Lab 2: Introduction to NI ELVIS, Multisim, and LabVIEW

Transcription

1 Page 1 of 19 Lab 2: Introduction to NI ELVIS, Multisim, and LabVIEW Laboratory Goals Familiarize students with the National Instruments hardware ELVIS Learn about the LabVIEW programming environment Demonstrate the use of MultiSim to simulate electronic circuits Identify ` Oscilloscope Record results in the laboratory notebook Pre-lab / lab reading ELVIS instruction manual Course Textbook Equipment needed Lab notebook, pen NI ELVIS Parts needed Jumper wires Oscilloscope probes Required Downloads lab1_lv2011.zip lab1_ms11.zip lab1-ni_elvis_environment.pdf lab2_lv2011.zip lab2_ms11.zip lesson2-ni_elvis_ii_digital_thermometer.pdf Lab safety concerns Do not make voltage measurements while the multimeter is set for current measurements-you will blow the current limiting fuse! Do not turn on the power supply until you have rechecked your circuit for correct wiring. Do not allow the test leads connected to the power supply to touch each other.

2 Page 2 of 19 Exercise 1: Measuring Component Values 1. Connect the NI ELVIS II workstation to your computer using the supplied USB cable. The box USB end goes to the NI ELVIS II workstation and the rectangular USB end goes to the computer. Turn on your computer and power up NI ELVIS II (switch on the back of the workstation). The USB ACTIVE (orange) LED turns ON. Then the ACTIVATE LED turns OFF and the USB READY (orange) LED turns ON. 2. On your computer screen, click on the NI-ELVISmx Instrument Launcher icon or shortcut. The NI ELVIS II instruments appears on the screen. You are now ready to make measurements. 3. Connect two banana-type leads to the digital multimeter (DMM) inputs (VA. ) and (COM) on the left side of the workstation. Connect the other ends to one of the resistors. 4. Click on the DMM icon within the Launcher to select the digital multimeter. Figure 1. Digital Multimeter, Ohmmeter Configuration

3 Page 3 of 19 You can use the DMM SFP for a variety of operations such as voltage, current, resistance, and capacitance measurements. Use the notation DMM[X] to signify the X operation. The proper lead connections for this measurement are shown on the DMM front panel. 5. Click on the Ohm button [W] to use the digital ohmmeter function, DMM[W]. Click on the green arrow [Run] box to start the measurement acquisition. Measure the three resistors R1, R2, and R3. Fill in the following data: R1 (1.0 kw nominal) R2 (2.2 kw nominal) R3 (1.0 MW nominal) To stop the acquisition, click on the red square [Stop] box. NOTE: If you click on the Mode box, you can change the {Auto} ranging to {Specify Range} and select the most appropriate range by clicking on the Range box. Exercise 2: Building a Voltage Divider Circuit on the NI ELVIS II Protoboard 1. Using the two resistors, R1 and R2, assemble the following circuit on the NI ELVIS II protoboard. Figure 2. Voltage Divider Circuit 2. Connect the input voltage, Vo, to the [+5 V] pin socket. 3. Connect the common to the [GROUND] pin socket. 4. Connect the external leads to the DMM voltage inputs (VW. ) and (COM) on the side of the NI ELVIS workstation and the other ends across the input voltage, Vo, to make the first measurement.

4 Page 4 of Check the circuit and then apply power to the protoboard by pushing the prototyping board power switch to the upper position [-]. The three power indicator LEDs, +15 V, -15 V, and +5 V, should now be lit and green in color. Figure 3. Power LED Indicators on NI ELVIS II Protoboard If any of these LEDS are yellow while the others are green, the resettable fuse for that power line has flipped off. To reset the fuse, turn off the power to the protoboard. Check your circuit for a short. Turn the power back on to the protoboard. The LED flipped should now be green. 6. Measure the input voltage, Vo, using the DMM[V] function. Press [Run] to acquire the voltage data. V0 (measured) According to circuit theory, the output voltage, V2 across R2, is as follows: V2 = R2/(R1+R2) * Vo. 7. Using the previous measured values for R1, R2 and Vo, calculate V2. Next, use the DMM[V] to measure the actual voltage V2. V2 (calculated) V2 (measured) 8. How well does the measured value match your calculated value?

