PC-Based Advanced Control ting

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PC-Based Advanced Control Supplementing DCS with the Unified User Interface

Yasuhiro Umehara Process Automation Systems Division Yokogawa Electric Corporation Musashino-shi Tokyo, 180-8750 Japan Noriko Kase Process Automation Systems Division Yokogawa Electric Corporation Musashino-shi Tokyo, 180-8750 Japan

Jun Nishida Process Automation Systems Division Yokogawa Electric Corporation Musashino-shi Tokyo, 180-8750 Japan Steve Lazok Industrial Automation Systems Division Yokogawa Corporation of America Stafford, Texas 77477

KEYWORDS
Advanced Control, DCS, PC, Controller, Control Bus

ABSTRACT
This paper describes advanced control using a personal computer (PC) working together with a traditional distributed control system (DCS). Because the PC control functions are similar to that of the DCS controller, engineering the user applications on the PC follow similar procedures. The human-machine interface of the DCS is used to operate and monitor the advanced controls. Close data connections, such as cascade control loops between the PC and DCS controller, can be achieved by directly connecting the PC to the redundant DCS control bus. With this architecture, a reasonable plant operating control structure, including a unified user-interface, is achieved.

INTRODUCTION
High operating plant efficiency is achieved with advanced control. During the last decade, the personal computer’s (PC) performance has improved dramatically. This has provided better opportunities to implement advanced control using a PC and integrating it into the entire control system architecture. One of the attributes of advanced control is that it may require large amounts of calculations. The new performance of PCs makes it good at solving intensive algorithms. Thus, if a PC is connected to the control bus of a DCS (Distributed Control System), then, while the DCS

controls the plant with high reliability, the PC performs advanced control. This architecture is explored and explained in detail. In addition, the advanced control PC should be integrated into the control system in a manner similar to the DCS controller. This can be accomplished if identical APIs (Application Programming Interface) for the DCS controller are provided for the PC. This covers both networking and system functions. Then it is possible to execute the same system programs as those of the DCS controller, and the PC user application can be made with the same engineering procedure and programming methods. Moreover, the operating and monitoring human-machine interfaces (HMI) are the same. For example, it is possible to display both application data from the PC and the DCS controller. In this paper, a PC user application example is explained. DCS control functions and advanced control applications are freely combined into the PC and are executed. Also, the data connection between the DCS controller and the PC is peer-to-peer (similar between two DCS controllers).

System Configuration of DCS with PC-Based Advanced Control
Figure 1 shows the DCS system configuration where a PC is connected to the control bus. The DCS controllers are directly wired to the plant. The PC is connected to the control bus, but there is not a direct link between it and the plant. This means the PC accesses the data of the DCS controller and does not access the data of the plant directly.
Ethernet

PC Engineering

PC Operation and Monitoring

PC Operation and Monitoring

Control Bus
Plant PC Advanced Control DCS Controller DCS Controller DCS Controller

Electric Wires

Figure 1. System Configuration of DCS with PC-Based Advanced Control

The DCS controller is used for non-stop, mission-critical control, such as regulatory control. The dual-redundant option of the DCS controller provides improved reliability in this area. Still, generally speaking, the CPU speed and memory capacity of the DCS controller is inferior to that of today’s PC computer systems. The PC reads data from the DCS controller and, using them as parameters, executes the control calculations, and then outputs the results back to the DCS controller. Even when the PC fails, the DCS controller continues with plant operation, since the PC is not directly connected to the plant. In the configuration shown in Figure 1, the DCS controllers are used for control that is essential for plant operation. The PC is used for additional purposes such as advanced control and efficiency improvement. This requires flexibility and a large amount of calculations. The PC with its highspeed CPU and large memory capacity is fit to take this role. Although the commercial PC typically cannot be made dual-redundant, this is not a fatal problem since the DCS controller continues to operate the plant when the PC stops. That is to say, even though the PC has inferior reliability, it is still suitable for advanced control because of its high performance with low price.

User Application Configuration on the PC
It is possible to duplicate the same DCS control functions together with advanced control applications in the PC. Figure 2 shows this configuration in detail. The duplicated DCS control functions consist of function blocks which perform regulatory control, sequential control, arithmetic calculation functions, and so on. The user application is also put into a function block using a programming language. In this paper, this function block is called the ‘user-defined algorithm block’. The user-defined algorithm block inherits basic features, such as mode transition and alarm handling.

