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C Code explanation

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wakaka

Joined: 11 Sep 2006
Posts: 27

C Code explanation

Posted: Wed Sep 20, 2006 2:00 am

Code:

#include <string.h>
#include <stdio.h>
/*=====================================================
PID Function

The PID function is used in mainly
control applications. PIDCalc performs one iteration of the PID
algorithm.

While the PID function works, main is just a dummy program showing
a typical usage.
======================================================*/

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;

/*=====================================================
PID
======================================================*/

double PIDCalc( PID *pp, double NextPoint )
{
double dError,
Error;

Error = pp->SetPoint - NextPoint;
pp->SumError += Error;
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
}
}

I not quite understand in some parts this code, can some explain on it??

1.

Code:

double PIDCalc( PID *pp, double NextPoint )
{
double dError,
Error;

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

What is the pp?? *pp?

2.

Code:

void PIDInit (PID *pp)
{
memset ( pp,0,sizeof(PID));
}

What is memset?

3.

Code:

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
}
}

PIDInit ( &sPID ), wat is &sPID ??
sPID.Proportion , the '.' means????

ckielstra

Joined: 18 Mar 2004
Posts: 3680
Location: The Netherlands

Posted: Wed Sep 20, 2006 2:19 am

Question 1 and 3 have to do with the concept of pointers, an important part of the C language. It would take a lot of text to explain the working of pointers here, every good C book will describe it in details.

Quote:

What is memset?

A standard function supplied with every C-compiler. Again, check your C language books for a detailed explanation.

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