DCS or PLC? — Seven questions to help you select the best solution

The convergence of PLC and DCS technologies has created a situation where it is more challenging than ever for process manufacturers to select the best technology for their application. A successful evaluation should start by developing a clear picture of the requirements of your application and the needs of your engineering, maintenance and operations personnel. To help clearly define these requirements and needs for your company, this paper outlines the seven key questions that will lead you to making the right choice.

The procedure for selecting the best automation technology is not as easy as it once was. In the past it was fairly easy to determine whether a PLC or a DCS was right for your application, because their strengths and weaknesses were well understood. In recent years this has become more difficult, thanks primarily to the advancement of the microprocessor, which has allowed the technologies to merge. With the trend toward flexible manufacturing in industry, many of the applications in the process industries now share the requirements traditionally thought to be exclusive to either DCS or PLC. These hybrid applications typically require a process control system that can deliver both PLC and DCS capabilities. Thus, understanding the merging of PLC and DCS functionality is important for selecting the best system for your company.

Let’s get technical stereotypes out of the way!

Selecting an automation system based on a review of available products is the typical course of action for someone in the market for a new automation system. The problem with this approach is that your perception of which systems ‘make the cut’ is often based on old stereotypes or influenced by the claims of the first salesperson in the door. Let’s look at the components of a DCS or PLC-based system to see how different (or similar) they really are.

At first glance the architectures look very similar. Both systems share the following components:

• Field devices
• Input/output modules
• Controllers
• Human machine interface (HMI)
• Engineering
• Supervisory control
• Business integration.

The technologies used in each system are, in fact, very similar; the difference becomes more apparent when you consider the nature and requirements of the application.

Figure 1: Typical DCS system architecture.

For example, in the DCS architecture of Figure 1, redundancy is often employed for I/O, controllers, networks and HMI servers. Since redundancy adds cost and sometimes complexity, DCS users must carefully evaluate their need for redundancy in order to achieve their required system availability and to prevent unplanned downtime.

The PLC architecture (Figure 2) illustrates one of its most common applications, the control of discrete field devices such as motors and drives. To effectively control motors and drives requires that the controller be able to execute at high speeds (typically a 10–20 ms scan rate), and that the electrical technician responsible for maintaining it be able to read and troubleshoot the configuration in a language that he is familiar with (relay ladder logic).

Figure 2: Typical PLC-based system architecture.

The seven questions to ask yourself before choosing a system

1. What are you manufacturing and how?

Typical factory automation applications, for which the PLC was originally designed, involve the manufacturing or assembly of specific items — ‘things’. These applications may employ one or more machines and a fair amount of material movement from machine to machine. A typical characteristic of this type of process is that the operator can usually monitor the ‘things’ visually as they progress through the manufacturing line. This type of process is often controlled by a PLC and human machine interface (HMI) combination.

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’. These applications may be composed of one or more process unit operations piped together. One key characteristic is that the operator can’t see the product. It is usually held within a vessel and may be hazardous in nature. There is usually a large amount of simple to complex analog control (ie, PID or loop control), although the response time is not that fast (100 ms or greater). This type of process is often controlled by a DCS, although the analog control capability of a PLC may be more than adequate.

There may also be sequential (or batch) control needs. A PLC can be used effectively 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. Again, the requirements of the batch application determine whether it is simple or complex. Is one product being manufactured or are multiple products? Are the recipe parameters constant or variable? Is there a single procedure or are the multiple (different) procedures? Is the equipment utilisation fixed or flexible? Are there frequent changes of formulas and recipes?

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, 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 selection should be DCS.

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 the product in 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.

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 keep the operator aware of what is happening in the process, which in many cases can run ‘lights out’.

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. The operator will change setpoints, operate valves or make a manual addition to move a batch to the next stage of production. In the event of a HMI failure, the plant could be forced to shutdown in order to keep people and equipment safe.

It all boils down to the vital need to have an operator ‘in the loop’ versus ‘out of the loop’. The DCS operator is the ultimate system stakeholder, whose upfront buy-in for the HMI design is essential for overall project success.

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 ms 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 ms range.

