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Engineering Projects with NI LabVIEW and Vernier © Vernier Software & Technology P4 - 1

Hot-Wire Anemometer

An anemometer is a weather instrument for measuring wind speed. There are many styles of anemometers including windmill, cup, and sonic, but this Project is based on the concept of a hot-wire anemometer. In a standard hot-wire anemometer, a thin wire is heated to a temperature above ambient by running an electrical current through it. After a short period of time, the wire will reach an equilibrium temperature such that the heat energy coming into the system from electricity will be matched by the loss of heat energy to the environment. The rate of heat loss depends on the wind speed of the surrounding environment. An increase in wind speed causes an increase in heat loss. In a hot-wire anemometer, wind flowing across the thin wire causes the temperature of the wire to decrease – the faster the wind moves, the more the wire cools. By measuring the temperature of the hot wire, you can indirectly determine wind speed.

PROJECT DESIGN REQUIREMENTS

In this Project, you will build a device that acts like an electric birthday candle based on the principle of a hot-wire anemometer. Use the Vernier Digital Control Unit (DCU) to supply power to a resistor, which will act as a heater (the “hot wire”) in your device. Add a red LED and a current-limiting resistor to your apparatus to represent the flame on a birthday candle. Write a LabVIEW program to monitor the temperature of the heating resistor with the Vernier Surface Temperature Sensor and illuminate the “candle flame” LED when the temperature exceeds a threshold value. Your program should make the LED act like a birthday candle – that is, when you blow on it, it should go out. Ev

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Project 4

P4 - 2 Engineering Projects with NI LabVIEW and Vernier

MATERIALS

SensorDAQ, LabQuest, or LabQuest Mini LabQuest or LabPro power supplyLabVIEW red LEDcomputer 100 resistorUSB cable 220 resistorVernier Surface Temperature Sensor adhesiveVernier Digital Control Unit (DCU) heat-shrink tubing (optional)

PROJECT SETUP

Construct a hot-wire anemometer

1. Wire the LED and resistors to the DCU cable as shown in the diagram below. Note that the flat side of (or the short wire on) the LED should be connected to GND.

Figure 1 Hot-wire anemometer wiring diagram

2. Secure the tip of the Surface Temperature Sensor against the heating resistor. If you have heat-conducting Epoxy cement available, use it. If not, use a small dab of any adhesive.

Figure 2 Hot-wire anemometer

3. Optional: Use short lengths of heat shrink tubing to cover your electrical connections, but make sure not to cause a short. The Surface Temperature Sensor and heating resistor should remain exposed to the open air.

Hot-Wire Anemometer

Engineering Projects with NI LabVIEW and Vernier P4 - 3

Connect the Surface Temperature Sensor and DCU to the interface

1. Connect the DCU to the DIG port of the interface.

2. Connect a power supply to the DCU.

3. Connect the Temperature Sensor to Channel 1 of the interface.

4. Connect the interface to the computer.

PROJECT BACKGROUND INFORMATION

In this Project, you are asked to build a hot-wire anemometer that will act like an electric birthday candle. A red LED will be used as a qualitative indicator of wind speed and will act as the “candle flame.” A resistor will serve as the “hot wire” to radiate heat as current flows through it. You should monitor the temperature of the hot wire with a Surface Temperature Sensor and control the illumination of the LED with the Vernier Digital Control Unit (DCU).

The DCU is a small box with a short cable that connects to the DIG port on the interface. On one side of the DCU is a socket that accepts a DC power supply for powering up to six electronic components. The top of the DCU is transparent revealing six red LEDs and one green LED inside the unit. The green LED illuminates when the DCU is properly connected and running a DCU program. The red LEDs indicate the status (on or off) of the six output lines of the DCU (D1-D6). A 9-pin D-sub socket cable with bare wires on one end is supplied with the DCU for use in building projects. There are connections for all six digital lines, plus a power connection and two ground connections. The color code of the wires is identified on a label attached to the cable.

When building devices such as a hot-wire anemometer, you should always keep the power limitations of the DCU in mind. You should not exceed 1000 mA total.

The Digital Express VI found in the Vernier functions palette can be used to control the DCU. In order to activate the DCU, you must send an output pattern to the Digital Express VI indicating which line(s) are to be turned on. When you place the Digital Express VI on the block diagram, a configuration window appears. If you select Output Lines 1–6 as the Device Selection, you will see a picture of the DCU. Change values from 0 to 15 in the DCU Pattern control for feedback on what lines are activated. As shown in the figure below, a pattern of “1” will turn on DCU line D1.

Figure 3 Digital Express VI configuration window for the DCU

Project 4

P4 - 4 Engineering Projects with NI LabVIEW and Vernier

You need to individually control the LED and the resistor acting as a heater with your DCU. The power to the heating resistor should remain on continuously, but the LED will turn on and off depending on the reading from the Surface Temperature Sensor.

Once your device is constructed, you must do some preliminary testing to determine a good threshold temperature. You will be using your own breath as the “wind” for your anemometer. When the device is first activated, the air temperature around the sensor will increase as current flows through the resistor. The LED should illuminate when a pre-defined temperature (commonly called the set point) is reached. At this point when you gently blow on the heating resistor, the LED should stay lit. You want to set a threshold temperature that requires you to blow somewhat vigorously (much like you would blow out the candles on a birthday cake) in order to cool the resistor wire to a point that the LED goes off.

