Requirements of a Control System

Stability, accuracy and speed of response are the three requirement s of a control system.

Stability:
A system is to be stable if the output of the system after fluctions, variation or oscillation, if any, settles at a reasonable value for any change in input or change in disturbance.

Accuracy:
A system is said to be 100 percent accurate if the error ( different between input and output ) is zero. An accurate system is costly. There is no point in going for 100 percent accurate system when that much of accuracy is not really required.

Example of accuracy:
When a variation of say 0.2 degree centigrade cannot be sensed by a human being, there is no need to have a home heating system of temperature variation equal to zero.

Speed of Response:
This refers to time taken by the system to respond to the given input and give that as the output. Theoritically the speed of response should be infinity, that is, the system should have an instantaneous response. This requirement is prime concern with follow-up systems.

Any ideal system is perfectly stable, 100 percent accurate and has instantaneous speed of response. Unfortunately, the requirements are incompatiable. Hence there should be a compromise between these requirements.

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Basic control system definitions

controlled variable:

the quality or condition characterizing a process whose value is held constant by a controller or is changed according to a certain algorithm designed with the interests of the nature of the function the system is performing.

Controlled medium:

the process material in the control system in which the variable to be controlled exists.

Command
An input that is established or varied by some means external to and independent of the feedback system.

Set point or Reference input:

A signal established as a standard of comparison for a feedback control system by virtue of it's relation to command. The setpoint either remains the same or is varied with respect to time depending on a preset algorithm.

Actuating signal:

An algebraic sum of reference input and primary feedback. It is also called error or control signal.

Manipulated variable:

The quality or condition that is varied as a function of the actuating signal so as to change the value of the controlled variable.

Primary feedback signal:

The function of thecontrolled output which is compared to reference input to obtain the actuating signal.

Error detector:

An element that detects feedback; essentially a summing point which gives the algebraic sum of it's inputs.

Disturbance:

An unwanted variable in the system which tends to affect the system adversely by changing the controlled variable. Disturbance may due to change in set point, supply, demand, environment and the other associated variables.

Feedback element:

An element of the feedback control system which establishes a functional relationship between controlled variable and feedback signal.

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Automatic Control Systems

With the development of technology, man has learnt to use very reliable and accurate systems that require least manpower. Automatic control systems are one such development. Control is defined as the science of regulation of a parameter by comparing it with a standard value. This is the aim of an automatic control system. They constantly monitor the output of system and if this output is found to deviate from the desired value, it produces a control signal that server to bring down the change by providing a driving energy to the component that is responsible for the deviation.

A system is said to be made of a number of components such that the behaviour of the overall combination can be predicted if

1. the behaviour of the each component can be predicted, and
2. the interaction between each component is known.

Hence, a system is obtained when a number of components are connected in a sequence to perform a specific function. Suppose in a system, the output quantity is controlled by altering the input quantity, then such a system is called control system.

The output quantity is called the controlled variable and the input quantity is called the input signal. Automatic control system have become an integral part of modern manufacturing and industrial processes.

Examples:

manufacturing industry : numerical control of machine tool.
Industrial process : control of pressure, temperature and flow.

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Laws of thermoelectricity

Actual applications of the thermocouple to measure require the consideration of following laws of thermo electricity.

First law of homogeneous circuit

An electric current cannot be sustained in a circuit of single homogenous by application of heat done. This law is generally accepted to an experimental. In thermocouple, an emf is formed by joining two dissimilar wires/metals.

Second law of intermediate metals

It states that intersection of third metal into a thermocouple circuit will have no effects, as long as a junction by the third metal with thermocouple at the same temperature.

Applications

This law makes it possible to use extension wires at metals different from the thermocouple because platinum extension wires are at the same temperature high cost, copper can be used without any change in performance.

The law enables an instrument to be introduced into the circuit to be means the emf produced.

This law allows the use of joining material such or hard solder(silver) in fabricating the thermocouple and junction.

Law of intermediate thermocouple

The emf generated in a thermocouple with junction at temperature T1 and T3 is equal to sum of emf generated by similar thermocouple one acting between T2 + T3 and where T2 between T1+T3

This law is used when making or reference junction temperature is different from temperature at which it is calibrated. Thus a thermocouple is calibrated with reference junction at 0’C is used and with the junction at 20’C.

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Download Instrumentation and control Ebooks

Hello guys, i have small set of books related to Instrumentation and control engineering that you can download now and if you have any problem use the comments link to post comments to me..

All the ebooks that i am going to give is in PDF format to have a adobe reader installed in your computer.



