Racket sport inertial sensor motion tracking

BACKGROUND

0001This invention relates generally to motion tracking and analysis, and specifically to tracking and analysis techniques for tennis and other racket sports. In particular, the invention concerns inertial sensor-based techniques for stroke reconstruction and analysis.

0002<patcit id="pcit0001" dnum="US2006184336A"><text>US 2006/184336</text></patcit> shows a highly miniaturized electronic data acquisition system which includes MEMS sensors that can be embedded onto moving device without affecting the static/dynamic motion characteristics of the device. The basic inertial magnetic motion capture (IMMCAP) module consists of a 3D printed circuit board having MEMS sensors configured to provide a tri-axial accelerometer; a tri-axial gyroscope, and a tri-axial magnetometer all in communication with analog to digital converters to convert the analog motion data to digital data for determining classic inertial measurement and change in spatial orientation (rho, theta, phi) and linear translation (x, y, z) relative to a fixed external coordinate system as well as the initial spatial orientation relative to the know relationship of the earth magnetic and gravitational fields. The data stream from the IMMCAP modules will allow the reconstruction of the time series of the 6 degrees of freedom for each rigid axis associated with each independent IMMCAP module. <patcit id="pcit0002" dnum="FR2829700A1"><text>FR 2829700 A1</text></patcit> shows a tennis racket player sequence movement analysis having movement detector with three plane static accelerometer measuring movement and transmitter transmitting movement size signals function measured components. <patcit id="pcit0003" dnum="US2004259651A1"><text>US 2004/259651 A1</text></patcit> shows a method and arrangement for detecting movement-parameters in a moving object. The parameters include acceleration and angular velocity of the object, the arrangement includes an Inertial Navigation System (INS) having at least one gyroscope and accelerometer for measuring an acceleration, angular velocity and effect of attraction of gravity on the sporting equipment.

SUMMARY

0003This invention is directed to a racket system for stroke analysis, and methods of using the system. The racket system includes a racket with a grip, a throat coupled to the grip, and a head coupled to the throat.

0004An inertial sensor is coupled to the racket, where the inertial sensor has an accelerometer array with three degrees of freedom in acceleration and an angular rate (or gyro) array with three degrees of freedom in rotation. A processor is connected to the inertial sensor, where the processor is configured to generate stroke profiles describing acceleration and rotation of the racket based on signals from the accelerometer and the gyro arrays. A memory device is connected to the processor, and configured to store the stroke profiles.

BRIEF DESCRIPTION OF THE DRAWINGS

0005<ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">FIG. 1A</figref> is a schematic view of an inertial sensor-based racket tracking and stroke analysis system.</li><li><figref idref="f0001">FIG. 1B</figref> is a schematic view of the racket tracking and stroke analysis system, in an external processor configuration.</li><li><figref idref="f0002">FIG. 2</figref> is schematic illustration of a body coordinate frame for the racket of <figref idref="f0001">FIGS. 1A and 1B</figref> .</li><li><figref idref="f0003">FIG. 3A</figref> is a plot of accelerometer time histories for a racket tracking system, in three spatial dimensions.</li><li><figref idref="f0003">FIG. 3B</figref> is an expanded plot of the accelerometer time histories in <figref idref="f0003">FIG. 3A</figref> , showing individual strokes.</li><li><figref idref="f0004">FIG. 4A</figref> is a plot of angular rate time histories for a racket tracking system, in three rotational dimensions.</li><li><figref idref="f0004">FIG. 4B</figref> is an expanded plot of the angular rate time histories in <figref idref="f0004">FIG. 4A</figref> , showing individual strokes.</li><li><figref idref="f0005">FIG. 5A</figref> is a plot of superposed stroke segments based on the accelerometer time histories of <figref idref="f0003">FIGS. 3A and 3B</figref> .</li><li><figref idref="f0005">FIG. 5B</figref> is a plot of superposed stroke segments based on the angular rate time histories of <figref idref="f0004">FIGS. 4A and 4B</figref> .</li><li><figref idref="f0006">FIG 6</figref> is a schematic illustration of a wind tunnel apparatus for aerodynamic analysis of aerodynamic forces and moments on the racket.</li><li><figref idref="f0007">FIG. 7</figref> is a schematic illustration of a motion capture system for calibrating a racket tracking system.</li><li><figref idref="f0008">FIG. 8A</figref> is a phase space plot defining a racket trajectory as a function of time, in three-dimensional position vector space.</li><li><figref idref="f0008">FIG. 8B</figref> is a phase space plot defining the racket trajectory in three-dimensional acceleration vector space.</li><li><figref idref="f0008">FIG. 8C</figref> is a phase space plot defining the racket trajectory in three-dimensional gyro angular rate vector space.</li></ul>

DETAILED DESCRIPTION

0006<figref idref="f0001">FIG. 1</figref> is a schematic view of racket tracking and stroke analysis system 10. System 10 includes inertial sensor 14 and processor 16 for racket 12. Sensor 14 provides an accelerometer array with three degrees of freedom in acceleration and a gyro array with three degrees of freedom in rotational angle.

0007Processor 16 is connected to sensor 14 for racket tracking and stroke analysis based on signals from the accelerometer and gyro arrays, including racket interactions with ball 18. Processor 16 performs real-time (on-line) racket tracking and stroke analyses for use in interactive play, training and competition, including aerodynamic analysis as described below.

0008Racket (or racket system) 12 is formed with head 20 connected to grip or handle 24 at neck or throat 22. Head 20 includes a generally oblong, round or oval face portion 26, extending between opposite sides of rim or frame 28. Sensor 14 and processor 16 are typically attached to throat section 22, grip 24 or frame 28, or mounted inside racket 12, for example in a recess or internal cavity.

0009In some configurations, tracking markers 30 are attached to head 20, throat section 22 or grip 24 of racket 12, for use with a motion capture system or other electronic tracking system, as described below. In other configurations, tracking markers 30 are absent.

0010In the particular example of <figref idref="f0001">FIG. 1A</figref> , face 26 of tennis racket 12 is formed as a web or bed of strings that are stretched or strung across frame 28 to form a contact surface for ball 18. Alternate configurations include stringed rackets 12 for racquetball, racquets, hard rackets, squash, badminton and other racket sports, and paddle-type configurations for table tennis (ping pong), paddle ball and other paddle games. The configurations of head 20, throat section 22 and grip 24 vary accordingly, and ball 18 encompasses both ball and shuttle-type components for use with the various rackets and paddles 12.

0011Sensor 14 provides six degrees of freedom for tracking rigid-body motions of racket 12 as a function of time, including three degrees of freedom in angular position or rotation, and three degrees of freedom in spatial position or displacement. Suitable sensor array devices include, but are not limited to, micro-electronic-mechanical-sensor (MEMS) inertial measurement sensors (IMU), magnetic sensors and piezoelectric sensors.

0012Processor 16 is connected to sensor 14 via wire or data bus 32, or using a wireless connection. Processor 16 includes a data interface with memory and microprocessor (cpu) components to communicate with sensor 14, and to store, process and analyze motion tracking data for racket 12. Processor 16 may also include a transmitter for transmitting raw sensor data, stroke profiles, stroke trajectories and other output to an additional linked microprocessor (µP) 34, for example a linked personal computer (PC), laptop, tablet computer, smartphone, or other portable data processing system linked to processor 16 of "smart" racket system 12.

