Kryzia Faith Calaguas ECE 5-1

Process Actuators and Output Devices

What Are Control Output Devices?These are devices that are associated with computer control

Actuators

They convert computer signals into movement.

Some devices are considered actuators themselves Other devices are connected to a computer which sends a response signal

to an intermediary actuator. The actuator would convert a computer signal into movement causing an action to happen.

For example: light bulb comes on.

Hardware devices that convert a controller command signal into a change in a physical parameter

The change is usually mechanical (e.g., position or velocity) An actuator is also a transducer because it changes one type of physical

quantity into some alternative form An actuator is usually activated by a low-level command signal, so an

amplifier may be required to provide sufficient power to drive the actuatorTypes of Actuators

1. Electrical actuators Electric motors

DC servomotors AC motors Stepper motors

Solenoids2. Hydraulic actuators

Use hydraulic fluid to amplify the controller command signal3. Pneumatic actuators

Use compressed air as the driving force

HYDRAULIC ACTUATORS

Hydraulic Actuators are used in industrial process control, employ hydraulic pressure to drive an output member.

Principle : Pascal’s Law

“Pressure exerted anywhere in a confined incompressible fluid is transmitted equally in all directions throughout the fluid, acts upon every part of the confining vessel at right angles to its interior surfaces”.

F = PxA

PNEUMATIC ACTUATOR A pneumatic actuator converts energy (typically in the form of compressed

Air) into motion. The motion can be rotary or linear, depending on the type

of actuator.

A Pneumatic actuator mainly consists of a piston, a cylinder, and valves or

ports.

Pneumatic systems are very common, and have much in common with

hydraulic systems with a few key differences

Electric Motors

Electric motors are the most common source of torque for mobility and/or

manipulation in machines.

The physical principle of all electric motors is that when an electric current

is passed through a conductor (usually a coil of wire) placed within a

magnetic field, a force is exerted on the wire causing it to move

Motors

Stepper motor- most common Motors can be used in automatic washing machines, automatic cookers,

can cause water to come into the heating systems, greenhouses and dishwashers.

They can also be used to move robot arms

Buzzers

Actuator connected from computer to buzzer. The purpose of the actuator is to switch the buzzer on or off.

They are used in automatic cookers. They are also used in mobile phones.

Lights

An actuator is connected from the computer to the light bulb. The purpose of the actuator is to switch the light on or off.

They are used in computer-controlled greenhouses in order to control the amount of light.

They are also used in security lamps and dimmers inside the house.Heaters

× An actuator is connected from the computer to the heater. × This is to turn the heater on and off.× Other uses of heaters are: 1. Automatic washing machines2. Automatic cookers3. Central heating systems4. Computer-controlled greenhouses.

Questions

1. These are devices that are associated with computer controlControl Output Devices

2. They convert computer signals into movementActuators

3. The most common source of torque for mobility and/or manipulation in machines.

Motors/Electric Motors

Jeena A. DionisioBSECE 5-1

SENSORS

Sensor - an electrical/mechanical/chemical device that maps an environmental attribute to a quantitative measurementattribute mixtures - often no one to one maphidden state in environment

Each sensor is based on a transduction principle - conversion of energy from one form to another

Also known as transducers

Types of Sensors

• Active– send signal into environment and measure interaction of signal w/ environment– e.g. radar, sonar

• Passive– record signals already present in environment– e.g. video cameras

Classification by medium used– based on electromagnetic radiation of various wavelengths– vibrations in a medium– concentration of chemicals in environment– by physical contact

Classification of measurement errors

A good sensor obeys the following rules:

Is sensitive to the measured property only Is insensitive to any other property likely to be encountered in its application Does not influence the measured property

Ideal sensors are designed to be linear or linear to some simple mathematical function of the measurement, typically logarithmic. The output of such a sensor is an analog signal and 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.

For an analog sensor signal to be processed, or used in digital equipment, it needs to be converted to a digital signal, using an analog-to-digital converter.

QUESTIONS: 1. What is a sensor? Give example.

Sensor - an electrical/mechanical/chemical device that maps an environmental attribute to a quantitative measurement

Tilt sensor, bend sensor, temperature sensor, light sensor

2. Define the major types of sensors. Active

- send signal into environment and measure interaction of signal w/ environment

Passive- record signals already present in environment

3. What are the rules that a good sensor obeys?

• Is sensitive to the measured property only• Is insensitive to any other property likely to be encountered in its application• Does not influence the measured property

Valerie Zenn D. MagbanuaBSECE 5-1

Introduction to Analog Process Control and Sensors

The technology of controlling a series of events to transform a material into adesired end product is called process control.

Process control can take two forms: (1) sequential control, which is an event-basedprocess in which one event follows another until a process sequence is complete;(2) continuous control, which requires continuous monitoring and adjustment ofthe process variables. However, Continuous process control falls into two categories: elementary On/Off action, and continuous control action.

Elementary On/Off actionOn/Off action is used in applications where the system has high inertia, whichprevents the system from rapid cycling. This type of control only has only two states,On and Off.

