Data acquisition system

Data acquisition systems are used for large number of applications in industrial and scientific areas like aerospace and telemetry industries. Data acquisition system rapidly collects bulky of data, processing it and stores or displays the desired result.

Data acquisition system

The generalized block diagram of data acquisition system is shown in figure. It consists of sensors with signal conditioning, multiplexing, data conversion stage and display system.

Transducer converts physical parameters such as temperature, pressure, acceleration, weight, displacement, etc,. Into electrical quantities like voltage, resistance or frequency, which is an acceptable by the acquisition system. Signal conditioner modifies or amplifies transducer output signal. It includes circuits for providing excitation power to the transducer.

Multiplexer accepts multiple analog inputs and sequentially connects them to any one of the measuring instruments. ADC (analog to digital converter) converts the analog voltage to its equivalent digital form. Either ADC output is fed to any of the measuring devices or it may feed to digital computer for data reduction and further processing.

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Pressure Measurement using U-tube Manometer

         A well known very simple device used to measure the pressure is the U-tube manometer. The name U-tube is derived from its shape. U-tube manometer is shown below,


Construction of U-tube Manometer:


Let me explain you about the construction about the u-manometer. This manometer consists of a U shaped tube in which the manometeric liquid is filled. The manometer is used to measure the pressure which is unknown by the balancing gravity force and acceleration due to gravity, g = 9.81 m/sec2

u-tube-manometer
The manometer consists of a steel, brass and aluminum material. It has a glass tube made up of pyralex glass. The graduations are made on the tube in terms of mm or in some condition it is graduated in kilo Pascal.

Working of U-tube Manometer:

The unknown pressure is applied in the one arm of the tube and the mercury in the tube or manometeric liquid filled in the tube moves in the tube or rises to the constant region and then the movement is stopped. The height of the liquid is measured and noted. The pressure is calculated by using the formula,

                                                                    P1-P2 = Pmhg

The above equation is arrived by

P1 = Pthg = P2+Pmhg
P1-P2 = hg(Pt – Pm)

P1 = applied pressure
P2 = 0
Pt = specific gravity of the liquid or water
g = acceleration due to gravity.
P1 – P2 is approximately equal to Pmhg.


Advantages of U-tube Manometer:

  1. Simple in construction
  2. Low cost
  3. Very accurate and sensitive
  4. It can be used to measure other process variables.

Disadvantages of U-tube Manometer:

  1. Fragile in construction.
  2. Very sensitive to temperature changes.
  3. Error can happen while measuring the h.

Characteristics of liquid used in U-tube Manometer:

  1. Viscosity should be low.
  2. Low surface tension is required.
  3. The liquid should stick on the walls.
  4. Should not get vaporized.

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Free Ebook on Industrial Automation a Pocket Guide

            A very cool handy pdf ebook that gives you a very brief knowledge about Industrial automation published by IDC Technologies. The Inustrial Automation Pocket guide is 172 page ebook.

The contents in the guide is as follow which are very important for a instrumentation engineer.

  1. I&C Drawing and Documentations
  2. Process controls
  3. Advanced process control
  4. Industrial data communication and wireless.
  5. HAZOPs Hazard Operations
  6. Safety Instrumentation and Machinery
  7. Hazardous Areas and Intrinsic Safety
  8. SCADA (supervisory control and data acquisition)
  9. Project Management of I&C Projects
  10. Latest Instrumentation and Valve Developments
  11. Forecasts and Predictions.
A pretty big vast area in Instrumentation is unreavealed here.
Download the Industrial Automation pocket guide

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Create Mosquito repeller – A mini project.

                For my loyal reader, A simple mini project on creating mosquito repeller.
Mosquito repeller basically consists of two important parts, they are heater and mat. Heater is used to heat the mat such that the chemical particles which are D-Elithrin in the mat vaporizes and gets mixed in the air. An 8.2K/7W flat shaped resistance is used as main heating component. Copper or aluminum metal plate is placed just above the flat shaped resistance so that the copper or aluminum gets heated but the heat from the resistor should not pass to the metal plate directly, for that purpose a mica sheet is used in between them.



                LED with a main lead is connected to a 100k/1/4W resistor so that it can be used as a power indicator which indicates the power supply flow inside the circuit. When the supply is given to the circuit the resistor blocks a massive amount of current flow through it which results in the heat and since the metal plate is kept above the resistor, the heat is transformed to the plate and the mosquito mat which is placed in the metal plate slowly gets heated and the D-Elithrin chemical which mosquito don’t like is mixed up with the air and as a result mosquito is avoided in that place.

             If you have any doubt, please don't hesitate to leave a comment.

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Transducer: Part 2 - Classification of transducers

This post is as a continuation of the previous post on transducers.
If you are not aware of transducers, please read the brief post about transducers and about it's classification on active transducers and passive transducers.

