Introduction to Introduction to Process Process

TRANSMITTERS - TRANSMITTERS - Part IPart I

transmittransmit : :

• effect conveyance of, pass on, effect conveyance of, pass on, communicate, serve as medium for communicate, serve as medium for the passage of. the passage of.

TransmissionTransmission

• involves a involves a transmittertransmitter, a , a transmission medium and a transmission medium and a receiverreceiver..

Transmit/Transmission/TransmitterTransmit/Transmission/Transmitter

Process transmitterProcess transmitter

• Combination of Combination of sensor and sensor and conditioning deviceconditioning device which together which together enable enable process parameter process parameter to be to be measured remotely from actual site of measured remotely from actual site of process measurement. process measurement.

• The transmitter enables The transmitter enables Process Process VariableVariable (PV) to be obtained at the (PV) to be obtained at the receiving end (for display, recording or receiving end (for display, recording or further analysis).further analysis).

Process Transmitter

• the general configuration of a measurement transducer consists of a sensing element combined with a driving element (transmitter).

• Transducers for process measurements convert the magnitude of a process variable (e.g., flow rate, pressure, temperature, level, or concentration) into a signal that can be sent directly to the controller.

• The sensing element is required to convert the measured quantity, that is, the process variable, into some quantity more appropriate for mechanical or electrical processing within the transducer.

Standard Instrumentation Signal-Levels

• Before 1960-70, instrumentation in the process industry utilized pneumatic (air pressure) signals to transmit measurement and control information almost exclusively.

• These devices make use of mechanical force-balance elements to generate signals in the range of 3 to 15 psig (0.2 to 1.0 kg/cm2).

• Since about 1960-70s, electronic instrumentation has come into widespread use.

Pneumatic transmitterPneumatic transmitter

• converts the converts the physical variablephysical variable measured to a measured to a standard pressure standard pressure signalsignal such as in the range 0.2 kg/cm such as in the range 0.2 kg/cm2 2

to 1.0 kg/cmto 1.0 kg/cm2 2 (3-15 psig). (3-15 psig).

• The pressure signal is transmitted The pressure signal is transmitted through pipes / tubes to gauges and through pipes / tubes to gauges and chart recorders and also used as chart recorders and also used as feedback signals in control feedback signals in control applicationsapplications

Pneumatic transmission: Pneumatic transmission: Problems?Problems?

• internal and external leakages,internal and external leakages,

• pressure drops, pressure drops,

• availability of clean, dry compressed availability of clean, dry compressed air (at proper supply pressures),air (at proper supply pressures),

• difficulty in transmitting pressure difficulty in transmitting pressure signals to longer distances,signals to longer distances,

• noise, mechanical vibrations,noise, mechanical vibrations,

• stuck-up faults, etc.stuck-up faults, etc.

• In control applications:In control applications:– quite a number of moving parts and quite a number of moving parts and

assemblies,assemblies,

– hysteresis, non-linearity,hysteresis, non-linearity,

– low response times,low response times,

– difficulty in attaining repeatability of difficulty in attaining repeatability of control,control,

– high frequency of service and associated high frequency of service and associated maintenance costs and inventory costs. maintenance costs and inventory costs.

Pneumatic transmission: Pneumatic transmission: Problems?Problems?

Electronic TransmittersElectronic Transmitters

• better accuracy,better accuracy,

• linearity and repeatability, linearity and repeatability,

• greatly simplified measurement & greatly simplified measurement & control control

• reduced operation and maintenance reduced operation and maintenance costs costs

• improved transmittability to greater improved transmittability to greater distances.distances.

ELECTRONIC ELECTRONIC TRANSMITTERSTRANSMITTERS

• Analog transmittersAnalog transmitters

• Digital transmitters Digital transmitters • Analog - Digital/hybrid (SMART) Analog - Digital/hybrid (SMART)

transmitterstransmitters

Analog TransmittersAnalog Transmitters

• Analog transmittersAnalog transmitters convert the process convert the process variable measured into an equivalent variable measured into an equivalent electrical signal voltage or currentelectrical signal voltage or current that is that is conveyed by means of cable/wires from conveyed by means of cable/wires from measurement site to the control room for measurement site to the control room for interfacing to display or process controller. interfacing to display or process controller.

