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Mechatroncis PDF UNIT -2 Sensors & Transducers.pdf
- 1. UNIT -2 Sensors and Transducers Dr. Shimi S.L Associate Professor, EE PEC(Deemed to be University), Chandigarh
- 2. • Sensors and transducers: Introduction, Performance characteristics of transducers, Transducer for displacement (Potentiometer, strain-gauge, Optical encoder, LVDT, Hall effect sensor); velocity (Tachogenerator), force (load cell), pressure (Piezoelectric sensors, Tactile sensor), liquid level (Floats, Differential pressure), Temperature (Bimetallic strips, RTDs, Thermistors, Thermocouples) and light sensor (Photovoltaic- transducer, LDR, Photodiode, Photo Transistor).
- 3. Learning outcomes • To explain and analyze the fundamental performance characteristics of transducers. • Demonstrate a deep understanding of transducers used for measuring displacement, velocity, force, pressure, liquid level, temperature and light . • Conduct laboratory sessions to work with actual transducers and sensor setups. • Develop the ability to integrate theoretical knowledge with practical applications in designing and implementing sensor systems for various engineering purposes. • Encourage collaborative projects where students design and build sensor systems and presenting clear justifications for their choice of transducers in specific engineering applications.
- 4. • An intelligent system works sequentially in closed loop system with measurement, comparison and decision followed by action. • Measurement being at the first stage; role of sensor/transducer is very important as sensor(s)/transducer(s) is responsible for collecting information regarding physical parameter(s) such as pressure, temperature ,voltage ,humidity ,force, distance, etc. 1.Sensors in Mechatronics System
- 5. • If sensor is selected properly as per requirement, data/information will be more accurate and system is expected to work properly if other stage components are good. • Choice of sensor depends on objective of the system under development, application, precision, working environment, cost, reliability, size, power consumption.
- 7. A sensor in response to a physical quantity /phenomenon such as force, temperature, speed, produces a corresponding proportional change in physical phenomena, such as electrical, mechanical and magnetic etc. in terms of resistance, inductance, capacitance, voltage and current. 2.Difference between Sensors & Transducers
- 8. • Transducer: It converts the change in one form of energy into a change in another form ( electrical) of energy. ie. converts the output signal of a sensor into an electrical signal. Transduction Sensing or detecting element and the transduction elements.
- 9. It includes sensors and transductor or transducer which receives energy from environment /system parameter or its change to produce an analog signal. Sensing Mechanism
- 10. Mercury thermometer; the mercury expands easily when the temperature increases to send a reading format for the user. There are no electrical signs or changes. So, it is originally a sensor. Sensor
- 11. •Loud speaker takes electrical energy in the form of electrical signals and converts it into sound waves that we can hear. •The electromechanical system of a speaker enables this conversion. It comprises several components, including voice coils, magnets, diaphragms, and a suspension system. Transducers
- 12. Here’s how it works: • An input signal (electrical energy) is sent to the speaker. • The voice coil interacts with the magnetic field created by the magnets. As a result, the speaker cone (diaphragm) moves back and forth. • This motion of the cone and suspension system converts mechanical energy into acoustic energy—the sound waves we perceive Transducers Transducers
- 14. General Classification of Transducers
- 15. • Classification based on the Principle of Transduction Classified by the transduction medium. The process of conversion of energy from one form to another is called transduction. The transduction medium may be resistive, inductive or capacitive depends on the conversion process / converts the input signal into resistance, inductance and capacitance resp ectively.
- 16. Primary and Secondary Transducers • Some transducers contain the mechanical as well as electrical device. • The mechanical device converts the physical quantity to be measured into a mechanical signal. • Such mechanical device are called as the primary transducers, because they deal with the physical quantity to be measured. • The electrical device then convert this mechanical signal into a corresponding electrical signal. Such electrical device are known as secondary transducers.