Continuous control actionContinuous process action is used to continuously control a physical outputparameter of a material. The parameter is measured with the instrumentation orsensor, and compared to a set value.

Kind of sensors– Principles of the sensors– Selection of sensors– Noise filter

INTRODUCTION TO SENSOR

WHAT IS SENSOR?– Sensor converts the physical quantity to signal that can be recognized by other components such as display, transmitter and etc.

SENSOR TYPES– Temperature: thermocouple, RTD, thermistor– Pressure: bellows, bourdon tube, diaphragm– Flow rate: orifice, venturi, magnetic, ultrasonic, Coliolis effect– Liquid level: float, differential pressure– pH: pH electrode– Viscosity: pressure drop across venturi or vane deflection– Composition: density, conductivity, GC, IR, NIR, UV

MEASUREMENT DEVICE Transducer: Sensor+Transmitter– Transmitter generates an industrial standard signal from the sensor output.– Standard instrumentation signal levels• Voltage: 1~5VDC, 0~5VDC, -10~+10VDC, etc.• Current: 4~20mA (long range transmission with driver)• Pneumatic: 3-15psig– Signal conversion• I/P or P/I transducer: current-to-pressure or vice versa• I/V (I/E) or V/I: current-to-voltage or vice versa• P/E or E/P: pressure-to-voltage or vice versa Analog-to-Digital (A/D) converter– Continuous signal converted to digital signal after sampling– Specification: sample rate, resolution (8bit, 12bit, 16bit)

TRANSMITTERS • Transmitter Gain (Km): adjustable – Amplification ratio: (output span)/(input span) • Span and Zero: adjustable – Span: magnitude of range of transmitter signal – Zero: lower limit of transmitter signal

FILTERING • Noise Source – Process nature (turbulence, vibration, oscillation…) – Various noise source from environment – Power line, electromagnetic force, etc.

• Removing noise – Analog filter – First-order filter analogy –The filter behaves as an interpolation between the measured output and previous filtered output.–If =1, the measured output is ignored. (constant)–If=0, the filtered output is same as the measured output (no filtering)–If F =0, =0 and no filtering is achieved.–If F=∞, =1 and the measured output is ignored.As F increases, heavier filter is applied.

1. The basic function of a __________ is to convert a reading from a transducer into a standard signal and transmit that signal to a controller or computer monitor. (1) recorder (2) transmitter (3) converter

2. 4–20 mA is the most common standard analog signal used in the process control industry today. Is this statement true or false? (1) True (2) False

3. Match the signal type in Column A with its example/application in Column B. (1) Analog signal (A) 3 –15 psig(2) Pneumatic signal (B) Fieldbus, Profibus and Modbus (3) Digital signal (C) 4-20 mA and 1 – 5 V

4. A customer would use __________to read the temperature of a process fluid on a display.(1) an indicator(2) a volt-meter(3) an actuator

Answers:1. 2 2. 2. 13. 3. C, A, B 4. 4. 1

CONTROL VALVES INTRODUCTION

A control valve regulates the supply of material or energy to a process by adjusting an opening through which the material flows; it is a variable orifice in a line. The formula based on Bernoulli's theorem for flow through an orifice is:

Where: Q = quantity of flow C = constant for the conditions of flow A = valve opening area P = pressure drop across the valve

Flow through the valve is proportional to the area of the opening and the square root of the pressure drop across the valve. Both factors vary; the area varies with the percent travel (position) of the valve and the pressure drop is related to conditions outside the valve and established by the process, such as layout and piping..

CONSTRUCTION

Actuator

The control valve consists of two major components:

. The actuator

. The valve

The actuator is made up of:1. Flexible diaphragm2. Spring and spring tension adjustment (range adjustment)3. Plate, stem and locknuts4. Housing

The valve is made up of a:1. Body2. Plug3. Stem4. Pressure tight connection

Q=CA√δ P

The actuator (refer to figure 7.1) is arranged as follows:The diaphragm is bolted to a dished metal head forming a pressure tight compartment. The controller output pressure is connected to this compartment. The motion of the diaphragm is opposed by a spring.

The valve stem is attached to the diaphragm so that any diaphragm movement results in the same valve plug movement.

Both diaphragm movement and plug can be direct or indirect. The action of the actuator may be such that an increase in air lifts the stem or a decrease in air may lift the stem.

The plug may be attached to the stem so that a lifting stem closes the valve, or a lifting stem opens the valve. Some plugs are reversible on the stem, others are not.

The determination of the valve action desired is usually based on what position the valve should take on an air failure. On one process it may be desirable to have the valve go wide open when the air fail, for example, a cooling process. On other processes it may be better to have the valve close, for example a heating process. When this is decided and the of plug is established, then the actuator action may be specified.

