Instrument Notes

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TRANSMITTER SELECTION

This section presents a number of considerations that should be viewed in selecting atransmitter. They include functional specifications, performance specifications,materialselection and desirable features. Also included are the definitions of thesespecifications and the relationship to functional and performance requirements.

Functional Specifications

1. Temperatures

2. Pressure

3. Environment

4. Hazardous Locations

5. Damping6. Zero Elevation or Suppression

7. Power Supply and Load Limits

Temperatures ^UP

Both the maximum process and ambient temperatures need to be considered. Oftenthe process temperature will exceed the limits of the sensing element. The sensingelement of most electronic pressure transmitters will not operate properly above 225F(107 C). This will require the use of good impulse piping practices to get thetransmitter temperature back within operating limits. High ambient temperatures on

solid state electronics adversely affect component life. Most electronics are not ratedfor service above 200 F (90C) and there are many components with a 185F (85C)rating. High temperatures tend to cause more electronic failures. Again, it is goodengineering practice to keep the electronics package as cool as possible.

Winterizing, either by steam tracing, electrical heaters, or heater controlled enclosuresmay also be a consideration.

Pressure ^UP

Both the operating pressure range and the maximum pressure should be considered.

Gauge pressure transmitters should have an overpressure rating of at least 150percent of the maximum rating operating pressure with no other ill effect than havingto recalibrate.

The minimum pressure should be also be considered. As part of the normal operation,a vacuum may be applied. Many transmitters have to be ordered special to obtain thiscapability.

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On differential pressure transmitters, overpressure may be accidentally applied toeither the high or low side of the unit when a three-value manifold is not sequencedproperly. High overpressure capability eliminates a possible shut-down while the unitis being recalibrated or repaired.

The static line pressure for differential transmitters should also be called out. Units areavailable on the market with standard line pressure capability from 500 to 6,000 psi.

Environment ^UP

The transmitter should be capable of operating in environments with 0 to 100%relative humidity.The working fluid and the ambient environment should be consideredfor corrosiveness. For instance, transmitters used on offshore oil rigs are subject tocorrosion from salt water. Another example is a transmitter in a steam or coolingwater system in the vicinity of acids or bases that tend to get into the atmosphere.The above applications have a non-corrosive working fluid, and a very corrosiveambient environment.

Hazardous Locations ^UP

Use of Instruments in Hazardous Locations:The Williams-Steiger Occupational Safety and Health Act of 1970 (OSHA), Subpart S,Electrical Considerations, has been in effect since 15 February, 1972. The purpose ofOSHA is to accelerate the adoption of national standards for occupational safety. TheAct given the Secretary of Labor two years to promulgate the adoption of suchstandards.

All electrical instruments or electrical equipment used in hazardous locations must nowbe approved. Equipment or an installation is acceptable to the Assistant Secretary ofLabor, and approved within the meaning of Subpart S if it is accepted, or certified, orlisted, or labeled, or otherwise determined to be safe by a nationally recognizedtesting laboratory, such as, but not limited to, Underwriters Laboratories Inc. andFactory Mutual Engineering Corp.

Definition of Hazardous Locations:

Class I, Division ILocations in which hazardous concentrations of flammable gases or vapors existcontinuously, intermittently, or periodically under normal operating conditions.

Class I, Division II

Locations in which volatile flammable gases are hazardous liquids, vapors or gases willnormally be confined within closed containers or closed systems from which they canescape only in case of accidental rupture or breakdown of such systems or containers,or in case of abnormal operation of equipment.

Class II Locations

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Locations which are hazardous because of the pressure of combustible dust.

Class III LocationsLocation in which easily ignitable fibers or materials producing combustible flyings are

present.

Group AAtmosphere containing acetylene.

Group BAtmospheres containing hydrogen or gases or vapors of equivalent hazards such asmanufactured gas.

Group CAtmospheres containing ethyl ether vapors, ethylene, or cyclopropane.

Group DAtmospheres containing gasoline, hexane, naptha, benzine, butane, alcohol, benzol,lacquer solvent vapors, or natural gases.

Group EAtmospheres containing metal dust, including aluminum, magnesium, and theircommercial alloys, and other metals of similar hazardous characteristics.

Group FAtmospheres containing carbon black coal or coke dust.

Group G

Atmospheres containing flour, starch, or grain dusts.

Explosion- Proof Enclosure

Explosion-proof enclosure means an enclosure for electrical apparatus which is capableof withstanding, without damage, an explosion which may occur within it, of specifiedgas or vapor, and capable of preventing ignition of specified gas or vapor surroundingthe enclosure from sparks or flames from explosion of specified gas or vapor within theenclosure.To make a system explosion-proof, the enclosure must be capable of withstanding anexplosion, and the system must be installed per national electrical code for hazardouslocations.

Intrinsically Safe Equipment

Intrinsically safe equipment and wiring are incapable of releasing sufficient electricalenergy under normal or abnormal conditions to cause ignition of specific hazardousatmospheric mixture. Abnormal condition will include accidental damage to any part ofthe equipment or wiring, insulation, or other failure of electrical components,application of overvoltage, adjustment and maintenance operations, and other similar

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conditions.

Equipment built for this requirement is designed with low energy storage componentsas outlined in ISA procedure RP12-2

Several advantages to the intrinsic safety approach are listed below. These advantageshave to be weighed against the initial higher purchase price. Today it is estimated that60 percent of these types of installations are classified as intrinsically safe.

Lower installation cost.

Less operator-dependent to maintain a safe system.

Easier to maintain and repair.

Accessible to repair without special precautions before opening the unit.

Damping ^UP

In some applications, pump or other process noise pulses must be dumped out to getgood control or indication. The more unit is damped , as specified by the cornerfrequency, the slower the response time. In other cases where the system dynamicsrequire fast transmittrer reponse for best performance. A review of the specificapplication is necessary to determine the requirements for adequate performance.However, in most cases this is not a serious problem.

( Figure 7)

Damping is defined in terms of corner frequency or "time constant." Corner frequencyis the junction of two confluent straight line segments of a plotted curve (see Figure

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7).

In a first-order system, the frequency at which the magnitude ratio is down 3db

Db = 20 log Eo / Ein

For the output of a first order system forced by a step or an impulse, the timeconstant is the time required to complete 63.2 percent of the total rise or decay.

In a Bode diagram, the breakpoint or corner frequency occurs where:

Fc = 1 / 2p T

Where T = Time constant,

Fc = Corner Frequency.

(figure 8)

Output ^UP

The standard output for two-wire transmitters is 4 - 20 mA dc or 10 - 50 mA dc. Thereare also four-wire transmitters that can provide zero-based voltage signals. The most

common is 0 - 5 V dc.Three-wire transmitters are also available, which can provide a4- 20 mA dc, 10 -50 mA dc, or zero-based signal.

The two-wire device, as the name implies, only has two wires to the transmitters.These wires are used for both power and signal (see figure 8).The two-wire unitrequires an external dc voltage power supply.

The three-wire device also requires an external dc voltage power supply with one lead

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as common for both signal and power.

The four-wire device has two wires for signal and two for power.The power required isusually 115V ac, and the unit has built-in transformer, rectifier, and regulator.

(Table 1)

Zero Elevation or Suppression ^UP

Zero Elevation-for an elevated zero range, the amount the measure variable zero isabove the lower range value. It may be expressed either in units of measuredvariables or in percent of span(see Table 1).

Zero Suppression-for a suppressed zero range, the amount the measured variable zerois below the lower range value. It may be expressed either in units of the measuredvariable, or in percent of span (see Table 1).

