i have read quiet a lot of articles for implementing PID controller.
what i understand is that initially i have to set the value of Kp,Ki and kd =0 and then adjust each value so as to decrease overshoot and undershoot and maintain temperature.

at present i heat the heater at a full 100%duty cycle.

iam using the following code
#include <stdio.h>
#include<math.h>

struct _pid {
int pv; /*integer that contains the process value*/
int sp; /*integer that contains the set point*/
float integral;
float pgain;
float igain;
float dgain;
int deadband;
int last_error;
};

struct _pid warm,*pid;
int process_point, set_point,dead_band;
float p_gain, i_gain, d_gain, integral_val,new_integ;;



/*------------------------------------------------------------------------
pid_init

DESCRIPTION This function initializes the pointers in the _pid structure
to the process variable and the setpoint. *pv and *sp are
integer pointers.
------------------------------------------------------------------------*/
void pid_init(struct _pid *warm, int process_point, int set_point)
{
struct _pid *pid;

pid = warm;
pid->pv = process_point;
pid->sp = set_point;
}


/*------------------------------------------------------------------------
pid_tune

DESCRIPTION Sets the proportional gain (p_gain), integral gain (i_gain),
derivitive gain (d_gain), and the dead band (dead_band) of
a pid control structure _pid.
------------------------------------------------------------------------*/

void pid_tune(struct _pid *pid, float p_gain, float i_gain, float d_gain, int dead_band)
{
pid->pgain = p_gain;
pid->igain = i_gain;
pid->dgain = d_gain;
pid->deadband = dead_band;
pid->integral= integral_val;
pid->last_error=0;
}

/*------------------------------------------------------------------------
pid_setinteg

DESCRIPTION Set a new value for the integral term of the pid equation.
This is useful for setting the initial output of the
pid controller at start up.
------------------------------------------------------------------------*/
void pid_setinteg(struct _pid *pid,float new_integ)
{
pid->integral = new_integ;
pid->last_error = 0;
}

/*------------------------------------------------------------------------
pid_bumpless

DESCRIPTION Bumpless transfer algorithim. When suddenly changing
setpoints, or when restarting the PID equation after an
extended pause, the derivative of the equation can cause
a bump in the controller output. This function will help
smooth out that bump. The process value in *pv should
be the updated just before this function is used.
------------------------------------------------------------------------*/
void pid_bumpless(struct _pid *pid)
{

pid->last_error = (pid->sp)-(pid->pv);

}

/*------------------------------------------------------------------------
pid_calc

DESCRIPTION Performs PID calculations for the _pid structure *a. This function uses the positional form of the pid equation, and incorporates an integral windup prevention algorithim. Rectangular integration is used, so this function must be repeated on a consistent time basis for accurate control.

RETURN VALUE The new output value for the pid loop.

USAGE #include "control.h"*/


float pid_calc(struct _pid *pid)
{
int err;
float pterm, dterm, result, ferror;

err = (pid->sp) - (pid->pv);
if (abs(err) > pid->deadband)
{
ferror = (float) err; /*do integer to float conversion only once*/
pterm = pid->pgain * ferror;
if (pterm > 100 || pterm < -100)
{
pid->integral = 0.0;
}
else
{
pid->integral += pid->igain * ferror;
if (pid->integral > 100.0)
{
pid->integral = 100.0;
}
else if (pid->integral < 0.0) pid->integral = 0.0;
}
dterm = ((float)(err - pid->last_error)) * pid->dgain;
result = pterm + pid->integral + dterm;
}
else result = pid->integral;
pid->last_error = err;
return (result);
}


void main(void)
{
float display_value;
int count=0;

pid = &warm;

// printf("Enter the values of Process point, Set point, P gain, I gain, D gain n");
// scanf("%d%d%f%f%f", &process_point, &set_point, &p_gain, &i_gain, &d_gain);



process_point = 30;
set_point = 40;
p_gain = (float)(5.2);
i_gain = (float)(0.77);
d_gain = (float)(0.18);



dead_band = 2;
integral_val =(float)(0.01);


printf("The values of Process point, Set point, P gain, I gain, D gain n");
printf(" %6d %6d %4f %4f %4fn", process_point, set_point, p_gain, i_gain, d_gain);

printf("Enter the values of Process pointn");

while(count<=20)
{



scanf("%d",&process_point);

pid_init(&warm, process_point, set_point);
pid_tune(&warm, p_gain,i_gain,d_gain,dead_band);
pid_setinteg(&warm,0.0); //pid_setinteg(&warm,30.0);

//Get input value for process point
pid_bumpless(&warm);

// how to display output
display_value = pid_calc(&warm);
printf("%fn", display_value);
//printf("n%f%f%f%f",warm.pv,warm.sp,warm.igain,warm.dgain);
count++;

}

}


the problems with this code are that i dont know how to use the function pid_setinteg and what value should i set to dead band.morever my setpoints and the process value are floats.if i change their data type and that of err to float the output for 31.1 to 31.9 and 32 remain the same.
please give suggestions on what can be done
thanks