PLC vs. DCS: Which is Right for Your Operation?

Over the past decade, the functionality of different control systems has been merging. Programmable logic controllers (PLCs) now have capabilities once found only in distributed control systems (DCSs), while a DCS can handle many functions previously thought more appropriate for PLCs. So what’s the difference between the two control approaches, where’s the dividing line and are there still reasons to choose one over the other?

PLCs grew up as replacements for multiple relays and are used primarily for controlling discrete manufacturing processes and standalone equipment. If integration with other equipment is required, the user or his system integrator typically has to do it, connecting human-machine interfaces (HMIs) and other control devices as needed.

The DCS, on the other hand, was developed to replace PID controllers and is found most often in batch and continuous production processes, especially those that require advanced control measures. The vendor handles system integration, and HMIs are integral.

As users demanded more production information, PLCs gained processing power and networking became common. PLC-based control systems began to function like a mini-DCS. At the same time, the DCS hybridized to incorporate PLCs and PCs to control certain functions and to provide reporting services. The DCS supervises the entire process, much like the conductor in an orchestra. Protocols, like OPC, have eased interactions between the two control systems.

由于PLC的价格较低，现在可以像DC一样表现出色，因此将所有内容转换为PLC是否有意义？就像自动化世界中的大多数事物一样，答案取决于您应用程序的需求。这是要考虑的六个关键因素：

1. Response time

PLCs are fast, no doubt about it. Response times of one-tenth of a second make the PLC an ideal controller for near real-time actions such as a safety shutdown or firing control. A DCS takes much longer to process data, so it’s not the right solution when response times are critical. In fact, safety systems require a separate controller.

2. Scalability

PLC只能处理几千I/O点以下。它不像DC那样可扩展，它可以处理数千个I/O点，并且更容易容纳新的设备，过程增强功能和数据集成。如果您需要高级流程控制，并且具有大型设施或一个流程，该工艺已分布在具有成千上万个I/O点的广泛地理区域，则DCS更有意义。

3.冗余

Another problem with PLCs is redundancy. If you need power or fault tolerant I/O, don’t try to force those requirements into a PLC-based control system. You’ll just end up raising the costs to equal or exceed those of a DCS.

4. Complexity

The complex nature of many continuous production processes, such as oil and gas, water treatment and chemical processing, continue to require the advanced process control capabilities of the DCS. Others, such as pulp and paper, are trending toward PLC-based control.

5. Frequent process changes
PLCs are best applied to a dedicated process that doesn’t change often. If your process is complex and requires frequent adjustments or must aggregate and analyze a large amount of data, a DCS is

typically the better solution. Of course, the very flexibility of a DCS system also makes it much more vulnerable to “meddling” by operators that can cause spurious shutdowns.

6. Vendor support

DCS vendors typically require users to employ them to provide integration services and
实现过程变化。

系统集成器对基于PLC的系统执行类似的功能。PLC供应商通过其系统集成伙伴网络提供支持服务也很普遍。

Process control has become increasing complex. It’s difficult for any individual to know everything about these sophisticated systems, increasing the need for vendor support. Manufacturers also continue to reduce factory staff and a generation of experienced process control personnel has begun to retire. As a result, the quality of support has become a critical factor in vendor selection.

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