Technical and economic comparison of DCS and PLC

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This post was last edited by dzhch on 2009-9-23 11:34. Technical and economic comparison of DCS and PLCs In the field of thermal automation in thermal power plants, DCS and PLC are two completely different concepts that are yet closely interconnected. Both DCS and PLC are products of the combination of computer technology and industrial control technology. DCS is used in the main control systems of thermal power plants, while PLCs are primarily applied in the auxiliary workshops of power plants. Both DCS and PLC have operator stations that provide a means for human-computer interaction; they both rely on computer-based controllers to carry out control calculations; they both use I/O cards to exchange data with primary components and actuating devices; and they both possess communication systems referred to as networks. DCS and PLC are so similar; why do they represent entirely different concepts? How should we make a choice in practical engineering applications? This article provides a review from several aspects such as historical development, technical features, and future directions, in the hope of offering insights for professionals in thermal engineering. Taking Keyuan’s NT6000 as an example for the DCS case, efforts are made to provide a detailed and clear explanation. 1. Historical development and core concepts of DCS and PLC DCS is the abbreviation for Distributed Control System (TOTAL DISTRIBUTED CONTROL SYSTEM). It refers to risk dispersion and data concentration. It entered the market in the mid-1970s, enabling analog control and replacing analog control instruments that relied primarily on PID calculations. It was instrument manufacturers who first proposed the concept of DCS, which was initially used primarily in the chemical industry. The PLC was developed in the late 1960s; it is known as a Programmable Logic Controller for logical operations, abbreviated as PLC. It is mainly used in the automotive manufacturing industry. 　　There are significant differences in the design principles of DCS and PLCs. PLCs evolved from imitating the control principles of traditional relays; PLCs in the 1970s had only digital logic control capabilities. It stores instructions for performing logical operations, sequential control, timing, counting, and arithmetic operations ； And various machines or production processes are controlled through digital input and output operations. The control programs developed by the user reflect the process requirements of the production process. It is stored in the user program memory of the PLC, and during operation, the instructions stored there are executed one by one to carry out the operations required by the process flow. 　　DCS was developed based on operational amplifiers. Design functional blocks for all functions and the relationships between various process variables. In the mid-1970s, DCS systems had only analog control. 　　The main difference between DCS and PLC controllers lies in the processing of digital and analog signals; even though there has been some overlap between the two over time, a distinction still remains. After the 1980s, in addition to logical operations, PLCs also incorporated some control loop algorithms; however, performing certain complex calculations remained difficult. PLCs are programmed using ladder diagrams, and performing arithmetic operations on analog values is not very intuitive in such programming methods, making it cumbersome to program. However, it shows advantages in terms of computation logic speed. DCS uses function blocks to encapsulate analog and logical operations; the representation of both logical and complex analog operations is very clear, yet its efficiency in handling logical operations is lower compared to PLCs. 　　The differences in their historical development between DCS and PLC are significant, and they had a major impact on their subsequent evolution. However, what has the greatest impact on subsequent development is not the difference in the underlying technology, but rather the difference in the underlying concept. The core concept of DCS is hazard dispersion, which refers to a computer-based control system in a data center. Therefore, the development of DCS involves the continuous use of the latest advancements in computer technology, communication technology, and control technology to create a comprehensive distributed control system. DCS offers users a complete, safe, reliable, efficient, and flexible solution for industrial control. The core concept of PLC is the programmable controller, which is designed to replace relays and carry out sequential control functions such as logic operations, timing, and counting, thereby creating flexible program control devices. Therefore, the main trend in the continuous development of PLCs is to continuously improve various performance metrics, thereby providing users with a control device that offers comprehensive functions and flexibility. 　　DCS is a system, while PLC is a device; this is the fundamental conceptual difference between the two. The impact of this difference is profound, as it permeates every aspect of technology economics. 