[Part 1] PLC and DCS
PLC
The evolution from switch control to sequential control and transport processing involves multiple functions such as bottom-up continuous PID control, with the PID controller located in the interrupt station.
One PC can be used as the master station, and multiple PLCs of the same type can be used as slave stations.
Alternatively, one PLC can be used as the master station, and multiple PLCs of the same type can be used as slave stations to form a PLC network. The advantage of this over using a PC as the master station is that when programming, users do not need to know the communication protocol; they only need to write the code according to the instruction manual format.
The PLC grid can function as an independent DCS or as a subsystem of the DCS.
PLCs are mainly used for sequential control in industrial processes, while newer PLCs also have closed-loop control functions.
DCS
Distributed control system (DCS) is a monitoring technology that integrates 4C (Communication, Computer, Control, CRT) technologies.
In a large, top-down tree-like topology system, communication is key.
In the interrupt station, the connection between the computer and the field instruments and control devices is a tree-like topology and a parallel continuous link structure. There are also a large number of cables running in parallel from the interrupt station to the field instruments.
Analog signals, A/D to D/A, and microprocessor-based hybrid signals.
One instrument has a pair of wires connected to the I/O, which is then connected to the local area network (LAN) from the control station.
DCS is a three-tiered architecture consisting of control (engineering station), operation (operator station), and field instruments (field measurement and control station). It is used for large-scale continuous process control, such as in petrochemical industries.
[Part Two] How to Choose Between PLC and DCS Systems
The choice between a programmable logic controller (PLC) and a distributed control system (DCS) depends on the specific circumstances, as different applications have different requirements for the control system.
The control system platform significantly impacts how automation systems meet requirements such as optimizing production, maintaining availability, and acquiring data. A lack of foresight in selecting a control system can also affect future expansion, process optimization, user satisfaction, and company profits. Beyond fundamental principles (such as process control), design teams must also consider factors like installation, scalability, maintenance, and upkeep.
Currently, while PLC systems may be the most cost-effective for small equipment, DCS systems offer more economical scalability and are more likely to achieve a higher return on initial investment.
A PLC is an industrial computer used to control manufacturing processes such as robotics, high-speed packaging, bottling, and motion control. Over the past 20 years, PLCs have added more functionality, creating greater efficiency for small factories and installations.
PLCs typically operate as standalone systems, but they can also be integrated with other systems, connecting them via communication. Because each PLC has its own database, integration requires some degree of mapping between controllers. This makes PLCs particularly suitable for small applications that don't have significant expansion needs.
DCS systems distribute controllers throughout the automation system, providing common interfaces, advanced control, system-level databases, and easily shareable information. Traditionally, DCS has been primarily used in process technologies and larger plants, where large system applications are easier to maintain throughout the plant's lifecycle.
A Power Control Logic (PLC) is developed from the principle of relay control. It stores instructions for performing logical operations, sequential control, timing, counting, and arithmetic operations; and controls various types of machinery or production processes through digital input and output operations. The user-written control program expresses the technological requirements of the production process and is pre-stored in the PLC's user program memory. During runtime, it executes the stored program line by line to complete the operations required by the technological process.
[III] Comparison of Engineering Analysis of PLC and DCS
The CPU of a PLC contains a program counter that indicates the storage address of a program step. During program execution, the counter automatically increments by 1 for each step executed. The program executes sequentially from the starting step (step number zero) to the final step (usually the end instruction), and then returns to the starting step for cyclical operation.
The time required for a PLC to complete one cycle of operation is called a scan cycle. The scan cycle time varies from 1 microsecond to tens of microseconds depending on the PLC model. This cyclic operation, such as the program counter, is not available in a DCS (Distributed Control System). This is also why PLCs have less redundancy than DCSs.
DCS was developed based on operational amplifiers. All functions and relationships between process variables are organized into function blocks (sometimes called expansion blocks in DCS systems). The main difference between DCS and PLC lies in the logic processing of digital signals and the calculation of analog signals. Even though there has been some overlap between the two, differences remain.
Since the 1980s, PLCs have significantly enhanced their algorithmic capabilities for control loops, in addition to logic operations. However, PLCs use ladder diagram programming, making analog quantity calculations less intuitive and relatively cumbersome. Nevertheless, they exhibit a speed advantage in logic decomposition, processing 1k logic programs in less than 1 millisecond at the microsecond level. They treat all inputs as switching quantities, with 16 bits (and sometimes 32 bits) representing a single analog quantity.