0013Whether alone or in combination with linked microprocessor 34, processor 16 provides racket tracking system 10 with on-line motion tracking and analysis and cueing capability based on real-time stroke reconstruction and analysis during training, interactive play, and competitive activities with racket 12. Racket tracking system 10 may also provide extended (off-line) motion tracking and analysis capability, as described below, for additional aerodynamic and kinematical studies of racket 12 and ball 18.

0014<figref idref="f0001">FIG. 1B</figref> is a schematic view of racket tracking system 10, in an external processor configuration. In this configuration, processor 16 is mechanically decoupled from racket 12, and connected to sensor 14 using a flexible wire connection 32, or a wireless connection. This reduces the on-racket mass contribution of tracking system 10, for improved balance and performance.

0015As shown in <figref idref="f0001">FIG. 1B</figref>, inertial sensor 14 may include a combination of inertial accelerometer sensors 14A, inertial gyro sensors 14G and magnetic sensors 14M. Typically, sensors 14A, 14G and 14M are provided in triple-axis array configurations, in order to generate signals for acceleration, rotation and magnetic field measurements along three different axes, with three degrees of freedom in each measurement.

0016Processor 16 can be provided with a battery pack or other power source 16P, for increased computational power and operational life. Transmitter/processor 16 may also include supplemental processor components 16S, such as a breakout board, data interface or flash memory system, and wireless or hard-wired transmitter 16T, for communication with linked microprocessor 34 via data port 36 and data link 37, via wireless signals, or a combination thereof. In these configurations, processor 16 is also referred to as transmitter 16, or a combined processor/transmitter 16.

0017Battery pack 16P, supplemental processor components 16S and transmitter 16T can be provided in lightweight, relatively low power configurations, so that processor 16 has substantially similar data processing and power usage capabilities in both integrated (on-racket) and external (off-racket) configurations. In particular, lightweight processor/transmitter components 16P, 16S and 16T may be internally mounted in throat section 22, grip 24 or frame 28 without substantially altering the balance, stroke and play characteristics of racket 12.

0018<figref idref="f0002">FIG. 2</figref> is schematic diagram illustrating body coordinate frame [X Y Z] for racket system (or "smart racket") 12. Body coordinate frame (or system) [X Y Z] is defined with respect to center of gravity (or center of mass) CG of racket system 12, for example in throat section 22.

0019Center of gravity CG is the point about which head 20 and grip 24 rotate in a free (unconstrained) racket system. Primary axis A of racket system 12 typically extends through center of mass CG, between head 20 and grip 24 in throat section 22.

0020In the particular example of <figref idref="f0002">FIG. 2</figref>, perpendicular coordinate axes [X Y] are defined in the plane of racket face or string bed 26, with the Y axis extending along major axis A of racket system 12. The Z axis is perpendicular to the X and Y axes, extending out of the plane of <figref idref="f0002">FIG. 2</figref> to define right-handed orthogonal coordinate frame [X Y Z].

0021Note, however, that this particular choice of axes is merely representative. Alternatively, the axes can be offset from center of gravity CG or transposed to define body coordinate frame [X Y Z] in either a light-handed or left-handed sense. Other body frames define skew or non-perpendicular coordinate axes, and use more generalized cylindrical or spherical coordinates.

0022Position vector RS represents the position of inertial sensor (or sensor array) 14 with respect to center of gravity CG, as defined in the body centered frame. As shown in <figref idref="f0002">FIG. 2</figref>, for example, sensor 14 is located in an internal cavity in grip 24, along major axis A, together with an integrated processor 16.

0023Sensor 14 is sensitive to three independent degrees of freedom in motion with respect to the position of racket system 12, as defined along coordinate axes [X Y Z], and three additional degrees of freedom with respect to rotation of racket system 12, as defined about coordinate axes [X Y Z]. Processor 16 is connected to sensor 14, and provides motion tracking and analysis based on the acceleration and gyro signals (motion data) from sensor 14.

0024In the wireless, internal processor/transmitter configuration of <figref idref="f0002">FIG. 2</figref>, processor 16 includes a radio frequency (RF), infrared (IR) or other wireless transmitter 16T to communicate with linked microprocessor 34. In some designs, hard-wired data port 36 is also provided, for offline data downloads, calibration, and additional tracking and analysis functions. In additional designs, processor 16 is connected to cueing device (cue generator) 38, in order to provide real-time audio, haptic (e.g., vibration) or optical (light) cues based on the stroke analysis output from processor 16, as described below. These cues are typically used by the player, or can be relayed to the opponent to enable cueing the player's actions.

0025<figref idref="f0003">FIG. 3A</figref> is a plot of accelerometer time histories for a racket tracking system, in three independent spatial dimensions based on body coordinate frame [X Y Z] of <figref idref="f0002">FIG. 2</figref>. In this particular convention, acceleration ax (bottom) is defined along the racket face, perpendicular to the major axis, acceleration ay (middle) is defined along the major axis, perpendicular to ax, and acceleration az (top) is transverse to the racket face, perpendicular to ax and ay.

0026Accelerations ax, ay and az are plotted along the vertical axis in arbitrary units (not necessarily using the same scale), as a function of time on the horizontal (also in arbitrary units, but using the same scale). The acceleration plots thus form stroke histories, with sharp peaks or spikes representing effects of impulse forces (momentum transfer) during ball strikes and interactions with the ground and net (or player). The stroke histories also include effects of inertial, gravitational, aerodynamic and player grip forces on the racket, which are present continuously during and between the individual ball strikes.

0027<figref idref="f0003">FIG. 3B</figref> is an expanded plot of the accelerometer time histories in <figref idref="f0003">FIG. 3A</figref>, showing individual impulse spike train during a stroke cycle. As shown in <figref idref="f0003">FIG. 3B</figref>, stroke and ball interactions can be substantially periodic or quasi-periodic, with variations about a base or fundamental frequency associated with the rhythm associated with the type of play such as groundstroke rally or volley play. Other stroke histories are intermittent or non-repeating, for example service strokes and certain approach shots, spikes, and lobs. Net and ground strikes provide additional intermittent impulse-type features.

0028<figref idref="f0004">FIG. 4A</figref> is a plot of gyro time histories for a racket tracking system. The gyro histories are given in terms of angular velocities ωx (bottom), ωy (middle) and ωz (top), defining gyro rates about coordinate axes X, Y and Z of <figref idref="f0002">FIG. 2</figref>, respectively. The gyro rates are plotted in arbitrary units on the vertical, versus time on the horizontal (also in arbitrary units).

0029The gyro plots of <figref idref="f0004">FIG. 4A</figref> represent rotational stroke histories, corresponding to the acceleration stroke histories of <figref idref="f0003">FIG. 3A</figref>. In particular, the peaks in <figref idref="f0004">FIG. 4A</figref> represent impulse moments (angular momentum transfers) during ball strikes and other racket interactions, corresponding to the simultaneous impulse forces representing linear momentum transfers in <figref idref="f0003">FIG. 3A</figref>. The ball typically strikes the racket with an offset from the CG, resulting in an offset in the impulse. The ball strike therefore causes a simultaneous change in linear and angular momentum of the racket.

0030<figref idref="f0004">FIG. 4B</figref> is an expanded plot of the gyro time histories in <figref idref="f0004">FIG. 4A</figref>, showing a combination of rhythmic (periodic or quasi-periodic; repeating) strokes and intermittent (aperiodic; non-repeating) racket interactions, as described above. The gyro plots also include continuous inertial, gravitational, aerodynamic and player grip moments (or torques), extending between and through the interaction peaks.