2. Technical characteristics of DCS and PLC and their mutual integration Different conceptual foundations and development paths result in distinct technical characteristics for DCS and PLC. Moreover, technological development is not isolated; learning from one another and mutual integration are constant aspects of this development process. 2.1 Control Processing Capacity We know that a PLC controller is usually capable of handling thousands of I/O points (up to over 8,000 I/Os). On the other hand, the controllers of DCS can generally handle only a few hundred I/O points (no more than 500 I/Os). Is it because the DCS developers lack sufficient technical skills? I’m afraid not. From the perspective of the requirements of distribution systems, centralized control is not allowed; controllers with too many I/O points are useless in practical applications. DCS developers do not need to create drivers for controllers with a large number of I/O points, as their main focus is on ensuring the reliability and flexibility of the system. PLC is different; as an independent flexible control device, the greater its processing capabilities, the higher its technical level. As for the level of application of the entire control system, that is primarily the responsibility of engineers and users, rather than the core goal of PLC manufacturers. 　　Another indicator of control processing capacity is processing speed, and it is generally believed that PLCs are much faster than DCSs. From a certain perspective, this is indeed the case: PLCs are highly efficient at performing logical operations, taking less than 1 millisecond to execute a 1K logic program, and their control cycle (taking direct transmission from DI inputs to DO outputs as an example) can be kept within 50ms ； DCS uses the same approach for handling logical and analog operations, and its control cycle is often over 100 ms. When we use the PID algorithm for comparison, we find that it takes a few milliseconds for a PLC to perform a PID calculation, while the T2550 controller in the NT6000 DCS also needs 1 millisecond to carry out a PID calculation. This shows that the actual computing capabilities of PLCs and DCS systems are comparable, with some models of DCS controllers being even more powerful. The differences in the control cycle are mainly related to the scheduling design of the controller. Large PLCs often use a secondary CPU to handle analog calculations, while the main CPU handles digital operations at high speeds; therefore, even though the speed of analog calculations is average, their performance in digital control is still very excellent. DCS processes digital and analog operations at the same speed, so the performance metrics regarding the control cycle are indeed not ideal. The new DCS controller incorporates the design principles of large-scale PLCs, resulting in a significant improvement in its performance regarding control cycles. Taking the T2550 controller of the NT6000DCS as an example. The controller can set four tasks with different priorities; the minimum operation cycle can be set to 10 ms, and with high-speed I/O cards, the control cycle can reach 15–20 ms. Analog operations are scheduled in other tasks with longer cycles. 2.2 Data Communication Exchange Data communication exchange mainly refers to the control system network and its forms of data exchange. In this regard, DCS has inherent advantages. The “decentralization” of a distributed control system is reflected in the independent controllers, while its “centralization” is evident in the human-machine interaction devices that possess complete data. It is the network that connects these decentralized and centralized elements to form a distributed control system. Therefore, from the early stages of DCS development, networking has become a key technical focus for DCS manufacturers; redundancy technology and narrowband transmission technology were among those first developed or successfully applied by DCS manufacturers. PLCs are primarily designed as standalone devices, and their “network” is actually serial communication. 　　The development and widespread use of industrial Ethernet technology have, in effect, narrowed the gap between DCS and PLC networks. On the surface, many DCS and PLCs use industrial Ethernet, but fundamental differences still exist. Take MODBUS-TCP, which is used in many PLCs, as an example. MODBUS is a serial communication protocol, not a network; there’s no doubt about that ； Is MODBUS-TCP a network? Many people have questions then. Upon close analysis, MODBUS-TCP is a communication method that incorporates the MODBUS communication protocol onto top of the Ethernet TCP protocol. Although it adopts a network-based structure, it still follows a master-slave management approach with a data table transmission format. As for DCS, taking the ELIN network of NT6000 as an example, although it is also based on industrial Ethernet, its application layer protocol is the masterless token LIN network protocol that has been developed over nearly 30 years, and it has seen long-term successful use in 1M OLIN as well as in 2.5M and 20M ARCNET networks. On ELIN, all stations are equal; there is no primary management station. Moreover, data communication involves structured data in module units, and its data management capabilities are beyond comparison to those of data tables. 　　