29-12-2012, 01:26 PM
Automatic Control Systems
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Open-loop control systems
Open-loop control systems: those systems in which the output has no effect on the control action.
The output is neither measured nor fed back for comparison with the input.
For each reference input, there corresponds a fixed operating conditions; the accuracy of the system depends on calibration.
In the presence of disturbances, an open-loop system will not perform the desired task.
Some comments on open-loop control systems
Simple construction and ease of
maintenance.
Less expensive than a closed-loop system.
No stability problem.
Recalibration is necessary from
time to time.
Sensitive to disturbances, so less accurate.
Closed-loop control systems
Closed-loop control systems are often referred to as feedback control systems.
The idea of feedback:
Compare the actual output with the expected value.
Take actions based on the difference (error).
This seemingly simple idea is tremendously powerful.
Feedback is a key idea in the discipline of control.
In practice, feedback control system and closed-loop control system are used interchangeably
Closed-loop control always implies the use of feedback control action in order to reduce system error
Comments on feedback control
Potential drawbacks of feedback:
cause instability if not used properly
couple noise from sensors into the dynamics of a system
increase the overall complexity of a system
[attachment=45740]
Open-loop control systems
Open-loop control systems: those systems in which the output has no effect on the control action.
The output is neither measured nor fed back for comparison with the input.
For each reference input, there corresponds a fixed operating conditions; the accuracy of the system depends on calibration.
In the presence of disturbances, an open-loop system will not perform the desired task.
Some comments on open-loop control systems
Simple construction and ease of
maintenance.
Less expensive than a closed-loop system.
No stability problem.
Recalibration is necessary from
time to time.
Sensitive to disturbances, so less accurate.
Closed-loop control systems
Closed-loop control systems are often referred to as feedback control systems.
The idea of feedback:
Compare the actual output with the expected value.
Take actions based on the difference (error).
This seemingly simple idea is tremendously powerful.
Feedback is a key idea in the discipline of control.
In practice, feedback control system and closed-loop control system are used interchangeably
Closed-loop control always implies the use of feedback control action in order to reduce system error
Comments on feedback control
Potential drawbacks of feedback:
cause instability if not used properly
couple noise from sensors into the dynamics of a system
increase the overall complexity of a system