• voltage transmittersvoltage transmitters– wire-resistance/impedance poses a significant wire-resistance/impedance poses a significant

problem, especially long transmission lengths problem, especially long transmission lengths – signal attenuated along the linesignal attenuated along the line– suitable only for short distancessuitable only for short distances– Typical signal ranges (corresponding to PV from Typical signal ranges (corresponding to PV from

0% to 100%)0% to 100%)• 0 to 10V0 to 10V• 0 to 5V0 to 5V• -10V to +10V-10V to +10V• -5V to +5V, etc. -5V to +5V, etc.

Analog TransmittersAnalog Transmitters

• current transmitterscurrent transmitters– transmitter is in essence a current source transmitter is in essence a current source

(current generator) that has very high output (current generator) that has very high output resistance. resistance.

– wire-resistance has no effect on the flowing wire-resistance has no effect on the flowing current in the current loop; current in the current loop;

– signal ranges 0-20ma, 4-20mA, 0-50 mA and 10-signal ranges 0-20ma, 4-20mA, 0-50 mA and 10-50 mA50 mA

– 4-20mA current signal is the industry standard. 4-20mA current signal is the industry standard. The 4 to 20mA represents an entire span with The 4 to 20mA represents an entire span with zero stimulus (0%) corresponding to 4mA and zero stimulus (0%) corresponding to 4mA and maximum (100%) corresponding to 20mA.maximum (100%) corresponding to 20mA.

Analog TransmittersAnalog Transmitters

Analog Conventional Transmitter

• A transmitter usually converts the sensor output to a signal level appropriate for input to a controller, such as 4 to 20 mA.

• Transmitters are generally designed to be direct acting.

• In addition, most commercial transmitters have an adjustable input range (or span). For example, a temperature transmitter might be adjusted so that the input range of the platinum resistance element (the sensor) is 50 to 150 °C.

Input Output

50 °C 4 mA

150 °C 20 mA

• This instrument (transducer) has a lower limit or zero of 50 °C and a range or span of 100 °C.

• For the temperature transmitter discussed above, the relation between transducer output and input is

20 mA 4 mAmA 50 C 4 mA

150 C 50 C

mA0.16 C 4 mA

C

mT T

T

Four wire transmittersFour wire transmitters

• To connect sensors of relatively low resistance To connect sensors of relatively low resistance such as piezo-resistors, RTDs, etc. to remotely such as piezo-resistors, RTDs, etc. to remotely located interface circuitslocated interface circuits

• connecting wires resistance pose a serious connecting wires resistance pose a serious problemproblem

• 4-wire method allows measurement of resistance 4-wire method allows measurement of resistance of connecting conductors.of connecting conductors.

• 2 wires connect to current source, and another 2 2 wires connect to current source, and another 2 wires to high input impedance voltage wires to high input impedance voltage measurement circuitry.measurement circuitry.

Digital transmittersDigital transmitters

• Pulse transmissionPulse transmission• Switch/contact-closure-digital Switch/contact-closure-digital

transmissiontransmission• Serial data transmission Serial data transmission

Pulse transmitterPulse transmitter

• Instruments /devices are incorporated Instruments /devices are incorporated with a sensing mechanism that with a sensing mechanism that produces pulse output in proportion produces pulse output in proportion to the variable being measured to the variable being measured (generally, motion).(generally, motion).

• The frequency of pulses is proportion The frequency of pulses is proportion to measurement.to measurement.

• capacitive proximity sensors,capacitive proximity sensors,• inductive transducers,inductive transducers,• magnetic reed-switches,magnetic reed-switches,• Magnetic proximity sensors,Magnetic proximity sensors,• optical encoders, photo-sensors,optical encoders, photo-sensors,

Pulse output mechanismsPulse output mechanisms

ExamplesExamples

Flowmeters:Flowmeters:• Turbine flowmeters, positive-Turbine flowmeters, positive-

displacement flowmeters, vortex displacement flowmeters, vortex flowmeters, etc.flowmeters, etc.

Displacement Encoders:Displacement Encoders:• tachometers, rotary position encoders, tachometers, rotary position encoders,

quadrature sensors, etcquadrature sensors, etc

• signals are normally weak, of low signals are normally weak, of low amplitude, improper / non-uniform amplitude, improper / non-uniform shapes, unsuitable for transmission shapes, unsuitable for transmission

• Conditioning: level shift, pulse-shaping, Conditioning: level shift, pulse-shaping, amplification, etc.amplification, etc.

