1 / 35

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.

lenka
Download Presentation

Measuring Temperature with Thermistors

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Measuring Temperature with Thermistors Academic Workshop Lab

  2. 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.

  3. Hardware Overview • CY8C3210-PSoCEval1 board. • MiniProg • Thermistor • 10k resistor • Breadboard wire

  4. Reference Material • AN2028 Ohmmeter • AN2017 Thermistor Based Thermometer • AN2239 ADC Selection

  5. Measuring Resistance • Unlike measuring voltage or current, measuring a a passive characteristic like resistance requires stimulus • 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.

  6. PSoC and Measuring Resistance • For this circuit the following equation holds. • Solving for Rtest results in: • 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 …

  7. PSoC and Measuring Resistance • And ADC resolution • For an n bit ADC the number of counts seen across aR is: • The reading is accurate to +/- .5 counts. • Overall resolution tolerance is: • For 14 bits and an attenuation of 15/16, the equation simplifies to:

  8. 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. 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.

  9. NTC Thermistors • ”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 temperature-measuring application. • But for small temperature differences the following holds:

  10. 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. Where: • A, B, and C are empirical constants. • TK & is temperature in Kelvin. • R is the thermistor’s resistance in Ohms .

  11. 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 Note: This is an example and not for the thermistor we are using

  12. Steinhart-Hart Equation • Apply the three data points to the following equation. To get the three following equations. • Solve to get: A = 0.11261637e-2 B = 0.23461776e-3 C = 0.85700804e-7

  13. 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.

  14. 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

  15. 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 P05 REFHI buf1 10k InputAtten V1_In P01 Buffer R ADC Therm 15R REFLO buf0 V2_Out P03

  16. EVAL1 Connections 10K P05 Therm P03 P01 Wire

  17. Starting a New Project • Open PSoC Designer • Select Start new project

  18. Starting a New Project • Select Project Type • Name The Project

  19. Starting a New Project • Select Device and Coding Method • CY8C29466-24PXI • C • OK

  20. Global Resource Settings

  21. 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

  22. 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.

  23. Select AMUX4 UM • Select an AMUX4 and rename it ADCMUX • Set its parameter has shown.

  24. 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

  25. Select LCD UM • Select and LCD UM and name it LCD. • Connect to Port 2 • BarGraph is not needed.

  26. Rename Buffers and Pins • Connect the AnalogOutBuf_1 to P05. • Renamethis pin V0_Out. • Connect the AnalogOutBuf_0 to P03. • Rename V2_Out. • Change PO1 to be an AnalogInput. • Rename it V1_In.

  27. (Cut and Paste from File on CD) Add Initialization Code • In the Initialization Section • Add code to start Buffer, InputAtten, ADC, and LCD. • Add code to connect REFHI to the column1analog 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.

  28. 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.

  29. 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. • ½ degrees are used for rounding. • Add the value zero at the end.

  30. Create a Look Up Table* • These values are used to make an ROM array WThermTable[ ] containing 81 values. *code is provided in lab file

  31. 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.

  32. Add Code CalculateR • If iV0<-iV1 (Open Circuit) • lRvalue = -1 • Else If iV1 <= iV2 (Short Circuit) • iRvalue = 0 • Else • Calculate Resistance • Add half denominator tonumerator to implementround off.

  33. Run • Build the Project. • Record RAM and ROM Usage. • Download to the Eval board and run. • Using the look up table determine the temperature.

  34. 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.

  35. Questions

More Related