Shepherd Pass: Ability Tuning for Augmented Sports
using Ball-Shaped Quadcopter
Kei Nitta1 , Keita Higuchi1 , Yuichi Tadokoro2 , and Jun Rekimoto1,3
The University of Tokyo
Tokyo Institute of Technology
3Sony Computer Science Labortory
1

2

ABSTRACT“Shepherd Pass” is a method of tuning sport abilities that focuses on passing a ball. The method can be used to overcome skill gaps between the players for design novel sport
games. Sports have been changed by technological innovations. Professional players are improving their skills using
updated tools, clothing, and training methods. These innovations can help those who enjoy leisure sports as a way to promote communication or better health. Augmented sports is
terms of designing novel sports with augmented fields, tools,or players using information technology. Our research involve the use of information technology to tune the sports
abilities of expert and non-expert players to fill gaps in theirskills and enable a freer sports design. We developed a selfactuated ball that flies via a ball-shaped quadcopter. The ball
can change its speed and trajectory based on a players skill.
In this paper, we explain the design concept and implementation of this system, and discuss the user experience and future
research directions.
Author Keywords

Augmented Sports; Quadcopter; Sports Design.
ACM Classification Keywords

H.5.2. Information interfaces and presentation: User Interfaces:Unmanned Aerial Vehicle
INTRODUCTION

Historically, technological innovations have been used to
evolve novel sports equipment (e.g., clothing). Informationand electronic technologies can also augment sports environments, including judging[12], training and watching[9, 13].
The relationship between electronic technology and sports
dates back to around 1936, when it was first officially used forelectric scoring in fencing. In the current era, a camera system is used to electronically judge tennis and soccer games.
Technologies are mostly employed to support high-level performance and competition.
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ACE ’15, November 16-19, 2015, Iskandar, Malaysia.
c 2015 ACM 978-1-4503-3852-3/15/11$15.00.
DOI:http://dx.doi.org/10.1145/2832932.2832950

Figure 1. Shepherd Pass: tuning ball game abilities using ball-shaped
quadcopter for multi-player augmented sports

However, since the late 1990s, some human computer interaction (HCI) researchers have focused on the creation of new
sports by augmenting fields and equipment using information
technology and electronic devices, to make sports more entertaining. This research area is called “augmented sports”. For
instance, Ishii et al. developed PingPongPlus [7, 25], which
is an athletic-tangible interface. The interface is a ping pongtable that includes eight microphones and a video projector,
which allows the player to enjoy dynamic graphics, alongwith audio generated by sound-based ball tracking technology. Mueller et al. developed a game that enables communication and play between multiple players in remote locations.
Players break blocks projected onto a screen by cooperatingwith other players through a wall screen with a camera. [15].
Pallot et al. discussed frameworks and models for augmented
sports in the context of distributed interactive multimedia environments[20]. Baudisch et al. invented Imaginary Reality
Gaming, which features a ball game similar to basketball[1],but without a ball. Instead, players participate in the gameusing voice feedback to determine things like “who has theimaginary ball”, “whether or not the goal was made”, and“whether the imaginary ball is inside or outside of the game
field”. This game is played based on this common understanding. In addition, some HCI researchers have introduced
robotic systems such as ground and aerial vehicles for sports
experience augmentation[3, 4, 24]. Augmented sports aim to
introduce information technology and electronics to actively
intervene in the game[18]. The goal of this research is torealize a new method of enjoying sports. It is important toprovide all of the players with an enjoyable experience.
However, when experienced and non-experienced players
participate in a sport together, their skills can be extremely
different. This skill difference presents a problem in provid-

ing the players with a serious game. It often blocks the creation of an enjoyable experience. For instance, an adult player
has to consider their throwing speed when playing with young
children, older people, or people with physical disabilities.
Therefore, filling this gap and augmenting their skills are important requirements in the augmented sports field to enable
more people to enjoy sports. We focused on the skill differences between the players in a sports game with the goal ofdeveloping an information technology to bridge the skill gapsof players.
In this paper, we propose “Shepherd Pass” to tune the ballgame abilities and fill the skill gaps between players using aself-actuated ball (Figure 1). First, this paper defines the skills
that are required to complete a pass action, which is a typicalactivity when playing ball games. We designed the Shepherd
Pass method to deliver the ability and experience of ball passing to all players. This paper describes the implementation of
the prototype, including the self-actuated ball, and the sports
environment. We developed a ball-shaped quadcopter as theball, which was constructed from a copter with four small
motors and a spherical exterior. This paper explains the user
experience of playing with the current implementation, andreports preliminary user feedback. Then, we outline improvements in tuning the sports abilities based on the feedback. Finally, we discuss future work, including improvements in the
self-actuated ball, multi-player augmented sports, and learning applications.
DESIGN SPACE

