A system is defined as the collection of blocks connected together to execute a process. In electronic theory, a system can be said as a group of interconnected electronic devices, in which each individual system having its own input – output relation, to control the other block by connecting its output further to its next system. This is called as “Control system”.
The control system can also be defined as the group of devices which are used to control or regulate the functioning of other system.
Ex: To control the production machinery we use production control systems, in heavy industries.
Sometimes, to attain the consistency and stability of a system and to produce the desired output of a control systems, we use feedback loop. A feedback is nothing but a part of output signal.
This concept is most frequent and important in control systems , to achieve stability of the output.
The feedback loop connected to the input of the system enables the system to adjust with its parameters to get the desired output response.
Based on the feedback connection, the control systems are classified into two types. They are Open loop control system and closed loop control system.
Open loop feedback system
The system which doesn’t have any feedback loops connected to it is called “Open loop feedback system”. These type of systems doesn’t depend upon its output i.e. in open loop feedback systems, output is not used as the control variable for the system. Open loop systems are one way signal flow systems.
As these systems doesn’t contain a feedback loops ,these are also known as Non-feedback systems. As there is no feedback loop, no data is transferred back to the open loop systems.
The output of the open loop system can be determined only by its present state input that means the input of open loop system will always return to zero, before the output returns to zero. There is no chance to correct the transition errors in open loop systems so there is more chance to occur errors.
We use open loop control systems in many applications of our day to day life. Some of the systems designed based on the concept of open loop control systems are given below.
- Automatic washing machine
- Electric bulb
- Electric hand drier
- Time based Bread toaster
- Volume of the audio system
- Water faucet
- TV remote control
- Clothes drier
- Shades or blinds on a window.
- Stepper motor or servo motor
- Inkjet printers
- Door lock system etc.
Real Time Example of open loop system
Some real time examples of open loop system are explained below.
Electrical cloth drier
We operate the cloth drier depending upon the amount of clothes to make dry. We set a timer to the system to do this operation, let’s say 20 mins. As it is an automatic open loop system, the machine stops working automatically after 20 mins, irrespective of the nature of the clothes, whether they are dry or damp.
The clothe drier is an open loop system, because it doesn’t consider the condition of clothes before it stops working. Then the task of clothes drying is an user issue. The user can adjust the drying process of the system by setting the timer controls as per his requirement. Observe the below block diagram for the open loop control system, electric drier
Here the electric drier system has three blocks, timer, heating elements and clothes. Primarily, the user sets the time for drying the clothes in the timer. And the timer control works combined with heating elements which are used to produce heat and dry the clothes.
As there is no feedback about clothes , humidity, the system stops working after the pre-set time, and it doesn’t consider the state of clothes. Even though the clothes are dry, the system will switch off after the allotted time.
As the system has no feedback, this is also known as “non feedback system”, in which the output of the system depends only on the input signal and no control action of the input signal is provided.
The output of the open loop system is not compared with the input of the system for checking errors occurred in the output. The output of non feedback signal faithfully depends upon its input and on no other circumstances or parameters of the external systems.
These systems may effect by large deviation in output when the pre-set value of the system drifts. As the open loop system doesn’t have any knowledge of the output occurrence and condition, it cannot correct errors itself. This is an major disadvantage of the open loop control system.
Another disadvantage is that these systems cannot handle the external disturbances and have very poor ability to oppose the adoption of changes external system parameters. For example, in this electric drier, it follows only the pre-set time which is set by user.
In the pre-set tie of 20m mins, if we keep open the machine door for some time and then closed, then also regardless of the situation of the clothes, the machine turns off. Even though the clothes are dry, the machine stops after particular time limit.
This happens because there is no feedback about the constant temperature to maintain in the machine. This is an another disadvantage of open loop system that it doesn’t consider the external effects that may decrease the efficiency of the system.
So we come across a discussion that the errors occurred in open loop systems will disturb the drying process and to maintain stability these systems requires the regular (continuous) attention of user.
This means the user needs to check the process and the temperature of clothes, frequently and take control of the machine when it deviates from the required conditional circumstances.
Mobile/ cellular phones
The mobile phone is an example of open loop system.At the time of incoming or outgoing call loop begins.In this loop the mobile phone is connected to the satellite directly.This continous till the user breaks the connection by ending the call.
Thus this system can be said as open loop system as it cannot break the connection by itself with out the user interference.The action is not done on an account of feed back but manually.
The word processor also an open loop control system. When we enter any information though the keyboard then the other blocks of the control system, like hardware components will process the user entered information and displays it on LCD.
If the operator stops typing the peripheral, there will be no input provided to the system and nothing is displayed on the display. So the word processor becomes an example for the open loop control system.
Advantages of Open loop control System
The main advantages of the open loop control system are listed below
- The open loop systems are very simple in design and easy to design.
- These are cheaper in cost than other type of control systems.
- Maintenance of open loop control system is very simple.
- These are stable for some extent of time.
- These types of systems are convenient to use.
Disadvantages of Open loop control System
The disadvantages of open loop system are
- The bandwidth of open loop control system is less.
- The non feedback system doesn’t facilitates the automation of process.
- Open loop systems are inaccurate in nature.
- As their output is affected by some of the external system disturbances, the non feedback systems are unreliable.
- It cannot correct the output deviations automatically.
Automatic washing machine
This we use in our home to wash and dry clothes. Internally, the machine runs according to the reset time and doesn’t depend upon the washing process, weather it is completed or not.
We all know that the electric bulb need some voltage or current to produce light. When the mains supply is available, by switching on the switch, we can make the bulb to work. And this process does not depends up on the temperature of bulb or any other surrounded parameters.
Electric hand drier
The hand drier that works on electric power supply and when we keep our hands in front of it, it will automatically dry our hands by blowing hot air, irrespective of how much our hand is dried.
The toasting machine runs as per our adjusted time period. And it toasts the bread piece irrespective of the toasting completed or not.
Volume of the audio system
We can adjust the volume of a audio system manually, irrespective of the range of output volume.