Practicum 1B

LabVIEW is a software application used to create software versions of hardware instruments. 

Consider a hand-held motor controller for a radio controlled car. 

If we were to create a VI version of the real hardware, the Front Panel would be a software representation of the knobs and joy sticks that allow an operator to input commands or settings, (e.g. Fig. 5.1a). 

The Block Diagram would be a software representation of the wires and electrical components that could be found inside the controller, (e.g. Fig. 5.1b).

Inspect the front panel of your VI, (Fig. 5.2a). You should see 5 fields where a user can input numeric values. The fields should be labeled Mass, Thrust, Buoyancy, Drag Coefficient, and Velocity (with appropriate units). These fields are call Numeric Controls, and they allow a user to set the parameters before (or while) running the VI. 

On the front panel there should also be a sixth numeric field labeled Acceleration. This is an instance of a Numeric Indicator, which is used to display the numeric value of output signals.

In Fig. 5.2b, all the function blocks corresponding to numeric controls (e.g. mass), have outputs that are connected to the inputs of a Formula Node function block. These connections are made with wires, the bright orange lines on the block diagram.

Above the numeric control function blocks on the block diagram is a Numeric Constant whose value is 9.81. This will be used for the gravity term of our acceleration calculation, and is wired to be an input to the formula node.

The output of the formula node block is wired to the input of the Acceleration numeric indicator block.

The central block is a formula node. Using the equation from section 4, complete the functionality of the block by double clicking the block and typing in the correct equation for acceleration as a function of mass, thrust, buoyancy, damping coefficient, and velocity. Ensure you end the equation with a semi-colon, and that you use the correct variable names (i.e. G, M, fT, fB, DC, V, and A). You may want to double check the correctness of your equation before using it here. (Questions  1 &2 on submission sheet)

Save the file by clicking File->Save. Or using the keyboard shortcut ctrl+S.

Run your program by clicking the arrow (Fig. 5.4) on the front panel or block diagram, (either will work). If all the values on the front panel are zero, your calculated acceleration will be NaN, (since m = 0 implies that you are dividing by 0). Try changing the values of the inputs, and note how the output changes values. 

Important: Running the program only makes a single calculation for acceleration. To make the acceleration output update in response to changing inputs, we will use a while loop, (later in section 5.2).

1. Click on File->New VI, or use the keyboard shortcut ctrl+N. This creates a new virtual instrument (VI).   The first thing we're going to do is save the VI and get it's name right: click on File -> Save As, double click on your library, and then specify the file name VDSim in the window that lab view creates.  This saves your new VI inside your library.  If you are not using a library in the future, you can always just save a VI anywhere on your computer: it will show up as a file with the .vi extension. Make sure your VDSim files are not floating.

2. Select the front panel. You should notice an accompanying window appear, named Controls. If it doesn't automatically show up, right click and make it persistent by clicking the pin icon in the top left corner.

3. Ensure that you are in the Modern tab of the Controls Palette and click on Numeric. Click and drag the Vertical Pointer Slide onto the Front Panel. You’ll notice that the slider will show up in both the front panel and block diagram.

This slider will be used to control the thrust force of your motor. Rename it Throttle by double clicking on the word Slide.

4. Next, right click on the slider in the front panel and click Display Format…, this will open a new window. In this window, click on the tab labeled Scale. Finally, change the min max range values of the slider to -0.5 and 0.5 respectively. Click Ok, and you should see the slider in the front panel updated with the new range.

7. Before trying to run the program, you will need to add a stop button. To do this, switch back to the front panel, navigate to the Controls Palette->Modern and click on Boolean. Next, drag and drop the stop button. Resize the button to make it larger and remove the text above. Select the button by highlighting it. Drag it to the left of the slider.

Next, switch to the block diagram window. You’ll see a new block with the words ‘STOP’. Click and drag this block to the bottom right of the while loop, next to the red stop sign.

8. We need to wire the two blocks together so that clicking the Stop button on the front panel will break the while loop and end the program.

Hover the mouse pointer over the center right side of the stop button block in the block diagram. A small image of a spool of wire should appear. Click and drag the wire to the center left side of the red button. Release the button click to make the wire appear.