5 Page 5 of 19 Exercise 3: Using the DMM to Measure Current According to Ohm s law, the current (I) flowing in the above circuit is equal to V2/R2. 1. Using the measured values of V2 and R2, calculate this current. 2. Perform a direct current measurement by moving the external lead connected to (VW. ) to the current input socket (A). Connect the other ends to the circuit as shown below. Figure 4. Circuit Modification to Measure Current 3. Select the function DMM[A] and measure the current. I (calculated) I (measured) 4. How well does the measured value match your calculated value? Exercise 4: Observing the Voltage Development of an RC Transient Circuit Using the DMM[C] function, measure a 1 μf capacitor. 1. Connect the capacitor leads to the Impedance Analyzer inputs, [DUT+] and [DUT-], found on the left lower wiring block of an NI ELVIS II protoboard. 2. For capacitance and inductance measurements, the protoboard must be energized to make a measurement. Switch the protoboard power ON. 3. Click on the capacitor button [ ] to measure the capacitor C with the DMM[C] function. Press the Run button to acquire the capacitance value. C (μf)

6 Page 6 of Select DMM[V] and click on RUN. Figure 5. RC Transient Circuit 6. When you power up the protoboard, the voltage across the capacitor rises exponentially. Set the DMM voltage range to {Specify Range} [10 V]. Turn on the protoboard power and watch the voltage change on the digital display and on the %FS linear scale. 7. It takes a few seconds to reach the steady-state value of Vo. When you power off the circuit, the voltage across the capacitor falls exponentially to 0 V. Try it! This demonstrates one of the special features of the NI ELVIS II digital multimeter it can still be used even if the power to the protoboard is turned off. Exercise 5: Visualizing the RC Transient Circuit Voltage 1. Change the voltage source of the circuit from the +5 V supply to the variable power supply [SUPPLY+]. Connect the output voltage, VC, to the first analog input socket, AI 0[+], and ground the AI 0[-] socket, as shown in Figure 6. Figure 6. RC Transient Circuit on NI ELVIS II Protoboard

7 Page 7 of 19 Close the NI ELVIS II SPFs and Instrument Launcher and start LabVIEW. Open the LabVIEW program, RC Transient.vi. This program uses LabVIEW APIs to turn the variable power supply to a set voltage of +5 V for 5 s and then to reset the VPS voltage to 0 V for 5 s while the voltage across the capacitor is measured and displayed in real time on a LabVIEW chart. Figure 7. Charging and Discharging the Waveform of the RC Transient Circuit This type of square wave excitation dramatically shows the charging and discharging characteristics of a simple RC circuit. 2. Look at the LabVIEW diagram window to see how this program works. Figure 8. LabVIEW Block Diagram for the Program RC Transient.vi

8 Page 8 of 19 In the first frame of the four-frame sequence, the NI-ELVISmx Variable Power Supplies VI (virtual instrument) outputs V to the RC circuit on the NI ELVIS II protoboard. The next frame measures 50 sequential voltage readings across the capacitor at 1/10-second intervals. In the For Loop, the DAQ Assistant takes 100 readings at a rate of 1000 S/s and passes these values to a cluster array (thick blue/white line). From the cluster, the data array (thick orange line) is passed on to the Mean VI. It returns the average value of the 100 readings. The average is then passed to the chart via a local variable terminal <<RC Charging and Discharging>>. The next frame sets the VPS+ voltage equal to 0 V. The last frame measures another 50 averaged samples for the discharge cycle. This program records one complete cycle of the charging and discharging of a RC circuit. To repeat the cycle, continually place the above program inside a While Loop. NOTE: This LabVIEW program is configured to connect to Dev1 for your NI ELVIS workstation. If your device is configured to another device name, you need to rename your NI ELVIS workstation to Dev1, in NI Measurement & Automation Explorer (MAX) or modify the LabVIEW programs to your current device name. Exercise 6: Design a Burglar Alarm Using Multisim Simulation In this exercise, design a burglar alarm for a house requiring three entry sensors and one window sensor. If the alarm system is activated, sound the alarm as soon as one of the sensors detects an open door or window. Signal to the front panel displays which door or window is open and sound an alarm. ASIDE: In practice, this is a simple system requiring only two wires to be connected to each door or window from a central alarm system. In your smart system, a loop design requires only one wire where each sensor switch shorts out or opens a sensor address resistor. The magnitude of the resistor defines which sensor (door or window) has been opened. Launch Multisim and open the file Alarm Design Version 0.ms11. The ON position of these switches (left side) signals when the door is closed. Click the switch to close or open a door or window. Your design consists of a power supply (+5 V), a digital multimeter, five resistors, and four switches. The four resistors, 1 kw, 2 kw, 4 kw, and 8 kw, are placed at the door or window locations with the resistor value as the address of that location. The circuit is a simple loop with the switches placed across the address resistors to simulate the opening and closing of a window or door. Finally, the resistor, R5, limits the current when all of the switches are closed. The current limiting resistor value is taken as half of the value of all of the address resistor values added in series (7.5 kw).

