Download Measuring Temperature with Thermistors

Measuring
Temperature with
Thermistors
Academic Workshop Lab
Objective
• Exploit PSoC topology to build inexpensive
digital thermometer.
• Understand the operation of a negative
temperature coefficient (NTC) thermistor.
• Understand how to calculate Steinhart Hart
constants for a specific thermistor.
• Calculate temperature using the Steinhart &
Hart equation.
• Calculate temperature using a look up table.
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Hardware Overview
•
•
•
•
•
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CY8C3210-PSoCEval1 board.
MiniProg
Thermistor
10k resistor
Breadboard wire
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Reference Material
• AN2028 Ohmmeter
• AN2017 Thermistor Based Thermometer
• AN2239 ADC Selection
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Measuring Resistance
• Unlike measuring voltage or current, measuring Vresponse
a a passive characteristic like resistance
requires stimulus
Rtest
• A Classic method is to push current into a
resistor and measure the developed voltage.
• Only as accurate as
• Current Source
• ADC Gain and Offsets
• Resistance limited to ADC range.
• Requires different current values for wide range
of resistors.
• Very popular when cost of accurate current
sources was less than the cost of
computation.
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Rtest 
ADC
Istim
Vresponse
I stim
PSoC and Measuring Resistance
• For this circuit the following equation
holds.
V0  V1 V1  V2

Rref
Rtest
V0
• Solving for Rtest results in:
Rtest  Rref
PSoC
Rref
V1
V1  V2
V0  V1
Rtest
V2
• Offset errors removed by difference
• Measurement offset voltages subtract out!
• Gain errors removed by quotient
• Measurement path errors divide out!
• Accuracy determined by an external
reference resistor …
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REFHI
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ADC
Atten
REFLO
PSoC and Measuring Resistance
• And ADC resolution
PSoC
V0
• For an n bit ADC the number of counts seen
across aR is:
a
n
2  Atten 
1 a
 12
REFHI
R
V1
• The reading is accurate to +/- .5 counts.
ADC
aR
V2
• Overall resolution tolerance is:
tol 
1
2
2 n  Atten
2

1 a

a
• For 14 bits and an attenuation of 15/16, the
equation simplifies to:
1  a 
1
tol 

