“Killer App” of Wearable Computing: Wireless Force Sensing Body Protectors for Martial Arts

Ed H. Chi*, Jin Song+, Greg Corbin+ *Palo Alto Research Center

3333 Coyote Hill Road, Palo Alto, CA 94304

echi@parc.com

Stanford University Taekwondo Program 375 Santa Teresa

Stanford, CA 94305 stanfordtkd@yahoo.com

+Impact Measurement 1400 Coleman Ave.

San Jose, CA ragmansong@cs.com

ABSTRACT Ubiquitous and Wearable Computing both have the goal of pushing the computer into the background, supporting all kinds of human activities. Application areas include areas such as everyday environments (e.g. clothing, home, office), promoting new forms of creative learning via physical/virtual objects, and new tools for interactive design. In this paper, we thrust ubiquitous computing into the extremely hostile environment of the sparring ring of a martial art competition. Our system uses piezoelectric force sensors that transmit signals wirelessly to enable the detection of when a significant impact has been delivered to a competitor’s body. The objective is to support the judges in scoring the sparring matches accurately, while preserving the goal of merging and blending into the background of the activity. The system therefore must take into account of the rules of the game, be responsive in real-time asynchronously, and often cope with untrained operators of the system. We present a pilot study of the finished prototype and detail our experience.

Categories and Subject Descriptors: H.5.2 [Information Interfaces and Presentation]: User Interfaces—Haptic I/O; C.2.1 [Computer-Communication Networks]: Network Architecture and Design---Wireless communication; H.5.2 [Information Interfaces and Presentation]: User Interfaces—Ergonomics; H5.m. [Information interfaces and presentation (e.g., HCI)]: Miscellaneous; C.5.3 [Computer System Implementation]: Microcomputers—Portable devices; J.7 [Computers in Other Systems]: Consumer Products. General Terms: Design, Experimentation, Human Factors, Performance.

Additional Keywords and Phrases: Wearable computing, Wireless computing, Extreme Sports, Taekwondo, Usability of Wearable Wireless Systems.

INTRODUCTION Ubiquitous computing has become a rapidly growing area of Human-Computer Interaction research. Championed by the late Mark Weiser at PARC, its goals are to blend computing into the surrounding environment while supporting all kinds of human activities [16]. Early research focused on various office-oriented activities such as embedded collaborative displays [5], tablet PCs [8], and location-aware badges [15]. Recent activities in the field has pushed the research into everyday environments such as the home [1], and a commuter’s car or mobile office [6], and wearable computing [11]. In this paper, we describe a novel wearable system for supporting an extreme human activity, namely taekwondo competition sparring.

Figure 1. Taekwondo is an extreme full-contact sport.

Recently inducted as an official Olympic sport, taekwondo has enjoyed enormous popularity in the last two decades. Because of this popularity, there has been increasing pressure to ensure fairness in judging and to make the sport more spectator-friendly.

This pressure has directly caused several rules changes [17], and the desire to utilize technology to ameliorate some problems inherent in scoring. The number one problem in achieving accurate scoring is the subjective judgment of what constitutes a valid scoring kick to the body. A scoring kick must be delivered “accurately and powerfully to the legal scoring of the body [17].” The subjective nature of the judging criteria has been a major impediment to the development of the sport, sometimes resulting in accusations of biased judges favoring players from certain countries1.

1 See for example, news stories at http://www.indiavarta.com/olympics/newHeadlines.asp?cat=Taekwondo: “Never-ending protests against taekwondo judges,” “More complaints about taekwondo bias towards home team,” and “Another day of disputes in Olympic taekwondo.”

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We have embarked on a mission to use wearable computing technology to help solve this problem. In this paper, we describe a wearable computing system, called TrueScore™ SensorHogu2, which uses piezoelectric sensors to sense the amount of force that has been delivered to a competitor’s body protector, and wirelessly transmits this signal to a computer that scores and displays the point. Complicating the system, each point must be confirmed by at least two judges using wireless scoring handsets as well.

There are a number of significant challenges. First, the system must be completely wireless. It is infeasible for the competitors to be tethered. In comparison, the standard foil fencing scoring system that has existed for several decades is tethered [7] and limits the players to essentially linear movements. The fencing system is essentially based on electrical switches that complete a circuit when the switches touch a metal vest. The tethered setup has limited the sport to linear forms of fencing, thus restricting the natural development of the sport. The human activity has been modified to suit the technology that was available. In comparison, the SensorHogu must not only measure the amount of force applied, it must also transmit this information wirelessly.

