Function Block Applications in Control Systems Based on IEC 61804

Transcription

1 Applications in Control Systems Based on IEC Christian Diedrich Department Control System Ifak Magdeburg, Germany Francesco Russo Standards Coordinator ENEL Milan, Italy Ludwig Winkel Terry Blevins Marketing Fieldbus Communication Principal Technologist Siemens Fisher-Rosemount Systems Karlsruhe, Germany Austin, Texas KEYWORDS s, Standards, Process Industry, IEC ABSTRACT A variety of fieldbus technologies and digital fieldbus devices have been introduced within the process industry over the last 10 years. There has been a gradually acceptance of the fact that a variety of communication technologies are needed to fully address the application requirements of a manufacturing facility. However, engineers responsible for the specification, engineer, implement of control system require that a common interface and functionality be proved in the control system. This capability should be independent of the underlying fieldbus technology or manufacturer of the fieldbus device. The draft IEC standard defines how a control system can be structured to provide this flexibility in the utilization of fieldbus technology. In this paper, we discuss how a consistent function block capability may be provided for all fieldbus technology utilized in a control system. Examples will be given of how this standard has been applied in modern control systems to give a consistent interface to Foundation Fieldbus and PROFIBUS. Some detail will be presented on the standard means that is defined for manufacturers to describe function block capability of a field device. An analysis is given of the impact and benefit that the IEC standard will have on the process industry and on manufacturers of control systems. INTRODUCTION The draft IEC standard is an end user driven specification of the requirements of distributed process control systems based on s. This requirement specification (which is defined as part 1) and its associated function block standard (part 2) originate from the power plant industrial sector. It is validated by applications in oil and gas, petrochemicals, pharmaceuticals and fine

2 chemicals, pulp and paper, food and beverage, waste water treatment plants, steel milling and others. The specification defines the requirements for function blocks to provide control, and to facilitate the maintenance and the technical management as applications, which interact with actuators and measurement devices. These parts were prepared by IEC SC65C WG7 and are available as draft IEC 61804, both parts with successful first CDs where the responsible national committees gave their comments. These comments will be resolved and integrated in the documents until end of The first CDV of draft IEC will be released end of Draft IEC61804-Part1 describes the requirements for process control function blocks and the full framework for their use as result reached by using top down and bottom up approach using inputs from end user views and views reflecting what is already offered with the today's digital systems using fieldbusses. Draft IEC Part 2 specifies function blocks by using the result of a harmonization work as regards several elements. First, the device model which defines the components of an IEC conformant device. Second, conceptual specifications of function blocks for measurement, actuation and processing. This includes general rules for the essential features to support control, while avoiding details, which stop innovation and specialization for different industrial sectors. Third, the Electronic Device Description technology that enables the integration of real product details using the tools of the engineering life cycle. The Draft IEC standard specifies a system in terms of architecture, models and the life cycle. The architecture is the "road map", names the components and presents the structure of the system. The models describe the details of the components i.e. their functions in the system. The life cycle makes visible, how the components work together during their use in different phases of the lifetime, i.e. make the operation visible. Figure 1 shows the different influences, basic specifications and technology support to the draft IEC from the top-down and bottom-up point of view. USER REQUIREMENTS PRIAM User Layer for process control ISA SP50 User Layer TR (implicit) Bottom-up WG7 METHODOLOGY Specify the application with FB independent of the technology supported Top-down Spec. ISA SP50 User Layer TR Fieldbus Foundation PROFIBUS Provide the knowledge to implement the application Detailed FB Specifications e.g. NOAH Figure 1: Influences to IEC TC65WG6 (IEC1499) generic FB model IEC TC65B WG7 Application Description Basic Models ISA 184 SC4 ACORN SC65C WG6 Fieldbus Specification

