introduction to Process transmitters

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introduction to Process transmitters

  1. 1. Introduction to Process TRANSMITTERS Part I
  2. 2. Transmit/Transmission/Transmitter transmit : • effect conveyance of, pass on, communicate, serve as medium for the passage of. Transmission • involves a transmitter, a transmission medium and a receiver.
  3. 3. Process transmitter • Combination of sensor and conditioning device which together enable process parameter to be measured remotely from actual site of process measurement. • The transmitter enables Process Variable (PV) to be obtained at the receiving end (for display, recording or further analysis).
  4. 4. Process Transmitter • the general configuration of a measurement transducer consists of a sensing element combined with a driving element (transmitter).
  5. 5. • 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.
  6. 6. 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 forcebalance elements to generate signals in the range of 3 to 15 psig (0.2 to 1.0 kg/cm 2). • Since about 1960-70s, electronic instrumentation has come into widespread use.
  7. 7. Pneumatic transmitter • converts the physical variable measured to a standard pressure signal such as in the range 0.2 kg/cm2 to 1.0 kg/cm2 (3-15 psig). • The pressure signal is transmitted through pipes / tubes to gauges and chart recorders and also used as feedback signals in control applications
  8. 8. Pneumatic transmission: Problems? • internal and external leakages, • pressure drops, • availability of clean, dry compressed air (at proper supply pressures), • difficulty in transmitting pressure signals to longer distances, • noise, mechanical vibrations, • stuck-up faults, etc.
  9. 9. Pneumatic transmission: Problems? • In control applications: – quite a number of moving parts and assemblies, – hysteresis, non-linearity, – low response times, – difficulty in attaining repeatability of control, – high frequency of service and associated maintenance costs and inventory costs.
  10. 10. Electronic Transmitters • • • better accuracy, linearity and repeatability, greatly simplified measurement & control • reduced operation and maintenance costs • improved transmittability to greater distances.
  11. 11. ELECTRONIC TRANSMITTERS • • • Analog transmitters Digital transmitters Analog - Digital/hybrid (SMART) transmitters
  12. 12. Analog Transmitters • Analog transmitters convert the process variable measured into an equivalent electrical signal voltage or current that is conveyed by means of cable/wires from measurement site to the control room for interfacing to display or process controller.
  13. 13. Analog Transmitters • voltage transmitters – wire-resistance/impedance poses a significant problem, especially long transmission lengths – signal attenuated along the line – suitable only for short distances – Typical signal ranges (corresponding to PV from 0% to 100%) • 0 to 10V • 0 to 5V • -10V to +10V • -5V to +5V, etc.
  14. 14. Analog Transmitters • current transmitters – transmitter is in essence a current source (current generator) that has very high output resistance. – wire-resistance has no effect on the flowing current in the current loop; – signal ranges 0-20ma, 4-20mA, 0-50 mA and 1050 mA – 4-20mA current signal is the industry standard. The 4 to 20mA represents an entire span with zero stimulus (0%) corresponding to 4mA and maximum (100%) corresponding to 20mA.
  15. 15. 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.
  16. 16. Input Output 50 °C 4 mA 150 °C 20 mA  20 mA − 4 mA  Tm ( mA ) =  T − 50 oC + 4 mA o o ÷  150 C − 50 C  mA  o  =  0.16 o ÷T C − 4 mA C  ( ) ( ) • 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
  17. 17. Four wire transmitters • To connect sensors of relatively low resistance such as piezo-resistors, RTDs, etc. to remotely located interface circuits • connecting wires resistance pose a serious problem • 4-wire method allows measurement of resistance of connecting conductors. • 2 wires connect to current source, and another 2 wires to high input impedance voltage measurement circuitry.
  18. 18. Digital transmitters • Pulse transmission • Switch/contact-closure-digital transmission • Serial data transmission
  19. 19. Pulse transmitter • Instruments /devices are incorporated with a sensing mechanism that produces pulse output in proportion to the variable being measured (generally, motion). • The frequency of pulses is proportion to measurement.
  20. 20. Pulse output mechanisms • • • • • capacitive proximity sensors, inductive transducers, magnetic reed-switches, Magnetic proximity sensors, optical encoders, photo-sensors,
  21. 21. Examples Flowmeters: • Turbine flowmeters, positive-displacement flowmeters, vortex flowmeters, etc. Displacement Encoders: • tachometers, rotary position encoders, quadrature sensors, etc
  22. 22. Pulse transmitters.. Signal Conditioning • signals are normally weak, of low amplitude, improper / non-uniform shapes, unsuitable for transmission • Conditioning: level shift, pulse-shaping, amplification, etc. • typical output peak-to-peak amplitudes 5V, 12V, 24V or bipolar ± 5V, ± 12V • Usually output wave-shape is square, or of fixed pulse-width, or sinusoidal;
