Engr. A. N. Aniedu Electronic and Computer Engineering Nnamdi Azikiwe University, Awka

Transcription

1 Engr. A. N. Aniedu Electronic and Computer Engineering Nnamdi Azikiwe University, Awka

2 INTRODUCTION In order to sense and measure physical variables such as pressure, flow, & motion, you need to use transducers (sensors), which convert physical variables into electrical signals and transmit these signals either to a signal conditioning device or directly to a data acquisition board. Instruments measuring physical quantities such as temperature, stress, displacement, pressure, flow, etc use respective sensors.

3 Introduction contd. The sensors receive an insignificant amount of energy from the medium and produces and electrical output depending in some way on the quantity being measured. The term transducer is also interchangeably used with the term sensor in practice. The transducer refers to a device that converts energy in one form to another, whereas the sensor refers to a device that receives a signal and responds with electrical signal Excitation Physical Quantity sensor Electrical output

4 DEFINITION A sensor (also called detector) therefore is a converter that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated glass tube. A thermocouple converts temperature to an output voltage which can be read by a voltmeter. For accuracy, most sensors are calibrated against known standards.

5 The relationship between the measurand and the transducer output signal is referred to as transducer sensitivity Transducer sensitivity = Defn. Contd. Output signal increment Measurand increment A sensor's sensitivity indicates how much the sensor's output changes when the measured quantity changes. For instance, if the mercury in a thermometer moves 1 cm when the temperature changes by 1 C, the sensitivity is 1cm/ C (it is basically the slope dy/dx assuming a linear characteristic).

6 Defn. Contd. Sensors that measure very small changes must have very high sensitivities. Technological progress allows more and more sensors to be manufactured on a microscopic scale as microsensors using MEMS (Micro- Electro-Mechanical Systems) technology. In most cases, a microsensor reaches a significantly higher speed and sensitivity compared with macroscopic approaches. Sensitivity of a transducer should be usually as high as possible since then it becomes easier to take the measurements

7 Defn. Contd. Ideal sensors are designed to be linear to some simple mathematical function of the measurement, typically logarithmic. The output signal of such a sensor is linearly proportional to the value or simple function of the measured property. The sensitivity is then defined as the ratio between output signal and measured property. For example, if a sensor measures temperature and has a voltage output, the sensitivity is a constant with the unit [V/K]; this sensor is linear because the ratio is constant at all points of measurement.

8 Sensors may output signals in different formats: Analog level (voltage or resistance) Analog waveform Digital level Digital waveform Most Modern computers require digital inputs

9 Actuators convert electrical energy into mechanical action. They are used as control elements in process control systems for controlling the opening or closing of valves, the precise positioning or movement of objects, the angle of rotation etc. The dc motors, ac motors, stepper motors, solenoids, and reed relays are a few examples of actuators

10 CLASSIFICATION Conventionally, sensors are classified into two broad groups based on energy requirements 1. Sensors generating electrical output on their own without the need of external power are classified into Active sensors 2. Sensors requiring external excitation for their operation are classified into Passive sensors Example, thermocouple which generates thermal emf is classified as active sensor and a resistive temperature detector (RTD) which requires a current to be passed through the sensor for its operation is classified as passive sensor.

11 Classification Contd. Classification based on the type of output Analogue transducers: These transducers convert the input physical phenomenon into an analogous output which is a continuous function of time Eg. Strain gauge, thermocouple, thermistor or a linear voltage differential transformer Digital transducers: These transducers convert the input physical phenomenon into an electrical output which may be in a form of pulse Eg. Shaft encoders, digital tachometers, limit switches

12 Classification Contd. Classification based on electrical principle involved Variable resistance type: Eg, strain and pressure gauges, thermistors, resistant thermometers, photoconductive cell etc Variable - inductance type: Eg, linear voltage differential transformer (LVDF), Reluctance pick-up, Eddy current guage etc Variable capacitance type: Eg, capacitor microphone, pressure gauge, dielectric gauge Voltage generating type: Thermocouple, photovoltaic cell, rotational motion tachometer, piezoelectric pick-up Voltage divider type: Potentiometer position sensor, pressure-actuated voltage divider