Second, it must function flawlessly in real-time. Multiple signals from each body protector could be delivered at the same time, and it must interpret these signals according to the rules. Third, the devices must withstand an extremely hostile environment. It must be small and secure, and can resist physical abuse and potential radio interference. Finally, it must be easy to use by untrained users.

We conducted two pilot studies during the development of the SensorHogu. First study is designed to understand what constitutes a valid score using the piezo sensor. We measured the different signals that depend on the competitor’s weight division, gender, and type of kick. Second, we used the SensorHogu in real sparring matches conducted with judges to see if it performs up to expectation. How does it actually function in real world?

In the paper, we first describe our high level goals. We then describe the design constraints. In the implementation section, we describe the architecture of the devices. Next we outline and present results of the user studies, and conclude by noting related work and future directions.

GOALS There are a number of significant goals for the system:

• Easy to use: A high priority of the design goals is for the SensorHogu to be easy to use. The system will be used by competitors of different genders and various heights, weight divisions, and age groups. For beginners and novice users, it must be easy to adjust

2 Hogu is the Korean word for a body protector.

and correct any errors. The handsets used by the three judges must be easy to operate.

• Accurate: In sparring, multiple valid points could be scored not only in rapid succession but simultaneously. It is imperative that the system interprets these data streams in a timely manner and conform to the scoring rules. The sensors must be calibrated so that it is appropriate for sensing various amounts of force, depending on the characteristics of the competitors.

• Robust Architecturally: The system must handle all of the data packets transmitted in real time and asynchronously. It must be able to withstand radio interference. The sensor must also coordinate with the handset signals to correctly interpret a valid scoring point. One hogu’s sensor data stream must not collide with another hogu’s data stream.

• Robust Physically: The system must withstand heavy abuse in extremely hostile environments. Sparring competitions occur in chaotic gyms with a large audience. Some environmental elements it must withstand are direct blows to the device, and sweat and spillage of water or sports drinks.

• Secure: The system must ensure the scoring is secure against hacking. Cheating is not unheard of in various sports. Competitors will go to great length to gain competitive advantages over their opponents, however slight the advantages might be. The system should guard against hacking by encrypting the data stream.

There are also some secondary goals of the project. These qualities are nice to have, but are not strictly necessary.

• Modular: The body protectors and the scoring system should be modular so that any problematic units can be replaced immediately, ensuring the smooth running of a tournament. It would be preferable for the transmitters to be detachable, so that a problematic transmitter can be replaced without taking off the hogu.

• Low-power: We would like to eliminate the need to change batteries frequently. At a minimum, the hogu should operate a whole day on a single charge.

• Low-cost: There are many amateur competitions for the sport. It would be nice if all such competitors can afford to obtain an SensorHogu at low cost.

DESIGN In this section, we describe the design and implementation of the SensorHogu scoring system. First, we will introduce basic knowledge of the sparring competition format. Then we will detail the architecture of the system.

Competition Environment As shown in Figure 2, the competition sport of taekwondo sparring is conducted on a square padded mat 12 meter wide on each side. The competitors, one wearing a blue body protector and the other red, face off against each other in a controlled environment. The competitors are moving rapidly around on a large mat, thus having them tethered is out of the question.

Figure 2. Competition ring of a taekwondo match consist of 3

judges and 1 referee, with one meter attention boundary.

Divisions: There are 8 weight divisions (or 4 combined divisions in Olympics) for both male and female. A single match lasts three rounds at three minutes each, with one minute of rest between rounds.

Scoring: A valid point should be scored “when a permitted technique is delivered accurately and powerfully to the legal scoring areas of the body.” A permitted technique is a punch with a clenched fist or a kick delivered by the foot. Legal scoring areas consist of mid-section of the trunk of the body that is covered by the trunk protector and the face. One point for an attack on the trunk, and two points for an attack to the face, with an additional point awarded when the contestant is knocked down.