The issue of analog control is important, but confusing. DCS was originally designed for delivering analog control, but to say the DCS has a lock on the analog control market reiterates the problem with traditional thinking. Increasingly, the PLC is capable of delivering simple to complex PID control, but the DCS is clearly the choice for applications with a large amount of advanced analog control, including cascade loops, model predictive control, ratio and feedforward loops.

6. What degree of customisation is required?

A systems integrator may be applying a PLC toward a palletising machine today and pointing it toward a laser cutting lathe tomorrow. 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.

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 often results in a design which trades unlimited functionality for repeatability and dependability. The significant tradeoff with the DCS is its inability to accept many custom modifications without creating compatibility issues.

7. What are your engineering expectations?

Factory automation engineers want customisable control platforms, which offer the individual components that can be quickly programmed together to accomplish the task at hand. The tools provided by a PLC are typically optimised to support a ‘bottom-up’ approach to engineering, which works well for smaller applications.

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 minimising 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.

Do you have a hybrid application?

Now that we have reviewed the key criteria for selection of a PLC or DCS, you may be thinking that your application requires capabilities from both a PLC and a DCS. If this is true, then you may need a process control system for hybrid applications, as shown below.

What is a hybrid application?

A hybrid application has been defined¹ in various ways:

• “The marriage of the discrete functions, which PLCs handled so simply and economically, with the sophisticated analog continuous control capabilities of the DCSs.”
• “Defined based on the industries in which the systems work and serve, like pharmaceutical, fine chemicals, food and beverage, and others.”
• “The architectural marriage of the PLC simplicity and cost with the sophisticated operator displays, alarm management, and easy but sophisticated configuration capabilities of the DCS.”

How to select a process control system for a hybrid application

We have described the key attributes and differentiators between classic PLC and DCS systems. This same information can be used to define the key requirements for a process control system that would be ideally suited for hybrid applications, such as those in the pharmaceutical, fine chemicals or food and beverage industries.

The key requirements for a process control system for use in hybrid applications are:

• Controller — can execute fast scan logic (10–20 ms), such as that required for motor control, and slow scan logic (100–500 ms), such as required for analog control, simultaneously in a single controller
• Engineering configuration languages — provides ladder logic, function block diagram and a powerful programming language for creation of custom logic from scratch.
• Flexible modular redundancy — offers the option of tailoring the level of system redundancy to deliver the required system availability by balancing up-front cost versus the cost of unplanned downtime.
• Modular batch from simple to complex — provides modular batch capability to cost-effectively address the continuum of simple to complex batch applications.
• Alarm management — offers alarm management tools to help operators respond effectively to plant upset conditions.
• System diagnostics and asset management — provides both a rich set of built-in system diagnostics, as well as asset management of all critical assets in the plant (transmitters, valve positioners, motors, drives, MCCs, heat exchangers etc).
• Scalable platform — hardware, software and licensing supports smooth and economical scale-up from small all-in-one systems (10’s of I/Os) up to large client-server systems (10,000s of I/Os).

Conclusion

Many of the stereotypes of yesterday are being replaced, thanks to the convergence of PLC and DCS. This convergence has opened up a new set of options for hybrid applications and for those process plants that traditionally used PLCs to control their electrical infrastructure such as motors, drives and motor control centres (MCCs), while utilising DCS for regulatory control. Remember, it’s not about the technology. Most importantly, it is about the requirements of your application and what supplier has the best solution, heritage, experience and breadth of knowledge to meet your needs, today and tomorrow.

You may find that a traditional PLC or DCS no longer meets your requirements. If you have a hybrid application, then you may need a process control system which combines the best of the PLC and DCS, and a supplier who can provide a full offering of both discrete and process capabilities, all based on a common platform.

Reference:
1. Intech, Sept 2007, ‘Hybrid control identity crisis: what is in a name?’

*Bob Nelson, PLC Marketing Manager, and Todd Stauffer, DCS Marketing Manager, for Siemens Energy & Automation, have over 40 years’ combined experience in the automation and control industry.
This article was previously published in the May 2008 issue of Control Engineering.

Siemens Ltd
www.siemens.com.au