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High-Pressure Shock Tube - Diagnostics and Controls
(U.S. Air Force Academy)
(U.S. Air Force Academy)
This section describes the two major components of a project I completed as a contract engineer working for the Hanson Collaborative, the prime contractor of a turn-key shock tube delivered to the U.S. Air Force Academy.
The designs of the diagnostics and controls are the sole property of the Hanson Collaborative (reproduced with permission).
Data Acquisition and Controls
A unique challenge of USAFA shock-tube project was the requirement for that of computerized control from an adjacent room. Additionally, whereas most combustion shock tubes reside in graduate-level research labs where they are run by expert-level operators, the controls for the USAFA shock tube had to be designed to be operated and understood by relatively inexperienced undergraduate researchers. The design and implementation of this control system was one of the tasks I was contracted to execute.
To meet these demanding requirements, I specified the necessary data acquisition and control hardware and implemented a LabVIEW-based control code, the graphical user interface (GUI) of which is shown below. I worked with Dr. Tom Hanson from the beginning of the project to ensure the facility was designed to include the instrumentation necessary to implement a safe and comprehensive control strategy.
The GUI for the control system was designed as a process and instrumentation diagram (P&ID) representing the facility. In this way, the GUI conveys the state of the shock tube in such a way as to provide the operator with an understanding of what is happening and how everything functions together. The colors of valves switch between green and red to reflect their states, and the gas lines change color to reflect what gas is passing through them at any given time. To the side (displayed on an adjacent screen in actual operation), the oscilloscope and shock-speed counter timers are displayed, such that all data acquisition and control functions can be handled through a single, integrated interface.
In the background, user inputs from the GUI pass through an event handler which produced queued messages to be processed by a state-machine-based control architecture. The state machine provides a first layer of logical control, limiting which operations can be completed in each of the five standard shock-tube states (Stand By, Vacuum, Gas Fill, Run Shock, Exhaust). Once the state has been validated, a second layer of logical checks is then performed to ensure that the operation is safe, given the specific states of other valves, pressure measurements, etc. In this way, the user is prevented from putting the tube into any state which may risk damaging equipment or the facility or otherwise be deemed unsafe.
As additional levels of safety beyond the software state machine and logical checks, special "Control Lock Out" and "Shock Abort" states were implemented. These special states can be entered either through the buttons on the software GUI or using a physical key switch and button (respectively) in the control room. Initialization of these states is treated on a priority basis by the message-queue handler, such that the states can be entered at any time during the operation of the tube. Continuous monitoring of the tube state is additionally implemented to automatically trigger the "Shock Abort" state if an unsafe condition is detected.
Optical Diagnostics
The USAFA shock tube additionally required two optical diagnostics. First, time-resolved fuel concentration measurements had to be provided using a laser-absorption-based diagnostic. A mid-infrared (MIR) HeNe laser was selected as the source providing benefits in ease of use and robustness compared to the MIR diode lasers becoming increasingly popular in high-temperature chemical kinetics research. Similarly, uncooled and amplified GaP detectors were selected, providing significant usability benefits compared to the liquid-nitrogen-cooled, cryogenic detectors sometimes used for infrared detection. Due to the spectral narrowness of the HeNe laser (and correspondingly high shot noise) a common-mode rejection scheme was implemented in order to lessen the impact of inherent laser noise. Both detectors used as part of the laser diagnostic were outfit with bandpass spectral filters to prevent effects of interfering emission on their measurements. Automated collection of dark-field measurements was additionally provided for through the incorporation of an electronic laser shutter integrated into the data acquisition system described above.
The second required diagnostic was an emission diagnostic package for monitoring emission from electronically excited hydroxyl radicals (OH*), a common marker of ignition in shock-tube experiments. To meet this need, an integrated diagnostic built using nested cage mounts was designed. This construction provided a balance between flexibility in adjusting the alignment of the type-II emission diagnostic while providing a robust package and ease of use. The diagnostic was designed to have spatial-resolution adjustment through the use of an adjustable slit and interchangeable spectral sensitivity using a magnetically mounted bandpass filter.
