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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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18
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
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
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}
\centering
@ -340,6 +344,7 @@ while the public internet and mobile networks are still offline and it is unaffe
telecommunication networks.
\subsection{Characterizing Grid Frequency}
\label{grid-freq-characterization}
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
@ -515,10 +520,13 @@ the meter's display after boot-up.
\label{fig_demo_sig_schema}
\end{figure}
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 might
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
To measure grid frequency in our demonstrator, we ported the same code we used in
Section~\label{grid-freq-characterization} to our demonstrator, again using the voltage measured using the
microcontroller's internal ADC but using a regular crystal instead of a crystal oven for the microcontroller's system
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.
We performed several successful experiments using a signature truncated at 120 bit and a 5 bit DSSS sequence. Taking the