CHAPTER 1
INTRODUCTION TO CONTROL SYSTEM
1.1 Describe an electrical, pneumatic and hydraulic control.
1.2 Identify general terms used in process control.1.1.1 Names the common application in electrical, pneumatic and hydraulic control.
1.2.1 Define process variables, dynamic variables, servo control, feedback control, process, regulation, set point, interference and error, controller, measurement elements and so on.1.3 Identify basic process control system:
1.3.1 Recognize the basic elements for process control (example level control in tank, temperature control etc).
1.3.2 Explain the process of measurement, evaluation and control elements from 1.3.1.
1.3.3 Draw and label block diagram of basic elements of process control.
1.3.4 Explain the purpose and use of open loop and closed loop block diagrams.
1.3.5 Differentiate open loop from closed loop process control by identifying negative and positive feedback.
1.3.6 Build examples of open loop and closed loop process control with the aid of block diagrams.
1.3.7 Define automatic control system.
1.3.8 Explain the purpose, advantages and disadvantages of automatic control system usage.1.4 Identify method of setting up a process control loop system.
1.4.1 Describe the need for a system control diagram.
1.4.2 Explain with the aid of diagrams:
(a) Free single variable loop.
(b) Interacting single variable loop.
(c) Multi -variable loop.
1.4.3 Identify the need of a cascade control system.
1.4.4 Explain the most important characteristics of a cascade control system with the aid of block diagrams.
1.4.5 Explain the necessity of quality in control system.
1.4.6 Define quality in terms of loop interference,stability, minimal error and minimal duration.
1.4.7 Analyze over-damped, critically damped and under-damped sketching.
CHAPTER 1 INTRODUCTION OF CONTROL SYSTEM
What is a control system?
Depending on the system configuration, there are two kinds of control systems
1)Open-loop control system
2)Closed-loop control system
Control system types.
There is THREE type control system that based on supply source.Systems are as follows:
i. Control system is pneumatic.ii. Control system is hydraulic.iii. Control system electric.
1.1.1 Names the common application in electrical, pneumatic and hydraulic control.
i. Pneumatic control system
Pneumatic control system is a system which uses air that in compress to generate force / energy to carry out work. Pneumatic system many in find out in system industrial like food industry,petrochemical and industry which uses robotics. Generally pneumatic system need:
- Air supply is compressed
- Control valve
- Tube link
- Transducer induce
The pneumatic control system can be handled human and automatic.
ii. Hydraulic control system
Hydraulic control system is a system which uses fluid to generate force / energy to carry out work. Hydraulic system many in use in automobile industry like power system, system brake, crane, jack car,satellite and so on. Fluid that commonly used is oil. Generally hydraulic system need :
Hydraulic control system is a system which uses fluid to generate force / energy to carry out work. Hydraulic system many in use in automobile industry like power system, system brake, crane, jack car,satellite and so on. Fluid that commonly used is oil. Generally hydraulic system need :
- Supply fluid is Hydraulic
- Control valve
- Cylinder
The hydraulic control system can be handled human and automatic.
iii. Electric control system Electric control system is a control system which uses electric current whether direct current (AT) or current shuttle as supply source.Generally Electric Control System requires:
- Electricity supply (AT) or (DC)
- Element entry (switch, sensor, valve,electronic component)
- Element product (Motor, light and so on)
- Connector cable
Electric Control System can be handled human and automatic.
Term used in process control
In this section we will give focus on terms that commonly used in process control.
- System : An interconnection of elements and devices for a desired purpose.
- Process : The device, plant, or system under control.
- Feedback control system : Feedback control system is a system where product always compared with entry set point and difference between value made foundation for control.
- Servo control : Servo control is a feedback control system where the product is position, velocity or acceleration.
- Dynamic variable : Dynamic variable is physical any parameter that variable the value spontaneously or through external influences.
- Error signal : Difference between entry signal and feedback signal/product.