Instrumentation and Control Fundamentals Handbook
Instrumentation and Control Volume 1 of 2
Instrumentation and Control Volume 2 of 2
Instrumentation and Control Tutorial 1
Instrumentation and Control Tutorial 2
Instrumentation and Control Tutorial 3
Instrumentation and Control Tutorial 4

Level and Pressure Instruments
Level Measurement In Liquids - Ultrasonic
Liquid Level Controllers

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Strain Gauge Pressure Cell:

Basic Principle:

When a closed container is subjected to the appilied pressure, it is strained (that is, its dimension changes). Measurement of this strain with a secondary transducer like a strain gauge ( metallic conductor) becomes a measure of the appilied pressure.

That is, if strain gauges are attached to the container subjected to the applied pressure, the strain guages also will change in dimension depending on the expansion or contraction of the container. The change in dimension of the strain guage will make its resistance to change. This change in resistance of the strain gauge becomes a measure of pressure appilied to the container (elastic container or cell).

There are two types of strain gauge pressure cells namely:

1. flattened tube pressure cell.
2. Cylindrical type pressure cell.

Flattened tube pressure cell.

The main parts of the arrangement are as follows:

An elastic tube which is flat and pinched at its two end as shown in diagram.
Two strain gauges are placed on this elastic tube: one is on the top and other is at the bottom of this elastic tube.
One end of the elastic tube is open to receive the appilied pressure and its other end is closed.

Operation:

The pressure to be measured is appilied to the open of the tube. Due to pressure, the tube tends to round off, that is, the dimension changes (strained). As the strain gauge are mounted on the tube, the dimension of the strain gauges also change proportional to the change in dimension of the tube, causing a resistance change of the strain gauges. The change in dimension of the tube is proportional to the applied pressure. Hence the measurement of the resistance change of the strain gauges becomes a measure of the appilied pressure when calibrated.

Cylindrical Type pressure cells:

Description

The main parts of this arrangement ar as follows:

A cylindrical tube with hexagonal step at its centre. This hexagonal step helps fixing this device on to place where the pressure is to be measured.

The bottom portion of this cylindrical tube is thearded at its external and is open to receive the pressure to be measured.

The top portion of this cylindrical tube is closed and has a cap screwed to it.

On the periphery of the top portion of the cylindrical tube are placed two sensing resistance strain gauges.

On the cap (unstrained location) are placed two temperature compensating strain gauges.

Operation.

The pressure to be measured is appiled to the open end of the cylindrical tube. Due to the pressure, the cylindrical tube is strained, that is its dimension changes. As the strain gauges are mounted on the cylindrical tube, the dimension of the sensing strain gauges also change proportional to the change in dimension of the cylindrical tube, causing a resistance changes of the strain gauges.

The change in dimension of the cylindrical tube is proportional to appilied pressure.

Hence the measurement of the resistance change of the sensing strain gauges becomes a measure of the appilied pressure when calibrated.

Appilications of the strain gauge pressure cells

the flattened tube pressure cell is used for low pressure measurement.
The cylindrical type pressure cell is used for medium and high pressure measurement.

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Microprocessor based data logging, processing and output – A mini Project

This post deals with using a microprocessor in instrumentation for collecting/acquiring data, processing it and displaying the output using the suitable display such as digits on an led or a computer screen. The basic elements of the data logger has been shown in the figure.

A typical data logger can handle 20 to 100 inputs. (some are even capable of handling around 1000 inputs). Such a unit is used to monitor the inputs from a large number of sensors or used to give outputs to number of display units or actuators. With the help of the signal conditioners, the output signals from the sensors are processed to make it suitable for measuring the input.

Mini Project based on Microprocessor

Lets assume the project is to data log the temperature of a liquid in a tank in a chemical industry and a thermocouple is placed inside the tank and the reading are to be seen in a display and recorded.

The output from the thermocouple is a small voltage is a small voltage in millivolts. Signal conditioning is done to convert this small voltage into suitable size current signal with noise rejection, linearisation and cold junction compensation for not being at 0'C.

The input and output devices are connected to a microprocessor system through ports. Inputs can be from sensors, switches, keyboards, etc.. and the output can be to displays, actuators, etc.,

Microprocessors require inputs that are digital. Hence, if the output from the sensor is analogue to digital converter ADC needed. Further, if the signal generated by he sensor is very small, amplification of the signal is first done before it is fed to an ADC. Even for digital signals, signal conditioning may be required to improve thier quality.

After suitable signal conditioning, the signal from the individual sensors are fed to a multiplexer. A multiplexer is circuit that can take-up inputs from a number of sources and then by selecting an input channel, give an output from just on of them.