The classification of transducers are made o the following basis:

Classification of transducers:


1. Based on the physical phenomenon,

  • Primary transducer
  • Secondary transducer
2. Based on the power type Classification,

  • Active transducer
  • Passive transducer
3. Based on the type of output the classification of transducers are made,

  • Analog transducer
  • Digital transducer
4. Based on the electrical phenomenon is a best Classification of Trasnducer,

  • Resistive transducer
  • Capacitive transducer
  • Inductive transducer
  • Photoelectric transducer
  • Photovoltaic transducer
5. Based on the non-electrical phenomenon Classification of transducer,

  • Linear displacement
  • Rotary displacement
6. Based on the transduction phenomenon,

  • Transducer
  • Inverse transducer.

Factor to be considered while selecting transducer:


• It should have high input impedance and low output impedance, to avoid loading effect.
• It should have good resolution over is entire selected range.
• It must be highly sensitive to desired signal and insensitive to unwanted signal.
• Preferably small in size.
• It should be able to work n corrosive environment.
• It should be able to withstand pressure, shocks, vibrations etc..
• It must have high degree of accuracy and repeatability.
• Selected transducer must be free from errors.

Requirements of a good transducers


• Smaller in size and weight.
• High sensitivity.
• Ability to withstand environmental conditions.
• Low cost.

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Moving coil galvanometer:

                   The principle of moving coil galvanometer is that when a conductor carrying current is placed in a magnetic field, it experience a force. The construction of a moving coil galvanometer is as shown in the figure. A rectangular coil is suspended in between the poles of a powerful magnet by means a phosphor bronze wire. The magnet is bored out cylindrically so that the field is radial. A soft iron core is placed inside the coil to induce large number of magnetic lines. A mirror M is fixed on the phosphor bronze wire. A spring is fixed at the bottom of the coil.


                     Let B be the strength of the magnetic field, n be the number of turns in the coil, A be the area of the coil and I be the current flowing through the coil.

            The deflecting couple produced in the coil
                            = BIAn.

        Due to this deflecting couple, the coil rotates and this couple is opposed by the torsional couple produced in the phosphor bronze wire.

                             The restoring couple produced in the coil
                                         =Cθ

                Where, C is couple per unit twist of the wire and θ is the angle of twist which can be measured by means of a lamp and scale arrangement.

  At equilibrium,

                             BIAn = Cθ

                             i.e., I = (C/BAn) * θ

            since, C,B,A and n are constants, the defelction in the coil is directional in the coil is directly proportional to the current through the coil.

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Types of Error

Error can be classified into three types:

1. Gross error
2. Systematic error
3. Random error

These errors occur mainly due to human mistakes in reading instruments, recording and calculating instruments. So whenever human being is involved in measuring some gross error are definitely committed. These errors must be detected and eliminated as possible.

1. Gross error

The gross error can be explained with an example.
An experimenter made transpose the reading, while reading the value, i.e., the value may to be 25.8 degree Celsius whereas the experimenter by mistake may read it as 25.5 degree Celsius. This information is known as gross error.

The gross error can be avoided in two ways,
Great care should be taken in reading and recording the data
For very large number of values its approximate value between the readings can be considered.

2. Systematic error


The systematic error can be classified into three types
• Instrumental error
• Environmental error
• Observation error

Instrumental error:


The instrumental error occurs due to three reasons
• Due to inherent short comings of the instruments
• Due to the misuse of the instruments
• Due to the loading effect.

Inherent shortcomings


These errors are due to construction, calibration and operation of instruments or measuring devices. Due to this error the instruments may read too high or too low.

For example, if the spring of permanent magnet has become very weak, then the instrument will always read very high.

To avoid these types of error

• The procedure of measurement must be carefully planned.
• Correction factors must be applied after determining the instrument errors
• An instrument can be re-calibrated carefully.

Misuse of the instrument


When a good instrument is used in a unintelligent manner then it results in misuse of the instruments, thereby producing poor initial adjustment and zero adjustments.

Loading effect


The loading effect occurs due to improper use of the instrument for measuring work.
For e.g. a well calibrated voltmeter may well mislead voltage when connected across a high resistance circuit. Similarly when the same voltmeter is connected across a low resistance value then it may give more dependable reading.

Environment error


These errors are due to environment factors like change in temperature, humidity and variation in pressure. This kind of error can be avoided by providing proactive cover or shield to the instrument.

Observation error


These errors are induced only by the observer and most common error is parallax error. These parallax errors are introduced while reading a meter scale.

This is due to the line of vision observer, which is not exactly above the pointer to avoid this error highly accurate meters are provided along with minus scale.

3. Random error


These types of error remain after gross error and systematic error which can be totally reduced. These errors are due to small factors which changes very often from instrument to the other instrument. These errors are also due to unknown cases which are also called residual error.

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Error Analysis

                Measurement is a process of comparing an unknown quantity with a standard quantity . No measurement can be made with perfect accuracy which results in the occurance of the errors.

