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 sweet spot position along a longitudinal axis of said bat;
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;
calculate a trajectory of said sweet spot position from said sensor data;
calculate a trajectory of said longitudinal axis from said sensor data;
determine a time of impact of said swing from said sensor data;
calculate a swing plane from said sensor data, 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 time of impact;
said trajectory of said sweet spot position;
said trajectory of said longitudinal 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 acceleration data; and
a three-axis gyroscope that generates angular velocity data.
3. The system of claim 1 , wherein
said bat geometry further comprises
a hand position along said longitudinal axis of said bat;
said processor is further configured to
calculate a trajectory of said hand position from said sensor data.
4. The system of claim 3 , wherein said processor is further configured to calculate one or more of said one or more swing quality metrics based on at least said trajectory of said hand position.
5. The system of claim 4 , wherein said processor is further configured to calculate a body tilt axis of said user from said trajectory of said hand position.
6. The system of claim 5 , wherein said body tilt axis is perpendicular to a plane passing through three different points of said trajectory of said hand position.
7. The system of claim 5 , 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.
8. The system of claim 7 , 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.
9. The system of claim 8 , 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.
10. 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 acceleration of said bat in a direction of 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.
11. The system of claim 10 , 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.
12. The system of claim 10 , 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.
13. 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.
14. 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.
15. 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 acceleration of said bat in a direction of said longitudinal axis.
16. The system of claim 15 , 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.
17. The system of claim 16 , wherein said fixed offset is in a range between 10 milliseconds and 50 milliseconds.
18. The system of claim 1 , wherein said processor is further configured to
obtain or calculate angular velocity data from said sensor data;
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.
19. The system of claim 18 , 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 acceleration of said bat in a direction of 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.
20. 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.