- 17. Primary transducers are mechanical devices that detect a physical quantity and convert it into a measurable signal. • Bourdon tube • Diaphragm • Bellows • Bimetallic thermometer • Liquid filled thermometer
- 18. Primary and Secondary Transducers • Ref fig in which the diaphragm act as primary transducer. • It convert pressure (the quantity to be measured) into displacement (the mechanical signal). • The displacement is then converted into change in resistance using strain gauge. • Hence strain gauge acts as the secondary transducer. Diaphragm
- 19. Analog and Digital Transducer Classified by output signals. It may be continuous or discrete. Analog Transducer – The Analog transducer changes the input quantity into a continuous function. Eg. strain gauge, L.V.D.T, thermocouple, thermistor Digital Transducer – These transducers convert an input quantity into a digital signal or in the form of the pulse(high or low power). Eg. shaft encoders, limit switches, digital tachometers
- 20. • Light Sensor used to Produce a Digital Signal The speed of the rotating shaft is measured by using a digital LED/Opto-detector sensor.
- 21. • Active transducer – No additional source for excitation signal is required. Develops theirs owns voltage or current. Piezoelectric sensors, photo diode, thermocouple, dynamos • Passive transducer - requires the power from an external supply source - RTDs, thermistors, photoconductive devices, PN junction diode, hall effector sensors opto-electronic devices, strain gauge
- 22. Resistance Temperature Detector (RTD)
- 23. Performance characteristics of transducers • Quality parameters of a sensor system (Selection of sensors) • Sensitivity: It is the ability of the measuring instrument to respond to changes in measured quantity. It is ratio of change of output to unit change of input parameter. S = ΔO / Δ I • I - Input, quantity to be sensed • O - Output, signal which can be recorded
- 24. Range : Maximum and minimum limits of physical variables under measurement. Pressure sensor may have a range of -400 to +400 mm Hg.
- 25. Span : This gives the difference of a sensor parameter variable measured from the maximum to minimum input values of a sensor. Pressure transmitter maybe re-scaled to read 4mA at 0.8 bar and 20 mA at 1.2 bar, span ranging from 0 to 0.4 bar
- 27. Resolution: It is defined as the smallest increment in the measured value that can be detected.
- 28. Error: It is the difference between measured values of a physical parameter and the true value of the same parameter. Systematic error, Random error and Gross error
- 29. Systematic error - instrument error cause due to misuse, loading, instrumental defect, environmental error, dust, temperature Random error – noise (unaware factors) Gross error - loose connections, human while reading, recording and estimating
- 30. Accuracy: Closeness of the measured value of a variable to its true value. Error in absolute term= Measured parameter value – true parameter value Error in relative term= Absolute error True value
- 32. • Precision: Precision is the ability of an instrument to reproduce a certain set of readings within a given deviation. • Repeatability: It is the ability to reproduce the output signal exactly when the same measured quantity is applied repeatedly under the same environmental conditions.
- 34. • Stability (Drift):It is the ability to give same output when a constant input is measured over a period of time. • Dead band: It is the range of input values for which there is no output.
- 35. Hysteresis: The difference between two output values that correspond to the same input depending on the trajectory followed by the sensor (i.e., magnetization in ferromagnetic materials)
- 36. ]
- 37. Hysteresis: • Hysteresis is defined as the magnitude error caused in the output for a given value of input, when this value is approached from opposite directions ; i.e. from ascending order & then descending order. Causes are backlash, elastic deformations, magnetic characteristics, frictional effects (mainly). • Hysteresis can be eliminated by taking readings in both direction and then taking its arithmetic mean.
- 38. • Backlash: Hysteresis caused by looseness in a mechanical joint. The maximum distance or angle through which any part of a mechanical system can be moved in one direction without causing any motion of the attached part.
- 39. Static and Dynamic Characteristics of a Sensor System • Static characteristics are the values given when steady state conditions occur. Input is not varying and output is constant. Output changes only due to drift. • Dynamic characteristics refer to time varying signal with corresponding time varying output.
