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Transduser

Dr Ir Dina Maizana MT

[email protected]

Mari kita berdoa menurut agama dan kepercayaan masing-masing sebelum kelas dimulai.

Doa dimulai…

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Jadwal Kandungan Jam

Minggu-1 Pendahuluan, penyampaian kontrak kuliah, Konsep-konsep pengukuran, Kesalahan-kesalahan pembacaan alat ukur

Selasa (9.40-12.10 wib) R. III.3 Selasa (19.00-20.30 wib) R. A.II.6

Minggu-2 Satuan pengukuran dan besaran standar listrik, Nilai dan fungsi satuan

Minggu-3 Besaran listrik, Alat ukur dengan termokopel, besi putar, elektrodinamis, elektrostatis dan induksi.

Minggu-4 Instrument penunjuk arus searah, Volt Ammeter DC, Prinsip kerja, Cara kerja, Penggunaan alat ukur DC

Minggu-5 Instrument arus bolak-balik, Voltmeter elektrostatis, Prinsip Kerja,Cara kerja, Penggunaan alat ukur untuk AC

Minggu-6 Pengukuran daya, Wattmeter, Pengukuran daya tanpa Wattmeter, Type alat pengukur daya

Minggu-7 Penggunaan jembatan Wheatstone, Prinsip dari jembatan wheatstone, Contoh dari jembatan wheatstone

Minggu-8 UTS

Minggu-9 Pengukuran dengan alat ukur oscilloscope

Minggu-10 Generator sinyal

Minggu-11 Alat-alat ukur digital

Minggu-12 Trafo Instrumentasi, Trafo arus untuk alat ukur, Trafo tegangan alat ukur, Kwh meter.

Minggu-13 Transduser

Minggu-14 Konversi Data analog ke digital

Minggu-15 Sistem data akusisi

Minggu-16 UAS

Rencangan Pembelajaran Semester (RPS) Sem. A ta 2019/20

CPMK

• Mahasiswa mampu menjelaskan transduser

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Content:

1. Introduction

2. Temperature Sensor

3. Optical Sensor

4. Other Sensor

Lesson Outcomes

At the end of the lesson, students should be

able to:

• define basic concept of transducers and sensors

• apply in-depth knowledge in transducer's and sensor’s application

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Objectives:

1. To get familiarize with several types of transducers

and selection criteria.

2. Able to apply basic principles of operation and

application of common transducer.

Introduction

Content:

• 1. Definition of Transducer

• 2. Types of Transducers

• 3. Application of Transducers

• 4. Advantage of Electrical

• 5. Transducers

• 6. Classification Of Transducers

• 7. Selecting A Transducers

• 8. Parameter

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Microphone Speaker Generator

DEFINITION OF TRANSDUCER & SENSORS

Transducer - device that converts one form of energy into

another form of energy sensor - device that measures a physical quantity and converts it

into a signal which can be read by an observer or by an instrument.

Sound > Electric Electrical > Sound Mechanical motion > Electrical Signal

Electrical Transducers

– Converts the input measurand into an electrical voltage/current

Mechanical Transducers

– Converts the input measurand into a mechanical energy

TYPES OF TRANSDUCERS

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Electrical Sensor

Device that capable to detect electrical signal and sent it to another

APPLICATION OF TRANSDUCERS

TRANSDUCER measurand electrical output

excitation

Electrical amplification and attenuation can be easily done

Mass-inertia effects are minimized

Effect of friction are minimized

ADVANTAGES OF ELECTRICAL TRANSDUCERS

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The output can be indicated and recorded remotely at a distance from the sensing medium

The output can be modified to meet the requirements of indicating or controlling units


The signal can be conditioned or mixed to obtain any combination with outputs of similar transducers or control signal


"Strive always to excel in virtue and truth." (Bukhari)

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Active Transducer

– Do not requires external power produce an analog voltage

CLASSIFICATION OF TRANSDUCERS

Active Transducer measurand electrical output

Passive Transducer

– Require external power source to operate

CLASSIFICATION OF TRANSDUCERS

Passive Transducer measurand electrical output

external power

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Operating Range Maintain range requirements and good resolution

Sensitivity Sensitivity enough to allow sufficient output

Environment Compatibility Ability to make it applicable and interactions

Accuracy Subject to repeatability and calibration error

Physical Condition Depend on its usage

Electrical Length and type of cable is required

SELECTING A TRANDUCER

Linearity Relationship between physical parameter and resulting electrical signal must be linear

Sensitivity Defined as the electrical output per unit change in physical parameter

Dynamic Range Operating range should be wide to permit it use under wide range of measurement condition

Repeatability Input or output relationship for a transducer should be predictable over a long period of time

Physical Size Minimum weight and volume

PARAMETER

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Potentiometer

• Electromechanical device containing a resistance element that is contacted by a movable slider

• The motion of the movable slider may be translatory or rotational.

