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Add details on frequency measurement

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jaseg 2021-07-30 18:37:50 +02:00
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paper/safety-reset-paper.tex
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@ -122,7 +122,11 @@ Spectrum (DSSS) modulation carried out on grid frequency through a large control
After forward error correction and cryptographic verification it re-flashes the meter's main microcontroller over the After forward error correction and cryptographic verification it re-flashes the meter's main microcontroller over the
standard JTAG interface. Note that our modulation technique is \emph{changing the grid frequency itself}. This is standard JTAG interface. Note that our modulation technique is \emph{changing the grid frequency itself}. This is
fundamentally different in both generation and detection from systems such as traditional PLC that superimpose a signal fundamentally different in both generation and detection from systems such as traditional PLC that superimpose a signal
on grid voltage, but leave the underlying grid frequency itself unaffected. on grid voltage, but leave the underlying grid frequency itself unaffected. The safety reset controller is an
off-the-shelf microcontroller much smaller than the one used for the meter's main application controller. It measures
grid frequency from a voltage waveform acquired using its internal analog-to-digital-converter (ADC) directly connected
to the mains voltage input through a resistive divider chain. The use of an off-the-shelf microcontroller keeps the
implementation overhead of our solution very low in both per-unit and engineering cost.
\begin{figure} \begin{figure}
\centering \centering
@ -340,6 +344,7 @@ while the public internet and mobile networks are still offline and it is unaffe
telecommunication networks. telecommunication networks.
\subsection{Characterizing Grid Frequency} \subsection{Characterizing Grid Frequency}
\label{grid-freq-characterization}
In utility SCADA systems, Phasor Measurement Units (PMUs, also called \emph{synchrophasors}) are used to precisely In utility SCADA systems, Phasor Measurement Units (PMUs, also called \emph{synchrophasors}) are used to precisely
measure grid frequency among other parameters. This task is much more complicated in practice than it might appear at measure grid frequency among other parameters. This task is much more complicated in practice than it might appear at
@ -515,10 +520,13 @@ the meter's display after boot-up.
\label{fig_demo_sig_schema} \label{fig_demo_sig_schema}
\end{figure} \end{figure}
Since we did not have an aluminium smelter ready, we decided to feed our proof-of-concept reset controller with an To measure grid frequency in our demonstrator, we ported the same code we used in
emulated grid voltage sine wave from a computer's headphone jack. Where in a real application this microcontroller might Section~\label{grid-freq-characterization} to our demonstrator, again using the voltage measured using the
take ADC readings of input mains voltage divided down by a long resistive divider chain, we instead feed the ADC from a microcontroller's internal ADC but using a regular crystal instead of a crystal oven for the microcontroller's system
$\SI{3.5}{\milli\meter}$ audio input. For operational safety, we disconnected the meter microcontroller from its clock. Since we did not have an aluminium smelter ready, we decided to feed our proof-of-concept reset controller with
an emulated grid voltage sine wave from a computer's headphone jack. Where in a real application this microcontroller
would take ADC readings of input mains voltage divided down by a long resistive divider chain, we instead feed the ADC
from a $\SI{3.5}{\milli\meter}$ audio input. For operational safety, we disconnected the meter microcontroller from its
grid-referenced capacitive dropper power supply and connected it to our reset controlller's debug USB power supply. grid-referenced capacitive dropper power supply and connected it to our reset controlller's debug USB power supply.
We performed several successful experiments using a signature truncated at 120 bit and a 5 bit DSSS sequence. Taking the We performed several successful experiments using a signature truncated at 120 bit and a 5 bit DSSS sequence. Taking the