Fig 7.1 Control valve construction

Opposing spring

Body

Air pressure from controller Diaphragm

Actuator or Diaphragm

StemYoke

Packing boxEqual percentage plug

Bottom

Flow

BASIC OPERATION OF CONTROL VALVE

The controller output serves as the input to the control valve if no positioner is used. The actuator converts the controller output to a valve opening. Refer to figure 7.2.

Suppose the controller output increases, this increase in pressure will cause the diaphragm to move against the spring. The diaphragm movement is relayed to the valve plug through the stem. The valve plug movement will regulate the flow as required.

Fig 7.2 Basic diagram of pneumatic control valve

PARTS OF CONTROL VALVE AND THEIR FUNCTION

A control valve is comprised of an actuator mounted to a valve.

Spring

Shaft (Stem)

Diaphram

Valve Plug

Input Pressure

Open to Atmosphere

SealedVolume

Flow

The valve modulates flow through movement of a valve plug in relation to the port(s) located within the valve body. The valve plug is attached to a valve stem, which, in turn, is connected to the actuator.

The actuator, which can be pneumatically or electrically operated, directs the movement of the stem as dictated by the external control device.

Pneumatic Actuator

Pneumatic Actuators are direct acting and utilize an air signal from an external control device to create a modulating control action. The force of the air signal is received into the actuator through a top port and distributed across the full area of the actuator’s diaphragm. The diaphragm presses down on the diaphragm plate and spring return assembly, which then moves the valve stem and plug assembly downward to stroke the valve. This actuator will move to a stem-out position in the event of air signal failure.The choice of valve action (Air-To-Close or Air-To-Open) will determine its signal failure position.

The part of the valve that connects to the process piping and carries the flow of fluid is called the valve body. Valve body must be able to withstand the same pressure and temperature as the process piping.

Pneumatic Actuator

Air to close

Pneumatic Actuator

Air to open

Valve body with

cage plug

Fig 7.3 Major Components of a control valve with pneumatic actuator

Position Indicator

Valve Stemassembly

Gear Train

valve

Motor

Microprocessor Control board

Actuator

Valve body

Electric Actuator

Electric Actuators are motor driven devices that utilize an electrical input signal to generate a motor shaft rotation.This rotation is, in turn, translated by the unit’s linkage into a linear motion, which drives the valve stem and plug assembly for flow modulation.In case of electric signal failure, these actuators can be specified to fail in the stem-out, stem-in, or last position.

Fig 7.4 Electric Actuators

TYPES OF CONTROL VALVE

The wide variety of control valves manufactured means that you can probably match the requirement of your application with a specific valve type. Among the control valve available are globe, cage, butterfly, ball, sliding-gate, Saunders, and split-body types. The relative advantages and disadvantages of each type are given in the following paragraphs.

1. GLOBE VALVES

The globe valve (so named because of its shape) is the most widely used control valve in process control. Its characteristics include:

provides tight shut-off (small leakage)

can withstand high differential pressure and requires a relatively small linear actuator

changes flow characteristics in response to changes in the trim

can be disassemble for easy maintenance

offers a wide range of sizes, pressure ratings, and flow characteristics.

The globe valve's major disadvantages are high pressure losses across the valve and potential for clogging.

Figure shows two different types of globe valve: single seated and double seated. Fluid flow through either valve is controlled by vertical movement of the throttling plugs. Double-seated valves have the advantage of being able to cancel the hydrostatic (non-moving fluid pressure) and hydrodynamic ( moving fluid pressure) forces exerted on the valve plug.

If you examine the double-seated valve shown in Figure 6, you can see that the flow and differential pressure directions are opposite. Because of this opposing forces on the plug and stem tend to cancel each other. Double-seated valves are also called balanced valves. This allows you to use a smaller actuator for a double-seated valve than for a single-seated valve of the same size.

One disadvantage of the double-seated valve is that it does not shut off or seal as tightly as single-seated valve. This is because thermal expansion and machining tolerances prevent both valve plugs from being seated at exactly the same time.

Single-seated globe valve Double-seated globe valve

VALVE ACTION: A direct acting valve opens as the pressure on the diaphragm is increased (air-to-open). A reverse-acting valve closes as the pressure on the diaphragm is increased (air-to-close).

2 CAGE VALVE

Cage valves are somewhat similar to globe valves in that both control flow by raising and lowering a plug. In a cage valve, the plug is inside a fixed, slotted cage through which fluid flows. You can change the valve's flow control characteristics by changing the shape or size of the slots cut in the cage. The cage valves are available in sizes from 1 to 12 inches (2.5 to 30.3 cm) and with a rating up to 600 psi.

Double-seated and single-seated cage valves are available.

Balanced cage valve

Quick Opening Linear Equal Percentage Types of Cage Valve Trim

3 BUTTERFLY VALVES

Figure below shows a butterfly valve with a rotary actuator. In general, butterfly valves have an economical, straight-through flow design with a low pressure drop and reduced likelihood on clogging. This makes them specially good in slurry processes. But butterfly valves also have some disadvantages. They do not shut off as tightly as globe, their

allowable differential pressures are lower than those of globe valves, and you cannot change their flow characteristics. Butterfly valves are available with diameters from 2 to 24 inches. (5.1 to 60.4 cm) and with pressure rating of 150, 300, and 600 psi.