- Process compound variable : process compound variable is process which contains more than one variable.
- Controller : Device which control process certain system and acts against error signal to reduce error to product like is wanted.
- Manipulated variables: These input variables are adjustable dynamically to keep the controlled variables at their set- points.
- Regulatory control: The set point is fixed at a constant value. When any disturbance enters the system, the manipulated variable is adjusted to drive the controlled variable back to its fixed set-point.
- Measurement: Refers to the conversion of the process variable into an analog or digital signal that can be use by the control system. The device that performs the initial measurements called a sensor or instrument.

1.3 Identify basic process control system:
Basic process of control element
process level control
- A level transmitter (LT), a level controller (LC), and a control valve (LV) are used to control the liquid level in a process tank.
- The purpose is to maintain the liquid level at some prescribed height (H) above the bottom of the tank.It is assumed that the rate of flow into the tank is random.
- the level transmitter is a device that measures the fluid level in the tank and converts it into a useful measurement signal, which is sent to a level controller.
- The level controller evaluates the measurement, compares it wiyh a desired set point (SP), and produces a series of corrective action that are sent to the control valve.
- The valve controls the flow of fluid in the outlet pipe to maintain a level in the tank.
process temperature control
1.3.2 Explain the process of measurement, evaluation and control elements from 1.3.1.
Measurement
- To control a dynamic variable in a process, must have information about the entiry or variable itself.This information is obtained by measuring the variable.
- Measurement refers to the conversion of the process variable into an analog or gigital signal that can be used by the control system.
- The device that performs the initial measurement is called a sensor or instrument.
- Typically measurements are pressure, level, temperature, flow , position and speed.
- The result of any measurement is the conversion of a dynamic variable into some proportional information that is required by the other elements in the process control loop or sequences.
Evaluation
- In the evaluation step of the process control sequences, the measurement value is examined, comparedwith the desired value or set point, and the amount of corrective action needed to maintain proper control is determined.
- A device called a controlle can be pneumatic, electronic or mechanical device mounted in a control panel or on the process equipment.
- It can also be part of a computer control.
- System, in which case the control function is performed by software.
Control element
- The control element in a control loop is the device that exerts a direct influence on the process or manufacuring sequence.
- This final control element accepts an input from the controller and transforms it into some proportional operation that is performed on the process.
- in most cases, this final control element will be a control valve that adjusts the flow of fluid in a process.
- Devices such as electrical motors, pumps, and dampers also used as control elements.
1.3.3 Draw and label block diagram of basic elements of process control.
What Is the Meaning of Open Loop System?
An open-loop system, also known as an open-loop controller or a non-feedback controller, is a kind of system that bases the input or start of the system without taking into consideration outside factors directly caused by the system itself. In other words, the feedback caused by the system does not factor into the decision of whether or not the system runs.
Two types of control systems
1. Open Loop
- no feedback
- difficult to control output with accuracy
- a common example of an open- loop control system is an electric toaster in he kitchen.
2. Close Loop
- must have feedback
- must have sensor on output
- almost always negative feedback
BENEFITS OF OPEN-LOOP SYSTEM
- Benefits of an open-loop system are often the small amount of cost associated with running the processes. It is simpler and more cost effective in most cases simply to start a repetitive process without worrying about factoring in feedback. For example, a process of a conveyor belt works more effectively without having to input feedback of the weight of every specific box that it is conveying. In this case, there is no need for feedback to be taken into consideration.
Open Loop System:
Advantages:
- Simplicity and stability: they are simpler in their layout and hence are economical and stable too due to their simplicity.
- Construction: Since these are having a simple layout so are easier to construct.
Disadvantages:
- Accuracy and Reliability: since these systems do not have a feedback mechanism, so they are very inaccurate in terms of result output and hence they are unreliable too.
- Due to the absence of a feedback mechanism, they are unable to remove the disturbances occurring from external sources.