13 Classification based on property: Classification Contd. Temperature - Thermistors, thermocouples, Resistant Temperature Detectors (RTD s), IC and many more. Pressure - Fibre optic, vacuum, elastic liquid based manometers, (Linear variable differential transofrmer) LVDT, electronic. Flow - Electromagnetic, differential pressure, positional displacement, thermal mass, etc. Level Sensors - Differential pressure, ultrasonic radio frequency, radar, thermal displacement, etc. Proximity and displacement - LVDT, photoelectric, capacitive, magnetic, ultrasonic. Biosensors - Resonant mirror, electrochemical, surface Plasmon resonance, Light addressable potentio-metric. Image - Charge coupled devices, CMOS Gas and chemical - Semiconductor, Infrared, Conductance, Electrochemical. Acceleration - Gyroscopes, Accelerometers. Others - Moisture, humidity sensor, Speed sensor, mass, Tilt sensor, force, viscosity.

14 Classification Contd. Classification based on Material and Technology Surface Plasmon resonance and Light addressable potentio-metric from the Bio-sensors group are the new optical technology based sensors. CMOS Image sensors have low resolution as compared to charge coupled devices. CMOS has the advantages of small size, cheap, less power consumption and hence are better substitutes for charge coupled devices. Accelerometers are independently grouped because of their vital role in future applications like aircraft, automobiles, etc and in fields of videogames, toys, etc. Magnetometers are those sensors which measure magnetic flux intensity B (in units of Tesla or As/m2).

15 Classification Contd. Classification based on Application: Industrial process control, measurement and automation Non-industrial use Aircraft, Medical products, Automobiles, Consumer electronics, other type of sensors.

16 Classification Contd. In the current and future applications, sensors can be classified into groups as follows: Accelerometers - These are based on the Micro Electro Mechanical sensor technology. They are used for patient monitoring which includes pace makers and vehicle dynamic systems. Biosensors - These are based on the electrochemical technology. They are used for food testing, medical care device, water testing, and biological warfare agent detection. Image Sensors - These are based on the CMOS technology. They are used in consumer electronics, biometrics, traffic and security surveillance and PC imaging. Motion Detectors - These are based on the Infra Red, Ultrasonic, and Microwave / radar technology. They are used in videogames and simulations, light activation and security detection.

17 Selection of sensors/transducers A good sensor must obey some basic rules. These include: It must be sensitive to the measured property only It must be insensitive to any other property likely to be encountered in its application It does not influence the measured property

18 The following major considerations need to be looked into while selecting a sensor: Mechanical suitability in terms of Physical size, weight and shape Mounting arrangement Ruggedness

19 Electrical suitability in terms of Sensitivity Frequency response Ease of signal transmission Environmental suitability in terms of Sensitivity to temperature and self-heating effects Magnetic fields Vibration, dust and humidity Supply frequency etc Transducer performance in terms of calibration accuracy

20 Desired measurement accuracy and range, power requirements, overload protection and vulnerability to sudden failure Purchase aspects

21 We will describe sensors for measuring Temperature light Pressure/force Flow Proximity Advanced operations The principles of operation, their characteristics, range of operation etc

22 TEMPERATURE SENSORS Thermocouple Resistive temperature detector (RTD) Thermistors lc temperature detector

23 Thermocouple widely used in industries for measuring temperatures from -200 C to o C. Works on the Seebeck principle (ie when two dissimilar metals or alloys are joined at both ends and one of the junctions is heated, current flow through the loop (fig 1a). If the loop is broken at the center, a potential difference (thermal emf) is generated between the functions (fig 1b). The magnitude of the potential difference depends on the material forming the function and is proportional to the temperature difference between the junctions Hence the measure of the emf is the measure of the temperature difference.

24 Junction being exposed to heat is known as hot junction or measuring junction while the other one is known as cold junction or reference junction. See fig 1. A _ + Hot cold B A Hot B B cold A (a) (b) Fig 1: Principle of operation of thermocouple

25 Characteristics of Thermocouples The thermal emf produced by thermocouples is very low and is in the range of-10mv to +50 mv Sensitivity is also low and is between 5 to 55uV/ o C. It needs high gain amplification. Usually, a high gain and high input impedance instrumentation amplifier is used. Cold junction introduces errors in measurements. It needs cold junction compensation. The temperature vs. voltage relationship of thermocouples is non-linear. It needs linearization.