9 Page 9 of 19 Figure 9. Multisim Smart Sensor Design To view the circuit operation, click on Run and open (1) and close (0) each switch, one at a time, using the mouse cursor. Fill in the following table: R1 R2 R3 R4 Voltage Table 1. Sensor Truth Table and Multimeter Voltage Readings Each switch when opened generates a unique voltage, which, when read by the voltmeter, reveals which window or door is open. Now that the design is complete, you can transfer the design into the real world as a test circuit built on an NI ELVIS II protoboard. Select five resistors as close to the design values as you have available. Launch NI ELVIS DMM(W) and measure the value for each of your chosen resistors.

10 Page 10 of 19 Fill in the following table: R1 R3 R3 R4 R5 (kw) (kw) (kw) (kw) (kw) Table 2. Table of Real Resistor Values Now go back to Multisim and replace the nominal resistor values with the measured (realworld) resistor values by double-clicking on each resistor in turn and entering the measured value. This becomes your new Alarm Design Version 1. Figure 10. Real-World Sensor Design You can now repeat your measurements of the predicted voltage readings when a window or a door is opened or closed. Use these resistors and five jumpers or push-button switches to construct a circuit similar to the one shown on an NI ELVIS II protoboard in Figure 11.

11 Page 11 of 19 Figure 11. Real-World Sensor Circuit on NI ELVIS II Protoboard Use the DMM[V] to verify its operation is similar to your real-world Multisim design, version 1. Exercise 7: LabVIEW Demonstration LabVIEW is a powerful programming language that you can use for many tasks including the measurement and control of circuits built on an NI ELVIS II protoboard. With one modification to the above circuit, you can route the alarm voltage levels to a LabVIEW program. Connect the voltage + pin (orange wire) to [AI 0+] socket pin and the GROUND to [AI 0-] socket pin. You can leave the DMM[V] connected if you wish to monitor the sensor voltage. The digital multimeter uses a different data acquisition card than NI ELVIS II analog inputs use. Imagine running the NI ELVIS suite of SFPs at the same time as a LabVIEW program is running. Launch LabVIEW and open the program House.vi for a unique view of the burglar alarm system. NOTE: This LabVIEW program is configured to connect to Dev1 for your NI ELVIS workstation. If your device is configured to another device name, you need to rename your NI ELVIS workstation to Dev1 in Measurement & Automation Explorer (MAX) or modify the LabVIEW programs to your current device name.

12 Page 12 of 19 Figure 12. LabVIEW Front Panel House.vi To operate the program, click on Run. If NI ELVIS II is connected and turned ON and power is applied to the protoboard, actions on the protoboard are signaled on the LabVIEW front panel. Each switch is mapped to a particular window or door. When open, an entry port appears black. Any open door or window sets off a red alarm along the eves trough. To end the program, click on the Alarm Off front panel slide switch. Figure 13. The LabVIEW Block Diagram for the Program House.vi The DAQ Assistant is programmed to read 100 consecutive voltage values at a rate of 1000 S/s. From the data cluster (blue/white line), select the array of voltages. The Mean.vi calculates the average value of this set of readings and sends it to the voltage trigger ladder. Whenever the voltage level falls between two limiting values (orange boxes), the corresponding condition is signaled on the front panel. The limiting values are picked as halfway between two neighboring trigger levels. The four-input OR function sets off the alarm if any door or window is opened. This design detects only the first occurrence of an open window or door. If you add a few more rungs to the limiting ladder, you can detect multiple openings and closings.

