Let’s forget about the classic stereotypes and look beyond technology to find the “sweetspots” where each system is best applied.  When considering one of these systems, you should ask yourself these 7 questions, designed to make you think about your company’s operating philosophy and application requirements. We will provide a list of possible answers per question.

To start: download the survey form

1 - What are you manufacturing and how?

This may seem like a very basic question, but the way a product is manufactured, the performance needed, along with any physical limitations of the process, all influence the system selection.



Typical factory automation applications involve the manufacturing and/or assembly of specific items – “things”. These applications may employ one or more machines and an amount of material movement from machine to machine. The “things” are usually visually monitored as they progress, and the process is very logic control intensive.



Process automation applications typically involve the transformation of raw materials through the reaction of component chemicals or the introduction of physical changes to produce a new, different product – "stuff." The operator can’t see the product, because it’s mostly held within a vessel. Depending on the size of the operation (plantwide VS single unit), this type of process is often controlled by a DCS.



Furthermore, a PLC can be used for “simple” batch applications, while a DCS is typically better suited for “complex” batch manufacturing facilities that require a high level of flexibility and recipe management.


Now select the responses that best fit your application.

2 - What is the value of the product being manufactured and the cost of downtime?

If the value of each independent product being manufactured is relatively low, and/or downtime results in lost production, but with little additional cost or damage to the process, the PLC is the likely choice. If the value of a batch is high, either in raw material cost or market value, and downtime not only results in lost production but potentially dangerous and damaging conditions, the decision should be DCS.


Now select the responses that best fit your application.

3 - What do you view as the "heart" of the system?

Typically the heart of a factory automation control system is the controller (PLC), which contains all of the logic to move to product through the assembly line. The HMI is often an on-machine panel or a PC-based station that provides the operator with supplemental or exception data.


In process automation, where the environment can be volatile and dangerous, and where operators can’t see the actual product, the HMI is considered by most to be the heart of the system. In this scenario, the HMI is a central control room console that provides the only complete “window” into the process, enabling the operator to monitor and control the processes which are occurring inside pipes and vessels located throughout the plant.



Now select the responses that best fit your application.

4 - What does the operator need to be successful?

In a PLC environment, the operator’s primary role is to handle exceptions. Status information and exception alarming help to keep the operator aware of what is happening in the process.


The DCS plant requires an operator to make decisions and continuously interact with the process to keep it running. In fact, leveraging the operator’s process knowledge is often critical to operational excellence and keeping the process running optimally.



It all comes down to the vital need to have an operator “in the loop” versus “out of the loop”.


Now select the responses that best fit your application.

5 - What system performance is required?

The speed of logic execution is a key differentiator. The PLC has been designed to meet the demands of high-speed applications that require scan rates of 10 milliseconds or less, including operations involving motion control, high-speed interlocking, or control of motors and drives. Fast scan rates are necessary to be able to effectively control these devices.


The DCS doesn’t have to be that quick, most of the time. The regulatory control loops normally scan in the 100 to 500 millisecond range. The extra cost for redundancy, an insurance policy of sorts, may be well worth it in the case of a typical DCS system, where high availability is mission critical.


Taking a PLC system offline to make configurations may have less impact, since the platform is not running continuously or because the process can be restarted easily. In contrast, configuration changes and tweak to the DCS system are done online, while the process is running virtually non-stop.

Now select the responses that best fit your application.

6 - What degree of customization is required?

The expectation and desire to be able to create a customized application varies greatly between DCS and PLC users.


Because the PLC was originally designed to be a jack of all trades, it’s understood that the development of customised routines and functions is required to meet the unique needs of an application. The PLC delivers a “toolkit” of functions and elemental building blocks that can be custom-developed and chained together to address the requirements of an application. Provisions are available to the integration of functions and products into a seamless architecture. Additionally, powerful programming languages are available to facilitate the creation of custom code from scratch.


Pre-engineered “solutions” consisting of standards, templates, and extensive libraries, are what DCS application engineers expect “out-of-the-box” when working with a new system. The highest priority of a DCS is to deliver reliability and availability, which results in a design which trades unlimited functionality for repeatability and dependability. The system is expected to function as a complete solution, which drives the use of standard functions already “baked in” to the platform.

Now select the responses that best fit your application.

7 - What are your engineering expectations?

Factory automation engineers want customizable control platforms, which offer the individual components that can be quickly programmed together to accomplish the task at hand. Often integrators and engineers just roll up their sleeves, and start programming. The tools provided by a PLC are typically optimised to support a “bottom-up” approach to engineering.

DCS engineers, on the other hand, are typically most effective using a “top-down” approach for engineering, which forces them to put significant effort into the upfront design. This focus on upfront design is a key to minimizing costs, compressing the project schedule, and creating an application that can be maintained by plant personnel over the long term.

Think about it this way – The PLC is controlling a machine, while the DCS is controlling the plant.

Now select the responses that best fit your application.

Wether you have found your solution or you still have some questions concerning the ideal solution for your application, contact us and our specialists will be happy to help!

Source: Siemens - Process Automation

Now select the responses that best fit your application.