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HART COMMUNICATIONS
A brief study
Mrs. Tannistha KapoorEngr. O&M/C&I
AGENDA
1. WHAT IS HART1. WHAT IS HART
2. HOW HART WORKS 2. HOW HART WORKS
3. HART COMMUNICATIONS3. HART COMMUNICATIONS
7. WIRELESS HART7. WIRELESS HART
5. CALIBRATION5. CALIBRATION
4. HART DATA4. HART DATA
6. BENEFITS OF USING HART6. BENEFITS OF USING HART
WHAT IS HART• Hart is an acronym for Highway addressable remote transducers.
• HART is a bidirectional master-slave field communications protocol developed in the late 1980's to facilitate communication between intelligent field instruments and host systems by Rosemount Inc. Later it was developed into an open protocol
• It makes use of the Bell 202 Frequency Shift Keying (FSK) standard to superimpose digital communication signals at a low level on top of the 4-20mA.
• It communicates without interrupting the 4-20mA signal and allows a host application (master) to get two or more digital updates per second from a field device
WHAT IS HART contd
A TYPICAL HART SETUP
WHAT IS HART contd
MULTIDROP
CONFIGURATION
POINT TO POINT
CONFIGURATION
HOW HART WORKSCONVENTIONAL PROCESS LOOP
PROCESS LOOP WITH HART ADDED
How Hart Works contd
• HART PROCESS TRANSMITTER:-.
The EEPROM stores fundamental hart parameters
UART is used to convert between serial & parallel data
Network interface is a current regulator which implement the two current sources.
How hart works contd.
HART COMMUNICATIONS The transmitting device begins by turning ON its carrier and
loading the first byte to be transmitted into its UART.
The UART converts each transmitted byte into a 11 bit serial character.
The serial character stream is applied to the Modulator of the sending modem.The Modulator operates such that a logic 1 applied to the input produces a 1200 Hz periodic signal at the Modulator output. A logic 0 produces 2200 Hz.After transmission of the first byte , the transmitter loads the next byte. And so on.After the last byte is serialized and transmitted the transmitter turns off the source.
HART COMMUNICATIONS
SIGNAL PATH:-
HART COMMUNICATIONS
PROTOCOL
Only one HART device can talk at a time.
A Master typically sends a command and then expects a reply. A Slave waits for a command and then sends a reply. A Slave accesses the network as quickly as possible in response to a Master. The command and associated reply are called a transaction.
There are typically periods of silence (nobody talking) between transactions.
Network access by Masters requires arbitration which is based on monitoring of network traffic and implementation of timers.
If two Masters are present and both are synchronized, then they will use the network alternately .
Timer Description SymbolValue (character
times)
Master Wait Before Re-Using Network
RT2 8
Primary Master Wait from Unsynched
RT1(0) 33
Secondary Master Wait from
UnsynchedRT1(1) 41
Slave Max time to Respond
TT0 28
Slave Time Between Bursts
BT 8
HART COMMUNICATIONS
A Slave (normally) has a unique address to distinguish it from other Slaves.
Addresses are either 4 bits or 38 bits.
The long address consists of the lower (least significant) 38 bits of a 40-bit unique identifier.
Each command or reply is a message, varying in length from 10 or 12 bytes to typically 20 or 30 bytes
HART COMMUNICATIONS
Part of MessageLength in
BytesPurpose
Preamble 5 to 20 Synchronization & Carrier Detect
Start Delimiter 1Synchronization & Shows Which
Master
Address 1 or 5Choose Slave, Indicate Which
Master, and Indicate Burst Mode
Command 1 Tell Slave What to Do
Number Data Bytes 1Indicates Number Bytes Between
Here and Checksum
Status0 (if Master)
2 (if Slave)
Slave Indicates Its Health and Whether it did As Master Intended
Data 0 to 253Argument Associated with Command
(Process Variable, For Example)
Checksum 1 Error Control
Hart communications contdHART MESSAGE STRUCTURE :-
Hart Communications contd.
The preamble is allowed to vary in length, depending on the Slave's requirements.
The status field (2 bytes) occurs only in replies by HART Slave devices. If a Slave does not execute a command, the status shows this and usually indicates why. Several possible reasons are:
1. The Slave received the message in error. (This can also result in no reply.)
2. The Slave doesn't implement this command.
3. The Slave is busy.
4. The Slave was told to do something outside of its capability (range number too large or small, for example).
5. The Slave is write-protected and was told to change a protected parameter.
Commands are one of 3 types: Universal, Common Practice, and Device Specific (Proprietary).
SLAVE REPLY ALGORITHM
Hart communications contd.
