a multi-element smart gas sensor

8/8/2019 A Multi-Element Smart Gas Sensor

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Sensors Expo -- June 2004 1

A Multi-Element Smart Gas Sensorwith IEEE 1451 Protocol

Darold Wobschall

State University of New York at BuffaloDept. of Electrical Engineering

and

Esensors, Inc.May version


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Presentation Topics

w Goals and Applications

w Review of gas sensor technologies

w Analog signal conditioners various technologies

w Smart (digital) sensor configurations

w IEEE 1451 protocol

w Multi-element gas sensor module design

w Networking considerations

w Test data


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Goals and Applications

w Measurement of gases for environmental

monitoring, industrial safety and homeland security

w Design a sensor pad which allows interchange ofsensors for various gases

w Convert gas sensor data to digital form (smart

sensor)

w Interface to various networks

w Configure automatically (plug and play)

w Use commercial, off-the-shelf sensor elements


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Gas Sensor Technologies

w Semiconductor resistive*

w Semiconductor voltage*

w Amperometric*w Catalytic*

w Infrared

w Photo-ionization

w Fluorescentw Surface acoustic wave (SAW) & vibrating beam

w Capacitive* and other

* Used in this multi-sensor


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Solid State (Semiconductor, Resistive)Characteristics

w Based on Tin Oxide (SnO2) or similar metal oxidesemiconductors

w Surface reaction with ambient gases when hot (350-500 oC)w Heater (e.g. 4 v @ 100 mA) heats substrate

w Adsorbed gas reduces grain-boundary potential barrier andthus increases conductivity (decreases resistance)

wDelta-R is a function (approx. log or square root) of gasconcentration (ppm)

w Resistance also decreases with temperature so temperaturecontrol needed for zero stability


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Solid state sensor construction

CityTech Ltd Semiconductor Sensor


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Solid State Resistive

Responses



Solid Electrolyte Gas Sensor

w Similar to semiconductor gas sensor but has voltage output

w Heater (5v @ 11.5 ohms)

w Has thermistor for temperature controlw Vsen increases 50 mv per factor of 10 change in gas conc

(220 to 490 mv at 350 ppm)

w Requires hi-Z amplifier

w Examples: Figaro TGS4160 (CO2)or Oxygen (zerconia)

w Periodic re-zeroing desirable



Amperometric Characteristics

w Chemical reaction involving gas releases electrons at

electrode (electrolysis reaction)

w Example: O2 + H2O + 2e- 2 OH-

w Gas is dissolved in electrolyte (e.g. H2O)

w Reaction is reversible so number of electrons released is

proportional to gas concentration (gas conc in electrolyte is

proportional to partial pressure of gas in air)

w Reaction occurs at specific applied voltage (e.g. 0.55 volts)

w Sensor current output is proportional to gas conc (ppm)



Amperometric Construction

City Tech Ltd Toxic Gas Sensors



Infra-red Principle

w Some gases absorb light at particular IR wavelengths

w I/Io = e-Ax

where I/Io is light absorbed during transmission,

x is path length and A is absorption coef. at specific wavelengthw Transmission filters select specific wavelength bands

w A is proportional to gas concentration

w IR sensors reproducible but not sensitive (need high conc or long paths)



Infra-red Construction

Gilway Visible/IR Lamps for NDIR Gas Sensors

Acoustical detection an attractive option



Photo-ionization

w High energy UV photons (> 3ev or



Fluorescent

w UV light impinges on some organics produces a

fluorescent light proportional to ambient gas

concentration (e.g. oxygen)w High sensitivity (because photo-detectors are sensitive)

w Applicable only to a few gases (but used with manybiological materials where it can be sensitive and selective)

w Few commercial sensors using this technology areavailable.



SAW and vibrating beam

w Surface acoustic wave (SAW) travel from transmitter to to receiver on

substrate surface

w Velocity depends on surface mass which is effected by adsorbed gases

w Positive feedback produces oscillation at frequency which depends onsound velocity and thus gas concentration



SAW and vibrating beam

continued

w Usually used in pairs (one not exposed

to gas) and difference (beat frequency)

measured

w Moderate sensitivity and selectivity

w Vibrating beam type (usually quartz)

resonance frequency varies with mass

loading and thus gas concentration.

wCan be small and low cost

w Few commercial products available.