3 The influences are international standards and projects, which are in the scope of WG7. These standards are either technology independent ones supporting the top-down approach or dedicated to a certain technology e.g. PLC or fieldbus. Both together will build the basement of the standard. The main purpose of this part 1 of draft IEC61804 is the harmonization of different views, models and starting points of end users, system providers and device manufacturer. It will be the reference document leading the discussions during the specification and the guideline for the readers of the part 2. A prerequisite to design, implement and operate a FB based process control system is, that the tools and the devices follow the same architecture based on a common specification. The architecture has to define the components of the systems e.g., device, data, data connections, and more as well as relations between these components. The PAS IEC model on which this requirement specification is related is able to provide these basic components for s for process control. One add-on to this PAS IEC standard is the specification of parameters and functions of s that may be implemented in devices. Part 2 contains a minimum set of function blocks that will be required for the process industries. This is divided into 2 sections: one which covers rich function blocks covering complex but common functions such as PID required by the majority of the process industries, and one which covers a set of elementary function blocks such as Boolean functions required to compose very specific and unique functionality. Application Components s are encapsulations of variables, parameters and their processing algorithms. The variables, parameters and algorithms are those required by the design of the process and its control system. They can be derived from the Pipe and Instrumentation Diagram (P&ID, Figure ). s perform the (control and monitoring) application. Level Control V3 T1 Temperature Control T2 L1 L2 F1 P1 Controlled process V1 P2 Pump Control V2 Analog Input AI-T C PID_SP Analog Output AO-V1 Analog Input AI-T C Control application PID % PID 1 Figure 2: structure is derived out of the process

4 The application can be distributed among several devices. The devices are connected via a communication network or a hierarchy of communication networks. For the purpose of this standard there are different types, which encapsulate specific functionality of devices performing an automation application. The Technology represents the process attachment of a device. It contains the measurement or actuation principles of a device. The technology block is composed of acquisition and transformation parts. The Application (here after called ) contains application related signal processing, such as scaling, alarm detection or control and calculation. Elementary functions and elementary function blocks contains mathematical and logical functions with specific additional exception handling procedures. The Device represent the resource of the device, which contains information and function about the device itself, the operation system of the device and the device hardware. The device shall have an interface to the communication system and may have system management facilities. Sensor/Actuator Device Device e.g. Device identification Device Status Message Technology s (Process attachment) e.g. Resistor Temp. Measurement System Management e.g. Application time synchronisation Application s e.g. Analog Input Analog Output Control Calculation Network Interface Management e.g. Communication loss Device type specific specification Common specification for all device types Not mandatory for all device types Figure 3: Device Structure All devices in the scope of IEC are expected to have the same logical device structure. The number and types of blocks, which are instantiated in a device, are device and manufacturer specific. There is a data flow chain from signal detection through the Technology and s and vice versa. The signals between the parts of the chain can be internal within the blocks or visible as linkages between blocks. The logical chain of technology and function block is called a channel.

5 description The description of an IEC is a list of algorithms that are encapsulated in the block together with the related input and output variables and parameters. There are algorithms that are related to the process signal flow and algorithms that are related to other block specific functions. The parameter table shows all the needed accessible parameters of the block, figure 4. Process signal flow Input Type Name Algorithms Output Algorithms Management Parameter Parameter_1 Parameter_2... Figure 4: IEC block overview Description of Parameter_1 Description of Parameter_2... The algorithm description is done individual for each algorithm by a device designer/manufacturer in the appropriate language, e.g. plain English, Harel State Diagram, IEC FBD (function block diagram) or IEC ST (structured text). The block variables, parameters and algorithms included in a block will be those that are significant for the algorithm and device. As a minimum, function blocks will include the variables and parameters defined in the P&ID. Application The technology and application blocks built a function chain along with the process signal flow. Together they built a measurement or actuation channel. Measurement and actuation channels perform together with control and calculation function blocks in the application (Figure 5). The technology blocks are technology dependent and the function blocks are technology independent. There are different implementations of an application, depending from the used technology of the devices. The application may be performed by implementing the application in measurement and actuation devices only (i.e. a complex device may perform measurement, control and actuation.) or in measurement and actuation devices together with controllers and other components of the system. A controller may be for instance integrated in the application as one calculation function block or an

