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Engr. A. N. Aniedu Electronic and Computer Engineering Nnamdi Azikiwe University, Awka
Engr. A. N. AnieduElectronic and Computer Engineering
Nnamdi Azikiwe University, Awka
INTRODUCTION
• In order to sense and measure physical variablessuch as pressure, flow, & motion, you need to usetransducers (sensors), which convert physicalvariables into electrical signals and transmit thesesignals either to a signal conditioning device ordirectly to a data acquisition board.
• Instruments measuring physical quantities suchas temperature, stress, displacement, pressure,flow, etc use respective sensors.
• 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
sensor Electrical outputPhysical Quantity
Excitation
Introduction contd.
DEFINITION
• A sensor (also called detector) therefore is aconverter that measures a physical quantity andconverts it into a signal which can be read by anobserver or by an instrument.
• For example, a mercury-in-glass thermometerconverts the measured temperature intoexpansion and contraction of a liquid which canbe read on a calibrated glass tube.
• A thermocouple converts temperature to anoutput voltage which can be read by a voltmeter.
• For accuracy, most sensors are calibrated againstknown standards.
• The relationship between the measurand and the transducer output signal is referred to as transducer sensitivity
• 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).
Transducer sensitivity = Output signal incrementMeasurand increment
Defn. Contd.
Defn. Contd.
• Sensors that measure very small changes musthave very high sensitivities.
• Technological progress allows more and moresensors to be manufactured on a microscopicscale as microsensors using MEMS (Micro-Electro-Mechanical Systems) technology.
• In most cases, a microsensor reaches asignificantly higher speed and sensitivitycompared with macroscopic approaches.
• Sensitivity of a transducer should be usually ashigh as possible since then it becomes easier totake the measurements
Defn. Contd.
• Ideal sensors are designed to be linear to some simplemathematical function of the measurement, typicallylogarithmic.
• The output signal of such a sensor is linearlyproportional to the value or simple function of themeasured property.
• The sensitivity is then defined as the ratio betweenoutput signal and measured property.
• For example, if a sensor measures temperature and hasa voltage output, the sensitivity is a constant with theunit [V/K]; this sensor is linear because the ratio isconstant at all points of measurement.
Sensors may output signals in different formats:
– Analog level (voltage or resistance)
– Analog waveform
– Digital level
– Digital waveform
Most Modern computers require digital inputs
• 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
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.
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
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
Classification Contd.
Classification based on property:
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.
Classification Contd.
Classification based on Material and Technology
Surface Plasmon resonance and Light addressablepotentio-metric from the Bio-sensors group are the newoptical technology based sensors.
CMOS Image sensors have low resolution as comparedto charge coupled devices. CMOS has the advantages ofsmall size, cheap, less power consumption and henceare better substitutes for charge coupled devices.
Accelerometers are independently grouped because oftheir vital role in future applications like aircraft,automobiles, etc and in fields of videogames, toys, etc.
Magnetometers are those sensors which measuremagnetic flux intensity B (in units of Tesla or As/m2).
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.
Classification Contd.
In the current and future applications, sensors can beclassified 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.
Classification Contd.
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
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
• 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
• Desired measurement accuracy and range, power requirements, overload protection and vulnerability to sudden failure
• Purchase aspects
• We will describe sensors for measuring – Temperature
– light
– Pressure/force
– Flow
– Proximity
– Advanced operations
• The principles of operation, their characteristics, range of operation etc
TEMPERATURE SENSORS
• Thermocouple
• Resistive temperature detector (RTD)
• Thermistors
• lC temperature detector
Thermocouple• widely used in industries for measuring temperatures from
-200°C to + 1750oC.• 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.
• 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.
Hot cold
_ +
Hot cold
A
B B B A A
(a) (b)
Fig 1: Principle of operation of thermocouple
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/oC. 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.
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.
• 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.
Vo = I(R+∆R)
I
R+∆RRTD
Fig 2: Temperature measurement with RTD
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.
Characteristics of RTD contd.
• Excitation current dissipates power equal to I2R 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.
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.
Characteristics of thermistors
• Range of measurement is limited to temperatures from -75 to +300oC only.
• They are available with resistances ranging from few ohms to 10Mohm.
• High resistance value thermistors (100kΩ to 500kΩ at 25oC) are used for measuring high temperatures (from 150-300oC)
• Intermediate value thermistors (10Ω to 2kΩ) are used for measuring temperatures (from 75 -150oC)
• Low resistance thermistors (100kΩ to 500kΩ at 25oC) are used for measuring high temperatures (from -75 to +75oC)
• 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 non-
linear 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 self-heating.
• 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
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
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 +150oC)
• Does not require complex signal conditioning circuits.
