Swing quality measurement system
A system that measures a swing of a bat with one or more sensors and analyzes sensor data to create swing quality metrics. Metrics may include for example rotational acceleration, on-plane efficiency, and body-bat connection. Rotational acceleration measures the centripetal acceleration of the bat along the bat's longitudinal axis at a point early in the rotational part of the swing; it is an indicator of the swing's power. On-plane efficiency measures how much of the bat's angular velocity occurs around the swing plane, the plane spanned by the bat and the bat's sweet spot velocity at impact. Body-bat connection measures the angle between the bat and the body tilt axis, which is estimated from the trajectory of the hand position on the bat through the swing; an ideal bat-body connection is near 90 degrees. These three swing quality metrics provide a simple and useful characterization of the swing mechanics.
1. A swing quality measurement system comprising:
a memory;
a processor coupled with said memory;
a first communication interface;
a sensor configured to capture sensor data associated with a bat during a swing of said bat by a user;
a bat geometry stored in said memory, said bat geometry comprising
a hand position along a longitudinal axis of said bat; and
a sweet spot position along said longitudinal axis;
wherein said processor is coupled to said sensor via said first communication interface, and wherein
said processor is configured to
obtain said bat geometry;
receive said sensor data from said sensor;
obtain or calculate acceleration data from said sensor data;
obtain or calculate angular velocity data from said sensor data;
calculate a trajectory of said hand position from said acceleration data and said angular velocity data;
calculate a trajectory of said sweet spot position from said acceleration data and said angular velocity data;
calculate a trajectory of said longitudinal axis from said acceleration data and said angular velocity data;
determine a time of impact of said swing from said acceleration data and said angular velocity data;
calculate a body tilt axis of said user from said trajectory of said hand position, wherein said body tilt axis is perpendicular to a plane passing through three different points of said trajectory of said hand position;
calculate a swing plane, wherein said swing plane is spanned by a velocity vector of said sweet spot position at said time of impact and said longitudinal axis at said time of impact; and,
calculate one or more swing quality metrics based on one or more of
said sensor data;
said acceleration data;
said angular velocity data;
said time of impact;
said trajectory of said hand position;
said trajectory of said sweet spot position;
said trajectory of said longitudinal axis;
said body tilt axis; and,
said swing plane.
2. The system of claim 1 , wherein said sensor comprises an inertial sensor that comprises:
a three-axis accelerometer that generates said acceleration data; and
a three-axis gyroscope that generates said angular velocity data.
3. The system of claim 1 , wherein
said sensor comprises one or more cameras; and,
said sensor data comprises images captured by said one or more cameras.
4. The system of claim 1 , wherein said sweet spot position comprises one or more of
an optimum location on said bat for striking a ball;
a position on said bat that maximizes energy transfer when striking said ball;
a position on said bat that maximizes ball speed when striking said ball; and,
a position on said bat that minimizes vibration when striking said ball.
5. The system of claim 1 , wherein said processor is further configured to
calculate a start time of centripetal acceleration comprising a time during said swing at or near a time when a longitudinal acceleration changes sign, wherein said longitudinal acceleration comprises said acceleration data projected onto said longitudinal axis.
6. The system of claim 5 , wherein said one or more swing quality metrics comprise a rotational acceleration metric comprising
a difference between
said longitudinal acceleration at an early rotation time, wherein said early rotation time comprises said start time of centripetal acceleration plus a fixed offset; and
said longitudinal acceleration at said start time of centripetal acceleration.
7. The system of claim 6 , wherein said fixed offset is in a range between 10 milliseconds and 50 milliseconds.
8. The system of claim 1 , wherein said processor is further configured to
define a swing plane coordinate system comprising
a z-axis along said longitudinal axis;
a y-axis normal to said swing plane; and
an x-axis orthogonal to said y-axis and to said z-axis;
transform said angular velocity data to said swing plane coordinate system, to yield a time series of transformed angular velocities, wherein each transformed angular velocity of said time series of transformed angular velocities comprises a vector sum of
a z-axis angular velocity;
a y-axis angular velocity; and
an x-axis angular velocity;
calculate a time series of rotation-on-plane ratios, each rotation-on-plane ratio of said time series of rotation-on-plane ratios comprising a ratio of
a magnitude of said y-axis angular velocity to
a magnitude of a vector sum of said y-axis angular velocity and said x-axis angular velocity; and,
calculate an on-plane efficiency metric of said one or more swing quality metrics based on said time series of rotation-on-plane ratios.
9. The system of claim 8 , wherein
said processor is further configured to
calculate a start time of centripetal acceleration comprising a time during said swing at or near a time when a longitudinal acceleration changes sign, wherein said longitudinal acceleration comprises said acceleration data projected onto said longitudinal axis; and,
said on-plane efficiency metric comprises
an average of said time series of rotation-on-plane ratios from said start time of centripetal acceleration to said time of impact.
