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Displacement Transducers

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Displacement is a basic variable whose value is measured and involved in many other physical parameters such as velocity, force, acceleration, torque and so on. The transducer used for the measurement of displacement can be classified in many ways. One of the most common classification is given below.

  • Mechanical
  • Pneumatic
  • Electrical
  • Optical

In order to obtain an electrical output, a mixture of two or more methods is also used. For example, optical methods using photo-detectors present the output as an electrical quantity like voltage, current and so on. Thus, the combined mechanical and optical method is desired.

Measurements can be made in the direct and indirect way. In direct method, the displacement is measured directly. But indirect methods are mostly used as the associated variables like force, acceleration, torque, velocity and so on can be obtained.

In electrical conversion method, the displacement is converted to an electrical quantity like voltage or current. This value is then recorded or displayed on a screen.

A basic displacement scheme is shown in the figure below.

Displacement Transducer
                                                      Displacement Transducer

Some of the most commonly used methods are listed and explained below. Though some of these methods can be used for the measurement of other physical quantities, the electrical signals derived from such transducers always depend on a displacement parameter.

Photo-Electric Transducers

Before explaining about the different photo-electric transducers, it is important to know the basics of photo-electric effect.
Photo-Electric Effect – It can be explained as the electric current produced when a light of certain intensity strikes on a piece of metal. The energy contained in the light is transferred to the surface of the metal and thus produce a movement of electrons. This movement produces a current. This effect cannot be produced by all colors in the spectrum. A
bright red colored light will not produce a current flow in the metal. But, a dim blue light can cause current flow. Thus, the only way to decide the colors producing the photo-electric effect is through the concept of photons. The idea was brought forward by Albert Einstein, and according to him, light is made up of small packets of energy that had the behavior of particles. These packets of energy were named photons. A red light does not move the electrons, as their individual photons does not have much energy. But a blue light can move electrons as their individual photons have more energy than that of red light. The electrons thus emitted in this manner are called photo electrons.

Some of the most commonly used displacement transducers through the application of photo-electric effect are explained below.

1. Vacuum Photo-Tube as Transducer

A schematic of the vacuum photo-tube transducer is shown in the figure below.

Vaccum Photo-Tube Transducer
                        Vaccum Photo-Tube Transducer

A displacement produced will modulate or change the intensity of the light intensity of the light incident on the photo-cathode. This changes the amount of voltage and thus, the proportional anode current is given to the resistor R. This changes the output voltage. The output voltage produced will be proportional to the amount of displacement given as input. This transducer is appropriate when there is an availability of a stable light source or an ac modulated light.

Advantages

  • Efficiency is fairly high.
  • Can take both static and dynamic measurements.

Disadvantages

  • Stability is achieved only for a short period.
  • If the light variations are subjected to high temperatures, there will be very little response.
  • Only suitable for applications having large displacements.

2. Photo-Diode as Transducer

The circuit for a photo-diode transducer is shown below.

Photo-Diode Transducer
                                          Photo-Diode Transducer

The arrangement is almost same as a photo-tube transducer except that the photo-tube has been replaced by a photo-diode and the lens has been replaced to make the light strike on the junction of the photo-diode. When a displacement is produced, it provides a force on the summing member and thus changes the quantum of light intensity incident on the photo-diode junction. The proportional change in anode voltage produces a current in the resistor R. Thus, an output voltage is produced which will be proportional to the displacement given as input.

3. Photo-Conductor as Transducer

The circuit is the same as a vacuum photo-tube arrangement except that the photo-conductor is placed instead of a photo-tube. The displacement causes a unique light intensity incident on the photo-conductor and causes a variation in current through resistor R. Hence the output voltage produced will be a linear function of the displacement produced. This device is not used much as the sensitivity produced remains stable only at the beginning and becomes poor at high frequencies.

4. Photo-Voltage Cell as Transducer

This device has great applications in electronic instrumentation and control circuits as it is an active transducer and needs no associated energy source. According to the light intensity striking on the photo-voltage cell, it illuminates and produces a small proportional voltage. The circuit diagram is the same as that given above except that the photo-tube is replaced by photo-voltaic cell and the source for voltage is taken away. The working is also the same as that explained above and it can be calibrated according to the amount of displacement produced.

Difference Between Sensors and Transducers

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The applications of sensors and transducers can be seen in all types of electrical and electronic instruments ranging from fancy gadgets to various plants, display units and so on. But, most people get confused whether a transducer and a sensor are the same thing, or are they different. What do they have in common? In this post we shall discuss about the basic working of the sensor and transducer with respect to each other.