0031<figref idref="f0005">FIG. 5A</figref> is a plot of superposed stroke profiles based on the accelerometer time histories of <figref idref="f0003">FIG. 3A</figref>, showing continuous inertial, acceleration, aerodynamic and player grip forces with impulsive momentum transfers during ball strikes and other racket interactions. <figref idref="f0005">FIG. 5B</figref> is a corresponding stroke profile superposition based on the gyro time histories of <figref idref="f0004">FIG. 4A</figref>, showing continuous grip and aerodynamic moments with impulsive angular momentum transfers.

0032To generate the stroke superposition plots of <figref idref="f0005">FIGS. 5A and 5B</figref>, ball strikes and other interaction impulses are identified in the sets of acceleration and gyro data (the stroke histories), and the stroke histories are subdivided (or segmented) into individual stroke segments or samples (stroke profiles) based on the impulse interactions. The interaction impulses are identified by pattern matching and other signal processing and pulse identification algorithms, and the stroke profiles are time-matched based on the leading edge or peak pulse height, or using other pulse matching techniques such as based on the trailing edge, mean interaction time or signal co-variation.

0033<figref idref="f0005">FIGS. 5A and 5B</figref> show that a number of stroke characteristics are shared in the acceleration and gyro stroke histories, while other characteristics are different. In one axis, for example (e.g., the X axis; bottom plots), the stroke profiles are substantially single-peaked in both the acceleration and gyro rate time histories; that is, there is generally one local maximum P between leading edge LE and trailing edge TE.

0034Saturation regions S can be observed in any of the acceleration or gyro rate plots, where the maximum signal amplitude of the inertial sensor or processor is reached. Saturation effects can be addressed by integrating the (saturated) pulse height, by resetting or recalibrating the electronic gain, or by reconfiguring the sensor array with a lower hardware gain.

0035<figref idref="f0005">FIGS. 5A and 5B</figref> also show that not all stroke profiles are the same. In particular, while the X axis acceleration and gyro profiles are typically single-peaked, some strokes have a double-peaked profile with local minima M between leading edge LE and trailing edge TE. These and other stroke characteristics are used to identify different classes or types of stroke profile, and to define stroke repertoires (or classes) based on the stroke characteristics. In addition, different stroke characteristics can be used to define analysis tools that are proper to a particular stroke type, for example forehand and backhand strokes, lobs, approach shots and service strokes.

0036In the Y axis plots (middle), the acceleration profiles are characteristically double peaked; that is, both the acceleration and gyro data generally exhibit first and second local maxima P1 and P2 between leading edge LE and trailing edge TE. In the acceleration data, moreover, the two peaks have similar magnitudes, with first peak P1 generally 10-50% higher than second peak P2, and peaks P1 and P2 separated by a strong local minima M.

0037In the gyro data, however, first peak P1 is substantially smaller than second peak P2, for example only 10-30% as high. In addition to sharp peak profile P2, moreover, with a steep rise and high amplitude, there is also a shallow peak profile P2', with a lower rise and peak amplitude. These additional features and characteristics allow the stroke profiles to be further divided into different classes, for example first and second service strokes, higher and lower lob strokes, and different types of forehand and backhand strokes, with relatively more or less spin (angular momentum) and velocity (linear momentum) transfer. The features come about through interaction of the player's imparted forces and the ball, and therefore also provide diagnosis about the player's skills and technique.

0038In the Z axis plots (top), both the acceleration and gyro time histories show rich pulse structure, with an initial pulse P0 followed multiple additional peaks PP between leading edge LE and trailing edge TE. Thus, the stroked profiles can be further classified according to the number of peaks and relative rise times (slopes) in the Z axis data, for example using a triple-peak feature discriminator with relatively steeper and shallower third (or higher-order) peaks PP. A mathematical model of the racket motion that encompasses all the forces allows estimates of the respective contribution of these forces to the sensed motion, and identifies the physical significance of these features to the player's technique.

0039<figref idref="f0006">FIG. 6</figref> is a schematic illustration of wind tunnel apparatus 40 for aerodynamic analysis of racket system 12. Wind tunnel apparatus 40 includes a turbine, blower or other means for generating airflow within housing 42. Racket system 12 is supported from fairing 44 by strut 46, for example by attachment to throat section 22 or grip 24.

0040Racket sensor 48 is used to determine the aerodynamic forces and moments (or torques) on racket system 12, as a function of racket orientation and flow velocity V. Alternatively, strut sensor 49 may be used, where strut sensor 49 is coupled to racket system 12 via strut 46 and vertical support 47, inside fairing 44.

0041In operation of wind tunnel apparatus 40, airflow is incident on each component of racket system 12, generating aerodynamic forces on head 20, throat section 22, grip 24, face 26 and frame 28. Depending on racket orientation and flow velocity V, therefore, racket sensor 48 and strut sensor 49 will measure different moments and forces.

0042Frame 28 and portions of throat section 22, for example, have substantially airfoil-shaped cross sections, as shown in <figref idref="f0006">FIG. 6</figref>, taken either in a horizontal section along the side of frame 28, or in a vertical section at the top (or bottom) of frame 28. These cross sections define angle of attack α and sideslip angle β, as measured in the horizontal and vertical planes, respectively, and with respect to transverse axis T, perpendicular to face 26 of racket system 12.

0043Flow velocity V and aero angles α and β thus determine the aerodynamic forces and moments on racket system 12 over a full range of different conditions, and across the different stroke profiles in the stroke portfolio. Note, however, that the aero angle definitions are arbitrary, and vary from application to application. In particular, because the airfoil portion of frame 28 is formed as a continuous hoop, it is also possible to take the defining cross sections at different locations on racket system 12, or to transpose aero angles α and β. To generate truth data for correcting the aerodynamic force and moment analysis, therefore, as described below, it is important to maintain consistent definitions, and to carefully transform the aero data between frames.

0044<figref idref="f0007">FIG. 7</figref> is a schematic illustration of motion capture system 50 for calibrating racket system 12. Motion capture system 50 includes one or more wireless motion or position detectors 52, mounted on rails or other support structures 54. Motion detectors 52 are connected to a computer or central motion capture processor (CPU) 56, in order to measure the position and orientation of racket system 12. Motion capture processor 56 utilizes the position and orientation data to generate truth data for racket system 12, including calibration of sensor 14 and verification of the racket tracking and stroke reconstruction algorithms utilized by processor 16.

0045Tracking markers 30 define the orientation and position of racket system 12, including a centroid location for face 26 and head 20. Motion detectors 52 use visual, infrared, radio, audio or other sensing techniques to determine the locations of tracking markers 30, and motion capture processor 56 uses these locations to generate truth data describing the position and orientation or racket system 12, as expressed in the stationary (ground-based) frame of motion capture system 50. Optionally, motion is tracked based on the racket shape, without dedicated markers 30. Additional truth data are provided by magnetic sensors, for example magnetic sensors 14M of <figref idref="f0001">FIG. 1B</figref>, in either an internal or external sensor configuration.

0046The truth data are used to calibrate sensor 14 during interactions with ball 18, and to generate high fidelity stroke reconstruction by verifying and tuning the racket tracking algorithm implemented in processor 16. This allows processor 16 to determine the aerodynamic forces and torques on racket system 12, and to distinguish between the aerodynamic effects, the inertial forces and the player reaction forces on grip 24, and the impulse forces due to the ball 18 and other external interactions.

0047High fidelity stoke reconstruction also provides more accurate stroke trajectories in position, velocity and acceleration space, for both linear and angular variables. More accurate stroke trajectories, in turn, provide additional information for the stroke characterization tools as described below.