Taking the PID module as an example, the basic data in it include PV, SP, and OP. Using the data table transmission method, you must first define the data addresses for PV, SP, and OP as 01, 02, and 03 respectively. Other stations also receive data via data tables, but what kind of data is at address 01? What data is 02? It must be restored through the data definition table. Managing data tables in this way is cumbersome and prone to errors; using such a method to handle tens of thousands of data entries in a large system, with them all stored in the data tables, is truly terrifying. The NT6000DCS features modularized management, treating each PID as a module; to access its PV value, one must first access its module, which is managed in the form of PID.PV. This groups all the tiled data into individual boxes, managing them by module and component, thereby **improving** management efficiency. 　　The problems with PLC data communication and exchange stem mainly from the fact that PLCs have long been developed as standalone devices, without a systematic approach ； Moreover, it is mainly used in small control systems, where the problems are not evident, so its development has been slow. Currently, some large-scale PLCs have made improvements in this area, but it will still take a considerable amount of time to reach the level of DCS. 2.3 Configuration Maintenance Function The configuration maintenance function includes logical configuration, downloading and modifying settings, operation and debugging, remote diagnosis, etc. 　　In the early days, PLCs used ladder diagrams primarily, while DCS systems relied on module function diagrams. After years of development, the International Electrotechnical Commission has specified five programming languages through the IEC1131-3 standard. Today’s mainstream DCS and PLC systems all claim to comply with this standard, supporting some or all of these programming languages. Considering development efficiency and program readability, module function diagrams and sequential function diagrams are increasingly becoming the primary programming methods, while ladder logic and structured text serve as development tools for custom modules. Large PLCs are becoming increasingly similar to DCSs in terms of configuration methods, with the gap between them narrowing gradually, while small PLCs still rely mainly on ladder diagrams. 　　Over the years of development, DCS has accumulated a large number of advanced algorithm modules. For example, the NT6000 features device-level modules that integrate the basic control functions and fault alarm capabilities specific to the device within a single module; network communication also takes place in units of these modules, **which improves the efficiency of software development. One device element is equivalent to a ladder logic value of 0.5K; for the PLC to achieve the same function, it has to go through much more complicated procedures. 　　PLCs lack solutions for downloading modifications, running debugging, and remote diagnosis. DCS, from its very inception, was designed with the system’s needs in mind, boasting well-developed solutions built up over many years. Taking the NT6000DCS as an example, the system allows for the modification of control strategies online, as well as the download of such strategies. These modifications and downloads do not affect the normal operation of the system. The NT6000DCS features comprehensive virtual DCS capabilities; it can be used not only for verifying logic configurations but also to create a complete virtual DCS that is connected to the model, enabling system simulation and debugging. The NT6000DCS features comprehensive security measures and offers a remote debugging solution based on wide area networks. 2.4 Hardware Packaging Structure PLCs generally feature a large-bottom-frame design, along with enclosed I/O modules. The enclosed structure helps to improve the reliability of these I/O modules, as well as their resistance to radio frequency interference, static electricity, and damage. The I/O count of PLC modules is 8, 16, or 32. 　　Most DCS systems consist of 19-inch standard chassis along with plug-in I/O modules, which have an exposed structure. Each module has 8 or 16 I/O points; 32-point modules are rarely used. 　　This structure of DCS stems from the fact that it is primarily used for large control systems; the 19-inch standard chassis facilitates dense arrangement, while the lower number of I/O points is due to the requirements regarding dispersion. The large-format frame of PLCs, with its enclosed modular structure, offers greater flexibility in management and configuration, as well as higher reliability for individual devices. Therefore, many DCS systems have also adopted the structural advantages of PLCs, using a packaging structure similar to that of PLCs; for example, the I/A modules use metal enclosures, while the NETWORK-6000+ uses conductive plastic enclosures. 