FEEDFORWARD CONTROL
Feedforward control
provides a direct solution to the control problem by measuring the disturbances
and compensating the controlled variable before it deviates from the set point.
Feedforward control or Open loop control is one of the basic automatic control strategies.
The feedforward control
measures the disturbances directly and its controller predicts the impact that
it will have on the process. The controller then generates the manipulated
variable in such a way to eliminate the impact of disturbance on controlled
variable. In feedforward control, the control action is taken once the
disturbance is detected, which is a contrast to feedback control. This control scheme provides perfect control,
theoretically.
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| Block Diagram of Feedforward Control |
Feedforward Control
as Predictive Control
Feedforward
control is sometimes called as predictive control as it has to predict the
controlled variable instead of measuring it. The feedforward controller cannot
be a conventional controller like P, PI and PID. The feedforward controller is a
special-purpose computing machine in which the mathematical model of the
process is implemented. The process model represents the behavior of the
process and it is used to predict the process output.
Example of
Feedforward Control
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| Process Fluid Temperature Contol with Feedforward Control |
Consider the
process outlet temperature control problem in a furnace. Here, the inlet flow rate of the process fluid is the major disturbance. To implement the
feedforward control, the major disturbance is first. Depending on the
measurement, he feedforward controller decides how to manipulate the control
valve for fuel gas in order to maintain the controlled variable at set point. In this example, the inlet temperature of the process fluid can also be included as a next major disturbance.
Advantages of
Feedforward Control
1. Feedforward
control takes corrective action before the disturbances entering into the
process.
2. The
feedforward control does not affect the stability of the system. A stable
system without feedforward control would remain stable even after including it.
3. It works well
for multi-capacity systems or with significant dead time.
Limitations of
Feedforward Control
1. The concept of
feedforward control demands the identification and direct measurement of all
possible disturbances. This is not possible for many processes. Also, it cannot
deal with unmeasured or unexpected disturbances.
2. The effectiveness
of feedforward control depends on a good design of process model. Perfect
process model is not practically possible for many systems.
3. The feedforward
controller requires sophisticated instruments and
control equipments making it expensive.
4. The inability
to prepare perfect model and take accurate measurements would make the
feedforward control accumulate errors. The accumulation of errors would self-destruct
the control mechanism.
5. The process
parameter changes (like catalyst deactivation, reduction of a heat transfer
coefficient) cannot be compensated by the feedforward controller.
Also read other control schemes
Also read other control schemes
Thank you so much for this! It's very hard to understand what a feedforward control is but I finally get it :)
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