0048<figref idref="f0008">FIG. 8A</figref> is a phase space plot defining a racket trajectory as a function of time, in three-dimensional position vector space. The centroid of the racket system follows a three-dimensional path in this vector space, for example in the ground-based reference frame of motion capture system 50 as shown in <figref idref="f0007">FIG. 7</figref>, above. Alternatively, the trajectory is defined along orthogonal racket coordinates [X Y Z], or in another reference frame or coordinate system attached to the player or racket system.

0049In contrast to the superposed stroke profiles of <figref idref="f0005">FIGS. 5A and 5B</figref>, the racket trajectory of <figref idref="f0008">FIG. 8A</figref> is continuous (unsegmented), and follows the racket over a number of consecutive stroke segments. The three-dimensional trajectory also combines the concurrent information from the three acceleration and gyro axes, rather than considering them individually as in <figref idref="f0003">FIGS. 3A</figref> and <figref idref="f0004">4A</figref>.

0050<figref idref="f0008">FIG. 8B</figref> is a phase space plot for the racket trajectory of <figref idref="f0008">FIG. 8A</figref>, in three-dimensional acceleration vector space. <figref idref="f0008">FIG. 8C</figref> is the corresponding phase space in three-dimensional gyro rate vector space. These data can be determined from raw accelerometer and gyro data, taken directly from the inertial sensor signals, and corrected based on truth data from a motion capture system or magnetic sensor array. Alternatively, the acceleration and gyro rate trajectories can be generated by applying appropriate kinematic transformation on the position space trajectory of <figref idref="f0008">FIG. 8A</figref>, where the truth data have already been incorporated into the trajectory reconstruction.

0051EXAMPLES. The following examples describe applications of racket tracking system 10 and smart racket system 12, as shown in <figref idref="f0001">FIGS. 1A, 1B</figref> and <figref idref="f0002">2</figref>, either alone or in combination with wind tunnel apparatus 40 of <figref idref="f0006">FIG. 6</figref> and motion capture system 50 of <figref idref="f0007">FIG. 7</figref>. In addition, the examples describe methods of stroke reconstruction and analysis based on tracking system 10 and smart racket 12, as illustrated in <figref idref="f0003">FIGS. 3A, 3B</figref>, <figref idref="f0004">4A, 4B</figref>, <figref idref="f0005">5A, 5B</figref>, <figref idref="f0008">8A, 8B and 8C</figref>.

0052These examples employ a variety of different motion tracking techniques and technologies, including micro-electronic-mechanical-sensor (MEMS) inertial measurement sensors (IMU), wind tunnels and wireless motion capture systems, in order to provide a more comprehensive understanding of racket and player performance during live play and competition. The examples also encompass additional performance factors including, but not limited to, racket weight, balance and rigid-body motion characteristics, head size, stringing characteristics, aerodynamic effects and biomechanical effects. Further, the examples can be applied in any order or combination to improve racket tracking and stroke and human performance analysis.

0053As a first step, racket tracking system 10 provides an integrated sensor, processor and transmitter configuration that can be embedded into a wide range of different racket and paddle designs, without substantially interfering with playability. As a second step, racket system 12 provides the processing capabilities needed to support real-time inertial sensor data analysis, for high fidelity stroke reconstruction and detailed, accurate stroke analysis. As a third step, racket tracking system 10 and racket system 12 provide methods of stroke and player analysis directed to the goal of identifying key relationships between physical racket characteristics and player performance, for better equipment/player matching and improved competitive ability. Such analysis can be executed offline, for post play analysis, or can be executed during play where the information from the analysis can be provided to the player in real time to provide cueing for training or shared with opponent player to enable interactive play features.

0054A number of technical innovations also provide for a more practical embedded motion tracking capability. These include primary capabilities for basic stroke analysis, and secondary capabilities based on the relationship between stroke analysis and physical racket characteristics. Advanced capabilities are also developed, for real-time interactive features blending racket characteristics, stroke analysis, and human (biomechanical) effects in live-game racket sports, both for training purposes and as applied to competitive play.

0055The racket systems and stroke analysis methods described here are also based on aerospace techniques, combining elements of aerodynamics, dynamics, kinematics, inertial and magnetic sensing technology, state vector (trajectory) estimation and finite element analysis. Under this approach, the racket is treated as an airframe, which is under the effect of external forces including gravity, aerodynamic forces, player forces on the grip, and interaction forces with the ball, ground or net. Under this more complete combined approach, these individual effects can be more effectively decoupled to enable detailed stroke reconstruction and analysis, for more complete feedback on player and equipment performance.

0056In particular, racket trajectories (stroke motions) are reconstructed using measurements from advanced, lightweight, inertial sensors, which provide three independent degrees of freedom in both translation and rotation, for a total of six degrees of freedom. The equations of motion are developed from the fundamental laws of rigid-body translation and rotation, and include gravity and aerodynamic effects across the full range of stroke profiles and trajectories.

0057This approach simultaneously determines force and moment (torque) contributions from the stroke impulse (or ball interaction) and the player's reaction on the grip, using the airframe analysis to account for additional aerodynamic forces and moments. This allows the stroke and grip data to be decoupled from the aero forces, in order to analyze the stroke profile and determine relationships between stroke motions and the physical characteristics of the racket, and to determine how each of the individual forces and moments contributes to overall racket and player performance.

0058EXAMPLE 1: Racket Hardware. As shown in <figref idref="f0002">FIG. 2</figref>, miniature MEMS inertial (IMU) sensor 14 provide six degrees of freedom (DOF) for inertial measurements of racket system 12, including three independent DOF for acceleration and three additional independent DOF for rates of angular rotation. With a mass of only a few grams or less, sensor 14 can be integrated into a wide range of racket systems 12 without substantially altering balance and other physical characteristics, and without substantial interference with playability and performance. In addition, advanced micro-controller designs also allow processor 16 to provide real-time stroke analysis and interactive performance feedback in both external and internal (integrated) processor/transmitter configurations, including audio, visual and haptic signals from cueing device 38.

0059More specifically, processor 16 provides first-stage analysis capability for real-time stroke (racket flight path) reconstruction based on inertial data from sensor 14, encompassing all the individual forces and moments acting on racket system 12. In addition, processor 16 also provides for automatic segmentation of the stroke data into individual stroke profiles, and identifies key stroke attributes and performance parameters such as racket speed, spin, turning rates, and force and moment impulses as well as variations in these parameters.

0060Data from processor 16 can also be used for second stage and offline data analyses, either alone on in combination with an additional linked microprocessor 34. In this stage of analysis, spin, impulse and other stroke attributes can be used to investigate different racket characteristics, including, but not limited to, racket mass, balance, head size, grip size, and string tension and composition.

0061Additional analysis tools are provided to support ensemble-based stroke pattern analysis, as described below, using statistical measures of the stroke characteristics to categorize individual strokes into different stroke types. Statistical pattern analysis also enables stroke recognition needed for new interactive cueing systems and live play analysis techniques.

0062EXAMPLE 2: Motion Sensing and Analysis. This example describes additional technologies and hardware instrumentations utilized in racket system 12 of <figref idref="f0001">FIGS. 1A, 1B</figref> and <figref idref="f0002">2</figref>. Sensor 14, for example, may incorporate a gyro, accelerometer and magnetic sensor/IMU with nine degrees of freedom, such as a SPARKFUN® - Razor IMU, available from SparkFun electronics of Boulder, Colorado. In this configuration, sensor 14 includes a triple-axis accelerometer sensor array 14A for linear accelerations along three independent axes, a triple-axis gyro sensor array 14G for rotations about the three axes, and a triple-axis magnetic sensor array 14M for magnetic field measurements along the three axes.

