Thrasher - 16oz Combat Robot


By Dale Heatherington

March 2004

Thrasher Thrasher

Thrasher is an Ant Weight combat robot built to compete in Ant and Sozbot events.

This is my second ant weight combat robot. My other bot, Amdroid-A has been very successful but has no active weapons and is somewhat boring. To add a little excitement I equipped Thrasher with a unique vertical spinner. It rotates at 1000 RPM at high torque. It has a 15 pound torque limiting clutch to prevent self inflicted damage. It's capable of tossing opponents 8 to 12 inches into the air. The bot is fast, agile and fully invertable.

I did almost all the machine work in my back yard shop. I have a lathe, vertical mill, band saw, drill press, sheet metal brake, shear and other tools. The two main Lexan vertical frame pieces were laser cut at


May 13, 2006

Thrasher wins 2006 AHRC Robot Rally Ant Combat competition
Photos here! Win/loss record is now 25/3.

Sept 4, 2005

Thrasher wins Dragon*Con Microbattles 2005
There were 15 ant weight bots at this event. Thrasher got a first round bye due to winning last year. In the second round Thrasher took on the powerful vertical spinner "Haze". Haze got in a few really good hits that tossed Thrasher across the arena doing some cosmetic and slight structual damage but nothing that affected performance. My new shock absorbing wheel mounts actually work! The fight ended in a draw with both bots in the pit. On the do-over Thrasher took Haze to the high speed wheel hazard twice. After the second wheel encounter Haze was on his back and the weapon was dead. I pushed him into the pit for a win.
Next we fought a new bot by Charles Guan called "Pop Quiz". It's a well built bot with a horizontal spinning bar on top. It's also very low profile. Unfortunatly Charles got some superglue in the weapon bearings and had to run Pop Quiz as a pushy bot. I flipped Pop Quiz a few times then propped him up against the wall. Eventually I was able to slip PQ down the wall (still on edge!) to the pushout for the victory.
For the championship Thrasher faced a new pushy bot named Da Froogin. A driving error put Thrasher in the pit. This lead to a 3 round, best 2 of 3 style final. Thrasher put Da Froogin in the pit twice and won the 1# championship. Win/loss record is now 22/3.
A video of all Thrasher fights at this event in RealPlayer format is here. (9.2 megs D/L)

Nov. 13, 2004

Thrasher wins Atlanta Area Bot Brawl 2
Thrasher first took on Alabama Sweeper, a modified dust pan and won by forcing the dustpan up against the wall. Next we fought Scalpel and won by flipping him on his back. Scalpel came back through the losers brackets so Thrasher had to fight him again for the championship. This time Scalpel was neatly proped up against the wall and counted out. Thrasher also won the rumble but suffered damage to both motors from the spinner on Criminal Prejudice. Thrasher needs some wheel protection! Win/loss record is now 19/3.
A 12 minute highlights video of this event in RealPlayer format is here. (20 megs D/L)

Sept 5, 2004

Thrasher wins Robot Microbattles at Dragon*Con Atlanta
Thrasher did very well at Dragon*Con Robot Microbattles taking 1st place. Due to the low number of competitors Kelly opted for a best of 2 out of 3 format. First fight was with vertical spinner bot Haze. Unfortunatly Haze had some intermittant electrical problems and tapped out after being flipped once. Next up was KCA(on left in photo) by team TestBot. KCA put up a good fight but eventually ended up in the pit. The final was against Scalpel. This bot was equipped with 4 plastic wedges similar to Thrasher. It also had a large plastic ring surrounding the back for protection against Thrashers rotary flipper. Thrasher managed to flip him once but fights were mostly push and shove with Scalpel ending up in the pit. Thrasher had slightly more traction and pushing power. Win/loss record is now 16/3. Thrasher did not win the rumble due to the antenna on 4F^2 becoming entangled in Thrashers spinner. Thrasher pushed 4F^2 into the pit but also fell in on account the entanglement.

May 29-30, 2004

Thrasher competes at Rocket City Robot Assault in Huntsville AL
I took Thrasher to RCRA in Huntsville to go up against some very good competition. There were 38 other bots in the Ant class. Thrasher did well but not well enough to win. We had 5 wins and 2 loses. Overall the record is now 13/3. Click here for a brief report, photos and videos.

May 15, 2004

Thrasher wins AHRC Ant Weight Competition
The annual AHRC Robot Rally© held at SciTrek included ant weight combat this year. Thrasher easily took the 1st place trophy after winning 4 matches. Three were by knockouts and one was by shoving the opponent into the pushout bin. Every opponent was disabled in less than 10 seconds! Thrashers win/loss record is now 8/1. Download and view a video here:

April 22, 2004

Thrasher gets a new brain!
I completely redesigned the controller to improve low speed handling and implemented it on a real pc board. Now thrasher has the low speed precision handling and torque of "VCA" combined with the speed and power of Amdroid-A. I've updated the specs and photos below to show the new enhancements.

April 3, 2004

Atlanta Area Bot Brawl

Thrasher wins Atlanta Area Bot Brawl at Hobbytown USA! This was a small event and a good way to test the bots battle worthyness. The battles took place in a brand new arena built by Chris Underwood and Lionel Vogt.

