Equipment required: Macintosh or IBM-PC; Vernier Universal Lab Interface wit voltage probe; Data Logger Software; Heath Voltage Reference Source or other DC/AC voltage source; 1.0 uF capacitor; 1.0 megohm resistor.
The Vernier Universal Lab Interface is a commercial data acquisition system consisting of an external box that connects to an IBM-PC or Macintosh computer via an RS-232 connection. The box contains its own microprocessor and 8 Kbytes of buffer memory for data storage. Technical specifications are given on page 31 of the ULI User's Manual. The manufacturer sells a general-purpose data acquisition software package called Data Logger for use with this equipment. A large number of sensor probes available from this company are listed on page 19 of the ULI User's Manual.
1. Install the ULI onto the Mac IIsi (next to the printer) according to the instructions in the ULI User's Manual (page 7). Connect the voltage probe to the DIN 1 input of the ULI and to the output of a Heath Voltage Reference Source (VRS ) set to 1 volt output.
2. Launch the Data Logger software (already installed on the Mac IIsi). The first thing we have to do is to tell the software what probe(s) to collect data from. Initially, we will collect data from only the first probe. Pull down the Collect menu, select Select inputs..., and click off "Probe in port 2" (i.e. make sure this check box is not checked), then click OK.
3. Click on the Start button on the bottom of the screen to start data acquisition. Note that the voltage on Probe 1 is indicated at the bottom of the screen and is plotted vs time on the graph. Vary the VRS voltage setting to see the effect. Experiment with changing the graph display. To change the graph limits of either the X or Y axes, click on the tickmark numbers and type in a new value. Alternatively, you can double-click anywhere on the graph to display the Axes dialog box. Pull down the Display menu to access the commands for changing the plotting symbols and lines.
4. What is the largest voltage that the ULI will measure accurately? How does it respond to negative voltages? How does the voltage range compare to the Protek digital multimeter that you used in a the RS-232 lab lab?
5. After you have acquired a few data points, select Data A Table from the Windows menu. This opens a window in which the collected data are displayed in a spreadsheet format. Experiment to discover how to change the number of digits displayed in the column. By expanding the number of digits displayed in the voltage column, determine the voltage resolution of the ULI (smallest change in voltage measurable).
6. Push in the "chopped" button on the VRS to generate a chopped waveform at the output rather than a simple DC voltage. In order to record the waveform, you will have to increase the sampling rate. Select Data Rate... from the Collect menu. Try selecting different data rates and observe the effect. Acquire approximately 100 samples of the VRS signal and print out a graph that clearly shows the waveform and period of the VRS output in chopped mode. Save the data as an ASCII text file (Save as...).
7. With the VRS output waveform displayed, select FFT... from the Analyze menu. From this display, estimate the frequency of the fundamental (lowest non-zero) frequency of the waveform.
8. What is the highest sampling rate that the ULI can achieve? Compare that to the maximum possible rate at which data could be transferred from the ULI to the computer via the RS-232 port. Hint: According to the technical specifications (page 31 of ULI manual), the ULI takes 12-bit data (that is, each sample contains 12 bits of binary data) and communicates its RS-232 data with 8 data bits and one stop bit. Since an RS-232 character can have a maximum of 8 data bits, how many characters minimum would have to be sent to transmit 12 bits? You may assume that the bit rate (baud rate) is 38.4 K bits/sec, the maximum baud rate given in. Assuming no delay between characters, what is the maximum possible rate at which samples could be transferred to the computer in real time? How would it be possible for the sampling rate of the ULI exceeds the rate maximum data transfer rate?
9. Disconnect the VRS from the ULI. Use the VRS to charge up a capacitor to 5 volts. Then disconnect the capacitor from the VRS and connect it to probe 1 of the ULI. Immediately begin taking data at a sampling rate sufficient to record the voltage decay as the capacitor is discharged by the input resistance of the ULI. Record about 50 points or so. Use the Fit... commands under the Analyze menu to fit an appropriate function to these data that will demonstrate the theoretically expected behavior of a first-order decay. (See page 39 of the Data Logger manual). Print out the data graph and the fit.
10. Compare the Vernier ULI to the Protek digital multimeter in terms of sampling rate, voltage range, and voltage resolution.
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