Logix5000 Controllers General Instructions - SLAC Confluence

518 Special Instructions (FBC, DDT, DTR, PID)Feedforward or output biasingYou can feedforward a disturbance from the system by feeding the.BIAS value into the PID instruction’s feedforward/bias value.The feedforward value represents a disturbance fed into the PIDinstruction before the disturbance has a chance to change the processvariable. Feedforward is often used to control processes with atransportation lag. For example, a feedforward value representing“cold water poured into a warm mix” could boost the output valuefaster than waiting for the process variable to change as a result ofthe mixing.A bias value is typically used when no integral control is used. In thiscase, the bias value can be adjusted to maintain the output in therange required to keep the PV near the setpoint.Cascading loopsThe PID cascades two loops by assigning the output in percent of themaster loop to the setpoint of the slave loop. The slave loopautomatically converts the output of the master loop into the correctengineering units for the setpoint of the slave loop, based on the slaveloop’s values for .MAXS and .MINS.Relay LadderStructured TextPID(master,pv_master,0,cv_master,0,0,0);PID (slave,pv_slave,0,cv_slave,master,0,0);Publication 1756-RM003I-EN-P - January 2007