0063As so configured, sensor 14 has a mass of about 7 grams. The accelerometer sensitivity range (maximum amplitude) is about 16g (about 150 m/s²), and the gyro range is about 300 deg/sec (about 5.25 rad/sec).

0064The accelerometer and gyro data from sensor 14 are calibrated using truth data from an independent tracking system, such as motion capture system 50 of <figref idref="f0007">FIG. 7</figref>. The magnetic field sensors also provide truth data for calibration and testing, measuring the local magnetic field strength with sufficient sensitivity to determine the relative orientation of the accelerometer and gyro axes with respect to a fixed (ground-based) reference frame.

0065Sensor 14 is connected to processor 16 via a data bus or cable, for example a universal serial bus (USB) or other cabling standard, or using a wireless connection. Processor 16 includes a microcontroller/microprocessor with an interface for data communications with sensor 14, a lightweight battery pack or other power supply 14P, transmitter 14T, and a memory system or other supplemental component 14S. In one particular example, processor 16 incorporates a GUMSTIX VERDEX PRO microcontroller, as available from Gumstix Inc. of Portola Valley, California, configured for writing to a microSD card or other flash memory system 14S, with communications to linked microprocessor 34 via a BLUETOOTH® transmitter 14T, or other wireless interface.

0066In operation of racket tracking system 10, sensor 14 and processor 16 are attached to an outside surface of racket system 12, or placed in an internal cavity. For example, racket tracking system 10 may provide sensor 14 and processor 16 as a motional analysis unit or module, separate from racket 12, where modular system 10 comprises sensor 14 and processor 16 for attachment to racket 12 using a strap, VELCRO, tape, adhesive or other means. Alternatively, racket tracking system 10 may comprise modular sensor 14 for attachment to racket 12 and modular processor 16 for wearing by or attachment to a player, for example using straps, VELCRO, tape, adhesives or other means to attach processor 16 to the player's wrist, arm or hip, and a data wire, USB cable, wireless connection or other means to connect processor 16 to sensor 14 for communications, data transfer, or both.

0067Processor 16 reads accelerometer, gyro and magnetometer data from sensor 14, and performs preliminary (first stage) motion tracking and stroke analysis in real time, writing stroke histories, stroke profiles, stroke trajectories or other output to flash memory system 14S. Depending on configuration, processor 16 can also download data to linked microprocessor 34 via transmitter 14T or data port 36, or a combination thereof, for additional (second stage, offline) analysis and processing.

0068In advanced configurations of Racket system 12, sensor 14 may have a mass of about 1 gram or less, with acceleration and angular measurement ranges suitable for high-performance players, for example 16-bit gyro sensors with a ±2000 deg/sec (about 35 rad/sec) sensitivity range, and 13-bit accelerometer sensors with a ±16 g (about ±150 m/s²) range. In addition, processor 16 may have a mass of about 11 grams or less, for example using GUMSTIX® OVERO® AIR microcontroller with a GUMSTIX® PINTO-TH breakout board.

0069In additional configurations, processor 16 may include one or more of a ARDUINO PRO interface 16S with a mass of 2 grams or less, as available from Atmel Corporation of San Jose, California, a polymer lithium ion battery pack 16P with a mass of about 18.5 grams or less, and a power converter 16S with a mass of 1.6 grams or less. In addition, processor 16 may include audio, visual or haptic signal cueing devices 38 for real-time feedback. Suitable cueing devices 38 include, but are not limited to, an audio signal generator such as a buzzer or beeper, a piezoelectric or electromagnetic vibrator, and an LED light or small LED or LCD screen.

0070Processor 16, whether alone or in combination with linked microprocessor 34, provides capabilities for real-time data acquisition, collection, storage and processing, including state estimation for stroke trajectories describing angular and linear accelerations, velocities and positions, and real-time interactive feedback. In these configurations, processor 16 includes one or more of a VERDEX PRO microcontroller, an OVERO AIR embedded computer board 16S, a PINTO-TH breakout board 16S, and an ARDUINO PRO interface 16S, as described above.

0071In each of these configurations, processor 16 provides sufficient computing power for real-time first-stage tracking and stroke analysis. In one such configuration, processor 16 runs a Linux-based operating system on CORTEX™-A8 processor with 256MB of RAM, as available from ARM Holdings plc of Cambridge, England (UK).

0072Breakout board 16S can be configured to provide a USB port 36, along with digital communication lines for reading and writing from sensor 14. Sensor data (motion and magnetic signals) are collected from sensor 14 at rates from 100-500 Hz, or up to 3.2 kHz or more, and stored in a microSD card or other flash memory system 16S with 1.6 GB or more memory for second-stage (offline) analysis. In addition, processor 16 may also include a transmitter 16T with Wifi or BLUETOOTH capabilities, or a combination thereof, for transferring data to a PC or other linked microprocessor 34. Such data transfer can be used to receive and send data to enable interactive play with opponent.

0073EXAMPLE 3: IMU Measurements, Data Processing and Acceleration. In use of racket system 12 as shown in <figref idref="f0002">FIG. 2</figref>, processor 16 analyzes raw acceleration, angular and magnetic field data from sensor 14. The accelerometers, gyros and magnetic field arrays generate sensor signals at the mounting point of sensor 14, given by position vector RS as defined with respect to center of gravity CG in racket body frame [X Y Z].

0074The acceleration in the racket body frame (a_s') is related to the acceleration in an "inertial" (ground-based) frame by: <maths id="math0001" num="[1]"><math display="block"><msubsup><mi mathvariant="normal">a</mi><mi>S</mi><mo>′</mo></msubsup><mo>=</mo><msub><mi mathvariant="normal">a</mi><mi>S</mi></msub><mo>−</mo><msub><mi mathvariant="normal">R</mi><mi>B</mi></msub><msub><mi mathvariant="normal">g</mi><mi>I</mi></msub><mo>,</mo></math><img file="EP3385937B1_D0001.tif" /></maths> where a_s is the acceleration in the ground frame, R_B is the rotation matrix or tensor defining the transformation from the ground frame to the racket body frame, and g_I = [009.81] is the three-dimensional gravitational acceleration vector (or covector), as defined in the ground frame.

0075The angular motion of racket system 12 is taken into account via angular rate vector Ω = [ω_x ω_y ω_z]', as defined in the racket body frame. Based on the kinematics of the respective reference frames, the acceleration in the racket body coordinate system is: <maths id="math0002" num="[2]"><math display="block"><msubsup><mi mathvariant="normal">a</mi><mi>s</mi><mo>′</mo></msubsup><mo>=</mo><mfrac><mi mathvariant="normal">F</mi><mi>m</mi></mfrac><mo>+</mo><mi mathvariant="normal">Ω</mi><mo>×</mo><mfenced separators=""><mi mathvariant="normal">Ω</mi><mo>×</mo><msub><mi>R</mi><mi>S</mi></msub></mfenced><mo>+</mo><mover><mi mathvariant="normal">Ω</mi><mo>.</mo></mover><mo>×</mo><msub><mi>R</mi><mi>S</mi></msub><mo>,</mo></math><img file="EP3385937B1_D0002.tif" /></maths> where F is the vector sum of the external forces (player grip forces, aerodynamic forces, gravity and ball impact or interaction forces). The second term in this expression is the centripetal acceleration, and the third term is the tangential acceleration.