Both the devices are used in electronic and electrical applications and are often encountered by mechanics. As the name implies, a sensor is a device that is used to measure various physical quantities like resistance, pressure, flow, level, humidity and so on and convert them into electronic signals (digital or analog) that can be easily read by the user or any other instrument.

As explained earlier, a transducer is a physical device that is used to convert a physical energy into another so as to gather information or to keep track of its measurement.

People get riddled by both terms as they do not understand why transducers are used in sensors. In a multi-operational device, a transducer converts one energy to another. This converted energy is measured or displayed to the user for other measurements using sensors.
It is confusing to see transducers being used in sensors as contact transducers for detecting energy levels and then converting them into electrical energy so as to be displayed on a screen. About twenty years before, we have seen the application of such transducers in tape heads of cassette players. They were used to transfer the magnetic information by direct contact with the magnetic tape. This data was then converted into electrical signals that were sent to loud speakers and then changed to sound format for the user to hear.

Other transducers that were used commonly were called immersion transducers and paintbrush transducers. Immersion type was used to measure energy in the form of sound, pressure and so on. Paintbrush transducers are also similar to the former, but they operate in air.

The one and only purpose to use a sensor is to convert an energy into a form that is noticeable by the user. To make this happen, a sensor may include a transducer as they are capable  of converting it from one form to another.

The simplest example of  a transducer is a Light Emitting Diode (LED) that converts light energy into its corresponding electrical energy. An example for a sensor is the sensors in bikes and cars that can sense the touch and also activates the sirens.

There are also cases when the sensor and transducer are the same. For example,a bi-metallic spring is used to measure the temperature change, and may well be the entire sensor if a pointer is attached to the bi-metalic spring.

Capacitive Transducers

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To learn about a capacitive transducer, it is important to know the basics of a parallel plate capacitor. Being the simplest form of a capacitor, it has two parallel conducting plates that are separated to each other by a dielectric or insulator with a permittivity of Ε (for air). Other than paper, vacuum, and semi-conductor depletion region, the most commonly used dielectric is air.

Due to a potential difference across the conductors, an electric field develops across the insulator. This causes the positive charges to accumulate on one plate and the negative charges to accumulate on the other. The capacitor value is usually denoted by its capacitance, which is measured in Farads. It can be defined as the ratio of the electric charge on each conductor to the voltage difference between them.

The capacitance is denoted by C. In a parallel plate capacitor, C = [A*Er*9.85*1012 F/M]/d

A – Area of each plate (m)

d – Distance between both the plates (m)

Er – Relative Dielectric Constant

The value 9.85*1012 F/M is a constant denoted by Eo and is called the dielectric constant of free space.
From the equation it is clear that the value of capacitance C and the distance between the parallel plates,d are inversely proportional to each other. An increase of distance between the parallel plates will decrease the capacitance value correspondingly. The same theory is used in a capacitive transducer. This transducer is used to convert the value of displacement or change in pressure in terms of frequency.

You may also like: Capacitance Transducer

As shown in the figure below, a capacitive transducer has a static plate and a deflected flexible diaphragm with a dielectric in between. When a force is exerted to the outer side of the diaphragm the distance between the diaphragm and the static plate changes. This produces a capacitance which is measured using an alternating current bridge or a tank circuit.

Capacitive Transducer
                                 Capacitive Transducer

A tank circuit is more preferred because it produces a change in frequency according to the change in capacitance. This value of frequency will be corresponding to the displacement or force given to the input.

Advantages

  • It produces an accurate frequency response to both static and dynamic measurements.

Disadvantages

  • An increase or decrease in temperature to a high level will change the accuracy of the device.
  • As the lead is lengthy it can cause errors or distortion in signals.

Linear Voltage Differential Transformer (LVDT)

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Like other inductive transducers, this transducer is also used for converting a linear motion into an electrical signal. The basic construction of an LVDT is explained and shown in the figure below.

Construction

LVDT Construction
                                                       LVDT Construction

The device consists of a primary winding (P) and two secondary windings named S1 and S2. Both of them are wound on one cylindrical former, side by side, and they have equal number of turns. Their arrangement is such that they maintain symmetry with either side of the primary winding (P). A movable soft iron core is placed parallel to the axis of the cylindrical former. An arm is connected to the other end of the soft iron core and it moves according to the displacement produced.

