DESIGN AND PRODUCTION PROCESS
PROJECT
Industrial
Engineering Assignment
By : Aniketh Pattnaik(roll 10) & Gaurav Pandey (roll 33)
Topic: SMART SENSORS TECHNOLOGY IN
INDUSTRY
Introduction
Transducers are sensors and actuators
in order that a computer system can interact with the physical environment. In
1982, Ko and Fung introduced the term “intelligent transducer” . An intelligent
or smart transducer is the integration of an analog or digital sensor or
actuator element, a processing unit, and a communication interface. In case of
a sensor, the smart transducer
transforms the raw sensor signal to a standardized digital
representation, checks and calibrates the signal, and transmits this digital
signal to its users via a standardized communication protocol. In case of an
actuator, the smart transducer accepts standardized commands and transforms
these into control signals for the actuator. In many cases, the smart
transducer is able to locally verify the control action and provide a feedback
at the transducer interface. With the advent of modern microcontrollers it
became possible to built low-cost smart transducers by using
commercial-off-the-shelf microcontrollers that provide a standard communication
interface, such as a UART (Universal Asynchronous Receiver/Transmitter). Thus,
the usage of smart transducers can become a cost decreasing factor for building
embedded control systems. The objective of this paper is to give a brief
overview in principles, communications, and configuration aspects for smart
transducers. The remainder of the paper is structured as follows. Discusses the
basic principles of smart transducers. It gives an overview on existing
communication interfaces for smart transducers. Section describes various
approaches for configuration support of smart transducer networks. The paper is
concluded in section Smart Transducer Principles
Two-Level
Design Approach
The smart transducer technology
introduces a two-level design approach that helps to reduce the overall
complexity of a system by separating transducer-specific implementation issues
from interaction issues between
different smart transducers.
The transducer manufacturer will deal
with instrumenting the local transducer and signal conditioning in order to
export the transducer’s service in a standardized way. Transducer manufacturers
are thus liberated from interoperability issues between sensors, naming
inconsistencies and the network topology of the total system. The user of a
smart transducer’s service can access its data via an abstract interface that
hides the internal complexity of the transducer hardware and software. Thus,
smart transducer applications can be built in a less complex way.
Interface
Design
The most critical factor for a smart
transducer design is the construction of the interfaces to smart transducers.
We distinguish the smart transducer interface and the transducer
communication interface.
The smart transducer interface is an
abstract interface that gives access to the transducer features, such as
measurement value or set value, respectively, but also vendor ID numbers,
diagnostic information and setup parameters. Usually, a smart transducer
interface provides different types of service, such as configuration, remote
diagnosis, and real-time measurement. In order to keep a clean separation of
functionalities and achieve a better understandability of a smart transducer
interface, Kopetz proposes the introduction of distinct interfaces for
functional different services
In
detail the following three interface types can be distinguished:
Real-Time
Service (RS) interface:
This interface provides the timely
real-time services to the smart transducer during the operation of the system.
Diagnostic and Management (DM) interface: This interface opens a communication
channel to the internals of a smart transducer. It is used to set parameters
and to retrieve information about the internals of a component, e. g., for the
purpose of fault diagnosis.
The DM interface is available during
system operation without disturbing the real-time service. Normally, the DM
interface is not time-critical. Configuration and Planning (CP) interface: This
interface is necessary to access configuration properties of a node. During the
integration phase this interface is used to generate the “glue” between the
autonomous smart transducers. The CP interface is not time-critical.
The
transducer communication interface defines the communication among the
transducers in the network. The idea to connect multiple transducers to a
single communication bus has its roots in the industrial fieldbus networks that
dateback to the early 1970s. An early example for a transducer communication
interface is the 4-20 mA current loop, an analog signal standard for the
point-to-point connection of analogue devices. The transducer communication
interface handles aspects such as the communication baud rate, data encoding,
flow control, and message scheduling. The difference between these two
interfaces becomes clear, when they are aligned to the 7-layer of the
International Standard Organizations Open System Interconnect (ISO/OSI) model.