9. In the block diagram of this new VI, navigate to the Functions palette and click on Select a VI…, located near the bottom. When prompted, select your library from the file explorer,  pick the AccelCalc.vi VI you just made, and then move to the block diagram to place the VI. Remember that you saved it in the directory named “Desktop/E79/Section X/Practicum YZ”, where X is your section number, Y is the practicum module number, and Z is the practicum module letter. For example: “Desktop/E79/Section 2/Practicum 1B”.

10. Hover the cursor over the input and output terminals of the VI icon you just added. These are located on the left and right edges of the VI icon, but won’t be visible until you hover the mouse pointer there. You should observe that they are all labeled. Wire up the Throttle slide control to the Thrust input terminal on the AccelCalc VI. 

12. Every time the while loop is executed, time is incremented. Velocity will be calculated by multiplying the acceleration with the time step associated with the while loop. We will assign a value of 20 milliseconds for this time step. To do this, from the block diagram, navigate to the functions palette. Search for Wait Until Next ms Multiple, located in Programming->Timing. Drag this into your window. 

To add the 20 ms delay, right click on the block’s input and click Create->Constant. Set the constant to 20. 

13. One method for numeric calculation of velocity at time step n, given previous velocity vn-1, acceleration an, time step size Δt is the backward Euler method:

15. To create Numeric Indicators on your VDSim VI that display your Acceleration, Velocity, and Position, right click on the output terminals of the DoubleIntegration block, then select Create->Indicator. See Fig. 5.16. Note that a constant block 0.02 (20 ms) must be defined for the time step size Δt. Create a constant for the DoubleIntegration input node labeled as deltaT. Set the value of the constant to be 0.02.

16. This next step is very important. In order to feed the calculated velocity back into the input of the AccelCalc block, create a wire from the input terminal on the AccelCalc block to the wire leaving the DoubleIntegration block Velocity output terminal. After doing this, LabVIEW should automatically add a Feedback Node. This block saves the value and passes it into the addition block after every iteration of the loop. See Fig. 5.17.

Note: The feedback node may be pointing in the wrong direction when compared with Fig. 5.17. To fix this, right click the feedback node, and click change direction.

Next, hover over the bottom of the feedback node, right click on the bottom terminal, and add a constant with a value of 0. This will be the initial value of velocity in the feedback node when you start the program. See Fig. 5.17. 

17. Now do a quick test to see if everything is working as expected. Switch to the front panel, input the approximate mass of your ROV (about 1 kg), the buoyancy (try 9.81 N), and the drag coefficient (use 1 kg/s). Click RUN. 

You should expect to see the acceleration, velocity, and position of your ROV change as you vary the thrust. Make sure that your simulation reaches steady state values for acceleration and velocity when the thrust is kept constant. Note what happens when you set the throttle to zero, mass to 1 and the buoyancy to 9.81. In this case, the buoyancy exactly cancels out the gravity term, (check your equation).

18. What’s the fun in simulations if there aren’t any visuals? Let’s add graphs to plot the acceleration, velocity, and position as a function of time. From the front panel, navigate to the controls palette, and add on Waveform Chart, located in Silver->Graph. Once it is in the front panel, right click on the chart and click Properties from the drop down menu. You’ll need to make two changes. The first is under the Display Format tab. Change type for the X-Axis to Relative Time. Next, under the Scales tab, check the box near Autoscale.

Click OK to exit.

19. Click on the chart, then copy and paste two more charts. These will be used to display the acceleration, velocity, and position of your simulated ROV as a function of time. Rename the charts.

20. Next, switch to your block diagram and wire the respective output to the input of the charts you just inserted. The result should look something like the image on the right.

Switch back to the front panel to test out your simulation. 

(Question 3 on submission sheet)

When your buoyancy force is balanced by your gravitational force (e.g. m=1, Buoyancy = 9.81), setting the throttle to a new value and leaving it at that new value should produce the following behavior:

• Velocity should exponentially approach some steady state value.

• Since acceleration is the change in velocity, it should become zero as the velocity approaches steady state.

