List of Symbols
4 CARIBIC OPC
4.4 Unit control and data acquisition
To control the OPC unit a Real-Time Embedded Controller (NI PXI-8106 RT) mounted into a 4-Slot DC-powered PXI chassis from National Instruments is used (www.ni.com;
last access: 20.07.2014). For system control and recording the data from the sensors and the KS-93, a relay card (NI PXI-6521), an M series multifunction data acquisition card (NI PXI-6220M), and a high speed R series data acquisition card (NI PXI-7851R) are mounted in the chassis, too. The above PXI components together are called “PXI system”. The software controlling the OPC unit and recording the data was designed by the author of this thesis and was mainly written by Thomas Conrath (TROPOS) in LabVIEW. The unit control works as follows.
During flight all measurement instruments in the CARIBIC container are controlled by the master computer, by providing the instruments with the measurement status of the container every 20 s. Generally, the master can send one of the following commands to the measurement instruments via an Ethernet network: ST (status), IN (initialize), SB (stand by), or MS (measure). When a slave instrument (in the following the OPC unit) receives a command from the master, it has to acknowledge its receipt, sends its current status to the master, and executes the command of the master. If the OPC unit receives no command from the master for three minutes, for safety reasons the OPC unit sets itself to IN mode. The IN mode is the basis mode of all measurement instruments, where only the computer has power to communicate with the master and all other power is shut off.
About 40 seconds after the OPC unit gets power the boot-up is finished and it is set to the IN mode. This status is send to the master, as response to the ST request the master sends directly after booting to all slaves. When the master receives the status of the OPC unit correctly, SB is send to the OPC unit thereafter. As described in Sec.
4.3.2, in SB mode the OPC unit is purged with filtered air to prevent contamination of the optics. Because the laser diode of KS-93 needs up to 10 minutes to stabilize, the KS-93 has already power in SB mode. When the CARIBIC aircraft reach a pressure level allowing to start the measurements16, the master sends the MS command. The OPC unit responds by switching the three-way valve to position 1-2 and enabling the flow control. Because both MFCs need about 40 seconds to adjust the volume flow to the set values (0.150 l/min for MFC 1 and 0.135 l/min for MFC 2, respectively), the OPC unit changes from SB first in the intermediate mode “begin measure” (BM). After 45 seconds the gas flow is stable and the unit mode changes to MS. When the pressure
16 currently 700 hPa, measured with a pressure sensor inside the master unit
43
increases above 850 hPa during landing the measurement status of the container changes to SB again. When the OPC unit receives the SB command from the master, the measurement is stopped and the unit is purged with filtered air, again.
The actual measurement mode of the OPC unit is indicated by four LEDs at the unit front plate as requested by the CARIBIC container concept. To track the communication between master and OPC unit, it is stored in a log file. Moreover internal error messages are logged in the same file to find possible reasons for errors in the OPC unit due to instrument malfunctioning. When the OPC unit is operated in the laboratory (e.g. for calibration measurements) the unit operation mode switch (Fig. 4.5, (3)) is switched from “aircraft” to “laboratory”. In the laboratory mode no master computer is needed to operate the unit.
In all modes (IN, SB, BM, MS) the data acquisition records the signals of the pressure and temperature sensors with 10 Hz resolution. In BM and MS mode additionally the current volume flow at the two MFCs are recorded with the same resolution. The amplified signal of the scattered light at the three SMB connectors of KS-93 (Sec. 4.2) can be recorded with a temporal resolution of up to 750 kHz for each channel. As the signal pulses have an average duration of ~ 60 µs (Sec. 5.1) the time resolution was set to 333 kHz to save disc space and computational power. With this resolution each signal pulse (roughly Gaussian shape) can be clearly resolved with 20 data points on average. When the signal intensity at the most sensitive channel 1 (Ch-1) increases above 0.35 V (trigger level), the signal intensity of all three channels are recorded until the voltage at Ch-1 decreases below the trigger level. The recording of the pulse form enables the post flight data analysis (Sec. 5.3.1) with the determination of the pulse maxima for each detected particle, the respective signal intensity and the exact detection time. Using these single particle data, the temporal, as well as the size channel resolution of the particle size distribution can be flexible set during later analysis.
After a sequence of usually four measurement flights the CARIBIC container is removed from the cargo bay of the aircraft and the raw data of all instruments are uploaded on a data server at the MPI-C in Mainz for post-flight analysis. To aid the transfer of the data from the OPC unit to the server, all data are stored on a Compact Flash (CF) card, which can be easily removed from the backside of the OPC unit without the need to boot the PXI system.
4.4 Unit control and data acquisition