When it comes to playing games against robots, the future doesn't look too bright for us humans. Machines will likely beat us, or are already beating us, at soccer, ping pong, chess, Go, baseball, basketball, rock-paper-scissors, iPhone games, and, of course, Jeopardy. Now add air hockey to the list.
Japanese researchers at Chiba University's Namiki Lab have developed an air-hockey robot that is skillful enough to compete against human players. It's not the first air-hockey robot developed, but the team led by Professor Akio Namiki has upped the ante: their robot changes its strategy based on its human opponent's playing style.
The system consists of an air-hockey table, a Barrett four-axis robotic arm, two high-speed cameras, and an external PC. It builds on the lab's work with high-speed tracking. Previously, the researchers (in collaboration with University of Tokyo's Ishikawa Oku Lab) paired a ultrafast vision system with a dexterous robot hand to juggle balls and fold towels, but here it tracks the puck and opponent's paddle. The position data from the camera images is then processed by the external PC, which determines the robot's next move. The robot is tracking the game at an insanely fast rate of 500 frames per second. Which means that, from the robot's point of view, its human opponent is moving at a laughably slow pace. It's like the robot is playing the game in a Matrix-style bullet-time frame.
To keep the game entertaining for human players, the researchers programmed the robot with a three-layer control system. The first layer is responsible for basic motion control at the hardware level. A second layer decides its short-term strategy—whether it should hit the puck, defend the goal, or stay still—to choose which motion can effectively counter the incoming trajectory of the puck. The third layer determines the machine's long-term strategy, and this is where things get interesting.
Basically, the robot observes the speed and position of the player's paddle in relation to the puck. This data can be described by what is known as a Motion Pattern Histogram (MPH). The robot uses this data to estimate whether its opponent is playing aggressively or defensively. Over the course of a game, the robot can detect these MPHs in real-time and compare them with reference patterns to help it figure out what you're doing.
If the robot is not adaptable to its opponent's style, the game can get boring. Say you are being offensive and the robot is being defensive; in this case, the game could become repetitive: you attack, the robot defends, you attack, the robot defends, and so forth. Conversely, if you're playing defensively and the robot offensively, the same problem arises. "To avoid this, the robot should be offensive when the opponent is offensive and should be defensive when the opponent is defensive," the roboticists write in a research paper.
So in a sense, by detecting and matching a given playing style, the robot isn't just physically playing the game against you: it's adding a psychological component to the match. A series of experiments showed that the robot was successful in detecting playing behaviors, and forcing them to change their strategies. As a result, players reported that this made the game more exciting, even if they're playing against a robot that is likely going to defeat them.
The Chiba researchers—Professor Namiki, Sakyo Matsushita, Takahiro Ozeki, and Kenzo Nonami—presented their paper, "Hierarchical Processing Architecture for an Air-Hockey Robot System," at the IEEE International Conference on Robotics and Automation (ICRA) last month.