I've been involved in robot combat (BattleBots) for the past three years now. My first robot Shish-Kabot was extremely successful with one 2nd place and two 3rd place finishes but it had one weakness. Every time it fought a flipper robot called Upheaval, it would get tossed around and lose via knockout. (see 2nd image) After spending way too much time thinking about how to beat Upheaval with Shish-Kabot I decided to build a better flipper bot...

Step 1: Background

I guess I should probably explain a few things about what I'm talking about.

Robot Combat is an sport often referred to as BattleBots, but that term is trademarked so I'm probably not supposed to say it. But the basic idea is two robots fight to the death in an enclosed bullet proof glass box for 3 minutes or until one can't move anymore, when a knockout is declared. There are basically three groups all robots can be categorized into. Spinners are probably the most prevalent. They have some spinning mass (think exposed lawnmower blade) and try to slice or blunt force the other robot into submission. Pushy bots are the simplest to build. They often have a wedge to get under the opponent. They win by dominating the flow of the fight. The third group is flippers or lifters. They are designed to get under the other robot and flip them over or chuck them across the arena and hope something breaks on impact. This is often done using pneumatic rams. Flippers are not very common anymore because the introduction of cheap imported Chinese brushless motors and the difficulties associated with pneumatics.

The design goals/constraints for Phoenix were

1. Powerful flipper weapon

2. Entertaining

3. Cost must be under $700 (I'm a high school student without a high paying job)

4. Must be under 30lb weight limit

Step 2: The Flipper

I design robots a little differently than most people. Many people start by making a CAD drawing or a cardboard mockup. I simply lay out the parts on the floor and begin arranging them until I find a layout that works, and design from there.

The arm is .25" thick 7075 grade aluminum with an adjustable .125" titanium spatula on the front to get under opposing robots.

The pneumatic system in Phoenix is a very simple. It's single direction flow with a bleed hole to retract the ram after its shot. It uses a modified 16oz paintball tank, and the solenoid valve is a Burkett 5404. The valve is actuated by a RC relay switch. The ram is 2.5" bore 4" stroke.

Step 3: Frame/Drivetrain

Now that I had the flipper weapon designed I could work on the frame for the robot.

I first made part of the frame that would hold everything. My material of choice is the 1/2"X1/2" box steel they sell at Home Depot. After welding the basic shape I once again test fit the components and worked on re-arranging them to make everything fit.

The drivetrain for Phoenix, as described earlier was two Harbor Freight 18 volt drill motors with 4" wheels, kitbots.com's hubs/motor mounts and one drill battery to power it. I decided on this drivetrain because it offered both adequate speed and power for the robot and was cheap. The speed controller I used was a Sabertooth 2X25 that I had won at a previous competition.

I then did a test assembly of the robot and then test drive...

Which reviled a major design oversight on my part. There was no weight on the drive wheels which made the robot almost impossible to drive. I needed to change that... with more steel....

I ended up welding 4lbs of steel to the front of the frame to help counteract the weight distribution issues. The added bonus was now it had 1/8" thick steel armor on the front.

Step 4: Armor/Assembly/More Testing

Between the weight of the frame, pneumatics, drivetrain and front armor/ballast. I didn't have much weight for the armor. I needed something light and strong... like... titanium. Titanium is really expensive and would have blown my budget, but I got really lucky because another robot builder was cleaning out his shop and had some old titanium armor from his 120lb bot that was too damaged for him to use but was still usable for a smaller robot like Phoenix.

In order to mount the armor I needed to make the frame grow...

I increased the width of the frame so that it could support the armor protect the wheels. There are three parts to Phoenix's armor. The steel plate welded on the front and the two 1/16" titanium body panels which are removable to allow access to the internals to recharge the batteries, refill the Co2 tank and so I can repair any damage from combat.

Phoenix only has 29 screws in it which makes it very easy to assemble and disassemble it, which is critical to success in robot combat competitions. But because of that ease of assembly I don't have any pictures of it partially assembled... so use your imagination.

After assembling it, I needed to do some testing, mostly of the pneumatics because I needed to find the optimum hole size for the bleed hole but also because there's something fun about launching heavy objects with a robot. I tried three different sizes; loose fitting, 1/16" and 1/32". The 1/16" worked the best because it brought the ram back down in a reasonable amount of time.

Here's a video of the 1/32" bleed hole test.

Through testing I discovered that I got about 12 flips before Phoenix runs out of gas.

Step 5: Phoenix in Combat

Stats:
Weight: 28lbs
Speed: 10mph
Weapon: Flipper (generates 2700lbs of force and gets about 12 flips before it runs out)
Armor: 1/8" steel and 1/16" titanium
Rank: 2nd
Cost: Around $800 spent, parts probably valued at around $1500

Phoenix is currently (as of 5/17/2012) the 2nd ranked 30lb bot in the US and Canada. Its got a fight record of 7/3 with all but one of its fights ending in a knockout. Its also finished in 2nd and 3rd in the two events its fought in.

Here's a compilation/highlight video of its best moments.

Step 6: More Information

For info about competing check out

The Builders Database This is the place where you can find out about and register for competitions

North East Robotics Club The premier robot combat event host in the North East

Combots/Robogames The largest events on the West Coast. They run weight classes from 1lb all the way up to 220lbs.

For a more complete and detailed build report of another 30lb combat robot (that beat Phoenix) check out Nyx

For parts check out

The Robot Market Place They sell everything and anything related to robots

KitBots.com for competitive 3-12lb kits and parts for robots up to 30lbs

FingerTech Robotics for 1-3lb robot parts and an awesome 1lb bot kit.

Holmes Hobbies for excellent 30lb robot speed controllers and other motors

HobbyKing for cheap stuff from China