As you already know, a feedback control system measures the controlled variable, compares the measurement with the setpoint or reference, and if there is a difference between them it changes its output signal to the manipulated variable in order to eliminate the error. This means that feedback control cannot anticipate and prevent errors, it can only initiate its corrective action after an error has already developed. Thus we can conclude that feedback control loops can never achieve perfect control of a process i.e, keep the output of the process continues at the desired setpoint value in the presence of load or setpoint changes.
Unlike
the feedback systems, the feed-forward control configuration reacts to
variations in disturbance variables or set point, predicts the disturbance’s
effects and takes corrective action to eliminate its impact on the process
output. Therefore, the feedforward controllers have the theoretical potential
for perfect control. But it is difficult to measure all possible disturbance
variables to predict their effect quantitatively feedforward control is
generally used along with feedback control in most cases, a combination of feedforward and feedback technique loops
are usually corrected by feedback trimming. with feedforward, the feedback
controller must only change its output by an amount equal to what the
feedforward system fails to correct.
Structure
of the feedforward control scheme
Above
figure shows the structure of a feedback control system and in comparison to
that general form of a feedforward control system. It measures the disturbance
directly and then it anticipates the effect that it will have on the process
output. Subsequently, it changes the manipulated variable by such an amount as
to eliminate completely the impact of the disturbance on the process output.
Control action starts immediately after a change in the disturbance has been
detected. It is clear from the figures that feedback acts ‘after the fact’, in
a compensatory manner, whereas feedforward acts ‘beforehand’ in an anticipatory
manner.
Now taking a practical example of a stirred tank heater, the control objective is to keep the temperature of the liquid in the tank at the desired value despite any changes in the temperature of the inlet stream.
The second figure shows the conventional feedback
loop, which measures the temperature in the tank and after comparing it with
the desired value, increases or decreases the steam pressure, thus providing
more or less heat into the liquid. A feedforward control system uses a
different approach as shown. It measures the temperature of the inlet stream
and adjusts appropriately the steam pressure. Thus it increases the steam
pressure if the inlet temperature decreases and the steam pressure decreases
when the inlet temperature increases.
Examples
of feedforward control
1. Heat Exchanger
The
objective is to keep the exit temperature of the liquid constant by
manipulating the steam pressure. There are two principal disturbances(loads)
that are measured for feedforward control- liquid flow rate and liquid inlet
temperature. Feedforward control can be developed for more than one disturbance
also. The controller acts according to which disturbance changed value.
The figure represents the general case of feedforward control with several loads and single controlled variables. The major components of load are entered into a model to calculate the value of the manipulated variable required to maintain control at the set point.
2. Boiler drum level control
Here the objective is to keep the liquid level in the drum constant. The two disturbances are the steam flow from the boiler, which is dictated by varying demand elsewhere in the plant, and the flow of the feedwater which is also the principal manipulated variable.
Advantages
1. Acts before the effect of a
disturbance has been felt by the system
2. Is good for slow systems or
with significant dead time
3. It does not introduce
instability in the closed-loop response
Disadvantages
1. Requires identification of
all possible disturbances are their direct measurement
2. Cannot cope with unmeasured
disturbances
3. Sensitive to process
parameter variations.
Comments
Post a Comment