• typical output peak-to-peak amplitudes typical output peak-to-peak amplitudes 5V, 12V, 24V or bipolar 5V, 12V, 24V or bipolar 5V, 5V, 12V 12V

• Usually output wave-shape is square, or Usually output wave-shape is square, or of fixed pulse-width, or sinusoidal;of fixed pulse-width, or sinusoidal;

Pulse transmitters..Pulse transmitters..

Signal Conditioning

• frequency of pulses is proportional frequency of pulses is proportional to instantaneous value of process to instantaneous value of process variable - speed, flow etc. variable - speed, flow etc.

• integration of process variable integration of process variable over a period of time is simple over a period of time is simple totalising/counting of the pulsestotalising/counting of the pulses

Pulse transmitters..Pulse transmitters..

Pulse Outputs are Popular

Switch/contact closure Switch/contact closure transmitterstransmitters

• Similar to pulse transmitters; but Similar to pulse transmitters; but frequency of pulse or switch/contact frequency of pulse or switch/contact closure is very small. closure is very small.

• Used to convey position, level, alarm Used to convey position, level, alarm status etc. to remote indicators, status etc. to remote indicators, anunciators and display systems.anunciators and display systems.

• Control applications, ON/OFF, etc.Control applications, ON/OFF, etc.

Serial data transmissionSerial data transmission• communicates serially between Computers, communicates serially between Computers,

peripheral devices, Instruments, etc.peripheral devices, Instruments, etc.

RS-232RS-232

• introduced in 1962 ,introduced in 1962 ,

• widely used throughout the industry.widely used throughout the industry.

• single-ended data transmission at relatively single-ended data transmission at relatively slow data rates (20 kBaud)slow data rates (20 kBaud)

• short distances (upto 50 ft.)short distances (upto 50 ft.)

Process Process TRANSMITTERS - TRANSMITTERS -

Part IIPart II

Smart TransmittersSmart Transmitters

• what is “smart” in transmitterswhat is “smart” in transmitters

• FeaturesFeatures

• Benefits / AdvantagesBenefits / Advantages

• SMART ProtocolsSMART Protocols

• Specifying for ProcurementSpecifying for Procurement

Coverage : Smart TransmittersCoverage : Smart Transmitters

SMART TRANSMITTERSMART TRANSMITTER

SMART SMART or or INTELLIGENTINTELLIGENT

The term is simply used to indicate The term is simply used to indicate presence of a presence of a microprocessormicroprocessor. .

This This microprocessormicroprocessor in the field-device in the field-device adds adds new or extra features new or extra features into the into the device, over and above what are present device, over and above what are present in a in a conventionalconventional non - microprocessor non - microprocessor based Process Transmitter.based Process Transmitter.

WHAT FEATURES TO BE WHAT FEATURES TO BE EXPECTED EXPECTED

IN A IN A SMART TRANSMITTERSSMART TRANSMITTERS

• CommunicationCommunication

Digital communication over the same two wires used for analog transmission.

The digital communication is two-way between the transmitter and the configurator - A hand-held communicator, a microprocessor based system or a computer.

HART Communication ProtocolHART Communication Protocol• Bell-202 standard Frequency-shift-

keying (FSK) • bit ‘1’ : 1200 Hz• bit ‘0’ : 2200 Hz

• Transfer rate :• 1200 bit/s

• Signal structure:• 1 start bit• 8 data bits• 1 bit for odd parity• 1 stop bit.

Software : Configuration OptionsSoftware : Configuration Options

• User can select from various options User can select from various options on Ranges, response, EU, Display on Ranges, response, EU, Display info, Outputs, etc….info, Outputs, etc….

StoreStore (and transmit when required) info such as :– tag - for identification of transmitter– date modified - date of last or next

calibration or installation– message - name of person or some special

precaution etc.– information on flange type, flange material O-

ring, seal type, sensor range etc.

Memory (EPROM) permits StorageMemory (EPROM) permits Storage

Sensor Characterisation DataSensor Characterisation Data

• Sensor linearization Coefficients or interpolation points for different T, P

• stored in the memory (EPROM) stored in the memory (EPROM)

• Sensor behaviour at various operating Sensor behaviour at various operating conditions is tested, and used for conditions is tested, and used for compensationcompensation

• Drastically reduces driftsDrastically reduces drifts

Processor: Enables Computations Processor: Enables Computations and Output Signal Optionsand Output Signal Options

• linear (for pressure, differential pressure, level measurement. ..),

• square root (for flow measurement with differential pressure meters),

• square root of third and fifth powers (for flow measurement in open channels..) etc,

• use values stored in table in the memory to calculate the value of process variable

Re-ranging, Turn-downRe-ranging, Turn-down• Adjust or Change the zero / span

• Send Command from Handheld/remote computer to the Process Transmitter

• Re-ranging can be performed without applying reference (pressure / temperature); uses Characterization data.

