California University of Pennsylvania. Department of Applied Engineering & Technology. Electrical / Computer Engineering Technology

Transcription

1 California University of Pennsylvania Department of Applied Engineering & Technology Electrical / Computer Engineering Technology EET 215: Introduction to Instrumentations Lab No.7 Temperature Measurement Using NTC Thermistor Names: Signature: Date

2 Instrumentation Amplifier Objective of the Experiment The main objective of this experiment is to understand NTC thermistors and to use them to calculate and display temperature readings. Learning Outcomes Students will demonstrate: - The ability to use Thermistor equations - the ability to derive linear equation for a Wheatstone Bridge - interpret data and design a digital thermometer using GUI Introduction Thermistors (thermally sensitive resistors) electrical resistors whose resistance changes with temperature. Thermistors are manufactured from metal oxide semiconductor material which is encapsulated in a glass or epoxy bead. Thermistors have a very high sensitivity, making them extremely responsive to changes in temperature. For example, a 2252 Ω thermistor has a sensitivity of -100 Ω/ C at room temperature. Thermistors also have a low thermal mass that results in fast response times, but are limited by a small temperature range. Thermistors have either a negative temperature coefficient (NTC) or a positive temperature coefficient (PTC). The first has a resistance which decreases with increasing temperature and the latter exhibits increased resistance with increasing temperature. Figure 1 shows a typical thermistor temperature curve: Figure 1: NTC Thermistor characteristics eference:

3 The resistance of a thermistor as a function of temperature is approximated by: (T) = 0 e β(1 T 1 T 0 ) Where: (T) is the resistance at a given temperature T: esistance as a function of temperature. β is the material constant T is the current temperature in o K T 0 is the room temperature (typically 25 o C = 298 o K Components Needed: - the IA circuit of last time KΩ resistors - 5 KΩ resistor (or close to it) - 9 KΩ resistor (or any value less than 10 KΩ but higher than 5 K) - Any value between 2KΩ and 6KΩ resistor (any value) - 2KΩ resistor (or close to it) - 10KΩ Thermistor Experiment A- Use the Instrumentation amplifier circuit of previous Lab (May have to rebuild it- see circuit below) With the power to the board initially turned OFF, a- SET G to 5KΩ. (instead of the previous value) b- Build the Wheatstone bridge circuit shown below. The 5V dc is from the DAQ board.

4 5V To V1 input on IA To V2 input on IA x OP-AMP1 V1 Vy LM741CN G Vout V2 Vx 50% OP-AMP3 LM741CN LM741CN OP-AMP2 Verification Step (Measure voltages using the DMM of the instrument launcher) c- Test to verify the circuit is still operational as follows: (Turn the power to the board ON) 1- measure Vout of the IA. It should be near zero volts? 2- Change x in the Wheatstone bridge to 5KΩ. Measure Vout.

5 Vout should obey the relationship: (1+2/G)(V2-V1) = (1+20K/5K)( ) = -4.15V Note: = 10KΩ (used in the IA circuit) If the two above steps agree to a great extent, then all is set to go! Otherwise, fix the problem. B- Develop a Linear elationship The goal here is to record measurements of Vout vs. X. This will allow us to develop a linear relationship that will be used to design the digital voltmeter. 1- Complete the table below: Example: a- Vary x and record measurements of Vout vs. x (may use fixed resistors) b- Calculate Vout Expected for each case and verify the measured value.) V1= 2.5V (since two equal resistors) At x = 9KΩ, V2 = 5*9K/(9K+10K) = 2.368V Vout=(1+20K/5K)*(V2-V1) = V You may use slightly different values of x than listed below. However, make sure to record the values used and recalculate the expected values. The example above explains the process x 9KΩ Vout (expected) V Vout (measured) 7KΩ KΩ KΩ Once the table is completed successfully, we are ready to develop the linear relationship. 3- Use Excel to develop a linear relationship between Vout and x. Will let x be the vertical axis (independent variable) and Vout be the horizontal axis (dependent variable). The reason for this will be apparent in later steps when we incorporate software.

6 4- Follow the steps below for guidance with EXCEL. a- Start EXCEL and fill in your data as shown (Use your values) b- Plot the data as follows: i- select the data as shown ii- click on INSET on the top menu then click on Line down arrow as shown iii-in the menu that appears, click on All Charts Types

7 iv- Pick the items shown and click ok v- a relatively linear plot appears as shown: vi- On the linear plot, place the cursor on the line, right click the mouse and choose Add Trend Line vii- in the menu that appears, click on Linear and on Display Equation on Chart and keep the Trend Line Automatic selection. Do not check others. See below:

8 viii- Click Close and record the equation that appears: (See example figure below) c- The equation that relates x to Vout is: (ecord Your Equation) Example (This example corresponds to the data from EXCEL above ) x = *Vout Ω To test the equation: At Vout = Voult,

9 x = *(-4.167) = 5.46KΩ (a little off the 5K expected.) d- Test your equation at a different value of Vout and see how close / far off from the actual x: At Vout = ?, x = ? Close or way off ?? (should be close) C- Thermistor Implementation In this portion of the lab, the following is accomplished: 1- x is replaced by the 10KΩ thermistor 2- The linear equation obtained by you will be used to calibrate software interface to the DAQ to better estimate the thermistor resistance. 3- using the thermistor resistance, the actual temperature is displayed. Here, some software calibration may be necessary. a- eplace x with a 10KΩ Thermistor b- From the course s page, download the LabVIEW VI called thermistor and save it to your N drive (in a folder ). Also, download the LabVIEW DAQ interface supplement document. c- Connect Vout of the instrumentation amplifier to Analog Input 0 + (AI0+) and connect the AI0- to ground. d- Turn the board power ON e- un LabVIEW from Start>>All Programs>>National Instruments>>LabVIEW. f- Once the Choose environment window comes up, click on Start LabVIEW (Not the robotics ) g- in LabVIEW, click File>>Open, then navigate to the VI you downloaded. To open it. h- Perform all of the necessary settings as mentioned in the LabVIEW DAQ interface supplement document i- click UN (on the arrow as shown below)

10 i- your VI should run nicely. You can verify that it is working by looking at the Vout reading in the indicator shown by the bubble above. This data should match your expected output. j- Click Stop (to stop running the VI) k- Enter your own equation as follows: 1- on the front panel of LabVIEW, click on Windows as shown and select Show Block Diagram 2- Edit the equation shown to enter your numbers. Make sure the equation ends with the semicolon ;

11 3- Click on Window in LabVIEW and select Show Front Panel 4- un the VI you should see good behavior Instructor approval of experiment completion: