pic: Beetleweight Combat Robot Progress

This is a work-in-progress of mine called Killer Oreo. It is a beetleweight (3lb) combat robot with a 1lb overhead bar that should (theoretically) top out at 12,000rpm. Whole robot is under 3" tall, current weight as it stands is just over 2lbs. As always any questions/comments/concerns are more than welcome (I dont bite)!

Do you have details on the electronics yet?

Drive motors are Silver Spark 50:1 gearmotors, weapon motor is a turnigy aerodrive 1200kv brushless outrunner, weapon esc is going to be a hobby king 30A, everything else is in the air right now. Any suggestions for electronics are more than welcome (and kind of needed).

Glad to see more people getting into combat robotics! There’s a facebook group you can join, named Combat Robotics. Lots of great info shared there.

For your drive motors you can use shelled Vex 29’s, I used those with larger motors and had great success, you just need to solder in an extra wire to provide ground for the PWM signal and supply separate battery power. You can also use Fingertech TinyESCs, which are more fitting for the size of motor you are using. You might run into some power issues with such small motors in a beetle but you can try it out.

For the spinner you are going to want to find an esc that can be flashed with special firmware named SimonK or BLHeli, you can buy these preflashed on Hobbyking. The firmware allows the esc to start up heavy inertial loads like the spinner - most airplane escs will detect the motor’s inability to rotate immediately as an issue and will refuse to start up. SimonK just juices it until it picks up the commutation direction.

If you’re not sure if your weapon motor has the grunt for the task, you can use this calculator to help figure that out. I used an aerodrive 3536 both 1400 and 1200kv for a .8lb weapon and it worked okay. You might have to be gentle with the throttle to avoid burning up the motor, they’re designed to operate with heavy airflow through them.

If you post more pics I’d be happy to comment further! Looks great so far :smiley:

Where are you located/where are you planning on competing?

Thanks for the advice! I will definitely post more progress pics as I go! I am located in central Indiana and I really don’t know where any local competitions are.

C.I.R.C is coming around again I think

Just to share some info, when I built my spinner which was similar to yours, I wanted to make the bar as heavy as possible for obvious reasons, but forgot that the the part of the robot with the lowest moment of inertia will (generally) spin up, for me, that part was the body :(. I would do some testing and make sure you don’t flipout when you try to spinuo because that’s all mine did ::ouch::

Here’s some vids of what I mean:

Buildersdb lists some competitions and also lets you register for them. Others are more off the record but are usually announced in the facebook group.

In any case, the torque for the spin up will have to be transmitted from the ground through the robot chassis to the spinner, which means the wheels will have to provide the torque.

This suggests another possible solution to the startup issue described above - start by spinning the robot with the rotor locked, then release the rotor and bring the robot chassis back to non rotating - then the spinner motor will have an already moving rotor to accelerate, rather than having to start from a standstill.

Sounds weird, but I know Dave ain’t afraid of weird.

Given the advice above about spinning, you may want to rethink the wheel placement as well. As shown, it will spin quite easily, and you don’t have much normal force to overcome it. Perhaps a treaded system might give you more grip, without adding a lot of complexity or weight?

Hmmmm… I never really thought about treads until now. Now I am really considering it::rtm::

Have you read through RioBotz Combat Robot Tutorial?
Here is the PDF that they published:
If you learn better with a paper book you can also buy a printed and bound version on Amazon for ~$15.

The electronics section in RioBotz is out of date as Brushless DC motors and LiPo batteries have totally changed the game. You get so much more energy density out of Brushless motors compared to traditional brushed motors. I really like the new BlHeli_32 firmware ESCs that have started coming out. They seem to handle startup current spikes better than SimonK and BlHeli_16 from what I have noticed.

I would highly recommend reading section 6.3 in RioBotz on tooth design. If you are not careful it is really easy to design a weapon that on paper has really high KE but can never actually deliver that KE to the opponent.

Do the math on the friction of your drivetrain before changing anything. The RunAmuk calculator that karlonpc posted is a great tools for finding the torque that your weapon will be experiencing and putting back into your drivetrain.

Think about how your robot will react when you hit someone, what direction will your robot go in? What forces does your weapon bar experience? How are those loads transmitted into your drivetrain? Nothing more frustrating than your robot destroying itself.

Another really important thing to think about what type of arena are you playing in.
Does the arena have a pit? Pits give advantage to wedge bots.
Is there an offset border wall on the arena? Will your robot get high centered on the edge of the wall?
Is there a ‘moat’ around the combat area? Gives advantage to flipper bots.
What is the floor made of? Is the floor magnetic? If the floor is magnetic you can use that add downforce to your drivetrain so you can spin up faster and hit harder.
How smooth is the floor? The rougher the floor the harder time wedges have.

Welcome to the wonderful hobby of Combat Robotics, it is ton of fun. Not quite as much fun FIRST but close.

You’re gonna want wayyyyy more power on your drive than two 50:1 sparks. My first BW was weak drive with 4 11:1’s (iirc) on 1.25" wheels. I would suggest either going to 4WD with each wheel individually driven by a spark or go up to the “standard” 1000rpm kitbots/servocity/ebay/chinese motors, 1 per side.


My usual rule of thumb is 3 SS’s = 1 Kitbots 1000rpm as far as power goes.