An Active Position Sensing Tag for Sports Visualization in American Football Darmindra D. Arumugam1, Michael Sibley2, Joshua D. Griffin3, Daniel D. Stancil4, David S. Ricketts5 1Jet Propulsion Lab., California Institute of Technology, Pasadena, CA, Email: [email protected] 2Tait Towers, Lititz, PA, Email: [email protected] 3Disney Research, Pittsburgh, PA, Email: joshdgriffi[email protected] 4,5North Carolina State University, Raleigh, NC, Email: [email protected] [email protected] Receive Loops Abstract—Remote experience and visualization in sporting 34 5 6 events can be significantly improved by providing accurate ADC tracking information of the players and objects in the event. Magnetic Field Lines Filter Sporting events such as American football or rugby have proved Amp. difficult for camera- and radio-based tracking due to blockage 2 7 of the line-of-sight, or proximity of the ball to groups of players. Magnetoquasistatic fields have been shown to enable accurate Loop Transmitter position and orientation sensing in these environments [1]–[3]. In Antenna Battery 1 1 8 0 this work, we introduce a magnetoquasistatic tag developed for 0 tracking an American football during game-play. We describe its 1 integration into an American football and demonstrate its use in game-play during a collegiate American football practice. Fig. 1. Magnetoquasistatic positioning system setup. The football is equipped with an integrated transmitter that emits quasistatic magneticfields.Eight I. INTRODUCTION receivers, positioned around region of the field extending from the back of the end-zone to the ten yard line, were used to track the ball from the back The use of camera-based and wireless localization technol- of the end-zone to approximately the 15 yard line. ogy in sports is growing for application such as athlete training, game analysis, and visualization [4]. However, localization in many sport environments is challenging because the line-of- and 34.2 m, whereas in [2], we showed a mean 3D position error of 0.77 m, a mean inclination orientation error of 9.67◦, sight (LoS) to the person/object to be tracked can be blocked ◦ by the bodies of players. This situation is common for ball and a mean azimuthal orientation error of 2.84 . tracking in American football or rugby where the ball can be AbriefoverviewofthesystemisprovidedinSectionIIand buried under several player’s bodies or held close to a player’s the design of the active RF tag is discussed in Section III. The body. In such situations, localizing the ball with a camera ball’s integration into a football is presented in Section IV; is difficult and conventional localization systems, such as the integrated tag’s performance under various conditions is ultra-wideband (UWB), satellite-based, and backscatter radio discussed in Section V; and a preliminary study of the specific frequency identification (RFID), will suffer reduced accuracy. absorption rate (SAR) induced in an adult male is reported in In an effort to meet these challenges, a tracking system Section VI. Finally, results from the system’s use in a collegiate based on magnetoquasistatic fields has been developed, and football practice are presented in Section VII. studied in detail, to determine both the position and orientation of a sensor when the LoS is blocked and the sensor is close II. SYSTEM DESIGN to lossy dielectrics (e.g., player’s bodies) [1]–[3], [5]. The The magnetoquasistatic tracking system is shown in Fig. tracking system measures the magnitude of the magnetic field 1. An electrically-small loop, or transmitter, is used to pref- generated by a small magnetic dipole antenna at multiple spa- erentially excite magnetoquasistatic fields that are sensedby tial location and then inverts the field equations to determine multiple receiving antennas whose positions and orientations the sensor’s position and orientation. are known. By knowing the magnetic moment of the trans- In this paper, we characterize the active RF tag for mitter, the position and orientation of the transmit loop can use in magnetoquasistatic position sensing of an American be determined using the governing field equations and a least- football during actual game-play. The tag is integrated into squares solver [1], [2]. an American football and its operation presented during an actual play. The technique, algorithm, and accuracy have been In the American football application, there are several requirements for the tracking system. The first is that the previously reported for the one-dimensional (1D) case in [1], two-dimensional case in [5], three-dimensional case in [2], receive antennas must be placed out of the active playing and for a goal-line play in [3]. In [1], we demonstrated a 1D field, resulting in a maximum transmitter-to-receiver