- 40. Dynamic Characteristics of a Sensor System
- 41. Response time: This is the time which elapses after a step input, when the transducer gives the output corresponding to 95 percentage of steady state value. Time constant: Time taken between application of input and 63.2 % of steady state value.
- 42. •Rise time: Time taken for the output to rise to some specified percentage of the steady state output. From 10% to 90%. •Settling time: This is the time taken for the output to settle within some percentage e.g. 2% of steady state value.
- 45. Resistive Transducers: Change in force/displacement causes change in resistance. Potentiometric Transducers types (i) Linear (ii)Rotary (iii)Helix
- 46. Displacement and Position measurement
- 47. Change in force causes change in resistance proportional to strain experienced by object. Strain Gauge Types:(i)Felt (ii)Helical (iii)Foil types Strain Gauge Bonded resistance type
- 49. Stress: Force experienced by an object per unit Area in order to distribute the internal force to resist the external force applied uniformly on the object. Strain: Amount of deformation ie. Changes in per unit length of an object under a force. 𝑆𝑡𝑟𝑒𝑠𝑠 = 𝜎 = 𝐹 𝐴 𝑆𝑡𝑟𝑎𝑖𝑛 =∈= ∆𝐿 𝐿 Gauge factor: 𝐺𝐹 = ∆𝑅 𝑅 ∆𝐿 𝐿 = ∆𝑅 𝑅 𝑆𝑡𝑟𝑎𝑖𝑛
- 50. • Length 0.008 to 4 in • Resistance 120-5000 ohms • (30Ω to 3 kΩ unstressed)
- 53. It requires a Wheatstone bridge arrangement to convert change in resistance into voltage . Measurement using Strain Gauge
- 54. Inductive Transducers: Linear Variable Differential Transducer (LVDT) / Transformer: Change in force /displacement causes change in voltage. Main Parts: (i)Primary coil (ii) S1&S2 secondary coils (iii) movable iron core (Annealed NiH2 magnetic material with high permeability). Frequency range (1-10kHz)
- 56. 𝑉 𝑠 = 𝑀 𝑑𝑖 𝑑𝑡 𝑉𝑠1 = 𝐾1sin(𝑤𝑡 − ∅) 𝑉𝑠2 = 𝐾2sin(𝑤𝑡 − ∅) 𝑉 𝑠 = 𝑉𝑠1−𝑉𝑠2= (𝐾1−𝐾2) sin 𝑤𝑡 − ∅ Very small linear and angular displacement, quality measurement Problematic in electromagnetic area 𝑉 𝑠 = 𝑉𝑠1−𝑉𝑠2= −(𝐾1 − 𝐾2) sin 𝑤𝑡 − ∅ 𝑉 𝑠 =0 𝑉 𝑠 = 𝑉𝑠1−𝑉𝑠2= (𝐾2−𝐾1) sin 𝑤𝑡 + (𝜋 − ∅ 𝑪𝒂𝒔𝒆 𝟏 𝑪𝒆𝒏𝒕𝒓𝒂𝒍 𝑪𝒂𝒔𝒆 𝟐 (𝑳𝒆𝒇𝒕 𝑴𝒐𝒗𝒆𝒎𝒆𝒏𝒕 𝐾1> 𝐾2) 𝑪𝒂𝒔𝒆 𝟑 (𝒓𝒊𝒈𝒉𝒕 𝑴𝒐𝒗𝒆𝒎𝒆𝒏𝒕 𝐾1< 𝐾2) (o/p Out of phase by 180o )
- 59. It consists of (i) a current carrying metallic plate (ii)a magnetic field perpendicular to the plate (iii) transverse voltage output. B -> Magnetic flux density n -> charge carrier density e ->electron charge 𝑅ℎ-> Hall coefficient Hall Effect Transducer 𝑉ℎ = 𝐵. 𝐼 𝑑. 𝑛. 𝑒 = 𝑅ℎ 𝐵. 𝐼 𝑑
- 60. velocity
- 62. Optical Encoder : Optical Encoder – Converts linear/angular position/ displacement into digital output.
