temperature sensor readout circuit for microheater
TRANSCRIPT
TEMPERATURE SENSOR READOUT CIRCUIT FOR MICROHEATER
PRESENTED BY SHYAMILI JOHN ROLL NO: 56 S7 F
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CONTENTS
INTRODUCTION MEMS BASED MICROHEATER ARRAY ON SOI WAFER MICRO HEATER PATTERN STRUCTURE HEATER DESIGN TEMPERATURE SENSOR SELECTION OF NTC THERMISTOR INTERFACE CIRCUIT ADVANTAGES DISADVANTAGES CONCLUSION REFERENCES
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INTRODUCTION
Micro heaters are used to elevate the temperature of gas sensor.
Micro heater array design can be more effective using thin film and MEMS technology.
Temperature measurement is recorded through resistive sensor, NTC thermistor.
For temperature readout circuit, NTC thermistor and heater is combined together in Wheatstone bridge.
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MEMS BASED MICROHEATER ARRAY ON SOI WAFER
The thickness of the microheater is 0.1µm and total length is 420µm.
The microheater is made of platinum. Temperature achieved is 200 0C . Power consumption is 20mW . Uniform temperature distribution over the microheater. Minimum heat loss through the substrate.
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CONTND…
Fig 1. Temperature VS power consumption graph of microheater
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MICROHEATER PATTERNS
Fig 2. Types of Geometries Structure
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MICROHEATER PATTERN STRUCTURESS-Shape – uniform temperature profile and sensing film
of small dimensions.Double spiral – to avoid radial temperature gradient of
conventional meander types.Honeycomb- redistribution of thermal energy.Meander shape- demonstrates undistributed hot spot at
high temperature.Plane plate- with central hole has a square hole in its
design.Fan shape- low power consumption and uniform
temperature profile.
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HEATER DESIGN
Copper is the heating element. Resistance is calculated by , R= ρL/A , ρ is resistivity in
/m, L for length of copper and A is a cross sectional area �of conductor.
The resistance of copper at different temperature can be measured by equation,
1. Rm = R0 [1+α 0 (T − T0)]
where R m : Resistance measured at T
R0 : Resistance given at To
α 0 : Temperature coefficient of metal
T0 : Room temperature
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MEANDER PATTERN HEATER
Fig 3. Temperature profile meander heater pattern
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CONTD…
The single MEMS heater is designed with 3μm width, 209 μm length and 0.15 μm thickness.
The maximum voltage from ambient temperature of 250C (300K) to 100 0C (375K) is 0.2V.
The amount of current supply is 3.3mA .
Power consumption is 0.6 mW .
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CONTD…
Fig 4. Temperature vs voltage graph Fig 5. Current vs Voltage graph
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TEMPERATURE SENSOR
Why Thermistor? Higher sensitivity. Less expensive than Resistance Temperature Detector. Better accuracy. Provides faster response. Reasonable output voltages. Very reliable and convenient to use.
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THERMISTORS
Thermistors are of two types-Negative temperature coefficient(NTC) and positive temperature coefficient(PTC)
NTC thermistor-used for temperature sensing applications because of its high stability.
PTC thermistor-used as heating element for small temperature controlled ovens, circuit protection applications etc.
NTC thermistor results non linear measurement.
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SELECTION OF NTC THERMISTOR
Minimum and maximum resistance values at the highest temperature.
4 Ω NTC thermistor.� � According to Steinhart-Hart equation , the resistance of
thermistor is determined at different temperatures, 2. 1/T= A+ B (loge R) + C (loge R)3 Dissipation factor should be maintained at low level.Excitation current calculated from, 3. Power dissipated= I2R = δ(T)
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RESISTANCE VS TEMPERATURE GRAPH
Fig 6. Resistance VS temperature graph of 4 Ω thermistor
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INTERFACE CIRCUIT
Wheatstone bridge is preferable than voltage divider.
Used to connect the heater and thermistor for temperature measurement.
Supply noise is fully eliminated.
Nonlinearity of R/T curve of thermistor leads to the use of this circuit in order to linearize the signal.
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WHEATSTONE BRIDGE CIRCUIT
V out = Vb – Vc = [R2/(R 1+R 2)] – [R 4/(R 3+R 4)]
Fig 7. Wheatstone bridge circuit
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RESULT
Fig 8. Circuit diagram of Wheatstone bridge
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CONTD…
Two voltage dividers. First voltage divider consists of two heaters with 8 Ω
resistance each. Total summed up voltages are 0.4 V. Current flow should minimize to avoid self heating
which is 0.1 A . Potentiometer of 4 Ω is used.
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OUTPUT VOLTAGE AND OUTPUT POWER
Total current consumption of heater and thermistor is 0.118A
Constant value of heater and thermistor is selected to have low power dissipation.
Fig 9. Reading of output voltage and output power
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OUTPUT VOLTAGE VS TEMPERATURE GRAPH
Fig 10. Voltage Output VS Temperature graph
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ADVANTAGES
Bridge circuit offers the simplest configuration Low power consumption especially in micro heater gas
sensor array application. Number of parts can be reduced. Sensor measurement is more accurate as the distance is
close with heater. Proper thermal isolation between sensor element and
substrate. Ease of microheater array fabrication and small size.
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DISADVANTAGES
Since thermistors are semiconductor devices, their operation is highly non linear.
Limited temperature range due to which they are rendered unsuitable for use at higher temperatures.
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CONCLUSION
Meander structure is suitable for heater . Resistance and current values to heat up the heater are
obtained from simulation of heater design. NTC thermistor resistance is selected based on the range
of temperature and power dissipation constant. Potentiometer is used to adjust the current and balance
the circuit. Maximum output voltage at 1000C is 0.4153V with
0.0049W requirement.
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REFERENCES
Solehah Md Hashim,Umadevi Chandaran, “Temperature Sensor Readout Circiut for Microheater”,International Conference on Electronic Design,2014
A. Datta. Gupta, C. Roy Chaudhuri, “Design and Analysis of MEMS Based Microheater Array on SOI Wafer for Low Power Gas Sensor Applications”, International Journal of Scientific & Engineering Research, vol 3, pp. 1-8, 2012.
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CONTD…
Avigyan Datta Gupta, Chiirashree Roy Chaudhuri.“Design and Analysis of MEMS Based Microheater Array on SOI Wafer Low Power Gas Sensor Applications” International Journal of Scientific & Engineering Research, vol 3. No 6. 2012.
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THANK YOU…