Feed-forward control is one of the process controller which predicts the disturbances or upsets in process and take corrective action to avoid or minimize it. As known fact that the feedback controller has the fundamental defect in that an error must exist before the controller before it take any action. In ISA notations, feedback control is linked with driving car by looking in rear view mirror. but this is not practically possible, the driver has to take necessary action before the error occurs. Likewise in process industries also for most industrial process industries feedback is entirely adequate. But for some critical control applications however ever the action has to be taken before the process upsets. This is what feed-forward controller does.
Any process control has three different process variables like control variable, manipulated variable and disturbance variable. In feedback control, the controlled variable is measured after the disturbance is occurred to the process and compared to the set-point and manipulates the end component to correct the process upset. But it is reverse in feed-forward controller as it measures the disturbances than can occur in the process and compares with the set-point and manipulates the end component to see that the controlled variable does not deviate from its set-point.
Design of feed-forward controller
To design a feed-forward controller requires a special knowledge about the process. The process disturbances can increase the controlled variable by some amount, and the manipulated variable which decreases the the controller variable by the same amount must be calculated.If the effects are identical and opposite then only the disturbance effect can be cancelled by the controller.
That’s why the special knowledge about the process is required for designing feed-forward controller. Not only steady state gain magnitude must be know, the dead time, integral time constant also should be known to design a perfect feed forward control system. These concepts can be understood by the following example.
Example: Design a feed-forward control system for tank level control which has three input flows and one output at the downstream. The controller has to placed at one input flow line.
Solution: The procedure in this and any other application of feed-forward controller is to develop an equation which gives the relationship between the three process variables i.e. controlled variable, manipulated variable and disturbance variable. In this example the controlled variable is tank level, Manipulated variable is the one input flow line and disturbance variables are input and output flows.
To calculate the disturbance causes to the controlled variable (Level) because of inlet and outlet flows, the tank total capacity should be know in addition to the current flow values. In this measurement of controlled variable is not necessarily like feedback control system. With the flow values and tank capacity the disturbance value that can be deviated from the controlled variable set-point will be calculated.
Disturbance value= (inlet flow – outlet flow)/ Tank capacity.
Manipulated variable = control algorithm(P, PI, PD, PID) of Disturbance value.
The manipulate is applied to actuator of control value placed in one of the input line to adjust the flow rate to maintain the controlled variable constant.
Numerical example ( 1 sec)
Input flow rates = 400, 200, 100 kg/sec
output flow rate = 700 kg/sec
tank capacity = 10000 liters.
length = 2 meters
if one of the input line flow increased to 200 to 300 kg/sec then the
Disturbance =((400+300+100)-(700))/10000 = 0.02meters
Adjustment value = control algorithm (0.02 meters).
The above example illustrates the calculation of adjustment values of 1 sec duration this will be continued for real time application.
It is important to remember that the feed-forward controller takes care of only those upsets that are measured by the feed-forward controller. In the above example the disturbance will be calculated by the inlet and outlet flows. It can not sense the deviation in controller variable(Level) because of the leakage flow from the tank.
So the general practice in applying feed-forward control with including feedback control to keep the controlled variable at its set-point in spite of unmeasured disturbances. Another requirement for feedback control is to eliminate the errors caused by feed-forward control equation. The equation is really a process model, and it may be very good in some areas so combined feedback and feed-forward control is used for some critical control systems in process industry.