This is a demonstration of a control system model of an outfielder catching fly balls. The fielder catches the ball by controlling the vertical and lateral velocity of the image of the ball on the retina. The fielder controls these variables by running toward or away from the ball (to keep vertical velocity increasing) and to the left or right of the ball (to keep lateral velocity at zero). The two plots on in left panel show the position of the fielder relative to the ball; the top plot is a side view of the action; the bottom plot is a top (bird's eye) view of the action. The plot in the middle panel shows the rate at which the fielder is running from the time the ball is hit until the ball is caught; the fielder starts slow, raches a peak velocity and then slows down to catch the ball. The plot in the right panel shows the projection of the path of the ball on the fielder's retina (relative to the projection of home plate -- the open square at the bottom of the plot).
Each press of the "Run" button launches a new hit.
Compare the "Retinal" plots obtained in this simulation to equivalent "Retinal" plots obtained by McBeath, Shaffer and Kaiser (reported in Science, 1995, v. 268, pp. 569-573, Figure 4.) using a shoulder mounted video camera on a real fielder catching fly balls (below).
Also compare the "Top view" plots of real fielder positions (also from the McBeath, et al article) to the "Top View" plots of fielder position in the simulation (below). Note that the simulated fielder sometimes backs up more than the real fielder but the lack of backing up by the real fielders may be a result of the particular fly ball trajectories used in the McBeath et al study and/or a result of the real fielders ability to use head movements to control the retinal variables.
Finally, compare the "Running Rate" plot in the middle panel to real running rates (collected by Dennis Shaffer and reported at his Web site: http://www.personal.kent.edu/~dshaffer/Outfielder.html.
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Last Modified: September 23, 1998
Richard S. Marken