PLC vs. DCS: Which control system is best suited to your operation?

Many of us engineers have asked ourselves, when implementing an automation project, which control system is the most suitable? Over the last decade, the functionality of different control systems has merged. Programmable logic controllers (PLCs) now have capabilities previously found only in distributed control systems (DCS), while a DCS can handle many functions previously considered more appropriate for PLCs.

Then:

• What is the difference between the two control approaches?
• Where is the dividing line?
• Are there still reasons to choose one over the other?

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

DCS, on the other hand, was developed to replace PID controllers and is most often found in batch and continuous production processes, especially those requiring advanced control measures where the supplier manages the system integration, and HMIs are integral.

As users demanded more production information, PLCs gained processing power and networks became common. PLC-based control systems began to function as a mini-DCS. At the same time, the DCS was hybridised to incorporate PLCs and PCs to control certain functions and provide reporting services. The DCS oversees the entire process, just like the conductor of an orchestra. Protocols, such as OPC, have facilitated interactions between the two control systems.

As PLCs are less costly and can now function as a distributed control system, Wouldn't it make sense to convert everything to PLC? The answer, as with most things in the world of industrial automation, is that it depends on the needs of your application and here are six key factors to consider:

1. Response time

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

2. Scalability

   A PLC can only handle a few thousand I/O points or less. It is simply not as scalable as a DCS, which can handle thousands of I/O points and more easily accommodate new equipment, process improvements and data integration. If you need advanced process control and have a large facility or a process that spans a wide geographic area with thousands of I/O points, a DCS makes more sense.

3. Redundancy

   Another problem with PLCs is redundancy. If you need power or fault-tolerant I/O, don't try to force those requirements on a PLC-based control system. You will simply end up increasing costs to match 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, continues to require the advanced process control capabilities of DCS. Others, such as process automation in pulp and paper mills, tend to move towards PLC-based control.
5. Frequent changes in the process

   PLCs are best applied to a dedicated process that does not change frequently. If your process is complex and requires frequent adjustments or must aggregate and analyse a large amount of data, a DCS is often the best solution. Of course, the very flexibility of a DCS system also makes it much more vulnerable to operator “meddling» that can cause spurious shutdowns.

6. Vendor support

   DCS vendors generally require users to employ them to provide integration services and implement process changes. System integrators perform similar functions for PLC-based systems. It has also become common for PLC vendors to offer support services through their network of system integrator partners.

Process control has become increasingly complex. It is difficult for anyone to know everything about these sophisticated control systems, which increases the need for supplier support. 

If you are interested in finding out more about the most appropriate control system, we invite you to find out which are the most suitable ones. benefits of distributed control systems in industrial plants.

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