4 BALL VALVES

Fig Butterfly valve

A ball valve is similar to a butterfly valve except that its flow-control element is a sphere instead of a disk, and thus has a greater seating area. Because of this, the ball valves allow higher differential pressure and have better sealing characteristics than the butterfly valves. But these improvements add to the cost of the valves. In other words, ball valves perform better than the butterfly valves but are more expensive. The ball valves are available with diameter of 1 to 8 inches (2.5 to 20.3 cm) and with pressure ratings of 150, 300 and 600 psi.

Fig Ball Valve

5 SLIDING-GATE VALVES

Sliding-gate valves have a straight-through flow design, but can be shut off almost as tightly as a globe valve. The sliding-gate valve shown is available with diameters of 0.25 to 6 inches (0.6 to 15.2 cm) and with pressure rating of up to 1440 psi. In addition to good sealing characteristic, sliding-gate valves have a pressure drop that is lower than that of a globe valve but not as good of a butterfly valve. It uses a linear actuator with a very short strike and you can easily change flow characteristics by replacing its trim (plate and disk).

Fig Sliding-Gate Valve

6 DIAPHRAGM VALVES

Diaphragm valves (also known as Saunders valves) uses a flexible rubber (or rubber-like) diaphragm to control flow. Their operating principle is similar to kinking a garden hose. Diaphragm valves have a simple, straight-through flow path, are self cleaning, require no stem packing and can shut off as tightly as a butterfly valve. They should not be used in high-pressure temperature applications because of the nature of the diaphragm material. The diaphragm valve is available with diameters of 1 to 6 inches (25 to 15.2 cm) in pressure rating of 125 to 200 psi. The usual process temperature limit is 185°F (85°).

Fig Diaphragm Valve

7 SPLIT-BODY VALVES

The split-body valve is a variation of the basic globe control valve. The main difference is that the two parts of the valve's body are bolted together for easy maintenance. If you need to service a standard globe valve, you must first remove its actuator, yoke, bonnet and trim. To service a split-body valve, you only need to remove the body bolts holding its body together. For this reason, split-body valves are often used in process where corrosion or valve clogging frequently occurs.

8 OTHER CONTROL VALVES

In addition to the control valves just described, you may encounter more specialized valves. These include:

three-way valves (fluid can exit either one of the two ports)

three-way mixing valves (fluid A and fluid B enter, some mixture of fluid exits)

angle-globe valves (inlet and outlet flanges are at right to each other)

pressure regulator/reducer valves (reduce inlet fluid pressure to some preset value)

Uses of Valve Positioner

Fig Split Body Valve

Fig Three-Way Valve

Valve positioners are used for the following purposes:

1. To compensate for the forcing effects of the fluids causing an unbalancing valve plug.

2. To minimize the friction effect.

3. To increase the speed of response of the control valve.

4. To split the travel of valves.

Positioner

Electro PneumaticActuator

Positioner

Questions (Control Valve)

1. Name the parts of the controlled valve in the diagram.

2 State the function of a control valve. What is the range of control signal used to operate a pneumatic operated valve?

3 Explain the principle of operation of a pneumatic operated control valve with the aid of a basic diagram.

4 A globe valve has different flow characteristics for different types of valve plug used. Name the flow characteristics and sketch their corresponding valve plug.

5 With the aid of a graph, describe the different flow characteristics in a globe valve

6 Name the two major components of a control valve.

7 Name the factors to be considered when selecting the control valve for a particular application.

8 Why do we need a valve positioner?

9 State the uses of a valve positioner.

Polytechnic University of the PhilippinesCollege of EngineeringDepartment of Electronics and Communications Engineering

Ecen 3264 Industrial electronics

- Smart transmitters -- Transducers -

- Process sensors -

Prepared by:

Borromeo, olen r.

bsece 5-1

Smart Transmitters

• It is a telemetry device which converts measurements from a sensor into a signal, and sends it, usually via wires, to be received by some display or control device located a distance away.

• Process transmitters isolate, filter amplify, and convert sensor signals to 4-20mA current  or 0-10V voltage signals for interfacing with controllers and other instrumentation.

• Field instruments or smart transmitters monitor process control variables, such as temperature, pressure, level and flow.

• A smart transmitter is a digital device that converts the analog information from a sensor into digital information, which allows the device to simultaneously send and receive information and transmit more than a single value.

Features of Smart Transmitters

1. Digital Communications - Smart transmitters are capable of digital communications with both its configuration device and a process controller. Digital communications have the advantage of being free of bit errors, the ability to monitor multiple process values and diagnostic information and the ability to receive commands.Most smart instruments wired to multi-channel input cards require isolated inputs for the digital communications to work.

2. Configuration - Smart transmitters can be configured with a handheld terminal and store the configuration settings in nonvolatile memory.