EXAMPLE OF OPEN-LOOP SYSTEM?
- One example of an open-loop system is a sprinkler system that turns on every day at a pre-programmed time. No matter the moisture level of the grass, the sprinkler system will continue to water it at a prescribed time. (For example, even if there was a heavy rain and the sprinklers do not need to be turned on, they will still water at their pre-programmed time.) That is an open-loop because the sprinklers will turn on no matter the feedback (in this case, the grass moisture). However, if someone were to install a moisture detector where the sprinklers only turn on once it reaches a certain point, then the entire system turns into a closed-loop system.
Open-loop Drying System
FIGURE 1: BLOCK DIAGRAM OF OPEN-LOOP SYSTEM
FIGURE 2: OPEN LOOP BLOCK DIAGRAM
CLOSED-LOOP SYSTEM
- The opposite of an open-loop system is a closed-loop system. This is a system where the feedback of the process determines the next part of the process. For example, if there is a light that turns on in a room if motion is detected in the room, then that is a closed-loop system since the process of turning on the light entirely depends on feedback, in this case the introduction of motion.
A Closed-loop Control System, also known as a feedback control system is a control system which uses the concept of an open loop system as its forward path but has one or more feedback loops (hence its name) or paths between its output and its input. The reference to "feedback", simply means that some portion of the output is returned "back" to the input to form part of the systems excitation.
Closed-loop systems are designed to automatically achieve and maintain the desired output condition by comparing it with the actual condition. It does this by generating an error signal which is the difference between the output and the reference input. In other words, a "closed-loop system" is a fully automatic control system in which its control action being dependent on the output in some way.Closed Loop System:
Advantages:- Accuracy: They are more accurate than open loop system due to their complex construction. They are equally accurate and are not disturbed in the presence of non-linearities.
- Noise reduction ability: Since they are composed of a feedback mechanism, so they clear out the errors between input and output signals, and hence remain unaffected to the external noise sources.
Disadvantages:
- Construction: They are relatively more complex in construction and hence it adds up to the cost making it costlier than open loop system.
- Since it consists of feedback loop, it may create oscillatory response of the system and it also reduces the overall gain of the system.
- Stability: It is less stable than open loop system but this disadvantage can be striked off since we can make the sensitivity of the system very small so as to make the system as stable as possible.
- Closed-loop Control
FIGURE 3: CLOSED LOOP BLOCK DIAGRAM
Advantages and disadvantages of open loop and Closed-Loop control System.
Open Loop
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Closed Loop
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1. Such systems are simple in construction.
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1. Accuracy of such system is always very high because controller modifies and manipulates the actuating signal such that error in the system will be zero
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2. Very much convenient when output is difficult to measure.
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2. Such systems senses environmental changes, as well as internal disturbances and accordingly modifies the error.
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3. Such systems are easy from maintenance point of view.
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3. In such system, there is reduced effect of non linearity and distortions.
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4. Generally these are not troubled with the problems of stability.
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4. Bandwidth of such system i.e. operating frequency zone for such system is very high
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5. Such systems are simple to design and hence economical.
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Differentiate between open loop and closed loop control system
Open Loop
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Closed Loop
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1. Any change in output has no effect on the input i.e. feedback does not exists.
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1. Changes in output, affects the input which is possible by use of feedback
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2. Output measurement is not required for operation of system
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2. Output measurement is necessary
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3. Feedback element is absent
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3.Feedback element is present
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4.Error detector is absent
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4. Error detector is necessary
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5. It is inaccurate and unreliable
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5. Highly accurate and reliable
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6. Highly sensitive to the disturbances
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6. Less sensitive to the disturbances
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7. Highly sensitive to the environmental changes
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7. Less sensitive to the environmental changes
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8. Bandwidth is small
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8. Bandwidth is large
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9. Simple to construct and cheap
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9. Complicated to design and hence costly
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10.Generally are stable in nature
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10. Stability is the major consideration while designing
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11. Highly effected by non linearity
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11.Reduced effect of non linearity
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The disadvantages of open loop and closed loop control systems are:
Open Loop
| Closed Loop |
1. Such systems are inaccurate and unreliable because accuracy of such systems are totally dependent on the accurate pre calibration of the controller.
