\section{Goals and Outcomes}

The goal of this research is to use formal methods to create high-assurance
hybrid control systems. Hybrid control systems have great potential for
implementation in autonomous control as they are able to change control laws
based on discrete triggers in the operating range of the controller. This allows
the autonomous controller to use several easily tractable control laws for different
regions in the state space, instead of using one controller over the entire
systems operating range. But, the discrete jumps between control laws in a
hybrid controller present challenges in proving stability and liveness
properties of the whole system. While tools from control theory can prove
properties for each individual control mode, they do not generalize when
switching between control laws is introduced.

This research will take a different approach to hybrid controller synthesis and
verification. Using tools from the formal methods community, we will create
controllers that are correct-by-construction and allow guarantees to be made
about the whole system's behavior. To demonstrate this, an autonomous controller
for a nuclear power plant start-up procedure will be created. Nuclear power is
an excellent test case for this work as the continuous piece of reactor dynamics
is well studied, while the discrete component of mode switching is explicitly
stated in regulatory requirements and operating procedures. Nuclear reactor
control today \textit{is} a hybrid control system--many functions in the control
room use automated controllers for basic tasks, but the engagement and selection
of these controllers is done by human operators referencing procedures to make
decisions.

The capability to create high-assurance hybrid control systems has the potential
to reduce the labor required to operate critical systems by removing the human
operator from the loop. Nuclear power stands to greatly benefit from greater
controller autonomy as the largest expense for reactors today is operations and
maintenance. Technologies such as microreactors and modular reactors will
improve the maintenance costs required through the use of factory-made
replacement components, but will suffer increased operating costs per megawatt
produced if they are required to staff the same way reactors today are staffed.
But, if increased autonomy can be introduced, these costs will be ameliorated.

If this research is successful, we will be able to do the following:

\begin{enumerate}

  \item
    \textbf{Formalize mode switching requirements as logical specifications that
    can be translated into discrete controller implementations.} The discrete
    transitions between continuous controller modes is often explicitly defined
    for critical systems in operating procedures and regulatory requirements.
    These statements will be translated into a temporal logic, which will then
    be synthesized into a discrete controller implementation for continuous
    controller mode switching.

  \item
    \textbf{Categorize different continuous controller modes by their strategic
    relavance.} Different control modes serve one of two purposes: they may be
    transitory or stabilizing. Knowing when to switch from one control mode to
    another is handled by the discrete component of the hybrid system, but this
    outcome will identify the properties the continuous components must satisfy
    for each controller mode.

  \item
    \textbf{Verify that continuous controller modes satisfy dynamic
    requirements.} For the discrete transitions between control modes to be
    useful, we must ensure that the continuous control modes will actually move
    the system to the transition, or if stabilizing, keep the system from
    leaving the control mode.

  \item
    \textbf{Prove that a hybrid system implementation achieve strategic goals
    across the entire controller operating range.} By creating discrete
    controller transitions from logical specifications that are
    correct-by-construction and validating that continuous components perform
    appropriately between discrete transitions, we can be confident that the
    hybrid system is free from defect and can be utilized as a critical
    autonomous controller.
  
\end{enumerate}
