The Design of a Portable Kit of Wireless Sensors for Naturalistic

Data Collection

Emmanuel Munguia Tapia, Stephen S. Intille,

Louis Lopez, and Kent LarsonMassachusetts Institute of Technology

@ Pervasive2006

概要• MITes (MIT environmental sensors)

– 無線センサデバイスシリーズ– 日常の活動に関するデータ収集が目的

– 目標• 使いやすい• 設置しやすい• 低コスト• 家など non-laboratory な環境でも使える

– 紹介・現段階の評価 (placelab で利用 )

mites

ダニ・・・

MITes

MITes センサ群• 環境側に設置するセンサ

– 動作(Ball, Mercury, Reed Switches)

– 使用状況取得を目的とした動作センサ ( 加速度 )

– 照度

– 温度

– 隣接・距離

– 電力消費量

• ウェアラブルセンサ– カラダの動作（加速度）

– 心拍数

– 紫外線量

– RFID リーダ・リストバンド

– 位置情報

Introduction

• 現代 pervasive computing の課題– コストの壁– non-laboratoryな環境で使えるような柔軟性の欠

如– しかし、ムーア氏いわく・・・

小さくてコストが低くて使いやすいモノの時代がくる

• 日常生活の活動を取得するためのセンサ– 目標を満たしたセンサ群はまだ現れていない– 分かりやすく、設置しやすく、低コストで、家などで

使える MITes を提案し、実際に作ってみた

家に設置するための Design Goals

1. Ease of Installation

2. Ease of Use

3. Adequate performance in natural settings

4. Affordability

Usability Goals

(What)

Sensor Design Goals (How) Benefit to Researcher(R) and Subject (S) (Why)

Ease ofinstallatio

n 

       Light weight, and small sensor nodes (portable)       Self-contained single point of contact to body/home       Single receiver to collect data from multiple sensors       Real-time simultaneous data -acquisition from high and low sampling rate sensors       No pre-configuration required or threshold setting       Good indoor Tx/Rx range and easy to detect if in range

       Minimizes installation time (R)       Subjects can install sensors themselves (R/S)       Subjects can re-install sensors if they dislodge, simplifying maintenance for researchers during studies (R/S)

 

Ease of use

       Real-time simultaneous data acquisition from multiple high and low sampling sensors with single receiver        Each sensor does one thing well       Convenient battery life       Robustness to environmental noise

          Facilitates maintenance (R/S)          Low training overhead (R/S)          Facilitates the addition of new sensors(R)          Reduces failure points (R)          Decreases probability of data loss (RS)

Adequate 　longitudinal performance in natural settings

       Convenient battery life/low power       Robustness to environmental noise        Self-contained, resistant packaging       Good indoor Tx/Rx range and easy to detect if in range       Performance valuated in a natural setting (Section 5.1)

          Decreases probability of data loss (R/S)          Facilitates design of data collection (R)

Affordable 　for researchers

       Design with low-cost components       Each sensor does one thing well

          Deployment of hundreds of sensors (R)

１． Ease of Installation （設置しやすく）

• Minimization of Installation time– 家にだいたいセンサノードを２００個置くとする– これまで (5-10min./node) x 200 = 16-32 man-hours

– センサの設置方法を簡易化する– ４点固定→１点固定（動いているかどうか知りたいだけ）

– これからは (5-60sec./node) x 200 = 1-2.5 man-hours

• 既存のモノは・・・– サイズがでかい（ snap-on boards, AA battery, antenna ）– 電波が弱い (low freq)

２． Ease of Use （使いやすく）• Robust communication protocols

• Good communication range– 余計な中継ノードを必要としない

• Infrequent (& easy) battery replace

• Add & removing sensors with little or no configuration.

• 既存のモノは・・・– ネットワークの構造・設定が複雑– low-rate for event detecting, high-rate for wearable sensors– 簡単に複数の種類のセンサのデータをまとめて見る手段がない

３． Adequate performance in natural settings （自然環境への適応）

• 温室育ちのセンサではダメ

• 頑強なケース

• 電波や騒音などのノイズに対する抵抗力

• 既存のモノは・・・– あまり論文にのっていない– 研究室向けのモノが多い

Affordability

• 安くないといけない

• 既存のモノは– $15,600(Motes x 200)– $26,000(+2 軸加速度 )

– ECO System は合格ライン

– u-part はライバルだけど– 自然環境での実験が行われていない

MITes ノードの概要

• 3.2 x 2.5 x 0.6cm• 8.1g (battery incl.)

