An embedded processing design for 192-channel 10-40 MS/s aero-engine optical tomography: progress and continued DAQ characterisation

We present ongoing work towards a 6-projection 126-beam tool for temporal and spatial diagnostic imaging of jet-engine exhaust plume gas species. At present, the optics of the system are customized for carbon dioxide (CO₂), however the design is intended to be scalable to other pollutant and green-house gases produced by such engines. For measurement of per-beam path concentration integrals (PCIs) of a gas species, tunable diode absorption spectroscopy (TDLAS) and wavelength modulation spectroscopy (WMS) are used. These techniques have been demonstrated, however this paper reports progress on the significant embedded, highly-distributed data acquisition (DAQ) systems that are integral to the instrument. Much of the mechanics and optics are finalized, tested and published, however we will briefly discuss these. The DAQ is parallel due to the high gas-speeds (100-200 m/s), and hierarchical to allow a multi-node system capable of both in-situ signal processing and scalability to higher numbers of projection angles, beams and exhaust gasses. We briefly describe the hardware and gate-level firmware, highlighting that TDLAS/WMS signals are demodulated using dual-harmonic digital lock-in amplifiers (DLIAs) implemented per-channel. Each distributed node can digitize up to sixteen analog channels at 10 to 40 MS/s (14-bit) with an analog bandwidth of 10 Hz to 3 MHz, which is tailored for TDLAS. The digital system, which uses regular interrupts to obtain and transmit data, is suitable for 20 frame/sec tomography, although the spectroscopy and image reconstruction is necessarily off-line. This work demonstrates progress towards a scalable multi-channel DAQ suitable for the harsh environment within jet-engine testing facilities.