3. Signal Conditioning - Smart transmitters can perform noise filtering and can provide different signal characterizations.

4. Self-Diagnosis - Smart transmitters also have self-diagnostic capability and can report malfunctions that may indicate erroneous process values.

Types of Field Instruments/Smart Transmitters

• Temperature Transmitter• Pressure Transmitter• Electromagnetic Flow Transmitter• Ultrasonic Flow Transmitter• Differential Pressure Flow Transmitter

Temperature Transmitter

Pressure Transmitter

Electromagnetic Flow Transmitter

Ultrasonic Flow Transmitter

Differential Flow Transmitter

Transducer

• A transducer is a device that transforms one form of energy into another.• Transducers are generally made as small as possible, and the energy being transferred is small.• Conversion between input and output is done quantitatively using a calibration process.• Transducers use basic physical laws to measure physical parameters using a sensing element that is the

heart of the transducer.• The parameters measured in a servo control systems are position and motion while the parameters

measured in process control systems are temperature, flow, level, pressure, pH, viscosity, color, salinity, and others.

Process Sensors

Things that we commonly measure are:Temperature PressureSpeed Flow rateForce Movement, Velocity and AccelerationStress and Strain Level or DepthMass or Weight DensitySize or Volume Acidity/Alkalinity

Sensors may operate simple on/off switches to detect the following:Objects (Proximity switch) Empty or full (level switch)Hot or cold (thermostat) Pressure high or low (pressure switch)

The block diagram of a sensor is shown below.

Temperature Transducers

1. Thermocouple

When two wires with dissimilar electrical properties are joined at both ends and one junction is made hot and the other cold, a small electric current is produced proportional to the difference in the temperature. Seebeck discovered

OUTPUTINPUTPrimary Transducer

this effect. It is true no matter how the ends are joined so the cold end may be joined at a sensitive millivolt meter. The hot junction forms the sensor end.

The picture shows a typical industrial probe with a flexible extension and standard plug.

Peltier showed that heat is absorbed at the hot end and rejected at the cold end. Thompson showed that part of the e.m.f. is due to the temperature gradient in the wire as well as the temperature difference between the junctions. Most thermocouple metals produce a relationship between the two temperatures and the e.m.f as follows.

e = (1 - 2) + (12 - 22

and are constants for the type of thermocouple. The relationship is nearly linear over the operating range. The actual characteristic and suitable operating temperatures depends upon the metals used in the wires. The various types are designated in international and national standards. Typical linear operating ranges are shown for standard types. It is important that thermocouples are standard so that the same e.m.f will always represent the same temperature.

Thermocouples come in several forms. They may be wires insulated from each other with plastic or glass fiber materials. For high temperature work, the wire pairs are put inside a tube with mineral insulation. For industrial uses the sensor comes in a metal enclosure such as stainless steel.

2. Resistance Type Sensors

These work on the principle that the electrical resistance of a conductor change with temperature. If a constant voltage is applied to the conductor then the current flowing through it will change with temperature. The resistivity of the conductor change with temperature. This usually means the resistance gets bigger as the conductor gets hotter. The following law relates the resistance and temperature.

R = Ro(1 + )

is the temperature coefficient of resistance. Ro is the resistance at 0oC. Sometimes the equation is given as

R = Ro(1 - 2)

A basic temperature sensor is made by winding a thin resistance wire into a small sensor head. The resistance of the wire then represents the temperature. This has an advantage over a thermocouple in that it is unaffected by the temperature of the gauge end. The main type of wire used is PLATINUM. The sensors are usually manufactured to have a resistance of 100 at 0oC and the value of is 0.00385 to 0.00390. A typical operating range is - 200 to 400oC.

A special type of resistance sensor is called a THERMISTOR. They are made from a small piece of semiconductor material. The material is special because the resistance changes a lot for a small change in temperature and so can be made into a small sensor and it costs less than platinum wire. The temperature range is limited. They are only used for a typical range of -20 to 120oC and are commonly used in small hand held thermometers for everyday use. The relationship between resistance and temperature is of the form R = AeB/

3. Liquid Expansion and Vapor Pressure Sensors

These are thermometers filled with either a liquid such as mercury or an evaporating fluid such as used in refrigerators. In both cases the inside of the sensor head and the connecting tube are completely full. Any rise in temperature produces expansion or evaporation of the liquid so the sensor becomes pressurized. The pressure is related to the temperature and it may be indicated on a simple pressure gauge.Ways and means exist to convert the pressure into an electrical signal. The movement may also directly operate a thermostat. These instruments are robust and used over a wide range. They can be fitted with electric switches to set off alarms.

4. Bimetallic Types

It is a well-known principle that if two metals are rigidly joined together as a two-layer strip and heated, the difference in the expansion rate causes the strip to bend.

In the industrial type, the strip is twisted into a long thin coil inside a tube. One end is fixed at the bottom of the tube and the other turns and moves a pointer on a dial. The outward appearance is very similar to the pressure type. They can be made to operate limit switches and set off alarms or act as a thermostat. (e.g. on a boiler).

