13-06-2014, 03:33 PM
Control System – An interconnection of components forming a system configuration that will provide a desired response.
1370333488-CONTROLSYSTEMSPRESENTATION.pptx (Size: 382.71 KB / Downloads: 16)
Process – The device, plant, or system under control. The input and output relationship represents the cause-and-effect relationship of the process
An open-loop (direct) system operates without feedback and directly generates the output in response to an input signal.
A closed-loop system uses a measurement of the output signal and a comparison with the desired output to generate an error signal that is applied to the actuator
Open loop system- Merits and Demerits
Merits:
Not much troubled with problems of stability
Simple in construction and design therefore economic
Easy from maintenance point of view
No sensor is required since there is no feed back
Demerits
Inaccurate since there is no tracking of output
No compensation for disturbances entering the system.
Closed loop system- Merits and Demerits
Merits:
1.It can provide decent accuracy in steady state since the output is tracked and correction is done.
2.They can operate in the presence of non linearities
3. Less sensitive to parameter variations.
4.Time response can be adjusted by design.
5. Due to increased band width it can operate in a large frequency range.
Demerits
1.Improper design may lead to unstable condition.
2.Reduces overall gain of the system.
3.Complex design.
Transfer function
The input- output relationship in a linear time invariant system is defined by the transfer function.
It is the ratio between the Laplace Transform of the o/p variable to the Laplace Transform of the i/p variable with initial conditions zero
Closed loop Transfer function
R(S) - reference input or desired output
C(S) or Y(s) – controlled output
B(s) – Feed back signal
E(s) - Error signal
G(s) – Forward path transfer function.
H(s) – Feed back path transfer function G(s) H(s) – Loop transfer function
Effects of feedback
a)Overall Gain:
Eqn. shows that the gain of the open loop system is reduced by a factor 1/(G(s) H(s) . Here the feed back signal is negative. If the feed back gain has negative value, the overall gain may increase.
`b) Stability:
A system is said to be Unstable, if its output is out of control. In eqn. ,
if G(s)H(s) = -1, the output of the system is infinite for any finite input. This shows that a stable system may become unstable for certain value of a feed back gain. Therefore if the feed back is not properly used, the system can be harmful.
c) Sensitivity:
This depends on the system parameters. For a good control system, it is desirable that the system should be insensitive to its parameter changes.
Sensitivity
This can be reduced by increasing the value of.
G(S)H(S) by selecting proper feed back
D) Increased bandwidth.
Band width can be increased by proper selection of feedback element. But increased bandwidth causes noise also to creep into the system.