SENSORS_UNIT-1.pdf
- 1. Sensors for Engineering Applications Course Code: 20EC001 Prepared by Dr. Jami Venkata Suman Department of ECE
- 2. Syllabus Unit I Sensor Fundamentals and applications Sensor Characteristics, System Characteristics, Instrument Selection, Data Acquisition and Readout, Installation, Acceleration, Shock and Vibration Sensors: Technology Fundamentals, Selecting and Specifying Accelerometers, Applicable Standards, Interfacing and Designs Sensor limitations, measurement issues and criteria Unit II Position and weight sensors Position, direction, distances measurement-large scale, distance travelled, and rotation. Force, Load and Weight Sensors: Quartz Sensors-Charge Mode High- Impedance Piezoelectric Force Sensor, Voltage Mode Low-Impedance Piezoelectric Force Sensor, Piezoelectric Force Sensor Construction, types of Strain Gage Sensors Special applications of strain gage sensors, gravitational sensing
- 3. Unit III Sound, Ultrasound and Infrasound sensors Principles, Audio to electrical sensors and transducers: moving iron microphone, moving coil microphone, capacitor microphones. Microphone problems, frequency and wavelengths. Electrical to audio transducers: moving iron transducer, moving coil transducer, Capacitor transducers. Ultrasonic transducers, Infrasound sensors. Building acoustics, Microphone Standards Unit IV Biosensors and Chemical Sensors Introduction to Biosensors, Applications of Biosensors, Origin of Biosensors, Bioreceptor Molecules, Transduction Mechanisms in Biosensors, Application Range of Biosensors, Technology Fundamentals of chemical sensors, chemical sensing techniques and applications. Evolution of Biosensors, smoke sensors
- 4. Text Books: 1. Jon Wilson, Sensor Technology Hand Book, Newnes, 2004. 2. Ian R.Sinclair, Sensors and Transducers, third edition, Newnes publications 3. Patranabis D, Principles of Industrial Instrumentation, TMH, 2nd edition References: 1. Patranabis D, Sensors and Transducers, 2nd Edition, PHI, New Delhi, 2010 2. E. O. Doeblin, Measurement System: Applications and Design, fourth edition, McGraw Hill Publications
- 5. What is a Sensor? A sensor is a device that converts a physical phenomenon into an electrical signal. As such, sensors represent part of the interface between the physical world and the world of electrical devices, such as computers. The other part of this interface is represented by actuators, which convert electrical signals into physical phenomena.
- 7. • The Sensors are devices that responds to a physical stimulus heat, light, sound, pressure, magnetism, motion, etc, and convert that into an electrical signal. • They perform an input function • The Devices which perform an output function are generally called Actuators and are used to control. some external device, for example movement. • Both sensors and actuators are collectively known as Transducers. Transducers are devices used to convert energy of one kind into energy of another kind.
- 8. Sensor Data Sheets The data sheet is primarily a marketing document. It is typically designed to highlight the positive attributes of a particular sensor and emphasize some of the potential uses of the sensor, and might neglect to comment on some of the negative characteristics of the sensor. In many cases, the sensor has been designed to meet a particular performance specification for a specific customer, and the data sheet will concentrate on the performance parameters of greatest interest to this customer.
- 9. The prospective user is generally forced to make a selection based on the characteristics available on the product data sheet. Many performance characteristics are shown on a typical data sheet. Many manufacturers feel that the data sheet should provide as much information as possible. Therefore the instrumentation engineer must understand the pertinent characteristics and how they will affect the measurement.
- 10. SENSOR CHARACTERISTICS 1. Transfer Function: The transfer function shows the functional relationship between physical input signal and electrical output signal.
- 11. 2. Sensitivity The sensitivity is defined in terms of the relationship between input physical signal and output electrical signal. It is generally the ratio between a small change in electrical signal to a small change in physical signal. As such, it may be expressed as the derivative of the transfer function with respect to physical signal. Typical units are volts/kelvin, millivolts/kilopascal, etc.. A thermometer would have “high sensitivity” if a small temperature change resulted in a large voltage change.
- 12. 3. Span or Dynamic Range: The range of input physical signals that may be converted to electrical signals by the sensor is the dynamic range or span. Signals outside of this range are expected to cause unacceptably large inaccuracy. This span or dynamic range is usually specified by the sensor supplier as the range over which other performance characteristics described in the data sheets are expected to apply. Typical units are kelvin, pascal, newtons, etc.