0076To define stroke histories, stroke segments, stroke profiles, stroke trajectories and stroke ensemble data, racket system 12 with sensor 14 was utilized in a variety of ground strokes, rally and volley play. Using one or both of processor 16 and linked microprocessor 34, motion tracking and stroke analysis data were generated to verify that the accelerometer, gyro and magnetic sensor ranges (and other specifications) were suited for applications in tennis and other racket and paddle sports.

0077Rather than treating all the stored data together as a bulk signal train, the accelerometer and gyro measurements (stroke histories) are segmented into stroke profiles based on pattern matching and identification of impulse peaks, and the stroke profiles are divided into stroke types based on selected stroke characteristics and racket motions. In this approach, individual stroke profiles are identified, stored and analyzed as single movement units, and grouped or assimilated into stroke ensembles. Statistical pattern analysis techniques are applied to the stroke ensemble data in order to provide more comprehensive stroke characteristics, such as dependence on physical racket parameters and player biomechanics, as well as the changes of these characteristics over time.

0078<figref idref="f0003">FIGS. 3A</figref> and <figref idref="f0004">4A</figref> show the linear acceleration and angular rate vector components for a complete rally, respectively, and <figref idref="f0003">FIGS. 3B</figref> and <figref idref="f0004">4B</figref> show an expanded time slice for selected strokes within the rally. In <figref idref="f0005">FIGS. 5A and 5B</figref>, the strokes are segmented and superposed, in linear acceleration space and angular rate space, respectively, in order to analyze stroke patterns and characteristics.

0079The superposition plots of <figref idref="f0005">FIGS. 5A and 5B</figref> show distinctive patterns in both the acceleration and angular rate (gyro) data. This demonstrates the feasibility these techniques for measuring different stroke characteristics, including single-peak, double-peak and multiple-peak analysis, as described above.

0080To increase utility for training, competition and research, the stroke profile data in <figref idref="f0003">FIGS. 3A, 3B</figref>, <figref idref="f0004">4A, 4B</figref>, <figref idref="f0005">5A and 5B</figref> are processed to extract actual stroke trajectories. Actual trajectory estimation and reconstruction requires substantial modeling and analysis. While similar techniques are utilized in flight control and navigation for aerospace applications, moreover, the complexity and nonlinearity of these techniques makes it difficult or impossible to accurately predict results for any given racket system 12, based on generic aerospace results. In particular, the grip, aerodynamic, gravitational and interaction (ball impact) force and moment contributions must all be considered in order to fully reconstruct the racket trajectories (kinematics), forces (dynamics) and moments (rotational effects), and this is impossible without considering the particular configuration of racket system 12.

0081EXAMPLE 4: Racket Motion and Stroke Trajectory Reconstruction. In general, the primary driver of any given stroke is provided by player forces on grip 24. Ultimately, however, the trajectory of racket system 12 is determined by the laws of motion, which require six degrees of freedom to represent rotation and translation in all three spatial dimensions. Full reconstruction of the stroke profile thus requires that all substantial forces and moments be considered, including grip forces, inertial, gravity and interaction impulses, as well as aerodynamic forces that depend on the flight mechanics of racket system 12.

0082Sensor 14 is used to measure the acceleration and angular (rotation) rates of racket system 12, as described above. Processor 16 provides the corresponding data processing capability for real-time (first-stage) and offline (second-stage) motion tracking and stroke analysis, either alone or in combination with linked microprocessor 34.

0083These processing capabilities include state estimation algorithms to reconstruct the three dimensional flight path (trajectory) of racket system 12, as well as aerodynamic modeling algorithms, for example based on data from wind tunnel apparatus 40 of <figref idref="f0006">FIG. 6</figref>. In addition, validation and calibration algorithms are utilized, based on truth data from magnetic sensors and other independent measurement systems, for example motion capture system 50 of <figref idref="f0007">FIG. 7</figref>. Processor 16 also provides force and moment estimation algorithms for reconstructing the external forces and moments acting on racket system 12, during individual stroke interactions.

0084Processor 16 utilizes these and other algorithms to determine angular motion parameters (roll, pitch and yaw angle), velocities (v_x, v_y and v_z), and trajectory or position data (x, y and z) from the acceleration and angular rates measured by sensor 14. In addition, processor 16 employs state estimation methods that combine the sensor measurements with a mathematical model of the racket dynamics to provide a more accurate computation of these quantities, including positions, velocities and accelerations in linear and rotational space.

0085The equations of motion are used to model racket system 12 as a rigid body, with known moment of inertia I and mass m. In the body frame reference system, with origin located at the center of gravity, the equations of motion are: <maths id="math0003" num="[3]"><math display="block"><mi mathvariant="normal">m</mi><mover accent="true"><mi mathvariant="normal">x</mi><mo>¨</mo></mover><mo>+</mo><mi>m</mi><mfenced separators=""><mi mathvariant="normal">Ω</mi><mo>×</mo><mover accent="true"><mi mathvariant="normal">v</mi><mo>¨</mo></mover></mfenced><mo>=</mo><mi mathvariant="normal">F</mi></math><img file="EP3385937B1_D0003.tif" /></maths> and <maths id="math0004" num="[4]"><math display="block"><mi mathvariant="normal">I</mi><mi mathvariant="normal">Ω</mi><mo>+</mo><mi mathvariant="normal">Ω</mi><mo>×</mo><mfenced><mi>IΩ</mi></mfenced><mo>=</mo><mi mathvariant="normal">M</mi><mo>,</mo></math><img file="EP3385937B1_D0004.tif" /></maths> where x is the racket position vector and Ω = [ωx ωy ωz]' is the racket angular rate vector. The external force and moment (or moment) vectors are F = [F_x F_y F_z]' and M = [M_x M_y M_z]', respectively.

0086The equations of motion result in six coupled, nonlinear differential equations for translations and rotations with respect to three independent axes X, Y and Z. The force and moment vectors include individual contributions from the gravitational force (F_g = R_Bg_I), aerodynamic forces F_a and moments M_a, impact/interaction (ball) forces F_i and moments M_i, and player grip (reaction) forces F_p and moments M_p.

0087Gravitational forces F_g are given by racket mass m, based on the composition and physical configuration of the racket. Aerodynamic forces F_a are a function of racket airspeed (relative velocity) and standardized aerodynamic coefficients, for example as extracted using wind tunnel apparatus 40 of <figref idref="f0006">FIG. 6</figref>. Impact forces F_i are estimated based on an impulsive force (large magnitude, short duration) model, making it possible to substantially isolate impact forces F_i and impact moments M_i from the other forces and moments.

0088EXAMPLE 5: Aerodynamic Forces and Moments. Racket head 20 may reach considerable speeds during individual strokes, for example up to 30-40 m/sec or more. This prompts racket manufacturers to minimize drag, and most racket systems 12 are designed with streamlined frame 28 and throat section 22.

0089Based on the relative air velocity, racket system 12 will thus produce lift and drag moments and forces during each stroke. The effects are similar to that of an airfoil, but again it is not practical to generalize generic results from aerospace research because of the very different racket geometry. This includes the hoop-shaped airfoil construction of frame 28, and the additional airfoil features of throat section 22, requiring the used of wind tunnel apparatus 40 as described above with respect to <figref idref="f0006">FIG. 6</figref>, or a similar engineering facility.

