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Data acquisition system soft DCSPLC - Solutions - Huaqiang Electronic Network
The data acquisition system is designed to collect real-time signals and send them to a control computer. However, it does not include any loop control function blocks within the computer itself. Its main purpose is for human-machine interface (HMI) display, making it relatively simple in design. In terms of hardware structure, it can be divided into one or two levels, typically consisting of I/O boards, controllers, and HMIs. For simpler applications, only basic I/O boards and an HMI are used.
However, certain industries have specific requirements. For example, in power plants, the boiler control system often uses a DCS (Distributed Control System), while other parts rely on a Data Acquisition System (DAS). Some manufacturers specialize in producing these systems. While the technology behind data acquisition systems is generally mature, power plants may require advanced features like event sequence recording (SOE). These systems must capture events with millisecond-level resolution and support fast interrupt functions—typically within 2 milliseconds. Although the number of such points is small (around 100), the ability to store and retrieve historical event data is essential. SOE functionality is usually provided by specialized manufacturers, and some DCS systems also integrate this feature.
One of the most widely used data acquisition systems is the OPTO22 product from the United States. It consists of small modules that handle both digital (DI/DO) and analog (AI/AO) signals. There are 42 different modules available, each supporting various voltage levels. These modules are mounted on larger boards (B1 and B2), with 8 analog channels per board and 16 digital channels per board. These boards also include serial communication ports. The boards are installed in cabinets, and the HMI runs on a Windows-based CRT display. Monitoring software such as Modbus drivers, FIX, and INTOUCH are commonly used. The HMI can read and display I/O signals from the modules. Over 20 U.S. plants produce similar products, comparable to the STD bus standard, though those systems are gradually being phased out due to complex user engineering tasks.
In purely data collection-based systems, OPTO products are frequently used due to their low maintenance and reliability. If closed-loop control is needed, a controller like the LC4 is added between the HMI and the I/O modules. This controller allows for configuration of various control strategies using function blocks. However, because the number of available function blocks is limited, OPTO22 systems are more common in smaller-scale applications rather than large systems with thousands of I/O points.
The most popular module for OPTO switching is the isolation module, which serves as a DCS input module. It helps protect the DCS from lightning strikes. Due to its straightforward manufacturing process, domestic users find it very practical, and it is significantly cheaper than imported versions.
In the early 1990s, with the advancement of computer technology, it became feasible to use standard PCs for control purposes. PCs offered sufficient computing power and memory to meet control requirements. This led to a shift where DCS or PLC systems no longer required separate controllers. Instead, control logic and ladder diagrams were transferred to PCs running under the NT operating system. However, software migration was challenging. These PCs were directly connected to I/O boards, acting as both HMI and controller. Some referred to these systems as "soft DCS" or "soft PLC." American company AB has developed a soft PLC solution, with each PC connected via Ethernet. Despite this, the adoption of such systems remains limited, with only smaller systems using soft DCS or soft PLC solutions.