Working

As shown in the figure above, an ac voltage with a frequency between (50-400) Hz is supplied to the primary winding. Thus, two voltages VS1 and VS2 are obtained at the two secondary windings S1 and S2 respectively. The output voltage will be the difference between the two voltages (VS1-VS2) as they are combined in series. Let us consider three different positions of the soft iron core inside the former.

  • Null Position – This is also called the central position as the soft iron core will remain in the exact center of the former. Thus the linking magnetic flux produced in the two secondary windings will be equal. The voltage induced because of them will also be equal. Thus the resulting voltage VS1-VS2 = 0.
  • Right of Null Position – In this position, the linking flux at the winding S2 has a value more than the linking flux at the winding S1. Thus, the resulting voltage VS1-VS2 will be in phase with VS2.
  • Left of Null Position – In this position, the linking flux at the winding S2 has a value less than the linking flux at the winding S1. Thus, the resulting voltage VS1-VS2 will be in phase with VS1.

From the working it is clear that the difference in voltage, VS1-VS2 will depend on the right or left shift of the core from the null position. Also, the resulting voltage is in phase with the primary winding voltage for the change of the arm in one direction, and is 180 degrees out of phase for the change of the arm position in the other direction.

The magnitude and displacement can be easily calculated or plotted by calculating the magnitude and phase of the resulting voltage.

Difference output Voltage Vs Displacement Curve
Difference output Voltage Vs Displacement Curve

The graph above shows the plot between the resulting voltage or voltage difference and displacement. The graph clearly shows that a linear function is obtained between the output voltage and core movement from the null position within a limited range of 4 millimeter.

The displacement can be calculated from the magnitude of the output voltage. The output voltage is also displayed on a CRO or stored in a recorder.

Advantages

1. Maintains a linear relationship between the voltage difference output and displacement from each position of the core for a displacement of about 4 millimeter.

2. Produces a high resolution of more than 10 millimeter.

3.Produces a high sensitivity of more than 40 volts/millimeter.

4. Small in size and weighs less. It is rugged in design and can also be assigned easily.

5. Produces low hysteresis and thus has easy repeatability.

Disadvantages

1. The whole circuit is to be shielded as the accuracy can be affetced by external magnetic field.

2. The displacement may produce vibrations which may affect the performance of the device.

3. Produces output with less power.

4. The efficiency of the device is easily affected by temperature. An increase in temperature causes a phase shift. This can be decreased to a certain extent by placing a capacitor across either one of the secondary windings.

5. A demodulator will be needed to obtain a d.c output.

Proximity Inductive Transducers

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This type of transducer is used for finding the linear displacement in terms of voltage or other digital parameters. Another transducer for finding the linear displacement is the Linear Motion Variable Inductance Transducer.

There are mainly two types of proximity inductance transducers. They are

Mutual Inductance Type Proximity Transducer

This transducer consists of a primary and secondary coil. The constructional diagram of the transducer is shown below.

Proximity Inductive Transducers

Proximity Inductive Transducers

Working

An ac source is given to the primary. This ac source excites the primary and a flux is produced. This flux is linked to the secondary coil and thus a voltage ‘V’ is induced. If the mutual inductance between the primary and secondary windings is represented by ‘M’ (Hertz) and the frequency of ac excitation is represented by ‘w’, then the voltage ‘V’ developed in secondary coil can be written as V = MwIp.
Ip – The current due to excitation in primary (Amperes).

As shown in the figure, a ferromagnetic displacement plate is placed very near to the windings. The current in the primary coil produces a magnetic flux that links with the secondary coil through the displacement plate. Thus, the movement of the ferromagnetic plate to the right causes a greater value of flux linkage between the two terminals. This in turn causes an increase in the resulting output voltage across the secondary terminal with a value of (T1-T2). This output is given to the input of the CRO or a recorder and the amount of displacement can be known in terms of voltage. A

Advantages

1. The device is small when compared to other transducers.

2. Wear and tear is minimized as there is no physical contact between the target and coil configuration.

3. The output value will be accurate for small displacements as there is a linear relation between the output voltage and linear displacement.

4. There will not be any external effects by contamination.

5. The device works accurately even at higher temperatures up to 400 degree Celsius.

Disadvantages

1. The accuracy decreases when it comes to the measure of large displacement. This is because the linear relation between voltage and displacement is less at higher ranges.

