Data Acquisition for Multi-Channel CST of Aero-Engine Exhaust Plume Species and Combustion Diagnostics

We present a tomographic system tailored to the measurement and study of aero-engine plume dynamics, exhaust chemistry and novel engine or fuel design. This aims to address aviation pollutant emission reductions. As the testing environment is not ideal for instrumentation and must not interfere with the operation or air flow of the engine, this data acquisition (DAQ) system utilises tunable-diode laser absorption spectroscopy (TDLAS) with wavelength modulation. Dithering increases SNR despite plume scintillation, but prompts exploration of accurate, at detector, real-time, digital lock-in (DLI) techniques. The readout options are restricted, forcing a custom, distributed architecture, embedded microprocessor control and Ethernet connectivity. The multi-channel chemical species tomography (CST) electronics performs simultaneous detection and is scalable for increased numbers of beams, sample rates and laser dither rates. The system uses 12 digitisation hubs around the imaging space, each with 16 parallel channels. These operate at 40MS/s, 14bit, giving a system capacity of 192 channels and input rates exceeding 107Gbit/s. To meet the future needs of spectroscopy, gas species or beam extensions, the diagnostic tool allows software control over many signal processing and acquisition parameters. This paper presents the design of the DAQ system through the entire custom electrical signal chain. Results are presented for a data interleaving approach aimed at reducing the impact of network packet loss when using UDP Ethernet protocols and high network utilisation. We also present gain and bandwidth results for all 192 analogue front-end circuits on the DAQ hubs, along with preliminary results for the optical detector and pre-amplifier circuits.