float iState = 0;
float lastTemp = 0;
#define PGAIN_ADR 0
#define IGAIN_ADR 4
#define DGAIN_ADR 8
#define WINDUP_GUARD_GAIN 10
float loadfloat(int address) {
// must be written
// this function return the float from EEPROM storage.
// This is used for the P,I, and D_GAIN settings.
// These are three values that need to be tuned after
// the machine up and running to make the PID loop
// work right.
}
float UpdatePID(float targetTemp, float curTemp)
{
// these can be cut out if memory is an issue,
// but they make it more readable
float pTerm, iTerm, dTerm;
float error;
float windupGaurd;
// determine how badly we are doing
error = targetTemp - curTemp;
// the pTerm is the view from now, the pgain judges
// how much we care about error we are this instant.
pTerm = loadfloat(PGAIN_ADR) * error;
// iState keeps changing over time; it's
// overall "performance" over time, or accumulated error
iState += error;
// to prevent the iTerm getting huge despite lots of
// error, we use a "windup guard"
// (this happens when the machine is first turned on and
// it cant help be cold despite its best efforts)
// not necessary. this makes windup guard values
// relative to the current iGain
windupGaurd = WINDUP_GUARD_GAIN / loadfloat(IGAIN_ADR);
if (iState > windupGaurd)
iState = windupGaurd;
else if (iState < -windupGaurd)
iState = -windupGaurd;
iTerm = loadfloat(IGAIN_ADR) * iState;
// the dTerm, the difference between the temperature now
// and our last reading, indicated the "speed,"
// how quickly the temp is changing. (aka. Differential)
dTerm = (loadfloat(DGAIN_ADR)* (curTemp - lastTemp));
// now that we've use lastTemp, put the current temp in
// our pocket until for the next round
lastTemp = curTemp;
// here comes the juicy magic feedback bit
return pTerm + iTerm - dTerm;
}
This work is licensed under a
Creative Commons Attribution-Noncommercial 3.0 Unported License.
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