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Datei: Elbe.tip Sprache: VDM

Original von: VDM^©

% USER.vdmrt

\subsubsection{Modelling the human operator -- The \texttt{User} class}

The \texttt{User} class is used to monitor the behavior of the user standing
on top of the ChessWay. The general idea here is that when the user moves
the handle bar forward or backward, then this influences the values measured
by the accelerometer, since gravity is no longer pointing straight downwards.
The \texttt{User} class converts the handle bar angle, which is provided from
the current scenario running in the \texttt{Environment}, into the values
measured by the 3-D accelerometer, by setting values to the appropriate
named interface elements in the \texttt{Environment}.

\begin{vdm_al}
class User

values
  gravity : real = 9.80665

instance variables
  -- link to the environment
  mEnvironment : Environment;

  -- link to both wheels
  mLeftWheel : Wheel;
  mRightWheel : Wheel;

  -- last evaluated at time step
  last : nat := 0;

  -- current deviation from upright
  angle : real := 0.0

operations
  public User: Environment * Wheel * Wheel ==> User
  User (pEnvironment, pLeftWheel, pRightWheel) ==
    ( mEnvironment := pEnvironment;
      mLeftWheel := pLeftWheel;
      mRightWheel := pRightWheel );

  public evaluate: () ==> ()
  evaluate () ==
    ( dcl user : real := mEnvironment.getValue("USER"),
          now : nat := time;
      -- compute and update the yaw rate
      def dt = (now - last) / World`SIM_RESOLUTION in
      def rate = if dt = 0 then 0 else (angle - user) / dt in
        mEnvironment.setValue("LEFT_YAW_RATE", rate);
      -- compute and update the acceleration
      def dx = MATH`cos(user) * gravity in
        mEnvironment.setValue("LEFT_ACC_X", dx);
      def dy = MATH`sin(user) * gravity in
        mEnvironment.setValue("LEFT_ACC_Y", dy);
      -- delta between the left and right wheel acceleration
      def dz = mLeftWheel.acc - mRightWheel.acc in
        mEnvironment.setValue("LEFT_ACC_Z", dz);
      -- update the angle
      angle := user;
      -- remember when we where executed
      last := now )

end User
\end{vdm_al}

In the \texttt{User} model shown here, we have abstracted away from the
acceleration caused by the wheels in the $X$ and $Y$ directions. However,
as a first order of magnitude approximation, the acceleration in the $Z$
direction is computed by calculating the difference between the
acceleration in either wheel, causing the device to steer in another
direction.

¤ Dauer der Verarbeitung: 0.32 Sekunden (vorverarbeitet) ¤

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