We did not consider instrumenting the facial areas for three reasons: (1) the United States Taekwondo Union and the World Taekwondo Federation did not want to instrument the facial area; (2) a facial mask would obscure the competitor’s vision and likely to face significant resistance from the competitors; (3) attacks to the face is much easier for judges to score, and they are not usually subjected to complaints.

Judging: There are three judges and one referee. The referee is responsible for conducting the match, while the judges are responsible for actually scoring the techniques. The judges are placed around the mat in a triangular shape, as shown in Figure 2. A point is only awarded when at least two judges confirm it within a one-second window.

Devices Figure 3 represents the relationships of the devices in the entire system. The system consists of a single wireless base station that is connected to a laptop, three judges’ scoring handsets, and two SensorHogu body protectors. Figure 4 depicts the wireless base station on the upper left, the battery charger on the upper right, and the judge’s handset for left hand on the bottom.

Figure 3. Overall System Flowchart.

Figure 4. The wireless base station on the upper left, the

battery charger on the upper right, and a single left hand red judge’s handset on the bottom.

Figure 5. The SensorHogu. The wireless transmitter is taken

outside of the left shoulder protective pouch.

Blue Hogu

Red Hogu

Judge 1 Judge 2 Judge 3

Base Station Collector

wireless asynchronous

signals

ScoreDisplay

Operator

Figure 6. The first generation proof-of-concept wireless

transmitter is the size of a small paperback.

Figure 7. The left picture shows the wireless transmitter

connected to the body protector, and the right picture shows the wireless transmitter after it has been stowed away in its

protective pouch.

The SensorHogu and the handsets are completely wireless. Figure 5 shows the wireless sensor body protector with its wireless transmitter on the upper left shoulder.

We went through several stages of development for the wireless transmitter. At first, we had envisioned the transmitter to be worn on the back on a strap near the spine. The transmitter of the first proof-of-concept prototype was about the size of a small paperback book, as depicted in Figure 6. Even though the spine is not a legal scoring area, it was discovered during the testing that the transmitter is likely to be damaged. The size of the transmitter was also a significant problem, as it was too bulky and hard to hide underneath the body protector. Some of the initial testers complained of discomfort as well.

From our experience of the first transmitter, we refined our design by first shrinking the transmitter to about 2 inches x 1 inch, with a power supply of 2 AAA-sized batteries. Through trial and error, we discovered that the best way to shield the transmitter from damage was a padded pouch on the shoulder. Figure 7 shows the second generation refined wireless transmitter and the protective pouch.

The SensorHogu are used in conjunction with scoring handsets for the judges. We developed the handset system to go along with the SensorHogu so that they all work wirelessly to the same base station. There are two handsets for each judge, one for scoring the red player on the left hand, and one for scoring the blue player on the right hand. Figure 8 shows the judge’s red handset held in the left hand. As shown, there are two buttons on the handset. A trigger button scores a point for the body, and a side button scores two points for a head blow. According to scoring rules, at least two judges must press the same button on their handsets within a one second window for the point to score.

Figure 8. Judge’s handsets are ergonomically designed to

eliminate hand fatigue, because competitions often run for an entire day.

Figure 9. Various prototypes of the judge’s handset.

These handsets also went thru several stages of refinement. Figure 9 shows the three developmental stages of different electronic boards of the handset wireless transmitter. The board on the very left shows the wireless evaluation board (developed with support from Texas Instruments) modified for our uses. The 3rd board from the left is the custom board we built for direct insertion into the plastic housing on the right.

Sensor For the actual sensor on the hogu, we needed something that is low-cost, low-powered, and rugged. We did not consider employing accelerometers because of our desire to directly measure the impacting force. The most common dynamic force detector is the piezoelectric sensor. The word “piezo” comes from Greek, which means to “squeeze”. A piezoelectric sensor when “squeezed” produces a voltage that is proportional to the force applied [10]. A piezoelectric sensor is essentially an electrically polarized material that when deformed creates an excess of surface charge. The signal generated by the material needs to be converted by an amplifier to a low impedance signal. This same principle is used in some piezoelectric microphones. With the right design, it is possible for the sensor to be low-powered.

Another advantage is that piezo sensors can be as rigid as a piece of solid steel. This stiffness and strength makes them particularly suitable in a harsh environment like a protective suit. This enables us to ensure the protector and its sensors can endure physical abuse over time. In our application, a single long piece of piezoelectric sensor is mounted onto a

plastic backing and inserted into the middle of a World Taekwondo Federation (WTF) approved body protector, as shown in Figure 10.