- 13. 4. Accuracy or Uncertainty: Uncertainty is generally defined as the largest expected error between actual and ideal output signals. Sometimes this is quoted as a fraction of the full-scale output or a fraction of the reading. For example, a thermometer might be guaranteed accurate to within 5% of FSO (Full Scale Output). “Accuracy” is generally considered by metrologists to be a qualitative term, while “uncertainty” is quantitative. For example one sensor might have better accuracy than another if its uncertainty is 1% compared to the other with an uncertainty of 3%.
- 14. 5. Hysteresis: Some sensors do not return to the same output value when the input stimulus is cycled up or down. The width of the expected error in terms of the measured quantity is defined as the hysteresis.
- 15. 6. Nonlinearity (often called Linearity): The maximum deviation from a linear transfer function over the specified dynamic range. There are several measures of this error. The most common compares the actual transfer function with the “best straight line,” which lies midway between the two parallel lines that encompass the entire transfer function over the specified dynamic range of the device.
- 16. 7. Noise: All sensors produce some output noise in addition to the output signal. In some cases, the noise of the sensor is less than the noise of the next element in the electronics, or less than the fluctuations in the physical signal, in which case it is not important. Many other cases exist in which the noise of the sensor limits the performance of the system based on the sensor. Noise is generally distributed across the frequency spectrum. Many common noise sources produce a white noise distribution, which is to say that the spectral noise density is the same at all frequencies. Since there is an inverse relationship between the bandwidth and measurement time, it can be said that the noise decreases with the square root of the measurement time.
- 17. 8. Resolution: The resolution of a sensor is defined as the minimum detectable signal fluctuation. Since fluctuations are temporal phenomena, there is some relationship between the timescale for the fluctuation and the minimum detectable amplitude. Many sensors are limited by noise with a white spectral distribution. In these cases, the resolution may be specified in units of physical signal/root (Hz). Sensor data sheets generally quote resolution in units of signal/root (Hz) or they give a minimum detectable signal for a specific measurement.
- 18. 9. Band Width : The bandwidth of a sensor is the frequency range between these two frequencies.
- 19. TOP SMART SENSORS • Temperature Sensors • Proximity Sensor • Pressure Sensor • Gas & Smoke Sensor • Accelerometer Sensors • Level Sensors • Image Sensors • Motion Detection Sensors • Optical Sensors • Gyroscope Sensors
- 30. SYSTEM CHARACTERISTICS • The sensor and signal conditioners must be selected to work together as a system. • Moreover, the system must be selected to perform well in the intended applications. • Overall system accuracy is usually affected most by sensor characteristics such as environmental effects and dynamic characteristics. • Amplifier characteristics such as nonlinearity, harmonic distortion and flatness of the frequency response curve are usually negligible when compared to sensor errors.
- 31. Instrument Selection 1. Selecting a sensor/signal conditioner system for highly accurate measurements requires very skillful and careful measurement engineering. 2. All environmental, mechanical, and measurement conditions must be considered. 3. Installation must be carefully planned and carried out. 4. The following are the guidelines to select and install measurement systems for the best possible accuracy.
- 32. 1. Sensor The most important element in a measurement system is the sensor. If the data is distorted or corrupted by the sensor, there is often little that can be done to correct it.
- 33. Check the following to operate the sensor satisfactorily in the measurement environment. 1. Temperature Range 2. Maximum Shock and Vibration 3. Humidity 4. Pressure 5. Acoustic Level 6. Corrosive Gases 7. Magnetic and RF Fields 8. Nuclear Radiation 9. Salt Spray 10. Transient Temperatures 11. Strain in the Mounting Surface
- 34. Check sensor characteristics to provide the desired data accuracy. 1. Sensitivity 2. Frequency Response 3. Resonance Frequency 4. Minor Resonances 5. Internal Capacitance 6. Transverse Sensitivity 7. Amplitude Linearity and Hysteresis 8. Temperature Deviation 9. Weight and size 10. Internal Resistance at Maximum Temperature 11. Calibration Accuracy 12. Strain Sensitivity 13. Damping at Temperature Extremes 14. Zero Measurand Output 15. Thermal Zero Shift 16. Thermal Transient Response
- 35. 2. Cable Cables and connectors are usually the weakest link in the measurement system chain. Check cable to operate satisfactorily in the measurement environment. 1. Temperature Range 2. Humidity Conditions Check the cable characteristics to provide the desired data accuracy 1. Low Noise 2. Size and Weight 3. Flexibility 4. Is Sealed Connection Required?
