Catching ground balls: Optical control heuristics used by humans and robots support a unified fielder theory

In past studies we examined optical control heuristics used by fielders and robots for interception of fly balls. Our findings support use of the heuristics of maintaining optical speed constancy and a linear optical trajectory (LOT) for balls projected above the horizon. In the current study we explore behavior of robots and fielders intercepting ground balls projected below the horizon. In robotic simulations, we confirmed that the same control heuristics demonstrated for fly balls are viable ones for intercepting ground balls. In the human experiment, we measured optical position of the ball with three skilled fielders using a head cam, and measured actual ball and fielder location using stationary external cameras. Our findings support that, for most easily caught grounders, fielders select a slow running pace and maintain the same optical control heuristics that they do for fly balls. This also results in an energy-efficient, near constant-speed running path. On a number of the more difficult trials, where fielders needed to run a long distance, the optical trajectory was much better accounted for as two phase: an initial constant-speed linear trajectory at a severe lateral angle, followed by a second constant-speed linear trajectory at a new diminished lateral angle. On these trials it appears fielders overshoot the first LOT and establish a second one with a more workable approach angle. The findings also support that fielders tend to establish a running path with a fairly constant velocity that remains roughly within a vertical plane perpendicular to their initial lineup with the ball. Overall, the findings are consistent with a unified fielder theory in which the same optical control heuristics are used to navigate to the interception destination for both fly balls headed above the horizon and grounders below it. Use of these heuristics holds promise in the creation of navigating mobile robots designed to achieve or avoid collisions.