2. The external magnetic field may cause harm to the ferromagnetic material.

Variable Reluctance Type Proximity Transducers

This device can be set up in two ways. Both the diagrams are shown below.

Variable Reluctance Type Proximity Transducers

Variable Reluctance Type Proximity Transducers

Working

The device consists of a coil that is wound on a core made up of ferromagnetic material. The displacement is given to the core through a target that makes an upward and downward movement according to the displacement produced. It does not touch the core of the coil and a smaller air gap is made between them.
When the target moves closer to the coil due to the displacement, the air gap becomes less causing the reluctance of the magnetic field to reduce and thus the coil inductance to increase. The value of inductance keeps on varying according to the variation in target movement. A CRO or a recorder takes these values and displays it to the user.

In the right side figure shown above, an E-type core is used for finding the displacement. The target is also pivoted at the central limb of the core. Thus, a single coil is divided into two turns and the end of each coil works as the arms of an inductance bridge.

As the displacement value changes, an output signal is produced. This is given to a CRO after amplification.

The biggest advantage of this device is that it shows a linear relationship between the output and the displacement.

 

 

 

Linear Motion Variable Inductance Transducer

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This transducer is used for displacement measurement. It is done by calculating the change in inductance in a single coil according to the variation in inductance. A schematic of the linear motion variable inductance transducer is shown below.

Linear Motion Variable Inductance Transducer
Linear Motion Variable Inductance Transducer

Working

The device consists of an arm that moves linearly according to the displacement produced. It also consists of a single coil wound on a former with ‘N’ number of turns. The end of the arm is connected to a soft iron core which moves linearly along the axis of the former. Thus, reluctance ‘R’ will be produced due to the flux path. The coil inductance of the device can be written by the equation, L= N2 /R.

A linear movement of the arm to the right decreases the reluctance ‘R’ of the flux path. Thus, according to the equation given above, the inductance increases due to the decrease in reluctance and vice versa. This inductance ‘L’ can be calculated or recorded with the help of an inductance bridge or through a recorder. Thus the measure of the displacement of the arm can be obtained from the corresponding change in inductance.
If the transducer is connected to the input of an oscillator tank circuit, the change in frequency ‘f’ of the oscillator could be taken as the measurement for the corresponding change in the displacement of the arm. A displacement of the arm changes the inductance and hence the frequency. Thus, the output can be measured in terms of inductance and frequency.

Advantages

1. There will not be problems due to mechanical hysteresis.

2. Provides a good response to static as well as dynamic measurements.

3. Provides a high output.

Disadvantages

1. The frequency response is controlled by the construction of force ring members.

2. Accuracy errors may occur due to the interference of external magnetic field.

Linear Potentiometer Transducer

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A linear potentiometer transducer consists of a potentiometer, which is short circuited by a slider. The other end of the slider is connected to a slider arm. The force summing device on the slider arm causes linear displacement of the slider causing the short circuit of a certain portion of the resistance in the potentiometer. Let the whole resistance positions on the potentiometer be ABC. Let the resistance position caused by the slider movement be BC. As the movement of the slider moves further to the right, the amount of resistance increases. This increase in resistance value can be noted according to the corresponding change in the linear displacement of the slider. The change in resistance can be calculated with the help of a Wheatstone bridge.

Another easy method than calculating the resistance with the help of a bridge connection is to connect a constant current source in series with the potentiometer. Thus a voltage will be developed. This voltage can be measured and hence the resistance, R = V/I.

Linear Potentiometer Transducer
Linear Potentiometer Transducer

Some of the most commonly used potentiometers for this purpose and their basic working is explained below.

1. Wire-Wound Potentiometer – The most commonly used resistance elements in this potentiometer are nickel, chromium or nickel copper. As these materials have a very low temperature coefficient of resistance, they can be used to handle large currents and also can be used up to 5 Hertz. They are also very cost effective. The winding of the resistance wire will depend on the different types of resistance changes due to the slider motion like linear, arithmetic, logarithmic and so on.

2. Cermet Potentiometer – This potentiometer is made from a material called Cermet which is made by mixing a paste of precious metal particles and a ceramic. Some of the most common mixtures used are palladium silver glass and palladium oxide glass. This device is used mostly for ac purposes as it has a low temperature coefficient of resistance and huge power ratings at high temperatures. Out of the lot, this device is mostly used as it is cost-effective.