6 actuation device may take parts of programmable functions from controller devices in terms of calculation function blocks. Setpoint Measurement Technology Sensor (s) Measurement (Application) Calculation (Application) Control (Application) Actuation (Application) Actuation Technology Actuator Sensor (s) Processrelated physics (process attachment Measurememt related principles (technologydependent) Process related application (almost technology-independent) Actuation related principles (technologydependent) Processrelated physics (process attachment) Figure 5: Application process signal flow There are different device characteristics, called device types. The main characteristic aspect is the execution control methods. Execution control of function block algorithms is a feature of each device. There are many possible execution policies within devices and in a distributed system: Free running Device internal time schedule (time synchronization) System wide time synchronization (time synchronization across the communication system) Communication service triggered Device internal event triggered System wide event triggered Distributed execution control Which execution control is used depends on the capability of the devices. Therefore the execution control is defined by concrete fieldbus systems. Device Descriptive Language The rapid development in the last few years of factory and process automation has made one basic concept inevitable: Engineers and technicians have to be supported by computer-based tools. This applies particularly for the so-called Distributed Control Systems (DCS). Fieldbuses carry the data between the controllers and the sensors/actuators and between the Engineering Station and all devices. In all the different phases of the engineering process one or several tools running on a PC, a workstation or a terminal need to know what kind of devices they are connected to. This covers the vendor information, the version of hard- and firmware release, the data format to be exchanged, the

7 reaction time, the physical unit et. al. In most cases the configuration, located in the engineering station also needs to know the communication abilities concerning responding time, supported baud rates et. al. With the development of remotely accessible devices most of the device features are put together in a device description, which is delivered in addition to the appliance. The Electronic Device Description defined by the IEC standard is a technology that allows off-line configuration and for devices to be integrated over the complete control system life cycle. Profiles Based on IEC The harmonization of function blocks is done at an abstract level that allow the specifier to define in an unique way the common features offered by the several technologies and some complementary features that answers user requirements. This abstract vision is here called the conceptual function block (FB) specification. Industry groups will map this to specific communication systems and their accompanying definitions. The draft IEC specification is based also on the abstract definitions of PAS IEC and ISO Abstract models IEC ISO Conceptual FB spec. for the process sector Solutions (technology) Profiles IEC FB specification FF FB Application PROFI- BUS PA Profile controlnet... FIP Companion Standards IEC EDD specification Implementation Products Figure 6: Position of IEC related to other standards and products The Electronic Device Description (EDD) language is ready for use. This fill in the gap between the conceptual function block specification and a product implementation. It allows the manufacturers to use the same description method for devices based on different technologies and platforms. In the part 2 annex of the standard, profile conformance statement for numerous technologies are defined including PROFIBUS 1 and Foundation Fieldbus 2. It is the intention of this standard, that future product developments of field devices will follow directly the definitions of this specification. 1 PROFIBUS is the trade name of PROFIBUS User Organisation (PNO). PNO is a non-profit trade organization to support the fieldbus PROFIBUS. This information is given for the convenience of users of this International Standard and does not constitute an endorsement by ISO/IEC of the trademark holder or any of its products. Compliance to this profile does not require use of the tradename PROFIBUS. Use of the tradename PROFIBUS requires permission of the tradename holder. 2 Foundation Fieldbus is the trade name of the consortium Fieldbus Foundation (non-profit organization). This information is given for the convenience of users of this International Standard and does not constitute an endorsement by ISO or IEC of the product named. Equivalent products may be used if they can be shown to lead to the same results.

8 Example Application The s resulting from the design of a process control system are abstract representations and may be implemented in different ways in different device types (Figure 7). s can be implemented e.g. in field devices, PLC, visualization stations and device descriptions. Additionally other applications such as system engineering and the supervisory system have to handle or interact with the s. s defined for a block in the conceptual model are not necessarily mapped one-to-one to the device; they can be mapped to the device and a proxy if the current technology doesn't solve it in the device. Engineering System Supervisory System IEC FB Visualisation FB Faceplate Commissioning Tool DD n DD 3 DD 2 DD 1 PLC Proxy FB (IEC 1131-FB-Library) IEC EDD FB e.g. AI_FB Member { Variable_1;... } FD 1 FD 2 FD 3 PA device FF device DD 1 DD 2 DD 3 FD n DD n DD - Device Description FD - Field Device PLC - Programable Logic Controller Figure 7: IEC s can be implemented in different devices One example of how the IEC specification may be applied to give a consistent function block interface to different technologies is the DeltaV Control system from Fisher-Rosemount Systems, Inc. The system architecture is based on Foundation Fieldbus function block specification and thus allows control strategies to be defined independent of whether the function block executes in the fieldbus device or in the DeltaV controller. Fieldbus devices that do not support function block such as those based on AS-Interface, Profibus DP, and DeviceNet are brought into the system using the concept of a proxy in the controller to add function block capability to the measurement or actuator value provided by the device To the end user, these proxy function blocks may be configured for monitoring and calculation application in the same manner as Foundation Fieldbus function blocks. The advanced signal processing and alarming that would normally be done by function blocks in the fieldbus device are done instead in the controller. Though the same functionality is provided through the proxy, there are differences in the dynamic associated with the signal processing. In many cases, these timing differences will have not impact on the application. An example configuration application that utilizes inputs and output from Foundation Fieldbus, AS-Interface, and Profibus DP is shown in Figure 8.