Control Bus
Read Data from PC to DCS Controller

Write Data from PC to DCS Controller

Control Function function blocks (same as controller’s) user-defined algorithm blocks

Control Function

function blocks

PC for Advanced Control

DCS Controller

Figure 2. User Application Configuration on the PC

Based upon their own knowledge and experience, users develop advanced control calculations with the user-defined algorithm block. To assist development, the PC’s system program should provide library functions, such as data input/output and alarm handling. Together the function blocks and the user-defined algorithm blocks are integrated into advanced control applications. These are then executed periodically on the PC. The advanced control application gets input process data from the DCS controller, executes the control calculations, and writes output calculation results back to the DCS controller. This keeps high reliability and robustness while improving plant performance.

System Program Integration on the PC
Figure 3 shows the PC system programming architecture. There are three main points: 1. The same API as that of DCS controller's network communication functions. 2. The same API as that of DCS controller's operating system functions. 3. The system programs are created from the same source files as those of the DCS controller.
PC for Advanced Control

Control Task

Communication Tasks

Created from the Same Source Files of DCS Controller’s

Library Functions with the Same API as DCS Controller’s Operating System Functions

Library Functions with the Same API as DCS Controller’s Communication Functions

General-Purpose Type Operating System (such as Windows 2000)

Network Communication Driver Network Communication Card for Control Bus

Control Bus

Figure 3. System Program Integration on the PC As previously shown is Figure 1, the engineering PC and the operation and monitoring PC are directly connected to the control bus. The same network communication card can be used for the advanced control PC. Library functions for network communication on the PC’s general-purpose type operating system have the same API as that of the DCS controller.

Real-time capacity is required to execute the control applications on the PC. But, it is not so severe as that required for the DCS controller. The DCS controller requires high-speed scanning as fast as ten milliseconds. But for advanced control, a scan period of seconds is good enough. Library functions, using a general-purpose type operating system (such as Microsoft Windows 2000), provide such a real-time capacity. In other words, PC based library functions do not accomplish the real-time capacity required for a DCS controller, but when uses are limited to advanced control, the library functions do possess sufficient real-time capacity. The PC system program consists of one control task and several communication tasks. The control task plays a central role in the advanced control function, and it executes function blocks periodically. The communication tasks handle data communication with other stations, such as a DCS controller or an operation and monitoring PC. Individual communication tasks exist for each data communication purpose. The control task uses the DCS controller’s API and the communication tasks also use the DCS controller’s API. Thus, if the DCS controller’s APIs are duplicated in the PC, then it is possible to execute the same control task and communication tasks in the PC. To make these tasks executable on the PC, the source files for the control task and the communication tasks are compiled into binary objects in the PC’s machine language. Although the PC is connected to the control bus, it does not have a direct link to the plant. The DCS controller needs the I/O access functions, but it is not necessary to prepare the I/O access functions for the PC.

The Same Engineering for Control Application
Engineering the user application for the PC is accomplished by combining function blocks with the control application builder. The detail-engineering procedures and programming methods are the same as those of the DCS controller. The engineering PC is used to create user applications, which are downloaded to the advanced control PC. Since the PC system programs are created from the same source files as those of the DCS controller, the advanced-control application has identical behavior and database format (Figure 4). An application engineer, who is familiar with DCS application engineering, can also create PC user applications.

PC for Engineering Control Application Builder

Control Application *1

Control Application *1

*1 The Same Database Format

Control Bus

PC for Advanced Control

Control Application *1

Control Application *1

DCS Controller

System Program to execute Control Application *2

System Program to execute Control Application *2

*2 Created from the Same Source Files

Figure 4. The Same Engineering

The Same Operation and Monitoring Human-Machine Interfaces
The operation and monitoring function accesses the advanced control PC’s data by way of the control bus. From the viewpoint of operation and monitoring, the advanced control PC and the DCS controller have the same communication interfaces and the same control functions (Figure 5). Detailed function block manipulation is also identical. This is beneficial for both the user application engineers and operators. The user application engineers can develop the operation and monitoring application in the same way as the DCS controller. The operators can operate and monitor in the same human-machine interfaces, so safety will be improved.

PC for Operation and Monitoring Panels and Windows for Operation and Monitoring *1 *1

Control Bus

*1 The Same Communication Interfaces

Control Application PC for Advanced Control

Control Application DCS Controller

Figure 5. The Same Operation and Monitoring

An Example of Advanced Control Application on the PC
On the advanced control PC, function blocks and the user-defined algorithm blocks (built with a programming language) are integrated into the entire application. A product composition control application is shown as an example for advanced control executing on the PC (Figure 6). The calculation results are output from a PC function block to a DCS controller function block, where there is a cascade connection between them. Figure 6 shows the user application configuration for this example. And Table 1 is the list of function blocks that are used in Figure 6.