Figure 10. The piezoelectric sensor is attached to a black

insert in the SensorHogu.

Architecture There were some electronic hardware architectural design issues. In the system, there are multiple asynchronous stimuli and activities. In a match combining two SensorHogus and three judges’ handsets, there are multiple stimuli that need to be processed simultaneously to correctly interpret the results of player exchanges. Our major issue, therefore, was the critical timing of the device signals. For accurate scoring, we must handle these signals in real time. We had to decide between two competing architectural designs:

• Central signal processing enables all sensor signal data to be collected in a single computation station and processed according to scoring rules. The advantage of the central processing model is the potential for lower cost by eliminating many of the distributed embedded processors. Its main disadvantage is that it is hard to build to ensure real-time accuracy. Any signal not processed immediately by the central processor is potentially an error in scoring.

• Distributed signal processing would instead embed processors in each device so that individual devices would perform their own signal processing. The advantage of this method is the ability to handle asynchronous and simultaneous signals, thus eliminating the need for a complex real-time operating system. Instead, a collector node would gather all processed signals and sends them to the computer running the user interface. The main disadvantage is potential high cost.

Because we wanted the system to be accurate, we decided that a distributed architecture more adequately suits our needs. This allows multiple signals to be process in parallel at each device before the processed data is transmitted to a base station. The base station then forwards the data to the computer running the operator interface and score display.

Figure 11 depicts the ElectronicHogo’s device architecture. The piezoelectric sensor’s high impedance signal is first fed to a preamplifier to be converted to a low impedance signal, which is then subsequently fed to an analog-to-digital signal

converter. The digital output of the converter is then fed to a low-powered processor for digital signal processing and readied for transmission. Each SensorHogu has a Texas Instrument MSP430 CPU microcontroller for this purpose. The MSP430 CPU is a 16-bit RISC mixed-signal processor that is ultra-low power and low-cost [14]. The output of the processor is fed to a transceiver to be sent out wirelessly. We used a Texas Instrument TRF6901 900Mhz ISM transceiver chip because it is low-power, cost-effective, and has relatively high transmitting power [4].

Figure 11. SensorHogu Architecture

For security, we used military grade encryption to ensure no competitors can temper with the wireless data packets. For robustness in face of radio frequency (RF) noise, we used Frequency Agile Spread Spectrum technology to encode the data stream. This is a common wireless technique where a code is used to generate a pattern of frequency hops, which helps avoid noise3.

Score Display and User Interface As the base station collects the signals from the two SensorHogus and the three judges’ handsets, a monitoring laptop is used to collect, display, and log the data. The time-stamped log tells us how hard each player has been hit, and whether any of the judges’ handset buttons are depressed. The laptop interprets these signals and updates the score board accordingly. The laptop is connected directly to a score display (an off-the-shelf LCD or CRT monitor). The user of the laptop can monitor the handsets and hogus to ensure they are transmitting data correctly. Moreover, when the referee signals for correction in the score, it can be immediately acted upon.

Design Summary There were numerous design goals for the SensorHogu system: easy to use, accurate, robust, secure, modular, low-cost, and low-powered. First of all, we made sure the system is easy to use and supports the human activity

3 For more information on spread spectrum techniques, see: Gitlin, http://www.columbia.edu/~rdg74/ee6713/Spread_Spectrum.PDF, course notes at Columbia University Spring 2001, and Peterson, Ziemer, and Borth’s Introduction to Spread Spectrum Communications. Prentice Hall, 1995.

Piezo Sensor

Pre-Amp

Analog-Digital Signal Converter

Digital Signal Processing and RF Baseband

Processing

(TI MSP430 Microcontroller) RF Analog Transceiver

(TI TRF6901)

wirelessly; the system cannot be tethered like the fencing scoring system. Judges’ handsets are ergonomically designed to ensure hours of use will not cause fatigue. We designed the system so that it can be used by adults and children alike. Second, by using embedded signal processors in each device, we ensure that the signals are processed in real-time and transmitted immediately for accurate scoring. Third, the whole system is designed with several criteria of robustness in mind. It is able to withstand the physical abuse of an extremely hostile environment as well as being able to handle potential radio interferences and malicious tampering of data packets. Fourth, the whole system is modular, low-cost and low-powered. Each device can run on two AAA-sized batteries for 3 or 4 days of competition. For modularity, each device can be individually replaced. For example, the sensored hogu and the transmitter unit in the pouch can be replaced separately. The SensorHogu is a novel wearable system that supports an extreme human activity.

