Harpy – Sumo Bots 2009

This will be a collec­tion of design infor­ma­tion, videos, and photos of Harpy, the robotic sumo plat­form I built for the Science Olympiad tour­na­ments in New York State.

Harpy’s drive­train uses the motors that were used in La Cucaracha, only paired with larger, stick­ier wheels. It also has more capa­ble, robust 4-cell 2.3Ah A123 lithium-iron-phos­phate batter­ies capa­ble of 70A contin­u­ous, but has a motor controller (Dimen­sion Engi­neer­ing Saber­tooth 2×25A) that is akin to complete crap. Videos on La Cucaracha‘s page will give you an idea of what Harpy would be like with a better motor controller.

Harpy took a lot of cues from lessons learned build­ing and compet­ing with previ­ous sumo bots. Its design goals were to be as simple and compact as possi­ble, mini­miz­ing espe­cially height and the length between the wheels and the front edge of the scoop. The first is to allow the bot to fit inside a foam-lined Plas­ti­case 628 procase, which were offered at a discount to Tech­ton­ics, and to lower the center of grav­ity. The second was to maxi­mize robot maneuverability—Harpy does not have swing around a large front “head” like many other robots do, but instead pivots about a point rela­tively fore to be bot. Its over­all length was reduced, but if more length was require for space, it would have been added to the rear.

Harpy’s weight distri­b­u­tion (which is to place a maxi­mal amount of weight upon tires and to mini­mize weight on ground contact points that do not contribute to push­ing force) was heav­ily consid­ered during design, but it was not a major design goal. Simplic­ity and robust­ness of the chas­sis were seen as more impor­tant and could not be compro­mised by design choices that improved weight distri­b­u­tion. Thus, a four-wheel design (driven by belts or gears from the two motors) was rejected, as well as a scoop with adjustable “ride height.”

Pursu­ing the goal of robust­ness and simplic­ity, Harpy features no flat hori­zon­tal load-bear­ing struc­tures in its chas­sis. That is, contrary to most “stan­dard” robot designs, there is no “base plate” or plat­form on which other compo­nents are mounted. Instead, Harpy has four one-quar­ter inch PETG plates stand­ing verti­cally. They clamp onto the motors and clip onto the ends of the battery pack. In this way, the sturdy gear­mo­tors and “shot­gun barrel” style A123 pack (built from kit from the now-defunct Model Elec­tron­ics Corp.) serve as the primary struc­tural elements of the chas­sis.

These verti­cal plates are also connected to another verti­cal piece running left-to-right and to the scoop. That is more or less the entire chas­sis, and it is excep­tion­ally sturdy and rigid compared to a tradi­tion (and easy to build) plat­form style design robot. In fact, even at the tour­na­ments, Harpy uses no glue to hold its plas­tic parts together.

Photo Gallery


Harpy has immense push­ing power, due to its 550-size gear­mo­tors and custom-molded wheels. Thanks to cylin­dri­cal gear­heads over the gear­mo­tors that are flush and inline to the motor cans, Harpy has wheels that fit over the gear­head casing, rather than protrud­ing out from the side of the gear­mo­tors. The wheel forms are made from poly­car­bon­ate tubing (razor-sawed by hand) with inserts laser-cut from PETG. They are then placed inside a mold of simi­lar construc­tion. Path­way Poly­mers Por-A-Mold S-111 (which has a cured hard­ness of Shore A 20) was then mixed and poured into the mold.

The wheel and tire assem­bly was then broken out of the mold by cutting away the outer mold tubing on two sides, break­ing the tube along one of the cuts, and fold­ing the tube along the other cut. Think open­ing up a wrist shackle.

The result­ing wheel and tire assem­bly were extremely sticky. After the wheel was mounted to the gear­mo­tor, each of which is about a pound, it was found that the assem­bly could stick to verti­cal painted drywall indef­i­nitely, and even upside-down to the ceil­ing. The robot, with clean wheels, can push more than 2 to 3 times its weight, depend­ing on the condi­tion of the wheels, align­ment, etc. In fact, if the robot were to drive into a wall from a stand­still, it can actu­ally pop apart all of its plas­tic parts at the tab-and-slot joints, and essen­tially disin­te­grate. That is why it does not use any glue or plas­tic weld cement—if the struc­ture can not relieve stress by popping apart at Lego-like joints, then it will crack and cleave.

New York City Region­als
Match 1 – Stuyvesant Team C vs. Townsend Harris Team B

Match 2 – Stuyvesant Team C vs. Townsend Harris Team A

Match 3 – Stuyvesant Team C vs. Unknown Team

Match 4 – Stuyvesant Team C vs. Cardozo Team A

Match 5 – Stuyvesant Team C vs. Unknown Team

Match 6 – Stuyvesant Team C vs. Cardozo Team A

Match 6 – Stuyvesant Team C vs. Cardozo Team A (Redo)

Match 7 – Stuyvesant Team C vs. Cardozo Team A

All in all, Harpy’s mechan­i­cal design was extremely satis­fac­tory, except that it has nowhere to mount the motor controller. Also, if the scoop is just a bit lifted up, then the rear rests on the batter­ies rather than the wheels, and it begins to lose trac­tion, which can be seen in the video for Match 7. The motor controller is unac­cept­ably unre­spon­sive and under­pow­ered, and a deci­sion was made to replace them with Victor 883s, from La Cucaracha. These and other defi­cien­cies resulted in Harpy II, which has not yet seen battle, but is a clear improve­ment upon Harpy I.
Harpy II

Some­what irrel­e­vant, but pretty:
Victor 883