email meOverview of Feedback Control:
Control theory is a very common concept and automatic control (involves
machine) has played a vital role in the advancement of engineering and science.
The human organism is an extremely complex feedback control system.
A component block diagram identifies the major component
of the system whileomitting details.
Ex. A block diagram of a room temperature control system is given:
In general , any operation to be controlled is a process.
Actuator Device that can influence the controlled variable of the process (i.e. gas furnace). Its pilot light involves feedback.
Feedback Control
An operation which in the presence of disturbances, tends to reduce the
difference between the output of a system and the reference input and which
does so on the basis of this difference.
Therefore, a feedback control system tends to maintain a prescribed relation between the output and the reference input by comparing these and using the difference as a means of control.
Open-loop Control Systems
Output has no effect upon the control action. Output is neither measured
nor fed back for comparison with the input.
Ex. Washing machine
Output is not compared with the reference input. Accuracy of the system depends on the calibration. The system must maintain calibration in order to be useful.
Shortcomings In the presence of disturbances it will
not perform the task - can be used only in practice only if the relationship
between input and output is known.
Ex. Traffic control by means of signals updating
on a time basis.
Fundamentals of Closed loop Control Systems
A closed loop system adjusts its output response in accordance to a control
input or reference signal and the feedback signal. This feedback signal is compared
to the reference signal in order to calculate the error signal.
Closed-loop Systems
Output signal has a direct effect upon the control action. These are feedback
control systems. Feedback action is used to reduce system error.
Ex. Servomechanisms, household refrigerators, and
automatic heating systems.
Advantage - Feedback makes the system response relatively insensitive to external disturbances and internal variations in system parameters.
Disadvantage Stability is not a major problem with open-loop control, however it is always a problem in the closed-loop control systems.
Which one do you use?
If the inputs are known ahead of time and there are no disturbances - open-loop
When unpredictable variations in system components are present - closed
Note: Dynamic characteristics of most central systems are not constant over time (i.e. deterioration of components).
Control Theory
The subject of control systems is of great importance to the field of bioengineering. A crucial problem is to develop a mathematical model of a given system, in order to relate the various variables in a quantitative manner. Since biological systems are dynamic in nature, their behavior will be described in the form of differential equations.