Open Loop System

In this tutorial, we will learn about an important concept in Control Systems known as Open Loop System. We will explore some basics of system and control system, what is an open loop system, what are its advantages and disadvantages and also few examples of open loop systems.

What is a System?

A System is defined as the collection of blocks (or components or objects) connected together to execute a process (or serve or perform an objective). Another simple definition of a System is it is a group of components (or elements) which are connected in a sequence to perform a specified function (or task).

System doesn’t mean only physical entity. It can also be an abstract or a phenomenon or a concept, something we often use in Mathematical and Economical domain. But a physical system consists of physical objects connected together to perform a function.

Consider an Electric Motor. It is a physical system with components like rotor, stator, a cage, wiring etc. It accepts electrical energy an input and produces a mechanical energy as output.

Based on this example, we can redefine the concept of System as an arrangement of components within a boundary, which work together by accepting an input (or multiple inputs) and provide an output.

What is a Control System?

A Control System is a System where the output is controlled by varying / adjusting the input. Another definition of Control System is it is a combination of subsystems and processes working together to obtain a desired output and performance with a specified input.

You can understand the definition of a Control System easily by considering an example. The control system we are going to talk about is an elevator.

Imagine you entered an elevator and pressed the key ‘4’ (as you want to go to the 4^th floor). The elevator control system takes this input and accordingly operates the motor and doors (outputs). The duration for which the motor runs depend on the input key pressed i.e., it varies as per the key pressed.

There are two more definitions of Control System which might be interesting. A Control System is a group of interconnected devices, where each device has its own input – output quantities and controls the next system by connecting its output.

The control system can also be defined as the group of devices which are used to control or regulate the functioning of other system. For example, to control the production machinery we use production control systems, in heavy industries.

There are several ways to classify Control Systems. Some of the popular classifications of control systems are:

• Open Loop System and Closed Loop System – based on feedback signal
• Linear and Non–Linear Systems – based on their differential equations
• Continuous and Discrete Systems – based on the type of signal in their system
• Time Varying and Time Invariant Systems – based on their time dependency

In this, we are interested in the feedback-based Control Systems i.e., Open Loop System and Closed Loop System and particularly, the Open Loop Systems, which is the main topic of this tutorial.

Sometimes, to attain consistency and stability of a system and to produce the desired output, a control system may use feedback. A feedback is nothing but a part of output signal being fed back to the input. Feedback is one of the important and frequently used concepts in Control Systems.

The feedback connected to the input of the system enables the system to adjust its parameters to get the desired output response.

Based on the feedback connection i.e., if feedback is used or not, the control systems are classified into two types. They are:

• Open Loop Control System
• Closed Loop Control System

Open Loop System

A Control System which doesn’t have any feedback connected to it is called as Open Loop System. These types of systems don’t depend upon its output i.e., in open loop systems, output is not used as a control variable for the system and it has no effect on the input.

Open loop systems are one way signal flow systems. As these systems doesn’t contain any feedback i.e., the output is not fed back to the input, these are also known as Non-Feedback Systems.

The following image shows a simple block diagram of an Open Loop System.

In an open loop system, the output can be adjusted / varied by varying the input but the output has no effect on the input. The output of the open loop system can be determined only by its present state input. If the output is affected due to some external noise / disturbance, the open loop system cannot correct it.

Also, there is no chance to correct the transition errors in open loop systems so there is more chance to occur errors.

Open Loop System Applications

We use open loop control systems in many applications of our day-to-day lives. Some of the popular systems, which are designed based on the concept of open loop control systems, are mentioned below:

• Washing Machine
• Electric Bulb
• Electric Hand Drier
• Time based Bread Toaster
• Automatic Water Faucet
• TV Remote Control
• Electric Clothes Drier
• Shades or Blinds on a window
• Stepper Motor or Servo Motor
• Inkjet Printers
• Door Lock System
• Traffic Control System

Real Time Examples of Open Loop System

Some real time examples of open loop system are explained below.

Electrical Clothes Drier

We operate the clothes drier depending upon the amount of clothes to make dry. We set a timer in 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 clothes drier is an open loop system, because it doesn’t consider the condition of clothes before it stops working. The user can adjust the operating duration of the system by setting the timer controls as per his / her 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 in combination with the heating element, to produce heat and dry the clothes.

As there is no feedback about clothes, humidity, the system stops working after the preset time and it doesn’t consider the state of clothes. Even if 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 based on the output is provided.

The output of the open loop control system is not compared with the input of the system for checking errors in the output. The output of non-feedback signal “faithfully” depends upon its input and doesn’t depend on any other circumstances or parameters external to the system.

These systems may be affected by large deviation in output, when the preset value of the system drifts away. As the open loop system doesn’t have any knowledge of the output, it cannot correct errors itself. This is a 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 to external system parameters.

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 require 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.

Traffic Control System

Most automated Traffic Control Systems are time-based open loop control systems i.e., each signal is allotted with a specific time slot during which it operates irrespective of the amount of traffic.

Washing Machine

Another example of an Open Loop Control System which we use in our daily lives is a Washing Machine. The operations of soaking, washing, rinsing and drying are time based and do not depend on the cleanliness of the clothes or if the clothes are dry or not.

Electric Bulb

We all know that when electric current passes through an electric bulb, it produces light. When the mains supply is available, by switching ON the bulb, we can make it to work. And this process does not depend up on the temperature of bulb or any other parameters.

Electric Hand Drier

The hand drier 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.

Bread Toaster

The Bread Toaster is another simple example of an Open Loop Control System as the machine runs based on the preset time period irrespective of the toasting of bread is completed or not.

Advantages of Open Loop Control System

The main advantages of the open loop control system are listed below:

• Open Loop Control Systems are very simple and easy to design.
• These are considerably cheaper than other types of control systems.
• Maintenance of an open loop control system is very simple.
• Generally, open loop systems are stable up to some extent.
• These types of systems are easy to construct and 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 facilitate the process of automation.
• Open loop systems are inaccurate in nature and also unreliable.
• If their output is affected by some external disturbances, there is no way to correct them automatically as these are non-feedback systems.

Conclusion

This was a brief introduction to the concept of Control Systems and in particular, the Open Loop System variant. You leaned what is a system, what is a control system, what is an Open Loop Control System, advantages and disadvantages of open loop systems and also some common examples of Open Loop System which we use / encounter daily.