- 36. 3. Power Supply Check the power supply to operate satisfactorily in the measurement environment. 1. Temperature Range 2. Maximum Shock and Vibration 3. Humidity 4. Pressure 5. Acoustic Level 6. Corrosive Gases 7. Magnetic and RF Fields 8. Nuclear Radiation 9. Salt Spray
- 37. Check that is this the proper power supply for the application? Check the following to confirm. 1. Voltage Regulation 2. Current Regulation 3. Compliance Voltage 4. Output Voltage Adjustable? 5. Output Current Adjustable? 6. Long Output Lines? 7. Need for External Sensing 8. Isolation 9. Mode Card, if Required
- 38. Check the power supply characteristics to provide the desired data accuracy. 1. Load Regulation 2. Line Regulation 3. Temperature Stability 4. Time Stability 5. Ripple and Noise 6. Output Impedance 7. Line-Transient Response 8. Noise to Ground 9. DC Isolation
- 39. 4. Amplifier The amplifier must provide gain, impedance matching, output drive current, and other signal processing. Check the amplifier to operate satisfactorily in the measurement environment. 1. Temperature Range 2. Maximum Shock and Vibration 3. Humidity 4. Pressure 5. Acoustic Level 6. Corrosive Gases 7. Magnetic and RF Fields 8. Nuclear Radiation 9. Salt Spray
- 40. Is this the proper amplifier for the application? Check the following 1. Long Input Lines? 2. Need for Charge Amplifier 3. Need for Remote Charge Amplifier 4. Long Output Lines 5. Need for Power Amplifier 6. Airborne 7. Size, Weight, Power Limitations
- 41. Check the following whether the amplifier characteristics provide the desired data accuracy. 1. Gain and Gain Stability 2. Frequency Response 3. Linearity 4. Stability 5. Phase Shift 6. Output Current and Voltage 7. Residual Noise 8. Input Impedance 9. Transient Response 10. Overload Capability 11. Common Mode Rejection 12. Zero-Temperature Coefficient 13. Gain-Temperature Coefficient
- 42. Data Acquisition and Readout ALL of previous check items, plus Adequate Resolution
- 43. Installation Even the most carefully and thoughtfully selected and calibrated system can produce bad data if carelessly or ignorantly installed.
- 44. Sensor Is the unit in good condition and ready to use? Check: 1. Up-to-Date Calibration 2. Physical Condition 3. Case 4. Mounting Surface 5. Connector 6. Mounting Hardware 7. Inspect for Clean Connector 8. Internal Resistance
- 45. Is the mounting hardware in good condition and ready to use? Check: 1. Mounting Surface Condition 2. Thread Condition 3. Burred End Slots 4. Insulated Stud 5. Insulation Resistance 6. Stud Damage by Over Torquing 7. Mounting Surface Clean and Flat 8. Sensor Base Surface Clean and Flat 9. Hole Drilled and Tapped Deep Enough 10. Correct Tap Size 11. Hole Properly Aligned Perpendicular to Mounting Surface 12. Stud Threads Lubricated 13. Sensor Mounted with Recommended Torque
- 46. Cement Mounting Check: 1. Mounting Surface Clean and Flat 2. Dental Cement for Uneven Surfaces 3. Cement Cured Properly 4. Sensor Mounted to Cementing Stud with Recommended Torque
- 47. Cable Is the cable in good condition and ready for use? Check: 1. Physical Condition 2. Cable Kinked, Crushed 3. Connector Threads, Pins 4. Inspect for Clean Connectors 5. Continuity 6. Insulation Resistance 7. Capacitance 8. All Cable Connections Secure 9. Cable Properly restrained 10. Excess Cable Coiled and Tied Down 11. Drip Loop Provided 12. Connectors Sealed and Potted, if Required
- 48. Power Supply, Amplifier, and Readout Are the units in good condition and ready to use? Check: 1. Up-to-Date Calibration 2. Physical Condition 3. Connectors 4. Case 5. Output Cables 6. Inspect for Clean Connectors 7. Mounted Securely 8. All Cable Connections Secure 9. Gain Hole Cover Sealed, if Required 10. Recommended Grounding in Use When the above questions have been answered to the user’s satisfaction, the measurement system has a high probability of providing accurate data.
