Sensorsand

Actuators

Professor Charlton S. InaoMechatronics

Defence Engineering College Bishoftu, Ethiopia

PE-4030Chapter 2/b Part two

Instructional Objectives To understand the working principle and applications of the following

sensors: 1. Liquid Flow Sensor

1.1 Orifice1.2 Turbine Flow Meter

2. Level Sensor2.1 Floats 2.2 Differential Pressure

3. Temperature Sensor3.1 Liquid in Glass3.2 Bimetallic Strip3.3 Thermistors

3.4 Electrical Résistance Thermometers3.5 Thermocouples

4. Light Sensor • To practice how to select sensor based on industrial requirements.

Flow Sensors

1.1 Differential Pressure Flowmeter

The orifice plate is simply a disc , with a central hole, which is placed in the tube through which the fluid is flowingThe pressure difference is measured between a point equal to the diameter of the tube upstream and a point equal to half of the diameter downstream. It does not work well with the slurries. The accuracy is typically about + 1.5% of full range and is non-linear.

1.1.1 Orifice

A concentric orifice plate is the simplest and least costly of the differential pressure devices.The orifice plate constricts the flow of a fluid and produces a differential pressureacross the plate (see Figure )

This results in a high pressure upstream and a low pressure downstream that is proportional to the square of the flow velocity. An orifice plate usually produces a greater overall pressure loss than other flow elements.

One advantage of this device is that cost does not increase significantly with pipe size

1.1.2 Venturi meter

Venturi tubes are the largest and mostexpensive differential pressure device.They work by gradually narrowingthe diameter of the pipe, and measuringthe pressure drop that results (seeFigure ). An expanding section of the differential pressure device then returns the flow to close to its original pressure. As with the orifice plate, the differential pressure measurement is converted into a corresponding flow rate. Venturi tubes can typically be used only in those applications requiring a low pressure drop and a high accuracy reading. They are often usedin large diameter pipes.

1.1.3 Flow NozzleFlow nozzles are actually a variationon the Venturi tube, with the nozzleopening being an elliptical restrictionin the flow, but having no outletarea for the pressure .

Pressure taps are locatedapproximately 1/2 pipe diameter downstream and 1 pipe diameter upstream.

The flow nozzle is a high-velocity flowmeter used where turbulence is high(Reynolds numbers above 50,000), asin steam flow applications. The pressure drop of a flow nozzle is between that of a Venturi tube and the orifice plate (30 to 95 percent).

1.2 Ultrasonic Flow Transducer

An ultrasonic flow meter is a type of flow meter that measures the velocity of a fluid with ultrasound to calculate volume flow. Using ultrasonic transducers, the flow meter can measure the average velocity along the path of an emitted beam of ultrasound, by averaging the difference in measured transit time between the pulses of ultrasound propagating into and against the direction of the flow or by measuring the frequency shift from the Doppler effect.

What is the Doppler Effect?The Doppler effect is observed whenever the source of waves is moving with respect to an observer. The Doppler effect can be described as the effect produced by a moving source of waves in which there is an apparent upward shift in frequency for observers towards whom the source is approaching and an apparent downward shift in frequency for observers from whom the source is receding.

. Ultrasonic flow meters are affected by the acoustic properties of the fluid and can be impacted by temperature, density, viscosity and suspended particulates depending on the exact flow meter. They vary greatly in purchase price but are often inexpensive to use and maintain because they do not use moving parts, unlike mechanical flow meters.

Ultrasonic-used to describe sounds that are too high for humans to hear (16KHz- 1 GHz) .

Sound is the propagation of smallest pressure and density variations in an elastic medium (gas, liquid, solid-state body). For example, a noise is generated when the air in a specific spot is compressed more than in the surrounding area. Subsequently, the layer with changed pressure propagates remarkably fast in all directions at speed of sound of 343 m/s.

Acoustic frequencies between 16 kHz and 1 GHz are referred to as ultrasound; in industrial settings we call it “ultrasonics”. To clarify: people are able to hear frequencies between 16 Hz and 20 kHz; i.e. the lower frequencies of industrial ultrasonics are audible, especially if secondary frequencies are generated. And what is more, ultrasonics is palpable when touching the weld tool. For ultrasonic welding, the frequency range is between 20 kHz and 70 kHz.Additional fields of application: Imaging ultrasound in the field of medical diagnostics ranges between 1 and 40 MHz. It is not audible or palpable. In the field of industrial material testing, ultrasonics is used at frequencies from 0.25 to 10 MHz.