5. Glass Thermometer

The ordinary glass thermometer is also a complete system. Again the bulb is the sensor but the column of liquid and the scale on the glass is the processor and indicator. Mercury is used for hot temperatures and colored alcohol for cold temperatures.

The problems with glass thermometers are that they areBrittleMercury solidifies at -40o C.Alcohol boils at around 120o C.Accurate manufacture is needed and this makes accurate ones expensive.It is easy for people to make mistakes reading them.

Glass thermometers are not used much now in industry but if they are, they are usually protected by a shield from accidental breakage. In order to measure the temperature of something inside a pipe they are placed in thermometer pockets.

Pressure Transducers

Pressure sensors either convert the pressure into mechanical movement or into an electrical output. Complete gauges not only sense the pressure but indicate them on a dial or scale.

Mechanical movement is produced with the following elements.- Bourdon Tube- Spring and Piston- Bellows and capsules- Diaphragm

1. Bourdon Tube

The Bourdon tube is a hollow tube with an elliptical cross section. When a pressure difference exists between the inside and outside, the tube tends to straighten out and the end moves. The movement is usually coupled to a needle on a dial to make a complete gauge. It can also be connected to a secondary device such as an air nozzle to control air pressure or to a suitable transducer to convert it into an electric signal. This type can be used for measuring pressure difference.

2. Piston Type

The pressure acts directly on the piston and compresses the spring. The position of the piston is directly related to the pressure. A window in the outer case allows the pressure to be indicated. This type is usually used in hydraulics where the ability to withstand shock, vibration and sudden pressure changes is needed (shock proof gauge). The piston movement may be connected to a secondary device to convert movement into an electrical signal.

3. Capsules and Bellows

A bellows is made of several capsules. These are hollow flattened structures made from thin metal plate. When pressurized the bellows expand and produce mechanical movement. If the bellows is encapsulated inside an outer container, then the movement is proportional to the difference between the pressure on the inside and outside. Bellows and single capsules are used in many instruments. They are very useful for measuring small pressures.

4. Diaphragms

These are similar in principle to the capsule but the diaphragm is usually very thin and perhaps made of rubber. The diaphragm expands when very small pressures are applied. The movement is transmitted to a pointer on a dial through a fine mechanical linkage.

5. Electrical Transducers

There are various ways of converting the mechanical movement of the preceding types into an electric signal. The following are types that directly produce an electric signal.

Strain Gauge typesPiezo electric typesOther electric effects

1. 4.1 Strain Gauge Types

The principles of electric strain gauges are covered later. Strain gauges are small elements that are fixed to a surface that is strained. The change in length of the element produces changes in the electrical resistance. This is processed and converted into a voltage. A typical pressure transducer would contain a metal diaphragm which bends under pressure.

1. 4.2 Piezo Electric Types

The element used here is a piece of crystalline material that produces an electric charge on its surface when it is mechanically stressed. The electric charge may be converted into voltage. This principle is used in the pick-up crystal of a record player, in microphones and even to generate a spark in a gas igniter. When placed inside a pressure transducer, the pressure is converted into an electric signal.

1. 4.3 Other Electric Effects

Other electric effects commonly used in transducers are CAPACITIVE and INDUCTIVE. In these cases, the pressure produces a change in the capacitance or inductance of an electronic component in the transducer. Both these effects are commonly used in an electronic oscillator and one way they may be used is to change the frequency of the oscillation. The frequency may be converted into a voltage representing the pressure.

Speed Transducers

Speed transducers are widely used for measuring the output speed of a rotating object. There are many types using different principles and most of them produce an electrical output.

1. Optical Types

These use a light beam and a light sensitive cell. The beam is either reflected or interrupted so that pulses are produced for each revolution. The pulses are then counted over a fixed time and the speed obtained. Electronic processing is required to time the pulses and turn the result into an analogue or digital signal.

2. Magnetic Pick Ups

These use an inductive coil placed near to the rotating body. A small magnet on the body generates a pulse every time it passes the coil. If the body is made of ferrous material, it will work without a magnet. A discontinuity in the surface such as a notch will cause a change in the magnetic field and generate a pulse. The pulses must be processed to produce an analogue or digital output.

3. Tachometers

There are two types, A.C. and D.C. The A.C. type generates a sinusoidal output. The frequency of the voltage represents the speed of rotation. The frequency must be counted and processed. The D.C. type generates a voltage directly proportional to the speed. Both types must be coupled to the rotating body. Very often the tachometer is built into electric motors to measure their speed.

Flow Meters

There are many hundreds of types of flow meters depending on the make and application. They may be classified roughly as follows.

POSITIVE DISPLACEMENT TYPESINFERENTIAL TYPESVARIABLE AREA TYPESDIFFERENTIAL PRESSURE TYPES

1. Positive Displacement Types

These types have a mechanical element that makes the shaft of the meter rotate once for an exact known quantity of fluid. The quantity of fluid hence depends on the number of revolutions of the meter shaft and the flow rate depends upon the speed of rotation. Both the revolutions and speed may be measured with mechanical or electronic devices.

