The Design of a Portable Kit of Wireless Sensors for...

The Design of a Portable Kit of Wireless Sensors for

Naturalistic Data Collection

Emmanuel Munguia Tapia, Stephen Intille, Louis Lopez, and Kent Larson

House_nMassachusetts Institute of Technology

Long-term goals

In complex, non-laboratory settings suchas real homes…

1. Explore whether some context-recognition detection problems can be robustly solved by using many small, easy-to-install, multi-modal sensors ubiquitously distributed

2. Enable novel context-sensitive applications to be built and piloted

Long-term vision

• End user brings home a box of small, non-intimidating sensors

• End user scatters them around the home without expert help; may wear a few as well

• Context detection algorithms then “learn” to detect useful contexts, and context-aware algorithms do useful things

Long-term vision

Problem: adequate wireless multi-modal sensors for doing pilot research on this problem were not available

Where are we?

Either…

• Single homes laboriously sensorized (e.g. Aware Home, The Neural Network House, MARC)

• Simulated home settings(e.g. The Smart Medical Home UR)

• Real homes wired with a relatively small number of single-mode sensors for short periods of time

Prior home sensing work

Nearly all prior work required either…

• Laborious installation(e.g. complex, multi-point wiring attachments)

• Skilled engineering knowledge to install and maintain the system(e.g. multi-hop wireless nodes, RFID tag placement)

Portable sensors

Because sensors are so difficult toinstall and maintain, those installed inreal homes…

• Are usually of a single type/mode• Are usually tested in single environment• Are usually tested with a single user

Portable sensors

Key usability goals

As described in paper, these goals benefitboth the end user and the researcher:

• Ease of installation• Ease of use• Adequate longitudinal performance in

natural settings• Affordable for research

(with hundreds of sensors in a single environment)

At first, we were reluctant to build another sensing platform…

But none we could obtain met the 4 usability goals.

(A review of prior systems is in the paper…)

Ease of installation

Problems:– Making each node multifunctional and expandable adds

size, weight and complexity to devices

– Dangling antenna facilitates dislodgement

Mica2DOT iMote

Mica2

iMote

Ease of use• Quite often, the use of mesh network

topologies that promise self-configuration and unlimited coverage area result in:– Increased cost– Increased size (including battery) – Increased installation complexity– Increased points of failure in real homes

• Few systems allow for easy integration of high sampling and low sampling and environmental and wearable sensors.

Adequate performance

Evaluation of• Real (not theoretical) battery life• Wireless Tx/Rx ranges• Effects of environmental noise

These parameters have not been clearly reported in the literature for most of the existing sensor platforms, making it hard to predict how they will work in practice in actual homes.

In summary

Researchers who want to deploy largenumbers of multi-modal sensors simultaneously in non-laboratory settingssuch as homes have limited optionsthat are robust and easy to use

MITes (MIT Environmental Sensors)

Goal: collect data from hundreds of multi-modal sensors (environmental and wearable) from single receiver in non-laboratory deployments

• Easy of installation• Ease of use • Adequate performance• Affordable for research• Well characterized/tested

MITes sensor kit includes

Six environmental sensors (low bandwidth)

Five wearable sensors (high bandwidth)

current sensingtemperatureobject-usage-detection

Proximitylightmovement

location beaconsheart rate

ultra-violet radiation exposure

RFID reader wristbandonbody acceleration

nRF24E1

1.33 MIPS MCU + Transceiver

125 Tx/Rx Channels at 2.4GHz

Microstrip

Antenna

ADXL2XX

MEM Accelerometer

24AA320 4K

EEPROM Memory

Crystal

16MHz

MITes Board

Connector for single Digital

sensor

Demonstration

Object movement MITes

Measures: Object movementBattery life 46 days @ 10Hz range: ±2g 2-axis cost: ≈ $28.43 US

Accelerometer represents ≈ 40% of the cost

Measure object movement in theenvironment using acceleration; allow forhundreds

Object usage MITes

Light and temperature MITes

Measures: Ambient light intensityBattery life: 14 days @ 1Hz Range: 0.003-1ku W/cm2 Cost: $21.0 US

Measures: TemperatureBattery life: 1309 days @ 1HzRange: –40C to 125C Cost: ≈ $20.3 US

Measure environmental conditionsin an environment

Proximity MITes (MERL)

Measures: Proximity to area (binary output)Battery life 47.5 days @ 2Hz (9V) Range: Circle with 0-2.6m varying radius

(by replacing lid)Cost: ≈ $33.1US

Determine if people underneath

Current flow MITes

Measures: Current consumption in electrical appliancesBattery life 14 days @ 1HzRange: 30mA to 28A Cost: ≈ $75.5 US

Current flow MITes

Onbody MITes

Real-time 3-axis motion of multiple limbs

Measures: Onbody accelerationBattery life 1.5 days @ 200HzRange: ±2g or ±10g 3-axis Cost: ≈ $44.3 US

Heart rate MITes Uses industry standard Polar

chest strap

Measures: Beats per minuteBattery life 2.5days @1-255bmpRange: 30-240 bmp Cost: ≈ $95.5 USD

RFID reader wristband

Measures: RFID tagged objects + wrist accelerationBattery life 0.2days @ 5HzRange: 10cm Cost: ≈ $181 US

Based on Intel Research Seattle RFID glove (Perkowitz ETAL ‘04)

Determine motion when holding an object

Location beacon MITes

Measures: Rough location with respect to a receiver nodeBattery life 5days @12HzRange: 2.5, 3.8, 4.8, and 9.4m outdoors, 0.7, 3, 4.5, and 6m indoorsCost: ≈ $48.5 US

