Data acquisition program

The data acquisition program was developed for recording experimental data in a consistent and easily analyzed format. The software collects data, applies sensor calibrations, and saves important experimental parameters. Further details are given in the following sections.

2.1.2.1.1 Inputs tab

A screen shot of the "Inputs" tab of the data acquisition program is shown in Figure 2-4. This tab is used for selecting sample parameters such as rate, sensor selection, trigger timing, and more.

Figure 2-4: Data acquisition program: "INPUTS" tab

Before running the data acquisition program the button labeled "Increment Shot?"

must be set to the desired state. It has a default value of ON. When this button is on, the output file is named "Shot###.txt," where ### is a number incremented with each consecutive shot. Each time a shot is incremented, a file named "ShotList.txt" is updated to show the entire list of recorded shots. If the "Increment Shot?" button is off the file will be saved as "Default.txt," and the file must be renamed before another shot is fired because the program will overwrite existing files. After this selection is made, the program can be started by pressing the run button (shown as an arrow) located in the upper left hand side of the screen.

The voltage range for the data collection can be adjusted from -10 to +10 volts by using the voltage inputs. A total number of samples are selected using the "Samples per Channel" input. Although the relevant data for a shock wave experiment is typically less than 10 milliseconds, 50,000 samples is typical for most experiments. These extra points ensure that the complete event is captured and also show any reflections/rarefactions that occur from the shock tube end configuration. A preset number of samples are taken before the trigger sensor is switched, which is set using the "Pre-Trigger Samples" input.

This establishes a clean baseline without interference caused by stress waves in the shock tube, which travel faster than the shock front. A sampling rate of up to 2.5 MHz can be selected using the "Sampling Rate" input, although 1 MHz is usually sufficient for most shock wave experiments.

The chassis houses two 6133 DAQ cards with 8 channels per DAQ. The available channels are listed in an array labeled "PXI Channel," and channel selection is done using the "Select Channel" array of buttons. There is also an array with pull-down lists for

selecting a sensor for each channel. This pull-down list labeled "Select Sensor" contains the serial numbers of many sensors, and selections are made for each active channel. The sensor location is also recorded using a write-in array labeled “Sensor Location.” Since it is write-in, other experimental notes can be added to this spot as well.

The hardware schematic in Figure 2-1 shows a trigger pulse being sent from the counter module to the DAQ and cameras. The "Trigger 0" slot is reserved for triggering the DAQ, but three more are available for triggering other systems (only two are shown in the figure). The trigger pulse duration and delay time can be adjusted for each trigger, which corresponds to an output pin on the digital counter.

2.1.2.1.2 Monitor channels tab

After all inputs have been selected, the "Monitor Channels" tab shown in Figure 2-5 should be opened. The graph shows real time data from all sensors, measured at 10 kHz.

These signals do not have calibrations applied, which allows the user to easily monitor noise level and troubleshoot problems before firing the shock tube. Sensors typically have a slight DC voltage offset, as shown on the figure. There is also a summary of the selected channels, selected sensors, sensor calibrations, sensor locations, and the average value of each signal shown on the right hand side of the screen. When a shot is ready to be fired, the "Look for Trigger" button should be pressed, causing the large light next to the button to turn bright green. The system is now armed and looking for a signal from the trigger. Upon receipt of the trigger signal the light changes to red which alerts the user that the system was triggered. The "Processing Data" light will activate while processing and turn off when the data is saved causing "Data Saved" to light up.

Figure 2-5: Data acquisition program: "MONITOR CHANNELS" tab

During the data processing phase, the DC baseline offset is calculated, and the signal baseline is adjusted to zero. Then the appropriate calibration value is applied to each sensor. These actions greatly reduce redundant calibration procedures required after a shot. Then the data are saved, the program is automatically stopped, and the data are ready for viewing. The format of the data is stored in a text file which contains the sensor data as well as a record of the user inputs, shock wave arrival time, and DC voltage offset.

2.1.2.1.3 Recorded signal tab

The recorded signal tab has a single graph which shows the calibrated profiles of the recorded data. This tab is not meant for detailed viewing of data but allows the user to quickly ensure that the expected data were collected. Detailed viewing and analysis of data are intended to be done using the data analysis program.

2.1.2.1.4 Sensor calibration tab

The sensor calibration tab shown in Figure 2-6 contains a list of sensors used in the shock facility along with their corresponding calibration value. Sensors with a linear voltage to pressure relation have their appropriate linear calibration listed in this tab.

Non-linearly calibrated sensors could have a calibration value set to unity, yielding a pure voltage output requiring further analysis.

Figure 2-6: Data acquisition program: "SENSOR CALIBRATION" tab

When the list of sensors in the "Sensor Calibration" tab is updated, the "Select Sensor" list in the "Inputs" tab must also be updated in the same numerical order to ensure that the proper calibration values are applied to the correct sensors. Failure to do this will cause the incorrect sensor calibrations to be applied.