3. Hot-Moulded Carbon Potentiometers – As the name implies, it is made by depositing a thin film of carbon and a thermosetting plastic binder. This device is mostly used for alternating current (ac) purposes.

4. Carbon Film Potentiometers – This potentiometer is made by coating a thin layer of carbon film on a non-conductive base. The temperature coefficient of resistance of this device is 1000 x 10-6 ohms/degree Celsius.

5. Thin Metal Film Potentiometer – This device is in the form of a thin vapour deposited layer of metal on glass or ceramic base. This is also used for ac applications.

Advantages

  • Cost-effective
  • Simple design and simple working
  • Can be used for measuring even large displacements.
  • The device produces a large output and hence can be used for control purposes without further amplification steps. Thus the whole operation is bounded to a single device.
  • Can produce a high electrical efficiency.
  • All devices other than wire-wound potentiometer can be used for a large frequency range.
  • Except wire wound, all other potentiometers can provide excellent resolutions.

Disadvantages

  • A huge force may be required for the slider movement.
  • Can produce unwanted noise due to alignment problems, wear and tear of the sliding contact. This may also affect the total life of the device.

Transducers

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A transducer is a device that is used to convert a physical quantity into its corresponding electrical signal.

In most of the electrical systems, the input signal will not be an electrical signal, but a non-electrical signal. This will have to be converted into its corresponding electrical signal if its value is to be measured using electrical methods.

The block diagram of a transducer is given below.

Transducer Block Diagram
                                                  Transducer Block Diagram

A transducer will have basically two main components. They are

1. Sensing Element

The physical quantity or its rate of change is sensed and responded to by this part of the transistor.

2. Transduction Element

The output of the sensing element is passed on to the transduction element. This element is responsible for converting the non-electrical signal into its proportional electrical signal.

There may be cases when the transduction element performs the action of both transduction and sensing. The best example of such a transducer is a thermocouple. A thermocouple is used to generate a voltage corresponding to the heat that is generated at the junction of two dissimilar metals.

Selection of Transducer

Selection of a transducer is one of the most important factors which help in obtaining accurate results. Some of the main parameters are given below.

  • Selection depends on the physical quantity to be measured.
  • Depends on the best transducer principle for the given physical input.
  • Depends on the order of accuracy to be obtained.

Transducer Classification

Some of the common methods of classifying transducers are given below.

  • Based on their application.
  • Based on the method of converting the non-electric signal into electric signal.
  • Based on the output electrical quantity to be produced.
  • Based on the electrical phenomenon or parameter that may be changed due to the whole process. Some of the most commonly electrical quantities in a transducer are resistance, capacitance, voltage, current or inductance. Thus, during transduction, there may be changes in resistance, capacitance and induction, which in turn change the output voltage or current.
  • Based on whether the transducer is active or passive.

Transducer Applications

The applications of transducers based on the electric parameter used and the principle involved is given below.

1. Passive Type Transducers

a. Resistance Variation Type

  • Resistance Strain Gauge – The change in value of resistance of metal semi-conductor due to elongation or compression is known by the measurement of torque, displacement or force.
  • Resistance Thermometer – The change in resistance of metal wire due to the change in temperature known by the measurement of temperature.
  • Resistance Hygrometer – The change in the resistance of conductive strip due to the change of moisture content is known by the value of its corresponding humidity.
  • Hot Wire Meter – The change in resistance of a heating element due to convection cooling of a flow of gas is known by its corresponding gas flow or pressure.
  • Photoconductive Cell – The change in resistance of a cell due to a corresponding change in light flux is known by its corresponding light intensity.
  • Thermistor – The change in resistance of a semi-conductor that has a negative co-efficient of resistance is known by its corresponding measure of temperature.
  • Potentiometer Type – The change in resistance of a potentiometer reading due to the movement of the slider as a part of an external force applied is known by its corresponding pressure or displacement.

b. Capacitance Variation Type

  • Variable Capacitance Pressure Gauge – The change in capacitance due to the change of distance between two parallel plates caused by an external force is known by its corresponding displacement or pressure.
  • Dielectric Gauge – The change in capacitance due to a change in the dielectric is known by its corresponding liquid level or thickness.
  • Capacitor Microphone – The change in capacitance due to the variation in sound pressure on a movable diagram is known by its corresponding sound.