• The output voltage of the position of the movable slider and is

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• Example: A displacement transducer with a shaft stroke of 30 cm is applied to the circuit. The total resistance of the potentiometer is 5k Ω . The applied voltage VS is 5V. Calculate the output voltage when the wiper is 9cm from B.

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Example:

• A potentiometer transducer with a shaft stroke of 8.0cm is used in circuit below. The applied voltage is 10V. The total resistance of potentiometer R1 and R2 is 6 kΩ. The total resistance of the potentiometer R3 and R4 is 4 kΩ. The initial position to be used as a reference point is set such that R1 is 4.5 cm and R3 is 3.5 cm of the shaft stroke length (from point A).

• i. Calculate the values of R1, R3 and VE at initial position

• ii. Calculate the displacements of potentiometer R3 and R4 in the case that VE =0.Then identify the direction of the displacement


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Exercise: A displacement transducer with a shaft of 2.0mm is used in the circuit as shown in figure below. The total resistance of the potentiometer R1 and R2 is 5000Ω and the applied voltage is 5.0V. The total resistance of the potentiometer R3 and R4 is also 5000Ω.The initial position to be used as reference point is set such that R1 = R2 (i.e. when the shaft is at mid-stroke). Initially, potentiometer R3 and R4 is adjusted so that the bridge is balanced (i.e. VE = 0). Assuming the shaft of the potentiometer R3 and R4 will be moved 0.5mm towards A, what is the value of VE?

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• Potentiometer senses displacement by means of sensing shaft, which is mechanically connected to the point or objects whose displacement, is to be measured.

Example: Petrol-tank level indicator.In this case, potentiometer is used to indicate/sense the petrol level in a tank as shown in Figure below. The output signal (voltage) is proportional to the petrol level,v

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Advantages & Disadvantages of Potentiometer

Capacitive transducer • A capacitor consists of two parallel plates separated

by an air space or by a dieletric (insulating material) as shown in figure below

• The capacitance of the pair of plates is measure of the amount of charge that can be transferred before a certain voltage is reached.

• If the capacitance is large, more charge is needed to establish a given voltage difference

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• The equation for capacitance of a parallel plate capacitor is given by:-

• the capacitive transducer works on the principle of changing of capacitance which may caused by:

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• Advantages :

1. Required extremely small forces to operate them and hence are very useful for use in small systems.

2. Extremely sensitive.

3. A good frequency response as high as 50kHz and useful for dynamic studies.

4. High input impedance therefore the loading effects are minimum.

5. The force requirements is very small and therefore require small power to operates them

• Disadvantages:

1. The metallic parts of the transducer must be insulated from each other in order to reduce the effects of stray capacitance, the frames must be earthen.

2. The output impedance of the capacitive transducers tends to be high on account of their small capacitance value this leads to loading effects.

Uses of Capacitive Transducer 1. It can be used for measurement of both linear and

angular displacements. 2. It can be used for measurement of force and

pressure. The force and pressure to be measured are first converted to displacement which caused a change in capacitance.

3. It can be used for measurement of humidity in gases since the dielectric constant of gases changes with change in humidity thereby producing a change in capacitance.

4. It is commonly used in conjunction with mechanical modifiers for measurement of volume, density, liquid level, weight and etc.


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Capacitive sensor is used to detect the presence of boxes on the conveyor belt

• Example: A capacitive transducer is used for the measurement of linear displacement, X, as shown in below. The parallel plate has a dimension of 5.0cm X 5.0cm and is separated by a distance of 1.0cm. The space between the plates is filled with a dielectric material of 1.0cm thick, which has a dielectric constant of 4.0. If the dielectric constant for air is 1.0cm, determine the value of the capacitance when x is equal to:

(i) 0.0cm

(ii) 2.0cm

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• Exercise:Figure below shows a capacitive transducer used for measurement of linear displacement, x. the parallel plates have a dimension of (4.0 cm x 4.0cm) and separated by a distance of 10 mm. the space between plates is filled with a dielectric material with constant of 3.0.If the dielectric constant for air is 1.0, determine the value of the capacitance when x is equal to:

• i) 0.0 cm

• ii) 2.0 cm

• iii) 4.0 cm

• What is the effect of capacitance when the displacement of dielectric is increased? Given εo = 8.854 x 10-12 F/m.

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Thermocouple • Thermocouple -thermal transducer.

• It consists of a pair of wire made of different metals that joined together at one end as shown in below.

• When there is a temperature difference between the two ends of wire, a voltage will be produced between the two wires – Seeback effect

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• The magnitude of voltage depends on :

i) the materials used for the wires

ii) the temperature difference between the joined ends and the other ends.

• The voltage of the thermocouple is given as

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• Normally the cold / reference temperature is set to 0oC as shown in figure below:

Cold junction compensation

• Thermocouple tables give the relationship between the voltage for a particular type of thermocouple and the measured temperature when the reference junction is at a particular reference temperature

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Thermistor

Thermistors - THERMally sensitive resISTOR are non-metallic resistors(semiconductor material) made by sintering mixtures of metallic oxides such as manganese, nickel, cobalt, copper and uranium.