I lost the first fight to "Inbred Son of Buck-toothed Hillbilly" due to a driving error. I ran a wheel off the edge of the pushout. Doh! This was my first time driving it in actual combat and Thrasher has a lot more steering sensitivity than Amdroid-A. Fortunately this was double elimination so I was able to work my way back to victory through the losers brackets.

Next I fought "Alabama Sweeper", a motorized dust pan. Thrasher flipped over the dustpan on the second hit and won by knockout in 15 seconds. Next up was "Inbred Son of Buck-toothed Hillbilly" again. I flipped him once and should have turned off the spinner. Unfortunately I flipped him again which put him back on his feet. Actually he runs both ways up but much worse inverted. The rest of the match was mostly push and shove and went to judges decision.

The next two fights were against "Very Crude Ant" by Charles. Yes, the same Charles that almost beat Invertabot at Dragon*Con last year. Because I came through the losers brackets I had to fight him twice to win. Fortunately VCA is slow and has lots of nice flat places for the spinner to bite. Thrasher managed to flip him several times and won both rounds for the Ant weight class victory.

The "Rumble" was fun. All the Ants were put in the ring with the last one running declared the winner. Thrasher disabled VCA by inversion then flipped Alabama Sweeper into the pushout bin. Next into the bin were Inbred son of Buck-toothed Hillbilly and Yee Haw. I went back, turned off the spinner and carefully pushed the inverted but still twitching VCA into the bin. There was also an immobile fairy weight bot in the ring just for fun and I carefully pushed that into the bin as well.

Now I know what needs to be improved before the next event. The steering needs to be desensitized and some changes need to be made to the front end to help the spinner make contact with opponents.

Here's a video of highlights of these fights.(about 8 megs download)



Thrasher Data Sheet
Size 5.75 in(W) x 6.5 in(L) x 5 in(H)
Weight 15.8 oz
Frame .093 in. Lexan
"C" ( AVR-GCC) Developed on SuSE Linux
CAD Software VariCAD and Eagle running on SuSE Linux 9.0
Radio Linx 900 mhz FM modules (in ham band)
Controller Hacked Sony Play Station 2 game controller
Data Link 9600 baud with CRC error checking
Power 14.8 volts from two 7.4v 250 mAh Li-Poly packs
Motors Two 12 volt 674 RPM gearmotors from Stock Drive Products
Part# D33S57M35G0011
Motor driver LMD18200T (2) driven by microprocessor
Motor Current Software limited to 1 amp max
Speed Control 28.8 khz locked anti-phase PWM with signed current feedback.
Drive System Direct 2 wheel drive from gear motor shaft
Tires Two 1.25 inch diameter Lego tires
Speed 60 inches/sec
Weapons Wedges and rotary flipper
Weapon Motor E100 Slot Car motor (12 volt, 24,000 RPM)
Weapon Motor Current Software limited to 600 mA max
Flywheel 1 inch diameter, 3/8 in wide brass. Weighs 0.6 oz. Energy: 6 Joules
Weapon Transmission 21:1 gear train with 15 lb torque limiter


Thrasher source code and schematics


900 mhz transmitter

This information is supplied in the hope it may be usefull but without any warranty; without even the implied warranty of merchanability or fitness for a particular purpose.

Creative Commons License
This work is licensed under a Creative Commons License.

Side view

Thrasher: Side view

Four hinged 2024 aluminum wedges drive under opponents and lift them into the 1000 RPM 4-40 cap screw flipper. The bot is fully invertable. The motion controls and weapon motor are automatically reversed when the bot is inverted.


Wedge closeup



The 1 inch brass flywheel stores about 6 Joules of energy at 20,000 RPM. It's geared down 21:1.


Weapon Transmission

This shows the weapon transmission gears. The ratio is 21.33 : 1. The motor and flywheel have 10 tooth pinion gears. The center gear has 32 teeth. It drives a 12 tooth pinion that drives the 80 tooth output gear. All gears are 48 pitch steel except the output gear which is aluminum. All are mounted on ball bearings. The "4-40 cap screw of death" is spun on an aluminum ring squeezed between the 80t gear on the left and aluminum plate on the right. This is the 15 pound slip clutch that prevents self destruction. There is a piece of teflon sheet between the gear and the ring.




Microprocessor Controller Board v2.0


This is the new controller I designed in April 2004. It has a much smarter motor control circuit and software. It continously measures the current pulled by each motor to determine the load torque and applies corrections to the PWM duty cycle to maintain constant speed. That plus exponential joystick response results in much more precise control at low speeds.
Board was made at PCB Express. I used the free Linux version of Eagle to do the layout.





The batteries are Li-Poly packs. Two packs are wired in series for 14.8 volts at 250 mAh. I bought them from Robot Marketplace.

Weapon motor


Weapon Motor


I used this cheap slot car motor to drive the weapon. With no load it runs at 24,000 RPM at 12 volts. It takes about 5 seconds to accelerate the flywheel to full speed. Without the flywheel this motor cannot flip an opponent. It's only purpose is to spin up the flywheel which does all the work.

Flip 0 Flip 0
Flip 0 Flip 0


Rotary flipper at work



This sequence of frames is from an early video demonstrating the effectiveness of the flipper. The wood block weighs 16 ounces.

Hacked radio control unit


Hacked PS2 game controller with transmitter

I use the the same controller with Invertabot.
Data is transmitted digitally at 9600 baud on 906.37 mhz.