0090With careful, system-specific measurements, however, aero forces F_a and moments M_a can be modeled using standard aerodynamic coefficients. The racket lift force, for example, is given by: <maths id="math0005" num="[5]"><math display="block"><mi>L</mi><mo>=</mo><mfrac><mn>1</mn><mn>2</mn></mfrac><msup><mi mathvariant="italic">ρV</mi><mn>2</mn></msup><msub><mi mathvariant="italic">SC</mi><mi>L</mi></msub><mo>,</mo></math><img file="EP3385937B1_D0005.tif" /></maths> where ρ is the air density, V is the relative air velocity and C_L is the racket lift coefficient. Similarly, the pitching moment is given by: <maths id="math0006" num="[6]"><math display="block"><mi>M</mi><mo>=</mo><mfrac><mn>1</mn><mn>2</mn></mfrac><msup><mi mathvariant="italic">ρV</mi><mn>2</mn></msup><msub><mi>S</mi><mi>C</mi></msub><msub><mi>C</mi><mi>M</mi></msub><mo>,</mo></math><img file="EP3385937B1_D0006.tif" /></maths> where C_M is the pitching (moment) coefficient and S_C is planform area times the chord length, or the projection of the planform area onto the chord length.

0091Racket head speed can be sufficiently large to produce substantial aerodynamic forces F_a and moments M_a, which depend on relative velocity V of airflow over the racket frame. Angle of attack α and side-slip angle β are thus relevant variables, but these vary over the stroke based on the relative velocity and orientation of the racket head with respect to the surrounding air, as described in <figref idref="f0006">FIG. 6</figref>, above. Analogous to an aircraft, the aerodynamic effects vary with angle of attack α and sideslip angle β, providing the player with some degree of control by adjusting the aerodynamic angles of the racket.

0092Existing techniques do not consider these effects on stroke technique and performance. Racket tracking system 10, in contrast, considers aerodynamic forces F_a and moments M_a as part of the overall stroke analysis, including the effect of aerodynamic control mechanisms on racket and player performance.

0093In particular, aerodynamic effects are the only other contribution, besides the player grip forces and moments, available to the player to affect the racket flight path. Thus, by considering aerodynamic forces F_a and moments M_a together with player grip forces F_p and moments M_p, racket tracking system 10 provides a more complete model of the equations of motion, in which unknown interaction forces F_i and moments M_i are more fully constrained, and can be unambiguously determined.

0094The aerodynamic forces and moments are described by the aerodynamic coefficients C_L (lift coefficient), C_D (drag coefficient), and C_M (pitching moment coefficient), as described above. The aero coefficients, in turn, are obtained from wind tunnel experiments, as a function of the measured lift L, drag D, and pitching moment M for different values of airspeed V, angle of attack α and side-slip angle β. Thus, <maths id="math0007" num="[7]"><math display="block"><msub><mi>C</mi><mi>L</mi></msub><mo>=</mo><msub><mi>f</mi><mi>L</mi></msub><mfenced><mi>V</mi><mi>α</mi><mi>β</mi></mfenced><mo>,</mo></math><img file="EP3385937B1_D0007.tif" /></maths><maths id="math0008" num="[8]"><math display="block"><msub><mi>C</mi><mi>D</mi></msub><mo>=</mo><msub><mi>f</mi><mi mathvariant="italic">DL</mi></msub><mfenced><mi>V</mi><mi>α</mi><mi>β</mi></mfenced><mo>,</mo></math><img file="EP3385937B1_D0008.tif" /></maths> and <maths id="math0009" num="[9]"><math display="block"><msub><mi>C</mi><mi>M</mi></msub><mo>=</mo><msub><mi>f</mi><mi>M</mi></msub><mfenced><mi>V</mi><mi>α</mi><mi>β</mi></mfenced><mo>.</mo></math><img file="EP3385937B1_D0009.tif" /></maths> This allows the individual aerodynamic coefficients to be extracted based on the wind tunnel data, for example using wind tunnel apparatus 40 of <figref idref="f0006">FIG. 6</figref>.

0095EXAMPLE 6: State Vector Estimation and Verification. One approach to state vector estimation is to use a Kalman filter. Similar techniques for generating state vector estimates are based on prior knowledge and real-time measurements, with variance or uncertainty included in the outputs. Regardless of approach, however, the combination of external (player grip), aerodynamic, gravitational and interaction (ball impact) forces and moments on racket system 12 presents substantial analytical challenges for processor 16, which are not adequately addressed by existing motion tracking and stroke reconstruction techniques.

0096In racket system 12, on the other hand, processor 16 takes advantage of particular features that are characteristic of the individual stroke profiles. In particular, the racket trajectory is constrained by the periodic nature of the stroke motion, and the impulsive nature of the ball (interaction) forces and moments allows a reliable state estimation algorithm to be developed across racket impacts. Processor 16 also utilizes racket state estimation algorithms that account for changes in the angular rates and accelerations outside of the interaction (or impact) window, providing reliable estimates of the player forces and moments on grip 24, based on a full understanding of the corresponding aerodynamic effects on frame 28 and the other components of racket system 12.

0097These state vector estimation algorithms are also applicable to biomechanical studies, where the changes in the player grip forces are particularly relevant. Truth data are used to tune and verify the state estimation algorithms, based on a combination of racket motion measurements and inertial motion data from sensor 14.

0098The truth data are obtained from a combination of magnetic sensor arrays 14M and independent motion tracking systems. In one such example, motion capture system 50 of <figref idref="f0007">FIG. 7</figref> utilizing a VICON motion system, as available from Oxford metrics Group plc of Oxford, England (UK).

0099EXAMPLE 7: Stroke Analysis. Racket tracking system 10 encompasses a number of different stroke analysis applications for racket system 12. In first stage analysis, processor 16 provides sufficient computational power for generating accelerometer and gyro time histories collected during a rally in real time, as shown in <figref idref="f0003">FIGS. 3A, 3B</figref>, <figref idref="f0004">4A and 4B</figref>.

0100In addition, processor 16 automatically segments the stroke data based on pulse matching and selected stroke characteristics, in order to generate superposed stroke profiles as shown in <figref idref="f0005">FIGS. 5A and 5B</figref>. The stroke profiles are further differentiated into stroke classes or stroke types based on selected stroke characteristics including, but are not limited to, stroke pace (frequency or period), spin (angular momentum impulse) and the motion of racket head 20 at impact, including normal and tangential velocities and accelerations. Additional stroke characteristics include impact intensity, based on linear and angular momentum transfers, and player forces F_g and moments M_g acting on racket grip 24.

0101Second-stage statistical methods are also used. In contrast to physics-driven analysis, this approach identifies additional stroke characteristics that are not obvious or accessible from intuition. In particular, higher-dimensional geometrical features of the data are exploited, including higher-order derivatives and non-physics-based combinations of the acceleration and gyro data.

0102Stroke features and characteristics (or descriptors) are also analyzed within identified patterns or clusters, in order to identify additional stroke performance metrics and related stroke characteristics. In this example, statistical stroke analysis identifies stroke patterns (or clustering) in the inertial sensor-based trajectories of <figref idref="f0008">FIGS. 8B and 8C</figref>, based on the three-dimensional acceleration and angular rates for an extended stroke history, encompassing a number of individual stroke profiles.

0103In the acceleration phase space diagram, for example, strokes tend to approach an attractor-type feature in the lower right (toward the front of <figref idref="f0008">FIG. 8B</figref>), at a median value of acceleration ax. In addition, the strokes exhibit a two-lobed structure extending along the ay/az plane. Neither of these features is evident in the accelerometer time histories of <figref idref="f0003">FIG. 3A</figref> or the superposed acceleration profiles of <figref idref="f0005">FIG. 5A</figref>, nor are they obvious based on a priori physics principles.