- 63. Incremental Encoder are used to detect changes in rotation with respect to a reference position – measurement of Angular Velocity Absolute Encoder are used to detect actual changes position, displacement or angular position.
- 66. Contd...
- 73. How many bit resolution encoder is required for measuring 0.03 degree in an absolute encoder?
- 74. Transducer for Velocity Measurement
- 75. Tachogenerator • Tachogenerator is used to measure angular velocity Variable reluctance tachogenerator • Toothed wheel of ferro magnetic material • Pickup coil wound on permanent magnet • Air gap between coil and ferro magnetic material changes ∅ = ∅0 + ∅𝑎 cos 𝑛𝑤𝑡 𝑒 = −𝑁 𝑑∅ 𝑑𝑡 = −𝑁 𝑑 𝑑𝑡 (∅0+∅𝑎 cos 𝑛𝑤𝑡) = 𝑁∅𝑎 𝑛𝑤sin 𝑛𝑤𝑡 e = 𝐸𝑚𝑎𝑥 sin 𝑛𝑤𝑡 𝑊ℎ𝑒𝑟𝑒 𝐸𝑚𝑎𝑥 𝑖𝑠 𝑁∅𝑎 𝑛𝑤
- 76. Variable reluctance tachogenerator DC tachogenerator AC tachogenerator Rotating conductor cuts the magnetic field it induces a voltage proportional to the shaft speed of motor
- 77. • Contact Type Tachometer • Non Contact Type Tachometer Laser beam is pointed on a reflective spot of an rotating shaft • DC type Tachometer • AC type Tachometer • Electronic speedometer Transistorized gauge to output a electrical signal through magnetic dial or a LED display
- 78. Transducer for Force Measurement
- 79. load cell – The stress is applied along the direction of S (shown by the arrow), the steel bar (active element) experiences a compression along that axis and an expansion along the X and Y axes. As a result, gauge A experiences a decrease in resistance, while gauge B undergoes an increase in resistance. Used to measure weight of heavy loads like trucks. Converts Force or Load into electrical signal
- 80. When these two gauges and the gauges on the two remaining sides of the steel are connected to form a bridge circuit, four times the sensitivity of a simple gauge bridge is obtained. This makes the load cell sensitive to very small values of applied stress, as well as to extremely heavy loads.
- 82. Measure Force, Acceleration and mechanical deformation Piezoelectric Crystal Application Piezoelectric elements are also used in the detection and generation of sonar waves.
- 83. Piezoelectric Crystal Force that stretches or compresses act on it, an electrical charge is produced on one of its surface and an opposite charge is also induced on it. Piezoelectric materials Quartz, Barium Titanate, Rochelle salt
- 84. Advantages • Self generating no external power required • High variable impedance 10kΩ for 100 kHz • High frequency Measurement • Output voltage α force applied
- 85. Capacitive Transducers: (i) Parallel separated metallic plate (ii) Dielectric filling 𝐶 = ε0 x εr x A 𝑑 Where, ε0 represents the permittivity of free space εr represents the relative permittivity or dielectric constant d represents the distance of the parallel plates and A represents the area of the plates.
- 86. Capacitive Tactile Sensor A piezoelectric material Quartz or Polyvinylidene fluoride (PVDF) ionic in nature is used. PVDF offer good mechanical strength for thickness of 5µm to 2mm
- 88. Light Sensor / Optical Transducers
- 89. A photovoltaic transducer converts light energy into electricity directly at the atomic level. Materials that exhibit this property are known as photoelectric materials, and the effect is called photoelectric effect.