PILOT USER TESTING There were numerous testing of the entire system during development. The first step was the testing of the judges’ wireless scoring handsets (i.e. the scoring system without the SensorHogu). Previous scoring systems used wired handsets, so we wanted to make sure the wireless system conforms to existing international established standards. Although unreported previously, the wireless scoring handsets have been in development for several years, and were demoed, tested, or used during several high profile tournaments, including US Taekwondo Union National Championships 2002/2003, US Junior Olympics 2002/2003, World University Championships 2002, World Military Championships 2002, and Canadian Junior Olympics 2002/2003. The system has been universally accepted by the international community.

The next step was the testing of the wireless SensorHogu along with the judges’ handsets. The first step of this testing was the understanding of the characteristic of the sensor when embedded in a body protector. How do factors of gender, weight division, and kick technique affect sensor output? We know that piezoelectric sensors are used in various force measurement applications, but do they work well in this particular setting? What sensor sensitivity settings should be used?

During the second phase, we tested the system at actual sparring matches. We ran some sparring matches with the system with videotape analysis to see if judges would generally agree with the sensor output.

Sensor Testing During the first pilot test, we concentrated on understanding the sensor characteristics. Our primary goal is to determine the suitability of the piezoelectric sensor for the purpose of measuring the amount of force that has been delivered to the body. A secondary goal is to understand if some

principles of the sensor sensitivity settings could be understood and applied in later phases of the development.

The difficulty of this test, however, is that there are a wide variety of different factors in the judgment of whether a hit is a scoring point or not. Depending on the weight division, gender, and type of kicking technique used, judges modify their criterion for a point accordingly. The current international standard for a point is if a competitor is able to deliver “accurately and powerfully to the legal scoring areas of the body.” While at least two judges must agree for a point to be scored, this criterion still clearly remains to be a subjective measure. As mentioned before, the subjective nature of the judgment of a point is one of the major reasons for the development of the SensorHogu. The desired goal of this testing phase is to calibrate our sensor to the current standards, as imperfect as it might be.

Subjects: Six subjects participated in this pilot study. Each subject is from different weight divisions, ranging from female finweight to male heavyweight. The subjects consisted of various ranks: two black belts (3rd and 2nd Dan), two red belts, a blue and a green belt. The green belt rank is when taekwondo practitioners typically start participating in sparring competitions.

Material: An anatomically-correct anthropomorphic dummy of a male upper torso is used to collect the testing data. The body protector is fitted to the dummy, and held in place while the participant attacked the dummy using a variety of techniques.

Procedure: Timed sensor data were collected using a laptop computer via the serial output of the prototype. The participants took turns kicking the dummy using a variety of kicking techniques during a two-hour period. A variety of kicking techniques were studied, including roundhouse, back, push back, axe, front, push front kicks, as well as both front hand and back hand punching. However, we particularly paid close attention to roundhouse kick, because it is by far the most common kicking technique used during competition. We tested the kicks on both left and right leg.

Analysis: 54 clean samples were recorded from the pilot study. Figure 12 shows direct readouts from the system of a typical output sample measured from a single kick. The upper blue curve measures the peak power detected by the system, while the red bottom curve measures the voltage detected at that particular point in time. Note that a piezo sensor’s voltage decays with a time constant defined by RC, where R is the load resistance of the measuring device and C is the piezo capacitance. Without going into detail of piezo sensor circuit design4, recall that P=V2/R=Fv, where P = power generated, V = voltage observed, R = resistance, F = force, v = velocity. In other words, the sensor output is

4 For some details, see: http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/node15.html

more or less directly observing an agent that exerts a force F while it moves with a velocity v.

The results were encouraging. First of all, we observed similar power waveforms from a variety of different kicking and punching techniques, with different subjects of various rank, weight, sex, and leg dominance. A low-level noise filter was able to filter out other motions such as bending or leaping so that they did not generate any significant signals.