While the issues on the smart transducer interface relate to the application
layer (layer 7) of the OSI reference model, the network communication relates
to the layers below 7, especially the physical layer (layer 1) and the data
link layer (layer 2). The intermediate layers 3-6 are usually not defined for
fieldbus systems. All interfaces have to be well-defined in the value and in
the time domain in order to enable desirable properties such as
interoperability, i. e., the ability of two or more devices, independent of the
manufacturer, to work together in one or more distributed applications , and
composability, i. e., if each subsystem implements well-defined interfaces in
the temporal and value domain, it can be guaranteed a priori that the subsystem
provides its specified service also in the composite system.
The IEC
worked out the IEC 61158 standard. It is based on the following existing
fieldbus systems:
Foundation
Fieldbus: A functional superset of WorldFIP.
The IEC 61158 standard defines also a Foundation Fieldbus High Speed Ethernet
type.
ControlNet:
ControlNet has been primarily designed to meet the requirements of high
speed real-time applications for automation and control. ControlNet features
the Control and Information Protocol that provides real-time and peer-to-peer
messaging.
Ethernet/IP:
EtherNet/IP is an open network based on the IEEE 802.3 Physical and Data
Link standard, the Ethernet TCP/IP protocol suite and the Control and
Information Protocol.
Profibus:
Profibus is a distributed control system for process automation.
Profibus is one of the most popular fieldbus protocols in this area. In 2001 it
claimed 53.6% of the revenue created in the fieldbus sector in Europe.
SwiftNet:
SwiftNet is a high performance fieldbus that was created as a
synchronous, high speed flight data bus for Boeing Commercial Airplane. SwiftNet
provides high data efficiency and clock synchronization among the communicating
nodes.
WorldFIP:
WorldFIP is designed with a strictly real-time capable control
scheme
based on a producer-consumer communication model. WorldFIP was published as a
French standard in the late 80s. No significant change has taken place since
the first French standard.
Interbus:
Interbus is a digital, serial communication system for communication
between control systems and transducer devices. Interbus is optimized, but not
limited to factory automation applications.
What is HART
The majority of smart field devices
installed worldwide today are HART-enabled. But some new in the automation
field may need a refresher on this powerful technology. HART (Highway
Addressable Remote Transducer) Protocol is the global standard for sending and
receiving digital information across analog wires between smart devices and
control or monitoring system. HART is a bi-directional communication protocol
that provides data access between intelligent field instruments and host
systems.
A DIGITAL UPGRADE FOR EXISTING PLANTS
HART technology offers a reliable, long-term solution for plant operators who seek the benefits of intelligent devices with digital communication – that is included in the majority of the devices being installed .Because most automation networks in operation today are based on traditional 4-20mA analog wiring, HART technology serves a critical role because the digital information is simultaneously communicated with the 4-20mA signal. Without it, there would be no digital communication.
HART technology offers a reliable, long-term solution for plant operators who seek the benefits of intelligent devices with digital communication – that is included in the majority of the devices being installed .Because most automation networks in operation today are based on traditional 4-20mA analog wiring, HART technology serves a critical role because the digital information is simultaneously communicated with the 4-20mA signal. Without it, there would be no digital communication.
There are several reasons to have a
host communicate with smart devices. These include:
- Device Configuration or re-configuration
- Device Diagnostics
- Device Troubleshooting
- Reading the additional measurement values
provided by the device
- Device Health and Status
How HART Works
“HART” is an acronym for Highway
Addressable Remote Transducer. The HART Protocol 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.
 |
| Figure 1. Frequency Shift Keying (FSK) |
This enables two-way field
communication to take place and makes it possible for additional information
beyond just the normal process variable to be communicated to/from a smart
field instrument. The HART Protocol communicates at 1200 bps without
interrupting the 4-20mA signal and allows a host application (master) to get
two or more digital updates per second from a smart field device. As the
digital FSK signal is phase continuous, there is no interference with the
4-20mA signal.
 |
| Figure 2. Two Communication Channels |
HART Communication occurs between two HART-enabled devices, typically a smart field device and a control or monitoring system. Communication occurs using standard instrumentation grade wire and using standard wiring and termination practices.
The HART Protocol provides for up to
two masters (primary and secondary). This allows secondary masters such as
handheld communicators to be used without interfering with communications
to/from the primary master, i.e. control/monitoring system.
Figure 3.