Comparison of characteristics of temperature sensors
SENSOR CHARACTERISTICSSIGNAL
CONDITIONING NEEDS
RANGE, OC
Thermocouple Low output voltage Cold-junction compensation,high amplification, linearization
-200 to +1600
RTD Low sensitivity, non-linear output, wide range, low resistance output, low sensitivity
Current excitation, linearization
-250 to +850
Thermistor Resistance 10 to 10ΩM NTC, high sensitivity, highly non-linear, fast response time
Voltage excitation, reference resistor, linearization
-100 to +200
IC temperaturesensor
High level voltage or current output, linear output
Power source, moderate gain amplifier
-50 to +150
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.
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.
Weight
Strain gaugesSpring steel strip
b a
Strain gauges Spring steel strip
(b)(a)
(c)
Figure 3: Strain gauge used to measure force:a) Side view b) Top view (expanded) c) circuit connections
• 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.
• 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.
Mutual Inductance Transducer
Figure 4: Mutual Inductance Transducer
• 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)
Linear Variable Differential Transformer (LVDT)
Figure 5: Linear Variable Differential Transducer (LVDT) a) and b) Composition and connectionsc) Pressure measurement in LVDT
• 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
• 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 180o 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).
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)
Touch Sensor: Principles of Operation
Fig 6: Touch sensor working principle
• 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 betravelled by the current is more.
• As a result, the resistance of the material is varied by changing theposition at which it makes contact with the fully conductivematerial.
• Generally, software are interfaced to the touch sensors. In such acase, a memory is being offered by the software.
• They can memorize the ‘last touched position’ when the sensor isdeactivated. They can memorize the ‘first touched position’ oncethe 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 theother end of mouse pad in order to move the cursor to the far sideof the screen.
Touch Sensor: Principles of Operation
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
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 “Infra-red” 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.
• 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.
• 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.
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.
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
• 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 methodscould 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.
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.
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.
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
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
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
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, ρA as the known fluid density, ρ as flowing fluid’s density, and g as the acceleration due to gravity.
Figure 11: Pitot Tube
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
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
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
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.
Figure 15: Coriolis Flow Meter
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
Figure 16: Vortex Shedding Flow Meter
PROXIMITY SENSOR
• A proximity sensor detects the presence ofobjects that are nearly placed without any pointof contact.
• Since there is no contact between the sensorsand sensed object and lack of mechanical parts,these sensors have long functional life and highreliability.
• The different types of proximity sensors areInductive Proximity sensors, Capacitive Proximitysensors, Ultrasonic sensors, photoelectricsensors, Hall-effect sensors, etc.
Proximity Sensor: Working Principles
• A proximity sensor emits an electromagneticor electrostatic field or a beam ofelectromagnetic radiation (such as infrared),and waits for the return signal or changes inthe field.
• The object which is being sensed is known asthe proximity sensor's target.
Fig 17: Ultrasonic sensor working principle
Proximity Sensor: Working Principles
Proximity Sensor: Types
• Inductive Proximity sensors– 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 objectwith a variation in an oscillation frequency. To detect the nearby object, theoscillation frequency is transformed into a direct current voltage which is comparedwith a predetermined threshold value. These sensors are preferred for plastictargets.
• 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 ultrasonicsignal from the concerned object. This helps in detecting the position, presence andmovement of objects. Since ultrasonic sensors rely on sound rather than light fordetection, it is widely used to measure water-levels, medical scanning proceduresand in the automobile industry. Ultrasonic waves can detect transparent objectssuch as transparent films, glass bottles, plastic bottles, and plate glass, using itsReflective Sensors.
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
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
ADVANCED SENSOR TECHNOLOGY
• Sensor technology is used in wide range in the field of Manufacturing. The advanced technologies are as follows:
Advanced Sensor Technology : Bar-code Identification
• The products sold in the markets has a UniversalProduct Code (UPC) which is a 12 digit code.
• Five of the numbers signify the manufacturer andother five signify the product.
• The first six digits are represented by code as light anddark bars.
• The first digit signifies the type of number system andthe second digit which is parity signifies the accuracy ofthe reading.
• The remaining six digits are represented by code asdark and light bars reversing the order of the first sixdigits. Bar code is shown in figure 3 below.
Advanced Sensor Technology : Bar-code Identification
Fig 18: Example of a bar code
Advanced Sensor Technology : Transponders
• In the automobile section, Radio frequency device isused in many cases.
• The transponders are hidden inside the plastic head ofthe key which is not visible to anyone. The key isinserted in the ignition lock cylinder.
• As you turn the key, the computer transmits a radiosignal to the transponder. The computer will not let theengine to ignite until the transponder responds to thesignal.
• These transponders are energized by the radio signals.The figure of a transponder is as shown in figure 4below
Advanced Sensor Technology : Transponders
Fig 19: Example of a transponder
• 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.
Advanced Sensor Technology : Others