• 低コスト防水ケース• 長いアンテナ無 (2.4GHz 帯 )• 超寿命 (mobileMITes>24hr,others>week)• $6 per Unit

• Receiver→USB or RS232→NotePC• C# 　 and JAVA available

Sensor Design Goals (What) Implementation (How) Benefit to Researcher (R) and Subject (S) (Why)

Light weight and small form factor

           Low-profile highly integrated chip components, 3cm Microstrip antenna (possible with 2.4GHz), 3.2x2.5x.0.6cm PCB design, 20mm coin cell battery          Total board size of 3.2x2.5x.0.6cm          Total weight (including battery) of 8.1g

          Facilitates installation (R)          Minimizes sensor dislodgement (S)          Improves portability (R)          Comfortably wearable (S)          Sensors fit on most household objects (R/S)

Self-contained and resistant packaging           Sensors embedded in low-cost water resistant plastic cases           Facilitates installation (R)          Physically robust (R)

Self-contained, with single point of contact to body/home

          Single, self-contained acceleration sensor to measure object usage          No dangling antennas          Minimize external sensors whenever possible

          Rapid installation (R)          Easy to reattach if dislodged (R/S)          Attach with only a small bit of putty (R/S)          No parts to break/yank (R/S)

Real-time simultaneous data acquisition from multiple high and low sampling sensors with a single receiver

          FDMA in wearable/high sampling rate sensors, and single channel shared by low sampling environmental sensors          Combination of TDMA and FDMA at receiver to collect data from sensors

          Data acquisition from environmental and wearable sensors (R)          Reduced costs – only one system needed (R)          Rapid installation (R)          Real-time applications possible (R/S)          Many wearable sensors can be used without bulky receiver devices or wires (S)

No pre-configuration required or threshold setting

          Simple star network topology          A featherweight MAC protocol with only one parameter that is fixed (num. of retransmissions) (See Section 4.3)          Receiver outputs simple data format via serial port or USB serial for easy programming

          Minimizes installation time (R)          Subjects can install sensors themselves (R/S)          Facilitates first time setup of system (R)          Decreases points of failure (R)

 

Good indoor Tx/Rx range and easy to detect if in range

          Transceiver with 0dB output power          PCB design that maximizes antenna ground plane          Optimally cut /4 monopole microstrip antenna          Extensive field measurements of outdoors and indoors range (see Section 5.1)

          Reduces cost since a typical home requires few receivers (R)          Facilitates deployment (R)          Facilitate subject mobility (S)

 

Convenient battery life (mobile MITES > 24 hr; receiver if attached to mobile device >24 hr; other sensors > weeks)

Node          Low power components          Low duty cycles          Embedded intelligence at the sensor node to broadcast information only when necessaryOverall System          Simple star topology – no overhead          A Featherweight MAC protocol – no overhead

          Long data collection deployments (R)          Reduce battery replacements (S)          Acceptable weight when worn (S)          Fits in pocket when receiver embedded in mobile device (S)

No dangling antenna           /4 Microstrip monopole onboard antenna and high Tx/Rx frequency of 2.4GHz           Facilitates installation (R)          Minimizes sensor dislodgement/breakage (S)          Easy to carry/pack (R)          Comfortable and aesthetical form factor (S)

Robustness to environmental noise           EMI reduction bead cores          Tantalum capacitors where required          Noise efficient PCB design

          Decreases probability of data loss (R/S)          Decreases probability sensor failure (R/S)          Permits deployment in natural settings when noisy appliances are being used (R/S)

Design with low-cost components           $3 integrated C and transceiver (RF24E1), $0.01 microstrip antenna, $0.30 CR2032 coin cell battery

          Easy to add as many sensors as desired (R)

Each sensor does one thing well           One sensor optimized per task (reducing complexity and cost of individual sensor)           Reduces learning-curve/implementation complexity (R)

通信 1/2

• Low Rate<10Hz , High Rate>10Hz

• Combining FDMA & TDMA– FDMA→HighRate TDMA→LowRate

• Data format– Head 2B, channel1B, Payload 4B 計 7bytes

• Star network topology with feather weight MAC

通信 2/2

• 500 個のノードで実験• 各ノードの発信レートとパケットロスの関係• 発信回数とパケット伝達確率の関係

• レートを 5sec. にすると 97 ％のパケットが伝達

• レートを 0.5sec. にすると 60 ％のパケットが伝達• ６回の再送信で 100 ％のパケットを伝達

2 3 4 5 6 7 80

0.2

0.4

0.6

0.8

1Reception probability vs. number of retransmissions

Number of retransmissions

Re

cep

tion

pro

ba

bili

ty

Ploss=0.016

Ploss=0.2

Ploss=0.4

Ploss=0.6

Ploss=0.8

Ploss=0.9

50 100 150 200 250 300 350 400 4500.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

Reception probability vs. number of sensors

Number of sensorsR

ece

ptio

n p

rob

ab

ility

0.5 Sec

1 Sec

5 Sec

10 Sec

(a) Plot of RxP

vs. number of retransmissions

(b) Plot of RxP

vs. number of sensors in the system.