• Position will tend to increase or decrease at a constant rate as the velocity reaches steady state.

Make sure your simulator does this. 

(Question 4 on submission sheet)

DEMONSTRATE THIS TO YOUR PROCTOR OR INSTRUCTOR, AND HAVE THEM CHECK THAT YOUR DATA (YOUR ALTERED VI) GETS SAVED TO THE DESKTOP.  THEN SAVE YOUR DATA TO A THUMB DRIVE OR TO THE CLOUD

In this section, you will be using a MyDAQ to acquire voltage readings from a linear, solid state temperature sensor. MyDAQs are data acquisition devices. They give you the ability to measure voltages in circuits (they also have many other features that make them very handy, such as multimeter, control, audio analysis, etc). Solid State Temperature Sensors are circuit elements that output a voltage which correlates to the temperature; thus, they can be used as a tool to measure ambient temperature.

Many of the analog/digital inputs on the MyDAQ are located on the right side. These inputs are where you will plug in the voltages you are measuring.

The solid state temperature sensor that you will be using for this practicum is the Microchip Technology MCP9701 Linear, Active thermistor. Simply put, it is a sensor that outputs a voltage which corresponds to the ambient temperature. As the name states, this relationship between voltage and ambient temperature is linear (in its region of operation, -40°C to 125°C). Thus, the output voltage and ambient temperature will have the following form shown in Fig. 6.3.

The temperature sensor that you receive will already be soldered to three wires. Red is connected to the source voltage (+5 Volts), black/green is ground (0 Volts), and blue/white is the output voltage. Refer to Figure 6.4 to the right.

Begin by connecting the blue/white wire to Analog Input 0+ (AI0+) in the MyDAQ, this is pictured below in Figure 6.5. Make sure that the wire is stripped at the end before you connect it.

Take a short black or green wire, one that is stripped on both ends, and insert it into the AI0- port, as shown in Fig. 6.6 (a). This wire will be connected to our circuit’s ground. Loop it back to the terminal labeled AGND as shown in Fig. 6.6 (b).

Insert the Ground wire of the sensor into AI0-. There should now be two wires coming into AI0-, one connected to the AGND terminal, and one leading to the Ground of the sensor, (see Fig. 6.6 (b)).

Next, you will need to provide power to the sensor from the 5V power supply of the digital side of the myDAQ. First, add a black/green wire that connects the AGND to the DGND so all voltages will be referenced to the same ground. See Fig. 6.7. Next, connect the 5V input line of the sensor (red wire), to the 5V supply on the myDAQ.

In order to read the data from this analog input, you will need to connect the MyDAQ to your computer using the provided USB cable and set up a quick LabVIEW program. Do this now.

Create a new VI and save it into your library with the name tempReader.vi. 

LabVIEW has a helpful MyDAQ Assistant, which makes it fairly simple to interface with the MyDAQ in LabVIEW. From the block diagram window, navigate to the functions palette and click Express->Input. Drag the DAQ Assistant into the block diagram. See Fig. 6.8.

Upon dropping the MyDAQ assistant into your block diagram, a screen should open, asking you to select whether you would like to Acquire or Generate a signal. Click on Acquire Signals->Analog Input->Voltage. Fig. 6.9a shows the DAQ Assistant window.

Next, select your myDAQ then ai0, as shown in Fig. 6.9b. Once it is highlighted, click Finish.

Another window should open. As shown in Fig. 6.10, change the Acquisition Mode to Continuous Samples. This gives you the ability to plot the temperature in real-time. 

Finally, click OK in the bottom right. LabVIEW will ask you to add a while loop; click Yes. Afterwards, your block diagram should look like the diagram in Figure 6.11.

LabVIEW will automatically generate a simple VI; however, we will not be able to plot our data yet. In your front panel, add a Waveform Chart from the controls palette in Silver->Graph->Waveform Chart.

From the block diagram, connect the Data output of the DAQ Assistant block to the input of the waveform chart.

To fix the scale of the y-axis on your waveform chart, right click on the chart, click on Properties; and under the Scales tab, select the Y-Axis in the drop down, turn off autoscale in the same tab, and set the range you would like to see, (0V to 2V is a good range).