PC for Advanced Control
IMAN(MAN) IMAN(MAN) IMAN(MAN)
by LOOP_CP by LOOP_CP by LOOP_CP

DCS Controller

LOOP_CP STBL Loop Coupling AUT CAS

IMAN(CAS) CAS

AUT

CAS

CAS_IN

CAS_IN

CAS_IN

COMP_CL
IN OUT

CNST_CL
OUT

IRRATE_CL
OUT

RRATE_CL PID Reflux Rate Control

UDEF Composition Control

UDEF Constraint Control

PID Internal Reflux Rate Control

COMP_ID
PV

PVI Composition Indication

Figure 6. Product Composition Control

Station
PC for Advanced Control DCS Controller

Table 1. List of Function Blocks in Figure 6 Function Block Type Tag Name
User-Defined Algorithm Block User-Defined Algorithm Block PID Controller Block Sequence Table Block PID Controller Block Input Indicator Block (UDEF) (UDEF) (PID) *1 (STBL) *1 (PID) *1 (PVI) *1 COMP_CL CNST_CL IRRATE_CL LOOP_CP RRTAE_CL COMP_ID

Role

Composition Control Constraint Control Internal Reflux Rate Control Loop Coupling Reflux Rate Control Current Composition Indicator

*1: Function block type included in DCS controller’s function On the DCS controller, the PID controller block RRATE_CL performs reflux flow control and the indicator block COMP_ID shows the current composition value. On the advanced control PC, the PID controller block IRRATE_CL is cascaded to RRATE_CL. The advance control application consists of two user-defined algorithm blocks, one PID controller block, and one sequence table block. The user-defined algorithm block COMP_CL inputs the current composition value from COMP_ID, and executes the calculation for composition control. The other user-defined algorithm block CNST_CL performs constraint control for the calculation result of COMP_CL. Composition and constraint controls are the kinds of the functions that many users want to develop on their own. This can be accomplished with a programming language contained in the user-defined algorithm block. Similar to function blocks on the DCS controller, on the PC, the PID controller block IRRATE_CL is used for internal reflux control, and the sequence table block LOOP_CP is used for loop coupling.

Data Connection between DCS Controller and PC
Close data connection, such as cascade control loops, can be made between the PC and the DCS controller via the control bus. In Figure 6, the DCS-PID controller block, RRATE_CL, receives control output from the PC-PID controller block, IRRATE_CL. And the data connection between the DCS controller and the PC is explained with an example of changing mode from CAS(Cascade) to AUT(Automatic) of RRATE_CL. IRRATE_CL receives the read-back value and status from RRATE_CL (Figure 7). When the mode of RRATE_CL is CAS, it accepts the control output from IRRATE_CL, but when the mode of RRATE_CL transforms into AUT, it does not accept the control output. This situation is reported back to IRRATE_CL with the read-back status. As a result, the mode of IRRATE_CL becomes IMAN(Initialization Manual). Finally, IRRATE_CL stops control output to RRATE_CL. In this paper, the term IMAN(CAS) means that the IMAN mode is established with the CAS mode. Such exchanging of values and status for the cascade connection between the PC and the DCS controller is the same as between two DCS controllers in peer-to-peer communication with each other. To sum up, the data connection between the PC and the DCS controller follows a close relationship as that of two DCS controllers.

PC for Advanced Control

DCS Controller

Readback Value and Status

Output
CAS_IN CAS_IN

IRRATE_CL
OUT

RRATE_CL PID Reflux Rate Control

PID Internal Reflux Rate Control

Figure 7. Readback Value and Status from DCS Controller to PC In Figure 6, the PC-sequence table block, LOOP_CL, handles loop coupling that follows the mode transition of the PC-PID controller block, IRRATE_CL. The sequence table block executes a user application described with a decision table. LOOP_CL changes the modes of COMP_CL, CNST_CL, and IRRATE_CL according with the mode transition of IRRATE_CL as shown in Figure 6. When function block specification for the PC and the DCS controller are the same, two merits arise: 1. The behavior related to data connection between the PC and the DCS controller is the same as that between two DCS controllers. 2. The behavior of function block on PC, like the mode transition etc, is the same as that of function block of the DCS controller. That is, for example, programming the decision table on the PC has the same method on the DCS controller. As a result, DCS application engineers can make use of their knowledge and experience. And operators, who have DCS experience in operation and monitoring, can operate and monitor the advanced control application on the PC.

CONCLUSION
The system described in this paper consists of a PC for advanced control and a DCS that controls the plant. This system has a unified interface for both the PC and DCS controller. This cover both the engineering and the HMI function. Making the DCS controller dual-redundant leads to a highly reliable system. On the other hand, the PC is making rapid progress in performance. The highly reliable DCS controller executes the essential control for the plant operation, and the flexible, high-

speed, large-capacity PC performs advanced control. With this combination of PC and DCS controller, a solid plant operating, system architecture is possible along with a unified user interface.

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