distance of approximately 50 m. The second is that the transmitter distance estimation error 11.74 cm for distances between 1.3 must be seamlessly integrated into the football so as not to D.D. Arumugam and M. Sibley performed this work at Disney Research. disturb its performance, in particular the transmitter’s weight Loop Mass vs. Number of Turns Charging circuitry 20 antenna 12 AWG Rechargeable 15 16 AWG Oscillator Switch Charging mode battery Transmit mode 20 AWG 10 24 AWG 28 AWG Fig. 2. Block diagram of the magnetoquasistatic tag and loop antenna. Mass (g) 5 30 AWG 32 AWG should be less than the variation of an average football, which 0 is approximately 28 g [6]. Finally, the transmitter must be 0 20 40 60 80 100 Number of Turns durable and accurate to provide tracking during play, when the football may be under physical stress, e.g., the weight ofa Fig. 4. Mass of the coil antenna as a function of the number of loop turns. body as well as when in close proximity to the player’s bodies. A. Antenna Design Our approach is shown in Fig. 1, where the loop is coiled around the air-bladder of the football, and the circuit and To preferentially excite a magnetic field, a coiled loop battery are located between the air-bladder and the outer antenna was used with the transmitter. The antenna was coiled leather enclosure. Here, we define the football coil antenna, around the air-bladder and underneath the outer leather of associated circuitry and battery as the RF tag as shown in the football as shown in the inset of Fig. 1. It consisted Fig. 2. The choice of tag operating frequency is determined of a multi-turn loop as depicted in the inset of Fig. 3. The by two competing requirements: 1) the quasistatic region (! coil antenna had a radius, r0,of8.25cmtoaccommodate λ,whereλ is the wavelength of the magnetic field) must the radius of the American football. We used 45 turns of be large and is achieved using a low frequency (which also closely wound, 30 AWG (American Wire Gauge) wire to limit reduces the effects of small, interfering objects and players); the weight to 10.33 g, and resonated the loop inductance and 2) a strong coupling between the transmitter and receivers with a series capacitor to obtain a resonant frequency of is required for a good signal-to-noise ratio (SNR), which can approximately 376 kHz. The real and imaginary impedance of be achieved at a higher frequency. We choose a frequency of the resonant loop antenna, measured using a vector network approximately 400 kHz to balance these two requirements and analyzer (VNA), is shown in Fig. 3a. The measured return stay below the AM broadcast band [1]. loss, shown in 3b, verifies the resonant frequency of 376 kHz. Figure 4 shows the calculated mass of the coil antenna as a III. FOOTBALL TAG DESIGN function of number of turns. The dashed lines indicate values for the 45-turn coil using 30 AWG wire, as used in the design. The transmitter integrated into the American football con- The magnetic moment of the coil antenna is dependent on the sists of an oscillator circuit connected to a multi-turn loop number of turns and current flowing through those turns. As we antenna. The oscillator circuit is powered by a rechargeable increase the number of turns, we must increase the wire gauge battery that is inductively charged using the same antenna to maintain our target weight of approximately 10g. A higher as shown in Fig. 2. Since the integrated transmitter must not wire gauge leads to a larger resistance, which reduced current disturb the dynamics of the ball during game-play, care must be when a class-E source is used. Thus, there is a diminishing given to the placement and weight of the integrated transmitter. return on increasing the number of turns and we found 45 The weight of the battery, circuitry, and antenna were chosen turns to be a good design point. A complete optimization of to be less than the weight tolerance of an American football turns and gauge is the subject of future work. (28 g) used in the National Football League [6]. B. Oscillator Circuit The principle requirements of the football oscillator are that it be lightweight, have high efficiency, and have relatively high output power. Through experimentation, we found that an output power of approximately 0.5 W was required to achieve adequate SNR for distances up to the width of the American football field. We satisfied these requirements using a45-turncoilloopantennadrivenbyaclass-Eoscillatorcircuit with power supplied through a 3.3V rechargeable battery, as shown in Fig. 5. A high-efficiency class-E design procedure Loop antenna Fig. 3. Impedance of the coil used as the transmitting loop antenna is shown in (a). The inset of (a) shows the multi-turn coil and capacitorfortheantenna, where r0=8.25 cm, and N = 45.