• Reranging done with reference will actually be more accurate calibration.

Limits / Alarm values Limits / Alarm values • High limit,

• low limit,

• high rate of change,

• low rate of change , etc.

depending on Make/Model

• Set output signal to 3.5mA or 21mA

Multidrop communication Multidrop communication Network More than one smart

transmitter using same two-wire loop.

Each transmitter configured a unique Address (Non-zero) “1 to 15”.

Each can be individually read, configured, reranged or calibrated. Each transmitter draws, outputs 4mA Only

In conventional analog mode, address set to “0”

Self-diagnosticsSelf-diagnostics • diagnostic to determine conditions of

sensor, communication line, power supply, configurations, etc.

• helps reduce trouble shooting efforts, improves servicing.

• Newer transmitters can sense impulse line plugging conditions.

• Rosemount : HART (Highway Addressable Remote Transducer)

• Honeywell : DE (Digitally Enhanced)

Common Smart Transmitter Communication ProtocolsCommunication Protocols

BENEFITS OF USING BENEFITS OF USING SMART TRANSMITTERSSMART TRANSMITTERS

• Improved SafetyImproved SafetyReranging, calibration, etc., can be done remotely without going to the actual transmitter site which may be in an hazardous or unsafe location.

• Time SavingsTime Savings• Remote communication implies

facility to rerange, reconfigure, etc. for one or more smart transmitters using the hand-held communicator or configurator;

means fewer trips to the field.

• Self - diagnostics,

implies lesser time spent for troubleshooting, repairs etc.

• High AccuracyHigh Accuracy• The process of analog-to-digital and

digital-to-analog conversion of the 4-20 signal are eliminated by the use of digital communication.

• Functions like sensor output compensation for drifts due to changing operating conditions,

• output linearisation

• or other computations, etc.

enable high accuracy of transmitted data.

• Reduced InventoryReduced Inventoryfacility to rerange the transmitter without loss of accuracy,

facility to configurate the transmitter when using a different process media,

computational abilities like square-root extraction, etc.,

imply that only one type of smart transmitter need to be purchased or maintained as spare for a wide range.

• Smart Transmitter Manufacturers:Smart Transmitter Manufacturers:

• Foxboro, Honeywell, Moore Products,

• Rosemount, Emerson Process Management

• SMAR, ABB, Yokogawa, FUJI,

Advanced CapabilitiesAdvanced Capabilities

• Embedded control (PID)

• Multi-variate transmitters

• FIELDBUS (Fully Digital, Multidrop Networking)

Embedded control Embedded control Built in PID FunctionsBuilt in PID Functions

Multi-variate Multi-variate

Smart TransmittersSmart Transmitters

HART ProtocolHART Protocol

HART Communication between HART Communication between master and slavemaster and slave

• The master sends messages with requests for actual/specified values, and/or any other data/parameters available from the slave device.

• The slave interprets these instructions as defined in the HART protocol.

• The slave responds with status information and data for the master.

HART Commands HART Commands

• Universal commands

• Common practice commands

• Device-specific commands

• HART follows the Open Systems Interconnections (OSI) model of the International Organization for Standardization (ISO).

• The HART protocol uses a reduced OSI model, implementing only layers 1, 2 and 7

• Layer 1, physical layer

• Layer 2, link layer

• Layer 7, application layer

Question TopicsOSI layer

How do we make the connection?

Plugs, sockets, cablePhysical

What signals can I send?

Voltage, current, frequencyPhysical

How do I address a message?

None (point-to-point), numerical address, tag

DataLink

When can I send a message?

Access rules: master-slave, token-passing, collision-detection

DataLink

What messages can I send?

Coding: bits, characters, parityDataLink

Data types: bits, integers, floating point, text

Application

What does a message mean?

Standard functionsApplication

Function blocks, Device Descriptions"User"*

Specifying a Specifying a Smart Smart

TransmitterTransmitter