In some appilications, there might be a need for measurements to be made at a number of different locations. In such a situation, instead of using a separate ADC and microprocessor for each measurement, a multiplexer is used to select each input one after the other, and switch it through a single ADC and microprocessor.

The output of the ADC is a digital signal which is processed using a microprocessor. The output from the microprocessor is displayed on a digital meter that indicates output and channel number or a printout record from a printer or the output of the microprocesor can be stored on a floppy disk or transferred to a computer for analysis but new need to program the microprocessor for every different output to needed.

A data logger can make around 1000 reading per second with an accuracy of 0.01% of full scale.

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Elastic diaphragm gauges

Basic principle of Elastic diaphragm gauges:

when an elastic transducer (diaphragm is this case) is subjected to a pressure, it deflects. This deflection is proportional to the applied pressure when calibrated.

Description of Elastic diaphragm gauges:

The main parts of the diaphragm which is a thin circular plate (made of springy metal) is fixed firmly around its edges. The diaphragm may either be flat or corrugated.

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Bourdon tube Pressure Gauge

Basic Principle of Bourdon tube pressure gauge:

when an elastic transducer ( bourdon tube in this case ) is subjected to a pressure, it defects. This deflection is proportional to the applied pressure when calibrated.

Description of Bourdon tube Pressure Gauge:

The main parts of this instruments are as follows:

An elastic transducer, that is bourdon tube which is fixed and open at one end to receive the pressure which is to be measured. The other end of the bourdon tube is free and closed.

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U-tube Manometer with equal unequal limb types

Description of manometer:

This is the most simple and precise device used for the measurement od pressure. It consists of a transparent tube constructed in the form of an elongated 'U', and partially filled with the manometeric fliud such as mercury. The purpose of using mercury as the manometeric fluid is that their specific gravity at various temperatures are known exactly and they dont stick to the tube. The two common types of manometers are the equal limb type and unequal limb type.

Operation of Manometer :

to measure the pressure of a fluid which is less dense and immisible with the manometeric fluid, it is appilied to the top of one of the limbs of the manometer while a reference fluid pressure (generally atmospheric ) is appilied to the other limb.

Let : P1 = unknown pressure with specific weight w1.
And : P2 = reference pressure with specific weight w2.
And : wm = specfic weight of the manometeric fluid.

The difference between the pressure on the two limbs of the manometer is a function of 'h', the difference between the levels of the manometeric fluid in the two limbs. 'h', can be read directly by placing a scale near the manometer.

For Equal Limb type:

the pressure balance equation is

P1 + gh * w1 = P2 + gh * wm

therefore, differential pressure

P = P1 – p2 = gh(wm – w1)

 

For unequal Limb type

the pressure balance equation is : (section-XX)

P1+w1(h1+h2+h) = P2 + w2h2 + wmh

Therefore, differential pressure:

P = P1 – P2 = w2h2 + wm * h – w1(h1 + h2 + h)

= wm*h [ 1 + (w2/wm * h2/h) – w1/wm ( h1+h2/h + 1) ]

= wmh * Cn

where; Cn = [ 1 + ( w2/wm – h2/h) – w1/wm ( h1+h2/h + 1)]

Cn is known as hydraulic correction factor.

Applications of Manometer :

• they are used to sense differential pressure in venturimeter and other flow meters.
• They are used as level devices to sense liquid heads.
• There are used as primary standard for pressure measurements.

Advantages of Manometer :

• they are simple is construction.
• They give accurate results.
• Wide range of manometeric fluids ar available such as mercury, water, aniline, tetrabromethane, bromoform and carbon tetrachloride.

Disadvantages of Manometer :

• They might break during transport.
• Certain manometeric fluids cause hazards when exposed to atmosphere.
• Error is introduced if the diameter of the tube is less.
• Leveling is required.

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Dead Weight Tester

Description

The dead weight tester apparatus consists of a chamber which is filled with oil free impurities and a piston – cylinder combination is fitted above the chamber as shown in diagram. The top portion of the piston is attached with a platform to carry weights. A plunger with a handle has been provided to vary the pressure of oil in the chamber. The pressure gauge to be tested is fitted at an appropriate plate.

Operation

the dead weight tester is basically a pressure producing and pressure measuring device. It is used to calibrate pressure gauges. The following procedure is adopted for calibrating pressure gauges. Calibration of pressure gauge means introducing an accurately known sample of pressure to the gauge under test and then observing the response of the gauge. In order to create this accurately known pressure, the following steps are followed.

The valve of the apparatus is closed.
A known weight is placed on the platform.
Now by operating the plunger, fluid pressure is applied to the other side of the piston until enough force is developed to lift the piston-weight combination. When this happens, the piston weight combination floats freely within the cylinder between limit stops.

