typedef struct PID
{
double SetPoint; // Desired Value

double Proportion; // Proportional Const
double Integral; // Integral Const
double Derivative; // Derivative Const

double LastError; // Error[-1]
double PrevError; // Error[-2]
double SumError; // Sums of Errors
} PID;

/*=========================================================================*
Perform One PID Iteration
*=========================================================================*/

double PIDCalc ( PID *pp, double NextPoint )

{ double dError, Error;

pp->SumError += (Error = pp->SetPoint - NextPoint);
dError = pp->LastError - pp->PrevError;
pp->PrevError = pp->LastError;
pp->LastError = Error;
return ( pp->Proportion * Error
+ pp->Integral * pp->SumError
+ pp->Derivative * dError
);
}

/*=========================================================================*
Initialize PID Structure
*=========================================================================*/

void PIDInit ( PID *pp )

{ memset ( pp,0,sizeof(PID) );
}

/***************************************************************************
Main Program
***************************************************************************/

double sensor ( void ) // Dummy Sensor Function
{ return 100.0;
}

void actuator ( double rDelta ) // Dummy Actuator Function
{}

void main ( void )
{ PID sPID; // PID Control Structure
double rOut; // PID Response (Output)
double rIn; // PID Feedback (Input)

PIDInit ( &sPID ); // Initialize Structure
sPID.Proportion = 0.5; // Set PID Coefficients
sPID.Integral = 0.5;
sPID.Derivative = 0.0;
sPID.SetPoint = 100.0; // Set PID Setpoint

for (;;) // Mock Up of PID Processing
{ rIn = sensor (); // Read Input
rOut = PIDCalc ( &sPID,rIn ); // Perform PID Interation
actuator ( rOut ); // Effect Needed Changes
}
}