18-08-2012, 11:14 AM
Feedback and Feedforward Control
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Feedback control is the action of moving a manipulated
variable m in response to a deviation or error e between the
controlled variable c and its set point r in such a way as to
reduce, or if possible, to eliminate the error. Feedback control
cannot anticipate and thereby prevent errors, because it can
only initiate its corrective action after an error has already
developed. Because of dynamic lags and delays in the response
of the controlled variable to manipulation, some time will
elapse before the correction will take effect. During this inter-
val, the deviation will tend to grow, before eventually dimin-
ishing.
FEEDBACK CONTROL
The purpose of any process control system is to maintain the
controlled variable at a desired value, the set point, in the
face of disturbances. The control system regulates the process
by balancing the variable load(s) with equivalent changes in
one or more manipulated variables. For the controlled vari-
able to remain stationary, the controlled process must be in
a balanced state.
Regulation through feedback control is achieved by act-
ing on the change in the controlled variable that was induced
by a change in load. Deviations in the controlled variable are
converted into changes in the manipulated variable and sent
back to the process to restore the balance. Figure 2.9a shows
the backward flow of information from the output of the
process back to its manipulated input. The load q is a flow
of mass or energy entering or leaving a process, which must
be balanced by an equal flow leaving or entering. It may have
more than one component—for example in a temperature
loop, both the flow and temperature of a stream are compo-
nents of its energy demand, but they may be balanced by a
single manipulated variable such as steam flow.
Limitations of Feedback Control
Feedback, by its nature, is incapable of correcting a deviation
in the controlled variable at the time of detection. In any
process, a finite delay exists between a changing of the
manipulated variable and the effect of the change on the
controlled variable. Perfect control is not even theoretically
attainable because a deviation in the controlled variable must
appear before any corrective action can begin. In addition,
the value of the manipulated variable needed to balance the
load must be sought by trial and error, with the feedback
controller observing the effect of its output on the controlled
variable.
Best-Possible Feedback Control
An absolute limitation to the performance of a feedback con-
trol loop is imposed by any deadtime in the loop. Figure 2.9b
describes the results of a step-load change applied to a process
whose dynamics consist of deadtime and a single lag in both
the load path gq and the manipulated-variable path gm. The
time scale is given in units of deadtime τd in the path of the
manipulated variable.
Integrated Error
The actual effectiveness of feedback control depends on the dynamic gain of the controller, which is a function of its control modes and their tuning. Although a high controller gain is desirable, the dynamic gain of the closed loop at its period of oscillation must be less than unity if the loop is to remain stable. In effect, then, the dynamic gain of the process dictates the allowable dynamic gain of the controller.
For each process and controller, optimum settings exist that
minimize some objective function such as Integrated Absolute
Error (IAE). However, most controllers are not tuned optimally,
for various reasons, such as process nonlinearities. Therefore,
controller performance needs to be stated in terms of the actual
settings being used. As an example, the integrated error pro-
duced by a load change to a process under ideal Proportional-
Integral-Derivative (PID) control will be evaluated based only
on its mode settings
FEEDFORWARD CONTROL
Feedforward provides a more direct solution to the control problem than finding the correct value of the manipulated variable by trial and error, as occurs in feedback control. In the feedforward system, 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. Figure 2.9c shows how information flows forward from the load to the manipulated variable input of the process. The set point is used to give the system a command. (If the controlled variable were used in the calculation instead of the set point, a positive feedback loop would be formed.)