Before running the VI and testing your connections, verify that your circuit is wired correctly and exposed wires are separated from each other.

Run your VI. 

(Optional) If your signal is very noisy, ask your proctor or professor for a  capacitor to place between the two analog inputs, as shown in Figure 6.13. It will be a little difficult to keep the wires inside, so ask a partner for assistance. 

Note: Disconnect your 5V power when making changes to a circuit.

If everything is wired correctly, you should see something like the plot shown in Fig. 6.14.

Warm up the thermistor with your hands and watch how the voltage signal responds.

Although the result is very interesting, it isn’t very meaningful. In order to get the temperature, you will need to rearrange the equation from Fig. 6.3, seen to the right here.

According to the data sheet shown in Figure 6.15, we get the information shown to the right.

We will use the arithmetic operations in LabVIEW to implement the equation from Fig. 6.15 and convert the output voltage from the MyDAQ into ambient temperature in °C. From the Functions Palette, under Numeric, find the Subtract block. See Fig. 6.16 below.

Click on the Subtract block and place one in your block diagram. Wire the output data terminal from the DAQ Assistant block to the top input terminal on the Subtract block. Also, right click on the lower input terminal of the Subtract block and select Create Constant. Set this constant to be 0.4. See Fig. 6.17.

Now add a Divide block from the functions palette Numeric tab. Use it to divide the signal by TC before plotting it on the chart. Fig. 6.18 shows the result.

1. Consider rescaling the chart y-axis to provide appropriate range of temperatures to plot. 

2. Note, you will be using these skills to measure water temperatures in your ROV later in the course.

(Question 5 on submission sheet)

In this section of the practicum, you will be using PWM signals to control the brightness of an LED. PWM stands for pulse-width modulation. In digital electronics, PWM is a method of quickly sending a signal that switches between on and off (a square wave) as a means of providing a desired voltage to a load. Fig. 6.19 shows you an example of this. PWM signals also have another important characteristic: their duty cycle. A duty cycle is the percentage of a waveform’s period that the signal is on. A duty cycle of 100% implies that the signal is always on, while a duty cycle of 50% implies that the signal is on for only half the period of the waveform. With a fast enough PWM signal, you can use digital signals to control the voltage supplied to motors, LEDS, etc.

For this practicum, you will be using Professors Spjut’s PWM.vi which can be found in your library. This VI uses the digital ports in the MyDAQ. Refer to Figure 6.20 below.

Cut and strip both ends of two fairly short wires; one for ground (black/green) and another for digital pin 0. Use a small flathead screwdriver to place them in the correct MyDAQ pins. Refer to Fig. 6.21 for a picture.

Using alligator clips, make the circuit illustrated in Fig. 6.22. LEDs are directional; therefore, you must make sure to wire them in the right direction. The longest pin on the LED will be the positive terminal; connect this pin to the MyDAQ’s digital output. The shorter pin is connected to the digital ground (DGND).

Connect the MyDAQ to your computer and open the PWM virtual instrument. This is what will be used to control your motors in the future; however, we now only use it to control an LED using Chan 0.

The following step is very important. In the front panel, there is a box labeled “Channel Settings”. Type (or copy and paste) this into the dropdown: myDAQ1/port0/line0:7 . Refer to Figure 6.23. You need to do this because the VI has been configured to use all of the output pins (we are only concerned with Pin 0).

Note: You may get an error when you run the IV that states that the DAQ is not connected. This error is because your DAQ might not be called myDAQ1. Somewhere on the error message it should display suggested DAQ number. Change the number to the suggested one in "Channel Settings." You can also delete the 1 in "Channel Settings" and the program should auto-suggest the correct one.

Once you have changed the output, you can now run your VI! Vary the Chan 0 slider and observe how the brightness of the LED changes. The slider is simply changing the duty cycle of the PWM signal.

Move the slider to the left. This is the effect of a shorter duty cycle. See Figure 6.24a.

Move the slider to the right. This is the effect of a longer duty cycle. See Figure 6.24b. You will eventually use a similar VI to control speed of your motors using digital signals.

(Question 6 on submission sheet)