Open -loop Control System
Open-loop control system is controlled directly, and only, by an input signal. The basic units of this type consist only of an amplifier and a motor. The amplifier receives a low-level input signal and amplifies it enough to drive the motor to perform the desired job.
Open-loop control system is controlled directly, and only, by an input signal. The basic units of this type consist only of an amplifier and a motor. The amplifier receives a low-level input signal and amplifies it enough to drive the motor to perform the desired job.
The open-loop control system is shown in basic block diagram form in
figure 2-1. With this system, the input is a signal that is fed to the
amplifier. The output of the amplifier is proportional to the amplitude
of the input signal. The phase (ac system) and polarity (dc system) of
the input signal determines the direction that the motor shaft will
turn. After amplification, the input signal is fed to the motor, which
moves the output shaft (load) in the direction that corresponds with the
input signal.
The motor will not stop driving the output shaft until the input signal
is reduced to zero or removed. This system usually requires an operator
who controls speed and direction of movement of the output by varying
the input. The operator could be controlling
Closed-loop Control System
Closed-loop Control System, also known as a feedback control system is
a control system which uses the concept of an open loop system as its
forward path but has one or more feedback loops (hence its name) or
paths between its output and its input. The reference to “feedback”,
simply means that some portion of the output is returned “back” to the
input to form part of the systems excitation.
Closed-loop systems are designed to automatically achieve and maintain
the desired output condition by comparing it with the actual condition.
It does this by generating an error signal which is the difference
between the output and the reference input. In other words, a
“closed-loop system” is a fully automatic control system in which its
control action being dependent on the output in some way.
So for example, consider our electric clothes dryer from the previous
open-loop tutorial. Suppose we used a sensor or transducer (input
device) to monitor the temperature or dryness of the clothes and fed the
signal back to the controller as shown below.
Characteristics of Closed-loop Control :
- To reduce errors by automatically adjusting the systems input.
- To improve stability of an unstable system.
- To increase or reduce the systems sensitivity.
- To enhance robustness against external disturbances to the process.
- To produce a reliable, repeatable performance.
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