Choosing between a Programmable Logic Controller (PLC) and a Distributed Control System (DCS) requires a specific analysis based on the situation, as different applications have varying requirements for control systems. When communicating with clients, we can approach the discussion from the following aspects!
PLC and DCSPLC
1. Development from switch control to sequential control, transport processing, and multifunctional continuous PID control from the bottom up, with PID in the interrupt station.
2. A single PC can serve as the master station, with multiple identical PLCs as slave stations.
3. A single PLC can also act as the master station, with multiple identical PLCs as slave stations, forming a PLC network. This is more convenient than using a PC as the master station: when user programming, there is no need to know the communication protocol; just follow the manual format.
4. The PLC grid can function as an independent DCS or as a subsystem of a DCS.
5. PLCs are primarily used for sequential control in industrial processes, and new PLCs also have closed-loop control capabilities.
DCS
1. The Distributed Control System (DCS) integrates monitoring technology with 4C (Communication, Computer, Control, CRT) technology.
2. It features a top-down tree topology system where communication is key.
3. PID is in the interrupt station, connecting computers with field instruments and control devices in a tree topology and parallel continuous link structure, with numerous cables running from the interrupt station to field instruments.
4. Analog signals, A/D—D/A, with microprocessor integration.
5. Each instrument connects to I/O with a pair of wires, linked to the control station and the local area network (LAN).
6. DCS has a three-level structure consisting of control (engineer station), operation (operator station), and field instruments (field measurement and control station). It is used for large-scale continuous process control, such as in petrochemicals.
How to Choose Between PLC and DCS Systems
Choosing between a Programmable Logic Controller (PLC) and a Distributed Control System (DCS) requires a specific analysis based on the situation, as different applications have varying requirements for control systems.
The choice of control system platform will influence how the automation system meets the demands for optimized production, maintaining availability, and data acquisition. A lack of foresight in selecting a control system may also affect future scalability, process optimization, user satisfaction, and company profits.
In addition to some basic principles (such as how to control the process), the design team must also consider various factors such as installation, scalability, maintenance, and upkeep.
Currently, while PLC systems may be the most cost-effective for small devices, DCS systems offer more economically scalable capabilities and are more likely to yield a higher return on initial investment.
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 functionalities, creating more benefits for small factories and installations. PLCs typically operate as standalone systems but can also be integrated with other systems through communication. Since each PLC has its own database, integration requires some degree of mapping between controllers. This makes PLCs particularly suitable for small applications that do not have significant scalability needs.
DCS systems, on the other hand, distribute controllers throughout the automation system and provide a common interface, advanced control, system-level databases, and easily shareable information. Traditionally, DCS is mainly applied in process industries and larger plants, where large system applications are easier to maintain throughout the lifecycle of the plant.
PLC is developed from relay control principles, storing instructions for executing logical operations, sequential control, timing, counting, and arithmetic operations; it controls various machinery or production processes through digital input and output operations. The user-defined control program expresses the process requirements of production and is stored in the PLC’s user program memory in advance. During operation, it executes the stored program step by step to fulfill the operational requirements of the process flow.
Comparative Engineering Analysis of PLC and DCS
The CPU of the PLC has a program counter that indicates the storage address of the program step. During program execution, this counter automatically increments by 1 after executing a step, and the program executes sequentially from the starting step (step number zero) to the final step (usually the end instruction), then returns to the starting step for cyclic operation. The time required for each complete cycle of operation is called a scan cycle. The scan cycle varies from 1 microsecond to several tens of microseconds depending on the model of the PLC. This cyclic operation of the program counter is something that DCS does not have, which is also why the redundancy of PLCs is not as robust as that of DCS.
DCS is developed based on operational amplifiers. It creates functional blocks for all relationships between functions and process variables (some DCS systems refer to these as bloated blocks). The main difference in performance between DCS and PLC lies in the logical resolution of discrete signals and the computation of analog signals. Even though there has been some overlap between the two, distinctions remain.
Since the 1980s, PLCs have significantly enhanced their algorithmic capabilities for control loops beyond logical operations, but programming in ladder logic for PLCs is not very intuitive for analog signal computations, making programming more cumbersome. However, PLCs excel in logical resolution, performing 1k logical programs in less than 1 millisecond at the microsecond level. They treat all inputs as discrete signals, with 16 bits (or 32 bits) representing one analog signal.
