The choice between a programmable logic controller ( PLC ) and a distributed control system (DCS) depends on the specific project and factory type. Different applications require different control systems.
The choice between a programmable logic controller (PLC) and a distributed control system (DCS) depends on the specific project and factory type. Different applications require different control systems.
For automation projects to succeed, automation engineers and design engineers must first work together to assess application requirements and then select the most effective control system platform. These decisions will have a long-term impact on factory performance, in some cases up to 25 years or more. Most control system decisions boil down to choosing between a programmable logic controller (PLC) or a distributed control system (DCS). Sometimes, a particular choice is perfectly suited to a factory, while in other cases it may not be. The more factors considered when selecting a control system, the better it is at achieving both short-term and long-term goals.
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.
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 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 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 do not 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.
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.
Process Scale: How many input/output (I/O) points are required? Small systems (<300 I/O points) may have limited budgets, making a PLC system more suitable. Applying a DCS system to smaller projects is not easy; conversely, it performs better in large-scale factory applications. Due to its global database, a DCS system is easier to manage and upgrade, and any changes are global.
Upgrade Plan: 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, and are therefore 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, interconnection becomes necessary. This is generally difficult to achieve because it typically requires data mapping using communication protocols. Integration itself isn't a problem, but changes in requirements create difficulties for the user: a change to one PLC system can cause communication breakdowns between two PLCs due to disruptions in data mapping. For DCS systems, mapping is unnecessary; configuration changes are a simple process, and the controller is integrated into the system.
High availability: For processes with high availability requirements, the DCS system can provide redundant configurations (see Figure 2).
Figure 2: For processes with high availability requirements, redundancy is crucial for long-term operation. Efficiency and ease of implementing redundancy are essential for keeping costs within budget.
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 these costs.
Balancing short-term needs with long-term vision is crucial for operational certainty and improving plant operation and maintenance.