- 92. The resistance of the photo resistive material decreases with increase in light intensity like potentiometric transducer. Photo Resistive Transducer Light Dependent Resistors (LDR)
- 93. Application •Control of Street Light, camera light, security alarm, heater, outdoor clocks, picture scanning, etc. •ORP12
- 94. Photodiode The incident light energy excites hole –electron pair of the photo diode allows to flow current in reverse biased mode .
- 96. 𝐼𝐿 =Load current output photocurrent 𝐼𝑠= reverse saturation current produced by electron-hole pair, 𝑣𝑑= Diode applied voltage 𝑘𝑏=Boltzman constant T=Absolute temperature h=Planks constant 𝑞𝑒=charge of electron P= Incident optical beam power 𝐼𝐿 = 𝐼𝑠 exp 𝑞𝑒𝑣𝑑 𝑘𝑏𝑇 − 1 − 𝑛𝑞𝑒𝑃 ℎ𝑓
- 97. Photo Transistor It consists of a light-sensitive CB junction, when base is exposed to light energy , base current flows proportional to light intensity.
- 98. Application • Opto-isolator or Opto-couplers • Optical Switches • Retro sensors Computer components – Floppy disc, printer margin control Industries – security system, light pen, encoders for speed measurements Consumers – audio-video games, remote controlled toys, etc.
- 100. Change in temperature causes shape change in Bimetallic strip type transducer and change in potential difference in the thermocouple type . Bimetallic Strip – Thermal expansion Thermocouple – Seebeck effect
- 101. 𝑙 = 𝑙𝑜(1+ α∆T), α is linear thermal expansion coefficient Bimetallic Strip Sensor Bimetallic Strip Sensor
- 102. • Consists of two strips of different metals with different temperature expansion coefficients • Usually made with steel (coefficient of thermal expansion - 12 x 10-6 K-1) and copper ( 16.6 x 10-6 K-1) (or brass 18.7 x 10-6 K-1) joined together (often welded) throughout the length
- 104. • Temperature Measurement • Seebeck effect Change in temperature α induced emf linearly 𝑒𝑚𝑓 = 𝛼 (𝑇2 − 𝑇1) Where, 𝛼 constant V/K 𝑇 , 𝑇 are junction temperature Thermocouples
- 105. 𝑒𝑚𝑓 = 𝐴𝑇 + 1 2 𝐵𝑇2 + 1 2 𝐶𝑇3 𝑒𝑚𝑓𝑎𝑣 = 𝐸1+𝐸2+𝐸3+𝐸4 4 = 𝐸𝑇 𝑛
- 106. RTDs Resistance of metals increases with temperature. Resistive element as coil. Response time 0.5 to 5s or more. 𝑅𝑡= 𝑅0(1+ α t)
- 107. • Mixture of metal oxides Chromium, cobalt, iron, manganese and nickel – oxides are semiconductors • Resistance decreases in a non-linear manner with increases in temperature. • Rugged, small and point detection • Large change in R / degree change in temperature Thermistors
- 108. PTC thermistors are used as self-resetting overcurrent protectors and self- regulating heating elements. NTC thermistors are widely used as inrush- current limiters and temperature sensors
- 109. Transducer for liquid level Measurement
- 110. Liquid Level Measurement Floats • Direct Method – Floats • Indirect Method – Measuring the weight (load cell) Leaver arm Float Slider • Leaver cause LVDT to displace / stretch or compress strain gauge 𝐴ℎ𝜌𝑔
- 111. (a) Pressure difference between liquid at the base of vessel and atmospheric pressure (open vessel) (b) Differential pressure cell Pressure difference between liquid at the base of vessel and the air/gas above the surface of liquid Liquid Level Measurement Differential pressure
- 112. Selection of Sensors • Linearity – i/p – o/p linear • Sensitivity – high change in o/p • Wide Range – o/p is linear over wide range • Accuracy – no change with age • Physical Size – small size • Cost - low cost • Reliability – without failure
- 113. THANKS