Hard2C Compact Mini-Sumo Robot

Hard2C (“hard to see”) is a miniature sumo robot that is small and painted black to make it difficult for opponents to locate.

Hard2C mini sumo robot

Hard2C mini sumo robot.

The top, bottom, and rear panels are made of 1/10th of an inch thick aluminum. The side panels are 1/4 inch thick PVC plastic.

Two Sharp GP2D12 infrared distance sensors are mounted on the rear to find the opponent. Unfortunately, because the GP2D12 distance sensors provide such a narrowly focused view, the robot struggles to locate opponents. It’s as if you were wearing glasses made of cardboard tubes from paper towel rolls. You’d need to constantly move your head around to scan the arena.

When Hard2C is fortunate enough to run into an opponent, there is a reflective sensor (red LED and photodetector) in the center of the base of the front of the robot to detect that it has made contact. If an opponent is detected, Hard2C can turn on two ultrabright red LEDs and two infrared LEDs on the sides of the front panel. Theoretically, the bright light can get under the opponent and fool it into thinking it has reached the bright edge of the sumo ring, causing the opponent to continuously back away.

Tiny scoop on the front of Hard2C robot

Tiny scoop on the front of Hard2C robot.

A short (1 mm) angled edge is intended to prevent sumo opponents from being able to get their scoop under Hard2C. Although it isn’t 100% effective, if the edge did not exist then nearly all opponents with scoops would be able to slide under Hard2C, resulting in Hard2C’s weight adding traction to the opposition’s tires.

That being said, the tiny edge is useless against arms such as on Number 2.

Underneath the front edge are two pairs of reflective sensors aimed at the floor. The sensors detect the white edge of the sumo ring, in order to avoid having the robot drive out of bounds. These are the same types of sensors that Hard2C tries to fool by flooding the opponent with bright red and infrared light from the front.

Tiny 4 mm reflective-pair edge sensor peeking through bottom plate

Tiny 4 mm reflective-pair edge sensor peeking through bottom plate.

Sumo Robot Controls

The rear panel of the robot contains the operator controls.

Rear panel showing DIP switch mode, power switch, start button, and bicolor LED

Rear panel showing DIP switch mode, start button, power switch, and bicolor LED.

  1. DIP switch that sets the mode
    1. Aggressive vs. passive
    2. Enable red and infrared emitters to fool opponent
    3. unused
    4. Sumo contest vs. pushing test (just drive forward)
  2. Pushbutton to start a round
  3. Power switch (recessed to avoid being accidentally turned off during a battle)
  4. Bicolor LED
    • Green - opponent detected on left
    • Red - opponent detected on right
    • Yellow/mixed - opponent detected by both sensors

The bicolor LED is useful when positioning the robot at the start of the round, to be sure that both sensors detect the opponent. The bicolor LED is also useful when reviewing contest footage, to debug the behavior of the robot.

Fatal Flaw

Although Hard2C performs exactly as designed, the robot does not usually win contests for three reasons:

  1. Difficulty finding opponent due to narrowly-focused sensors
  2. No real scoop
  3. Very little weight on the wheels
Side view of mini sumo robot on ring

Side view of mini sumo robot on ring.

As you can see, the robot has small wheels mounted at the rear. This was intended to allow the robot to continue to push even if it were tilted at the front due to a scoop.

Unfortunately, I didn’t consider that the majority of the Hard2C’s weight would be resting on the front of the robot. Not only does that mean the robot wastes a lot of mechanical power due to front friction, but it also means the rear wheels spin a lot due to lack of grip of the tires.

Roundabout sumo robot has most of the mass over the wheels

Roundabout sumo robot has most of the mass over the wheels.

Compare Hard2C’s side profile to Roundabout in a sumo configuration (pictured above). The motors, battery, circuit board, and even a small weight are positioned on top of the wheels for maximum traction. Obviously this is easier to accomplish when the wheels themselves take up half the length of the robot.

The fatal flaw of poor mass position makes it very difficult for Hard2C to win a sumo contest, since the sport mostly comes down to which robot has greater pushing power. Hard2C is unable to move the weight of most opponents even if they aren’t pushing back, as Hard2C’s wheels simply spin in place. The few rounds that Hard2C has won have been due to the opponents driving themselves off the edge, or other opponent flaws like scoops that are too long.

Other Information

You can find additional pictures and information about Hard2C on the repair page. Years after being built, Hard2C needed some repairs to eliminate gaps due to two threaded parts, square the corners of a PCB for a better fit, and fix a loose right-angle gear.

The guts of Hard2C, including the motor arrangement and internal battery placement, can be found in figures 12-6, 18-14, 18-15, and 18-16 of Intermediate Robot Building.