26 Resistive Temperature Detector (RTD) This works on the property that resistivity of metals increases with increase in temperature. RTDs use metals like platinum, nickel, tungsten and copper Because resistivity of metals is very small and the temperature coefficient of resistance is still smaller, metals used in the RTDs should have relatively high resistivity and high temperature resistance coefficient. Platinum is the ideal metal for RTDs.

27 I R+ R RTD V o = I(R+ R) Fig 2: Temperature measurement with RTD The basic method of measuring temperature with the RTD is illustrated in Fig 2. A constant current is passed through the sensor and the voltage across the sensor is measured. As the voltage across the sensor is proportional to the resistance and the resistance in turn is proportional to the temperature, the voltage can be calibrated in terms of temperature.

28 Characteristics of RTD Sensitivity of the RTD is related to temperature resistance coefficient of the metal used in the RTD. It is a very small value therefore it needs signal-conditioning that effectively increases the measurement sensitivity. Bridge circuits are normally used for measuring the small changes in resistance values. Resistance vs. temperature relationship is non-linear for wide range of temperature. It needs linearization for wide range of measurements. Since, the RTD is a low-resistance device, the connecting lead-wires also contribute their resistances considerably in measurements. So, the signal-conditioning circuits for the RTD should eliminate the effect of lead wire resistances. The RTD requires excitation current. Hence, a stable current source is essential.

29 Characteristics of RTD contd. Excitation current dissipates power equal to I 2 R in the device that leads to sensor selfheating. Hence, it is required to keep the current through the device at a minimum value. Response is very slow and typically the response time varies from 0.5 to 5 seconds. This is mainly because, the temperature of the device attains the temperature of the medium by thermal conduction. Used for measurements in the temperature range from -250 to +850 C. More accurate and more linear than thermocouple Excellent stability and repeatability Expensive Highly suitable for very precise temperature measurements.

30 Thermistor Thermistor is a temperature sensitive resistor made of semiconducting materials Their resistance normally decreases as the temperature of the device increases (negative temperature coefficient, NTC) They have relatively high resistance and high NTCs.

31 Characteristics of thermistors Range of measurement is limited to temperatures from -75 to +300 o C only. They are available with resistances ranging from few ohms to 10Mohm. High resistance value thermistors (100kΩ to 500kΩ at 25 o C) are used for measuring high temperatures (from o C) Intermediate value thermistors (10Ω to 2kΩ) are used for measuring temperatures (from o C) Low resistance thermistors (100kΩ to 500kΩ at 25 o C) are used for measuring high temperatures (from -75 to +75 o C)

32 Thermistors are highly sensitive and have high NTC. Comparatively, they are much more sensitive than RTDs. They have very fast response time Variation of resistance with temperature is highly nonlinear hence it needs signal conditioning for linearization of output. They are used in applications where precise measurement is not required. They require excitation current which leads to selfheating. Thermistors with positive temperature coefficients (PTC) are also available. They have different temperature vs. resistance characteristics and are used in switching applications. They are used as thermostats to sense and regulate oven temperatures

33 IC Temperature Sensor These are active sensors and require power for operation just like any other IC component They consist of electronic circuits that exploit the temperature characteristics of active semiconductor junctions. They produce current or voltage outputs proportional to temperature Examples are AD590, LM135, LM235, LM335

34 Characteristics of IC temperature sensors Available in sensors producing voltage or current proportional to the temperature. Produces comparatively high level outputs Mostly linear Inexpensive Current devices have remote sensing capabilities Temperature range is very limited (-50 to +150 o C) Does not require complex signal conditioning circuits.

