Control System

What is a control system?

When a number of elements are combined together to form a system to produce desired output then the system is referred as control system. As this system controls the output, it is so referred. Each element connected to the system has its own effect on the output. It is a system of devices or set of devices, that manages, commands, directs or regulates the behavior of other device(s) or system(s) to achieve desire results.

What are the requirements of Good Control System?
  • Accuracy :  Accuracy is the measurement tolerance of the instrument and defines the limits of the errors made when the instrument is used in normal operating conditions. Accuracy can be improved by using feedback elements. To increase accuracy of any control system error detector should be present in It.
  • Sensitivity :  The parameters of control system are always changing with change in surrounding conditions, internal disturbance or any other parameters. This change can be expressed in terms of sensitivity. Any  system should be insensitive to such parameters but sensitive to input signals only.
  • Stability : It is an important characteristic of control system. For the bounded input signal, the output must be bounded and if input is zero then output must be zero then such a control system is said to be stable system.
  • Bandwidth : An operating frequency range decides the bandwidth of control system. Bandwidth should be large as possible for frequency response of good control system.
  • Speed : It is the time taken by it to achieve its stable output. A good control system possesses high speed. The transient period for such system is very small.
  • Oscillation : A small numbers of oscillation or constant oscillation of output tend to system to be stable.
Describe the types of control system with examples?

There are basically two types of control system. These are :

  • Open Loop System
  • Closed Loop Systemopen control system

Open Loop Control System: It is a system in which the control action is totally independent of output of the system then it is called open loop control system. Manual system is also an open loop control system. Fig  shows the block diagram of it in which process output is totally independent of controller action.

Practical Examples of Open Loop Control System
  1. 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.
  2. Automatic Washing Machine – This machine runs according to the pre-set time irrespective of washing is completed or not.
  3. Bread Toaster – This machine runs as per adjusted time irrespective of toasting is completed or not.
  4. Automatic Tea/Coffee Maker – These machines also function for pre adjusted time only.
  5. Timer Based Clothes Drier – This machine dries wet clothes for pre-adjusted time, it does not matter how much the clothes are dried.
  6. Light Switch – Lamps glow whenever light switch is on irrespective of light is required or not.
  7. Volume on Stereo System – Volume is adjusted manually irrespective of output volume level.

Closed control systemClosed Loop Control System: The 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  system can be converted in to closed loop system by providing a feedback. This feedback automatically makes the suitable changes in the output due to external disturbance. In this way this system is called automatic control system. Figure below shows the block diagram of it in which feedback is taken from output and fed in to input.

Practical Examples of Closed Loop Control System
  1. Automatic Electric Iron – Heating elements are controlled by output temperature of the iron.
  2. Servo Voltage Stabilizer – Voltage controller operates depending upon output voltage of the system.
  3. Water Level Controller – Input water is controlled by water level of the reservoir.
  4. Missile Launched and Auto Tracked by Radar – The direction of missile is controlled by comparing the target and position of the missile.
  5. An Air Conditioner – An air conditioner functions depending upon the temperature of the room.
  6. Cooling System in Car – It operates depending upon the temperature which it controls.

What are the advantages and disadvantages of Open and Closed Loop System?

Advantages of Open Loop Control System
  1. Simple in construction and design.
  2. Economical.
  3. Easy to maintain.
  4. Generally stable.
  5. Convenient to use as output is difficult to measure.
Disadvantages of Open Loop Control System
  1. They are inaccurate.
  2. They are unreliable.
  3. Any change in output cannot be corrected automatically.
Advantages of Closed Loop Control System
  1. Closed loop  systems are more accurate even in the presence of non-linearity.
  2. Highly accurate as any error arising is corrected due to presence of feedback signal.
  3. Bandwidth range is large.
  4. Facilitates automation.
  5. The sensitivity of system may be made small to make system more stable.
  6. This system is less affected by noise.
Disadvantages of Closed Loop Control System
  1. They are costlier.
  2. They are complicated to design.
  3. Required more maintenance.
  4. Feedback leads to oscillatory response.
  5. Overall gain is reduced due to presence of feedback.
  6. Stability is the major problem and more care is needed to design a stable closed loop system.
Describe the types of control system on the basis of driving medium, with advantages and disadvantages?

On the basis of driving medium there are three types of it:

  • Electrical
  • Pneumatic
  • Hydraulic

Advantages of Electrical Control System

  1. No air compressor and associated machinery required.
  2. High efficiency since there is no leakages.
  3. Very little affected by normal temperature variations.
  4. Very accurate and quick response.
  5. Electric cables are cheap and easy to lay.
  6. No time lag or transmission delay.
  7. No contamination in control medium

Disadvantages of Electrical Control System

  1. The system requires uninterrupted power supply with no voltage fluctuations.
  2. Complex networks
  3. Standby batteries required in case of power failure.
  4. There is risk of fire due to overheating. Expensive intrinsically safe / explosion proof equipment may be required.
  5. Difficult maintenance and test kit requirements
  6. Moisture may cause damage to the system.
  7. Damage readily occurs in the event of a fire.

Advantages of Pneumatic Control System

  1. Not affected by ship’s power supply variations for short term.
  2. Low cost of installation
  3. No heat generated and hence no risk of fire.
  4. Simplicity of the components and no complex structure
  5. Actuators are cheaper and accurate than electric systems.
  6. Simple control air piping system.

 Disadvantages of Pneumatic Control System

  1. Require air compressor and associated systems.
  2. Copper piping is vulnerable to damage
  3. A clean, dry and oil free supply of good quality air is essential for satisfactory operation of the system.
  4. Good quality air require driers, filters with drains, etc. which increases maintenance.
  5. May have transmission lags in large systems.
  6. Pipe-couplings can give rise to leaks in certain ambient conditions
  7. Potential noise

Advantages of Hydraulic Control System

  1. High horsepower-to-weight ratio
  2. Its reliable to operate
  3. Leaks easier to find
  4. Maintains constant torque and force
  5. Hydraulic power can be transmitted in long distances
  6. Motion reversal is fast
  7. Handles strong, heavy loads, shock forces
  8. Costs less energy to operate
  9. Lesser overall wear because of oil lubrication
  10. Does not generate sparks
  11. Smooth operation for lifting and movement of loads
  12. Costs less energy to operate
  13. Operates in hot environments

Disadvantages of Hydraulic Control System

  1. More expensive initial installation than pneumatic system 
  2. Slower motion than pneumatic.
  3. Noisy
  4. Risk of contamination
  5. Leakage of oil problem
  6. Requires more energy to operate
  7. Requires more maintenance
  8. Heavier components, parts
  9. Hydraulic fluid dangerous to humans
  10. Hydraulic fluid not environment friendly
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Abhishek Kumar

He is working as an engineer in Fleet Management Ltd. He is alumni of Marine Engineering & Research Institute (MERI) Mumbai. He loves writing blog & exploring new places. Specialty: Tankers and Bulk Ship

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