INTRODUCTION
2.6 Data Acquisition System
The data acquisition system consists of several interconnected components. Figure 2-6 presents a general system diagram, showing the components and their interconnections. Select components are used depending on the type of experiment being performed and there are no cases where all components are used simultaneously.
The focal point of the data collection system is a PC that serves as the master controller.
This computer runs a custom software package (OPO v125) that orchestrates the entire experiment.
It communicates through RS-232 serial interfaces with the AFC-100, the EG&G dual-phase lock- in analyzer, the optical parametric oscillator, and the acetone seeding system. It also incorporates a Measurement Computing DAS-08 card which provides digital I/O for interfacing with the Andor ICCD controller, as well as analog inputs for recording slowly changing or DC signals. The software interface provides extensive control of the OPO including features for general tuning, wavelength scans, and crystal peaking. It also incorporates a comprehensive interface to the AFC- 100. Built-in routines provide features such as automated combustion response scans using the AFC-100, PMT and lock-in analyzer. The software also features an internal scripting language that allows the operator to write scripts that run experiments fully autonomously. This was deemed advantageous since many of the experiments are long and arduous.
The workhorse of the data collection system is the AFC-100 (acoustic forcing controller).
This is a custom instrument that was engineered and built entirely from scratch. It was designed to solve many of the issues uncovered with the experimental technique when it was initially developed by Pun and Palm in the late 1990s. A detailed description of its construction, capabilities and the problems that it solved is covered in appendix F. Photos of the instrument can be seen in Figure 2- 7. Its primary functions include:
EG&G Dual Phase Lock-In Analyzer
Computer 1 (Master Controller)
With DAQ
AFC-100
Drive Amplifier (Mackie 1400i)
PMT and Lens Assembly Signal to
Acoustic Drivers
from Test Section
Transimpedance Amplifier
DC AC
REF DRV
PMT Signal
Computer 2 (Camera Controller)
Andor ICCD Camera
Camera Gate Controller
Laser Q-Switch
Signal Trigger Signal to Laser
Camera Gate Enable To Acetone
Seeding System
Figure 2-6: Data acquisition general system diagram. PMT and ICCD camera systems are used independently depending on the experiments being performed.
a) Pressure Sensor Signal Multiplexing and Conditioning – The AFC-100 provides eight inputs for piezoelectric pressure transducers or electret microphones. Each input has an independent DC current source to power the transducer. The eight inputs are buffered and then fed to two signal multiplexers. The output of each multiplexer constitutes one of the signal channels that is selectively conditioned/amplified, analyzed and recorded.
b) Acoustic Drive Control – The instrument closed-loop controls the frequency and amplitude of the standing wave in the chamber using feedback from strategically located pressure transducers in the test section. The feedback transducer must be multiplexed to channel 1.
c) Laser Timing Control (Phase Targeter) – A laser trigger signal is generated by the unit to initiate each laser shot. In targeting mode, the laser shot timing is jittered in order to collect
PLIF images at specific acoustic phase angles while still maintaining an average laser firing rate of 10 Hz.
d) ICCD Enable Control (Camera Gate Enable) – The CGE signal is generated by the laser timing system and the “skip” register. Setting the skip register to a value other than zero inhibits the camera gate signal and data collection every N shots where N is the skip register value. This grants the ICCD the required read-out time when that time exceeds the time between laser shots.
e) Waveform Collection – During data collection, the instrument samples and records sections of waveforms from each of the two channels. These sections or windows encompass the instant of the image collecting laser shot. Along with this, a “bit pointer”
is recorded indicating the sample at which the laser shot occurred. The recoded waveform data provides 14 bit resolution and the laser shot information provides better than one degree phase accuracy.
Figure 2-7: The AFC-100 acoustic forcing controller. The instrument was custom engineered and built to solve many of the difficulties previously discovered in the experimental technique. Details of its design and resulting features can be found in appendix F.
Lastly, an EG&G model 5206 dual-phase lock-in analyzer is used to process the signal from the photomultiplier tube during the bulk chemiluminescence experiments. Because of the periodic nature of the signal being measured, the lock-in analyzer provides exceptional sensitivity and a dynamic range that spans six decades. With the dual lock-in feature, the unit provides both amplitude and phase information on the measured signal.
The photomultiplier is coupled to the lock-in analyzer through a transimpedance amplifier.
This device serves two functions. First, it performs the conversion from the current signal provided by the PMT to the voltage signal required by the lock-in analyzer. The second function is to split the AC and DC components of the signal and provide them on separate output connectors. By design, the crossover frequency from AC to DC is chosen to be at 0.5 Hz. Although the impedance looking into the amplifier input is 50 ohms, the actual transimpedance between the input and the outputs is 100 kΩ for the AC channel and 20 kΩ for the DC channel. The signal from the AC channel is directed to the lock-in analyzer while the signal from the DC channel is fed to one of the inputs on the DAS-08 for logging of the average chemiluminescence signal.
C h a p t e r 3