Capacitive

w Dielectric constant of polymer increase with absorbed gas

such as water vapor (K is 80 for water, 2-3 for polymer)

w Typically C increase by 10-30% as relative humidity

(RH) varies from 0 to 100%.



Other Gas Sensor Technologies

These techniques have few commercial product available

w Polymer resistance

w Fiber optic

w ChemFET

w Miniaturized versions of mass spectrometers



MEMS Sensors

w Micro Electronic Mechanical Systems (MEMS) type

sensors are miniaturized versions of types already described

w Promise much smaller size, lower power and lower costthan conventional gas sensors

w Many under development but few commercially available.


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Linearization by shunt resistors

Rs has log response vs T

(thermistor)


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Linearization by load resistors

Semiconductor sensor response

0

5

10

15

20

25

30

35

40

45

0 20 40 60 80 100 120

Conc (ppm)

Resistance(kohm)


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Signal Conditioner forVoltage Type Sensors


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Signal Conditioner forAmperometric Type Sensors


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Generic Smart Sensor Block Diagram

BUS/NETWORK

DATA

LOGGER

(optional)

CALIBRATION

/ ID MEMORY

SENSOR

ELEMENT

MICRO-

CONTROLLER

BUS/NETWORK

INTERFACE

BUS/NETWORK

INTERFACE

COMPUTER (READOUT,

DATA STORE)

ANALOG

SIGNAL

CONDITIONER

A/D


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Need for Network Standards

w Smart sensors require a digital network

w Over 50 sensor networks and busses in common use

wUsers and manufactures would like one standard to reducemanufacturing/installation costs and for plug&play capability

w No single local network is likely to dominate in near future due to

divergent needs

w The Internet via Ethernet will likely be one of the dominate networks

(but cost and complexity are problems)w The IEEE 1451 standard for sensor interfacing overcomes many of the

complications of multiple networks


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IEEE 1451 Parts

w IEEE 1451.0 Protocols/formats (approval process underway)

wIEEE 1451.1 Object model (approved 1999)

w IEEE 1451.2 Interface (approved 1997)*

w IEEE 1451.3 Local network (approved 2003)

w IEEE 1451.4 Analog & TEDS (just approved)

w IEEE 1451.5 Wireless (early approval process)

w IEEE 1451.6 Canbus (just started)

* Enhancement /revision working group in process


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Present (1997) IEEE 1451.2System Block Diagram

Dot2


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IEEE p1451.2 TEDS Blocks--Transducer Electronic Data Sheet --

Machine Readablew Meta-TEDS (mandatory)

wChannel TEDS (mandatory)

w Calibration

w Physical Layer Meta (proposed)

w Physical Layer Channel(proposed)

Note: One TEDS per channel forChannel and Calibration

Human Readablew Meta-ID TEDS

w Channel-ID TEDS

w Calibration-ID TEDS

w Application Specific

End Users Application-Specific

TEDSw Future Extensions

Industry Extension TEDS

Dot2

New Tuples format TEDS approved by Dot 2 working group


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Advantages of the

IEEE 1451 Standardw Continuing network interface and microcontroller cost

reductions have made interface more attractive.

w The sensor industry is closer to recognizing thenecessity for a sensor network standard.

w The general concept of the IEEE 1451 approach,

especially TEDS, is supported by many.

w Working groups are addressing the dot2 problems andexpanding the standard via dot3, dot4, and dot5.


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TEDS Memory Types

w Option #1 Standard dot2 TEDS

* Meta-TEDS (binary/machine readable)

[Meta is all channel]* Meta-ID-TEDS (ASCII)

* Channel-TEDS (binary)

Option #2 Modified dot4* Basic TEDS (8 bytes, binary)

* ID TEDS (user provided 24 bytes ASCII)* Standard templates available but special used here

Dot2


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IEEE 1451.4 (Dot4) Interface


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Basic TEDS

Basic TEDS (8 bytes)

w Manufacturer ID (14 bits)

w Model Number (15 bits)

w Version Letter (5 bits, A-Z)

w Version Number (6 bits)

w Serial Number (24 bits)