NOTE:

1.3 Drag Force Flowmeter

• The turbine flowmeter consists of a multi-bladed motor that is supported centrally in the pipe along which the flow occurs.The fluid rotates the motor , the angualr velocity being approximately proportional to the flow rate. The rate of the revolution of the rotor can be determined using a magnetic pick up which produces an induced emf pulse every time the rotor blade passes it as th e blades are made from magnetic material or have small magnets mounted at their tips.

1.4 Turbine Meter

The pulses are counted and so the number of revolutions of the rotor can be determined. The meter is expensive with a n accuracy of typically about + 0.3%

Turbine-Based Flow Sensors

Turbine and propeller type meters use the principle that liquid flowing through theturbine or propeller will cause the rotor to spin at a speed directly related to flow rate.Electrical pulses can be counted and totaled. These devices are available in full bore, line-mounted versions and insertion types where only a part of the flow being measured passes over the rotating element.

Turbine flow meters, when properly specified and installed, offer good accuracy, especially with low viscosity fluids.

Insertion types are used for less critical applications; however, they are often easier to maintain and inspect because they can be removed without disturbing the main piping.

Turbine flowmeters use the mechanical energy of the fluid to rotate a “pinwheel” (rotor) in the flow stream. Blades on the rotor are angled to transform energy from the flow stream into rotational energy. The rotor shaft spins on bearings. When the fluid moves faster, the rotor spins proportionally faster.

Turbine flowmeters now constitute 7% of the world market.Shaft rotation can be sensed mechanically or by detecting the movement of the blades. Blade movement is often detected magnetically, with each blade or embedded piece of metal generating a pulse.

Turbine flowmeter sensors are typically located external to the flowing stream to avoid material of construction constraints that would result if wetted sensors were used. When the fluid moves faster, more pulses are generated. The transmitter processes the pulse signal to determine the flow of the fluid. Transmitters and sensing systems are available to sense flow in both the forward and reverse flow directions.

1.5 Electromagnetic Flow SensorElectromagnetic Flow Sensors

Operation of these sensors is based upon Faraday’s Law of electromagnetic induction, which says that a voltage will be induced when a conductor moves through a magnetic field.The liquid is the conductor, and the magnetic field is created by energized coils outside the flow tube. The voltage produced is proportional to the flow rate. Electrodes mounted in the pipe wall sense the induced voltage, which is measured by the secondary element.Electromagnetic flow meters are applied in measuring the flow rate of conducting liquids (including water) where a high quality, low maintenance system is needed. The cost of magnetic flow meters is high relative to other types of flowmeters. They do have many advantages, including: they can measure difficult and corrosive liquids and slurries, and they can measure reverse flow.

1.6 Laser Doppler Anemometter

The laser doppler anemometer (LDA) is a well-established technique that has been widely used for fluid dynamic measurements in liquids and gases for well over 30 years. The directional sensitivity and non-intrusiveness of LDA make it useful for applications with reversing flow, chemically reacting or high-temperature media, and rotating machinery, where physical sensors might be difficult or impossible to use. This technique does, however, require tracer particles in the flow.

The laser doppler anemometer (LDA) is a well-established technique that has been widely used for fluid dynamic measurements in liquids and gases for well over 30 years. The directional sensitivity and non-intrusiveness of LDA make it useful for applications with reversing flow, chemically reacting or high-temperature media, and rotating machinery, where physical sensors might be difficult or impossible to use.This technique does, however, require tracer particles in the flow.

1.7 Hot Wire Anemometter

The Hot-Wire Anemometer is the most well known thermal anemometer, and measures a fluid velocity by noting the heat convected away by the fluid.

The principal of a hot wire anemometer is based on a heated element from which heat is extracted by the colder impact airflow.

Thermal anemometry is the most common method used to measure instantaneous fluid velocity.

Typically, the anemometer wire is made of platinum or tungsten and is 4 ~ 10 µm (158 ~ 393 µin) in diameter and 1 mm (0.04 in) in length.Typical commercially available hot-wire anemometers have a flat frequency response (< 3 dB) up to 17 kHz at the average velocity of 9.1 m/s (30 ft/s), 30 kHz at 30.5 m/s (100 ft/s), or 50 kHz at 91 m/s (300 ft/s).

Due to the tiny size of the wire, it is fragile and thus suitable only for clean gas flows. In liquid flow or rugged gas flow, a platinum hot-film coated on a 25 ~ 150 mm (1 ~ 6 in) diameter quartz fiber or hollow glass tube can be used instead, as shown in the schematic .