Gage pressure ranges are usually expressed in pounds per square inch gage (i.e., 0-100 psig). The range may have a suppressed zero (i.e., 50 to 100 psig), or it may be acompound range (i.e., 20 inHg vacuum to 50 psig).

Absolute ranges are usually expressed in inches of mercury absolute or psia (i.e., 0-30HgA or 0-100 psia). The most common range with a suppressed zero is the barometricrange (i.e., 28 to 32 in HgA).

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(Figure 9).

Power Supply and Load Limits ^UP

The choice of a power supply for two-wire transmitters will depend on the load. Mosttransmitters are capable of operating over a wide range of load limits (see Figure 9).

Wiring should consist of twisted pairs. Most transmitters do not require shielding, but itis recommended to eliminate noise pickup from electric motors, inverters, or othernoise generating electrical equipment in the area getting into the receiver.

The size of the wire is usually not critical; 18-gauge is usually sufficient. The resistanceof the wire adds to the total load, and in most cases simply requires a power supply ofsufficient voltage to handle the entire voltage drop across the system.

Special considerations should be given when using computers, especially wheresampling times are short. Electronic transmitters often have internally generatesdriving frequencies, which may show up as high frequency noise on the output.As anexample, assume the sample time in 85 microseconds, and assume some 50 kHzfrequency noise on the line, which would have an equivalent period of approximately6.4 microseconds. The integration time to average out this noise is only 13.4 cycles.This does not allow enough time to completely integrate a high level noise. If it issignificantly high, a filter should be considered

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Types of Level Sensors

Conductivity/Capacitive-Level Sensors

Float Type Sensor

Heat Transfer Level Sensor

Inductive Level Transducer

Photoelectric

Pressure Type

Ultrasonic sensors

Vibrating Element

Weighing

Conductivity/Capacitive-Level

Sensors ^Conductivity and capacitive level sensors serve as a continuous and point-level sensors by measimpedance between two electrodes immersed in the liquid or between one electrode and theelectroconductive tank's wall.

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Capacitive level sensors with two (a) or one (b) electrodes. L = level, Z = impedance, 1 = tank, 3 and 4 = electrodes.

Float Type Sensor^

In a float-type level sensor the buoyancy force holds the float on the surface of the liquid. The flo

a member having a magnetic coupling with a transduction element (coil, magnetic reed, or Hall-switch), that is mounted on the outside wall of the tank and can be actuated by the proximity of In some designs, the float mechanically links the switching mechanism through the sealing in thebellows). The switching system can respond to the restraining force developed by a spring elemeconnected to the float or by an actuator of a force-balance servo system.

Float-type sensors with magnetic coupling (a) or mechanical link (b). L = level,1 = tank, 2 = liquid, 3 = float, 4 = magnet, 5 = magnetic armature, 6 = contacts,7 = bellows, 8 = lever.

Heat Transfer Level

Sensor ^

Heat-transfer level sensors are built from a heated (usually self-heated) wire, thermistor, or therwhose heat transfer undergoes a step change when the transition from gas to liquid takes place.change causes the change in the element's resistance or electromotive force.

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Heat-transfer level sensor. L = level, R = resistance, 1 = tank, 2 = liquid, 3 = resistive heated e

Inductive LevelTransducer ^

An inductive-level transducer finds its application in the measurement of the level of liquid metaelectroconductive liquids. In one of the designs, a coil is wound around a tube containing the liquinductance of this coil changes rapidly as the liquid moves and approaches the coil. In another d

transducer is introduced by a transformer with a primary coil wound on one limb of a twin-limbeThe other limb is enclosed by a tube containing the liquid and forming one turn of the secondaryThe effective resistance of this turn is inversely proportional to the height of the liquid column inThe change in the height can be sensed by measuring the power consumption at the primary coi

Variable-inductance level transducer (a), and transformer-type level transducer (b).L = level, Z = impedance, 1 = tank, 2 = liquid, 3 = coil, 4 = core.

Photoelectric^

Photoelectric level sensors operate in transmittance or reflection modes. In the transmittance mosensing system, including a light beam source and a photodetector, responds to the interruptionattenuation of the light beam when the liquid breaks the beam path from the source to the detecreflection mode, an optical prism mounted inside a tank changes the reflectance of the light wheimmersed in the liquid. The construction of the transducer is arranged so that a light source andphotodetector for sensing the change in the light's intensity are mounted on the outside wall of tThe light beam passes through and is reflected from the faces of the prism.

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Transmittance-mode (a) and reflectance-mode (b) photoelectric level sensors. L = level, 1 = tanliquid, 3 = light source, 4 = photodetector, 5 = prism.

Photoelectric level sensors operate in transmittance or reflection modes. In the transmittance mosensing system, including a light beam source and a photodetector, responds to the interruptionattenuation of the light beam when the liquid breaks the beam path from the source to the detecreflection mode, an optical prism mounted inside a tank changes the reflectance of the light wheimmersed in the liquid. The construction of the transducer is arranged so that a light source andphotodetector for sensing the change in the light's intensity are mounted on the outside wall of tThe light beam passes through and is reflected from the faces of the prism.

Transmittance-mode (a) and reflectance-mode (b) photoelectric level sensors. L = level, 1 = tanliquid, 3 = light source, 4 = photodetector, 5 = prism.

Pressure Type

^

A pressure-type level sensing system contains a pressure transducer mounted at the bottom of afilled tank. The transducer responds to the pressure developed by the weight of the liquid's colum

pressure is directly proportional to the measured height.

Pressure-type level sensing system. L = level, 1 = tank, 2 = liquid, 3 = pressure transdu

Ultrasonic sensors^

Several sensing techniques are used in ultrasound-level sensors, including:

Oscillations of quartz, ceramic or magnetostrictive elements at an ultrasound frequency hgreater amplitude in gas than in liquid. Wetting the elements causes a decrease in the amproviding the detection of the liquid level.

Point-level or continuous-level sensing is provided by measuring the time lapse between ttransmission and reception of the ultrasound pulses generated by ceramic crystals at the the tank. Usually one crystal acts, alternately transmitting and receiving pulses that pass liquid height and are reflected from the surface back to the tank bottom. Some constructseparate elements for generating and receiving the pulses.

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A point-level detection is also performed by two piezoceramic crystals oriented toward eaacross the inside of a tank. One of the crystals transmits ultrasonic waves and the other oreceives them. The transmission is intensified when the liquid wets the crystals. The increoutput voltage of the receiving crystal indicates that the level has reached the specific po

Ultrasound-level sensors, a, b, and c = level-sensing systems with one crystal at side (a), bottomtwo crystals at side (c) of tank; L = level, 1 = tank, 2 = liquid,3 = piezoelectric crystal, 4 = pulse generator, 5 = pulse receiver.

Vibrating Element

^

In a vibrating-element level sensor, the oscillations of a member (paddle) are damped when it isin the liquid. The attenuation of oscillations indicates that the liquid has reached the measured leoscillations are stimulated and sensed by electronic means.

Vibrating-element level sensor. L = level, 1 = tank, 2 = liquid, 3 = vibrating paddle, 4 = excitati

Weighing^

A weighing sensing system for measuring level determines the level with load cells placed underbottom of the tank or connected to the tank by a mechanical link. If the tank's weight and liquidare known, the level is readily calculated using data obtained with the cells.

Weighing sensing system for measuring level. L = level, 1 = tank, 2 = liquid, 3 = load cell.

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TECHNICAL UTILITIES

Types of Level Sensors

Conductivity/Capacitive-Level Sensors

Float Type Sensor

Heat Transfer Level Sensor

Inductive Level Transducer

Photoelectric

Pressure Type

Ultrasonic sensors

Vibrating Element

Weighing

Conductivity/Capacitive-Level

Sensors ^

Conductivity and capacitive level sensors serve as a continuous and point-level sensors by measimpedance between two electrodes immersed in the liquid or between one electrode and theelectroconductive tank's wall.