Primary and Secondary Masters
The HART Protocol permits all digital
communication with field devices in either point-to-point or multidrop network
configurations:
Figure 4.
Point-to-Point Configuration
Multidrop Configuration
There is also an optional
"burst" communication mode where a single slave device can continuously
broadcast a standard HART reply message. Higher update rates are possible with
this optional burst communication mode and use is normally restricted to
point-to-point configuration.
Benefits of Using HART
Communication
- Leverage the capabilities of a full set of
intelligent device data for operational improvements.
- Gain early warnings to variances in device,
product or process performance.
- Speed the troubleshooting time between the
identification and resolution of problems.
- Continuously validate the integrity of loops
and control/automation system strategies.
- Increase asset productivity and system
availability.
Increase Plant Availability
- Integrate devices and systems for detection of
previously undetectable problems.
- Detect device and/or process connection
problems real time.
- Minimize the impact of deviations by gaining
new, early warnings.
- Avoid the high cost of unscheduled shutdowns
or process disruptions.
Reduce Maintenance Costs
- Quickly verify and validate control loop and
device configuration.
- Use remote diagnostics to reduce unnecessary
field checks.
- Capture performance trend data for predictive
maintenance diagnostics.
- Reduce spares inventory and device management
costs.
Improve regulatory compliance
- Enable automated record keeping of compliance
data.
- Facilitates automated safety shutdown testing.
- Raise SIL/safety integrity level with advanced
diagnostics.
- Take advantage of intelligent multivariable
devices for more thorough, accurate reporting.
The standard features of HART technology
range from simple compatibility with existing 4-20mA analog networks to a broad
product selection:
- Compatibility with standard 4-20mA wiring
- Simultaneous transmission of digital data
- Simplicity through intuitive menu-driven
interfaces
- Risk reduction through a highly accurate and
robust protocol
- Ease of implementation for maximum “up-front”
cost effectiveness
- Broad product selection, with compatible
devices and software applications from most process automation providers
- Platform independence for full
interoperability in multi-vendor environments
HART Specifications
. The current version of the HART
Protocol is revision 7.3. The "7" denotes the major revision level
and the "3" denotes the minor revision level.
The HART Protocol implements layers
1,2, 3, 4 and 7 of the Open System Interconnection (OSI) 7-layer protocol
model:
The HART Physical Layer is
based on the Bell 202 standard, using frequency shift keying (FSK) to
communicate at 1200 bps. The signal frequencies representing bit values of 0
and 1 are 2200 and 1200Hz respectively. This signal is superimposed at a low
level on the 4-to-20mA analog measurement signal without causing any
interference with the analog signal.
The HART Data Link Layer defines
a master-slave protocol - in normal use, a field device only replies when it is
spoken to. There can be two masters, for example, a control system as a primary
master and a handheld HART communicator as a secondary master. Timing rules
define when each master may initiate a communication transaction. Up to 15 or
more slave devices can be connected to a single multidrop cable pair.
The Network Layer provides
routing, end-to-end security, and transport services. It manages
"sessions" for end-to-end communication with correspondent devices.
The Transport Layer: The
Data-Link Layer ensures communications are successfully propagated from one
device to another. The Transport Layer can be used to ensure end-end communication
is successful.
The Application Layer defines
the commands, responses, data types and status reporting supported by the
Protocol. In the Application Layer, the public commands of the protocol are
divided into four major groups:
asurement Devices
Measurement devices
Measurement devices perform a complete measuring function, from
initial detection to final indication. Two important aspects of a measurement
system are the sensor and the transmitter. A third is the transducer.
- Sensor:
Primary sensing element
- Transducer:
Changes one instrument signal value to another instrument signal value
- Transmitter:
Contains the transducer and produces an amplified, standardized instrument
signal
Signal
types
In most existing plants pneumatic and electronic signals are predominant. Pneumatic signals
are normally 3-15 pounds per square inch (psi), and electronic signals are
normally 4-20 milliamps (mA). Optical signals are also used with fiber optic
systems or when a direct line of sight exists.
Radio and hydraulic signals
are also used, though they are not as common because of inherent problems such
as radio signal interference and leakage of hydraulic systems. However, radio
signals commonly are used when sensors and transmitters are great distances (on
pipelines, for example) from control centres.