Some of the most common listed below.Rotary piston typeVane typeLobe type or meshing rotorReciprocating piston typeFluted spiral gear

1.1. Meshing Rotor

The MESHING ROTOR type consists of two rotors with lobes. When fluid is forced in, the rotors turn and operate the indicating system.

1.2. Inferential Type Meters

The flow of the fluid is inferred from some effect produced by the flow. Usually this is a rotor which is made to spin and the speed of the rotor is sensed mechanically or electronically. The main types are:

Turbine rotor typesRotary shunt typesRotating vane typesHelical turbine types

1.2.1. Turbine Type

The pictures show two industrial flow meters.

The turbine type shown has an axial rotor which is made to spin by the fluid and the speed represents the flow rate. This may be sensed electrically by coupling the shaft to a small electric tachometer. Often this consists of a magnetic slug on the rotor which generates a pulse of electricity each time it passes the sensor.

1.2.2. Rotating Vane Type

The jet of fluid spins around the rotating vane and the speed of the rotor is measured mechanically or electronically.

1.2.3. Variable Area Type

There are two main types of this meterFloat type (Rotameter)Tapered plug type

1.2.3.1. Float Type

The float is inside a tapered tube. The fluid flows through the annular gap around the edge of the float. The restriction causes a pressure drop over the float and the pressure forces the float upwards. Because the tube is tapered, the restriction is decreased as the float moves up. Eventually a level is reached where the restriction is just right to produce a pressure force that counteracts the weight of the float. The level of the float indicates the flow rate. If the flow changes the float moves up or down to find a new balance position.

When dangerous fluids are used, protection is needed against the tube fracturing. The tube may be made of a non-magnetic metal. The float has a magnet on it. As it moves up and down, the magnet moves a follower and pointer on the outside. The position of the float may be measured electrically by building a movement transducer into the float.

1.2.3.2. Tapered Plug Type

In this meter, a tapered plug is aligned inside a hole or orifice. A spring holds it in place. The flow is restricted as it passes through the gap and a force is produced which moves the plug. Because it is tapered the restriction changes and the plug takes up a position where the pressure force just balances the spring force. The movement of the plug is transmitted with a magnet to an indicator on the outside.

1.2.4. Differential Flow Meters

These are a range of meters that convert flow rate into a differential pressure. The important types conform to BS 1042 and are

ORIFICE METERS.VENTURI METERSNOZZLE METERSPITOT TUBES

The diagram shows a cross section through the four types of differential flow meters.

The working principle for all these is that something makes the velocity of the fluid change and this produces a change in the pressure so that a difference p = p2 - p1 is created. It can be shown for all these meters that the volume flow rate Q is related to p by the following formula.

Q = K(p)0.5K is the meter constant. A full explanation of these meters is covered in the tutorials on fluid mechanics. The picture shows an industrial differential flow meter. Extra instrumentation heads can be fitted to produce an electrical output (4 – 20 mA) or a pneumatic output (0.2 – 1 bar).

Force Sensors

The main types of force sensors areMechanical typesHydraulic types

Electrical strain gauge types

1. Mechanical Types

Mechanical types are usually complete measuring systems involving some form of spring such as in a simple spring balance or bathroom scale. It is a basic mechanical principle that the deflection of a spring is directly proportional to the applied force so if the movement is shown on a scale, the scale represents force.

2. Hydraulic Types

Hydraulic types are often referred to as hydraulic load cells. The cell is a capsule filled with liquid. When the capsule is squeezed, the liquid becomes pressurized. The pressure represents the force and may be indicated with a calibrated pressure gauge. The capsule is often a short cylinder with a piston and the pressure produced is given by p = F/A where F is the force and A the piston area.

3. Strain Gauge Type

A typical load cell consists of a metal cylinder with strain gauges fixed to it. When the cylinder is stretched or compressed, the strain gauges convert the force into a change in resistance and hence voltage. Since the elements require a supply voltage, the cell usually has 4 wires, two for the supply and two for the output.Position Sensors

Position sensors are essential elements in the control of actuators. The position of both linear and rotary actuators is needed in robotic type mechanisms. There are three principle types.

RESISTIVEOPTICAL

INDUCTIVE

1. Resistive Types

A potentiometer is a variable electrical resistance. A length of resistance material has a voltage applied over its ends. A slider moves along it (either linear or rotary) and picks off the voltage at its position or angle. The tracks may be made from carbon, resistance wire or piezo resistive material. The latter is the best because it gives a good analogue output. The wire wound type produces small step changes in the output depending on how fine the wire is and how closely it is coiled on the track.

2. Optical Types

Optical types are mainly used for producing digital outputs. A common example is found on machine tools where they measure the position of the work table and display it in digits on the gauge head. Digital micrometers and verniers also use this idea. The basic principle is as follows. Light is emitted through a transparent strip or disc onto a photo electric cell. Often reflected light is used as shown. The strip or disc has very fine lines engraved on it which interrupt the beam. The numbers of interruptions are counted electronically and this represents the position or angle. This is very much over simplified and you should refer to more advanced text to find out how very accurate measurements are obtained and also the direction of movement.