There are two types of error,

  • Absolute error
  • Percentage error

Absolute error:

           Absolute error may be defined as the difference between the variable and measured value of the variable it can be expressed as e=yn-xn

Where, e = absolute error
Yn= expected value
Xn = measured value

Percentage error:

Percentage error = absolute error*100/expected error

Where, A = yn-xn*100/yn

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Mistakes made in Instrumentation and steps to avoid them

In this is post i am interested to give you an ebook which is provided from ABB control engineering.

This book tells us how to avoid the basic mistakes made in instrumentation, it a cool 7 page ebook with good explanation and diagram.

The four Mistakes made in instrumentation are

  1. Selecting the wrong sensor
  2. Installing sensors incorrectly
  3. Generating gibberish
  4. Quitting too soon
Download the mistakes made in instrumentation and control engineering

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What are Transducers ?

        Transducers play an important role in the field of instrumentation and control engineering. Any energy in a process should be converted from one form into another form to make the communication from one rectification sector to another.

            Transducer is a device which converts one form of energy into another form i.e,. the given non-electrical energy is converted into an electrical energy.

Types of transducers:

There are two types of transducers, they are:

  • Active transducers
  • Passive transducers
Active transducers:
Active transducer is a device which converts the given non-electrical energy into electrical energy by itself. Thermocouple, Photovoltaic cell and more are the best examples of the transducers


Passive transducers:
Passive transducer is a device which converts the given non-electrical energy into electrical energy by external force. Resistance strain gauge, Differential Transformer are the examples for the Passive transducers.

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Linear Variable Displacement Transducer (LVDT):


Update:Linear Variable Displacement Transducer (LVDT):

A very basic transducer which is always useful in the field of instrumentation, I have studied about this in my college days. Now let me explain about the LVDT with its Principle of Operation and I will explain how it is constructed for its well known operation and you can understand the working of LVDT.


Principle of LVDT:

LVDT works under the principle of mutual induction, and the displacement which is a non-electrical energy is converted into an electrical energy. And the way how the energy is getting converted is described in working of LVDT in a detailed manner.


Construction of LVDT:

LVDT consists of a cylindrical former where it is surrounded by one primary winding in the centre of the former and the two secondary windings at the sides. The number of turns in both the secondary windings are equal, but they are opposite to each other, i.e., if the left secondary windings is in the clockwise direction, the right secondary windings will be in the anti-clockwise direction, hence the net output voltages will be the difference in voltages between the two secondary coil. The two secondary coil is represented as S1 and S2. Esteem iron core is placed in the centre of the cylindrical former which can move in to and fro motion as shown in the figure. The AC excitation voltage is 5 to 12V and the operating frequency is given by 50 to 400 HZ.



Working of LVDT:

Let's study the working of LVDT by splitting the cases into 3 based on the iron core position inside the insulated former.


Case 1:


On applying an external force which is the displacement, if the core reminds in the null position itself without providing any movement then the voltage induced in both the secondary windings are equal which results in net output is equal to zero


i.e., Esec1-Esec2=0


Case 2:

When an external force is appilied and if the steel iron core tends to move in the left hand side direction then the emf voltage induced in the secondary coil is greater when compared to the emf induced in the secondary coil 2. Therefore the net output will be Esec1-Esec2



Case 3:

When an external force is applied and if the steel iron core moves in the right hand side direction then the emf induced in the secondary coil 2 is greater when compared to the emf voltage induced in the secondary coil 1. therefore the net output voltage will be Esec2-Esec1


Advantages of LVDT:


* Infinite resolution is present in LVDT
* High output
* LVDT gives High sensitivity
* Very good linearity
* Ruggedness
* LVDT Provides Less friction
* Low hysteresis
* LVDT gives Low power consumption.


Disadvantages of LVDT:

* Very high displacement is required for generating high voltages.
* Shielding is required since it is sensitive to magnetic field.
* The performance of the transducer gets affected by vibrations
* Its is greatly affected by temperature changes.

Applications of LVDT:

LVDT is used to measure displacement ranging from fraction millimeter to centimeter.

Acting as a secondary transducer, LVDT can be used as a device to measure force, weight and pressure, etc..

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Download Pocket Guide for Instrumentation by R.R Lee

A very important Ebook that will be very helpful for the instrumentation engineers is the Pocket Guide for Instrumentation by R.R Lee. The ebook consists for 171 pages with the 6 chapters listed below:

  • Control Devices
  • Instrument Valves
  • Instrument Fittings and Monitors
  • Storage Batteries and Solar Modules
  • Miscellaneous Electrical and Instrumentation Items
  • Bar Coding Systems
Every topics explained with detailed pictorial explanation that you can easily understand.

Download the Pocket Guide for Instrumentation by R.R Lee here.
Thanks for the globalautomation.info for providing the link.

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