\chapter{State of the Art}

The conventional methods of measuring the mass or volume flow of bulk materials \cite{protogerakisInterview2022} are using so-called \textit{belt scales} or \textit{belt weighers}. These typically either employ the gravimetric method or nuclear method in order to determine the mass or volume flow of bulk materials.

As already detailed in \autoref{chap:intro}, gravimetric belt scales use load cells to transform the compression due to the weight of the belt, into electrical signals.

Nuclear belt scales \cite{elias1980} function principally by measuring gamma ray attenuation through the bulk material. While these type of scales have their advantages over the gravimetric conventional method, such as ease-of-installation and calibration, there are also other severe disadvantages. Most importantly, the handling of radioactive products must be carried out by certified personnel. Secondly, the chemical composition of the bulk material must also be homogeneous. 

The interest in the implementation of \textbf{optical methods} for the purposes of measuring bulk material is not novel. The reasoning is clear: conventional methods are intrusive and costly. 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.\ \cite{green1997} were already experimenting with non-contact methods to calculate mass flow rates. In that time, they resorted to using electrodynamic sensors. These electrodynamic sensors were used to estimate both velocity and concentration, which in turn were used to derive mass flow rates. They also used a cross-correlation method to determine material velocity. 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.

In 2014, Fojtik \cite{fojtik2014} 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.

Independently, Zeng et al.\ \cite{zeng2015} too released their paper on the use of laser scanning for measuring the volume flow of bulk material. The focus of their paper was using these technologies to increase energy efficiency. In that paper, they claim that non-contact methods of measuring the volume flow of bulk materials increased energy efficiency by up to \SI{30}{\percent} and reduced maintenance costs by up to \SI{20}{\percent}.

Although they differed slightly in their precise approaches, both Fojtik and Zeng et al.\ used the same fundamental principle to determine 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*}.