The Hot-Wire Anemometer is the most well known thermal anemometer, and measures a fluid velocity by noting the heat convected away by the fluid. The core of the anemometer is an exposed hot wire either heated up by a constant current or maintained at a constant temperature (refer to the schematic ). In either case, the heat lost to fluid convection is a function of the fluid velocity.

By measuring the change in wire temperature under constant current or the current required to maintain a constant wire temperature, the heat lost can be obtained. The heat lost can then be converted into a fluid velocity in accordance with convective theory.

LEVEL

SENSORS

Level Sensors

A direct method of monitoring the level of liquid in a vessel is by monitoring the movement of the float. . The displacement of the float causes a lever arm to rotate and so move a slider across the potentiometer. The result is an output of voltage related to the height of the liquid.

2.1 Floats

2.0 Indirect Method 1. Monitoring of the weight of the

vessel by load cell Weight= AhρgNote: hρg = P

2. Measurement of pressure at some point in the liquid P= hρg

Float Swtich

2.2Differential PressureThe differential pressure cell determines the pressure difference between the liquid at the base of the vessel and atmospheric pressure, the vessel being open to atmospheric pressure.

The differential pressure cell monitors the difference in pressure between the vase of the vessel and the air or gas above the surface of the liquid.

Level Transmitters

Level Sensors

Radar Level Sensor

Guided-Wave Radar (GWR)Guided-wave radar (GWR) is a contacting level measurement method that uses a probe to guide high-frequency electromagnetic waves from a transmitter to the media being measured (Figure 2).GWR is based on the principle of time domain reflectometry (TDR). With TDR, a low-energy electromagnetic pulse is guided along a probe. When the pulse reaches the surface of the medium being measured, the pulse energy is reflected up the probe to circuitry that then calculates the fluid level based on the time difference between the pulse being sent and the reflected pulse received. The sensor can output the analyzed level as a continuous measurement reading via an analog output, or it can convert the values into freely positionable switching output signals.Unlike older technologies, GWR offers measurement readings that are independent of the chemical or physical properties of the process media with which it is in contact. Additionally, GWR performs equally well in liquids and solid

GWR is suitable for a variety of level measurement applications including those that involve:Unstable process conditions—Changes in viscosity, density, or acidity do not affect accuracy.Agitated surfaces—Boiling surfaces, dust, foam, and vapor do not affect device performance. GWR also works with recirculating fluids, propeller mixers, and aeration tanks.High temperatures and pressures—GWR performs well in temperatures up to 315°C and can withstand pressures up to 580 psig.Fine powders and sticky fluids—GWR works with vacuum tanks filled with used cooking oil as well as tanks holding paint, latex, animal fat, soybean oil, sawdust, carbon black, titanium tetrachloride, salt, and grain.

GWR technology measuring liquid level in process vessel  

Ultrasonic Technology

Ultrasonic transmitters operate by sending a sound wave generated from a piezoelectric transducer to the surface of the process material being measured. The transmitter measures the length of time it takes for the reflected sound wave to return to the transducer. A successful measurement depends on the wave, reflected from the process material and moving in a straight line back to the transducer. Because factors such as dust, heavy vapors, tank obstructions, surface turbulence, foam, and even surface angles can affect the returning signal when using an ultrasonic level sensor, you must consider how your operating conditions can affect the sound waves.

Ultrasonic transmitter mounted on top of tank  

GWR is suitable for a variety of level measurement applications including those that involve:

Unstable process conditions—Changes in viscosity, density, or acidity do not affect accuracy.

Agitated surfaces—Boiling surfaces, dust, foam, and vapor do not affect device performance. GWR also works with recirculating fluids, propeller mixers, and aeration tanks.

High temperatures and pressures—GWR performs well in temperatures up to 315°C and can withstand pressures up to 580 psig.

Fine powders and sticky fluids—GWR works with vacuum tanks filled with used cooking oil as well as tanks holding paint, latex, animal fat, soybean oil, sawdust, carbon black, titanium tetrachloride, salt, and grain.

Gravimetric Level Sensor

DescriptionGravimetric measurement of level with SIWAREX weighing technology produces high-precision weight measurement results without any contact with the material. The weight of your product is correctly determined independently of the temperature, container shape, material density, shift in the center of gravity and agitators or the like. Bridging, heaped objects, hopper flow, foam, steam and dust have no effect on the gravimetric measurement.These advantages enable SIWAREX weighing technology to be used in legal-for-trade plants. Measuring points in potentially explosive areas can be very easily realized with standard components.

