night before. Anyway, my wheels slipped quite a bit during the battle. Another competitor suggested that I use rubber no-slip shelving liner on the outside of my wheels.
The size of your wheel affects both your torque (pushing power) as well as your speed. The two are inversely proportional, meaning that if you have small wheels, your robot won't go as fast, but you'll have more power to work against the other robot.  On the other hand, with large wheels, you'll go faster, be able to ram your opponent at higher speeds, and get more traction.
These wheels were placed on a 3/8" threaded rod axle and secured in place with lock washers and a pair of nuts on each side. A bushing was required to fill the gap between the outside of the rod and the inside of the hub. The front caster wheel has no lock washers or nuts, although I placed a regular washer on each side to prevent the hub from rubbing against the axle mounts.
Items:
Item Cost Supplier
6 Lawnmower wheels @$3: $18  Home Depot
Professional grade duct tape (Used throughout construction) $5 Northern
Packing foam Free borrowed from someone who had some
Carpet $3 Home Depot
Threaded Rod $2 Home Depot
Bushings $3 Ace Hardware
Rubber Cement $6 Target
Dental Floss $1 Target
Silicone Glue $3 Target
 

Frame

My frame is made from 1" PVC tubing, the kind that normally carries water around your house. PVC has many advantages--it's light weight, strong, easy to cut, cheap, and snaps together like tinkertoys. It's strong enough that I could have bought the cheaper, lighter grade of PVC and still have been OK. I think that aluminum might be a little better for next time, though.
The drill motor mounts are pieces of a metal shelf hanger. They are each secured in place with two screw-wingnut combinations. The drills, in turn are secured to the motor mounts with a screw clamp. There is duct tape on the drill, motor mounts, and PVC frame to prevent slipping. The front axle is secured in a similar fashion.
The front part of the frame, the part that attaches to the pool, is reinforced with duct tape.
I used a chunk of plastic fan grill to mount all of my electronics. The grill is tied down with plastic cable ties. The electronics are secured to the grill with Velcro tape, for easy removal. I don't think that any of my components became dislodged from the grill, so the convenience of the Velcro mounts was a big trouble saver. The receiver batteries did pop loose, but I put some duct tape over them, which fixed that problem.

Items:
Item Cost Supplier
PVC Tubing @$0.60/foot: $10 (I bought much more than I needed) Home Depot
PVC T-Connectors @$0.50:$7 Home Depot
Steel Shelving Bar for Wheel Mounts $2 Home Depot
Misc. screws, nuts, screw clamps, Velcro, plastic fasteners $10 Ace Hardware, Home Depot, Office Depot
Plastic Mounting Grid for Electronics Free Found on an old fan in my back yard

Armor and weaponry

There's no size limit in the robot battle, and I wanted to take full advantage of this rule. I used the biggest round plastic thing I could think of: a 5' kiddie swiming pool. The pool is attached to the frame with screw clamps at three points: two on the corners where the motors sit and one on the opposite side. I ran some 8' plastic coated aluminum garden stakes through the pool to give it even more size. I suspended some fishing nets from the stakes in hopes that my opponent would become ensnared in the net, with the net wrapping around robot's wheels and gears. To promote this kind of entanglement, I stuck some duct tape loops in strategic places around the net. Finally, as the afternoon wore on, I added my fish decorations, which included cardboard fish cutouts from greeting cards, and a tissue-paper string of fish. Actually, these weren't just for decoration, they had duct tape all over them as well.
Items:
Item Cost Supplier
Swimming Pool $12 Toys R Us
Garden Stakes $6 Home Depot
Nets $5 Party Land
Fish Decorations $5 Party Land
 
A close-up of the Tunabomber.
Photo courtesy of Nora C. Hogan.

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