MicroHTR (Part 5)
Few more lines of code are required to build a fully operational PID. In the case of a PWM control, we need to get an output value ranging from 0 to 100% (Duty cycle). But other applications may require different ranges. That is the reason why we shall set extra global parameters and use them in the following way:
/* Compute output */
double outputValue = (_Kp * error) + (_Ki * _integralError) + (_Kd * dError);
outputValue *= (_limitUpp - _limitLow);
outputValue += _limitLow;
/* Constrain output signal to predefined values */
if (outputValue < _limitLow) { outputValue = _limitLow; } else if (outputValue > _limitUpp) {
outputValue = _limitUpp;
}
In this way, the PID function output shall be constrained within the _limitLow and _limitUpp bounds. Finally with get a fully operating PID which will work fine for controlling heating systems.
double PlainPID::PIDOutput(double processVariable)
/* Compute output signal */
{
_actualPointValue = processVariable;
/* Timing stuff */
uint32_t now = millis();
double timeInterval = (now - _lastTime) / 1000.0;
/* Auto adjust set point in order to prevent windup and spikes */
if (_firstPass == 0x01) {
_computedSetpoint = processVariable;
_firstPass = 0x00;
}
if (_computedSetpoint != _setpoint) {
/* Reset integral of errors */
_integralError = 0;
if (_computedSetpoint < _setpoint) { _computedSetpoint = (processVariable + 1); if (_computedSetpoint > _setpoint) {
_computedSetpoint = _setpoint;
}
} else if (_computedSetpoint > _setpoint) {
_computedSetpoint = (processVariable - 1);
if (_computedSetpoint < _setpoint) {
_computedSetpoint = _setpoint;
}
}
}
/* Compute error */
double error = (_computedSetpoint - processVariable);
/* Compute error derivative */
double dError = (error - _lastError) / timeInterval;
/* Compute error integral */
_integralError += ((error + _lastError ) * timeInterval) / 2.0;
/* Compute output */
double outputValue = (_Kp * error) + (_Ki * _integralError) + (_Kd * dError);
outputValue *= (_limitUpp - _limitLow);
outputValue += _limitLow;
/* Constrain output signal to predefined values */
if (outputValue < _limitLow) { outputValue = _limitLow; } else if (outputValue > _limitUpp) {
outputValue = _limitUpp;
}
/* Save last values in volatile variables */
_lastTime = now;
_lastError = error;
return(outputValue);
};