The measurements are quite consistent between kicks. We deduced, however, that the sensor gave very different peak power readings depending on the rank of the competitor and the type of technique delivering the hit. For example, with roundhouse kick, green and blue belt subjects typically generate about 2/3 power when compared to a red or black belt subjects (4 watts vs. 12 watts).

Right Roundhouse Kick

-2

0

2

4

6

8

10

12

0 6 12 18 24 30 36 42 48 54

Time

volts

or w

atts Voltage

Power inWatts

Figure 12. A sample output by the sensored hogu prototype

Certain kicks generate similar power peaks regardless of the subject characteristics. For example, a front kick typically register a reading at 3 or 4 watts, and an axe kick at 8 or 10 watts in all subjects. It is well-known that in current judging standards higher-level competitors rarely score with a front kick because of its potential for power delivery is low. Competitors usually score with back and roundhouse kicks, because these two techniques afford the most power and quickness. Thus these are reasonable measurements.

In determining a threshold for a valid “powerful” technique, there are arguments for both an absolute threshold that every technique must meet, as well as a relative threshold that varies according to experience. With 6 subjects we were only able to sample the space to gain an understanding of the characteristics of the measurements. Clearly more data is needed to understand this issue. Our initial analysis shows that the scoring threshold should probably be based on the experience of the competitor, which roughly equates to the rank of the competitor. Our current recommendation is 8 watts as the power threshold for a scored hit for advanced competitors (red and black belts), and 4 watts for intermediate and beginning competitors (green to blue belts). However, our prototype allows the threshold to be adjusted directly by the scoring display operator using the connected laptop.

Sparring Match Testing During this testing phase, we invited high-level competitors to several rounds of sparring with the SensorHogu.

Subjects: We purposely chose competitors with a lot of prior experience to get feedback on the system. The subjects were four national level champions5, two local tournament competitors, and four Stanford black-belt competitors. We also recruited three trained black-belt judges to score the matches.

Procedure: First, the participants familiarize with the system by kicking targets that have been instrumented with the sensor. This enabled them to get a sense of how hard one must kick to score. Real sparring matches were then conducted and recorded on videotape. Each subject participated in at least 2 rounds of competition.

Results: The videotapes were analyzed in slow motion by independent judges and the players themselves. Accompanying this paper, we produced and edited a video of the actual sparring matches using the SensorHogu equipment. The video shows each scored points in slow motion for in depth analysis of whether the points should have been awarded. Pictures from Figure 13 are taken from this video.

Figure 13. Screenshots from the videotape of the sparring test

study showing various scoring kicks.

5 Subjects agreed to allow us to divulge their identities for this trial. We had Kevin McCullough, 96 US National Lightweight Gold Medalist, 98 US National Team Trial Silver Medalist. We also had Chris Ariagos, a US Collegiate Champion, and Akiko Rod, 93 National Finweight Champion, 95 Olympic Festival Medalist, and 98 US Collegiate Silver Medalist. As well as James Song, 2000 National Junior Olympic Silver Medalist, and 1999 and 2000 California Junior Olympic Gold Medalist.

Over the two hour period, the system performed extremely well. Given that a single match is extremely tiring for the competitors, we were not able to accumulate enough point data to evaluate the system more quantitatively. After viewing the videotapes, the participants generally agreed with the scoring made by the system. In the tests conducted at Stanford, the system worked perfectly. The judges surveyed after the matches did not disagree with the scoring done by the system in the one-hour period they used the system. Feedbacks from all subjects were quite positive. In fact, as a result of these tests, we were invited by the US Taekwondo Union (USTU) to demo the system at the 2003 US Taekwondo Invitational at US Olympic Training Center in Colorado. Unfortunately, the event was canceled in May due to the SARS outbreak.

User Study Summary We conducted two major user studies on the feasibility of the wearable computing prototype for sensing a scoring kick in taekwondo. The first user testing showed that a wireless body protector with piezoelectric sensors can be used to accurately measure different amounts of power delivered to the body. The second user testing showed that the whole system performed well in real sparring matches, enabling judges to more reliably determine when sufficient power has been delivered for a scoring hit.

OVERALL DEVELOPMENT EXPERIENCE Our experience of moving an UbiComp system into actual practice in the field showed that this path is laden with unexpected problems. Here we will describe a few.

