There are basically two types of control system: the open loop system and the closed loop system. Both open- and closed-loop control systems try to maintain a variable at a predetermined value. They can both be represented by block diagrams.
Open-loop control system: it is a control system in which the output has no effect upon the control action. A control system in which the control action is totally independent of output of the system then it is called open loop control system. Manual control system is also an open loop control system. Figure below shows the block diagram of open loop control system in which process output is totally independent of controller action.
Block diagram of open loop control system
Examples:
1. Consider an open-loop system for switching on and off the street lamps along a road. The control required is that when darkness falls, the lights turn on, and when it is light, the lights turnoff. The control signal would be pre-set to accordingly switch the lights on or off at set times in the evening and morning, using a timing device. This system would probably work acceptably for a few weeks. However, because the hours of darkness change over the year, the pre-set signal would soon be incorrect and the lights would be switching on and off too early or too late. Figure below shows this open-loop street lighting system in flow diagram form.
Timer based street lighting system
2. Electric Hand Drier: Hot air (output) comes out as long as you keep your hand under the machine, irrespective of how much your hand is dried.
3. Automatic Washing Machine: This machine runs according to the present time irrespective of washing is completed or not.
4. Bread Toaster: This machine runs as per adjusted time irrespective of toasting is completed or not.
Open-loop control systems are in general relatively simple in design and inexpensive. However, they can be inefficient and require frequent operator intervention. Under many circumstances pre-set values become incorrect due to the parameter they are controlling changing in some way, and so they need resetting.
Advantages:
1. Simple in construction and design. 2. Less expensive than closed-loop control system.
3. Easy to maintain. 4. Generally stable.
Disadvantages:
1. They are inaccurate. 2. They are unreliable.
3. Any change in output cannot be corrected automatically.
Note: any control system which operates on a time basis is open-loop control system,
e.g. washing machine, traffic light …etc.
Closed-loop control system: it is a control system in which the output has an effect on the input quantity in such a manner that the input quantity will adjust itself based on the output generated is called closed loop control system. Open loop control system can be converted in to closed loop control system by providing a feedback. This feedback automatically makes the suitable changes in the output due to external disturbance. In this way closed loop control system is called automatic control system. Figure below shows the block diagram of closed loop control system in which feedback is taken from output and fed in to input.
A feedback is a common and powerful tool when designing a control system. Feedback loop is the tool which takes the system output into consideration and enables the system to adjust its performance to meet a desired result of system. it’s the information that tells you how you’re doing , the feedback used the difference between the Process Variable and the Set point (the error) tells the controller how well it’s doing in automatic(closed) controller , but not used in Open loop control .
Example
Air Conditioner – An air conditioner functions depending upon the temperature of the room.
Water Level Controller– Output water is controlled by water level of the reservoir.
Water Heater Controller–Input steam is controlled by temperature of the reservoir
Advantages of Closed Loop Control System
1. More accurate than the open-loop control system.
2. This system is less affected by noise.
Disadvantages of Closed Loop Control System
1. They are expensive. 2. They are complicated to design.
3. Required more maintenance. 4. Stability is the major problem.
Comparison of Closed Loop and Open Loop Control System
Open Loop Control System
In any closed control system, output is affected due to change in environmental condition or any kind of disturbance. So one signal is taken from output is fed back to the input. This signal is compared with reference input and then error signal is generated. This error signal is applied to controller and output is corrected. Such a system is called feedback system. Figure below shows the block diagram of feedback system. When feedback signal is positive then system called positive feedback system. For positive feedback system, the error signal is the addition of reference input signal and feedback signal. When feedback signal is negative then system is called negative feedback system. For negative feedback system, the error signal is given by difference of reference input signal and feedback signal.
Feedback Control Theory
? Output of the system y(t) is feed back to the reference value r(t) through measurement of a sensor
? Controller C takes the difference between the reference and the output and determines the error e
? Controller C changes the inputs u to Process under control P by the amount of error e
A control loop consists of three parts:
(1) Measurement by a sensor connected to the process , (2) Decision in a controller element , (3) Action through an output device ("actuator") such as a control valve.
Example Thermostat Controller
? recognized the house is too cold
? sends signal to the furnace to turn on and heat the house
This is an example of what is commonly called a feedback control loop. The thermostat has a temperature gauge that senses when the house is cold. It sends a signal to the furnace to turn on and heat the house until the temperature in the house reaches a point where the temperature sensors reach a pre-determined set point and the thermostat then sends a signal to turn off the furnace until the house gets too cold again the process repeats itself. This is a feedback loop since the controlled variable, in this case the house
temperature, is fed back through the thermostat controller to the furnace.
Others Examples .
distillation tower
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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