c. Inductance Variation Type

  • Eddy Current Transducer – The change in inductance of a coil due to the proximity of an eddy current plate is known by its corresponding displacement or thickness.
  • Variable Reluctance Type – The variation in reluctance of a magnetic circuit that occurs due to the change in position of the iron core or coil is known by its corresponding displacement or pressure.
  • Proximity Inductance Type – The inductance change of an alternating current excited coil due to the change in the magnetic circuit is known by its corresponding pressure or displacement.
  • Differential Transformer – The change in differential voltage of 2 secondary windings of a transformer because of the change in position of the magnetic core is known by its corresponding  force, pressure or displacement.
  • Magnetostrictive Transducer – The change in magnetic properties due to change in pressure and stress is known by its corresponding sound value, pressure or force.

d. Voltage and Current Type

  • Photo-emissive Cell – Electron emission due to light incidence on photo-emissive surface is known by its corresponding light flux value.
  • Hall Effect – The voltage generated due to magnetic flux across a semi-conductor plate with a movement of current through it is known by its corresponding value of magnetic flux or current.
  • Ionisation Chamber – The electron flow variation due to the ionisation of gas caused by radio-active radiation is known by its corresponding radiation value.

2. Active Type

  • Photo-voltaic Cell – The voltage change that occurs across the p-n junction due to light radiation is known by its corresponding solar cell value or light intensity.
  • Thermopile – The voltage change developed across a junction of two dissimilar metals is known by its corresponding value of temperature, heat or flow.
  • Piezoelectric Type – When an external force is applied on to a quartz crystal, there will be a change in the voltage generated across the surface. This change is measured by its corresponding value of sound or vibration.
  • Moving Coil Type – The change in voltage generated in a magnetic field can be measured using its corresponding value of vibration or velocity.

 

Instrument Parameters

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Units and Standards

In order to avoid confusion and to obtain a consistent result, a set of units and standards have been commonly followed by all countries. Each instrument used is given a separate symbol which makes it easier for its identification and also for process control drawings. All the lists have been developed by The Instrument Society of America (ISA) and is being used worldwide.

The units that are used for the measurement f different variables fall mainly under two categories. One is the International system, SI (Systéme International D’Unités) and the other is the English system. The problem is that the latter is followed by very few countries including USA, but the former is followed by most of the other countries.

Parameters

There are some parameters that are to be checked during a process. They are all explained below.

  • Accuracy – It is defined as the difference between the indicated value and the actual value. The actual value may be a known standard and accuracy is obtained by comparing it with the obtained value. If the difference is small accuracy is high and vice versa.  Accuracy depends on several other parameters like hysteresis, linearity, sensitivity, offset, drift and so on. It is usually expressed as a percentage of span, percentage of reading or even absolute value. The standard value is set by the government so as to maintain the standard.
  • Reading accuracy is the deviation from true at the point the reading is being taken and is expressed as a percentage. Absolute accuracy of an instrument is the deviation from true as a number not as a percentage.
  • Span – It can be defined as the range of an instrument from the minimum to maximum scale value. In the case of a thermometer, its scale goes from −40°C to 100°C. Thus its span is 140°C. As said before accuracy is defined as a percentage of span. It is actually a deviation from true expressed as a percentage of the span.
  • Precision – It may be defined as the limits within which a signal can be read. For example if you consider an analog scale, which is set to graduate in divisions of 0.2 psi, the position of the needle of the instrument could be estimated to be within 0.02 psi. Thus the precision of the instrument is 0.02 psi.
  • Range – It can be defined as the measure of the instrument between the lowest and highest readings it can measure. A thermometer has a scale from −40°C to 100°C. Thus the range varies from −40°C to 100°C.
  • Reproducibility – It can be defined as the ability of an instrument to produce the same output repeatedly after reading the same input repeatedly, under the same conditions.
  • Sensitivity – It can also be called as the transfer function of a process. It is the ratio between the change in the output of an instrument to the corresponding change in the measured variable. For a good instrument or process, the sensitivity should always be high, thus producing higher output amplitudes.
  • Offset – Offset is the reading of an instrument with zero input.
  • Drift – Drift is the change in the reading of an instrument of a fixed variable with time.
  • Hysteresis – It can be defined as the different readings taken down when an instrument approaches a signal from opposite directions. That is the corresponding value taken down as the instrument moves from zero to midscale will be different from that between the midscale and full scale reading. The reason is the appearance of stresses inside the instrument material due to the change of its original shape between the zero reading and the full scale reading.
Hysterisis

Hysteresis

  • Resolution – It is the smallest difference in a variable to which the instrument will respond.
  • Repeatability – It is a measure of the closeness of agreement between a number of readings (10 to 12) taken consecutively of a variable, before the variable has time to change. The average reading is calculated and the spread in the value of the readings taken.
  • Linearity – It can b defined as a measure of the proportionality between the actual values of a variable being measured to the output of the instrument over its operating range.