13 Page 13 of 19 Exercise 8: Measurement of the Resistor Component Values 1. Launch NI ELVISmx Instrument Launcher. 2. Select digital multimeter (DMM) from the SFP strip of instruments. 3. Click on the Ohm button. 4. Connect the test leads to DMM(V,W,. ) and (COM) side sockets. 5. Measure the 10-kΩ resistor and then the thermistor. 6. Fill in the following chart: 10 k Ω Resistor Ohms Thermistor Ohms 7. With the thermistor still connected, place the thermistor between your finger tips to heat it up and watch the resistance changes. It is especially interesting to watch the changes on the display bar scale (%FS). The fact that the resistance decreases with increasing temperature (negative temperature coefficient) is one of the key characteristics of a thermistor. Thermistors are manufactured from semiconductor material whose resistivity depends exponentially on ambient temperature and results in a nonlinear response. Compare the thermistor response with an RTD (100 W platinum resistance temperature device) shown in the following figure. Figure 14. Resistance-Temperature Curve of a Thermistor and an RTD Exercise 9: Operating the Variable Power Supply Complete the following steps to set a voltage level on one or both variable power supplies. 1. From the strip menu of SFPs, select the [VPS] icon. There are two controllable power supplies with NI ELVIS II, 0 to -12 V and 0 to +12 V, each with a 500-mA current limit.

14 Page 14 of 19 Figure 15. Virtual SFP for Variable Power Supplies In the default mode, you can control the VPS with the virtual panel shown above. Set the output voltage on the virtual knob and click on the [Run] box. The output voltage is shown (blue in color) in the display area above your chosen power supply. When you click on the stop button, the output voltage is reset to zero on the protoboard. NOTE: To sweep the output voltage through a range of voltages, make sure that you have clicked the [Stop] button. Select the Supply Source (+ or -), Start Voltage, Stop Voltage, Step Size, and Step Interval, and click on [Sweep]. For manual operation, click on the Manual box and use the knobs on the right side of the NI ELVIS II workstation to set the output voltages. To view the output voltage in the display area, click on the white box now appearing next to the LabVIEW label. 2. Connect the leads from the protoboard strip connector sockets labeled Variable Power Supplies [Supply +] and [Ground] to the DMM voltage inputs.

15 Page 15 of Select DMM[V] and click on RUN. Select VPS front panel and click on RUN. Rotate the virtual VPS control for Supply + and observe the voltage changes on the DMM[V] display. Note: You can use the [RESET] button to quickly reset the voltage back to zero. 4. Click on the Manual box to activate the real controls on the right side of the workstation. The virtual controls are grayed out. Observe that the green LED Manual Mode on the NI ELVIS II workstation is now lit. 5. Rotate the + voltage supply knob and observe the changes on the DMM. NOTE: VPS- works in a similar fashion except the output voltage is negative. Exercise 10: A Thermistor Circuit Complete the following steps to build and test the thermistor circuit. 1. On the workstation protoboard, build a voltage divider circuit with the 10 kω resistor and a thermistor. The input voltage is wired to [Supply +] and [Ground] sockets. The voltage across the thermistor goes to the DMM[V] leads. Figure 16: Temperature Measuring circuit using a Thermistor (left). Real Thermistor circuit on NI ELVIS protoboard (right). 2. Make sure the Variable Power Supply voltage levels are set to zero. Apply power to the protoboard and observe the voltage levels on the DMM display. Increase the voltage from 0 to +5 V. The measured voltage across the thermistor, VT, should increase to about 2.5 V. 3. Reduce the power supply voltage to +3 V. This ensures that the self-heating (Joule heating) inside the thermistor does not affect the reading of the external temperature. 4. Heat the thermistor with your finger tips and watch the voltage decrease.

