The program ACQUIRE is a customized data acquisition "virtual instrument"
(VI) module written in the framework of the commercial program LabVIEW,
distributed by National Instruments. It provides the software interface
between the instruments used to make the necessary measurements and any
required computer analysis of the data. Voltages from the multimeters or
other hardware are sent through a computer interface box, to a data acquisition
board that resides inside the PC. Below is a brief description of the various
components of the data acquisition system and instructions for use of the
software.
Experiment to Computer Interface
Starting the program ACQUIRE
Analog Channels
Analog Scale
Sampling Frequency and Number of Points
Trigger
Graph
Saving Data to a File
Shutting Down the Program
Experiment to Computer
Interface
The computer measures the voltage of electrical signals. The necessary electrical connections from your experiment are made to what is referred to as the "computer interface box". Measurements can be performed on one or two different electrical signals. The connections between the experiment and the computer interface module are done in the following way:
An extra electrical signal can be used to start (trigger) the acquisition
of data. That signal should be connected to the external trigger terminals
of the interface module. It is also possible to start the data acquisition
by pressing the red knob labeled EXTERNAL TRIGGER on the interface module.
Once the signals have reached the back of the PC, they are sent to an analog-to-digital converter board that resides inside the computer. The board used in the Physics Laboratories is either the Keithley DAS-8-PGH or the Computerboards, Inc. CIO-DAS08-PGH (a less-expensive copy of the Keithley board). Either board has 8 channels of 12-bit analog input, with a programmable analog scale.
Generalities and Starting
the program ACQUIRE
The program ACQUIRE will allow you to
Using the drop-down menu in the area labeled Analog Channels, select the number of channels you would like to acquire. If you have only one signal, choose channel 0. If you have two signals, then select channel 0 to channel 1 from the drop-down menu by holding down the shift key and highlighting both items in the menu list. At present, the computer interface box can only send 2 channels to the CIO-DAS08, although there are 8 available channels on the board.
Pull down the drop-down menu labeled Analog scale. This item permits selection of the range of voltages that can be measured without saturation (voltage outside the acceptable range of values). The default range is from -5 to +5 volts. This range should accommodate many measurement situations. If another range is needed, click on this item and choose from the list in the drop-down menu. For bipolar ranges -X to +X, the numerical values produced by the analog to digital (A/D) converter and recorded in the memory of the computer range between -2048 and +2048. For unipolar ranges 0 to X, these values range between 0 to +4096. Choosing the appropriate analog scale allows you to match the resolution of the A/D to the voltage signals from your experiment.
Sampling Frequency
and Number of Points
These two items permit selection of the number of measurements to be performed every second (sampling rate), and the total number of measurements to be performed on each channel. You should choose these two parameters in an appropriate combination according to the experiment. Typically this is a tradeoff between how much data you want to try to digest (and how long you want to wait for the acquisition) and how rapidly the signals in the experiment are changing. For example, you may wish to collect 1000 data points in a total time of 2 seconds. Then you would choose a scan rate of 500 Hz, and 1000 samples/channel.
You may choose a sampling rate from 15 to 8000 samples/second. There is no limit on the total number of samples, but the file size will obviously increase with the total number of samples. For practical reasons you should limit yourself to less than 10,000 samples.
If you wish to have data taking to start automatically as soon as you click on TAKE DATA, choose INTERNAL for the trigger setting. See the drop-down menu labeled Trigger, under the analog scale. If you would like to control the start of data taking through the computer interface box, choose EXTERNAL. In this mode, data taking will start when the signal going into the TRIGGER of the computer interface box makes the transition from 0 to 5 V. Once you have set up all of the above parameters, you are now ready to begin taking data. Click on the button labeled TAKE DATA to start acquisition.
After data taking has stopped, the channels in which you have collected data can be plotted in the graph at the bottom of the page. Click on the button PLOT DATA. If you have acquired 2 channels of data, you can also make an X/Y plot of the two channels. Use the drop-down menus in the upper left portion of the graph to choose what you would like to be displayed. Change the scales on the graph either by double-clicking on the lower and upper limits on each axis, or by using the buttons provided in the upper left. You may also change the appearance of the graph by right-clicking on area labeled "Plot 0". When you are satisfied with the plot, make a hardcopy by going to File/Print Window from the main drop-down menus.
Click on the SAVE DATA button to save data to a file. The data are saved in tab-delimited ASCII format in 5 columns (time, channel 0, channel 1, channel 2, channel 3). The files should be ported to some other program for further analysis (Excel, Mathematica, etc.). You may append data to an existing file or write data to a new file. Note that you must write either to a floppy disk or to the C:\SCRATCH area on the local drive.
When you have finished with the program, click on the STOP button to stop the program. Then from the main menus, use File/Exit to shut down the program altogether.