\chapter{State of the Art}

The interest in the implementation of optical methods for the purposes of measuring bulk material is not novel. The reasoning is clear: conventional methods are intrusive and costly. Being a non-contact, non-intrusive approach, makes any sort of optical solution to the measurement problem very desirable.

As early as 1997, Green et al.\ were already experimenting with non-contact methods to calculate mass flow rates. In that time, they resorted to using electrodynamic sensors. Although a far cry from the resolution afforded by contemporary sensors, Green et al.\ and their electrodynamic sensors demonstrated the potential of non-contact sensing for bulk materials\cite{green1997}.

In 2014, Fojtik released his paper on using laser scanning to measure the volume of bulk material on a conveyor belt. Fojtik focused on the measurement of wood chips, which required special consideration to the volume fluctuations due to humidity\cite{fojtik2014}. 

Independently, Zeng et al.\ too released their paper on the use of laser scanning for measuring the volume flow of bulk material.\cite{zeng2015} The focus of their paper was using these technologies to increase energy efficiency.

Although they differed slightly in their precise approaches, both Fojtik and Zeng et al.\ used the same fundamental principle to determining volume flow, namely the derivation of the cross-sectional area of material based on the difference between an empty and laden belt. Both of them also are similar in their use of SICK LMS industrial laser scanners.

Both Min et al.\ in 2020\cite{min2020}, and Qiao et al.\ in 2021\cite{qiao2022} too have published their analyses and results on solving this problem. They both take novel approaches, however, using not only laser scanning but a hybrid solution involving regular optical imaging to supplement the analysis of the material surface. They both also attempt to implement more advanced mathematical models, using 3D reconstruction and neural networks.

As shown by the papers above, this work is not novel in its use of optical methods to solve the problem of measuring bulk material volume flow. This project does set itself apart, however, by
\begin{enumerate*}[label=\alph*)]
\item focusing on the use of commercially-available hardware
\item being an all-in-one solution and not requiring any additional sensory information, such as belt velocity
\end{enumerate*}.