35 Comparison of characteristics of temperature sensors SENSOR CHARACTERISTICS SIGNAL CONDITIONING NEEDS Thermocouple Low output voltage Cold-junction compensation, high amplification, linearization RTD Thermistor IC temperature sensor Low sensitivity, non-linear output, wide range, low resistance output, low sensitivity Resistance 10 to 10ΩM NTC, high sensitivity, highly non-linear, fast response time High level voltage or current output, linear output Current excitation, linearization Voltage excitation, reference resistor, linearization Power source, moderate gain amplifier RANGE, O C -200 to to to to +150

36 FORCE AND PRESSURE TRANSDUCERS To convert force or pressure (force/area) to a proportional electrical signal, two most common methods are used: strain gauges or linear variable differential transformers (L VDTs). Both of these methods involve moving something. This is why we refer to them as transducers rather than sensors.

37 Strain Gauges and Load Cells A strain gauge is small resistor whose value changes when its length is changed. It may be made of thin wire, thin foil, or semiconductor material. Figure 3a shows a simple setup for measuring force or weight with strain gauges. One end of a piece of spring steel is attached to a fixed surface. A strain gauge is glued on the top of the flexible bar. The force or weight to be measured is applied to the unattached end of the bar. As the applied force bends the bar, the strain gauge is stretched, increasing its resistance. Since the amount that the bar is bent is directly proportional to the applied force, the change in resistance will be proportional to the applied force. If a current is passed through the strain gauge, then the change in voltage across the strain gauge will be proportional to the applied force.

38 Strain gauges Spring steel strip Strain gauges Spring steel strip (a) Weight b (b) a (c) Figure 3: Strain gauge used to measure force: a) Side view b) Top view (expanded) c) circuit connections

39 Unfortunately, the resistance of the strain-gauge element also changes with temperature. To compensate for this problem, two strain-gauge elements, mounted at right angle as shown in Fig 3b are often used. Both of the elements will change resistance with temperature, but only element A will change resistance appreciably with applied force. When these two elements are connected in a balance-bridge configuration, as shown in Fig 3c, any change in the resistance of the elements due to temperature will have no effect on the differential output of the bridge. However, as force is applied, the resistance of the element under strain will change and produce a small differential output voltage. The full-scale differential output voltage is typically 2 or 3m V for each volt of excitation voltage applied to the top of the bridge. For example, if 10v is applied to the top of the bridge, the full-load output voltage will be 20 or 30mV. This small signal can be amplified with a differential amplifier or an instrumentation amplifier. Strain-gauge bridges are used in many different forms to measure many different types of force and pressure. If the strain-gauge bridge is connected to a bendable beam structure, as shown in 3a, the result is called a load cell and is used to measure weight.

40 Mutual Inductance Transducer A two-coil mutual inductance transducer consists of two coils: an energizing coil X and a pick-up coil Y (fig 4). A change in the position of the armature by a mechanical input changes the air gap. This causes a change in the output from coil Y, which may be used as a measure of the displacement of the armature, ie the mechanical input.

41 Figure 4: Mutual Inductance Transducer

42 Linear Variable Differential Transformer (LVDT) LVDT is a passive inductive transducer and is commonly used to measure force (or weight, pressure and acceleration etc which depends on force) in terms of the amount of direction of displacement of an object. The LVDT consists of: one primary winding (p), two secondary windings (s1 and s2) which are placed on either side of the primary winding and mounted on the same magnetic core, And a magnetic core (fig 5a and b)

43 Figure 5: Linear Variable Differential Transducer (LVDT) a) and b) Composition and connections c) Pressure measurement in LVDT

44 When the core is in the center (called reference position) the induced voltage E1 and E2 are equal and opposite hence they cancel out and the output voltage Vo is zero. When the external applied force moves the core towards coil s2, E2 is increased but E1 is decreased in magnitude they are still antiphase with each other. The net voltage available is (E2-E1) and is in phase with E2. Similarly, when the movable core moves towards coil s1, E1>E2 and Vo = E1-E2 and is in phase with E1

45 The magnetic core is free to move axially inside the coil assembly and the motion being measured is mechanically coupled to it. The two secondaries s1 and s2 have equal number of turns but are connected in series opposition so that emfs (E1 and E2) induced in them are 180 o out of phase with each other and hence, cancel each other out. The primary is energized from a suitable A.C. source Hence the magnitude of Vo is a function of the distance moved by the core and its polarity or phase indicates which direction it has moved. If the core is attached to a moving object, the magnitude and polarity of Vo gives the position of the object (fig 5c).

