Micro-sensing Modalities

Hongtao Du

August 31, 2004

Introduction Sensor

Devices that transform (or transduce) physical quantities such as sound, pressure or acceleration (called measurands) into output signals (usually electrical) that serve as inputs for control systems [1].

Sensors must satisfy a difficult balance between Accuracy Robustness Manufacturability Low cost Small size

Sensor Network Smart Sensor Web (SSW)

SSW exploits the information from commercial products distributed throughout an area (local TV cameras and other commercial devices already transmitting over commercial airwaves and the Internet), and seed the battlefield by rapidly deploying military sensors via air drops, robotic vehicles, pre-positioned assets, soldier platforms,UAVs, or overhead surveillance [2].

Micro-sensor Node

Sensors can be classified by … Working principle

Electro-magnetic: acoustic sensor, seismic sensor

Conductivity: CCD, temperature sensor

Chemical reaction: biosensor

Application Automotive sensors

Powertrain Chassis Body

Working Principles of Microphones Microphones are used to detect acoustic signals and

produce a voltage or a current proportional to the sound.

Source Compression waves (330m/sec) Microphones

Dynamic Ribbon

Crystal microphone: based on piezoelectric effect of crystal Condenser microphone: a capacitor with two plates

Crystal Condenser

Directional Patterns Omni-directional:

sensitive in all directions Bi-directional: front and

rear, 90 each Cardioid: the specific

direction it points to

Acoustic Sensing Phenomenology

Scattering

Sound waves are scattered into all directions when they reach an obstacle.

Reflection Law of reflection: the

angle of incidence equals the angle of reflection.

Refraction The bending of waves when

they enter a medium where their speed is different.

Doppler Effect When the source or the receiver is moving relatively

to each other, the frequency of a wave observed at a receiver changes.

sources

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Example

Microphone used in WINS NG 2.0

sensor platform from Sensoria Co.

1-second sample Power Spectral Density (PSD)

Geophone /Accelerometer

Structure of geophone

Geophone Electro-magnetic Low frequency:

< 100Hz

Accelerometer Piezoelectric effect High frequency

> 100Hz

Seismic Waves Body wave: travels the earth’s inner layers at a

higher speed and propagates in three dimensions Compression (P) waves Shear (S) waves

Surface wave: moves along the surface of the ground and propagates in two dimensions Surface waves propagate slower than body waves Love waves Reyleigh waves

Seismic Sensing Phenomenology

A: reflection B: direct path

C: P-S wave conversion D: refraction

Example

Geophone examples

1-second sample Power Spectral Density (PSD)

Magnetic Sensing Only detect presence, strength, direction of

magnetic fields Ferrous object (vehicle, airplane) disturbance in

uniform field Very useful in navigation control system

Pressure Sensor Principle: Piezoelectric effect Fabrication process

Batch fabricated and a thousand or more per wafer Piezoresistive strain-sense elements are implanted in

appropriate areas of an etched silicon diaphragm The stain-sense elements are electrically connected

into internal circuit, thereby providing a means of detecting pressure acting on the diaphragm.

Infra-red Sensing Infra-red radiation is an electromagnetic wave. Used in field security, alarm system, remote

control, etc. Infra-red motion detectors

Passive infra-red (PIR) Active infra-red (AIR)

PIR A lens allows the sensor to divide the field of

view into several zones. For the best performance, the target should move

across the two sensing elements within the sensor coverage.

Example

PIRs from Visionic Ltd.

AIR Two units

Infra-red photodiode and Infra-red sensitive phototransistor

Infra-red reflector

Optical Sensing

Principle: conductivity Charge-Coupled Detector

(CCD) Cell Electron-Hole Pair (EHP)

Other Sensors

Thermal sensor MicroFLIR Weight: 70g, volume:12 cubic inches, power:

540 milliwatts. Sponsored by US Army Night Vision and

Electronics Directorate (NVESD)

Temperature Sensor Silicon

Single-Crystal silicon Restriction of 150 C

Important Technology in Sensor Developments

Micro, Electro-Mechanical Systems (MEMS) and Micro,Electro-Optical-Mechanical Systems (MEOMS). Economy of batch processing Miniaturization Integration of on-chip electronic intelligence

Tendency System-on-chip: Integration sensing unit

and processing unit Low voltage analog/digital circuits

– save power by square

Sensor die size is shrinking, wafer diameters is increasing.

– both help to lower the cost of sensor manufacture

Reference Norton, H., “Transducer fundamentals”, in Handbook of

Transducers, Englewood Cliffs, NJ:Prentice Hall, 1989. Paul, J.L., “Smart Sensor Web: Web-based exploitation of sensor

fusion for visualization of the tactical battlefield”, IEEE Aerospace and Electronic Systems Magazine, Vol.16, No.5, pp.29 - 36, May 2001.

Thank you!

Questions?