We put together the correct load cells and built-in components from our product range for gravimetric measurement to match the particular application, load range and accuracy requirements. Our range extends from platform load cells, bending beams and shear beams to can compression cells in the load classes from 3kg to 280t.

Service-proven load cell technology and separation of the medium mean maximum service life with no special maintenance. This improves plant availability and reduces the operating costs on a permanent basis.

The load cell signals are evaluated by SIWAREX weighing electronics which are seamlessly integrated in the SIMATIC automation system. This enables very easy handling and use of the advantages provided by SIMATIC such as flexibility, a diagnostic interrupt system and much more.Don’t forget that the safest engineered level measurement solution includes switches for back-up, overfill, low level and dry run protection.

Capacitance Level Sensor

• With the tank empty, the insulating medium between the two conductors is air. With the tank full, the insulating material is the process liquid or solid. As the level rises in the tank to start covering the probe, some of the insulating effect from air changes into that from the process material, producing a change in capacitance between the sensing probe and ground. This capacitance is meas ured to provide a direct, linear meas urement of tank level.

Hydrostatic Level Sensor• Principles of Operation

A hydrostatic level sensor is a submersible or externally mounted pressure sensor that determines level by measuring pressure above it, which increases with depth. From this measurement, together with knowledge of the liquid's density / specific gravity, it is possible calculate the liquid level above the sensor in the vessel. Temperature compensation will take into account changes in specific gravity due to variations in temperature.

Advantages of Hydrostatic Level Sensors• Easy to install and relatively low cost• Good overall accuracy and long-term stability• Applicable to a wide variety of fluids

Limitations of Hydrostatic Level Sensors• Not suitable for solids or liquids with suspended solids• Can only read level above the transmitter• Need to know the density / specific gravity of the liquid being measured

Hydrostatic Head Level Sensor

• For decades, DP-type instruments—long before the DP cell—were used to measure liquid level. Orifice meters, originally designed to measure differential pressure across an orifice in a pipeline, readily adapted to level measurement.

• Today’s smart DP transmitters adapt equally well to level measurements and use the same basic principles as their precursors.

• With open vessels (those not under pressure or a vacuum), a pipe at or near the bottom of the vessel connects only to the high-pressure side of the meter body and the low-pressure side is open to the atmosphere.

• If the vessel is pressurized or under vacuum, the low side of the meter has a pipe connection near the top of the vessel, so that the instrument responds only to changes in the head of liquid .

• DP transmitters are used extensively in the process industries today. In fact, newer smart transmitters and conventional 4– 20 mA signals for communications to remote DCSs, PLCs, or other systems have actually resulted in a “revival” of this technology. Problems with dirty liquids and the expense of piping on new installations, however, have opened the door for yet newer, alternative methods.

• Hydrostatic Tank Gauging. It is an emerging standard way to accurately gauge liquid inventory and to monitor transfers in tank farms and similar multiple-tank storage facilities. HTG systems can provide accurate information on tank level, mass, density, and volume of the contents in every tank. These values can also be networked digitally for multiple remote access by computer from a safe area.

• The level transmitter, with its probe installed at an angle into the bottom portion of the tank, is an innovative way to detect accumulation of water, separated from oil, and to control withdrawal of product only. Moreover, by measuring the water-oil interface level, the LT provides a means of correcting precisely for the water level, which would incorrectly be measured as product.

Though the DP transmitter is most commonly used to measure hydrostatic pressure for level measurement, other methods should be mentioned. One newer system uses a pressure transmitter in the form of a stainless steel probe that looks much like a thermometer bulb. The probe is simply lowered into the tank toward the bottom, supported by plastic tubing or cable that carries wiring to a meter mounted externally on or near the tank. The meter displays the level data and can transmit the information to another receiver for remote monitoring, recording, and control.Another newer hydrostatic measuring device is a dry-cell transducer that is said to prevent the pressure cell oils from contaminating the process fluid. It incorporates special ceramic and stainless steel diaphragms and is apparently used in much the same way as a DP transmitter. 

