DDC evolved from PLC and was designed by manufacturers specifically for niche markets. Its biggest advantage over PLC is that it has a fixed set of control programs.
Note: DDC (Direct Digital Control) is commonly referred to as a DDC controller. A typical DDC system consists of a central control unit (central control computer, color monitor, keyboard, printer, uninterruptible power supply, communication interface, etc.), field DDC controllers, a communication network, and corresponding sensors, actuators, control valves, and other components.
Regarding PLCs and DDCs, which is more advantageous in building automation systems? A comparison is made below:
1. Application Areas
DDC (Directional Control) evolved from PLC (Programmable Logic Controller). PLCs are specifically designed for industrial automation, and almost all industrial production line control systems, thermal power plant control systems, and steel plant control systems in China use PLC systems. Currently, a significant number of building control systems also utilize PLCs. Building automation DDC systems were developed by manufacturers based on the characteristics of building automation from PLCs. The difference between DDC and PLC lies primarily in the fact that a portion of the program is fixed internally, but DDC also lacks the flexibility and ability to handle complex electromagnetic interference environments of PLCs.
2. Structural differences
Through years of development, modern PLCs are essentially the same as DDCs in terms of networking. They also support multiple protocols, have a hierarchical structure, and can achieve point-to-point communication. PLCs distributed at various sites in the field can be managed through communication without the need for a host computer.
3. Software Characteristics
The host computer software for DDC systems is mostly proprietary, which, in fact, indicates incompatibility. Each manufacturer's software differs, and many are in English, making it a nightmare for technicians. In contrast, PLC system host computer software can be both proprietary and general-purpose configuration software. Currently, most general-purpose configuration software in China is entirely in Chinese, offering flexible and convenient configuration. General-purpose configuration software can handle complex industrial control systems, so building control is a piece of cake. Furthermore, both PLC and DDC systems are configured by professional technicians, who then train the owner on operation and management. For the owner, it's essentially the same process. On the other hand, general-purpose configuration software can achieve all the functions of proprietary software, as well as many functions that proprietary software cannot (such as highly realistic interfaces, human voice alarms, and user-customized functions).
4. Professionalism
Many building control projects now utilize PLC systems. It has been proven that PLC systems can also perform the functions described above for DDC (Distributed Control Center). Experienced PLC building control system implementers have accumulated complete control flow programs, energy management programs, and energy-saving programs. Furthermore, because they possess the source code for all flow programs, they can quickly create customized functions for different projects. Since readily available flow programs are available, on-site debugging is much shorter and smoother than DDC debugging. This is because DDC's built-in programs only have interface functions in a fixed format. If special requirements arise, it's necessary to contact the manufacturer's engineers located far away, and even then, the solution is not guaranteed.
5. Scalability
Everyone knows that PLC systems are universal and open systems. Currently, most PLC systems and most DDC system operator stations use the TCP/IP protocol, allowing access as long as there is a network. However, DDC system software currently charges exorbitant fees based on the number of users, making it unattainable for most completed building control systems to have sub-control operator stations! PLC systems have this advantage.
6. Security
The problem is simple to imagine: a thermal power plant is generating electricity, and suddenly a PLC module fails. If, as mentioned above, the entire system needs to be shut down to replace it, what would the consequences be? The boiler could explode! Haha! The consequences would be far more serious than those of building control systems! Many modern PLC systems feature hot-swappable modules and hot-standby redundancy (these are features that building control systems seem to have adopted from PLCs in recent years). PLCs are developed for industrial environments; in complex industrial control environments, module failures or system shutdowns can lead to major accidents or casualties. Their reliability requirements are far higher than those of DDCs! I don't know where the above argument came from. A PLC control core can operate reliably and without failure for extended periods in harsh environments. It is easy to wire, easy to maintain, has good isolation, strong corrosion resistance, can adapt to a wide temperature range, and has a mean time between failures (MTBF) greater than 15 years.
7. Adjust the complexity level
Ladder diagram language is now the most widely used language for PLC programming. This language is visual and intuitive, and does not require English proficiency; ordinary electricians can learn it.
Modern PLC systems, like DDC systems, can be fully debugged from a comfortable server room using only a laptop and a network cable. As for accuracy, a glance at the product's technical specifications will tell you! I just want to say: do you think industrial control requires less precision than building control? I doubt anyone would think that! Haha! Modern industrial production lines often have control systems with tens of thousands of points (mostly using PLCs, not a single one using a DDC), and require millisecond-level network connectivity. Can a DDC achieve that?
Overview
In fact, DDC evolved from PLC and was designed by manufacturers specifically for a niche market. Its biggest advantage over PLC is only in that it has a fixed set of control programs; in other aspects, its performance is inferior to that of PLC. Because DDC targets only the specialized market of building control, the national market size is not large, which is the fundamental reason why DDC's performance is not outstanding, but its price is relatively high.
A fixed portion of the control program in the DDC (Digital Control Center) was initially conceived by manufacturers because the contractors of building control systems are mostly low-voltage system integrators, a group with relatively little exposure to automation technology. Therefore, manufacturers needed to create an easy-to-debug product for them to debug. However, this also resulted in a loss of flexibility.
If you encounter an experienced automation engineer, the built-in programs of their product can actually become a burden on their ability to perform. After the building control system is completed and delivered to the user, the user's maintenance engineer is more familiar with PLCs in the market. PLC programming now most commonly uses ladder diagram language, which is visual and intuitive, does not require English proficiency, and can be learned by ordinary electricians. Therefore, its maintenance technology and cost are actually lower.