- 49. Accelerometers are sensing transducers that provide an output proportional to acceleration, vibration and shock. These sensors have found a wide variety of applications in both research and development arenas along with everyday use. In addition to the very technical test and measurement applications, such as modal analysis, NVH (noise vibration and harshness) and package testing Accelerometers are also used in everyday devices such as airbag sensors and automotive security alarms. Acceleration, Shock and Vibration Sensors
- 50. Whenever a structure moves, it experiences acceleration. Measurement of this acceleration helps us gain a better understanding of the dynamic characteristics that govern the behavior of the object. Modeling the behavior of a structure provides a valuable technical tool that can then be used to modify response, to enhance ruggedness, improve durability or reduce the associated noise and vibration. Acceleration, Shock and Vibration Sensors
- 51. The most popular class of accelerometers is the piezoelectric accelerometer. This type of sensor is capable of measuring a wide range of dynamic events. Many other classes of accelerometers exist that are used to measure constant or very low frequency acceleration such as automobile braking, elevator ride quality and even the gravitational pull of the earth. Accelerometers rely on Piezoresistive, Capacitive and Servo technologies.
- 52. Piezoelectric Accelerometer • Piezoelectric Accelerometer works based on the use of a piezoelectric material as the sensing element for the sensor. • Piezoelectric materials are characterized by their ability to output a proportional electrical signal to the stress applied to the material.
- 53. The basic construction of a piezoelectric accelerometer is shown below
- 54. The active elements of the accelerometer are the piezoelectric elements. The elements act as a spring, which has a stiffness k, and connect the base of the accelerometer to the seismic masses. When an input is present at the base of the accelerometer, a force (F) is created on piezoelectric material proportional to the applied acceleration (a) and size of the seismic mass (m).
- 55. The sensor is governed by Newton’s law of motion F = ma. The force experienced by the piezoelectric crystal is proportional to the seismic mass times the input acceleration. The more mass or acceleration, the higher the applied force and the more electrical output from the crystal.
- 58. Piezoelectric accelerometers can be classified into two main categories based on their mode of operation. 1. Internally amplified accelerometers or IEPE (internal electronic piezoelectric) Internally amplified accelerometers or IEPE (internal electronic piezoelectric) contain built-in microelectronic signal conditioning. 2. Charge mode accelerometers Charge mode accelerometers contain only the self-generating piezoelectric sensing element and have a high impedance charge output signal.
- 59. IEPE sensors incorporate built-in, signal-conditioning electronics that function to convert the high-impedance charge signal generated by the piezoelectric sensing element into a usable low-impedance voltage signal that can be readily transmitted, over ordinary two-wire or coaxial cables, to any voltage readout or recording device. 1. Internally amplified accelerometers or IEPE (internal electronic piezoelectric)
- 60. IEPE sensor circuitry can also include other signal conditioning features, such as gain, filtering and self-test features. The simplicity of use, high accuracy, broad frequency range, and low cost of IEPE accelerometer systems make them the recommended type for use in most vibration or shock applications. Limitation in temperature range. The routine upper temperature limit of IEPE accelerometers is 250°F (121°C); however, specialty units are available that operate to 350°F (175°C).
- 61. The electronics within IEPE accelerometers require excitation power from a constant current, DC voltage source. This power source is sometimes built into vibration meters, FFT analyzers and vibration data collectors. A separate signal conditioner is required when none is built into the readout. In addition to providing the required excitation, power supplies may also incorporate additional signal conditioning, such as gain, filtering, buffering and overload indication. The typical system set-ups for IEPE accelerometers are shown in below figure.
- 62. 2. Charge Mode Accelerometers Charge mode sensors output a high-impedance, electrical charge signal that is generated directly by the piezoelectric sensing element. This signal is sensitive to corruption from environmental influences and cable-generated noise. Therefore it requires the use of a special low noise cable. To conduct accurate measurements, it is necessary to condition this signal to a low-impedance voltage before it can be input to a readout or recording device. A charge amplifier or in-line charge converter is generally used for this purpose.
