A closed loop control system uses a measurement of the output and feeds it back through the feedback path to compare it with the input (reference or desired output).
If there is any difference between the two responses ( measured output and the desired output) 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.
Example . Autopilot mechanism and the airplane it controls.
Objective - to maintain a specified airplane heading despite of atmospheric changes.
It performs the task by continuously measuring the actual airplane heading, and automatically adjusting the airplane control surfaces so as to bring the actual airplane heading into correspondence with the specified heading.
The human pilot is not a part of the control system.
Feedback Control
Feedback is the property of a closed loop system which permits the output (or some other controlled variable) to be compared with the input to the system (or an input to some other internally situated component or sub-system) so that the appropriate control action may be formed as some function of the output and the input.
Advantages of feedback
• Increased accuracy of the system.
• Reduced sensitivity of the ratio of output to input to variations in system parameters and other characteristics.
• Reduced effects of nonlinearities.
• Reduced effects of external disturbances or noise.
• Increased bandwidth.
Disadvantages of Feedback
Closed loop control systems also has several disadvantages such as:
• They have above advantages only when unpredictable disturbances and/or unpredictable variations in the system components are present.
• Tendency toward oscillation or instability.
• Cost is high.
Feed forward Control
Even though feedback is an effective way of eliminating disturbances, it has the disadvantage that the controller does not react to a disturbance before a control
error has already occurred. In many cases it is possible to measure the value of a
disturbance before it gives rise to a control error.
A feed forward control configuration measures the disturbance directly and takes control action
to eliminate its impact on the process output.
An illustration for feed forward control is given in Figure.
Figure chows illustration of feed forward control
Let’s consider an example in our everyday life, in which we can apply feed forward control.
Suppose that you are driving a vehicle and there is a bump on the road.
So, if you see the bump before you hit it, then you can take an evasive action to avoid the bump.
Basic terms used in control systems
System: A system is a combination of components that act together and perform a certain objective. We can use the word system not only for physical ones but also for dynamic phenomena such as those encountered in economics.
Input: An input is an excitation applied to a system as an input from an external source. This is our desired output of the system.
Output: The output is a variable or variables we use to observe the plant or process. We can call this as the actual response of the system.
Summing junction or error detector: Summing junction is a physical device that algebraically sums the incoming signals to produce an output signal. In a normal feedback control system, summing junction has 2 input signals, input signal and the feedback signal. In electrical systems, usually an operational amplifier is used as the summing junction.
Controller: According to the error signal, the controller determines the actions required to get the plant (process) to the desired state.
Actuator: The controller changes the state of the plant through the actuator. i.e. the actuator is a device used to perform the control action or to exert direct influence on the process. It receives signals from the controller and performs some type of operation on the process to change its
state. Examples are valves, motors, pumps, power amplifiers etc.
Sensor . Sensor measures the output signal and determines the actual state of the plant.
Error signal :The error signal is the algebraic addition of the input signal and the feedback signal. In feedback control systems the control action is generated by the actuating signal. In an open loop systems the actuating signal is equal to the input signal as there is no feedback.
Actuating signal: Actuating signal is the output signal of the controller.
Disturbance: A disturbance is a signal that tends to adversely affect the value of the output of a system. If a disturbance is generated within the system, it is called internal, while an external disturbance is generated outside the system.
Different types of control systems
Continuous & Digital control systems
The signals in control system, e.g. the input and the output waveforms, are typically functions of some independent variable usually time, denoted by t.
A continuous-time signal will contain a value for all real numbers along the time axis and if a control system has this type of signals, we call it as a continuous control system.
In contrast to this, a discrete-time signal is often created by using the sampling theorem to sample a continuous signal. So it will only have values at equally spaced intervals along the time axis. We call these signals as discrete signals and in digital control systems these signals are used. Usually a digital computer is used as a controller in digital control systems.
Non-linear control systems
A non-linear dynamic system is one that changes in a seemingly random way. For example, the time taken to process each person in line at a bank teller is very random - some people will have quick transactions and will be processed in short time, other people will have lengthy transactions and will take longer. Thus, the system (the bank queue) is an example of a non-linear system. Also, it is “dynamic” because the collection of people in line is continually changing. So, the bank queue is an example of a non-linear dynamic system.
Causal Systems / non-causal systems
Causal signals are signals that are zero for all negative time and non-causal signals are signals that have nonzero values in both positive and negative time.
If all the variables of a control system are causal, then we call that system as a causal system.
In this course we concentrate only on linear, causal and continuous systems. Also we study only about classical methods which are used to analyze these systems.