Our first prototype transmitter was about the size of a small pencil box, and we had mounted in the center of the player’s back since the spine area was supposedly off limits to attacks due to safety reasons. We discovered that incidental contacts were enough to potentially damage the wireless transmitter. It was only after severely redesigning the packaging and reducing the size of the transmitter that we were able to place the transmitter over the right shoulder. We also discovered that to modify a body protector to accommodate our sensors and transmitters was difficult if we wanted it to fit and feel the same as before. Luckily we found a good seamstress.

Another problem was a complete surprise. Anticipating heavy abuse, we had padded down the transmitter as well as the battery housing. Foam pieces were used to compact the battery so that it does not move during use. To our surprise, intermittent contact in the battery housing was still causing all kinds of electrical problems. Our solution was to suspend the battery in between two springs (instead of the standard housing which uses one spring and a metal plate on the other side.) Our intermittent contact problem was solved by suspending the battery and allowing the battery to vibrate inside the housing laterally as well as horizontally.

Overall, our experience is that to move a prototype to the field, one must pay attention to all kinds of details,

particularly the interfaces between mechanical and electrical components. In future adoption of this technique, these connections all require tamper-proof connections and verifiable trusted components.

RELATED WORK Researchers in ubiquitous computing started focusing on wearable computing devices in early efforts, most notably Active Badges [15], a system that wirelessly transmit people’s locations in a office building using wearable devices. This field has now blossomed into conferences such as IEEE ISWC Wearable Computing conference, now in its 7th year. For example, recent works have focused on platforms, methods of interaction design and user-centered studies of these wearable applications [12].

In the area of martial arts, our work is most similar to Lieu’s pioneering work with piezoelectric accelerometer and sensors for martial arts at UC Berkeley. First, Lieu used piezo accelerometers at the center of mass of an instrumented anthropomorphic head to measure the energy and terminal velocity of taekwondo kicks [9, 2]. Although not yet formally reported, Beck and Lieu also investigated the use of thin-film piezoelectric sensors for the purpose of scoring systems [3], but their system did not include wireless scoring electronics. Judges can only visually identify how hard each competitor was hit based on a LED indicator at the top of each competitor’s headgear. It was informally reported that this system quickly ran out of battery power, and occasionally judges do not see the scoring indicators due to occlusion. Moreover, it was difficult in this system to see multiple scoring points in a short amount of time. Since no central computing device records the signals, it is impossible to log the data for later analysis. To our knowledge, our wearable wireless computing devices in extreme sports is novel.

OTHER POTENTIAL APPLICATIONS A force sensing body suit can easily be applied in entertainment applications. For example, current game phenomenon “Dance Dance Revolution” allows the use of two pads in two user mode to sense both players’ feet locations. We can easily imagine that games could be build such that players will actually have to make contact with one another, thus enabling dancers to actually interact with each other, e.g. placing a hand on each other’s hips, or grabbing each other’s shoulders.

Our suit could also be used in military, police, or self-defense training scenarios. For example, a dummy outfitted with our sensors could be used to train soldiers in correct punching techniques. A course with multiple body targets could be set up, and soldiers would have to correctly attack each dummy with accurate and powerful punches before moving on to the next training stage. In self-defenses classes, pressure sensors could be used to identify whether correct techniques are being applied. For example, stomping on the opponent’s foot is one of the most effective

self-defense techniques. A dummy foot with sensors could be build to sense whether enough force is in the stomp to break the foot bones.

CONCLUSION Taekwondo is an extreme sport, and supporting the activity with wearable devices in a seamless way is non-trivial. We have succeeded in blending the scoring wireless devices as part of the competition environment. Our system uses piezoelectric force sensors that transmit signals wirelessly to enable the detection of when a significant impact has been delivered to a competitor’s body. Such a capability might be used in the future for other entertainment application in which low amount of impact is applied to the body. Our goal was to support an extreme human sport while merging and blending into the background of the activity. In our view, we have succeeded in our goal and believe this is the first step toward other future devices that might support other extreme sports and entertainments.

ACKNOWLEDGMENTS We thank Stanford Taekwondo Program for their support of this project. In particular, Coach Tim Ghormley provided much needed support during sparring classes for the testing and development of the system. Kent Noble helped conduct the user studies, and numerous members helped in testing the equipment.

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