Capacitive level sensors with two (a) or one (b) electrodes. L = level, Z = impedance, 1 = tank, 3 and 4 = electrodes.

Float Type Sensor

^

In a float-type level sensor the buoyancy force holds the float on the surface of the liquid. The floa member having a magnetic coupling with a transduction element (coil, magnetic reed, or Hall-switch), that is mounted on the outside wall of the tank and can be actuated by the proximity of In some designs, the float mechanically links the switching mechanism through the sealing in the

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bellows). The switching system can respond to the restraining force developed by a spring elemeconnected to the float or by an actuator of a force-balance servo system.

Float-type sensors with magnetic coupling (a) or mechanical link (b). L = level,1 = tank, 2 = liquid, 3 = float, 4 = magnet, 5 = magnetic armature, 6 = contacts,7 = bellows, 8 = lever.

Heat Transfer Level

Sensor ^

Heat-transfer level sensors are built from a heated (usually self-heated) wire, thermistor, or therwhose heat transfer undergoes a step change when the transition from gas to liquid takes place.change causes the change in the element's resistance or electromotive force.

Heat-transfer level sensor. L = level, R = resistance, 1 = tank, 2 = liquid, 3 = resistive heated e

Inductive LevelTransducer ^

An inductive-level transducer finds its application in the measurement of the level of liquid metaelectroconductive liquids. In one of the designs, a coil is wound around a tube containing the liquinductance of this coil changes rapidly as the liquid moves and approaches the coil. In another dtransducer is introduced by a transformer with a primary coil wound on one limb of a twin-limbeThe other limb is enclosed by a tube containing the liquid and forming one turn of the secondaryThe effective resistance of this turn is inversely proportional to the height of the liquid column in

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The change in the height can be sensed by measuring the power consumption at the primary coi

Variable-inductance level transducer (a), and transformer-type level transducer (b).L = level, Z = impedance, 1 = tank, 2 = liquid, 3 = coil, 4 = core.

Photoelectric

^

Photoelectric level sensors operate in transmittance or reflection modes. In the transmittance mosensing system, including a light beam source and a photodetector, responds to the interruptionattenuation of the light beam when the liquid breaks the beam path from the source to the detecreflection mode, an optical prism mounted inside a tank changes the reflectance of the light wheimmersed in the liquid. The construction of the transducer is arranged so that a light source andphotodetector for sensing the change in the light's intensity are mounted on the outside wall of tThe light beam passes through and is reflected from the faces of the prism.

Transmittance-mode (a) and reflectance-mode (b) photoelectric level sensors. L = level, 1 = tanliquid, 3 = light source, 4 = photodetector, 5 = prism.

Photoelectric level sensors operate in transmittance or reflection modes. In the transmittance mosensing system, including a light beam source and a photodetector, responds to the interruptionattenuation of the light beam when the liquid breaks the beam path from the source to the detecreflection mode, an optical prism mounted inside a tank changes the reflectance of the light wheimmersed in the liquid. The construction of the transducer is arranged so that a light source and

photodetector for sensing the change in the light's intensity are mounted on the outside wall of tThe light beam passes through and is reflected from the faces of the prism.

Transmittance-mode (a) and reflectance-mode (b) photoelectric level sensors. L = level, 1 = tan

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liquid, 3 = light source, 4 = photodetector, 5 = prism.

Pressure Type

^

A pressure-type level sensing system contains a pressure transducer mounted at the bottom of afilled tank. The transducer responds to the pressure developed by the weight of the liquid's columpressure is directly proportional to the measured height.

Pressure-type level sensing system. L = level, 1 = tank, 2 = liquid, 3 = pressure transdu

Ultrasonic sensors

^

Several sensing techniques are used in ultrasound-level sensors, including:

Oscillations of quartz, ceramic or magnetostrictive elements at an ultrasound frequency hgreater amplitude in gas than in liquid. Wetting the elements causes a decrease in the amproviding the detection of the liquid level.

Point-level or continuous-level sensing is provided by measuring the time lapse between ttransmission and reception of the ultrasound pulses generated by ceramic crystals at the the tank. Usually one crystal acts, alternately transmitting and receiving pulses that pass liquid height and are reflected from the surface back to the tank bottom. Some constructseparate elements for generating and receiving the pulses.

A point-level detection is also performed by two piezoceramic crystals oriented toward eaacross the inside of a tank. One of the crystals transmits ultrasonic waves and the other oreceives them. The transmission is intensified when the liquid wets the crystals. The increoutput voltage of the receiving crystal indicates that the level has reached the specific po

Ultrasound-level sensors, a, b, and c = level-sensing systems with one crystal at side (a), bottomtwo crystals at side (c) of tank; L = level, 1 = tank, 2 = liquid,3 = piezoelectric crystal, 4 = pulse generator, 5 = pulse receiver.

Vibrating Element

^

In a vibrating-element level sensor, the oscillations of a member (paddle) are damped when it isin the liquid. The attenuation of oscillations indicates that the liquid has reached the measured le

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oscillations are stimulated and sensed by electronic means.

Vibrating-element level sensor. L = level, 1 = tank, 2 = liquid, 3 = vibrating paddle, 4 = excitati

Weighing

^

A weighing sensing system for measuring level determines the level with load cells placed underbottom of the tank or connected to the tank by a mechanical link. If the tank's weight and liquidare known, the level is readily calculated using data obtained with the cells.

Weighing sensing system for measuring level. L = level, 1 = tank, 2 = liquid, 3 = load cell.

Back

Introduction

If you think Fieldbus is merely a digital replacement for the 4-20 mA signals thatnowadays link field devices to the control room equipment, may it be a single stationcontroller or DCS, you are in for a shock. That is only a fraction of the beauty ofFieldbus, since it brings so much more.

The Reason for Fieldbus

Fieldbus is not only a new intelligent/smart transmitter protocol. Fieldbus ischaracterized by three criteria:

Completely digital replacement of 4-20 mA.

Control, alarm, trend and other functions distributed to devices in the field.

Interoperable multivendor.

Open system; specification available without licensing agreement

Fieldbus devices should not even be called smart devices, because that might misleadthe user to believe that existing intelligent transmitters can do the same thing.

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Fieldbus is a complete system, with the control function distributed to equipment inthe field, while still allowing operation and tuning from the control room using thedigital communication. It replaces the traditional 4-20 mA and the classic DCS wherethe control function was centralized to one or more 'control cards.'

Some manufacturers argue that their DCS has had Fieldbus for many years, but noneof them meet the four criteria above.

Figure 1.1

Direct Digital Control (DDC) system -- The first computerized system wherecontrol is centralized to a single computer in the control room.

Figure 1.2Distributed Control System (DCS) -- The control partially distributed to a few

control cards, still in the control room, each having several loops.

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Figure 1.3

Fieldbus system -- Control totally distributed to field with loops in individual

devices.

Fieldbus is an interoperable multivendor protocol, which started as standardizationwork by ISA, the Instrumentation Society of America, just like the 4-20 mA standard.Fieldbus is expected to get worldwide recognition. All major instrument manufacturers

have pledged their commitment to a single Fieldbus standard. The approach was toestablish the standard before commercial products were in place; as with moststandards, it looks as if that will not be the way.