46 Touch Sensor A touch sensor acts as a variable resistor as per the location where it is touched. The figure is as shown in fig 1 below. A touch sensor is made of: Fully conductive substance such as copper Insulated spacing material such as foam or plastic Partially conductive material (see fig 1)

47 Touch Sensor: Principles of Operation Fig 6: Touch sensor working principle

48 Touch Sensor: Principles of Operation The partially conductive material opposes the flow of current. The main principle of the linear position sensor is that the current flow is more opposed when the length of this material that must be travelled by the current is more. As a result, the resistance of the material is varied by changing the position at which it makes contact with the fully conductive material. Generally, software are interfaced to the touch sensors. In such a case, a memory is being offered by the software. They can memorize the last touched position when the sensor is deactivated. They can memorize the first touched position once the sensor gets activated and understand all the values related to it. This act is similar to how one moves the mouse and locates it at the other end of mouse pad in order to move the cursor to the far side of the screen.

49 Touch Sensor: Application The touch sensors being cost effective and durable are used in many applications such as Commercial Medical, vending, Fitness and gaming Appliances Oven, Washing machine/dryers, dishwashers, refrigerators Transportation Cockpit fabrication and streamlining control among the vehicle manufacturers Fluid level sensors Industrial Automation Position and liquid level sensing, human touch control in automation applications Consumer Electronics Provides a new feel and level of control in various consumer products

50 LIGHT/PHOTO SENSORS A Light Sensor generates an output signal indicating the intensity of light by measuring the radiant energy that exists in a very narrow range of frequencies basically called light, and which ranges in frequency from Infrared to Visible up to Ultraviolet light spectrum. Light sensors are more commonly known as Photoelectric Devices or Photo Sensors because they convert light energy (photons) into electricity (electrons). Photoelectric devices can be grouped into two main categories, 1. Those which generate electricity when illuminated, such as Photo-voltaics or Photo-emissives etc, and 2. Those which change their electrical properties in some way such as Photo-resistors or Photo-conductors. This leads to the following classification of devices.

51 Photo-emissive Cells: These are photodevices which release free electrons from a light sensitive material such as caesium when struck by a photon of sufficient energy. The amount of energy the photons have depends on the frequency of the light and the higher the frequency, the more energy the photons have converting light energy into electrical energy. Photo-conductive Cells: These photodevices vary their electrical resistance when subjected to light. Photoconductivity results from light hitting a semiconductor material which controls the current flow through it. Thus, more light increase the current for a given applied voltage. The most common photoconductive material is Cadmium Sulphide used in LDR photocells.

52 Photo-voltaic Cells: These photodevices generate an emf in proportion to the radiant light energy received and is similar in effect to photoconductivity. Light energy falls on to two semiconductor materials sandwiched together creating a voltage of approximately 0.5V. The most common photovoltaic material is Selenium used in solar cells. Photo-junction Devices These photodevices are mainly true semiconductor devices such as the photodiode or phototransistor which use light to control the flow of electrons and holes across their PN-junction. Photojunction devices are specifically designed for detector application and light penetration with their spectral response tuned to the wavelength of incident light.

53 Some photo-junction devices include: 1. Photodiodes : "Photodiodes" are semiconductor junction diodes which are connected into a circuit in reverse bias, so giving a very high resistance, so that when light, falls on the, junction the diode resistance drops and the current in the circuit rises appreciably A photodiode can be used as a variable resistance device controlled by the light incident on it. These diodes have a very fast response to light 2. Photo resistor : It has a resistance which depends on the intensity of the light falling on it, decreasing linearly as the intensity increases. The cadmium sulphide photoresistor is most responsive to light having wavelengths shorter than about 515 nm and the cadmium selinide photoresistor for wavelengths less than about 700 nm. An array of light sensors is often required in a small space in order to determine the variations of light intensity across that space.