30,720
a
2
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Atten
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REFLO
Tol (%)
a
0.332%
0.01
0.0399%
0.1
0.013%
1
0.0399%
10
0.332%
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Thermistors
• A negative temperature coefficient thermistor (NTC) is a
semiconductor device that becomes less resistive as its
temperature increases. The change in resistance is
“roughly” expressed by the equation below.
R(T1 )
 A (T1 T2 )
R(T2 )
Where:
• A is some empirical value less than one for negative
temperature coefficient (NTC) thermistors.
• T1 & T2 are temperatures measured in Kelvin.
• R(T1) & R(T2) are the thermistor’s resistances at
these temperatures.
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NTC Thermistors
R(T1 )
 A (T1 T2 )
R(T2 )
• ”Roughly” is defined as a good approximation for
an academic introduction to thermistors.
• It shows the temperature/resistance relationship
to be ideally exponential.
• It won’t hold up for real world temperaturemeasuring application.
• But for small temperature differences the
following holds:
 T1  T2 
R
  RT1   RT2 
 2 
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Steinhart-Hart Equation
• The Steinhart-Hart equation describes the resistance
change of a thermistor as related to its temperature. The
equation below shows it to be a 3rd order logarithmic
polynomial using three constants.
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 A  B  ln( R)  C  ln( R)3
TK
Where:
• A, B, and C are empirical constants.
• TK & is temperature in Kelvin.
• R is the thermistor’s resistance in Ohms .
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Steinhart-Hart Equation
• Many thermistors come with these three parameters
defined.
• For this particular thermistor they are in the datasheet
• If not they must be calculated.
• This is done by taking three points in the conversion table and
solving for these constants.
• It makes most sense to use the minimum, maximum, and a
middle value for the temperature range for which you are
interested. From the Thermistor Table
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Tc
Resistance
0°C
32,660 ohms
40 °C
5,325 ohms
80 °C
1,257 ohms
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Note: This is an example and not
for the thermistor we are using
Steinhart-Hart Equation
• Apply the three data points to the following equation.
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 A  B ln( R)  C  ln( R) 3
TC  273.15
To get the three following equations.
.366099e-2
A  10.3939  B  1122.89  C
.319336e-2
A  8.58017  B  631.666  C
.283166e-2
A  7.13648  B  363.456  C
• Solve to get:
A = 0.11261637e-2
B = 0.23461776e-3
C = 0.85700804e-7
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Thermistors
• The cost of thermistors is primarily determined by the
accuracy of the thermistor’s resistance. This is where
the exponential nature of thermistors works out to your
advantage.
• A thermistor’s resistance tolerance shows up as a
temperature shift. This can be calibrated out with a
single point calibration.
• In test, bring the thermistor to 25˚C and measure its temperature.
• Suppose it reads 26.2˚C
• Software needs to store a 1.2˚offset in memory.
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Thermistors
• In consumer products this calibration is many times left
to the user.
•
The user interface allows access to the temperature offset
register.
• The user sets this if they think the temperature is a bit low or a
bit high.
• A good rule of thumb is that a thermistor resistance
uncertainty of n% works out to a temperature shift of
approximately (n/3)˚C. This will help determine if any
calibration is needed. Temperature calculations are only
as accurate as the resistance measurement of the
thermistor
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Let’s Get Started
Desired Topology
• Connect 10k Ohm from P05 to
P01.
• Connect 10k Ohm thermistor
from P01 to P03.
• Start Designer
• Name the Project Therm.
PSoC
V0_Out
buf1
REFHI
10k
InputAtten
V1_In
P01
Buffer
R
Therm
V2_Out
15
P05
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15R
P03
buf0
REFLO
ADC
EVAL1 Connections
10K
P05
Therm
P03
P01
Wire
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Starting a New Project
• Open PSoC Designer
• Select Start new
project
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Starting a New Project
• Select
Project Type
• Name The
Project
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Starting a New Project
• Select Device and
Coding Method
• CY8C29466-24PXI
• C
• OK
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Global Resource Settings
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Select PGA UM
• Select PGA and name it InputAtten
• Insert into ACB00
• Set the PGA parameters to:
• Atten Value set to 15/16
• Reference to AGND
• Input connected to column
MUX to read all three points
on the resistor string
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Select Second PGA UM
• Select PGA and name it Buffer
• Insert into ACB01
• Set the PGA parameters to:
• This UM generate API
in multiplex the input
lines.
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Select AMUX4 UM
• Select an AMUX4 and
rename it ADCMUX
• Set its parameter has
shown.
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Select ADCINC UM
• Select an ADCINC UM and rename it ADC.
• Select a single modulator and place it in
ASC10.
• Select the clock to be VC2.
• Place the digital block in DBB0.
• Input connects to Buffer.
• PWM is not used
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Select LCD UM
• Select and LCD UM and name
it LCD.
• Connect to Port 2
• BarGraph is not needed.
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Rename Buffers and Pins
• Connect the AnalogOutBuf_1 to P05.
• Rename this pin V0_Out.
• Connect the AnalogOutBuf_0 to P03.
• Rename V2_Out.
• Change PO1 to be an AnalogInput.
• Rename it V1_In.
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Add Initialization Code
•In the Initialization Section
• Add code to start Buffer,
InputAtten, ADC, and
LCD.
• Add code to connect
REFHI to the column1
analog bus.
• Add code to connect
REFLO to the column0
analog bus.
• Declare iV0, iV1, iV2,
iRvalue to be global
variables.
• Enable global interrupt.
• Declare bTempValue to
be a global 8 bit variable.
• Add LookUp table.
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(Cut and Paste from File
on CD)
LookUp Table Temperature Conversion
• The Steinhart-Hart equation requires using the
floating point math library. Floating point is slow and
uses buckets more ROM compared to integer math.
• An alternative is to use a look up table. For any
particular thermistor, the manufacturer either supplies
a R/T conversion table, or supplies the three
Steinhart-Hart coefficients. If only the coefficients are
supplied, a table can be generated from them. This
particular thermistor has a R/T conversion table that
supplied.
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Create a Look Up Table
• Excel file ThermTable.xls contains the
81resistance values for temperature for 0°C to
80 °C.
• Calculate half values for ½°C to 79 ½°C
using the following equation.
Rn  12   Rn Rn  1
• ½ degrees are used for rounding.
• Add the value zero at the end.
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Create a Look Up Table*
• These values are used to make an ROM array
WThermTable[ ] containing 81 values.
Rn  12   Rn Rn  1
*code is provided in lab file
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Add Code Control Loop
•In the Control Loop
• Set ADCMUX to P05.
• Run ADC.
• Wait for data.
• Place in iV0
• Set ADCMUX to P01.
• Run ADC.
• Wait for data
• Place in iV1.
• Set ADCMUX to P03.
• Run ADC.
• Wait for data
• Place in iV2.
• Calculate Resistance.
• Display on LCD.
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Add Code CalculateR
• If iV0<-iV1 (Open Circuit)
• lRvalue = -1
• Else If iV1 <= iV2 (Short Circuit)
• iRvalue = 0
• Else
• Calculate Resistance
• Add half denominator to
numerator to implement
round off.
 A  12 B 
A

int 
  int   .5 
B

 B 
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Run
• Build the Project.
• Record RAM and ROM Usage.
• Download to the Eval board and run.
• Using the look up table determine the
temperature.
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Summary
• PSoC makes measure resistance a cost
effective option.
• Temperature can easily be measure using
thermistor with the Steinhart-Hart equation or
look up table.
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Questions
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