DESIGN AND PRODUCTION PROCESS PROJECT

DESIGN AND PRODUCTION PROCESS PROJECT

                                                                   Industrial Engineering Assignment

                               By : Gaurav Pandey (roll 33) & Aniketh Pattnaik(roll 10)

       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.

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.

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.

HART technology is a master/slave protocol, which means that a smart field (slave) device only speaks when spoken to by a master. The HART Protocol can be used in various modes such as point-to-point or multidrop for communicating information to/from smart field instruments and central control or monitoring systems.

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.

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:

In order to control a dynamic variable in a process, there must be information about the variable itself. This information is obtained from a measurement of the variable. A measurement system is any set of interconnected parts that include one or more measurement 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.

Signal Transmission for Electronic and Pneumatic Signals

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

The figure above illustrates functions of a smart transmitter. 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
• Re-ranging
• Characteristics
• Signal conditioning
• Self-diagnosis

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