【Daily Question 20090401】Discuss electrical integration in the field of automated applications

1. DCS is a “distributed control system,” whereas PLC (Programmable Logic Controller) is merely a control “device””； The difference between the two is that of “system” and “device”. 2. PLCs focus on logical control, but the number of analog input points is limited (up to 500 points). And DCS solves this problem. 3. The DCS is designed with a large number of scalable interfaces, making it very convenient to connect external systems or expand the system ； Once the entire system connected to the PLC is completed, it is very difficult to add or remove operator stations as desired. 4. In terms of safety and redundancy, DCS has significant advantages over PLCs. 5. In the practical application of DCS, control wires should be separated from power wires, and the control wires must be properly shielded; otherwise, interference issues will directly affect normal operation.

Using PLCs to control motors represents one of the main applications of PLC technology. PLCs began to be used in industrial production in the 1980s, primarily to replace relays in order to carry out relay-based logic control. Thanks to their advantages such as having no contacts and the ability to expand the number of auxiliary contacts, they were quickly adopted in the field of machine tools. Machine tools often have multiple motors, and it is crucial to ensure that these motors operate in the correct sequence. By using PLCs for control, the failure rate is reduced, wiring requirements are minimized, and space is saved. Currently, the application areas of PLCs far exceed their original scope.

Using a PLC to control motors represents one of the main applications of PLCs. PLCs began to be used in industrial production in the 1980s, primarily to replace relays for implementing relay-based logic control. Thanks to their advantages such as being contactless and allowing for a large number of auxiliary contacts, they were quickly adopted in the field of machine tools. Machine tools often have multiple motors, and it is crucial to ensure that these motors operate in the correct sequence. By using PLCs for control, the failure rate is reduced, wiring is simplified, and space is saved. Currently, the application areas of PLCs far exceed their original scope.

The improved PLC-driven servo motor: The original device was entirely composed of transistor circuits; the controller is implemented using a touch screen and a PLC, which reduces the development time and improves operational reliability

In industrial process control, there are three major control systems, namely PLC, DCS, and FCS. DCS (Distributed Control System) is a distributed control system, also known as a distributed control framework. PLC (Program Logic Control) refers to a programmable logic controller. FCS (FieldBus Control System), a fieldbus control system. DCS evolved from traditional dashboard monitoring systems. Therefore, DCS systems are inherently more focused on the control of instruments; for example, the ABB Freelance2000 DCS system we use does not even have any limit on the number of PIDs (PID, or proportional-integral-differential algorithm, is the standard algorithm used for closed-loop control of control valves and frequency converters, and typically the number of PIDs determines the number of control valves that can be used). PLCs evolved from traditional relay circuits; the earliest PLCs did not even have the capability to process analog signals. Therefore, logic processing capabilities were emphasized in PLCs from the very beginning.

Let’s give a few examples. The project I am currently debugging uses a Siemens 400 CPU to connect to Siemens ET200S stations via Profibus DP; the ET200S stations are equipped with 5–6 integrated motor starters. Standard low-power motor control, not exceeding 7.5K. During the motor startup phase, it is only necessary to supply AC380V as input and have AC380V as output. What’s left is to issue commands to the virtual IO addresses within the CPU. It is possible to read commands indicating whether the motor starter is ready, whether it is running in forward or reverse direction, whether there is a fault or an alarm, and the motor current can also be calculated. All of this no longer needs to be implemented through hard wiring. I tried Turck’s motor starters in the previous project as well; they are also based on the DP bus. Apart from the inability to detect motor current, it is not very different from the motor starter of the ET200S. The maximum supported power is slightly higher, reaching up to 11KW. I also tuned Schneider Auma motor starters 3 years ago. That isn’t a DP bus; it’s a Modbus bus, with a slightly lower speed. For the next project, I plan to continue using Siemens 400 with an ET200S interface. However, there are 2 small, reversible motors with forward and reverse rotation. There are several very small frequency converters used on the assembly line. It is uniformly mounted on the ET200S station and communicates with the CPU via Profibus DP.

At the moment, I am providing consulting services for a laboratory platform to a foreign company. The ABB AC800M is used to connect to the ABB ACCS800 frequency converter. Every motorcycle must be tested on a testing bench before it leaves the factory. The CPU main controller communicates with the ACS800 via the ModuleBus fiber optic bus. The variables in the ACS800 can be directly passed to the AC800M. It’s just like the remote Io of the AC800M. Hardwiring of Io points such as AI, AO, DI, and DO is no longer used to establish communication between the CPU and the frequency converter. More importantly, the client requires a control cycle every 10 ms. Only high-speed buses such as ModuleBus can ensure the real-time transmission and reception of data. If it is connected via IO wiring, it simply won’t work. Just AI processing alone takes 20ms faster. DI also needs more than 5ms. Ao is also slow, about 20ms. These have no problems in regular applications. However, on the testing platform, it is not possible to meet the client’s requirements; this can only be achieved by using high-speed communication between the motor’s frequency converter and the CPU.

In a power plant, with an air-cooling system, there are approximately 48 frequency converters. It is to control 48 cooling fans. Install in 12 cabinets. Two sets of Controllogix 5000 systems were added specifically for these 48 frequency converters. Each set is redundant and connects 24 frequency converters. Devicenet bus communication is used. Between the PLC and the frequency converter, in addition to communication, there is also a Do signal used for starting and stopping. All the other signals are sent to the PLC via communication. Objectively speaking, I wasn’t the one who worked on that project, but I went to check it out twice and found it to be a complete failure. The root cause lies in the problem with the Devicenet bus. Take a look – as the top and second-largest players in the global market, what buses do ABB and Siemens use for variable frequency drives and DC drives? Prifubus DP. . . The bus medium can be redundant and can form a ring. The communication distance can be relatively long, unlike Devicenet which has a maximum distance of 500 meters. When the communication distance is short, the speed can be set relatively high; for example, at a distance of 1.5 meters, 500 K is quite normal, while Devicenet can only reach 500 K at a distance of 100 meters. It’s barely possible! Moreover, there are many drive devices that support DP, while very few support Devicenet. ABB’s AC800F and AC800M, as well as Siemens’ S7, can all be directly connected to DP-type drive equipment. For the redundant Logix5000 system of AB, when it is connected to its own drive system, an additional 1756-DNB Devicenet communication module must be installed at the Io station as well. And that’s how it works: CPU -- ControlNet -- Devicenet --- drive equipment. Obviously, the levels are quite different.