Sunday 10 March 2013

Closed-Loop (Feedback Control) Systems

The disadvantages of open-loop systems, namely sensitivity to disturbances and
inability to correct for these disturbances, may be overcome in closed-loop systems.
The generic architecture of a closed-loop system is shown in Figure 1.6(6).
The input transducer converts the form of the input to the form used by the
controller. An output transducer, or sensor, measures the output response and
converts it into the form used by the controller. For example, if the controller
uses electrical signals to operate the valves of a temperature control system, the
input position and the output temperature are converted to electrical signals. The
input position can be converted to a voltage by a potentiometer, a variable resistor,
and the output temperature can be converted to a voltage by a thermistor, a device
whose electrical resistance changes with temperature.
The first summing junction algebraically adds the signal from the input to the
signal from the output, which arrives via the feedback path, the return path from the
output to the summing junction. In Figure 1.6(6), the output signal is subtracted from
the input signal. The result is generally called the actuating signal. However, in
systems where both the input and output transducers have unity gain (that is, the
transducer amplifies its input by 1), the actuating signal's value is equal to the actual
difference between the input and the output. Under this condition, the actuating
signal is called the error.

The closed-loop system compensates for disturbances by measuring the output
response, feeding that measurement back through a feedback path, and comparing
that response to the input at the summing junction. If there is any difference between
the two responses, the system drives the plant, via the actuating signal, to make a
correction. If there is no difference, the system does not drive the plant, since the
plant's response is already the desired response.
Closed-loop systems, then, have the obvious advantage of greater accuracy
than open-loop systems. They are less sensitive to noise, disturbances, and changes in
the environment. Transient response and steady-state error can be controlled more
conveniently and with greater flexibility in closed-loop systems, often by a simple
adjustment of gain (amplification) in the loop and sometimes by redesigning the
controller. We refer to the redesign as compensating the system and to the resulting
hardware as a compensator. On the other hand, closed-loop systems are more
complex and expensive than open-loop systems. A standard, open-loop toaster
serves as an example: It is simple and inexpensive. A closed-loop toaster oven is
more complex and more expensive since it has to measure both color (through light
reflectivity) and humidity inside the toaster oven. Thus, the control systems engineer
must consider the trade-off between the simplicity and low cost of an open-loop
system and the accuracy and higher cost of a closed-loop system.
In summary, systems that perform the previously described measurement and
correction are called closed-loop, or feedback control, systems. Systems that do not
have this property of measurement and correction are called open-loop systems.

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