ELEC 243 Lab

Experiment 4.1

Labview as a Source of Signals

Last week we looked at the ability of Labview to measure and display external signals. These included voltages and other physical variables which we could represent as voltages. This week we will start by looking at the other direction: using Labview to generate signals. These will also be voltages, which we can then use in their own right or transform into other physical variables.

In this Experiment we will practice signal generation techniques on a new, blank VI. When we are confident of our skills, we will use them to make improvements to the VI we built last week in Experiment 3.2.

Part 1: Generate a Fixed Voltage

Let's start with something simple: a VI that puts out a constant 1 volt signal. Not very exciting, but we we can add enhancements until it does something useful.



Step 1:

 Recall that the DAQ card has two analog voltage outputs: dac0 and dac1. Unlike the analog inputs which have different sensitivities, there's no real difference between these, so we'll arbitrarily choose dac0 for our output.

Use a BNC patch cable to connect J1-1 to CH 1 of the oscilloscope. On the interface module connector strip, connect J1-1 (socket strip pin 1) to dac0 (pin 51).

Step 2:

 Start Labview and open a new, blank VI. Since we want this VI to run continuously, go to the block diagram window and create a large while loop.

Step 3:

 We create D/A output blocks in nearly the same way we did A/D input blocks, using the DAQ Assistant. From the Functions palette, select Output, then Place the resulting block inside the while loop, somewhere in the right-hand half.

Step 4:

 Once it is placed, you will get a Create New ... wizard, just as you did with the input block in Experiment 3.2. This time, select Analog Output, then Voltage. From the list of Supported Physical Channels, select ao0, then click the Finish button at the bottom of the frame.

Step 5:

 When the DAQ Assistand dialog appears, set the Output Range to have a Max of 10 Volts and a Min of -10 Volts. Set the Generation Mode to 1 Sample (On Demand). When finished, click the OK button.

Step 6:

 There will be a brief flurry of activity, the DAQ Assistant block will expand, and a white band containing the word data will appear.

Step 7:

 On the left-hand side of the while loop, place a numeric constant. Set its value to 1.

Step 8:

 Wire the constant to the data input of the D/A output block. Our first signal generating VI is finished.

Step 9:

 Press Run and verify on the oscilloscope that the output is set to 1 volt.

Part 2: Input from the Front Panel

As we'll see shortly, being able to output/ produce a fixed voltage is a useful capability. But being able to change the value of that voltage would make it even more useful. We could do this by editing the value of the numeric constant, but we would have to stop the VI each time we wanted to make a change. It would be much easier if we could do this from the front panel. Fortunately, we are able to do this by using a numeric control, which is the dual of a numeric indicator, i.e. instead of displaying a value it allows you to enter one.


Step 1:

 Go to the front panel and press the Stop button. VIs can't be edited while they are running.

Step 2:

 Right click to bring up the Controls palette. Move the cursor to the Num Ctrls button to bring up the Numeric Controls palette.
This looks a bit like the Numeric Indicators palette, except that these are tools for producing rather than displaying numeric values. Click on the Num Ctrl button
and place the control in a convenient location on the front panel. Change the label to Vout.

Step 3:

 Return to the block diagram and notice the orange icon labeled Vout.
(An alternate way to get to the icon associated with a control or indicator from the front panel is to right click over the object and select Find terminal. This will bring up the block diagram with the associated icon selected.) This icon looks similar to the numeric indicator icons we used in Experiment 3.2, but the small triangle denoting the connection terminal is on the right-hand side.

Step 4:

 Delete the numeric constant and the wire from the diagram. Place the icon for the numeric control in the left-hand half of the diagram and wire its output to the input of the DAQ Assistant block.

Step 5:

 Return to the front panel and press the Run button. If the numeric control still contains the default value (zero), the signal displayed on the scope should fall to zero.

Step 6:

 With the VI still running, double click over the Vout control and type in a new value (between -10 and 10). Press Enter or click the check box. The signal on the scope should jump to the value that you entered.

Step 7:

 Try typing in a few more values. When the excitement has faded, stop the VI.