HART DATA OVERVIEW
DIGITAL DATA: 35-40 valuable data items standard in every HART device DEVICE IDENTIFICATION: device tag, supplier, device type and revision, device serial number CALIBRATION DATA: upper and lower range values, upper and lower sensor limits, PV damping, last calibration date PROCESS VARIABLES: primary variable plus secondary measurements and multivariable parameters STATUS/DIAGNOSTIC ALERTS: device malfunction, configuration change, power fail restart, loop current fixed or saturated, primary or secondary variable out of limits, communication error, plus more
HART DATA OVERVIEW CONTD
PROCESS VARIABLE VALUESPrimary Process Variable (analog) - 4-20 ma current signal continuously transmitted to host Primary Process Variable (digital) - Digital value in engineering units, IEEE floating point, up to 24 bit resolution Percent Range - Primary Process Variable expressed as percent of calibrated range Loop Current - Loop current value in milliamps Secondary Process Variable 1 - Digital value in engineering units available from multivariable devices Secondary Process Variable 2 - Digital value in engineering units available from multivariable devices Secondary Process Variable 3 - Digital value in engineering units available from multivariable devices
HART DATA OVERVIEW CONTD
COMMANDS FROM HOST TO DEVICE
Set Primary Variable Units
Set Upper Range
Set Lower Range
Set Damping Value
Set Message
Set Tag
Set Date
Set Descriptor
Perform Loop Test - Force loop current to specific value
Initiate Self Test - Start device self test
Get More Status Available Information
Codes vary by manufacturer/device
STATUS AND DIAGNOSTIC ALERTS:-Device Malfunction - Indicates device self-diagnostic has detected a problem in device operation Configuration Changed - Indicates device configuration has been changed Cold Start - Indicates device has gone through power cycle More Status Available- Indicates additional devices status data available Primary Variable Analog Output Fixed - Indicates device in fixed current mode Primary Variable Analog Output Saturated - Indicates 4-20mA signal is saturated Secondary Variable Out of Limits - Indicates secondary variable value outside the sensor limits Primary Variable Out of Limits - Indicates primary variable value outside the sensor limits
HART DATA OVERVIEW CONTD
HART DATA OVERVIEW CONTD
DEVICE IDENTIFICATION:-
Instrument Tag - User defined, up to 8 characters
Descriptor - User defined, up to 16 characters
Manufacturer Name (Code) - Code established by HCF and set by manufacturer
Device Type and Revision - Set by manufacturer
Device Serial Number - Set by manufacturer
Sensor Serial Number - Set by manufacturer
HART DATA OVERVIEW CONTD
CALIBRATION INFORMATION FOR 4-20MA TRANSMISSION OF PRIMARY PROCESS VARIABLE
Date - Date of last calibration, set by user
Upper Range Value - Primary Variable Value in engineering units for 20mA point, set by user
Lower Range Value - Primary Variable Value in engineering units for 4mA point, set by user
Upper Sensor Limit - Set by manufacturer
Lower Sensor Limit - Set by manufacturer
Sensor Minimum Span - Set by manufacturer
PV Damping - Primary Process Variable Damping Factor, set by user
Message - Scratch pad message area (32 characters), set by user
Loop Current Transfer Function - Relationship between Primary Variable digital value and 4-20mA current signal
Loop Current Alarm Action - Loop current action on device failure (upscale/downscale)
Write Protect Status - Device write-protect indicator
HART ADVANTAGES
Key benefits of this unique open standard communication technology are: -4-20mA compatibility with simultaneous digital information available - Easy to use and understand -Low risk - highly accurate and robust -Cost-effective implementation for both users and suppliers -Available in a wide variety of device types - Supported by most industry device and systems suppliers -Fully interoperable and reliable
CALIBRATION
• TRANSDUCER BLOCK:- Generates the actual digital signal representation of the process parameter.
• ZERO & SPANNING BLOCK:- The upper and lower range values are used to produce the transducer value from above to correspond to a 4mA signal for the lower range and a 20mA signal for the upper range in the % form. In addition an appropriate transfer function (e.g., linear, square root, quadratic, cubic spline, etc.) may be applied .
• DAQ BLOCK:- Produces the 4-20ma signal, insuring that 0% equals exactly 4 ma and 100% equals 20ma.
WIRELESS HART
Wireless HART contd
• Wireless HART Networks consists of WirelessHART field devices, at least one WirelessHART gateway, and a WirelessHART network manager.
• These components are connected into a wireless mesh network supporting bi-directional communication from HART host to field device and back.
Wireless HART contd• Network Manager
The Network Manager is an application that manages the mesh network and Network Devices. The Network Manager performs the following functions:
- Forms the mesh network - Allows new devices to connect to the network - Sets the communication schedule of the devices - Establishes the redundant data paths for all communications - Monitors the network
• GatewayThe Gateway Device connects the mesh network with a plant automation network, allowing data to flow between the two. The Gateway Device provides access to the WirelessHART devices by a system or other host application.
• Field DevicesThe Field Device may be a process connected instrument, a router or Hand Held device. The WirelessHART network connects these devices together. -Router Device:A device to improve network coverage (to extend a network) capable of forwarding messages from other Network Devices. - Process Connected Instrument:Typically a measuring or positioning device used for process monitoring and control. It is also capable of forwarding messages from other Network Devices. - WirelessHART Adapter:A device that allows a HART instrument without wireless capability to be connected to a WirelessHART network.
Wireless HART contd
• TECHNOLOGY BASICS:-
- Time Synchronized Communication:-WirelessHART devices communicate using Time Division Multiple Access. All device-to-device communication is done in a pre-scheduled time window which enables very reliable (collision-free), power-efficient, and scalable communication
- Self-Organizing and Self-Healing:-It means every device has the intelligence to discover neighbors, measure RF signal strength, acquire synchronization and frequency hopping information, and then establish paths and links with neighboring devices. This enables very simple and robust network installation, reliable long-term performance, and simple network expansion.
- Frequency Hopping Spread Spectrum:-It uses the unlicensed part of the radio spectrum in the 2.4GHz ISM band.- Secure CommunicationsThere are three pillars of secure communication: encryption, authentication and integrity. Encryption keeps the information carried by the message from being read by other parties; authentication ensures that the sender is actually the sender; and integrity ensures that the message was delivered unaltered.
-Redundant Mesh RoutingWirelessHART implements a “full-mesh” topology in which every device has multiple redundant communication paths.
HART SYSTEM
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