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| -rw-r--r-- | paper/safety-reset-paper.tex | 18 |
1 files changed, 13 insertions, 5 deletions
diff --git a/paper/safety-reset-paper.tex b/paper/safety-reset-paper.tex index c71ec26..a7d4344 100644 --- a/paper/safety-reset-paper.tex +++ b/paper/safety-reset-paper.tex @@ -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 |