Opponent Detection

When detecting the state of the world around our MiniSumo we are looking at two broad areas, the arena (dohyo) and the location of the opponent. This article concerns itself with opponent detection generally, more detail on types of detection and specific application will follow.

Sensor Range

The sensor range is important, if we have nothing but contact sensors we will have a limited means of detecting the opponent where as long range sensors may be susceptible to detection of objects out side the arena. If we consider the specification given to the susceptible we see how important it is to maintain the defined dohyo exterior and limit the range of detection for reliable operation. It can be difficult for competition organisers to maintain the dohyo exterior when the gathering is informal, we have all a tendency to squeeze in to see better.

To illustrate these limits I'll assume that we have two minisumo with diameters of 100mm, which are right at the interior edge of the dohyo white line. This edge location has been chosen as it's the point at a minisumo might detect and turn from the arena edge. As pictured we see that our minisumo are 520mm apart but also a minisumo with 520mm range on it's sensors is detecting to a point 5mm with the the dohyo exterior. Therefore a minisumo is not distracted by objects outside this area but also a minisumo central to the dohyo might be distracted by a person standing at the edge of the arena. While we can reduce the range of our sensors this will impact upon our competitive ability to detect the opponent, in our documentation I will assume that out competition organisers make an effort to maintain the exterior.


Practically we may not need to detect to work as far as 520mm as the opponent will tend to move away from the dohyo edge. However the shorter the range the more vulnerable we will be come, especially if this is combined with significant blind spots in our sensing. Some sensors such as Sharp's IR Rangers or Sonar may have much longer ranges than we require, the response can be limited in the software. Where we are using IR TV Remote hardware to construct a sensor then the limitation needs to be part of the construction, this may not be straight forward as the response depends upon ambient conditions.

Beam Shape

The range is just one of several factors that may effect what your minisumo detects, to ensure that your minisumo is going to react in the way you expect you must understand the area of detection. These sketches show a simplified cone shaped area, the first depicts sensors that are point straight ahead and second with the sensors pointing out a little to spread the detection area and to give better discrimination between left and right detection.

Forward facing beamsSpread Beams

With centre detection activating both left and right hand sensors, the dotted circles depict how far off centre the opponent might be before central detection is lost. The detection areas need not overlap but when spreading detection it's worth looking for dead zones, these may be addressed in your movement or provisional sensors.

With single range, or ON/OFF, sensors an opponent may pass through your detection with our giving indication of it direction of travel. When using two sensors your likely to have one sensor triggered before the other, giving a relative direction of travel.

Data sheets will give you an indication of the area that might be detected by you sensor, usually it's more tear drop as shown below but testing will help you tune your application of the chosen device. The shapes depicted below are based upon the data sheets for TSOP TV IR sensors used on many commercial MiniSumo, noted the line is based on a constant signal level we'll high light some issues that mean this may not hold true in practice. Nominally the picture on the left represents several positions that may give the same detection.

Detection ShapeShaped Beams

Considering the right hand sketch and the wide area of detection you can see how the discrimination may be improved by blocking some of the field of view on each of the three areas depicted.

A number of off the shelf minisumo use low cost TV IR remote components for detection, many use multiple IR transmitters and a single detector to save on cost but at the expense of possible sensor confusion. We depict this on the left where the minisumo would transmit to the left and turn that way upon detection and similarly to the right. If we transmitted to the right but detected IR from a minisumo on the left we could turn in the wrong direction!

False Detection Single BeamFalse Detection Dual Beam

An additional detector will remove the problem as detection to the left or right will represent either a single reflection or direct transmission from the opponent. We will cover TV Remote based sensors in more detail later but it's worth remembering that signal transmission from your opponent will be stronger than your reflected signal and white objects, such as Tee Shirts, outside the dohyo may prove a problem with odd reflections especially as these devices are aimed at operation upto 300m!

Blind Spots

If all your sensors are forward facing then the biggest blind spots are to the side and rear of your minisumo. Some of this may be addressed by the action of your minisumo where the opponent is not detected within a specified time period but additional detection may help address the issue. Local rules may also dictate various constraints upon the starting position, while many are head to head, others start facing opposite or more random directions. The sketch below represents starting in opposite directions but with a minisumo that will detect something to it's right. The side and rear sensors on this minisumo would indicate that it may need to turn 90° or 180° to get the opponent in the forward facing sensors.

All round Sensing

Given the wide angle of detection these are rough angles but it must be remembered that you opponent is likely to be on the move too.

Speed Of Operation

When we first build our minisumo we might not consider speed of operation but it must factor in our consideration, especially when we (or our opponents) start running at higher speeds. While minisumo based upon modified servos travel around 0.1 m/sec there are some builders pushing speeds of 1.5 m/sec.

The time a sensor takes to react will be listed in it's data sheet, often in milli-seconds or micro-seconds. If you and your opponent move at just 100mm/sec then there may be almost 8mm of movement in the 38 milli-seconds (typical for one of Sharps analogue distance measuring sensors) but if your opponent is actually managing 1.5 m/sec time in the 60mm is much more significant - it might be out of sight by the time the next measurement is taken.

A simpler on/off detection may react in 8 milli-seconds or even 400 micro-seconds (again typical of two Sharp products), now the relative movement of the high speed minisumo is just 13mm or 0.6mm respectively. The speed and how your minisumo acts on this information is important but the right choice of sensor is going to be critical to your ability to practically sense the opponent.

You'll find some sensor discussion on the MiniSumo Forum.
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