Relative Humidity RUNNING AVERAGE Lab

INTRODUCTION

In this exercise, you will create a VI that will monitor current Relative Humidity (R.H.) along with the average R.H. on a waveform chart. It will employ a while loop and shift register to do this. Elements added to the shift register will store R.H. data from previous iterations, and be used to calculate the average R.H.

      • Create a New VI (or use the VI from your previous lab)
      • Add a while loop to the Block Diagram
      • Wire a Boolean Stop Control and a Boolean NOT function to the conditional terminal
      • Add two Digital R.H. Acquisition Sub VIs; one inside the loop and one outside
           o The Digital R.H. Sub VI that is outside the loop is only read once at the beginning of the first iteration and is
              only used to initialize the register to read the initial room humidity.
           o The R.H. Sub-VI must be created before this lab can be completed
      • You will add the following Sub-VI, and replace the random number simulated temperature Sub-VI used in the
         previous lab.

Note: You will receive error messages if you do not create the exact tasks.  The section below describes how to create
the required tasks.

In order to acquire external data, a Data Acquisition (DAQ) device must be wired and connected to the computer running LabView.  We will be using the NI-USB- 6808 DAQ unit.   The follow steps are required to configure the device to read external humidity readings from an Omega RH71 hygrometer.

Make sure the NI-USB 6808 unit is plugged into the USB port on your computer, and a blinking link light is established before continuing.  Note: if a link light is not present, the driver must be installed from the CD that came with the device.
 
1. Open the Measurement and Automation (MAX) software.
2. Click on Devices and Interfaces and expand the selections.
3. The NI USB-6808 should be shown as an available device.
4. Right click on the NI-USB-6808 device icon and select CREATE TASK.
 
 

5.  The following screen should appear:  Select ANALOG INPUT.

6.   The following screen should appear:  Select VOLTAGE.

 

7.  The following screen should appear:  Select AI4 (analog input number 4):

8.  Enter the NAME of the TASK as shown below in the example:

9.  The VOLTAGE INPUT SETUP screen will appear.   In the terminal  configuration, select RSE (Reference Single End).

Next, a scale must be created to correspond to the desired representation of acquired data.
The analog input voltage varies from 0 to +1; therefore, the desired scale factor will be 100.

10.  Click on SCALE, and then CREATE NEW as shown below

 

 11.  Next click on LINEAR as shown below:

12.  Enter 100 for the slope of the linear scale.  Note the Y intercept is at 0.

13. Click ok  then enter the VOLTAGES LEVELS AS SHOWN
Note:   MAX = 1,  MIN = 0 and make sure the newly created CUSTOM SCALE is set to the named scale you just created.

14. On the bottom tab, click on CONNECTION DIAGRAM to verify connections as shown:

15.  Close the connection diagram and then click on TEST (green arrow at top)

You should be reading a voltage corresponding to the HUMIDITY READING shown on the screen of the Omega RH71 Hygrometer device.

1.  Create the VI shown below.

             2.  Next add terminals to make the R.H. Sub-VI capable of being wired to the main VI.
             3.  On the FRONT PANEL, right click on the ICON in the upper right hand corner, and select SHOW CONNECTOR.
 
 

             4. Right click again on the connector icon and select PATTERNS, then select a two terminal block as shown below.

          5. Next, On the FRONT PANEL, using the wiring tool, wire from the Relative Humidity Indicator to the
              RIGHT terminal as shown below (note it should change color).

RETURN TO THE MAIN VI
• Add a Shift Register to the loop by moving the cursor over the loop and right clicking
• Add four additional Elements to the Shift Register by right clicking while the cursor is over the loop and select ADD ELEMENT
• Add a Wait (ms) function to the Block Diagram as shown and make it so that the loop iterates every 1/2 second
• Add a Waveform Chart to the Front Panel
Your completed Block Diagram and Front Panel should look like the ones shown below:

                                       R. H. Data Acquisition Front Panel

o Add the Waveform Chart's Digital Displays by selecting the Waveform Chart > pop-up > Visible Items >Digital Display:

• Add Free Labels to the Waveform Graph Digital Displays
     o Select the Edit Text Tool on the Tools Palette > select the location for the text next to the digital displays > type the
        text>click Automatic Tool Selection on the Tools Palette
• Modify the X and Y scale Label and Waveform Chart Label text to read as shown in the above example
• Add a Compound Arithmetic function to the Block Diagram and Expand it to have five Inputs
     o Pop-up in the Block Diagram > Functions > Numeric > Compound Arithmetic

o Drag and stretch the Compound Arithmetic icon to include more inputs:

• Add a Divide Function and wire a Numeric Constant with a value of five to the loop
• Add a Bundle Function to the Block Diagram
• Add a CASE STRUCTURE with TRUE  as shown.  (Note right click on the block diagram and select from STRUCTURES
   then select CASE STRUCTURE.

• Complete the wiring as shown in the above example
• Run the VI and observe the results
• Save the VI as Humidity_Running_Average.vi

The next section describes how to add a write to spreadsheet function so that acquired data can be exported to Excel.

Write To Spreadsheet File

Purpose:  The WRITE TO SPREADSHEET FILE function provides the ability to output data that can be written to spreadsheets such as EXCEL.   For this part of the laboratory, the function will be added to the previously written VI.

Background and format for writing a DOUBLE PRECISION variable to an EXCEL compatible format:
The WRITE TO SPREAD SHEET FILE converts a 2D or 1D array of strings, signed integers, or double-precision numbers to a text string and writes the string to a new byte stream file or appends the string to an existing file. The data type you wire to the 2D data input or 1D data input determines the polymorphic instance to use. You also can transpose the data. The VI opens or creates the file before writing to it and closes it afterwards. You can use this VI to create a text file readable by most spreadsheet applications.

Double

 
format specifies how to convert the numbers to characters. If the format is %.3f (default), the VI creates a string long enough to contain the number, with three digits to the right of the decimal point. If format is %d, the VI converts the data to integer form using as many characters as necessary to contain the entire number. Use the format string syntax.

file path is the path name of the file. If file path is empty (default) or is <Not A Path>, the VI displays a dialog box from which you can select a file. Error 43 occurs if you cancel the dialog box.

2D data contains the data the VI writes to the file if 1D data is not wired or is empty.

1D data contains the data the VI writes to the file if this input is not empty. The VI converts the 1D array into a 2D array before proceeding. If transpose? is FALSE, each call to this VI creates a new line or row in the file.

If append to file? is TRUE, the VI appends data to an existing file. If append to file? is FALSE (default), the VI replaces data in an existing file. If there is no existing file, the VI creates a new file.

If transpose? is TRUE, the VI transposes the data after converting it from a string. The default is FALSE.

delimiter is the character or string of characters to use to separate fields in the spreadsheet file. For example, a value of , specifies a single comma as the delimiter. The default is \t, which specifies a single tab character as the delimiter.

new file path returns the path to the file.
 

Procedure:

1.  Open the previously created VI used to acquire temperature data.  Remember this laboratory exercise incorporated SHIFT REGISTERS and provided a means to average 5 points then plot both raw and averaged values to a WAVEFORM CHART.   An example is shown on the following page.

2. Under FUNCTIONS, FILE I/O, select the WRITE TO SPREADSHEET options and place inside the while loop.   Note the function location is shown below.

3. From FUNCTIONS, ARRAY, select the CLUSTER TO ARRAY FUNCTION
And drag inside the while loop.   The location of this function is shown below.

4. Add a BOOLEAN (TRUE) constant inside the while loop.
5. Add a PATH constant inside the while loop (location is shown below).
 

6. Wire the BLOCK DIAGRAM (an example is shown below):

7. Type in the desired path in the PATH CONSTANT box.  Note this example in the PATH BOX contains the following path:
  C:\Documents and Settings\ballaaron\My Documents\ET 472_SP09\LabView_Temp_Running_Average_with_Overtemp_Control\Tempdata

      NOTE: “Tempdata” is the spreadsheet file name to be written to.

8. Run the VI for a few seconds (allow enough time to collect data).
9. Find the file in the folder or path specified, and open with Excel
10. Verify that data is present in the Excel file.   Note:  Alternately, you can add the .XLS file extension to your file name so that it will be associateD with EXCEL.
11. Write a lab report showing screen captures of your front panel, block diagram and Excel file Containing the collected data.