54 3. Photo transistors : The phototransistors have a light-sensitive collector-base P-N junction. When there is no incident light there is a very small collector-to-emitter current. When light is incident, a base current is produced that is directly proportional to the light intensity. This leads to the production of a collector current which is then a measure of the light intensity. Phototransistors are often available as integrated packages with the photo transistor connected in a Darlington arrangement with a conventional transistor (Fig. 6). Since this arrangement gives a higher current gain, the device gives a much greater collector current for a given light intensity. Figure 7: Photo Darlington

55 To control the flow rate of some material in electronics factory, it needs to be measured. Depending on the material, flow rate, and temperature, different methods could be used One method of measuring flow is with a differential pressure transducer, as shown in Figure 8.A wire mesh or screen is put in the pipe to create a difference in pressure between the two sides of the screen. The pressure transducer gives an output proportional to the difference in pressure between the two sides of the resistance. In the same way that the voltage across an electrical resistor is proportional to the flow of current through the resistor, the output of the pressure transducer is proportions to the flow of a liquid or gas through the pipe.

56 FLOW SENSORS Flow is defined as the rate (volume or area per unit time) at which a substance travels through a given cross section and is characterized at specific temperatures and pressures. To control the flow rate of some material in electronics factory, it needs to be measured. The instruments used to measure flow are termed flow meters. The main components of a flow meter include the sensor, signal processor and transmitter. Flow sensors use acoustic waves and electromagnetic fields to measure the flow through a given area via physical quantities, such as acceleration, frequency, pressure and volume. As a result, many flow meters are named with respect to the physical property that helps to measure the flow.

57 Common Types of Flow Meters The flow rate as determined by the flow sensor is derived from other physical properties. The relationship between the physical properties and the flow rate is derived from fundamental fluid flow principles, such as Bernoulli s equation. 1. Differential Pressure Flow Meters These sensors work according to Bernoulli s principle which states that the pressure drop across the meter is proportional to the square of the flow rate.

58 Examples include: 1a Orifice Meter Orifice plates are installed in flow meters in order to calculate the material balances that will ultimately result in a fluid flow measurement on the sensor. An orifice plate is placed in a pipe containing a fluid flow, which constricts the smooth flow of the fluid inside the pipe. By restricting the flow, the orifice meter causes a pressure drop across the plate. By measuring the difference between the two pressures across the plate, the orifice meter determines the flow rate through the pipe. Orifice meters used in conjunction with DP (Differential Pressure) cells are one of the most common forms of flow measurement. Orifice meters are not only simple and cheap, they can also be delivered for almost any application and be made of any material. Figure 8: Orifice Meter

59 1b Venturi Meter These can pass 25 50% more flow than an orifice meter. Here the fluid flowrate is measured by reducing the cross sectional flow area in the flow path, generating a pressure difference. After the constricted area, the fluid is passes through a pressure recovery exit section, where up to 80% of the differential pressure generated at the constricted area, is recovered. The Venturi meter is most commonly used for measuring very large flow rates where power losses could become significant. It has a higher start up cost than an orifice, but is balanced by the reduced operating costs. Figure 9: Venturi Meter

60 1c Flow Nozzle Flow nozzles are often used as measuring elements for air and gas flow in industrial applications. At high velocities, Flow Nozzles can handle approximately 60 percent greater liquid flow than orifice plates having the same pressure drop. Hence for measurements where high temperatures and velocities are present, the flow nozzle may provide a better solution than an orifice plate. Its construction makes it substantially more rigid in adverse conditions and the flow coefficient data at high Reynolds numbers is better documented than for orifice plates. Liquids with suspended solids can also be metered with flow nozzles. However, the use of the flow nozzles is not recommended for highly viscous liquids or those containing large amounts of sticky solids. The turndown rate of flow nozzles is similar to that of the orifice plate. The flow nozzle is relatively simple and cheap, and available for many applications in many materials. Figure 10: Flow Nozzle

61 1d Pitot Tubes Pitot tubes measure the local velocity due to the pressure difference between points 1 and 2 as in fig below. Unlike the other differential flow meters, the pitot tubes only detect fluid flow at one point rather than an overall calculation. Each pitot tube has two openings, one perpendicular to the flow and one parallel to the flow. The impact tube has its opening perpendicular to the fluid flow, allowing the fluid to enter the tube at point 2, and build up pressure until the pressure remains constant. This point is known as the stagnation point. The static tube, with openings parallel to the fluid flow gives the static pressure and causes a sealed fluid of known density to shift in the base of the tube. Pressure drop can be calculated using the height change along with the fluid densities and the equation below. with Δp as the pressure drop, ρ A as the known fluid density, ρ as flowing fluid s density, and g as the acceleration due to gravity. Figure 11: Pitot Tube