Step 8:

 Typing in a number is a convenient way to precisely set a value, but for quick changes, something like a knob is more suitable. Right click over the Vout control and select Replace. This will bring up a palette of objects with which this control may be replaced. From this palette, select Num Ctrls, then a Vertical Pointer Slide.


Step 9:

 Start the VI. Place the cursor over the white triangular portion of the slider, left click, and drag the pointer up and down. The signal on the scope should rise in fall in response.

Step 10:

 Change the lower limit on the slider from the default value (0) to -10. To do this, double click over the number you wish to change. This will select it and allow you to enter the new value. You should now be able to vary the output over the full range of -10 volts to +10 volts.

Step 11:

 Try replacing the slider with some of the other controls and examine their behavior.

Part 3: A Function Generator VI

With enough practice you should be able move the slider of the VI you built in the previous part in such a way as to produce waveforms like sinusoids and triangles. A better approach is to have your lab partner do this, leaving your hands free to work on less repetitive parts of the experiment. However, on those occasions when two pairs of hands and a sine wave (or any number of hands and a 1 kHz sine wave) are required, it would be nice to have something more automatic.

This should be fairly easy. We have Labview compute the value of $\sin(\omega t)$ at regularly spaced values of $t$ , then set the D/A output to that value during the corresponding interval. The tricky part is keeping the values of $t$ at which we update the output regularly spaced. In Part 4 of Experiment 3.2 we took samples of the A/D input value at 1 second intervals by placing a 1 second delay block inside the while loop. This is satisfactory for long intervals (like 1 second), but as we try to generate more closely spaced samples (think of that 1 kHz sine wave) the scheduling uncertainty of Windows will make the sample spacing very irregular.

The solution is to do the sampling in hardware, on the DAQ card, directly at the A/D or D/A converter. If we buffer a sufficient number of samples, irregularities in processing them can be smoothed out and the actual input and output values will change at consistently correct times. In order for this to work, we need to generate and process samples in blocks or buffers containing a fixed number of samples. Labview provides for this with the array data type.


Step 1:

 Stop the VI. Go to the block diagram and disconnect the slider output from the D/A converter input. Move the slider icon out of the way, but don't delete it; we'll use it again a bit later.

Step 2:

 From the Functions palette, select Input, then Simulate Sig. Place the Simulate Signal block to the left of the DAQ Assistant block and left click to place it. In the Configure Simulate Signal accept the default values and click OK A white band labeled Sine will appear at the bottom of the block. Wire the Sine output of this block to the data input of the D/A block. Double click on the D/A block or right click and select Properties from the menu. Change the Generation Mode from 1 Sample (On Demand) to Continuous. Click OK and wait for everything to settle down.

Step 3:

 Click Run. You should see a 10.1 Hz sine wave on the oscilloscope.

Step 4:

 Again, it would be convenient if we could change the parameters of the waveform (e.g. frequency or amplitude) without having to stop the VI, edit the Simulate Signal block, and restart. Fortunately that's easy to do.

Stop the VI. Move the cursor over the small double arrow in the middle of the bottom edge of the Simulate Signal block. It will turn into a small black square and the cursor will become a resize arrow.

Left click and drag down slightly until the dashed outline grows by one increment, then release. Another white band, labeled error out will appear (the pink color indicates the data type of the output). Left click in this new box to bring up the menu of choices. This is the list of all available inputs and outputs for this block. Select Frequency.

Step 5:

 Wire the slider output to this new input. Edit the slider to set its range to be 0 to 100.

Step 6:

 Go to the front panel and start the VI. Adjust the slider and observe that the frequency of the sine wave changes corresponding to the position of the slider.

Step 7:

 You may notice that the output sine wave is initially discontinuous and that the response to changes in the slider position is very sluggish. The former is caused by a lag in synchronizing the generation and output processes and the latter is due to the amount of buffering that Labview provides. We can speed things up by increasing the sample rate.

Double click on the Simulate Signal block. In the Timing sub-panel, set Samples per second (Hz) to 10000. Restart the VI. Things should now be a bit more responsive.

Step 8:

 If you feel so inclined, place another slider (or knob) on the front panel and use it to control the amplitude of the sine wave.

Step 9:

 Stop the VI and save it in a persistent location.