In contrast, DCS treats all inputs as analog signals, with 1 bit representing a discrete signal. The resolution of a logic operation in DCS occurs in the range of hundreds of microseconds to several milliseconds. For PLCs, the computation of a PID operation takes several tens of milliseconds, which is comparable to the computation time of DCS.
Regarding grounding resistance, the requirements for PLCs may not be stringent, but for DCS, it must be below several ohms (usually below 4 ohms). Analog signal isolation is also very important.
For systems with the same number of I/O points, using PLCs is generally less expensive than using DCS (approximately 40% savings). PLCs do not have dedicated operator stations; their software and hardware are general-purpose, so maintenance costs are significantly lower than those of DCS. If the controlled objects mainly involve equipment interlocks and relatively few loops, using PLCs is more appropriate.
If the primary requirement is for analog signal control and there are many functional computations, it is best to use DCS. DCS excels in redundancy for controllers, I/O boards, communication networks, and in meeting advanced computations and industry-specific requirements compared to PLC. Due to the use of general monitoring software, PLCs are easier to integrate into the management information systems of enterprises.
PLC and DCS systems are generally suited for discrete and process manufacturing, respectively. Discrete manufacturing facilities using PLC systems typically consist of individual production units, primarily for assembling components, such as labeling, filling, or grinding. Process manufacturing facilities usually employ automation systems to produce continuously and in batches according to recipes rather than by individual items. Large continuous processing equipment, such as refineries and chemical plants, utilize DCS automation systems. Mixed applications often use both PLC and DCS systems simultaneously. Choosing a controller for a specific application requires consideration of the scale of the process, scalability, future upgrade plans, integration needs, functionality, high availability, and the return on investment over the entire lifecycle of the plant.
Factors Influencing the Choice
Process Scale:How many input/output (I/O) points are needed? Small systems (<300 I/O points) may have a lower budget, making PLC systems more suitable. Applying DCS systems to smaller projects is not easy; conversely, they perform better in large plant applications. With a global database, DCS systems are easier to manage and upgrade, as any changes are global.
Upgrade Plans:Smaller industrial processes can use PLC systems, but if the process requires expansion or upgrades, more PLC hardware and databases will need to be added, requiring separate maintenance. This is a time-consuming and labor-intensive process that is prone to errors. DCS systems are easier to upgrade, as user access can be managed from a central hub, making maintenance and upkeep simpler (see Figure 1).
Figure 1: DCS system structure with a single database allows users to maintain and operate the system from a central control station.
Integration Needs:For standalone devices, PLC systems are the ideal choice. When a plant is configured with multiple PLC systems, interconnection requirements arise. This is generally difficult to achieve, as it often requires mapping data using communication protocols. Integration is certainly possible, but when changes are needed, it can become troublesome for users: if one PLC system is changed, it may disrupt communication between two PLCs due to affected data mapping. For DCS systems, mapping is not necessary; configuration changes are a simple process, as the controllers are built into the system.
High Availability:For processes with high availability requirements, DCS systems can provide redundancy configurations (see Figure 2).
Figure 2: Redundancy is crucial for long-term operation in processes with high availability requirements.
Efficiency and ease of achieving redundancy are critical for keeping costs within budget.
Functional Requirements:Some industries and facilities require historical databases, streamlined alarm management, and centralized control rooms with common user interfaces. Others may require integration with Manufacturing Execution Systems (MES), advanced control, and asset management. DCS systems come with these applications built-in (see Figure 3), making it easy to add them to automation engineering applications without the need for additional servers or increased integration costs. From this perspective, DCS systems are more economical and can enhance productivity while reducing risks.
Figure 3: Each system platform has unique database requirements.
Lifecycle Return on Investment:Facility requirements vary by industry. For smaller process engineering projects with no expansion needs and no requirement for integration with other process areas, PLC systems offer a better return on investment. DCS systems may have higher installation costs, but over the entire lifecycle, the increased output and safety benefits provided by DCS systems can offset some of the costs. Balancing short-term needs with long-term vision is crucial for operational certainty and improving plant operations and maintenance.