16 Page 16 of 19 You can rearrange the voltage divider equation to calculate the thermistor resistance as follows: RT = R1 * VT /(3 -VT) At an ambient temperature of 25 C, the thermistor resistance should be about 10 kω. With this equation, called a scaling function, you can convert the measured voltage into the thermistor resistance. You can easily measure VT with the NI ELVIS II DMM or within a LabVIEW program (VI). In LabVIEW, the above scaling equation is coded as a subvi and looks like the following block diagram. Figure 17. Block Diagram for Scaling Function The thermistor response curve demonstrates the relationship between device resistance and temperature. It is clear from this curve that a thermistor has the three following characteristics: The temperature coefficient DR/DT is negative. The response curve is nonlinear (exponential). The resistance varies over many decades (refer to Figure 14). You can produce a calibration curve by fitting a mathematical equation to the response curve. LabVIEW has many mathematical tools to fit such a relationship. When you find the correct equation, you can calculate the temperature for any resistance within the calibrated region. The following calibration VI is typical for a thermistor and demonstrates how you can use the LabVIEW formula node to evaluate mathematical equations. Figure 18. For this thermistor, the calibration equation is R = exp( T)

17 Page 17 of 19 Exercise 11: Building an NI ELVIS Virtual Digital Thermometer The digital thermometer program Digital Thermometer.vi activates the VPS to power up the thermistor circuit. It then reads the voltage across the thermistor, converts it into a temperature, and displays its value in a variety of formats on the front panel. Measurement, scaling, calibration, and display occur in sequence within the while loop. VoltsIn.vi measures the thermistor voltage. Scaling.vi converts the measured voltage to resistance according to the scaling equation above. Convert R-T.vi uses a known calibration curve to convert the resistance into temperature. Finally, the temperature is displayed on the LabVIEW front panel as a number, meter reading, and thermometer display. The Wait function of 100 ms ensures that the voltage is sampled every one-tenth of a second. All of these actions occur within the while loop until you click the [Stop] button on the front panel. Figure 19. Block Diagram for Digital Thermometer Program Thermistors, like resistors, create heat (Joule heating) as a current passes through them. For a thermistor that is trying to report the external temperature, this self-heating can be a problem. The trick is to minimize the current so that the temperature effects outside the thermistor dominate the self-heating. For your 10 kw thermistor, a driving voltage of +3 V meets this requirement. With a LabVIEW Express VI, you can program the VPS on the NI ELVIS II workstation. The value 3 in the orange box sets a +3.0 V output on VPS+. One extra line, green in color, connected to the STOP icon ensures the VPS is reset to zero volts when the program ends.

18 Page 18 of 19 Complete the following steps to open and view the components and code in the digital thermometer VI: 1. Start the LabVIEW program Digital Thermometer.vi. 2. Open the block diagram (Window»Show Block Diagram) and subvis (double-click on the icons) to view the program flow and see how the subvis and the Read and Convert functions are coded. NOTE: This LabVIEW program is configured to connect to Dev1 for your NI ELVIS workstation. If your device is configured to another device name, you need to rename your NI ELVIS workstation to Dev1, in Measurement & Automation Explorer (MAX) or modify the LabVIEW programs to your current device name. With the calibration curve for your thermistor, you can update the subvi (Convert R-T) with the proper equation and use it to achieve a functioning digital thermometer. If you want to write your own program, find the VPS API function in the Functions palette (Functions»Measurement I/O»NI-ELVISmx»NI-ELVISmx Variable Power Supplies). Figure 20. Functions Palette

19 Page 19 of 19 Challenge Exercise: Design a Passion Meter Using the Thermistor Circuit When an individual becomes embarrassed, excited, or just plain hot, blood flows to the skin to keep body s core temperature constant a sort of an internal air conditioning. The in-rush of blood to the skin appears as a reddened patch, and the skin temperature of that patch becomes hot to the touch. Telling a joke can lead to burning earlobes for some people. By placing the thermistor on that reddened part, you can measure this temperature rise. Design a LabVIEW program to measure the body skin temperature. The normal body temperature is 38.5 C. Use this value as the maximum scale reading on a LabVIEW thermometer control. Use the ambient room temperature (25 C) as the lower limit. Be creative with your front panel labels. Open the LabVIEW program, Passion Meter.vi. NOTE: This LabVIEW program is configured to connect to Dev1 for your NI ELVIS workstation. If your device is configured to another device name, you need to rename your NI ELVIS workstation to Dev1, in Measurement & Automation Explorer (MAX) or modify the LabVIEW programs to your current device name. Figure 21. Front Panel for Passion Meter.vi