Balls are the most popular equipment in sports because they
can be used in various ways, including throwing, hitting,rolling, and kicking. For this reason, they are key piecesof equipment in sports, as well as other types of entertainment such as juggling. Some research has been conductedin the augmented sports field to develop next-generation balls
to enhance ball-related entertainment using technological improvements[5, 17, 21, 23]. Izuta et al. created a ball for augmented sports called “Bouncing Star”[8]. Bouncing Star has
a wireless module, acceleration sensor, sound sensor, and infrared LEDs, and is observed by a camera. This system is able
to recognize the balls motion and track its position. Shootball, an original sport developed by Sugano et al., is a goalbased ball game played in a field surrounded by four screens
linked to each other[22]. The ball used in this sport contains
an embedded wireless shock sensor. The player throws theball to a goal area projected on the screen in order to earnpoints. Horita et al. developed a ball with built-in camerasthat can capture the balls point-of-view during flight to augment a spectators experience in American football[6]. Thesestudies augmented the ball-game experience using visual andsound effects.
Several recent studies have focused on developing new balltypes to enhance sport abilities. Ohta et al. invented aball-type trajectory change device using gas ejection called
TAMA[19]. This ball includes a gas tank, Arduino Nano, acceleration sensor, gyro sensor, and wireless module. TAMA
can recognize its own orientation and change its trajectory.
However, TAMA cannot measure its own absolute position,and cannot hover and fly through the air because of the limited capacity of its gas tank. In contrast, we introduce small
quadcopters, which are classified as multi-rotor unmanned
aerial vehicles (UAV). We also use absolute position sensing
in a sports field. These make it possible for a ball to freely
move through the air and realize interaction between players
and the ball. Previously, HoverBall introduced a quadcopterand ball-shaped exterior to determine the feasibility of imaginary dynamics, which allowed non-contact interaction withthe ball as an augmentation of the balls functions[17]. However, HoverBall did not have sufficient robustness and flightcapability for multi-player sports (e.g., throwing and catching). Thus, HoverBall was implemented and discussed in relation to single-player interaction, even though most existingball games are multi-player activities. The goal of this study
was to support multiple player interaction to provide enjoyable sports experiences for all the players via tuning theirsports abilities and filling the skill gaps. In particular, we
focus on ball passing, which is a major activity in ball games.
Passing is the most important strategy in many games, andhelps human-human communication, training, and healthpromotion in itself (e.g., playing catch). As shown in Figure
2, there are certain abilities that are required to deliver the ballto a catcher. (a) The thrower needs to estimate the distance
between himself and the catcher. (b) The thrower should havethe throwing ability to deliver the ball to the catcher. (c) The
thrower needs to predict the movement of the catcher after
throwing. (d) The thrower has to consider how to throw theball so that its trajectory and speed allow it to be caught by
the catcher. The catcher also needs to catch the thrown ball.
In addition, the players need to practice to achieve consistentsuccess in passing. However, this can be hard for first-timeplayers, young children, and seniors. Therefore, our goal wasto deliver the needed passing skill sets to all players.

Figure 2. Skill requirements for passing: (a) estimating distance between players, (b) throwing ability, (c) prediction of catcher movements
on field, (d) consideration of throwing speed and catchers catching ability

Imaginary Dynamics

Sports and training using a floating ball often appear in theworld of science fiction (SF) and fantasy (for example, theHarry Potter series, written by J. K. Rowling). As these examples demonstrate, by unleashing the trajectory of the ball fromthe conventional physical laws, the manner in which players
and balls interact becomes richer, and more strategic games
can be designed. On the other hand, the trajectories of conventional balls are determined by aerodynamics and gravity.

Conventionally, the vocabulary of ball sports includes kicking, throwing, catching, and rolling. An interaction between
a player and a ball comes into existence through contact with
their body. In our research, by providing autonomous mobility to the ball itself, it is possible to program and add new
artificial physical laws; we refer to such laws as “imaginary
dynamics”. This allowed a ball to utilize a virtual force fieldto simulate attraction, repulsion, and anti-gravity by controlling the acceleration of the ball. Imaginary dynamics augments the space for interaction with a player and expands the
vocabulary of conventional ball sports. It creates new ballplaying terms such as attracting and hovering.
Shepherd Pass