  23. 23. Pulse transmitters.. Pulse Outputs are Popular • frequency of pulses is proportional to instantaneous value of process variable - speed, flow etc. • integration of process variable over a period of time is simple totalising/counting of the pulses
  24. 24. Switch/contact closure transmitters • Similar to pulse transmitters; but frequency of pulse or switch/contact closure is very small. • Used to convey position, level, alarm status etc. to remote indicators, anunciators and display systems. • Control applications, ON/OFF, etc.
  25. 25. Serial data transmission • communicates serially between Computers, peripheral devices, Instruments, etc. RS-232 • introduced in 1962 , • widely used throughout the industry. • single-ended data transmission at relatively slow data rates (20 kBaud) • short distances (upto 50 ft.)
  26. 26. Process TRANSMITTERS Part II Smart Transmitters
  27. 27. Coverage : Smart Transmitters • • • • • what is “smart” in transmitters Features Benefits / Advantages SMART Protocols Specifying for Procurement
  28. 28. SMART TRANSMITTER SMART or INTELLIGENT The term is simply used to indicate presence of a microprocessor. This microprocessor in the field-device adds new or extra features into the device, over and above what are present in a conventional non - microprocessor based Process Transmitter.
  29. 29. WHAT FEATURES TO BE EXPECTED IN A SMART TRANSMITTERS
  30. 30. • Communication Digital communication over the same two wires used for analog transmission. The digital communication is twoway between the transmitter and the configurator - A hand-held communicator, a microprocessor based system or a computer.
  31. 31. HART Communication Protocol • Bell-202 standard Frequency-shiftkeying (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.
  32. 32. Software : Configuration Options • User can select from various options on Ranges, response, EU, Display info, Outputs, etc….
  33. 33. Memory (EPROM) permits Storage Store (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 Oring, seal type, sensor range etc.
  34. 34. Sensor Characterisation Data • Sensor linearization Coefficients or interpolation points for different T, P • stored in the memory (EPROM) • Sensor behaviour at various operating conditions is tested, and used for compensation • Drastically reduces drifts
  35. 35. Processor: Enables Computations and 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
  36. 36. Re-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.
  37. 37. 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
  38. 38. 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”
  39. 39. Self-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.
  40. 40. Common Smart Transmitter Communication Protocols • Rosemount : HART (Highway Addressable Remote Transducer) • Honeywell : DE (Digitally Enhanced)
  41. 41. BENEFITS OF USING SMART TRANSMITTERS
  42. 42. • Improved Safety Reranging, calibration, etc., can be done remotely without going to the actual transmitter site which may be in an hazardous or unsafe location.
  43. 43. • Time 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.
  44. 44. • High Accuracy • The process of analog-to-digital and digital-to-analog conversion of the 420 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.
  45. 45. • Reduced Inventory facility to rerange the transmitter without loss of accuracy, facility to configurate the transmitter when using a different process media, computational abilities like squareroot extraction, etc., imply that only one type of smart transmitter need to be purchased or maintained as spare for a wide range.
  46. 46. • Smart Transmitter Manufacturers: • Foxboro, Honeywell, Moore Products, • Rosemount, Emerson Process Management • SMAR, ABB, Yokogawa, FUJI,
  47. 47. Advanced Capabilities • Embedded control (PID) • Multi-variate transmitters • FIELDBUS (Fully Digital, Multidrop Networking)
  48. 48. Embedded control Built in PID Functions
  49. 49. Multi-variate Smart Transmitters
  50. 50. HART Protocol
  51. 51. HART Communication between master 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.
  52. 52. HART Commands • Universal commands • Common practice commands • Device-specific commands
  53. 53. • 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
  54. 54. Question How do we make the connection? What signals can I send? How do I address a message? When can I send a message? Topics Plugs, sockets, cable Voltage, current, frequency None (point-to-point), numerical address, tag Access rules: master-slave, tokenpassing, collision-detection Coding: bits, characters, parity What messages can I send? What does a message mean? OSI layer Physic al Physic al DataLi nk DataLi nk DataLi nk Data types: bits, integers, floating point, Applic text ation Standard functions Function blocks, Device Descriptions Applic ation "User" *
  55. 55. Specifying a Smart Transmitter

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