Some advantages of bi-directional digital communications, over 4-20 mA, and othergood features are known from existing smart transmitter protocols:

Higher accuracy and data reliability

Multi-variable access

Remote configuration and diagnostics

Reduction of wiring

Use existing 'analog wiring'

Higher accuracy can be attributed to digital communication, since the microprocessorsin, e.g., a transmitter and a controller may talk directly, rather than going throughD/A and A/D conversions, of which there may be many in a closed loop. Status ispassed along with measurement and control data. It is therefore possible todetermine if the information is reliable or not. All data are checked and guaranteedfree from distortion due to noise or an impedance mismatch that may affect analogundetected signals. Multi-variable access, meaning, e.g., that a pressure transmitteris not limited to a single output for pressure, but also informs process temperature.Another example is access to setpoint and manipulated variable of a controller in thesame device, or multiple channel inputs for a temperature transmitter.

The digital communication allows the complete configuration to be changed remotely.Calibration can be done in operation without having to apply any input or measurethe output. Similarly, the status of the selfdiagnostics may be interrogated.Reduction in wiring and simplification is achieved through connection of severaldevices on a single pair of wires -- multidropping. Connection is a simple task, sinceeverything is in parallel and terminal number matching is at a minimum. This meanslow cost and easy replacement of old transmitters.

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Some problems and disadvantages of existing protocols, in comparison to 4-20 mAtechnology, have also been seen:

Communication speed too low for closed loop control

Poor or no Interoperability between devices of different type andmanufacturer.

Devices must be polled for status

Not interdevice

The slowest option for Fieldbus is 25 times faster, and far more efficient, than themost common smart transmitter protocol, ensuring tight closed loop control. The lowspeed version of Fieldbus was designed to use the same type of wiring as analog andsmart transmitters, being able to easily replace them. However, it should be notedthat in order to use the multidrop feature, transmitters must be connected in parallel.

Figure 1.4With analog signals -- each individual variable has a physical 4-20 mA

connection.

Figure 1.5With intelligent/smart hybrid of analog and digital signal -- individual

variables involved in closed loop control have a physical 4-20 mA connection.Superimposed on it is a digital signal for configuration and diagnostics.

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Figure 1.6With Fieldbus pure digital communication -- a single physical wire is the

medium for multiple variables having logical connections.

Many smart transmitter protocols are proprietary and unique to a singlemanufacturer. For the user this lack of standard means he is locked to a singlemanufacturer, if he selects their system. If that manufacturer cannot provide anurgent replacement, or does not have a particular type of transmitter, the user's onlyoption is to return to 4-20 mA. The user depends on a single vendor that cannotprovide the latest technology and features. The vendor is in a position where he canset any price.The ideal opposite to a proprietary system is an 'open' system. Open systems arebased on off-the- shelf standards enabling multiple vendors to provide interoperablehardware and software.The ability of 4-20 mA devices to replace any other device of the same type is called

'interoperability' roughly meaning compatible. Fieldbus offers the same capability. Abrand 'Y' transmitter can be replaced by a brand 'X' transmitter of the same type anytime, without loss of functionality, and can interface to another brand 'Z' device.Fieldbus forces interoperability between the devices, which are complying with thisstandard and it is also available to all manufacturers and users without licensingagreements. It is 'open', it is fully disclosed, there are no 'secrets'. The user or a thirdparty can make their own configurations and software. Users may now select a devicebased on price, performance, quality and delivery time. They may mix and match thebest of each type, just as they could with the 4-20 mA. They do not have to choose adevice manufactured by a certain company just to match other devices of the samebrand already installed (without Fieldbus, users would have been forced to developspecial communication drivers in this case). Another benefit of interoperability is thatsystem software does not have to be upgraded when new products are introduced.

The lack of standard means that smart features of existing intelligent field devices arelargely unused. Communication is only used for calibration. For example, there arevery few existing systems that have managed to make use of the multidrop capabilityof existing smart transmitters. Though smart devices have self diagnostics, status isonly informed when device is polled. This is rare in most applications sincecommunication is normally done only at calibration. One must suspect and query for afailure before one finds out about it.

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The Fieldbus advantages for users are so apparent that if they were the ones todecide, Fieldbus would probably have already been chosen as the internationalstandard. End users have had very little information and opportunity to affect thedevelopment of the standard. It appears that they are facing the problem of having tochoose between one technology made available by a group of manufacturers, or

another, as it emerges. Such a situation is very similar to the decision users wereforced to make some years ago about which video recording technology they wouldbuy for their homes. The user's fear is to choose the 'wrong' protocol in the samefashion some chose the 'wrong' video technology.Many manufacturers, on their end, also want Fieldbus to be ready. The manufacturersthat really can benefit from Fieldbus are those that do have complete control systempackages, but only a good transmitter or a good valve. They do not have to ventureinto system design, developing a complete DCS. Field device manufacturers arerelieved from man-machine interfaces such as configuration and supervisorysoftwares. These can be written by software companies that are specialists on thatfield. The latter are also relived from the never-ending development of

communication drivers for every new protocol they come across. Manufacturers wantFieldbus ready because they know that many users hold back purchase of newsystems awaiting Fieldbus, in fear of buying the 'wrong' system.HART(TM) is a smart transmitter protocol that has reached almost worldwideacceptance, and is strengthening its position each year that Fieldbus is delayed.Though it meets the requirement of intrinsic safety and reaches a fair amount ofinteroperability, (more than most believe) its speed is a major limitation. PROFIBUSand FIP are other standardized open and fast protocols, but they are not intrinsicallysafe and the bus cannot provide power for the devices, thereby requiring four wires.

Turning to Fieldbus

The impact on the user, when turning to Fieldbus will be great. The main points are:

Even higher reliability operation

Virtually unlimited flexibility

Reduction in equipment cost.

Reduction in installation cost.

Mass of information.

The speed by which Fieldbus will capture the market depends largely on how fast theusers are trained. One does not feel confident to buy something if one does notunderstand how it works.The simplicity of analog technology makes it easy to be understood. That is the main

reason people feel so comfortable with it. 4-20 mA devices may be operated usingonly a screwdriver and tested with the most basic current meter. Almost anybodycould configure and troubleshoot such devices.Field devices may report failures and problems immediately, enabling maintenancepersonnel to pinpoint errors instantly or even before they can cause any harm.The multivariable capability of Fieldbus allows control, totalization and other signalprocessing in the field. Therefore, a separate controller or other signal conditioning

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equipment is not necessary. The host may be a simple off-the-shelf PC with MMIsoftware. Multidropping of several devices on a single wire may drastically reduce theamount of needed cable. In many factories, a device can be a kilometer or evenfurther away from the control room. Since every loop needs at least two pairs of wires(one for the transmitter, the other for the actuator) a refinery can have several

hundreds of such loops. In all, the cabling saving for a medium or large factory isimmense. Transmitters and actuators are often located next to each other, but farfrom the operator console, an ideal situation for multidropping. Though prices ofFieldbus devices may initially be high, the reduction in number of devices and wiring,with associated cable trays and marshaling boxes, will yield a less expensive system.Manufacturers can no longer rely on proprietary technology to keep prices high.Fieldbus will bring open competition, which will eventually reduce prices.The multivariable access will virtually flood the control room with information. Classicrecorders will not be able to handle the job. Paperless recorders and computerstorage will take over. Such information may be used for statistical process controland other process management.

Fieldbus has, on top of this, software function blocks which replace many functionstoday performed by hardware. This provides tremendous flexibility since the controlstrategy may be edited without having to rewire or change any hardware. Oncephysically connected, logical connections between function blocks may be changed,function blocks can be added and removed. More advanced devices may execute avirtually unlimited number of function blocks. If a system has to be expanded orimproved, the need for additional hardware is minimized, just by letting existingdevices execute more blocks.