Temperature Sensing Devices

Temperature Scales• Celsius(º C)- common SI unit of relative temp• K=C +273• Kelvin(K)-Standard SI unit of absolute

thermodynamic temperature• Fahrenheit-(º F)English unit of relative

temperature. T= 9/5C +32• Rankine(ºR) English system unit of absolute

thermodynamic temperature. R=F +460

Temperature Measurements

Temperature Sensors

• 3.1 Liquid in Glass-A simple non electrical temperature measuring

device which typically uses alcohol or mercury as the working fluid, which expands and contracts relative to the glass container. When making measurements in a liquid, the depth of immersion is important

Temperature Sensors• 3.2 Bi–Metallic StripAnother nonelectrical temperature

measuring device. I tis composed of two or more metal layers having different coefficient of thermal expansion. Since these layers are permanently bonded together, the structure will deform when temperature changes, due t to the difference in the thermal expansions of the two metal layers. The deflection can be related to the temperature of the strip.

The mechanical motion of the strip makes or breaks an electrical contact to turn a heating or cooling system On or OFF.

Temperature Sensors

• 3.3Resistance Temperature Detector(RTDs)RTD is constructed of metal wire wound around

a ceramic or glass core and hermetically sealed. The resistance of the metallic wire increases with temperature. The resistance Temperature relationship is approximated by the following linear expression:

R=Ro[1 +α(T-To)]

Where To=reference temperature Ro= resistance at the reference temperature α=calibration constant

The reference temperature is usually the ice point of the water(0º C).

The most commonly used metal in RTD is platinum, because of its high melting point, resistance to oxidation, predictable tem characteristics, and stable calibration values.

The operating range of typical platinum RTD is –220 deg centigrade to 750 deg centigrade.

3.4 Thermistor-is a semiconductor device whose resistance changes exponentially with temperature. Thermistors have much narrower operating ranges than RTDs.

Its resistance –temperature relationship is usually expressed in the form

R= Roe [β(1/T-1/To)]

Where To= reference temperature β =a calibration constant called the characteristic temperature of the material

Temperature Sensors

• 3.5 ThermocouplesTwo dissimilar metals in contact

form a thermoelectric junction occur in pairs, resulting in what is called thermocouple.This is known as Seebeck effect.The thermocouple voltage is directly proportional to the junction temperature difference

V= α(T1-T2)

Where α is called the Seebeck coefficient; T1 and T2 is the junction temperature of metals A and B.

Thermocouple Circuit

Thermocouple Configuration

Thermocouple Data

Thermocouple

Thermocouple Type, Materials, Range, Sensitivity

Thermocouple Junction Temperature and Output voltage

Junction Temperature (C) Output Voltage (mV)

0 0

10 0.507

20 1.019

30 1.536

40 2.058

50 2.585

60 3.115

70 3.649

80 4.186

90 4.725

100 5.268

Light Sensors

Since distance is velocity multiplied by time, wavelength can be expressed as the velocity of electromagnetic waves multiplied by the time of one cycle of frequency f. Since the accepted speed of light is 186,000 miles per second or 300,000,000 meters per second, this is: ë(in meters) = 300,000,000 meters/sec × 1/f(in seconds) or, ë(in meters) = 300/f(in MHz) If visible light (white light) is passed through a prism, , the visible light separates into its color components.

The electromagnetic spectrum is divided into radio waves and light waves by frequency. Light waves are further divided by into infrared, visible, ultraviolet and X-rays. The spectrum is either expressed in frequency or wavelength. Wavelength is the distance that an electromagnetic wave travels through space in one cycle of its frequency.

The Electromagnetic Spectrum

The frequency of visible light is from 400 million megahertz to 750 million megahertz. The wavelength is from 750 nanometers (10−9) to 400 nanometers. Light sensors extend into the infrared frequency range below visible light and into the ultraviolet light frequency range above visible light. Cadmium sulfide sensors are most sensitive in the green light region of visible light, while solar cells and phototransistor sensors are most sensitive in the infrared region.

Light Sensors

Light sensor diodes make the resistance of the circuit decreases and the current increases as the light/illuminance increases, at constant voltage.

• Used in control of street lamps

• Used in the automatic /digital camera

• Used in the automotive and military industry

Selection of Sensors1. Identify the nature of the measurement

required• Variable to be measured• Nominal value• Range of Value• Accuracy required• The required speed of measurement• Reliability required• Environmental conditions

2. Identify the nature of the output required from the sensor, this determining the signal conditioning requirements in order to give suitable output signals from the measurement.

3. Identify the possible sensors, taking into account such factors as range, accuracy, linearity, speed of response, reliability, maintainability, life, power supply requirements, ruggedness, availability and cost.

4.Identify the signal conditioning requirements. Eg. Measurement of level of a corrosive acid in a vessel.

Using a load cell, which gives an electrical output, calibrated to the level, ie. When empty and when full.

The End