Determine proximity to a receiver

Beacon MITes

Ultraviolet light MITes

Measures: Body ultraviolet radiation exposure Battery life 2.58 days @ 1HzRange: 0-28UV Cost: ≈ $93.5 US

Continuously measure exposure to sun

MITes receiver

Single receiverUSB or serialconnector

Serial output

Design decisions

Keep it simple: star network topology

SingleRx

Tx Tx

TxTx

TxTxTx

Tx

SingleRx

Receiver sampling implications

Ch0

Ch1

Ch4

Ch7retransmission

No collisions

Up to 125 channels (fewer in practice)

TDMA at receiver to collect datafrom sensor nodes


Ch0

Ch1

Ch4

Ch7retransmission

No collisions

tLISTEN

SingleRx



Ch0

Ch1

Ch4

Ch7retransmission

No collisions

tLISTENSingle

Rx



Ch0

Ch1

Ch4

Ch7retransmission

No collisionstLISTEN

SingleRx



Ch0

Ch1

Ch4

Ch7

No collisions

tLISTEN

SingleRx


SingleRx

Low sampling rate nodes

High sampling rate nodes

Design decisions

Keep it simple: featherweight MAC protocol

Allow hundreds of lowsampling nodes

Ch0

Ch1

Ch4

Ch7retransmission

No collisions

(M. Feldmeier and J. A. Paradiso ‘04)

Featherweight appropriate

• Sensor sampling rate is low• Nodes in star topology• Data flow unidirectional (sensors to Rx)• Propagation delays (ms) are tolerable• Sporadic loss of data from single sensor

usually ok (due to sensor density)

Appropriate for activity recognition!

Featherweight advantages

• Cost savings– No receiver, carrier detection circuit or

precision clock needed on sensors

• Energy savings– No time spent listening for control packets

of forwarding data on sensors

• Fast network setup– No network parameter tuning

• Simple implementation– Adds robustness

Disadvantages of approach

• Due to TDMA:– List of channels needs to be specified

beforehand for high SR sensors– The more high SR sensors there are, the

lower the effective SR for each(6 accelerometers at 180Hz; SR = 180/6 = 30Hz)

• Due to Featherweight retransmission:– Possible loss of data due to collisions during

high activity periods

Advantages of approach

Collecting data from real homes with

• Hundreds of low sampling rate sensors attached to objects

• Different types of low SR sensors • Many (e.g. six high SR sensors)• Good battery life for high and low SR

sensors

(All at an affordable cost for research)

Possible sources of error

• Channel impairments due to environmental noise, shadowing, fading, reflection, refractions, obstacles, etc.

• Collisions due to simultaneous transmission of multiple sensors

Collisions in object movement MITes

• In practice, probably rare in real homes(in our prior work with switch sensors on objects in homes, out of 10,458 activations, only 1.2% were within 2s)

• Still, a concern when simultaneously manipulating several objects(e.g. opening a drawer with many objects inside)


Collision noise can be overcome withretransmission

In practice, retransmission quite good

8870

9560 (PRx=40)10030

Theoretical PRx(%)Six retransmissions

Assuming Ploss(%) one transmission

Worst-case simulation: 500 sensorsFire randomly every 5, 1 and 0.5 secs

Single transmission (worst case still good)600.5

951

975

PRx(%)Time window (s)


MITes performance in real settings

Wireless Tx/Rx range outdoors

Rx =?distance

Tx =180 packets

Parallel orientation

Rx =?distance

Tx =180 packets

Random disposition

Wireless Tx/Rx range outdoors

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 re

ceiv

ed

GigantMITesLinxYageo

20 40 60 80 100 1200

20

40

60

80

Tx/Rx Range LOS antennas in random disposition

Distance in meters

Per

cent

age

of p

acke

ts re

ceiv

ed GigantMITesLinxYageo

38.1 mRandom disposition

106.6 mParallel orientation

MITes microstrip ($0.01)

Tx/Rx range measured at distance where PRX=90%

Wireless Tx/Rx range indoorsTested in thePlaceLab

• A 16.5x5.2m living laboratory condominium with interior walls constructed from steel frame and drywall.

• The environment has several kilometers of digital and electrical wiring

Wireless Tx/Rx range indoors

Floorplan view


Six receivers scattered around apartment


31 Tx sensor nodes installed at worse case locations throughout the apartment

(1) random orientation and (2) random distance from the floor


R1


R2


R3


R4


R5


R6


R1 R2 R3 R4 R5 R688.6 88.4 93.5 98.4 70.9 75%


R1 R2 R3 R4 R5 R688.6 88.4 93.5 98.4 70.9 75%

Mean each receiver:

Impact environmental noise

• % packets lost at the PlaceLab when WLAN was on and the following devices were turned on

– Vacuum cleaner: 3.7% drop– Microwave: 4.3% drop– Cordless 2.4GHz telephone: 1.2% drop– WLAN vs. no WLAN: 0.006% drop

MITes installation time

• Across two subjects installing 175 sensors total, measured an average installation time of only 36 seconds per sensor

Take away

• Simple strategy (TDMA + featherweight MAC) performs well in a real environment

• Simplicity of design contributes to robust performance, low price point, and ease of use

• Getting the devices to work as a demonstration in the lab was substantially easier than getting them to work in practice in uncontrolled conditions in a real home; in situ testing was critical to development

Thank you!

• MITes hardware and softwarespecifications are available online– http://architecture.mit.edu/house_n/MITes

• Questions? Contact:

– Emmanuel Munguia Tapia [email protected]

– Stephen [email protected]

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