Fieldbus requirements

The main requirements for Fieldbus were to overcome the problems of smarttransmitter protocols while maintaining the advantages of 4-20 mA standards (the

main advantage of 4-20mA is tight closed loop control).By providing various options for communication speed and device powering, therequirements for both intrinsic safety and minimum communication delay can be met.By optimization of network use, tight closed loop control can also be achieved whereintrinsic safety is required.With 4-20 mA technology it is possible to build a control loop containing only atransmitter, a controller and an actuator. Fieldbus devices must also be capable ofdoing so, as well as acting in a larger control system. Fieldbus must be multipurposeand as versatile as 4-20 mA. Fieldbus devices must therefore be able to operate bythemselves with a simple user interface in order to be economical in small systems.The cost of a host computer with dedicated software. and certainly a DCS. cannot be

justified for a small system, even though costs are going down. There would also be alogistics problem for both users and manufacturers if they were forced to keep usinganalog technology in small systems.The possible complexity of a system where so many devices can be connectedtogether (and where each device can perform the function of several conventionaldevices) requires a friendly user interface. The user must be freed from manualaddress assignment, as seen in smart transmitter protocols, and the painstaking job

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of tracking bits, bytes, words and memory addressees, as it is done in PLCs.The function block model is the choice of all Fieldbus proposals. The user can easilyrelate to it since the device is now represented by blocks, just like blocks in ISA andSAMA control diagrams. Physical wiring will now be logical connections or 'soft wiring,'links between blocks. Though technically different, it appears very familiar, and users

shall feel comfortable with it. Device address and parameter indexes areautomatically assigned. Some systems, including Smar's CD600 controller, alreadyimplement a similar philosophy.Standardization ensures interoperability, but if it is too rigid it may have adverseeffects for the user. There must be room for manufacturer differentiation. If thestandard forces compliance to every conceivable detail, the user would actually havenothing to choose from, because the devices from all manufactures would be exactlyalike. If one manufacturer came up with a great idea, he would not be able toimplement it, or forced to go through a process to have it included in the standard,whereby his competitors would learn about it.

Fieldbus specifies the basic functionality requirements, but must allow a manufacturerto add unique features to their device. These features benefit the user, and themanufacturer may use them as marketing tools. Likewise, if newer models aredifferent from their predecessors, devices communicating with it must know that. Inshort, Fieldbus must not hinder development and improvement of products. Fieldbusprovides a mechanism that ensures that interoperability is maintained for themanufacturer specific features as well.The cost of shutting down a system can be very high in terms of production loss. Tobe able to configure the system while in operation is therefore a requirement met byFieldbus.

THE BASICS OF FIELDBUS

FIELDBUS BASICSMORE THAN ONE FIELDBUS STANDARD

FOUNDATION FIELDBUS TECHNOLOGYACHIEVING INTEROPERABILITY

FIELDBUS BENEFITS

FIELDBUS BASICS ^UPFieldbus is a digital, two-way, multi-drop communication link among intelligent

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control devices that will replace the 4-20 mA standard. Let's break this definitiondown to give a little clearer idea about why this is such a drastic departure fromtoday's technology.

(FIGURE 1)

First of all, fieldbus is digital. Although computers, programmable logic controllers(PLCs), and remote terminal units (RTUs) communicate with each other digitally,most end devices (e.g. valves, pressure transducers, switches, etc.) still useanalog signals to communicate.

For example, an analog value of 4 mA might correspond to a pressure of no flowwhile a value of 20 mA might correspond to a 1000 GPM flow value. With discretedevices, the presence of a signal might represent a "closed" or "alarm" conditionwhile the absence of a signal might represent "open" or "normal".

Two-way communications means that a value can not only be read from the end

device but it is now possible to write to the device. For example, the calibrationconstants associated with a particular sensor can now be stored directly in thedevice itself and changed as needed.

The multi-drop capability of a fieldbus will perhaps result in the most immediatecost saving benefit for users. With analog devices, a separate cable needs to berun between the end device and the control system because only a single analogsignal can be represented on the circuit.

Modern distributed systems partially solve this problem by locating remotemultiplexing devices out in the field. The ultimate solution, however, is to be ableto connect a reasonable number of sensors all located in the same area to the

same cable. This is exactly what fieldbus allows.

Finally, fieldbus will replace the 4-20 mA standard, although this will not happenovernight. There are millions of instruments in the world using this standard rightnow, which does in fact have some advantages. It is simple and well understood.Devices from different suppliers using the 4-20mA standard can easily operatetogether (ie. Interoperate). More about this later.

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Ultimately, however, a digital standard is necessary to realize the benefits offieldbus. Some hybrid protocols, such as HART , are now being used to bridgethis gap between analog and digital technology. Using the HART protocol, theanalog signal is still used to transmit the process value while a digital value issuperimposed on top of the analog value for exchanging additional informationwith the device.

(Figure 2)

MORE THAN ONE FIELDBUS STANDARD ^UPThe term "fieldbus" is in fact a generic term. Unfortunately, there are variety offieldbuses in use or evolving today. These buses fall into two general areas:

1>Sensorbuses

2>Fieldbuses.

Sensorbuses : Sensorbuses are currently used primarily in manufacturingapplications or other areas such as building heating, ventilation and airconditioning (HVAC) control where such issues as security, intrinsic safety, andcritical process control are not a major concern. Sensorbuses often work withPLCs and provide a cost effective solution where a large amount of discrete orsimple analog data acquisition and control is taking place. Major sensorbusestoday include DeviceNet and LONWorks .

Fieldbuses : Fieldbuses are designed to meet the stringent requirements ofthe process industries. In addition to the more stringent requirements forconnection to the fieldbus itself as mandated by the IEC 1158-2 standard,fieldbuses are designed to include more features in their protocol to addressissues of performance, security, and error detection.

Depending on the fieldbus used, the protocol provides a large suits of functions atthe user layer that facilitate distributing control from the central control system

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out to the field devices themselves.

FOUNDATION FIELDBUS TECHNOLOGY ^UP

The rest of this description discusses FOUNDATION fieldbus technology. Othertechnologies are similar, but not as comprehensive, as FOUNDATION fieldbus.

Communications over a fieldbus is achieved by installing a fieldbus interfacedevice in all of the devices communicating over the fieldbus. One end of thisinterface device connects to the fieldbus and the other end to the sensorelectronics or the host computer system.

Initially, these fieldbus interface devices are small printed circuits that fit into thedevice or the host computer system. Ultimately, these devices will be little morethan a single integrated circuit that can be included in the device.

Fieldbus is designed to allow for both low-speed and high-speed communications.Low-speed, or H1 fieldbus, is currently in use in the industry. High-speed fieldbusis currently under development and will use high-speed Ethernet at a speed of100 Mbits/s. Linking devices will be developed for bridging between low-speedand high-speed fieldbus segments. These linking devices will also be used towhich handle high-speed contact I/O.

(Figure 3)

ACHIEVING INTEROPERABILITY ^UP

One of the primary goals of FOUNDATION fieldbus technology is interoperabilityamong different manufacturers. FOUNDATION fieldbus takes a rigorous approachto the area of interoperability using Function Block and Device DescriptionLanguage (DDL) Technology.

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Function blocks are provided through the use of a standard function block shellincluded in the user layer of the stack. These function blocks provide standardfunctions such as analog input, digital input, PID control, analog output, digitaloutput, etc., that can be included in the field devices.

Currently, there are over 30 standard function blocks supporting most functionsever encountered in the process industries. These function blocks were developedby a team of different manufacturers working together so they do not merelyrepresent the requirements of a single control system supplier. Any company thatdevelops a device cab be assured other manufacturers as long as they simplyconfigure their application using these standard function blocks.

To simplify the process for smaller companies, a set of standard device profileshave been defined for common devices such as flow meters, temperaturetransmitters, valves, etc. These companies can simply refer to one of thesestandard device profiles rather than having to having to define all the individualparameters for their device.