Normally, when you blow out a candle, the flame is completely extinguished; but your Project will be more interesting and amusing if you treat your device as a “trick” candle. If you leave DCU line D2 on continuously, the current through the heating resistor will never stop. The wire will reheat to the threshold temperature when you stop blowing on it, and the LED will illuminate again waiting for you to blow it out again.

PROJECT TIPS

1. LEDs are light emitting diodes. Like all diodes, an LED will only conduct an electrical current in one direction. The short wire or flat side of the lens should be connected to ground.

2. The tip of the Vernier Surface Temperature Sensor should be near or touching the resistor.

3. Since you are using a resistor instead of a very fine wire, your anemometer may be slow to respond. Be patient in waiting for the temperature reading to stabilize.

4. Avoid using the Abort Execution button to stop your VI, because some of the DCU lines may remain on.

5. A good data collection rate for the Surface Temperature Sensor is 10 samples per second.

6. The Analog Express VI collects data for a fixed amount of time. For an indefinite data collection length, check the Repeat option in the Analog Express VI’s Set Timing configuration window.

7. Refer to Appendix E for additional information about the Vernier Surface Temperature Sensor and DCU.

PROJECT TROUBLESHOOTING

1. Make sure the DCU is receiving power. The green LED in the top of the DCU box will be lit when the DCU is powered on.

2. Double-check the DCU cable connections against the color-coded label attached to the cable.

3. Make sure you are sending the proper output pattern to turn on one or more digital lines.

4. Both the “stop (F)” and “stopped” terminals on the Vernier Analog and Digital Express VIs must be wired in order for the program to execute and shut down properly.

Hot-Wire Anemometer

Engineering Projects with NI LabVIEW and Vernier P4 - 5

CHALLENGE DESIGN REQUIREMENTS

Note: Do not attempt the Challenge until you have completed the Project Design Requirements.

Modify your Project program to include a birthday greeting sound file. (LabVIEW has the ability to play any file with the .WAV extension.) Place a new button on the front panel to allow the user to play the sound file when the “candle flame” LED lights up. You can record your own personal greeting, download a file from another source, or use the sound file, Birthday.wav, included on the CD that came with this book.

CHALLENGE BACKGROUND INFORMATION

LabVIEW contains several example files to help you learn how to play and/or record sound files. The function you are most likely to use in this exercise is the Play Sound File subVI (found in the Programming ► Graphics & Sound ► Sound ► Output functions palette). When using this subVI, you must specify the path to the location of your sound file. You can designate the path as a constant or you can use the path functions from the Programming ► File I/O function palette to build a unique path name based on the location of your main program.

CHALLENGE TIPS

1. The Play Sound File subVI has a timeout input. A timeout value of 0 allows the While Loop to continue without pause.

2. Play the sound file when the temperature is above the set point and the user has pressed the button. A Case Structure is a good structure for this type of logic.

CHALLENGE TROUBLESHOOTING

Make sure the volume on your computer is turned on and set to an acceptable level.

Project 4

P4 - 6 Engineering Projects with NI LabVIEW and Vernier

EXTREME CHALLENGE DESIGN REQUIREMENTS

Instead of using your breath to blow out the “candle flame” LED, incorporate a small DC fan into your apparatus that will turn on and blow out the LED when the temperature reaches the threshold value. Since you are already using DCU lines D1 and D2 for the LED and resistors, you will need to wire the fan to line D3.

ADDITIONAL MATERIALS

small fan EXTREME CHALLENGE SETUP

Connect a DC fan to the DCU

1. Wire a small DC fan between DCU line D3 and GND.

Figure 4 DC fan wiring diagram

2. Position the fan so that it blows on the heating resistor in your anemometer.

EXTREME CHALLENGE BACKGROUND INFORMATION

The DCU can be used to control the fan as well as the components in your hot-wire anemometer. The fan should come on when the LED is illuminated because the purpose of the fan is to “blow out” the “candle flame” LED. This means that when the threshold value is exceeded all three components (the fan, the resistor, and the LED) should be on.

The first 8 DCU output patterns correspond to a binary number system as shown in the table below. If the switch is in the + position, current will flow and the device connected to that line will be on. If the switch is in the – or X position, the device will be turned off. If your circuit is wired correctly, turning on all three lines simultaneously should cause the fan to blow on the heating resistor and cool it down so that the LED can go out.

Hot-Wire Anemometer

Engineering Projects with NI LabVIEW and Vernier P4 - 7

Output Binary D1 D2 D3 D4 D5 D6

0 0000 — — — — X X

1 0001 + — — — X X

2 0010 — + — — X X

3 0011 + + — — X X

4 0100 — — + — X X

5 0101 + — + — X X

6 0110 — + + — X X

7 0111 + + + — X X

Table 1 Digital output patterns for the DCU for use in this Challenge

EXTREME CHALLENGE TROUBLESHOOTING

If your LED seems to be taking a long time to go out, adjust the position of the fan. The fan should blow directly on the heating resistor rather than the LED or the Surface Temperature Sensor.

Vernier Lab Safety Instructions Disclaimer

THIS IS AN EVALUATION COPY OF THE VERNIER STUDENT LAB.

This copy does not include:

Safety information

Essential instructor background information

Directions for preparing solutions

Important tips for successfully doing these labs

The complete Engineering Projects with NI LabVIEW™ and Vernier manual includes 12 projects as well as essential teacher information. The full lab book is available for purchase at: http://www.vernier.com/cmat/epv.html

Vernier Software & Technology

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