We propose a method for tuning sports abilities called Shepherd Pass to assist with passing between multiple players using imaginary dynamics. This method provides an automatic
trajectory to the ball during the game. The ball automatically moves to the catcher after being thrown, and stops for a
moment in front of the catcher, which makes it more catchable. This method supports the players skills, including theirthrowing power, distance estimation, movement prediction,and catching ability. Figure 3 shows the concept of Shepherd
Pass. (A) The ball tracks the catchers movements after being
thrown. (B) The ball also changes its trajectory and speed after being thrown to become a catchable pass for the catcher.
This method can create a novel sports experience for exercise
and enjoyment.

the ball and players on the field and streams position information to the game manager and quadcopter controller. We usean optical motion capture system for global sensing, which
can measure the positions and rotations. The game manager
detects player activities, including throwing and catching, using the positions, directions, and movements of the players.
The manager also generates trajectory and speed information
about the ball to deliver the ball to the catcher using the quadcopter controller.
We instrumented the motion capture system (OptiTrack
S250e) using infrared light cameras on the field. It tracks at
120 frames per second to measure the positions and rotations,with errors of less than 1 mm. We also placed retro-reflective
makers on the ball and players hands. The size of the playing
field was 5 ⇥ 5 m.
The trajectory and speed can be decided based on predefinedplayer information such as age, gender, and ability. The quadcopter controller connects with the ball-shaped quadcopter
through a wireless data link. The controller also sends movement commands to the ball to control its trajectory and speed
based on information from the game manager.

Figure 4. System and data flow
Figure 3. Concept of Shepherd Pass: (A) automatic tracking with selfactuated ball and (B) tuning sports abilities for each player

We designed a mechanism that included a self-actuated balland its control field. The ball has the ability to fly and moveby itself in mid-air. The field uses position sensing to measure the ball and the players positions and direction, and detects the players activities to activate the balls flying. Whenthe thrower releases the ball, the ball automatically tracks thecatcher. The field can distinguish between the thrower andcatcher based on their positions and movements.
IMPLEMENTATION

We developed a prototype to provide the Shepherd Pass experience. The main components of the prototype were a ballshaped quadcopter as the self-actuated ball and a controllingsystem, which included global position sensing, a game manager, and the quadcopter controller.
Figure 4 shows the system configuration and data flow. To
develop this system configuration, we examined the configuration of The Flying Machine Arena project, which is anindoor research space built specifically for the study of aerial
robotics. Our system also uses multi-player positions and activities[11]. Global sensing measures the absolute position ofBall-shaped Quadcopter

We constructed the self-actuated ball from a small quadcopter
and truncated icosahedron grid shell, as shown in Figure 5.
We modeled it on an existing state-of-the-art small spherical helicopter. Briod et al. proposed a flying robot witha three-axis gimbal structure in a protective cage to reducethe impact of the friction force on the attitude of the innerframe[2]. Figure 5shows the concept of our self-actuated ball,which includes an exterior three-axis-gimbal structure and thequadcopter used to create the needed mobility. To reduce theweight of the ball, we changed the gimbal structure to only asingle axis, which still permits throwing and catching by theplayers, and bouncing and rolling on the ground.
This quadcopter consists of the frame, main computer, four
motors, wireless data link receiver, electronic speed control
drivers, and battery that weighs 125 g. The frame has a backbone and joint arms to connect the exterior and support otherparts. The computer is an OpenPilot CC3D Board, which has
three-axis gyros and a three-axis accelerometer for stabilized
altitude control. The computer receives control commands
from the quadcopter controller, and applies them to the four
motors. To construct the truncated icosahedron grid shell,
which is similar to a soccer ball, we used 102 carbon fiberto the aim point using the PID controller. The flight path of
the ball has some waypoints at the midpoint between the startand goal positions. The system selects a target point fromthe waypoints based on the current time. The ball changes its
moving direction to the target position. A game designer orplayers can control the goal position and arrival time to arrange the trajectory of the ball. For example, the ball canmove straight according to the waypoints when the player
throws the ball in front, as shown in Figure 7(A). If a player
throws the ball in a different direction, the ball changes its
moving direction to the goal after being thrown (Figure 7(B)).

Figure 7. (A) Tuning imaginary dynamics by interval waypoints and (B)automatic tracking with self-actuated ball

USER OBSERVATION

Figure 5. Our self-actuated ball: (1) implementation of ball-shaped
quadcopter, (2) soncept of ball-shaped exterior with three-axis gimbal,(3) 3D printed frames, (4) truncated icosahedron grid shell with 102 carbon fiber rods and 62 3D printed joints, (5) constructed quadcopter withwireless data link sender/receiver, and (6) bouncing and rollingrods and 62 3D printed joints, each 57 mm long. The shell is
280 mm in diameter and weighs 70 g. Thus, the total weightis 174 g. The battery is a 450-mAh, 7.4-V lithium polymer
battery. Figure 6 shows the relationship between the rotational speed and static thrust. The maximum power is 3.4 N(340 g) with the four motors. A flight time is about 2 min.

