Those in the electromechanical industry often encounter this question: How to choose the right PLC? Is a higher-end PLC always better? What about reliability? Stability? Information security? Network connectivity...?
Siemens, Schneider Electric, and AB are often referred to as the three major high-end PLC brands, but what exactly does "high-end" mean here? Does it refer to harsh environments, high speed, or high performance? Where exactly does the "high-end" aspect of a PLC lie?
So, how do you choose the best option?
1. PLC for Industrial Field Information Acquisition and Communication Management
We know that there are two types of measurement and control systems in industrial sites. One type is called production process control system, or DCS for short; the other type is called power supply equipment and power measurement and control system, or SCADA.
The image below shows the technical specifications for ABB's mid-range PLC, including its communication interfaces and protocols.
For example, there is a water pump in an industrial site. Monitoring and controlling the pump's flow rate, pressure, and other parameters falls under the scope of a DCS (Distributed Control System). Currently, most measurement and control parameters are executed using frequency converters. The DCS monitors the parameter information uploaded by the frequency converter and issues control commands to the frequency converter in real time; the measurement and control of the operating parameters of the high-voltage motor driving the water pump, as well as various power supply and distribution switches, falls under the scope of SCADA (Supervisory Control and Data Acquisition).
The changes in parameters during the production process are relatively slow, and the interference is also weak. For example, the change cycle of flow rate, temperature, pressure, materials, etc., is generally several seconds, or even longer. In contrast, the changes in parameters during power measurement and control processes are rapid, and the interference is strong. For example, the opening and closing of switching devices, voltage spikes, current spikes, and short-circuit current surges, etc. In these processes, the change cycle is generally less than 1 second, or even less than 5 milliseconds.
Whether it's SCADA or DCS, there are PLC cabinets on-site for monitoring production process parameters. The purpose of these cabinets is to collect information and perform communication management and information exchange tasks.
My work involves SCADA, and we always use a communication management unit on-site. Initially, we used a regular PLC, but we encountered numerous problems, mainly related to memory allocation and communication program interrupt handling. Sometimes, it would even automatically reset and restart, causing a huge and disruptive impact on the field.
In addition, the EMC rating of a PLC is particularly important. Many low-end PLCs may even be damaged or spontaneously combust in industrial settings, especially in the high electromagnetic environment of substations.
Therefore, under these operating conditions, a mid-range PLC must be used. This type of PLC must have sufficient communication interfaces and communication speeds, and its power supply must also have adequate interference immunity.
2. Inter-transfer redundancy system
The so-called mutual redundancy refers to the simultaneous operation of two PLC CPUs, one as the primary CPU and the other as a backup. If the primary CPU fails, the backup CPU must seamlessly switch over.
This type of PLC is commonly used for generator rotor measurement and control.
Everyone knows that the frequency of alternating current is 50 Hz, so you can imagine how fast the rotor speed of a generator is. If the CPU of the PLC performing the measurement and control fails, it must be replaced by a backup CPU within a few clock cycles.
This type of dual-CPU PLC is a high-end PLC, also known as PLC hardware redundancy.
PLCs also have software redundancy, allowing them to operate in lower-speed environments. For example, ABB typically uses software redundancy for the PLCs supplied to station power distribution rooms (power supply distribution room, environmental control distribution room, traction distribution room) in subway projects. Even though the speed is relatively slow, a mid-range PLC must still be used.
AB's PLCs are used extensively here. See the image below for an example of an AB PLC:
These two examples show that the type of PLC used is entirely determined by the working characteristics of the controlled object and its working environment. It has nothing to do with the personal preferences of the design engineer.
Therefore, when reviewing and determining the performance of a PLC, it is necessary not only to look at its sample parameters, but also at its type test parameters, especially EMC performance, system stability and reliability.
Differences and relationships between 3DCS, PLC , and SCADA
DCS, PLC, and SCADA are different things and different concepts, and there is basically no subordinate relationship between them.
The evolution of DCS (Distributed Control System) has progressed from pneumatic unit instrumentation to electric single-loop controllers, and finally to DCS. It targets process control. There was a brief period of centralized control, but the centralized risks were deemed unacceptable, leading to the development of DCS . The hardware structure includes field instruments, junction boxes, grouping cabinets (sometimes absent), I/O card cabinets, controller cabinets (sometimes combined with I/O card cabinets), power supply cabinets, relay cabinets, and safety barrier cabinets (for intrinsically safe systems), etc. Modern DCS systems can also perform logic judgments and sequential control functions. DCS typically also has communication interfaces with other PLCs, SIS (System-on-a-Channel System), and higher-level data networks.
The evolution of PLCs has progressed from solid-state logic to relay groups, and finally to Programmable Logic Controllers (PLCs). Its initial purpose, as its name suggests, was to handle logical judgments, sequential control, and batch processing. Later, PID control was added (allowing it to perform some functions of a DCS). While the exact evolution of PLCs may not be entirely accurate, the difference between them and DCSs is clear. Large-scale chemical plants would not use PLCs for core process control; they would only use SIL-certified PLCs for System-on-System (SIS) and individual control of certain equipment packages.
SCADA, as its name suggests, is primarily used for data acquisition and monitoring. For example, it's used in the monitoring systems of the MCC (Mechanical Control Center) for electrical systems in chemical plants. Another example is the use of SCADA systems at various pressurization and regulating stations along long-distance oil and gas pipelines to centralize data into a control center. The main difference is that SCADA focuses on information collection and display, while PLCs and DCSs primarily output control signals. I'm not very familiar with SCADA.
PLC and DCS are two different things with different strengths. Although they have become more intertwined in their development, their applications are still quite different.
