The PID block can prevent reset-windup through the use of the Feedback Tag. Reset-windup typically occurs when there is a hardware failure in the measurement device. This causes the deviation to be larger than the parameters of the block would normally produce in a full output swing.
The following figure illustrates how the Feedback Tag ensures that the controller does not get saturated under these conditions. By indicating that the PID block's output is not maintaining the set point, the Feedback Tag can help to avert serious control upsets.
If your processing scheme requires a cascade loop, the PID block can include master and slave PID blocks, as the following figure shows. The slave PID block can fetch the master PID block's adjusted output value for its set point.
The cascade loop, shown in the preceding figure, illustrates control in which the output of one PID block (master) provides the set point value for another PID block (slave). The Master PID block drives the set point of the slave PID block, which, in turn, controls the flow of fuel oil based upon the readings from a temperature transmitter. To accomplish this, specify the master block's name and the F_CV field in the slave block's Set Point Value field.
NOTE: If you specify the slave block in the master's Next Block field, the master can only send the slave a value as a variable and cannot output a value to the process.
