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% 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.
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