Digitally Balanced Detection for Fast Optical CT in Fluidised Beds

R. Hafiz and K. Ozanyan

School of Electrical and Electronic Engineering, The University of Manchester, UK, Email: k.ozanyan@manchester.ac.uk

ABSTRACT

Analogue Balanced Detection is primarily aimed at removing the laser intensity noise and existing commercial products are optimised for this task. Furthermore, their design and cost is not suitable for multi channel operation required for fast Tomography. We have implemented Digitally Balanced Detection (DBD) on the reconfigurable hardware of a fully digital tomography system, which allows the use of this technique for applications other than just laser noise removal.

The application of DBD for fast optical CT in fluidised beds is demonstrated. Fluidised beds provide a challenging environment for optical measurements due to laser beam occlusion by the moving particles in the fluidised medium. The use of Gated Digitally Balanced Detection (G-DBD) is proposed as an effective method to drastically increase the signal-to-noise ratio. The proposed technique suppresses the effects of the particle occlusion by balancing a signal beam with a reference beam, overlapping in the measurement medium. The measurements taken in various regimes of fluidisation, including turbulent ones, indicate the feasibility of digital balancing methods for species-selective imaging in fluidised beds. We show that Dual Colour G-DBD in fluidised beds is possible due to the available hardware resources of the digital tomograph. The high bandwidth and high data acquisition rates provide the option, in a dense bed, to evaluate the character of the data in real time between two measurement events. By employing Dual Colour G-DBD, it was possible to considerably lower the line-of-sight measurement level in the fluidised bed. Thus, DBD and its variant G-DBD in particular show promise for optical sensing and imaging modalities in fluidised beds.

Keywords Balanced Detection, Optical tomography, Fluidised beds, Two-colour measurements