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PID Control Loop using PIC24F

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ysba

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Hi,

I'm working with a closed loop application and I'm getting stucked at the PID control. The PID itself is not my problem, but how to implement it in the MCU (I'm using PIC24FJ). I'm using a 250kHz PWM, and running the core at 32MHz, so my duty cycle resolutin is only 6 bits. So, how can I convert the 10 bits AD result in a 6 bits PWM duty cycle? And also, which variable type is best for the control calculations? Could someone give me a good reference? Thanks.
 

Jinzpaul4u

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Hi there,

/*
1. Calculate difference of setpoint and actual value:
*/
diff=actual-setpoint;

/*
2. Sum up for integral part:
*/
sum+=diff;
/*
You may want to set sum to zero if controller is disabled:
*/
if (0==enabled) sum=0;
/*
You may want to check the sum for limits to prevent numerical underflow.

3. Calc change of diff for derivative part:
*/
deltadiff=diff-olddiff;
olddiff=diff;
/*
4. Now Combine P,I and D parts:
*/
output=kp*diff+ki*sum+kd*deltadiff;
/*
You may want to check output against limits which are reasonable for the controlled device,
e.g. 0-100 percent.
Make sure the above function is carried out periodically.

- - - Updated - - -

The following is a pretty basic C PID controller, but it has worked very well with good stability when tuned correctly. The code doesn't have alot of features and uses a independant gains equation, but you can dress it up or down as much as you want. I just pasted it from another application, adapt as needed. Good Luck.


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


/*----------------------------------------------------------------- -------
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(a, pv, sp)
struct _pid *a;
int *pv, *sp;
{
a->pv = pv;
a->sp = sp;
}


/*----------------------------------------------------------------- -------
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(a, p_gain, i_gain, d_gain, dead_band)
struct _pid *a;
float p_gain, i_gain, d_gain;
int dead_band;
{
a->pgain = p_gain;
a->igain = i_gain;
a->dgain = d_gain;
a->deadband = dead_band;
a->integral=0.0;
a->last_error=0;
}

/*----------------------------------------------------------------- -------
get_gains

DESCRIPTION Returns the gains and dead band in a _pid control structure
in the locations pointed to by the p_gain, i_gain, d_gain,
and dead_band pointers.

ALSO SEE pid_tune
------------------------------------------------------------------- -----*/
void get_gains(a, p_gain, i_gain, d_gain, dead_band)
struct _pid *a;
float *p_gain, *i_gain, *d_gain;
int *dead_band;
{
*p_gain = a->pgain;
*i_gain = a->igain;
*d_gain = a->dgain;
*dead_band = a->deadband;
}



/*----------------------------------------------------------------- -------
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(a,new_integ)
struct _pid *a;
float new_integ;
{
a->integral=new_integ;
a->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(a)
struct _pid *a;
{
a->last_error = *(a->sp) - *(a->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"
main() {
struct _pid PID;
int process_variable, set_point;
pid_init(&PID, &process_variable, &set_point);
pid_tune(&PID, 4.3, 0.2, 0.1, 2);
set_point = 500;
pid_setinteg(&PID,30.0);
process_varialbe = read_temp();
pid_bumpless(&PID);
for(;;) {
process_variable = read_temp();
output( pid_calc(&PID) );
wait(1.0);
}
}
------------------------------------------------------------------- -----*/
float pid_calc(a)
struct _pid *a;
{
int err;
float pterm, dterm, result, ferror;
err = *(a->sp) - *(a->pv);
if (abs(err) > a->deadband) {
ferror = (float) err; /*do integer to float conversion only once*/
pterm = a->pgain * ferror;
if (pterm > 100 || pterm < -100)
a->integral = 0.0;
else {
a->integral += a->igain * ferror;
if (a->integral > 100.0) a->integral=100.0;
else if (a->integral < 0.0) a->integral=0.0;
}
dterm = (err - a->last_error) * a->dgain;
result = pterm + a->integral + dterm;
}
else result = a->integral;
a->last_error=err;
return (result > 100.0 ? 100.0 : (result < 0.0 ? 0.0 : result));
}
 
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    ysba

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ysba

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Thanks man! This will be very useful. My application turns on and off the control many times, and I was not considering basically three things:

1) Clear the sum when the control is disabled
2) Check for overflow in the sum
3) Smooth the bum when the control is re-enabled

I'll keep working. Thanks again.
 

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