Some companies may have special features in their devices that make themunique. Manufacturers can make this unique data available to the system bydefining a device description (DD) for the device using the device descriptionlanguage (DDL). The host system can then read this special data from thisdevice, without using any special programming, by using a program called devicedescription services (DDS) to read and interpret the DD.

Both function blocks and device descriptions would be of limited use if there wasno method of enforcement. Conformance and interoperability test tools areavailable to allow third party testing of both communication stacks and actualdevices themselves to assure that they meet all the requirements for

interoperability.

The Fieldbus Foundation will provide a service of registering and verifying bothcommunication stacks and devices to assure that they meet all the requirementsfor interoperability.

FIELDBUS BENEFITS ^UP

Fieldbus is already proving that it can have dramatic benefits for end users.Wiring cost savings of 80 to 90% over conventional installations are beingrealized.

The myriad of configuration and diagnostic information available in fieldbusdevices is greatly reducing device commissioning time. This additional diagnosticinformation often makes it possible to remotely diagnose a field device problem,thus saving a costly trip to the field.

Finally, the benefits of moving control functions from the central control room outto the fieldbus devices are resulting in better, more reliable control as well as a

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less complex centralized control system.

How Fieldbus Works

There are two major parts to the Fieldbus system architecture: interconnection andapplication. Interconnection refers to the passing of data from one device toanother, may it be a field device, operator console or a configurator. This is thecommunication protocol part of Fieldbus. The application is the automation functionthe system performs. By standardizing part of the application, Fieldbus has gonefurther than any other communication standard, ensuring interoperability betweenconforming products.

Overview

The Fieldbus application architecture supports distribution of automation tasks tothe devices in the field which are interconnected by a network. The most basic

functions performed by a device are modeled as blocks. The blocks cooperate andare interconnected with each other, supporting the propagation parametersbetween devices, and the operator.The Fieldbus interconnection architecture is based on a three-layer subset of thearchitecture from the OSI (open systems interconnect) reference model developedby ISO (International Organization for Standardization). The OSI application andsystem management, and likewise the Fieldbus application architecture, models arebased on Object Oriented Programming (OOP) concepts. Both OSI and OOP usemodels to simplify understanding of functionality. Both are also briefly introduced inthis tutorial to achieve a better understanding of Fieldbus.

OSI model

The OSI reference model is an internationally recognized standard for networkarchitecture on which open networks are based. The standard is developed as amodel for telecommunications on all levels. All the functions (facilities such asaddressing, error checking and encoding/decoding) of a network have beengrouped into logical sets called layers, altogether seven. The layers are piled on topof each other and are together called the protocol stack. A layer only interfaceswith layers immediately above and below in the stack. The stack interfacesupwards to the application, and downwards to the transmission media. The part ofthe whole application performed by the system that is performed in a device iscalled the application process (AP). The AP consists of two parts, one user portion,which is the functionality, and one communication portion. In Fieldbus the userportion is the actual device function, such as measurement or control (functionblocks), or the user interface.

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Figure 2.1

OSI Protocol Stack

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Each layer provides services for the layer above it, and communicates with thecorresponding layer in the stack of the other station -- itspeer, calledpeer-to-peercommunication. The set of facilities a layer provides to the above layer is calledservices.When transmitting data from one application to another, the data is passed from

the top to the bottom of the stack, being processed by each layer obtaining thephysical layer frame. The frame is passed over the media. On the other end, whenreceiving, the data is passed from the bottom to the top of the stack. Theprocessing performed by the layers in the transmitting end is reversed by therespective layer on the receiving end, through which it obtains the originalapplication data.Layers 3 through 6 are not used because Fieldbus, (and most other LANs) has nointerconnection between networks, which is the purpose of these layers. Thissimplification makes Fieldbus faster and easier to implement in devices with limitedprocessor power, such as field instruments.

The three remaining layers, and provided functional and procedural characteristicsare:

1. Physical Layer (PhL): media independent activation, maintenance, anddeactivation of physical links that transparently pass the bit stream forcommunication; it only recognizes individual bits, not characters or multicharacterframes. The standard defines types of media and signals, transmission speed andtopology including number of nodes, and device power (only in Fieldbus).

2. Data Link Layer (DLL): transfers data between network entities; activation,maintenance and deactivation of data link connections, grouping of bits intocharacters and message frames, character and frame synchronization, errorcontrol, media access control and flow control (allowing several devices to share

the network). The standard defines type of media access control, frame formats,error checking and station addressing*.

7. Application Layer (APP): gives access to a set of local and communicationservices for serving the distributed system -- interconnection between the APs andthe user. Standard defines message formats and services available to the AP.

*Note: Addressing is actually part of the Network layer, which is not defined inFieldbus, where it instead is done in the Data Link Layer.

Some of the functionality of layers 3 through 6 are within the Fieldbus protocol,implemented in the application layer.

The OSI network management is an extension to the OSI layers, reaching over allthe layers. System management monitors and controls operation of networkresources. It is divided into system management functional areas. For Fieldbus, themost important functional area is network configuration. System management isaccessed from one station to another. It is modeled as a managing system (playingthe manager role), a managed system ) that plays the agent role), which has aManagement Information Base (MIB). MIB is the logical store for information andresources used to support network management

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Fieldbus Physical Layer

Closed loop control with performance like a 4-20 mA system, and variable speeddrives require high data transmission speed. Since higher speed means higherpower consumption, it then clashes with the need for intrinsic safety in someapplications. Therefore one moderately high communication speed was selected,and other faster non-intrinsically safe options where also made available, cateringto all applications. The system was designed to have a minimum of communicationoverhead to meet control requirements even with the low speed option.

There are several combinations for the physical layer, each with their relativemerits. All devices on a bus must use the same options for media, connection andtransmission rate. However, bus or non-bus powered devices may be mixed, aswell as intrinsically safe or nonintrinsically safe ones.

Physical media options:

Wire

Fiber optics (pending)

Radio (pending)

Transmission rate options:

31.25 kbit/s

1 Mbit/s

2.5 Mbit/s

Common Media Characteristics

The data are interchanged as a synchronous serial half-duplex signal. A devicetransmits and receives on the same media, but not simultaneously. The signal is

self-clocking, using Manchester (a.k.a. Biphase L) encoding. Since the transmissionis synchronous, no start and stop-bits are required. In Manchester coding clock anddata are combined so that a rising edge represents logic 0 (zero) data, and a fallingedge represents logic 1 (one) data.

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Figure 2.2

Manchester encoding

When transmitting, apreamble, equivalent to a phone's 'ring' signal, is firsttransmitted to synchronize the receivers of other devices. The beginning and end ofthe message are indicated by start and end delimiters respectively. The delimitersare not Manchester encoded, only data is, and can therefore be uniquely identified.The non-encoded bits in the delimiters are called N+ (nondata positive), and N-(nondata negative). The preamble and the delimiters added by the physical layer inthe transmitting device, are stripped off by the physical layer of the receivingdevice.

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Figure 2.3

Fieldbus physical layer frame.

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Redundancy may be achieved by duplication of physical layer and media. Voting(based on timeout and validity) controls which of the two media a device is using.

Wire Media Characteristics

The wire media uses electrical signals on a normal twisted pair wire, and is alreadyapproved as an IEC/ISA standard since 1992.

The maximum distance allowed between two devices on wire media depends on thetransmission rate selected.

31.25 kbit/s: 1,900 m

1 Mbit/s (voltage mode): 750m

1 Mbit/s (current mode): 750m

2.5 Mbit/s: 500m

The device must isolate the communication hardware, Media Attachment Unit(MAU), from ground to avoid electrical ground loops when devices aremultidropped.

