03-12-2012, 05:48 PM
Feedback and Feedforward Control
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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. To simplify the illustration, the dead-
times in both paths are identical (this is not essential—any
Control Theory
The best-possible load response for a process having deadtime and a single lag.
deadtime in the load path, or none, will produce the same
response, simply shifting its location in time). Also for sim-
plification, the steady-state gains in both paths are made equal.
At time -1, a step decrease in load q enters the process.
After the deadtime in the load path expires, at time zero, the
controlled variable c responds, beginning to rise along an expo-
nential curve determined by the gain and lag in the load path.
If the controller were left in Manual, it would continue to a
new steady state. In this example, Kq is 2.0, leaving the final
value of c in Manual having changed twice as much as load
q; the time constant τq of the load lag in this example is 2.0τd.
These values of eb and Eb are the best that can be obtained
for any feedback controller on a process whose dynamics
consist of deadtime and a single lag.
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.)
A system, rather than a single control device, is normally
used for feedforward loops because it is not always convenient
to provide the computing functions required by the forward
loop with a single device or function. Instead, the feedforward
system consists of several devices if implemented in hardware
or several blocks of software if implemented digitally. The
function of these blocks is to implement a mathematical model
of the process.