Features and capabilities of hybrid controllers

Is there a difference between a DCS and a hybrid control system? Bela Liptak and his cadre of automation experts suggest where to apply each technology in this installment of Ask the Experts.

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QUESTION:

I AM A PROJECT Engineer at PETROPERU's Talara Refinery. We are in the process of doing the basic design engineering prior to purchasing a control system for our UDV unit. I need to obtain definitions of the difference between a DCS and a Hybrid Controller, since we have in the UDV unit a Moore APACS Hybrid Controller and we've had problems that are now being solved in the two years since installation. We need to know how to avoid recurrence of our problems, and where to apply each technology: DCS and Hybrid Controllers.

Ing. Humberto Paredes R., Petroperu Talara Refinery


ANSWERS:

IN VOLUME 3 (Process Software and Digital Networks) of my <i>Instrument Engineers Handbook<i>, Section 3.9 describes the features and capabilities of Hybrid Systems. Systems that combine both DCS and PLC functions used to be called hybrid.

In the 3rd edition of Volume 2 (Process Control), Sections 7.7 to 7.14, in the 4th edition, Sections 4.6 to 4.14 are devoted to the detailed description of DCS systems. With a DCS you are pretty much locked to a single supplier, because in most cases it is still impossible to choose the workstation software and you still cannot connect third party devices to the DCS networking.

The noted material in my handbook is comprehensive, requires good knowledge of the English language, and is not vendor specific. If you had described the nature of your problems with the Moore APACS Hybrid Controller in some detail, it would be easier to give you advice. In many cases, when I take a closer look, I find that the problem is not with the controller, but with the process or with the way the control system is configured.

Béla Lipták



THE TERM
“Hybrid Controller" is no longer used in the industry. Today, the Moore APACS will be considered as a DCS. Lately, there is another type of controller that is emerging called PAC (Programmable Automation Controller). PAC is based on PLCs but has enhanced functions and greater flexibilities. 

Asish Ghosh, Vice President, Manufacturing Advisory Services, ARC Advisory Group 



DCS AND HYBRID
Controller are general classifications at best, similar to the term SUV to describe a class of vehicles. My advice is to put 80% of your system selection effort into describing the functionality that is needed and the conditions under which the components must perform and be maintained. Your ideal system will be the one that meets the user and system functional specifications. Most often one of the big system builders has a standard model that fits just fine. Sometimes a custom system provides the best functionality or return on investment. Only a good functional specification can help determine which is best for your application. Don't try to make a system decision based on the vendor's classification of its system.

G. Kevin Totherow, Sylution Consulting



DCSes WERE
developed as a replacement for large numbers of single loop PID controllers. PLCs were developed as a replacement for large numbers of relays. These days the difference between these two architectures is not very big, and both could be called hybrid systems. Both have a CPU card (controller module) and an I/O subsystem with I/O modules. In the past, a PLC was purely logic while the DCS was a purely continuous controller. The PLC was programmed in ladder and the DCS in function blocks. Today, both handle all kinds of I/O and can be programmed in multiple languages.

In the past, a DCS included servers and workstation software, whereas for the PLC the HMI software was purchased separately; i.e., with a DCS you got an integrated system whereas with PLC you did system integration. In the past, a DCS used only proprietary networking whereas a PLC used open networking, making it possible to connect third party hardware. In the past, DCS applications were proprietary whereas the PLC was an open system. With the DCS all applications were tailored for its native hardware, minimizing configuration work but making it impossible or unfeasible to add hardware and software from third parties. The PLC could freely use third party hardware and software, and required lots of configuration work, but at least it was possible.

Today, PLCs use OPC to make data available to the control software as a single integrated database with little or no duplicate work. At the same time, DCSes implement OPC as a gateway that makes access to some data possible, although it is still impossible to choose the workstation software and you still cannot connect third party devices to the DCS network. These days, most PLC manufacturers have either bought or aligned themselves with HMI software companies, thus supplying a total solution.

Other differences in the past included far better diagnostics and redundancy in the DCS, but this gap has been closed. Today, many PLCs are used in applications where in the past only a DCS could be used. Historically a DCS was also far more expensive, but the competition from PLCs and new architectures have driven the initial price of DCSes down. The long term cost may be higher, because with a DCS, you are pretty much locked into a single supplier.

Jonas Berge, SMAR



1) WHAT IS THE
difference between a Hybrid controller and a pure DCS? This is a very difficult question to answer because most DCSes today claim to be hybrid systems (Moore APACS, Emerson DeltaV, etc.) so they can address a bigger part of the market, and because it describes their capabilities.

In general, I think the hybrid controller of today incorporates 99% of the features of a classic DCS plus adds many new features not seen in the classic DCS. One example in my mind would be in the providing of ladder logic as one of the IEC configuration languages that can be used to configure the control strategy. APACS provided this capability and it is much appreciated by people in a maintenance/technician position, who are typically more familiar with this language than function block or the other languages of a classic DCS.

In general, I do not consider the hybrid control system of today to have any less capability than the DCS of today.

2) Petroperu's Problems with APACS: Being familiar with APACS (as a former employee of Moore Products), I cannot think of any reason why APACS could not be applied in a classic DCS application. There are numerous DCS customers (such as refineries, HYCO facilities and power generating stations) that have successfully applied APACS.

I would be happy to help Humberto if he would like to follow up with someone from Siemens/Moore Products regarding his concerns.

It seems like he is drawing the conclusion that, just because he is having some problems with an APACS system (of which we do not know), hybrid control systems in general can't be applied to his application. I think this is incorrect reasoning and may lead him to make some future decisions that are not optimum for his company.

Todd Stauffer, Siemens Energy & Automation

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