0104Similarly, the angular rate data exhibit an attractor-type feature in the central region of <figref idref="f0008">FIG. 8C</figref>, with two distinct spiral structures extending along the ωx/ωz and ωx/ωy planes. Again, these features are not shown in the individual gyro histories of <figref idref="f0004">FIG. 4A</figref> or the superposed angular rate profiles of <figref idref="f0005">FIG. 5B</figref>, and it is not obvious how these features would have been predicted a priori, based on generic physics and engineering principles. These features, however, are relevant since they are manifestations of the interactions between the racket, the player and the ball.

0105EXAMPLE 8: Motion Tracking and Stroke Analysis Methods. The stroke analysis methods described here can also be used to investigate the effects of racket design on the stroke profiles, and ultimately on the player's performance. For example, mass, weight distribution (balance), racket frame aerodynamics, string type, gauge and tension affect the forces and moments the player experiences through the racket grip, the aerodynamic forces and moments, and the forces and moments from the ball impact with the racket face and therefore the racket trajectory. By explicitly accounting for these characteristics in the analysis of the stroke profiles, it is possible to determine what physical characteristics of the racket will help a particular player achieve best play performance.

0106Statistical analyses can be used to identify these stroke characteristics and racket parameters that affect player performance, without physical model. In contrast to physics-based analysis, these methods represent a data-driven approach where the goal is to discover unknown relationships between stroke profiles and player performance, where the relationships are not obvious from first principles.

0107These stroke analysis methods are also used to provide real-time feedback during live play, training and competition. In particular, stroke analysis can be used to identify target or model stroke profiles, features and characteristics, with audio, haptic and visual cuing to provide positive reinforcement based on matching the model profiles, reinforcing good stroke techniques. Stroke analysis can also be applied to kinesthetic and biomechanical methods, including methods to determine relationships between particular stroke profiles corresponding effects on muscle fatigue, injury and recovery.

0108To practice these methods, smart racket system 12 provides an integrated motion sensor 14 and data processing/transmitter system 16, with combined racket tracking and stroke analysis capabilities including data storage, data retrieval and preprocessing, and automatic stroke data segmentation. Processor 16 also performs state vector estimation methods to reconstruct individual strokes and corresponding racket flight paths, and verifies stroke reconstruction accuracy and fidelity using truth data from magnetic sensors and an independent motion tracking system. This allows processor 16 to accurately estimate and determine all the substantial forces and moments acting on racket system 12, including player grip (reaction) forces and moments on grip 24, interaction (impulse) forces and moments on head 20, and gravitational and aerodynamic forces and moments that act on racket system 12 as a whole.

0109These methods encompass both primary (first-stage) and secondary (second-stage) motion tracking and stroke analysis techniques. The first stage techniques generate stroke profiles based on physical stroke characteristics, using racket-based physics analysis (kinematics and dynamics) to extract stroke characteristics including impact point, head speed and top spin from the inertial sensor data.

0110Second-stage methods employ data-driven statistical analysis to characterize additional stroke characteristics based on pattern recognition in order to characterize and identify dominant (primary), secondary and tertiary stroke patterns and profiles, and to build stroke repertoires and portfolios based on the different stroke patterns and profiles.

0111These statistical methods are also used to determine the effects of physical racket parameters on the racket trajectories and stroke characteristics, and to establish relationships between different stroke characteristics and racket parameters. In this approach, the relevant racket parameters include, but are not limited to, racket mass, balance, head size, grip size, string composition, string gage and string tension, as described above.

0112Live play methods apply stroke analysis to provide real-time haptic, audio and visual cuing based on real-time stroke segmentation and stroke feature characterization. For these methods, the relevant stroke characteristics and features include head speed, racket face orientation, impact point on the racket face, momentum transfer (or impulse), and spin (angular momentum transfer or moment impulse).

0113Additional methods utilize these stroke analysis and trajectory reconstruction techniques to provide online interactive analysis, including stroke profile processing and additional real-time feedback based on audio signals, haptic (vibration) cues, and visual displays. In multi-player applications, data from two or more individual smart racket systems are used to provide cross-cuing capability, for example to cue a first player based on stroke analysis of a second player.

0114In these methods, real-time stroke analysis provides player, coach and competitor cuing based on additional stroke features including, but not limited to, spin, topspin, backspin, racket speed, racket trajectory, ball speed, ball trajectory, and the magnitude and direction of the linear and angular momentum impulses measured during stroke interactions.

0115Real-time stroke trending analysis is also encompassed, including changes or trends in a stroke pattern or feature, and changes or trends in the variability of a stroke pattern or feature. Additional methods include statistical analysis of stroke variability for injury prevention, injury prediction, and variability-based learning.

0116Injury prediction and diagnosis include cuing based on changes or trends in a stroke profile, or changes or trends in the variability of a stroke profile. For these methods, variability is determined according to statistical methods applied to individual stroke profiles and stroke portfolios/repertoire, for example a variation, coefficient of variation, standard deviation, dispersion, likelihood or confidence level determined for a particular stroke feature or characteristic, as described above.

0117Auditory (sound), haptic (e.g., vibrational) and visual cuing methods also provide stroke feedback based on amplitude, pitch or frequency modulation, in order to indicate differences, changes or trends in a stroke profile or feature, or in the variability of a stroke profile or feature. These differences and changes can be determined with respect to a model stroke profile, for example using racket tracking data for a model player or coach. Alternatively, the differences and changes are based on a computer-generated stroke model, or a stroke history generated by comparing past and present stroke profiles for a particular player. Training, rehabilitation and injury prediction or diagnosis techniques are also provided, based on the stroke analysis, with or without real-time auditory, visual or haptic cueing.

0118Exteroceptive, proprioceptive (or proprioception-based) and kinesthesiology-based stroke analysis methods are also included. Depending on application, these methods utilize differences, changes and trends in stroke profiles, based on perceived, sensed, desired or otherwise selected stroke profiles (or goals), as compared actual stroke profiles (truth data). Alternatively, stroke analysis is based on differences, changes or trends determined with respect to goal profiles and biomechanical limits, where the biomechanical limits include absolute physics limits, injury-based limits selected for rehabilitation purposes, and training-based limits selected to improve player performance.

0119Additional methods include determining a variability range within a particular stroke profile, and determining perturbations and perturbation directions from the stroke profile, either within or outside the variability range. Further methods include real-time play analysis by classifying stroke profiles into stroke classes based on the corresponding racket trajectories and stroke characteristics, enumerating and differentiating the stroke classes, and establishing a stroke repertoire based on the enumerated stroke classes. This approach provides for additional training techniques, including identifying individual strokes within a given stroke repertoire, and providing audio, visual or haptic cuing to train players to add or improve strokes in the repertoire, or to avoid strokes in the repertoire.

0120With respect to racket selection, these methods provide for stroke analysis based on racket parameters including, but not limited to, weight, head size, grip size, racket length, string type (polymer, gut, or mixed), racket composition, and balance or weight distribution. In these analyses, player performance is evaluated based on the identified stroke profiles and repertoire, in order to select a racket based on one or more of the racket parameters, to identify a parameter for modification, or to signal the need for racket maintenance or replacement.

0121While this invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the spirit and scope of the invention. In addition, modifications may be made to adapt particular situations or materials to the teachings of the invention, without departing from the essential scope thereof. Thus, the invention is not limited to the particular examples disclosed here, but encompasses all embodiments falling within the scope of the appended claims.