In our pilot tests, we used our current implementation ofShepherd Pass to receive user feedback and discuss the potential of multi-player augmented sports. We observed two
scenarios: (1) two players passed each other the self-actuatedball with the automatic trajectory and speed adjustment. (2)The system also made it possible to pass the ball between
three players. We tested basic functions of the current implementation of Shepherd Pass. We recruited five participants(aged 22-30) to verify the applicability of the system and provide feedback based on their experiences. We focused on
three questions. (1) Can the ball move to the catcher after
being thrown? (2) Can the participants catch the ball? (3)Can the system distinguish between three players in the field,
and control the trajectory and speed of the self-actuated balldepending on the players?
We obtained permission from the research ethics committee of our organization to conduct the two tests. The tests
were carefully conducted; we always watched the experimental field, participants, and ball, and could quickly stop the testat any time. We also explained the experiment contents to theparticipants and their parents and obtained their consent.

Figure 6. Static thrustControl Method of Shepherd Pass

The control method for the quadcopter uses a proportionalintegral-derivative (PID) controller that responds to position
data from the motion capture system. The speed of the quadcopter is affected by its pitch and roll angles. The system cancontrol the speed of the self-actuated ball based on changing angles. The ball accelerates in proportion to the distance

Figure 8. User observations: (1) players A and B passing self-actuatedball and (2) players C, D, and E passing ball

In observation I, players A and B threw the self-actuatedball to each other. As shown in Figure [fig:observations] (1),the self-actuated ball could move to the catcher after beingthrown, and the catcher could easily catch the ball, because
the ball slowed down in front of the catcher. Figure 8 (1)shows that the ball tracked the movement of player Bs hands.
Thus, he succeeded in catching the ball. Figure 9 shows the
trajectory of the self-actuated ball and waypoints.

the following statements: “This was an extremely novel experience. I was really enjoying it,”,and “I could control the flying ball. I think this is a new type of sport.” We also received
negative feedback, including the following: “My throwing
was limited. It would be more enjoyable if it would allow
a strong physical impact,” and “I felt scared of the propeller
rotation of the quadcopter. The ball should look safe.”
In addition, we also conducted a test where a child played andinteracted with the flying ball to observe how the child reacted
to the self-actuated ball. In this test, a boy (age: 2.5) enjoyedinteracting with the self-actuated ball very much. The ball
kept flying around the boy. As shown in Figure 11, we observed several interesting reactions such as looking, jumping,striking, and kicking. After the test, the boy said that the experience was fun.

Figure 9. User observation I: trajectory of self-actuated ball (blue) andwaypoints (red)Observation II aimed to check the passing activity of threeplayers. Figure 8(2) shows that players C, D, and E threw
and caught the self-actuated ball. The system could distinguish the three players during the observation. Further, wetested the ball, and found that it dynamically changed its trajectory to player D. As shown in Figure 10, the ball turnedin front of player E after being thrown by player C, whichallowed player D to catch it and throw it back to player C.
In the observation, the three players usually succeeded in thepassing action using our system. However, when the participants threw the ball hard, it often fell to the ground. Thus,
we conclude that the current implementation could provide
the basic functions of Shepherd Pass, but the players had tothrow the ball softly.

Figure 11. Observation of child: (A) looking, (B) jumping up to touchball, (C) striking, and (D) kicking

DISCUSSION

Here, we discuss the next generation of the method, including the self-actuated ball, multi-player augmented sports, andlearning applications.
Requirements for the self-actuated ballFigure 10. User observation II: trajectory of self-actuated ball (blue)
and waypoints (red)We asked the players about their experience with the system.
They all replied that the experience was enjoyable. The players also liked the novelty. Their positive feedback included

The current implementation of the ball mainly needs two improvements to the quadcopter exterior. First, the ball could
be more robust to throwing and shock. The current exterior
has a one-axis gimbal, which can only rotate in one direction.
As shown in Figure 5 (2), when using a three-axis gimbal
exterior, the quadcopters main body is hardly affected by outside forces. Second, the ball-shaped exterior could be covered
with a mesh to protect against contact between the propellers
and players fingers/hands. The players mentioned that theyfelt scared by the propeller rotation. We need to design the
self-actuated ball to be safer. As shown in Figure 12, we already attempted to prototype a mesh-covered exterior for thequadcopter, which could fly. Therefore, we plan to combine
the three-axis gimbal and mesh cover in the next quadcopter
exterior. The current flight time is too short to play a ballsport game. However, we could not mount a large-capacity
battery because of the payload limitation. Therefore, somereserves are needed when used in a game.