- 63. These devices utilize high-input-impedance, low-output- impedance charge amplifiers with capacitive feedback. Adjusting the value of the feedback capacitor alters the transfer function or gain of the charge amplifier. Typically, charge mode accelerometers are used when high temperature survivability is required.
- 64. PIEZOELECTRIC SENSING MATERIALS Two categories of piezoelectric materials that are used in the design of accelerometers, those are 1. Quartz and 2. Polycrystalline ceramics. Quartz is a natural crystal, while ceramics are man- made. Each material offers certain benefits. The material choice depends on the particular performance features desired of the accelerometer.
- 65. Quartz Quartz is widely known for its ability to perform accurate measurement tasks and contributes heavily in everyday applications for time and frequency measurements. Examples: wristwatches and radios to computers and home appliances.
- 66. Quartz is naturally piezoelectric, it has no tendency to relax to an alternative state and is considered the most stable of all piezoelectric materials. This important feature provides quartz accelerometers with long-term stability and repeatability. Also, quartz does not exhibit the pyroelectric effect (output due to temperature change), which provides stability in thermally active environments. Because quartz has a low capacitance value, the voltage sensitivity is relatively high compared to most ceramic materials, making it ideal for use in voltage-amplified systems. Conversely, the charge sensitivity of quartz is low, limiting its usefulness in charge-amplified systems, where low noise is an inherent feature
- 67. Polycrystalline Ceramics A variety of ceramic materials are used for accelerometers depending on the requirements of the particular application. All ceramic materials are man-made and are forced to become piezoelectric by a polarization process. This process, known as “poling,” exposes the material to a high-intensity electric field. This process aligns the electric dipoles, causing the material to become piezoelectric.
- 68. If ceramic is exposed to temperatures exceeding its range, or large electric fields, the piezoelectric properties may be drastically altered. High-voltage-sensitivity ceramics that are used for accelerometers with built-in, voltage-amplified circuits. There are high-charge-sensitivity ceramics that are used for charge mode sensors with temperature ranges to 400°F (205°C). High-temperature piezoceramics that are used for charge mode accelerometers with temperature ranges over 1000°F (537°C) for monitoring engine manifolds and superheated turbines.
- 69. Structures for Piezoelectric Accelerometers: Three primary configurations in the structures of Piezoelectric Accelerometers 1. Shear 2. Flexural beam and 3. Compression
- 70. Shear mode accelerometer designs bond, or “sandwich,” the sensing material between a center post and seismic mass. Under acceleration, the mass causes a shear stress to be applied to the sensing material. This stress results in a proportional electrical output by the piezoelectric material. The output is then collected by the electrodes and transmitted by lightweight lead wires to either the built-in signal conditioning circuitry of ICP sensors, or directly to the electrical connector for a charge mode type. 1. Shear Mode
- 71. 2. Flexural Beam Mode Flexural mode designs utilize beam-shaped sensing crystals, which are supported to create strain on the crystal when accelerated. The crystal may be bonded to a carrier beam that increases the amount of strain when accelerated. The flexural mode enables low profile, lightweight designs to be manufactured at an economical price. Generally, flexural beam designs are well suited for low frequency, low gravitational (g) acceleration applications such as structural testing.
- 72. 3. Compression Mode Compression mode accelerometers are simple structures which provide high rigidity. A pre-load stud or screw secures the sensing element to the mounting base. When the sensor is accelerated, the seismic mass increases or decreases the amount of compression force acting upon the crystal, and a proportional electrical output results. The larger the seismic mass, the greater the stress and, hence, the greater the output.
- 73. Comparison of Piezoelectric Accelerometers
- 74. Piezoresistive Accelerometers A piezoresistive accelerometer produces resistance changes in strain gauges that are part of the accelerometer's seismic system. Piezoresistive accelerometers have a very wide bandwidth which allows these to be used for measuring short duration (high frequency) shock events such as crash testing.
- 75. A piezoresistive accelerometer has a stable structure composed on a silicon chip created by the micromachining and semiconductor production technology. A mass and a beam on which a set of the piezoresisters are created on a silicon chip. A set of electrical bridged is formed by such piezoresistive resisters to generate signals proportional to the applied acceleration.