10. The system of claim 1 , wherein said processor is further configured to
calculate a time series of body-bat angles, wherein each body-bat angle of said time series of body-bat angles comprises
an angle between said longitudinal axis and said body tilt axis.
11. The system of claim 10 , wherein said processor is further configured to
calculate a connection-at-impact metric of said one or more swing quality metrics comprising said body-bat angle at said time of impact.
12. The system of claim 11 , wherein said processor is further configured to
calculate a disconnection-at-impact metric of said one or more swing quality metrics comprising an absolute value of a difference between said connection-at-impact and ninety degrees.
13. The system of claim 11 , wherein said processor is further configured to
calculate a start time of centripetal acceleration comprising a time during said swing at or near a time when a longitudinal acceleration changes sign, wherein said longitudinal acceleration comprises said acceleration data projected onto said longitudinal axis; and,
calculate a connection-early metric of said one or more swing quality metrics comprising said body-bat angle at said start time of centripetal acceleration.
14. The system of claim 13 , wherein said processor is further configured to
calculate a disconnection-early metric of said one or more swing quality metrics comprising an absolute value of a difference between said connection-early metric and ninety degrees.
15. The system of claim 13 , wherein said processor is further configured to
calculate a disconnection-at-impact metric of said one or more swing quality metrics comprising an absolute value of a difference between said connection-at-impact metric and ninety degrees;
calculate a disconnection-early metric of said one or more swing quality metrics comprising an absolute value of a difference between said connection-early metric and ninety degrees; and,
calculate an average-disconnection metric of said one or more swing quality metrics comprising an average of said disconnection-at-impact metric and said disconnection-early metric.
16. The system of claim 1 , further comprising
a database of swing quality metrics for a plurality of players; and
an analytics system configured to compare a player or a potential player to said swing quality metrics for said plurality of players.
17. A swing quality measurement system comprising:
a memory;
a processor coupled with said memory;
a first communication interface;
a sensor configured to capture sensor data associated with a bat during a swing of said bat by a user;
a bat geometry stored in said memory, said bat geometry comprising
a hand position along a longitudinal axis of said bat; and
a sweet spot position along said longitudinal axis comprising one or more of
an optimum location on said bat for striking a ball;
a position on said bat that maximizes energy transfer when striking said ball;
a position on said bat that maximizes ball speed when striking said ball; and,
a position on said bat that minimizes vibration when striking said ball;
said processor coupled to said sensor via said first communication interface, and configured to
obtain said bat geometry;
receive said sensor data from said sensor;
obtain or calculate acceleration data from said sensor data;
obtain or calculate angular velocity data from said sensor data;
calculate a trajectory of said hand position from said acceleration data and said angular velocity data;
calculate a trajectory of said sweet spot position from said acceleration data and said angular velocity data;
calculate a trajectory of said longitudinal axis from said acceleration data and said angular velocity data;
determine a time of impact of said swing from said acceleration data and said angular velocity data;
calculate a body tilt axis of said user from said trajectory of said hand position, wherein said body tilt axis is perpendicular to a plane passing through three different points of said trajectory of said hand position;
calculate a swing plane, wherein said swing plane is spanned by a velocity vector of said sweet spot position at said time of impact and said longitudinal axis at said time of impact;
calculate a start time of centripetal acceleration comprising a time during said swing at or near a time when a longitudinal acceleration changes sign, wherein said longitudinal acceleration comprises said acceleration data projected onto said longitudinal axis;
define a swing plane coordinate system comprising
a z-axis along said longitudinal axis;
a y-axis normal to said swing plane; and
an x-axis orthogonal to said y-axis and to said z-axis;
transform said angular velocity data to said swing plane coordinate system, to yield a time series of transformed angular velocities, wherein each transformed angular velocity of said time series of transformed angular velocities comprises a vector sum of
a z-axis angular velocity;
a y-axis angular velocity; and
an x-axis angular velocity;
calculate a time series of rotation-on-plane ratios, each rotation-on-plane ratio of said time series of rotation-on-plane ratios comprising a ratio of
a magnitude of said y-axis angular velocity to
a magnitude of a vector sum of said y-axis angular velocity and said x-axis angular velocity;
calculate a time series of body-bat angles, wherein each body-bat angle of said time series of body-bat angles comprises
an angle between said longitudinal axis and said body tilt axis;
calculate one or more swing quality metrics comprising
a rotational acceleration metric comprising
a difference between
said longitudinal acceleration at an early rotation time, wherein said early rotation time comprises said start time of centripetal acceleration plus a fixed offset; and
said longitudinal acceleration at said start time of centripetal acceleration;
an on-plane efficiency metric comprising
an average of said time series of rotation-on-plane ratios from said start time of centripetal acceleration to said time of impact;
a connection-at-impact metric comprising
said body-bat angle at said time of impact; and,
a connection-early metric of said one or more swing quality metrics comprising
said body-bat angle at said start time of centripetal acceleration.