MORI Hiroshi1 MIZUSHIMA Fuyuki1 TOMITA Shoji1 HAYASHI Shunsuke1
We have developed fieldbus communication functions for our distributed control system CENTUM CS that comply with the specifications of the Fieldbus Foundation. These new functions enable cascade control of the CENTUM CS control function and the field device control function, and also support the operation and monitoring functions of the field device control function. The fieldbus communication module (ACF11) interfaces with the fieldbus. It can also be connected to a node on the remote IO bus (RIO bus), thereby making the system construction flexible. This paper introduces the characteristics of the CENTUM CS fieldbus system, with the main focus on its communication functions.
A fieldbus is a bidirectional, multidrop, digital communication system developed to replace the conventional 4-20 mA communication lines that connect intelligent field devices with a control system. Fieldbus is continually reviewed in an effort to standardize the fieldbus specifications to comply with the specifications of the IEC and ISA.1
The features of the fieldbus include a reduction in the amount of cable required, field-distributed control functions, and bidirectional transfer of control data. Among these, the fielddistributed control functions are attracting particular attention as field devices become increasingly "digital" and more and more "advanced." This means that the control functions implemented in a distributed control system are being distributed throughout the field. Drastic innovations in control systems are not always favorable though, and we believe there needs to be a way of shifting gradually from the existing systems to more advanced systems.2
The CENTUM CS fieldbus communication functions were developed with the aim of reducing the burdens of plant operation. This was achieved by standardizing the operation and monitoring functions, including the functions for the control of field devices, in an environment where the fieldbus coexists with a conventional analog communication line. One major technical challenge in this achievement was communication between field devices and higher-order control equipment while prioritizing the characteristics of fieldbus communication functions among other things.
The CENTUM CS fieldbus communication functions are designed to comply with the fieldbus standards specified by the Fieldbus Foundation, while still upholding Yokogawa Electric's traditional ideas of instrumentation. Accordingly, these functions integrate fieldbus capabilities into the CENTUM CS control, operation and monitoring functions, to form one of the key technologies for the Enterprise Technology Solutions (ETS) offer.
The CENTUM CS fieldbus system features:
|Figure 1 CENTUM CS System Configuration|
1. System Configuration
Figure 1 shows the system configuration. As shown in the figure, the CENTUM CS fieldbus system consists of the following equipment.
2. Fieldbus Communication Module (ACF11)
Figure 2 is an external view of the ACF11 fieldbus communication module. The ACF11 module is installed in the node of an RIO bus and can be used in combination with conventional 4-20 mA analog modules. The functions provided by the ACF11 module include:
Table 1 Hardware Specifications of ACF11 Module
|Number of modules that can be installed||80 modules/FCS (i.e., [10 modules/RIO node] 5 8 nodes)|
|Number of fieldbuses||1 segment/ACF11 module|
|Communication speed||31.25 Kbps in the voltage wire mode|
|Connection||By means of 4-mm screw terminals|
|Built-in bus power supply||Output voltage: 18 to 20 V DC (fixed to 19 V DC)
Continuous output current: 80-mA maximum (with an overcurrent limiter)
Transient output current: 100-mA maximum (for a maximum duration of 2 ms)
|External dimensions||188 (D) 5 215 (H) 5 43.6 (W) (mm)|
|LED indicator||RDY, CTL, RCV and SND|
|Withstanding voltage||1500 V AC, tested between the fieldbus and the system|
|Maintenance||Online maintenance is possible. (Equipment to be maintained: Module itself and cable connectors)|
|EMC standard||Complies with CE marking (as part of a CENTUM CS system)|
Figure 2 External View of
For momentary power failures that last less than the specified period of time, this function can prevent FCS function blocks from entering the MAN mode. Table 1 summarizes the hardware specifications of the ACF11 module. The module features:
1. Process I/Os
|Figure 3 Block Connection|
Figure 3 shows how FCS function blocks are connected to fieldbus function blocks. If viewed from the FCS function blocks in the figure, it is possible to handle the I/Os of the fieldbus function blocks in the same way as conventional process I/Os. In addition, these I/Os provide the following necessary functions for cascading between the FCS function blocks.
With these functions, the new fieldbus system enables flexible application design even in cases where control functions are distributed throughout the field. Though different from fieldbus function blocks, the FCS function blocks have the capability of initialization handshaking based on the data status. This allows the FCS function blocks to be connected to the fieldbus function blocks without problems.
2. Operation and Monitoring of Field Device Control Functions
|Figure 4 Operation and Monitoring
Using a Faceplate Block
As a means of monitoring the operating condition of field device control functions (namely, the internal parameters of fieldbus function blocks), the CENTUM CS fieldbus system can display and set the parameters of fieldbus function blocks using on-demand communication functions.
Like the conventional process I/Os, each parameter of the fieldbus function blocks can be dealt with using a terminal number. By linking this terminal number with a faceplate block, the parameter can be viewed and set on a parameter tuning panel like those of FCS function blocks. This implements ICS-based operation and monitoring functions for fieldbus function blocks.
Figure 4 shows how each parameter of the PID control block relates to the process variable (PV), setpoint (SP), manipulated variable (MV) and mode of the faceplate block in a control loop consisting of an analog input (AI) block, PID control block and analog output (AO) block. Figure 5 shows a display of the faceplate block tuning panel.
3. Alarms and Events
Alert messages (alarms and events) issued by field devices are received by the ACF11 module and transferred through an FCS up to an ICS. These alerts are then stored in the ICS as historical messages. These messages can be referenced using the "historical report function" or viewed using the device management tool.
|Figure 5 Faceplate Block Tuning Panel|
The fieldbus engineering function is implemented by means of a tool that runs under Windows NT (fieldbus engineering tool) and coexists with other general-purpose tools. This tool enables the following:
Figure 6 shows an example of the various engineering tool builder screens.
|Figure 6 Engineering Tool Builder Screens|
Fieldbus technology is expected to spread dramatically by the year 2000 as a replacement for 4-20 mA analog transfer technology. Yokogawa Electric strongly supports fieldbus standardization, participating in the Fieldbus Foundation's development of fieldbus specifications and demonstrating the usefulness of fieldbus technology in experiments and other activities.
The CENTUM CS fieldbus communication functions discussed in this paper have been developed as a forerunner of fieldbus technology on the basis of Yokogawa Electric's previous experience and expertise. We believe these new functions will contribute significantly to the spread and development of fieldbus technology.
We will continue to research further functional improvements and cost reductions in new fieldbus systems, taking into consideration a balance of fieldbus systems and distributed control systems, in order to offer users the full advantages of fieldbus technology.
Since it was released in 1993, CENTUM CS is widely applied in the plants of oil refinery, petrochemical, chemistry, iron and steel, non-ferrous metal, metal, cement, paper pulp, food and pharmaceutical industries, and power, gas and water supply as well as many other public utilities.
Our distributed control system (DCS) enables automation and control of industrial processes and enhanced business performance. Over 10,000 plants entrust Yokogawa DCS to deliver their production goals.