Dot4


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Dot 2 to Dot 4 TEDS Conversion

w Dot4 TEDS read over 1-wire (specific sensor head)

w Contains standard TEDS and special (manufacturer specific) TEDS

wSpecial head configuration data used for signal conditioner setup

w A/D data read in and converted to floating point (Dot2 option)

w Calibration data from Dot4 TEDS used to convert to engineering units

w Data from Dot4 standard TEDS used to prepare tuples style Dot2 (Dot0)

TEDS (Meta, Channel, Meta-ID, and Channel ID)

w Parameters (fields) not in Dot4 TEDS inserted into Dot2 TEDS

w UUID or Universal Unique Identification (10 bytes) consists of 6-byte

Dot4 TEDS as the least significant + 4 bytes (FFFF0000h), which will

not occur using the specified Dot2 formula


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Multi-Element Gas Sensor

Design Challenges

w Sensor elements for different gasses use differenttechnologies, and thus signal conditioners, making sensorhead/element interchange difficult.

w Many reliable off-the-shelf sensor elements require largeamounts of power (heaters) thus reducing battery life.

w Multiple communications channels (Wireless, Internet via

Ethernet) may be neededw Auto configuration (plug and play)

w Should be easy to use

w Moderate cost desirable


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System Block Diagram


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Sensor Pod Board Organization


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Sensor Pod Block Diagram


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Sensor Head Block Diagram

* Technologies Accommodated

Semiconductor with Heater (e.g. CO) 3 Types

Amperometric (e.g. O2) 2 Types

Catalytic (e.g. Methane) 2 Types


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Gas Sensor List

HVAC/Environmental

w Carbon dioxide

w Humidity /Temperature

w Smoke

Decontamination/Industrial gases

w VOC/Methyl bromide

w Ozone

w Hydrogen peroxide

w Oxygen

w Combustible gases

w Carbon monoxide

Toxic gas sensors

w Hydrogen sulfide

wSulfur dioxide

w Chlorine (chlorine dioxide)

w Hydrogen cyanide

w Nitric oxide

w Nitrogen dioxide

w Hydrogen chloride


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Sensor Signal Conditioner- Semiconductor type -


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Sensor Signal Conditioner- Amperometric type -


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Sensor Signal Conditioner- Microcomputer section -


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Humidity/Temperature Sensorwith digital output


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Communication Options

w Internet via Ethernet

TCP/IP protocol similar to websensor

w RF Point-to-point initial wireless version

900 MHz spread spectrum (Chipcom)

w Full-feature wireless network (IEEE 802.15.4/Zigbee)

Scheduled transmissions for power reduction, node-to-node

hopping, collision recovery, error handling


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NCAP Block Diagram


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Test data

w VOC response to ethanol measured

(100 to 3000 ppm)

w Method: small volume solvent

injections into closed container

w Solid state VOC sensor resistance

change converted to ppm and

transmitted digitally

w Date from computer plotted

Sensor Response

0

500

1000

1500

2000

2500

0 500 1000 1500 2000 2500

C o n c ( p p m ) - - a c t u a l

Series1

Not final data

Printout line here


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References

w R. Johnson, et al A Standard Smart Transducer Interface

http://ieee1451.nist.gov/Workshop_04Oct01/1451_overview.pdf

w Philip N. Bartlett & Julian W. Gardner Electronic Noses: Principles and

Applications, Oxford Univ. Pr; (March 1999)

w R. Frank Understanding Smart Sensors, 2nd edition, Artech House

(2000)

w D. Wobschall IEEE 1451 Prototype Dot 2 and Dot 4 NCAPs with

Internet Access, Proc. Sensors Expo (Sept 2003)

w www.eesensors.com/IEEE1451

Experimental Dot2 TIM and NCAP demo at IEEE 1451 booth


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Summary

w Interchangeable gas sensor elements/heads of variedtechnology requires adaptable signal conditioners

wA microcomputer-based smart sensor with the requiredsignal conditioners was developed for this purpose.

w Transmission of sensor data over a network wasdemonstrated

w IEEE p1451 protocol was used for simplified signal readout

and plug and play capability.

Further information: [email protected]

a multi-element smart gas sensor

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