A major difference between electronic and pneumatic transmission
systems is the time required for signal transmission. In an electronic system
there are no moving parts, only the state of the signal changes. This change
occurs with virtually no time lost.
As we stated previously, mechanical movement takes place whenever
any pneumatic process signal changes. When devices move mechanically, time is
lost. In addition, pneumatic systems, because they contain moving parts, are
higher maintenance and subject to vibration, as well as rotational or
gravitational mounting problems. However, pneumatic systems are still in place
in many plants because they are safer than electrical systems in certain
environments containing potentially explosive atmospheres.
A transmitter's gain, that is the ratio of the
output of the transmitter
to the input signal, is constant
regardless of its output. In other words, an electronic transmitter's gain will
remain constant whether it's output is 0% of span (4 mA) or 100% of span (20
mA) or any other point between those extremes.
So far, the discussion has centred around electronic and pneumatic
transmitters. The input and output of both of these types of transmitters is an analog signal
-- either a mA current or air pressure, both of which are continuously
variable. There is another kind of transmitter -- the "smart"
transmitter.
Smart Transmitter Components and Function
They can convert analog signals to digital signals (A/D),
making communication swift and easy and can even send both analog and digital signals at the same time as denoted by D/A.
A smart transmitter has a number of other capabilities as well.
For instance, inputs can be varied, as denoted by A/D. If a temperature
transmitter is a smart transmitter, it will accept millivolt signals from thermocouples and
resistance signals from resistance
temperature devices (RTDs), and thermistors.
Components of the smart transmitter are illustrated in the lower
figure. The transmitter is built into a housing about the size of a softball as
seen on the lower left. The controller takes the output signal from the
transmitter and sends it back to the final control element. The communicator is
shown on the right.
The communicator is a hand-held interface device that allows
digital "instructions" to be delivered to the smart transmitters.
Testing, configuring, and supply or acquiring data are all accomplished through
the communicator. The communicator has a display that lets the technician see
the input or output information. The communicator can be connected directly to
the smart transmitter, or in parallel anywhere on the loop.
Smart transmitters also have the
following features:
Configuration
Smart transmitters can be configured to
meet the demands of the process in which they are used. For example, the same
transmitter can be set up to read almost any range or type of thermocouple,
RTD, or thermistor. Because of this, they reduce the need for a large number of
specific replacement devices.
Re-ranging
The range that the smart transmitter
functions under can be easily changed from a remote location, for example by
the technician in a control room. The technician or the operator has access to
any smart device in the loop and does not even have to be at the transmitter to
perform the change. The operator does need to use a communicator, however. A
communicator allows the operator to interface with the smart transmitter. The
communicator could be a PC, a programmable logic controller (PLC), or a
hand-held device. The type of communicator depends on the manufacturer.
Re-ranging is simple with the smart
transmitter. For instance, using a communicator, the operator can change from a
100 ohm RTD to a type-J thermocouple just by reprogramming the transmitter. The
transmitter responds immediately and changes from measuring resistance to
measuring millivoltage.
There is a wide range of inputs that a
smart transmitter will accept. For instance, with pressure units, the operator
can determine ahead of time whether to use inches of water, inches of mercury,
psi, bars, millibars, Pascal’s, or kilopascals.
Characteristics
Another characteristic of a smart
transmitter is its ability to act as a stand-alone transmitter. In such a
capacity, it sends the output signal to a distributed
control system (DCS) or a PLC.
Signal conditioning
Smart transmitters can also perform
signal conditioning, scanning the average signal and eliminating any "noise"
spikes. Signals can also be delayed (dampened) so that the response does not
fluctuate. This is especially useful with a rapidly changing process.
Self-diagnosis
Finally, a smart transmitter can
diagnose itself and report on any problems in the process. For example, it can
report on a circuit board which is not working properly.
Summary
There are distinct advantages in using
a smart transmitter. The most important include ease of installation and
communication, self-diagnosis, improved and digital reliability. Smart
transmitters are also less subject to effects of temperature and humidity than
analog devices. And although vibration can still affect them, the effects are
far less than with analog devices. Smart transmitters also provide increased
accuracy. And because can replace several different types of devices, using
them allows for inventory reduction.
No comments:
Post a Comment