DCS treats all inputs as analog signals, with each bit representing a digital signal. It processes a logic operation in the range of hundreds of microseconds to milliseconds. In contrast, a PLC processes a PID operation in tens of milliseconds, a time comparable to that of a DCS.
Regarding grounding resistance, the requirements may not be high for PLCs, but for DCS, it must be below a few ohms (usually below 4 ohms). Analog isolation is also very important.
For systems with the same number of I/O points, using a PLC is less expensive than using a DCS (saving approximately 40%). PLCs do not have dedicated operator stations; their software and hardware are generic, resulting in significantly lower maintenance costs compared to DCS. If the controlled objects primarily consist of equipment interlocks and have relatively few loops, a PLC is more suitable.
If the control is primarily analog and involves numerous function calculations, a DCS (Distributed Control System) is preferable. DCS offers significantly better redundancy than PLCs in terms of controllers, I/O boards, and communication networks, as well as in handling advanced computations and industry-specific requirements. PLCs, on the other hand, are easier to design for enterprise management information systems due to their use of general-purpose monitoring software.
PLC and DCS systems are generally suitable for discrete and process manufacturing, respectively. Discrete manufacturing facilities using PLC systems typically consist of individual production units primarily used for assembling parts, such as labeling, filling, or grinding. Process manufacturing facilities typically use automated systems to produce according to recipes rather than by piece in continuous and batch processes. Large continuous processing facilities, such as oil refineries and chemical plants, use DCS automation systems. Hybrid applications often use both PLC and DCS systems. Selecting a controller for an application requires considering many factors, including process size, scalability and future upgrade plans, integration requirements, functionality, high availability, and return on investment throughout the plant facility's lifecycle.
[IV] Relevant Factors Influencing How to Make a Decision
Process Scale: How many input/output (I/O) points are needed? Small systems (<300 I/O points) may have limited budgets, making PLC systems more suitable. Applying a DCS system to smaller projects is not easy; instead, it performs better in large-scale factory applications. Due to its global database, DCS systems are easier to manage and upgrade, and any changes are global. Upgrade Planning: Smaller industrial processes may be suitable for PLC systems, but if the process needs to be expanded or upgraded, more PLC hardware and databases will be required, along with separate maintenance. This is a time-consuming, labor-intensive process, and prone to errors. DCS systems are easier to upgrade, for example, by managing user accounts from a central hub, making them easier to maintain and service (see Figure 1).
Figure 1: DCS system architecture with a single database, allowing users to maintain and operate the system from a central control station.
Integration Requirements: For standalone devices, PLC systems are ideal. However, when a factory has multiple PLC systems, interconnectivity becomes crucial. This is often difficult to achieve because it typically requires data mapping using communication protocols. Integration itself isn't a problem, but changes can cause issues: a change to one PLC system can prevent two PLCs from communicating properly due to disruptions in data mapping. DCS systems, on the other hand, eliminate the need for mapping; configuration changes are a simple process, and the controller is integrated into the system. High Availability: For processes with high availability requirements, DCS systems can provide redundant configurations (see Figure 2). Efficiency and ease of redundancy implementation are critical for keeping costs within budget.
Figure 2: For processes with high availability requirements, redundancy is crucial for long-term operation.
Functional Requirements: Some industries and facilities require historical databases, streamlined alarm management, and a central control room with a universal user interface. Others require integration with Manufacturing Execution Systems (MES), advanced control, and asset management. DCS systems have these applications built-in (see Figure 3), making them easy to add to automation engineering applications without requiring additional servers or increasing integration costs. In this respect, DCS systems are more economical and can improve productivity while reducing risk.
Figure 3: Each system platform has unique database requirements.
Lifecycle ROI: Facility requirements vary by industry. For smaller process engineering projects with no expansion needs or integration with other process areas, PLC systems offer a better ROI. DCS systems may have higher installation costs, but over their entire lifecycle, the increased output and safety benefits they provide offset some of those costs. Balancing short-term needs with long-term vision is crucial for operational certainty and improving plant operation and maintenance.