62 2. Direct Force Flow Meters These flow meters are governed by balancing forces within the system.examples include: 2a Rotameter A rotameter is a vertically installed tube that increases in diameter with increasing height. The meter must be installed vertically so that gravity effects are easily incorporated into the governing equations. Fluid flows in through the bottom of the tube and out through the top. Inside the glass tube there is a float that changes position with the flow rate. When there is no liquid flow, the float rests in the bottom of the meter. Figure 12: Rotameter

63 2b Turbine Meter A turbine wheel is placed in a pipe that holds the flowing fluid. As the fluid flows through the turbine, the turbine is forced to rotate at a speed proportional to the fluid flow rate. A magnetic pick-up is mounted to the turbine wheel, and a sensor records the produced voltage pulses. Voltage information can then be translated into the actual flow meter reading. main advantages of the tubine meter over conventional differential head devices is they are more accurate registration of flow in the low flow range of process operation. This results from the registration being proportional to the velocity rather than the velocity square Another advantage to using this type of flow meter is reliability. Additionally, the turbine flow meter does not have a high installation cost. However, due to the turbine wheel motion, a low to medium pressure drop can result. Turbine wheel replacement may also be required due to abrasion caused by particles within the fluid. Figure 13: Turbine Meter

64 2c Propeller Flow Meter Propeller flow meters have a rotating element similar to the wheel in turbine meters, hence rotation is caused by fluid flow through the propeller, and voltage pulses are created as the propeller passes a magnetic or optical sensor. Similarly, the frequency of the pulses is proportional to flow rate of the fluid and the voltages can be directly correlated with the fluid flow rate Propeller flow meters are often used specifically with water, though other fluids may also be used. Low cost coupled with high accuracy make propeller flow meters a common choice in many applications. Figure 14: Propeller Flow Meter

65 2d Coriolis Mass Flow Meter A Coriolis flow meter harnesses the natural phenomenon wherein an object will begin to drift as it travels from or toward the center of a rotation occurring in the surrounding environment. A merry-go-round serves as a simple analogy; a person travelling from the outer edge of the circle to its center will find himself deviating from his straight-line path in the direction of the ride s rotation. Coriolis flow meters generate this effect by diverting the fluid flow through a pair of parallel U-tubes undergoing vibration perpendicular to the flow. This vibration simulates a rotation of the pipe, and the resulting Coriolis drift in the fluid will cause the U-tubes to twist and deviate from their parallel alignment. This Coriolis force producing this deviation is ultimately proportional to the mass flow rate through the U-tubes. where Fc is the Coriolis force observed, w is the angular velocity resulting from rotation, and x is the length of tubing in the flow meter. Because the Coriolis flow meter measures the mass flow rate of the fluid, the reading will not be affected by fluctuations in the fluid density. Furthermore, the absence of direct obstructions to flow makes the Coriolis flow meter a suitable choice for measuring the flow of corrosive fluids. Its limitations include a significant pressure drop and diminished accuracy in the presence of low-flow gases.

66 Figure 15: Coriolis Flow Meter

67 3. Frequency Flow Meters These flow meters use frequency and electronic signals to calculate the flow rate. Examples include 3a Vortex Shedding Flow Meter A blunt, non-streamline body is placed in the stream of the flow through a pipe. When the flow stream hits the body, a series of alternating vortices are produced, which causes the fluid to swirl as it flows downstream. The number of vortices formed is directly proportional to the flow velocity and hence the flow rate. The vortices are detected downstream from the blunt body using an ultrasonic beam that is transmitted perpendicular to the direction of flow. As the vortices cross the beam, they alter the carrier wave as the signal is processed electronically, using a frequency-to-voltage circuit. Vortex-shedding flow meters are best used in turbulent flow with a Reynolds number greater than 10,000. One advantage of using this type of flow meter is its insensitivity from temperature, pressure, and viscosity. The major disadvantage to using this method is the pressure drop caused by the flow obstruction

68 Figure 16: Vortex Shedding Flow Meter

69 PROXIMITY SENSOR A proximity sensor detects the presence of objects that are nearly placed without any point of contact. Since there is no contact between the sensors and sensed object and lack of mechanical parts, these sensors have long functional life and high reliability. The different types of proximity sensors are Inductive Proximity sensors, Capacitive Proximity sensors, Ultrasonic sensors, photoelectric sensors, Hall-effect sensors, etc.