Bus topology (Figure 2.4), tree topology (Figure 2.5) and point-to-point topologiesare supported. Tree topology is only supported by the low speed version. The bushas a trunkcable with two terminators. The devices are connected to the trunk viaspurs. The spurs may be integrated in the device giving zero spur length. A spurmay connect more than one device, depending on the length. Active couplers maybe used to extend spur length. Active repeaters may be used to extend the trunklength.

Figure 2.4

Bus Topology

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Figure 2.5Tree Topology

The terminators are designed to have an impedance of 100 each around thetransmission frequency. A device transmits by modulating current in the network

according to the Manchester encoded signal. Receiving devices sense the voltagedrop generated over the two terminators as the current is modulated. Themodulated current is 15 to 20 mA pk-to-pk for the low speed version, with areceiver sensitivity of 150 mV.

Figure 2.6Wire media signal modulation

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31.25 kbit/s wire media characteristics

The lowest speed wire media option, 31.25 kbit/s, is the most versatile andexpected to be the most widely used type. It offers versions for intrinsic safety anddevice powering by bus. The number of devices is limited by this choice.

Intrinsically safe / nonintrisically safe

Bus powered / Separately powered

The typical number of devices is indicated in table 2.1, the actual number variesfrom device type to device type.

Table 2.1

Maximum number of network nodes

In intrinsically safe systems, the safety barrier should be placed between the powersupply and the power supply end terminator.

Devices may be powered by the bus, only requiring two wires for supply andcommunication. A single power supply, common to all devices, is connected to thenetwork at either end of the trunk. The voltage may be in the range 9-32 VDC. Theimpedance of the power supply must be a minimum of 3 k around thetransmission frequency in order not to short circuit the communication signal. AManchester coded signal has a duty-cycle of exactly 50% and can be seen as a ACsignal. The DC power consumption (power drawn) of a device is therefore constant.

Fieldbus Data Link Layer -- FDL

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The Fieldbus Data Link Layer consists of two sublayers: the lower portion is theFieldbus Media Access Control (FMAC) and the upper portion is the Fieldbus DataLink Control (FDLC).

A device on the Fieldbus network is either one of two station types:

Master station

Siave station

A master station has the right to access the media (initiate communication). Slavesonly have the right to respond to a request from a master.

Fieldbus Media Access ControlThe Fieldbus medium access is a fusion of the Token- passing and Polling principles.Several devices on a network may be master stations. Only the station that has thetoken is permitted to initiate communication. The master may poll (masterrequests, slave responds) the slave devices while it has the token.

The token is passed to the next master in a special frame.

The devices are given individual station addresses. All frames contain thedestination address (DA) and the source address (SA) for the message.

The Fieldbus has services that free the user from the responsibility of assigning andkeeping track of addresses.

A requirement for reliable control is reliable data. A two-byte Frame CheckSequence (FCS) is calculated on all frame data using a polynomial in thetransmitting device, and added to the frame. The receiving device performs thesame calculation and compares the result with the FCS, thereby detecting any

error. The FCS is equivalent to the parity-bits and Cyclic Redundancy Checks ofasynchronous protocols.

When the above layer requests the FDL to pass a message, the message priority ispassed along with it. There are two priorities: high, e.g., alarms, and low, e.g.configuration and diagnostics data. The FDL transmits the high priority messagesfirst.

Fieldbus Data Link Control

The FDLC provides various possibilities for the application layer to send data toother stations.

There are two message types that can be identified in a Fieldbus system:

Operational

Background

Operational traffic is data transferred between devices as part of the controlstrategy, e.g. process variables. It is characterized as low volume, time critical andcyclic. Background traffic is data transferred between a device and the operatorinterface, e.g. configuration and diagnostics. It has the opposite characteristics of

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operational traffic: high volume, not time critical and is acyclic (sporadic).

Object Oriented Design

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When dealing with a complex system such as a Fieldbus application, the whole mayappear unfathomable. By decomposing it to parts, and even more primitiveelements in a hierarchic nature, to a suitable level of abstraction (recognizingessential characteristics), complex interacting parts are brought to order and thesystem becomes easier to grasp. That is achieved since one only needs tocomprehend a few simple parts. A technique, called Object Oriented Design (OOD),

was used to design the application layer and the function block application process.

There are many 'keywords' in OOD. However, for Fieldbus study, Object and Classare sufficient. Objects are entities with a well defined behavior. Systems are brokendown to objects which can be said to be 'parts of' the system. In OOD, software isbased on objects that do things or change when one sends them messages oroperates upon them. Therefore, OOD is not based on algorithms (execution steps).Objects often represent entities in the real world, e.g. a file. Objects may beclassified according to their function and other properties they have in common. Aclass defines various 'kinds of' objects. Unique properties of an object defines aninstance of the class.

To order or rank abstractions, both objects and classes are organized in a hierarchy(levels of abstraction or complexity). One being built on top of the other, each levelis understandable on their own. Inheritance is class hierarchy -- a subclass (lowerclass) shares structure and behavior of a superclass (higher class). Multipleinheritance is possible, and common. Aggregation is object hierarchies: objects arebuilt from subobjects.

For example: In a large control system one can find management system,supervisory system and field equipment; all parts of the control system. A fielddevice may have subparts such as sensor, electronics and casing. In a system theremay be a LD302, which is a kind of pressure transmitter which is a kind oftransmitter, which is a kind of field equipment. The LD302 inherits the propertiesand behavior of the pressure transmitter class. Therefore, an operator familiar with

pressure transmitters can operate the LD302 in a matter of minutes, only having tolearn the unique properties of the LD302.

OOD yields smaller systems through this reuse of common mechanisms,aggregation and inheritance.

Models are used extensively in Fieldbus and in all engineering because they makeabstraction, decomposition and hierarchical ordering easier.

OOD in Fieldbus

The Fieldbus control system has been decomposed down to the so called simplevariables which is a suitable level of abstraction. Examples of those are float,integer and string. Simple variables are used on their own but also as parts ofdatastructures such as the function block l/O parameters and function block links. Againthese data structures are parts of the function block data type which is also a datastructure.

Function blocks are part of the function block application process, which is part thefield device which finally is part of the system.

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Variables may be classified in many ways:

float, integer or string,

static or dynamic

read or write

etc.

Variables is only one of many types of objects, but the most important, defined inFieldbus. Since they exist in the AP, they are calledApplication Process Objects(APO).

Fieldbus Application Layer

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The distributed application processes in the system need to communicate. TheFieldbus provides logical communication paths (channels) between the applicationprocesses. Various types of connections with various combinations of characteristicsare available to meet the various communication needs. Several connections mayexist simultaneously, enabling multivariable access.

The Fieldbus connections are modeled in two ways:

Client-server model

Publisher-subscriber model

The client-server server model is used to describe acyclic data transfer. From acommunication point of view a clientis an AP which is using a remote AP'sfunctionality. The remote AP is called the server. For example, if the operatorconsole wants to read a tuning parameter in a controller in the field, the AP in theconsole is the client, and the AP in the controller is the server.

The publisher-subscriber model is used to describe cyclic data transfer. From a

communication point of view a subscriberis an AP which is using a remote AP'sfunctionality. The remote AP is called thepublisher. The publisher is actuallyproducing (publishing) data, a subscriber is consuming that (subscribing to) thatdata. For example, a transmitter is publishing a process variable which is consumedby a controller. The controller is publishing an output which is consumed by anactuator. Transmission is controlled by a third party, the requestor, which issues arequest to the publisher to publish its data.

Note: The publisher-subscriber model is derived from the more common producer-consumer model.