2.5 Human-Computer Interaction Devices In the early days, as a system, the DCS relied on specialized devices provided by the DCS manufacturers for human-computer interaction. PLC manufacturers generally do not provide human-machine interaction devices; instead, system integrators usually use general-purpose monitoring software to achieve this purpose (such as ifix, Intouch, KingView). Human-computer interaction devices integrated with DCS typically feature specialized functions and good stability, but they are also quite expensive. With the rapid development of PC technology, some general-purpose monitoring software has seen rapid growth, and its functions and performance have gradually surpassed those of the specialized devices provided by DCS manufacturers. As a result, many DCS manufacturers have gradually abandoned dedicated human-machine interaction devices and, like PLCs, started using general-purpose monitoring software instead. When DCS manufacturers use general monitoring software, it is not a simple matter of piecing things together; rather, based on such software, they develop solutions through collaboration, thereby preserving and inheriting their years of accumulated network communication technologies and system self-diagnosis techniques in the form of specialized software packages. For example, the NT6000 used to employ a T1000 human-machine interaction system based on a dedicated operating system in its early versions, whereas it currently uses a T3500 human-machine interaction system based on FIX/IFIX or INTOUCH. The LINPOLL network communication packet is integrated during development. 3. Market situation and development trends of DCS and PLC In the field of thermal process automation, DCS is used in control systems for main plant buildings without exception. PLC is only used in the auxiliary workshop. The main reason is that early DCS systems were extremely expensive; it was believed that the operation of auxiliary workshops could be intermittent, with lower requirements for reliability, and fewer needs for analog control. To reduce costs, PLCs were often chosen to build the control systems. The control systems for boilers, turbines, and generators require long-term stable and reliable operation, and the signals contain a significant proportion of analog values; considering the performance requirements of such systems, people were forced to opt for expensive DCS systems. 　　Furthermore, by analyzing the market competition in the DCS systems of the main plant and the control systems of the auxiliary workshops, we discover an interesting phenomenon. Competition for the DCS in the main plant usually takes place among suppliers or agents of different brands; it is fierce, and the prices of DCS keep dropping. The competition in auxiliary workshop control systems often takes place among various manufacturers of PLCs from the same brand; the entry barriers are low, so the competition is fierce. However, the decline in PLC prices is not as significant as that of DCS. The main reason is that DCS manufacturers are directly involved in competition, and under intense market pressure, they continuously reduce the costs of equipment manufacturing and project implementation. The PLC manufacturers do not participate in this competition directly, and engineering firms can only reduce their limited project costs, leaving little room for reduction. Judging from the current situation, the price gap between DCS and high-end PLCs is no longer significant; auxiliary workshops still rely heavily on PLCs, due to market inertia. 　　With the continuous expansion of the installed capacity of power plants in China and the advancement of power system reforms, the requirements for control in auxiliary workshops are also increasing. In this context, it has become a trend for DCS systems to be used for controlling auxiliary workshops. Due to its comprehensive technical and economic advantages, the NT6000DCS is playing, and will continue to play, an increasingly important role in the control of auxiliary workshops. 　　PLCs, which are widely used in auxiliary workshops, will not disappear from the history of thermal process automation. Unprecedented competitive pressures will drive PLC manufacturers to align their technologies with DCS standards and to make greater efforts in terms of price. The result of market competition will bring greater benefits to users. 4. Conclusion As products of the combination of computer technology and control technology, DCS and PLC have each made their contributions to improving the thermal automation level in thermal power plants. Due to the significant similarities in their applications, the technical or price advantages of each at different times will directly affect their market position. And the market’s reaction will also be reflected, sooner or later, in respective technological developments and price adjustments. Overall, the technical integration and advancement of DCS and PLCs will be the main trend in competition, while continuous improvements in cost-performance will also be the driving force behind development.