description={(computing) is a model for enabling ubiquitous, convenient, on-demand access to a shared pool of configurable computing resources}
}
\newglossaryentry{datasheet}{
name=datasheet,
description={a document that summarizes the performance and other technical characteristics of a product, machine, component (e.g., an electronic component)}
@ -82,7 +82,7 @@ As the two waveforms are no longer “in-phase”, they must therefore be “out
also power factor correction
\subsection{Measuring the electric power}
\subsection{Electric power measurement}
Measuring the electric power makes most sense on the customer appliances. The first reason is, that they generally consume power that is purchased on contract. The energetic company measures all the power used up by the end customer, but customer has no easy way to see how much and how \textit{effectively} is power used by the appliances. The second important reason is that the appliances has a standardised connector (plug) that is guaranteed to fit in all the area using it, which is not a case for example on battery powered devices (batteries has different sizes, connectors and general properties.
When it comes to measuring the electrical power, the first and the most important thing to discuss is safety. Only after all the safety precautions had been made clear, the theory can be clarified and subsequently, the practice can be applied.
@ -122,10 +122,31 @@ If not handled with care, operating or manipulating with voltage can cause perma
%some words about sampling too
\subsection{Power measuring Integrated Circuits}
Although it is possible to construct a circuit out of discrete components that would measure the mentioned quantities, and such a solution would probably be the cheapest solution out there, it would be highly impractical due to multiple reasons.
The most importantly, the obtained accuracy of the measurements would be dependent on the implementation and used components. It is safe to assume, that without multiple design iterations, the accuracy may be too low to be used in practice.
Another point is that, there is no definitive guide, ready to follow, about how to design such circuit. The reason of this is the vast amount of components available on the market and a lot of design considerations to take into account, depending on the requirements.
A special purpose \glspl{ic} are being developed for the exact purpose of measuring the real, apparent and reactive power, the power factor, and in most cases, gathering some other relevant information.
\caption{The simplified block diagram for a power measurement \gls{ic}}\label{f:meas_IC_diag}
\end{figure}
From the block diagram \ref{f:meas_IC_diag}, it can be seen that the power measuring \gls{ic} is just a specialised microcontroller. It takes the data from the sensing circuitry, which in case of voltage can be measured \textit{directly}, provided that the galvanic isolation is included, for the sake safety. The current however, must be measured \textit{indirectly}. There are three common ways of doing so:
\begin{enumerate}
\item\textbf{shunt resistor} - a resistor with a very small but precise value, that causes a voltage drop with a current passing through it due to the Ohm's law, regardless of frequency. The actual voltage drop is so small, that it can be assumed insignificant. However, the voltage drop is still present and may cause some issues, if not taken into account. The advantage is really low price. External galvanic isolation must be provided.
\item\textbf{current transformer} - a current passing wire inside a current sensing coil. Since it is a magnetic induction based transformer, the galvanic isolation is naturally present. The disadvantage is, that the transformer has a cut-off after which it's effect diminishes rapidly. External magnetic fields can cause problems too. Suitable for measuring current of a fixed (or non-decreasing) frequency.
\item\textbf{Hall-effect sensor} - a sensor measuring absolute electromagnetic field in a conductor. In contrast to the current transformer, this sensor is able to measure low frequency currents, down to \gls{dc}, which is a feat that the shunt resistor possesses too. Can be placed anywhere near the current path and doesn't require physical connection, thus providing galvanic isolation too. The price increases with operating currents range and precision. Prone to external magnetic fields too.
\end{enumerate}
Using dedicated power measuring IC has another advantage apart from being more accurate. In fact, the part \gls{datasheet} can be consulted and if all application notes and advices are abided, the specified accuracy can be guaranteed.
\newpage
@ -199,7 +220,7 @@ The simplified view on the \Gls{linux} \gls{system} structure can be seen on \re
\subsection{OpenWRT}
OpenWrt is an \gls{os} (in particular, an embedded \gls{os}) based on the \Gls{linux}\gls{kernel}, primarily used on embedded devices to route \gls{network} traffic. It has been optimized for size, to be small enough for fitting into the limited storage and memory available in home \glspl{router}.
OpenWrt is configured using a command-line \gls{interface} (ash \gls{shell}), or a web \gls{interface} (LuCI). There are about 3500 optional \gls{sw} packages available for installation via the opkg package management \gls{system}.
OpenWrt is configured using a command-line \gls{interface} (ash \gls{shell}), or a web \gls{interface} (LuCI). There are about 3500 optional \gls{sw} packages available for installation via the \texttt{opkg} package management \gls{system}.
\subsection{Components of the OpenWRT}
The main components are the \Gls{linux}\gls{kernel}, \texttt{util-linux-ng}, \texttt{uClibc} and \texttt{BusyBox}. The \Gls{linux}\gls{kernel} was already mentioned. \texttt{util-linux-ng} is self explanatory - it is a set of \gls{linux} utilities.
Peter Babič
9 years ago