Multi-modal Sensor Types

• RS232, I2C, SPI output のセンサなら拡張 OK

• Mobile, Current sensing, Ultraviolet exposure, Temperature, Light, Heart rate, Location beacons, Proximity, RFID wrist-band

MITES Type Measures Sensor Range Res Battery life (days)

Cost ($)

Object usage Object manipulation Accelerometer. ADXL202

2g 2-axis 0.005 46, 10Hz 28.43

Mobile OnbodyAcceleration (Acc)

Accelerometer. ADXL202/10

2g or 10g 3-axis 0.005 1.5, 200Hz 44.3

Temperature Temperature MAX6677 –40C to 125C ±1.5 1309, 1Hz 20.3*

Light Ambient light intensity

DigitalTSL235R

0.003-1ku W/cm2at 320-1050nm

16bit 620, 1Hz 21.0*

Current sensing Current consumption

Split-core current transformer

30mA to 28A 10bit 14, 1Hz 75.5*

Heart rate Beats per minute WearLink Polar chest strap/receiver

30-240 bmp 1 2.5 @1-255bmp9Vbattery

95.5*

Ultraviolet exposure

Onbody UV exposure

UV PhotodiodeEryf by Sglux

0-28UV 0.027 2.58, 1Hz 93.5*

Location beacons Rough location with respect to a receiver node

Tx beacon and Rx node counting packets received

2.5, 3.8, 4.8, and 9.4m outdoors0.7, 3, 4.5, and 6mindoors

- 5, 12Hz 48.5*

Proximity Proximity to area(binary output)

PIR motionKC7783R

Circle with 0-2.6m varying radius(by replacing lid)

1 47.5, 2Hz9Vbattery

33.1*

RFID wristband Acc + RFID tagged objects

ADXL202/10M1 Mini SkyTek

10cm - 0.2, 5Hz 4.7 Li-Po.

181*

Table 3. Summary of MITes types available and performance parameters.

Evaluation1 屋外無線実験

• Line of sight range (LOS)• 屋外で実験• 180 packets per sec. 発信してパケ落ちを計

測• 無線 channel0(2.4GHz)

  MITes microstrip ($0.01)

Linx Chip($1.5)ANT2.45CHP

Yageo chip ($3)

Monopole Gigant ($35)

Parallel orient. 106.6 m 60.96 m 15.24 m 114.3 m

Random disp. 38.1 m 15.24 m 7.62 m 30.48 m

  

    Titanis is a /4 monopole antenna manufactured by Gigant corp

  Microstrip /4 monopole embedded antenna 

The Linx embedded chip antenna 

The Yagui embedded chip antenna 

Figure X. MITes Antennas installation and antenna types

  Titanis Yagui (Blue) Linx (white)

Central frequency 2.4-2.5GHz 2.45GHz 2.45GHz

Bandwidth 310MHz > 100MHz 180MHz

Impedance 50 50 50

Polarization Linear Linear Linear

Maximum power Not specified in datasheet 1W Not specified in datasheet

Efficiency (Typ) 65%    

Gain 4.2dBi 0dBi 0.8dBi

VSWR < 1.5:1 < 2 Not specified in datasheet

Temperature (C) -40 to 85 -55 to 125 -25 to 85

Antenna type Swivel Embedded Embedded

Size 20x19.5x62.5mm 8x3.5x.9mm 6.5x2.2x1.0mm

Weight 7.5g Not specified in datasheet

Not specified in datasheet

Table 1. Presents the different antenna properties or parameters that are important to consider while selecting an antenna.

    

 

0 50 100 1500

20

40

60

80

100Tx/Rx Range outdoors LOS antennas parallel

Distance in meters

Per

cent

age

of p

acke

ts r

ecei

ved

GigantMITesLinxYageo

 

20 40 60 80 100 1200

20

40

60

80

Tx/Rx Range LOS antennas in random disposition

Distance in metersP

erce

ntag

e of

pac

kets

rece

ived

GigantMITesLinxYageo

(a) LOS range outdoors with antennas parallel.

（ b ） LOS range with the Tx antenna in a random disposition.

Evaluation2 　無線ノイズ実験

• 自然界のノイズを入れてみる• 無線 LAN + α の時のパケ落ち率

• + 掃除機　 3.7 ％落ち• + 電子レンジ　 4.3 ％落ち• + 電話（子機）　 1.2 ％落ち

Evaluation3 　電池・インスタレーション実験

• 平均発信回数寿命– 新品 CR2032 を使って　 20.85million 回発信

• 平均インスタレーション所要時間– 75 個 /45 分　 100 個 /60 分– 36sec. per node

まとめ• placelab を始めとして、様々なプロジェクトで活用中

• 医療関係への展開– 日常生活の中での状態を同時に複数の視点から見れる– TV と心拍数の変化– UV MITes で　皮膚ガンの研究

• ミーティングのコンテキストをリアルタイムで取得

• non-laboratory environment• たくさん設置して使える

• 買いたい場合は著者に連絡• 現段階での最も難しいのは、購入・センサの取り付け・プログラムの焼きこみ　のプロセスである。