In this condition of equilibrium, the pressure force of fluid is balanced against the gravitational force of the weights puls the friction drag.

Therefore, PA = Mg + F

Hence : P = Mg + F / A

where, P = pressure
M = Mass; Kg
g = Acceleratoion due to gravity ; m/s²
F = Friction drag; N
A = Eqivalent area of piston – cylinder combination; m²

Thus the pressure P which is caused due to the weights placed on the platform is calculated.

After calculating P , the plunger is released.

Now the pressure gauge to be calibrated is fitted at an appropriate place on the dead weight tester. The same known weight which was used to calucated P is placed on the platform. Due to the weight, the piston moves downwards and exerts a pressure P on the fluid. Now the valve in the apparatus is opened so that the fluid pressure P is transmitted to the gauge, which makes the gauge indicate a pressure value. This pressure value shown by the gauge should be equal to the known input pressure P. If the gauge indicates some other value other than p the gauge is adjusted so that it reads a value equal to p. Thus the gauge is calibrated.

Applications:

It is used to calibrated all kinds of pressure gauges such as industrial pressure gauges, engine indicators and piezoelectric transducers.

Advantages:

it is simple in construction and easy to use.
It can be used to calibrated a wide range of pressure measuring devices.
Fluid pressure can be easily varied by adding weights or by changing the piston cylinder combination.

Limitations:

the accuracy of the dead weight tester is affected due to the friction between the piston and cylinder, and due to the uncertainty of the value of gravitational constant 'g'.

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Pressure Measurement Devices

Gravitational transducers

1. A dead weight tester
2. Manometer

Elastic transducers ( Force summing Transducers)

1. Bourdon tube pressure gauge
2. elastic diaphragm gauges
3. bellow gagues
4. Bellow gaugues to measure gauge pressure
5. Bellow gauge to measure differential pressure

Strain gauge pressure cells.

1. Flattened tube pressure cell (PINCHED TUBE).
2. Cylindrical type pressure cell.

Mcleod vacuum gague.

1. Thermal conductivity gauges.

Pirani gauge.

1. Thermocouple typre conductity gauge.

Ionisation gauge

1. bulk modulus or electrical resistance pressure gauge.

The above instruments are used in following situations:

Type of pressure to be measured	Pressure Measuring instrument to be used
Low pressure	Manometer
High and medium pressure	Bourdon tube pressure gauge. Diaphragm gauge. Bellows Gauges.
Low vacuum and ultra high vacuum	Mcleod vacuum gauge thermal conductivity gauges. Ionisation gauges.
Very high presures	Bourdon tube pressure gauge. Diphragm gauge. Bulk modulus pressure gauge.

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Terms related to pressure

Atmospheric pressure:

The pressure due to air surrounding the earths surface is called as atmosperic pressure.

Absoulte pressure:

It is known that pressure is force per unit area when the interaction of fluid particles among themselves is zero, a zero pressure intensity will occur. This is possible only when the population of molecules is negligibly small which is nothing but perfect vacuum. Hence the pressure intensity measured from a state of prefect vacuum is called as absoulte pressure.

Gauge Pressure:

A pressure measuring instrument generally measures the difference between the uknown pressure (p) and the atmospheric pressure (pa). When the unknown pressure (P) is greater than the atmospheric pressure (Pa), the pressure measured by the instrument is called as the gauge pressure.

Vacuum pressure:

A Pressure measuring instrument generally measures the difference between the unknown pressure (P) and the atmospheric pressure (Pa). When the atmospheric pressure (Pa) is greater than the unknown pressure(P), the pressure mesured by the instrument is called as the vacuum pressure.

Static Pressure:

the pressure caused on the walls of the pipe due to a fluid at rest inside the pipe or due to the flow of a fluid parallel to the walls of the pipe is called as static pressure. This static pressure is measured by inserting a pressure measuring tube into the pipe carrying the fluid, so that the tube is at right angle to the fluid flow path.

Total or Stagnation pressure:

the pressure which is obtained by bringing the flowing fluid to rest isentropically is called as total or stagnation pressure. Hence the pressure will be a sum of static pressure and impact pressure.

Dynamic – or – Impact – or – Velocity pressure.

The pressure due to fluid velocity (flow speed) is called as impact pressure.

Impact pressure = Total pressure – static pressure.

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Introduction to pressure measurement

It is time to know about pressure measurement..

    In general, pressure is represented as force per unit area. The measurement of pressure is one of the most important measurements, as it is used in almost all industries. Some important appilications of pressure measurement is listed.

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