3. Inductive Types

The most common of these is the Linear Variable Differential transformer or LVDT. The transformer is made with one primary coil and two secondary coils, one placed above and the other below the primary. The coils are formed into a long narrow hollow tube. A magnetic core slides in the tube and is attached to the mechanism being monitored with a non-magnetic stem (e.g. brass). A constant alternating voltage is applied to the primary coil. This induces a voltage in both secondary coils. When the core is exactly in the middle, equal voltages are induced and when connected as shown, they cancel each other out. When the core moves, the voltage in one secondary coil grows but reduces in the other. The result is an output voltage which represents the position of the core and hence the mechanism to which it is attached. The output voltage is usually converted into D.C. With suitable electronic equipment for phase detection, it is possible to detect which direction the core moves and to switch the DC voltage from plus to minus as the core passes the center position. These can be very accurate and are widely used for gauging the dimensions of machined components.

Depth Gauges

Depth gauges measure the depth of liquids and powder in tanks. They use a variety of principles and produce outputs in electrical and pneumatic forms. The type to use depends on the substance in the tank.

Here are a few.

The ultrasonic system reflects sound waves from the surface and determines the depth from the time taken to receive the reflected sound. The electronic version uses a variety of electrical affects including conduction of the fluid and capacitance. The pneumatic version bubbles air through the liquid and the pressure of the air is related to the depth. A simple pressure gauge attached to a tank is also indicates the depth since depth is proportional to pressure.

Strain Gauges

Strain gauges are used in many instruments that produce mechanical strain because of the affect being measured. In their own right, they are used to measure the strain in a structure being stretched or compressed. The strain gauge element is a very thin wire that is formed into the shape shown. This produces a long wire all in one direction but on a small surface area. The element is often formed by etching a thin foil on a plastic backing. The completed element is then glued to the surface of the material or component that will be strained. The axis of the strain gauge is aligned with the direction of the strain. When the component is stretched or compressed, the length of the resistance wire is changed. This produces a corresponding change in the electrical resistance.

Let the length of the gauge be L and the change in length be L. The mechanical strain = L/L. Let the resistance of the gauge be R (typically 120 ) and the change in resistance be R.

The electrical strain = R/R.

The electrical and mechanical strains are directly proportional and the constant relating them is called the gauge factor (typically 2).

Gauge Factor = Electrical Strain/Mechanical strain = /= L R/R LAssessment Questions

1. It is a digital device that converts the analog information from a sensor into digital information, which allows the device to simultaneously send and receive information and transmit more than a single value.Answer: Smart Transmitter

2. Give the four features of smart transmitters.Answer: Digital Communication, Configuration, Signal Conditioning, Self-Diagnosis

3. Give the different types of Field Instruments/Smart Transmitters.Answer:• Answer: Temperature Transmitter• Pressure Transmitter• Electromagnetic Flow Transmitter• Ultrasonic Flow Transmitter• Differential Pressure Flow Transmitter

4. It is a device that transforms one form of energy into another.Answer: Transducer

5. Conversion between input and output in a transducer is done quantitatively using what process?Answer: Calibration Process

6. Process transmitters isolate, filter amplify, and convert sensor signals to _______________ signals for interfacing with controllers and other instrumentation.Answer: 4-20mA current or 0-10V voltage

7. It is a special type of resistance sensor made from a small piece of semiconductor material.Answer: Thermistor

8. It either converts the pressure into mechanical movement or into an electrical output. Answer: Pressure Sensor

9. The element used here is a piece of crystalline material that produces an electric charge on its surface when it is mechanically stressed.Answer: Piezo Electric Types

10.These are widely used for measuring the output speed of a rotating object.Answer: Speed Transducers

11.This type of speed transducer uses a light beam and a light sensitive cell. The beam is either reflected or interrupted so that pulses are produced for each revolution. The pulses are then counted over a fixed time and the speed obtained.Answer: Optical Type

12.These types of flow meters have a mechanical element that makes the shaft of the meter rotate once for an exact known quantity of fluid.Answer: Positive Displacement Types

13.In this type of flow meter the flow of the fluid is inferred from some effect produced by the flow. Usually this is a rotor which is made to spin and the speed of the rotor is sensed mechanically or electronically.Answer: Inferential Type Meters

14.Give the different types of force sensors.Answer: Mechanical Types, Hydraulic Types, Electrical Strain Gauge Types

15.It measures the depth of liquids and powder in tanks.Answer: Depth Gauge

Resources:

D.J.Dunn, Instrumentation and Control, Tutorial 2 – Sensors and Primary TransducersPractical Process Control, “Fundamentals of Process and Instrumentation Control”Peng Zang, Industrial Control Technology: A Handbook for Engineers and Researchers