Instrumentation System

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The basic need of instrumentation in a process is to get the best and most amount of information so as to successfully complete the process. When referring to the completion of the project with reference to instrumentation, it basically means maximum efficiency with minimum production expense and desired output quality.

The information that is achieved from these processes may be very simple and may mostly involve a direct measurement method. But as the process becomes more complex, direct measurement may seem to be impracticable and so indirect methods must be used for measurements. These methods involve a derived relationship between the measured quantity and the result that is needed.

Most of the indirect methods involve electrical techniques as they have high speed and also simple processing methods. The output from such methods is easier to link to computers.

The obtained information may not necessarily be the direct value of a measured quantity. That is, the value obtained may be a variation of the value with respect to other parameters. It may also be a signal corresponding to the end limit. It could also be a specific value with an indicating hand over a suitable scale. Thus, one instrument may be needed to perform the required operations individually or a number of them at a time.

When it comes to industrial measurements, the measurands are all physical variables which is used to determine the flow of energy in these dynamical units. If so, they can be classified as

1. Flow through or per- variables

Flow trough variables can be measured from a single point in space. Some of the most measured variables using this method are force, momentum, flow, charge, current, volume and so on.

2. Across or trans-variables

Trans-variables need a referencing point and a measuring point. Some of the measured variables are displacement, velocity, pressure, temperature, level and voltage.

Instrumentation Systems

If we are mentioning instrumentation systems based on industrial applications it can be broadly classified into two. They are automatic type and manual type. The former works automatically without any help and the latter will need the assistance of an operator. If viewed from the system design view, the instruments will be classified into self-operated type and power operated type.

Whatever maybe the performance of an instrument, there will be some basic building blocks for its functioning. The correct combination of these blocks in a measurement system helps in converting a process condition into a suitable indication.

These blocks are also called as functional units and are present in all instrumentation systems.

All together, instrumentation systems can be classified into two. They are

1. Analog Instrumentation System

The block diagram is shown below.

An analog instrumentation system includes three functional units. They are

Analog Instrumentation System
                                   Analog Instrumentation System
  • The Primary Element/Transducer

The input receives the quantity whose value is to be measured and is converted into its proportional incremental electrical signal such as voltage, current, resistance change, inductance or even capacitance. Thus, the changed variable contains the information of the measured variable. Such a functional element or device is called a transducer.

  • The Secondary Element/Signal Processing Unit

The output of the transducer is provided to the input of the signal processing unit. This unit amplifies the weak transducer output and is filtered and modified to a form that is acceptable by the output unit. Thus this unit may have devices like: amplifiers, filters, analog to digital converters, and so on.

  • The Final Element/Output Unit

The output from the signal processing unit is fed to the input of the output unit. The output unit measures the signal and indicates the value to the reader. The indication may be either through: an indicating instrument, a CRO, digital computer, and so on.

2. Digital Instrumentation System

All the functional units that were used in an analog system will also be used here. He basic operation in a digital system includes the handling of analog signals, making the measurements, converting and handling digital data, programming and also control. The block diagram and functional units are given below.

Digital Instrumentation System
                                        Digital Instrumentation System
  • Transducer

All the physical input parameters like temperature, pressure, displacement, velocity, acceleration and so on will be converted into its proportionate electrical signal.

  • Signal Conditioning Unit

This working of this unit is exactly the same as that of a signal processing unit in an analog instrumentation system. It includes all the balancing circuits ad calibrating elements along with it.

  • Scanner/Multiplexer

Multiple analog signals are received by this device and are sequentially provided on to a measuring instrument.

  • Signal Converter

It is used to convert an analog signal to a form that is acceptable by the analog to digital converter.

  • Analog to (A-D) Digital Converter

The analog signal is converted into its proportional digital signal. The output of an A-D converter is given to a digital display.

  • Auxiliary Equipment

All the system programming and digital data processing functions are carried out by this unit. The auxiliary equipment may be a single computer or may be a collection of individual instruments. Some of its basic functions include linearizing and limit comparison.

  • Digital Recorder

It is mostly a CRO or a computer.