In the tests, the players mentioned that they felt scared anduncomfortable because of the loud sound from the propeller
rotation. This unpleasant problem should be eliminated toprovide a comfortable and enjoyable user experience. To reduce the sound from the propellers, we plan to use three-blade
propellers on the quadcopter instead of the current two-blade
propellers.

Figure 12. Quadcopter with mesh-covered exterior

Competitive

In our tests, the players enjoyed the Shepherd Pass experience, including throwing to each other and the changing trajectory and speed of the ball. On the other hand, it is important to design games that are competitive, where two or moregroups are competing with each other. We believe that Shepherd Pass could help tune and enhance abilities in competitive
augmented sports using the self-actuated ball. For instance,
Figure 8 (2) shows a fake pass with a trajectory change toaugment trick plays. This is important to create more exciting augmented sports with multiple players.
In the multiple-player augmented sports field, one of the goals
is for heated competition to bring out each players best performance, even when each player has a different skill level.
Augmented sports technology supports players abilities byadjusting each players skill level to balance and enhance theexcitement of the game. In related research, Muller et al.
proposed a distributed jogger support system for balancing
the experiences of joggers with different fitness levels[14].
We are focused on the creation of a novel ball-based gamefor players at the same location using augmented sport technologies. We plan to design competitive multiple-player augmented sports to introduce the Shepherd Pass mechanism forbalancing players skill differences.
For Learning, Training, and Rehabilitation

In this paper, we focused on an enjoyable passing experience
for all sports players using an ability tuning method. On theother hand, we believe that our system will be able to realize
learning applications such as for training and rehabilitation.
In the well-known space opera movie “Star Wars Episode
IV: A New Hope”, Luke Skywalker trained with an aerial
spherical robot. The robot floats in the air and attacks him
using laser beams. In our future vision, a self-actuated ballwill support throwing training for children. In this training,
the ball will be able to make situational changes in its behavior, including alternating between a normal ball and theself-actuated ball. For instance, the ball should act like a normal ball when a child is throwing it. However, after the ballstops, it could begin flying to return to the childs hands. Thisis able to allow the child to repeatedly practice throwing theball without the need to pick it up, which would save time.
Area of Sports Field

The area of our sports field used to perform the trajectory
control tests and check the feasibility of the passing activities using the self-actuated ball was 5 m ⇥ 5 m. However,for actual sports, we need to increase this size. Potentially,
the motion capture system can recognize objects at a maximum distance of 30 m. Thus, the field could be extended
to approximately the size of a tennis court by increasing thenumber of cameras. Another potential option is to introduce
a self-localization algorithm for use in a field with an unlimited size. Recent technologies allow high-precision and highspeed self-localization using cameras and sensors (e.g., GPS
and inertia) mounted on UAVs[10, 16] . We need to selectposition sensing methods based on the size of the field andtypes of sports.
Remote Sport Experience

Our self-actuated ball can be applied to playing ball sports
over a distance by transmitting and copying remote players
physical interactions (e.g., throwing). Muller et al. utilized
remotely mediated video to realize sports with a shared digital context[15]. In contrast, our ball can simulate the outcomeof a remote players throwing and catching, which makes it
possible to share a physical context. Figure 13 shows an example of players using the ball to participate in a game fromremote places.

Figure 13. Ball sports over distance with synchronized ball movement

CONCLUSION

This paper introduced a method for tuning sports abilities bycontrolling the balls speed and trajectory. The method aims
to fill the gaps between the abilities of expert and non-expert
players. We focused on passing, which is a typical activity inball games. The method includes a self-actuated ball and controlled field. To implement a prototype, we developed a ballshaped quadcopter, along with a system for controlling thequadcopter trajectory using position sensing. We recruitedparticipants for user tests of the current implementation, andalso asked them to give us feedback. During the tests, thesystem can provide five players with an enjoyable passing experience using changes in the trajectory and speed of the selfactuated ball. A child also enjoyed interacting with the ball.
However, we found several problems and limitations with thecurrent implementation. Finally, we discussed future work,
including a new-generation quadcopter, the need to design

a competitive multiple player augmented sport, and learning
applications. In the future, we will improve the function of
the ball and design a proper sports game.
ACKNOWLEDGMENTS

We thank to Kensaku Kawauchi, Shinji Kawauchi, Michihiko Ueno, Takamitsu Hamajo, Junichi Kasahara and Kentaro
Ayuzawa for their support.
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