Measurement of rotational property for a baseball in flight with acceleration sensors

Abstract

We attempt to measure the rotational characteristics of a ball in flight by using a baseball-like ball with a built-in acceleration sensor unit which consists of one three-axis sensor for low G and three sensors for High G. Two approaches for obtaining the rotational speed and its axis of the ball from outputs of the sensors are formulated; one is based on rotational component in the acceleration outputs and the other is based on translational one. We employ in the present approaches the assumption that the time variation of rotation axis can be neglected because it is sufficiently slow. In flight experiments, we obtain nine time series data of acceleration for straight and curve ball trajectories, respectively. By using these experimental data, we estimate the rotational speed and rotation axis with the present two algorithms. As the results, it is shown that the rotation axis keeps almost constant during the flight in each algorithm’s result and this means the validity of our assumption on rotation axis. The difference in the rotation axis obtained by the two algorithms ranges from 3.5 to 10.6 degrees in the straight ball data while it does from 3.9 to 18.8 degrees in the data of curve ball. Also, the averaged rotational speed in the nine trials is 12.56 rps in the straight data, and the difference between the algorithms indicates 0.10 rps (after pitching) and 0.17 rps (before catching). In the curve ball data, the averaged value and the difference between the algorithms are 19.22 rps, 0.10 rps (after pitching) and 0.14 rps (before catching), respectively. The present result shows the decrease of the rotational speed along flight trajectories, and it is probably the effect of aerodynamic torque.

Keywords

• measurement
• rotational speed
• rotation axis
• acceleration sensor
• baseball
• flight experiment
• sports engineering