70 Proximity Sensor: Working Principles A proximity sensor emits an electromagnetic or electrostatic field or a beam of electromagnetic radiation (such as infrared), and waits for the return signal or changes in the field. The object which is being sensed is known as the proximity sensor's target.

71 Proximity Sensor: Working Principles Fig 17: Ultrasonic sensor working principle

72 Inductive Proximity sensors Proximity Sensor: Types They have an oscillator as input to change the loss resistance by the proximity of an electrically conductive medium. These sensors are preferred for metal targets. Capacitive Proximity sensors They convert the electrostatic capacitance variation flanked by the detecting electrode and the ground electrode. This occurs by approaching the nearby object with a variation in an oscillation frequency. To detect the nearby object, the oscillation frequency is transformed into a direct current voltage which is compared with a predetermined threshold value. These sensors are preferred for plastic targets. An ultrasonic sensor They are used to detect the presence of an object. It achieves this by emitting ultrasonic waves from the device head and then receiving the reflected ultrasonic signal from the concerned object. This helps in detecting the position, presence and movement of objects. Since ultrasonic sensors rely on sound rather than light for detection, it is widely used to measure water-levels, medical scanning procedures and in the automobile industry. Ultrasonic waves can detect transparent objects such as transparent films, glass bottles, plastic bottles, and plate glass, using its Reflective Sensors.

73 Proximity Sensor: Applications Sensors are used in many kinds of applications such as: Used in automation engineering to define operating states in process engineering plants, production systems and automating plants Used in windows, and the alarm is activated when the window opens Used in machine vibration monitoring to calculate the difference in distance between a shaft and its support bearing Shock Detection Machine monitoring applications

74 Proximity Sensor: Applications contd. Vehicle dynamics Low power applications Structural Dynamics Medical Aerospace Nuclear Instrumentation As pressure sensor in Mobiles touch key pad Lamps which brighten or dim on touching its base Touch sensitive buttons in elevators

75 ADVANCED SENSOR TECHNOLOGY Sensor technology is used in wide range in the field of Manufacturing. The advanced technologies are as follows:

76 Advanced Sensor Technology : Bar-code Identification The products sold in the markets has a Universal Product Code (UPC) which is a 12 digit code. Five of the numbers signify the manufacturer and other five signify the product. The first six digits are represented by code as light and dark bars. The first digit signifies the type of number system and the second digit which is parity signifies the accuracy of the reading. The remaining six digits are represented by code as dark and light bars reversing the order of the first six digits. Bar code is shown in figure 3 below.

77 Advanced Sensor Technology : Barcode Identification Fig 18: Example of a bar code

78 Advanced Sensor Technology : Transponders In the automobile section, Radio frequency device is used in many cases. The transponders are hidden inside the plastic head of the key which is not visible to anyone. The key is inserted in the ignition lock cylinder. As you turn the key, the computer transmits a radio signal to the transponder. The computer will not let the engine to ignite until the transponder responds to the signal. These transponders are energized by the radio signals. The figure of a transponder is as shown in figure 4 below

79 Advanced Sensor Technology : Transponders Fig 19: Example of a transponder

80 Advanced Sensor Technology : Others Electromagnetic Identification of Manufactured Components This is similar to the bar code technology where the data can be coded on magnetic stripe. With magnetic striping, the data can be read even if the code is concealed with grease or dirt. Surface Acoustic Waves This process is similar to the RF identification. Here, the part identification gets triggered by the radar type signals and is transmitted over long distances as compared to the RF systems. Optical Character Recognition This is a type of automatic identification technique which uses alphanumeric characters as the source of information. In United States, Optical character recognition is used in mail processing centres. They are also used in vision systems and voice recognition systems.