- 76. Features • Measurements are possible over the wide frequency range covering down from the ultra low up to the high frequency as of several kHz • Compact and light weight • High sensitivity • Highly acceleration resistant
- 77. Capacitive Accelerometers Capacitive accelerometers, also known as vibration sensors, rely on a change in electrical capacitance in response to acceleration. Accelerometers utilize the properties of an opposed plate capacitor for which the distance between the plates varies proportionally to applied acceleration, thus altering capacitance.
- 79. • Most capacitive accelerometers contain built-in electronics that inject a signal into the element, complete the bridge and condition the signal. • For most capacitive sensors it is necessary to use only a standard voltage supply or battery to supply appropriate power to the accelerometer. • One of the major benefits of capacitive accelerometers is to measure low level (less than 2 g’s), low frequency (down to dc) acceleration with the capability of withstanding high shock levels, typically 5,000 g’s or greater. • Some of the disadvantages of the capacitive accelerometer are a limited high frequency range, a relatively large phase shift and higher noise floor than a comparable piezoelectric device.
- 80. Comparison of Piezoelectric, Piezoresistive and Capacitive Accelerometers
- 82. Selecting and Specifying Accelerometers Comparison of accelerometer types
- 85. In order to select the most appropriate accelerometer for the application, you should look at a variety of factors. First you need to determine the type of sensor response required. The three basic functional categories of accelerometers are 1. IEPE, 2. Charge Mode and 3. DC responding. The first two categories of accelerometers, the IEPE and Charge Mode type of accelerometers, work best for measuring frequencies starting at 0.5 Hz and above. The IEPE is a popular choice, due to its low cost, ease of use and low impedance characteristics, . The Charge Mode is useful for high temperature applications. There are advantages of each design.
- 86. When looking at uniform acceleration, as may be required for tilt measurement, or extremely low frequency measurements below 1 Hz, capacitive or piezoresistive accelerometers are a better choice.
- 87. These sensors may contain built-in signal conditioning electronics and a voltage regulator, allowing them to be powered from a 5–30 VDC source. Some manufacturers offer an offset adjustment, which serves to null any DC voltage offset inherent to the sensor. Capacitive accelerometers are generally able to measure smaller acceleration levels.
- 88. The most basic criteria used to narrow the search, once the functionality category or response type of accelerometer has been decided, includes: • sensitivity, • amplitude, • frequency range and • temperature range.
- 89. Sensitivity for shock and vibration accelerometers is usually specified in millivolts per g (mV/g) or picocoulombs per g (pC/g). This sensitivity specification is inversely proportional to the maximum amplitude that can be measured (g peak range.) Thus, more sensitive sensors will have lower maximum measurable peak amplitude ranges.
- 90. The minimum and maximum frequency range that is going to be measured will also provide valuable information required for the selection process.
- 91. Another important factor for accelerometer selection is the temperature range. Consideration should be given not only to the temperatures that the sensor will be exposed to, but also the temperature that the accelerometer will be stored at. High temperature special designs are available for applications that require that specification.
- 92. Physical characteristics can be very important in certain applications. Consideration should be given to the size and weight of the accelerometer. It is undesirable to place a large or heavy accelerometer on a small or lightweight structure. This is called “mass loading.” Mass loading will affect the accuracy of the results and skew the data. The area that is available for the accelerometer installation may dictate the accelerometer selection. There are triaxial accelerometers, which can be utilized to simultaneously measure acceleration in three orthogonal directions. Older designs required three separate accelerometers to accomplish the same result, and thus add weight and require additional space.
- 93. Consideration should be given to the environment that the accelerometer will be exposed to. Hermetically sealed designs are available for applications that will be exposed to contaminants, moisture, or excessive humidity levels. Connector alternatives are available. Sensors can come with side connections or top connections to ease cable routing. Some models offer an integrated cable. Sensors with field-repairable cabling can prove to be very valuable in rough environments. Interfacing and Designs
- 94. Accelerometer mounting may have an effect on the selection process. Most manufacturers offer a variety of mounting alternatives. Accelerometers can be 1. stud mounted 2. adhesively mounted or magnetically mounted. Stud mounting provides the best stiffness and highest degree of accuracy, while adhesive mounts and magnetic mounting methods offer flexibility and quick removal options.