9 Foundation Fieldbus AS-Interface Profibus DP Figure 8: Example for Foundation Fieldbus, AS-Interface, and Profibus DP Another example of how the IEC specification may be mapped to a real application using PROFIBUS and the DCS system from SIEMENS, named PCS7, is briefly explained. The Transducer s and s of IEC are mapped to the Profile PROFIBUS PA of PROFIBUS International. This specifies the function block view of PROFIBUS PA fieldbus devices. Such fieldbus devices communicate through PROFIBUS to host systems to control a plant. Such a controller use function blocks to perform control. Five fieldbus function block types are enough to build a proxy of fieldbus devices based on Profile PROFIBUS PA. These are Analog Input and Output and Discrete Input and Output and a Totalizer (PA_IN, PA_OUT, PA_DI, PA_DO). A HMI is linked to the controller and get information from the controller and sent for example setpoints. Visualization is based on faceplates, which represents for example a motor, valve, or PID controller. Coming back to the proxies within a controller. Such a PA_AO represents a fieldbus device like a valve. Based on a tag name, PA_AO write a setpoint and read feedback information including quality code to and from a device, independent of a specific vendor or AO type. There is no explicit communication specific configuration. But how to configure the fieldbus devices? An engineering tool performs this. But how to learn about the thousands of parameter of fieldbus devices? Each vendor has to write an Electronic Device Description, which can be imported by this engineering tool to learn about options and manufacturer specific behaviors. So it is not necessary to specify more precisely a fieldbus devices in a standard. The device type behavior is described in an electronic descriptive description language.

10 CONCLUSION Process control systems that use fieldbus, function blocks and device descriptive languages already exist in the market. It is clear that the majority of these systems will continue to use function blocks into the foreseeable future, as they are modular and re-usable. However control system manufacturers, implementers and users are faced with: - Multiple technology platforms that are changing rapidly. - Multiple communication standards. The approaches are similar, but the technologies are different. According to the strategic approach adopted by IEC65C-wg7, the IEC draft standard for process control function blocks was produced by identifying the common concepts of today's function blocks and of the device descriptive languages and by harmonizing them. All the industrial actors may use the resulting conceptual specifications as a common way to describe what is needed and what is solvable. They understand each other, irrespectively of the specific terminology and of the evolution of it. The harmonized view includes function blocks representing different elements of the application: control, maintenance and technical management. They may be mapped on the several technologies and the system engineering may be supported with the same device descriptive tool. As for the mapping on the several technologies, an important role is played by the proxy approach. It is expected that process plants users use the standard as a point of reference in specifying their control system requirements and in evaluating technology to meet these requirements in formal manner. Provided that some function blocks are not completely solved in the existing devices, the proxy approach allows the system integrator to complement these devices by solving the missing functionalities with some additional software (the proxy) installed in the controller. System integrators are able to define the proxies needed for the different applications and for the different industrial sectors. Applications can be solved with today's devices. Control system manufacturers, device manufacturers and Industrial groups have the evidence of "what else" should be solved in future devices and are able to decide "if", "when" and "how" to solve them within new devices. Thus IEC allows the users to carry a top-level view of their control systems forward into the future and the manufacturers to answer with evolving solutions. REFERENCES 1. IEC 1804 Part 1 General Requirements Committee Draft (SC65CWG7(PT1CD)2) 2. IEC 1804 Part 2 Specification Committee Draft (SC65CWG7(PT2CD)1)