As mentioned in the OOD introduction, a Fieldbus system is broken down into

variables. There is a set of services that lets an AP use the functionality of an AP inanother device, such as getting the value of a variable or otherwise manipulate theobject.

The primary intention of Fieldbus is to build the application using function blocks.This would be done in the Function Block Application Process (FBAP). However,within a Fieldbus device it is possible to have other types of APs, e.g. ladder logic orstructured text, though no such definition has been made yet.

From a Fieldbus point of view, a device is not its hardware parts as they are tohumans. For example, a pressure transmitter is not an assembly of pressuresensor, electronics and a housing, but a network node containing parameters. Thisnetwork view is called the Virtual Field Device (VFD). A device (station) contains

only one FBAP. The FBAP may contain several VFDs to device a device's applicationinto individual loops to make it easier for the operator to overview.

The VFD is the Interface between protocol stack and function block AP. The VFD isthe part of the real application that is visible and accessible through the network,the communication objects such as variables and blocks etc.

Before a device can access communication objects (e.g. variables) in anotherdevice, it must first know which objects are available. and their structure. Knowingthe structure is important because there is no point in asking for a variable if you

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System and Network Management

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The purpose of network management is to provide services for central configurationand control of the network protocol stack, such as maintenance and start-up of theFieldbus system. For example, the network management manages the connections.The system management is split into two parts: a kernel which provides the basicfunctionality that a control application can be built upon, and a part that providesoptimization of operation and diagnostics of problems. It coordinates functions in all

layers, controls overall device operation and startup.

The purpose with system management kernel is to provide functions for:

Device Tag assignment

Station address assignment

Clock synchronization

Scheduling of distributed APs

Function block binding

In a system, each of the above functions can only be managed by one device

(though one device may handle many of them), the others act as agents. In case amanager fails, one of the agents will assume the manager role. For the systemmanagement to perform its task, it must cooperate with the system managementin other stations on the network. A simple device may implement only a part of thesystem management functions.

Physical Device Tag assignment

Before a device is put on the network, the user must first assign a physical devicetag to the device (i.e. it is done off-line). The tag may be up to 16 characters,typically in accordance with normal instrumentation practices, e.g. PT-10270.

Station address assignmentAutomatically assigns and ensures that each device on the network has a unique

address. An 'uninitiated' device has a default address. Configuration devices detectnew devices and will assign a station address, after check for duplicate tags.bringing the device to the 'standby' state. A temporary device such as a hand heldconfigurator selects its own address if there is no traffic on the network.

Function block binding

The network automatically finds the device (station address) for a given functionblock. It checks for multiple tags. This function is used when resolving linksbetween block outputs to inputs (identified by the block's tag, and the parametername) to the short address and index reference.

Clock synchronizationFor the Fieldbus system to perform scheduling and other time related functionssuch as time stamping of alarms and events, there is a distributed time base(clock) in each device, providing a common sense of time among all devices --'system time.' System management provides a mechanism for synchronization ofthe time in each device. This is done from a 'master clock' which provides thecorrect time.

Scheduling

The purpose of scheduling is to minimize delays due to communication. Such delaysare pure dead time which make control difficult. Scheduling also insures thatvariables are sampled and function blocks are executed on a precisely period basis,

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Figure 2.7

Scheduling

In the example in figure 2.7 a simple control loop consisting of input (Al), control(PID) and output (AO), with the Al and PID in the same physical device. Nointerdevice connection is necessary for the process variable. In this example, theexecution period for the AO block is shorter in order to illustrate that block

execution time depends on block.

Communication is scheduled in the master device which controls traffic andrequests communication. Function blocks are scheduled in the individual device.

Equation 2.1

Execution period/time, where x is a signed integer: -128 to 127.For example. x = -3 results in 125 ms

Function Block Application Process

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The Function Block AP (FBAP) is where the user configures their measurement andcontrol application. Parts of it is distributed to the various devices in the field. It isnot executed in a single control card as it is done in a DCS.

The functionality of a Fieldbus device is modeled as objects. The block object hasthree classes which again have sub-classes under which the various blocks are

grouped.

Block objectFunction block object

Input function blockOutput function blockControl function blockCalculate function block

Transducer block objectInput transducer blockOutput transducer blockDisplay transducer block

Physical block object Alarm object

Event object

Trend object

Display list

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Figure 2.8FBAP architecture

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The part of the FPAP, which is standardized by Fieldbus, is called the function blockshell. For example, the block algorithms are not standardized. For each block thereis a set of parameters that, to a certain extent, defines what minimum functionalitya block will have. However, the manufacturer may implement such block in theirown way. For example, in the PID control block there must be a GAIN parameterand therefore, the manufacturer may use this parameter as gain or proportional

band.

Function Block

The blocks model the user configurable part of the entire application. Typically,these functionalities were previously available in individual physical devices. Now,several are included in form of software blocks in a single device. Together in aFieldbus system the different types of function blocks provide all the functionalitynecessary for most control systems. The user can build control strategies suitablefor their application by linking these function blocks.

In general, function blocks can be said to use an algorithm and containedparameters in order to process input parameters, and producing as results, output

parameters. Again, the block is just an abstraction of software and data. There areno blocks inside the device to be seen. The function block concept was designedaround the PID block since it is the most complex block. The concept oflocal/remote setpoint, automatic/manual output, cascade (remote setpoint) and thealgorithm has been carried on to other blocks, which may have appeared strange atfirst. A particular selection of setpoint and output is called the block mode. Thealgorithm does not refer to the PID algorithm in the PID block alone, but in generalto the processing function of all blocks.

Each block is identified in the system by a tag assigned by the user. This tag mustbe unique in the Fieldbus system. Each parameter in a block has a name thatcannot be changed. All parameters in the system are uniquely defined by the blocktag plus parameter name.

Inputs from other blocks arrive asynchronously. When a block is executed a 'snapshot' of the inputs is therefore taken. This also prevents input data to changeduring block execution. After executing the block algorithm, its outputs are updatedand broadcasted on the network and read by inputs of blocks using thisinformation. This way, the output has to be communicated only once, even if it isconnected to many inputs. The execution time for the block is expressed just likethe execution period in equation 2.1.

Configuration is basically assignment of tags and building of the control strategy byselecting blocks (installation), linking them and adjusting the contained may bedone by a simple hand held configurator or through the use of a computer with agraphical user interface, which allows users to draw the configuration as a controldiagram. Configuration may be done in advance or during operation.

Analog input block

Provides the functionality of what is known as a transmitter. It makes themeasurement performed by a device available to the Fieldbus system. Italso optionally applies calibration, damping, and a transfer function such asa square root of a measured differential pressure, enabling inferred

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Figure 2.9

Analog input function block

Function block links

By linking function block outputs to inputs of other function blocks, controlstrategies can be built. When such a link is made the input 'pulls' the value fromthe output, thereby obtaining its value. Links can be made between function blocksin the same device or in different devices (see figure 2.10). An output may beconnected to many inputs. These links are purely software, and there is basicallyno limitation to how many links can travel along a physical wire. Links cannot bemade with contained variables. Analog values are passed as floating point inengineering unit, but are scaled to percentage (e.g., in the PID control block) toenable dimensionless tuning parameters. Digital values are passed as Boolean, 0 or255. The analog scaling information may also be used in operator interfaces toprovide a bar-graph readout.

An output value is always accompanied by a status informing if, e.g., a value

received from a sensor (forward path) is suitable for control or as the feedback(backward path) informing if, e.g., the output does not move the final controlelement. The status is determined by the source. Note that the pull system is usedfor backward paths as well. This way, the receiving function block can take anappropriate action.

Links are uniquely defined by the name of the output parameter and th