What kind of PLC can be considered a high-end PLC?
Reliability? Stability? Information security? Network connectivity... Which of these are the most important factors we need to consider for high-end PLCs in the future?
Let's discuss this together!
First example
Industrial field information acquisition and communication management PLC
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.
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, the purpose of which 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.
The second example
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 monitoring and control. Everyone knows that the frequency of alternating current is 50 Hz, so you can imagine how fast the generator rotor rotates. If the CPU of the PLC performing the monitoring and control malfunctions, 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.
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.