Thermistor - type of resistance thermometer, uses the change in the electrical resistance to determine the temperature.

Thermistors have a Negative Temperature Coefficient (NTC) – resistance decrease as temperature rises as shown in below.

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Strain gauge • Strain gauge - passive transducer that uses the variation in

electrical resistance in wires to sense the strain produced by force on wires.

• It is used for measuring weight, pressure, mechanical force and displacement.

• A bonded strain gauge consists of a fine wire looped back and forth on a mounting plate which is usually cemented to the member undergoing stress as shown below

Bonded strain gauge

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• Strain gauge is generally uses as one arm of a bridge is shown Figure below.

• This method is capable to measure the change in resistance when the wire is under strain.

• In some cases, strain gauges are used in pairs (active gauge and dummy gauge) to provide temperature compensation as in Figure below. However, only the active gauge will respond to stress.

• The dummy gauge is mounted in an insensitive orientation to provide some compensation for temperature effects.

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Strain gauge 1 is stretch Strain gauge 2 is compressed

APPLICATION OF STRAIN GAUGE

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EXAMPLE OF STRAIN GAUGE

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• The strain will cause:

• i) The change in length ΔL

• ii) The change in gauge resistance ΔR


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Linear Variable Differential Transformer

-Inductive position sensor • When an AC excitation signal is applied to the Primary Coil (P), voltages

are induced in the two Secondary Coils (S). The MAGNETIC CORE inside the COIL WINDING ASSEMBLY provides the magnetic flux path linking the Primary and secondary Coils.

• Since the two voltages are of opposite polarity, the Secondary Coils are connected series opposing in the center, or Null Position. The output voltages are equal and opposite in polarity and, therefore, the output voltage is zero. The Null Position of an LVDT is extremely stable and repeatable.

• When the MAGNETIC CORE is displaced from the Null Position, an electromagnetic imbalance occurs. This imbalance generates a differential AC output voltage across the Secondary Coils which is linearly proportional to the direction and magnitude of the displacement.

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• As shown in the figure, when the MAGNETIC CORE is moved from the Null Position, the induced voltage in the Secondary Coil, toward which the Core is moved, increases while the induced voltage in the opposite Secondary Coil decreases.

• LVDTs possess the inherent ruggedness and durability of a transformer and truly provide infinite resolution in all types of environments. As a result of the superior reliability and accuracy of LVDTs, they are the ideal choice for linear motion control.

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• Advantages -LVDT compared to a resistive potentiometer are that its linearity, that is its voltage output to displacement is excellent, very good accuracy, good resolution, high sensitivity as well as frictionless operation and is sealed against hostile environments

Example

• An AC LVDT has the following data. Input = 6.3V, output=5.2V range +/- 0.5 in. Determine:

a) Calculate the output voltage vs core position for a core movement going from +0.45 in to -0.30in.

b)The output voltage when the core is -0.25in from the centre.

Solution

a)0.5 in core displacement produces 5.2V,therefore a 0.45 in core movement produces(0.45x5.2)/0.5 = 4.68V

At -0.30 in core movement produces

(-0.30x-5.2)/(-0.5) = -3.12V

b) -0.25 in core movement produces

(-0.25x-5.2)/(-0.5) = -2.6V

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Resistive Temperature Detectors (RTD).

RTD's are precision temperature sensors made from high-purity conducting metals such as platinum, copper or nickel wound into a coil and whose electrical resistance changes as a function of temperature, similar to that of the thermistor

Also available are thin-film RTD's. These devices have a thin film of platinum paste is deposited onto a white ceramic substrate.

They have poor sensitivity, that is a change in temperature only produces a very small output change for example, 1Ω/oC.

• RTD is a resistive device - need to pass a current through them and monitor the resulting voltage.

• Any variation in resistance due to self heat of the resistive wires as the current flows through it, I2R, (Ohms Law) causes an error in the readings.

• To avoid - RTD is usually connected into a Whetstone Bridge network which has additional connecting wires for lead-compensation and/or connection to a constant current source.

• Relationship between temperature and resistance of conductors :-

» Rt=Rref(1+αΔt)

Rt – resistance of conductor at temperature t degree

Rref – resistance of the reference temperture, usually 0 degree

α – temperature coefficient of resistance

Δt – difference between operating and reference temperature

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Exercise

• A platinum resistance thermometer has resistance of 180Ω at 20 degree Celsius. Calculate its resistance at 60 degree Celsius. (α20 = 0.00392) ans:151.78 Ω

• A platinum resistance thermometer has a resistance of 100 Ω at 23 degree Celsius. Find its resistance at 50 degree Celsius. The resistance temperature coefficient of platinum is 0.00392 Ω/ Ω celsius. If the thermometer has a resistance of 200 Ω, calculate the value of temperature.

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