| 1. Such systems are complicated and time consuming from design point of view and hence costlier. |
2. Such systems give inaccurate results if there are variations in the external environment i.e. such systems cannot sense environmental changes.
| 2. Due to feedback, system tries to correct the error time to time. Tendency to over correct the error may cause oscillations without bound in the system. Hence system has to be designed taking into consideration problems of instability due to feedback. The stability problems are severe and must be taken care of while designing the system. |
3. Similarly they cannot sense internal disturbances in the system, after the controller stage. | |
4. To maintain the quality and accuracy, re calibration of the controller is necessary, time to time. |
- 1.3.7 Automation Control SystemAn automatic control system is a preset closed-loop control system that requires no operator action.This assumes the process remains in the normal range for the control system. An automatic control system has two process variables associated with it: a controlled variable and a manipulated variable.
A controlled variable is the process variable that is maintained at a specified value or within a specified range. In the previous example, the storage tank level is the controlled variable. - A manipulated variable is the process variable that is acted on by the control system to maintain
the controlled variable at the specified value or within the specified range. In the previous example, the flow rate of the water supplied to the tank is the manipulated variable. - Functions of Automatic Control
- In any automatic control system, the four basic functions that occur are:
- i)Measurement
- ii)Comparison
- iii)Computation
- iv)Correction
- In the water tank level control system in the example above, the level transmitter measures the level within the tank. The level transmitter sends a signal representing the tank level to the level control device, where it is compared to a desired tank level.The level control device then computes how far to open the supply valve to correct any level control device correct any difference between actual and desired tank levels.
- Elements of Automatic Control
- The three functional elements needed to perform the functions of an automatic control system are a measurement element.An error detection element is a final control element.
- Advantages Of Automatic Control System
- Accuracy: They are more accurate than open loop system due to their complex construction. They are equally accurate and are not disturbed in the presence of non-linearity.
- Noise reduction ability: since they are composed of a feedback mechanism, so they clear out the errors between input and output signals, and hence remain unaffected to the external noise sources.
- Construction: They are relatively more complex in construction and hence it adds up to the cost making it costlier than open loop system.
- Stability: It is less stable than open loop system but this disadvantage can be stroked off since we can make the sensitivity of the system very small so as to make the system as stable as possible
Method of Setting up A Process Control Loop System
Control System Design
- Establish control goals
- Identify the variables to control
- Write the specifications for the variables
- Establish the system configuration and identify the actuator
- Obtain a model of the process, the actuator and the sensor
- Describe a controller and select key parameters to be adjusted
- Optimize the parameters and analyze the performance
1.4.3 Identify the need of a cascade control system.
The needed of a cascade
control system
- A cascade control system is multiple-loop system where the primary variable is controlled by adjusting the set point of a related secondary variable controller. The secondary variable then affects the primary variable through the process.
- Feedback control - Corrective action taken regardless disturbance source.
the set point.
- Problematic for
processes with large time constants and/or long
time delays.
- Disturbance must
be explicitly measured.- Feed forward control - Corrective action can be taken before the output has deviated from the set point.
Cascade control system contain integrated sets of control loops.
- Primary Loop: Monitors the control variable and uses deviation from its set point to provide an output to the secondary loop.
- Secondary Loop: Receives its set point from the primary loop and controls the reference variable accordingly.
- Effectively accounts for external disturbances
- Reduces dead time in variable response
- Compatible with other control system, such as feed-back and feed-forward control architectures.
- Multiple control loop make physical and computational architecture more complex.
- Additional controllers and sensors can be costly.
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