Microcontrollerbased interface circuit

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Microcontroller-based interface circuit for
metal-oxide gas sensors
M. Brugia, A. Scorzoni, M. Baroncini, P. Placidi,
L. Verducci Dipartimento di Ingegneria
Elettronica e dellInformazione, University of
Perugia G.C Cardinali, I. Elmi CNR IMM, Bologna
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SUMMARY

• Introduction
• System architecture
• System optimization
• Experimental results
• Conclusions and future developments

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INTRODUCTION

• Target necessity to monitor the air quality
• Micromachined gas sensor
• Detection principle sensing layerresistance
  change when thegas is present (5k? ?15M?)
• Sensitivity and selectivity dependon the
  temperature of the sensinglayer determined by
  the heater
• An electronic interface is necessary

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INTRODUCTION

• Starting condition
• PC-based acquisition system
• sensor resistance measured using A/D conversion
  through an expensive PCI board
• heater resistance analogically controlled ?
  relatively high power consumption due to power
  dissipation in the output stage of the controller

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SYSTEM ARCHITECTURE
Specifications

• Low cost
• Reduced power consumption
• Single voltage supply (5V)
• Reduced sensitivity to ambient temperature change
• Self-calibration capability

Proposed approach

• Microcontroller-based prototype

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SYSTEM ARCHITECTURE
Block diagram
RH heating element resistance PH power
supplied to the heater Vref programmable DC
voltage
Rs sensing element resistance Top operating
temperature
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SYSTEM ARCHITECTURE
Heater control circuit

• Based on a previously determined RH vs. T
  calibration curve
• Top ? RH (Top) ? Rref (set point)

heater 1
Rref (Top)
heater 2
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SYSTEM ARCHITECTURE
Heater control circuit
TCU Time control unit PCU Power control
unit IDC DC current source (? Imeas) IDCE IDC
module enable CE Control signal for
self-calibration

• Pulsed control
• Calibration curve ? RH (Top)
• Measurement of RH at constant (low) current
  Imeas RH VH / Imeas
• Comparison Rref ? RH ??? Vref ? VH
• Programmable Vref

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SYSTEM ARCHITECTURE
Voltage reference generation

• DAC emulation exploiting the ?C PWM module
• Specifications
• - Top range 200C ? 500C ? 170mV ? Vref ?
  280mV
• - Top error ? 5C ? ?Vref ? 1.6mV

• Procedure

• DC component extraction from PWM wave
• Programmable duty cycle

• Results

• Estimated error ? 3C
• Resolution 1C

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SYSTEM ARCHITECTURE
Data acquisition circuit

• Voltage divider
• Auto-range procedure
• Noise reduction
• - Ileak (PIC) 1?A ? low-leakage nMOS
  transistors
• - Interfering signals ? LPF
• - AC power supply ? average of 8 acquired
  values in 20ms
• Result accuracy on measured resistance better
  than 1

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SYSTEM OPTIMIZATION
Self-calibration
Variations with respect to the nominal value of
Top
Rs

• Self-calibration procedure
• Hardware substitution of the sensor with an
  external circuit featuring a
  resistance of known value
• Software measuring and elaboration procedure
  automatically managed by the ?C

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SYSTEM OPTIMIZATION
Two different methods for data acquisition from a
sensor

• Constant temperature method
• - measurement of RS when the operating
  temperature has a
• constant value TH (e.g. 400C) ? RS ?
  gas

• Pulsed temperature method
• - measurement of RS in correspondence of
  a sequence of temperature values between
  ambient temperature and a fixed value TH
• ? Rs1, Rs2, Rs3 ? gas

• Time discrete intervals of 20ms
• User-programmable timing parameters

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The prototype
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EXPERIMENTAL RESULTS
Test of the system constant T

• Controlled gaseous environment gas sequence

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EXPERIMENTAL RESULTS
Test of the system pulsed T

• Controlled gaseous environment NO2

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CONCLUSIONS AND FUTURE DEVELOPMENTS

• Conclusions
• Realization of a data acquisition and control
  system for a gas
• sensor, based on a microcontroller, that
  fulfills the specifications
• Simple interface based on RS-232
• Self-calibration capability
• Two methodologies of data acquisition constant
  and pulsed T
• Test of the system in chamber with controlled
  environment
• Future developments
• Microcontroller-based multisensor system to
  compensate for the effect of interfering gases
• wired or wireless LAN interface based on standard
  IEEE 1451

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APPENDIX
PIC16F873 microcontroller Microchip development
tools

• Characteristics
• -  Integrated peripherals ADC (10bit), PWM
  (10bit), USART, etc
• -  Program memory (FLASH) 4096x14 bit
• -  Data memory (EEPROM) 128 Byte
• -  Data memory (RAM) 192 Byte
• -  In/Out lines 22 (25 mA maximum current)
• Execution time 1?s / instruction (clock at
  4MHz)
• Programming languages Assembler, C

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