Expert Witness Forensic Engineer Industrial Controls Automation New Orleans
Arthur Zatarain
Industrial Control & Computer Systems
 Programmable - Solid State - Relay Logic

Industrial Controls: Arthur Zatarain has experience with both patents and accidents involving Programmable Logic Controller (PLC), Distributed Control Systems (DCS), and even "old fashioned" hard-wired relay logic. This experience includes forensic engineering as well as expert witness and litigation support. Sample engagements include:

Industrial control systems often use a programmable logic controller, or "PLC," which has become the default industrial control device for small and large applications. A programmable logic controller can be used as a stand alone control device to perform all aspects of system control. Alternately, a PLC can be used as part of a larger Distributed Control System (DCS) along with computer workstations, process controllers, and network communications. The input/output (I/O) count in a single system can range from less than eight to more than several thousand.

A related programmable logic device is a programmable automation controller PAC that offers more computer-like features to a industrial control environment. The term PLC has become so common that it is often applied to any programmable industrial controller, including process controllers and "soft-PLC" workstation computers. 


The field of industrial controls and instrumentation covers a wide range of technologies, ranging from simple off-on switches to networked computerized controls. Many industrial controls still employ electro-mechanical relays to form the logic and power-driving aspects of the system. These "hard wired" systems have been around for over one hundred years yet are still found in almost every industrial control situation. However, the ever-decreasing size and cost of PLC, PAC, and DCS components has made computerized controls practical for almost every industrial application.

A PLC is programmed with specialized workstation software that produces a logic program in the hardware language of the particular controller. The most common form of programming is done with "Ladder Logic" or a "ladder diagram." These programs graphically represent traditional relay logic that consists of "rungs" having individual logic statements. A small program may have only a few dozen lines of code; larger programs may have several thousand.

Although programmable logic controllers have traditionally been programmed with proprietary development software, a standardized industry format called IEC-61131 provides for ladder logic programming as well as several other industrial control languages. The IEC standard provides Ladder Diagram (LD), Function Block Diagram (FDB), Structured Test (ST), Instruction List (IL), and Sequential Function Chart (SFC). A sample of ladder logic diagram is shown below:


Ladder logic has its roots in the PLC world, while FDB has roots in DCS. The availability of both languages in any system using the IEC standard reflects the merger of PLC and DCS technologies. Ladder diagrams and ladder logic are often considered the most common industrial control language for PLC in the USA. Other geographical areas use ladder logic as well as the other IEC languages, with Function Block Diagram (FDB) being the most common.

Industrial controllers and other devices exchange data using both proprietary and open architecture data exchange protocols. Some of these are known as fieldbus interfaces, with common ones being Foundation Fieldbus, Modbus, Profibus, CAN, and LonWorks. A more complex and powerful interface is OLE for Process Control, or OPC. This system allows data interchange via standard Windows software interfaces to link controllers, workstations, and servers in local and wide area networks.


Operators interact with a PLC or DCS via a Human Machine Interface (HMI), formerly known as a Man Machine Interface (MMI). The interface programs operate on workstation computers to collect data from the industrial controller and possibly store it for later recall. The interface software can also be used to alter data variables in the connected controllers to adjust process actions and vary process "setpoints" to suit changing conditions.


The engineering required to design, fabricate, and program industrial controls can involve electrical, electronic, pneumatic, and hydraulic component, often coordinated by computerized logic solvers. Although an industrial controls engineer may excel in one or more specialties, an engineering generalist is often useful when combining diverse pieces into reliable and practical system.

A control system failure can result from problems with the control components including hardware, software, and communications. The control system failure may not be evident from historical documents due to changes in hardware and software that affect the programmable systems. An analysis of the control system failure, and the circumstances of the failure, can reveal latent defects caused by the original design or subsequent modifications. The control system failure investigation involves analysis of historical documents as well as current configurations, often requiring on-site inspections and testing of the installed hardware and software.

The following section is for key term and general interest only.

Artzat Consulting

Industrial control system

Engineering for industrial control systems, and for industrial automation systems, involves a wide range of technologies. Although electrical engineering is often the basis of modern control, many other technologies play a role in the complete systems. An industrial control system may include elements of computerized control as found in a programmable controller, often called a programmable logic controller (PLC). Another common computerized control unit is a distributed control system, often termed a DCS. A safety PLC is a special form of programmable controller used for high reliability installations.

Ladder Logic

A PLC is usually programmed in ladder logic, a term derived from the similarity of the graphic programming language to the traditional wiring diagrams used for electrical relay logic. Ladder logic can be very complex in a large system, with the entire program divided into main programs and subroutine programs. The IEC 61131 programming languages can also be used to program a PLC, with variants including Ladder diagram LD, Function Block Diagram FBD, Structured Text ST, Instruction List IL, and Sequential Function Chart SFC.

PLC Patent

There are many patent issues for intellectual property related to industrial controls. Some of the patents involve the hardware components such as input output I/O as well as electronic components for the computer and memory circuits. Other patents related to the software used in the logic solver, network communications, and file systems.

PLC Accident

An industrial control system is often involved in accidents related to process control, manufacturing, and other industries such as bulk material handling. The accident may relate to the control system itself, as in a failure to properly control. The accident may also relate to improper operation of the control system, as when it is used in a lockout tagout loto situation. Other instances involving an accident with industrial control can relate to an emergency stop, or estop or e-stop.

Industrial Control Manufacturers
Leading players in the industrial control industry include Rockwell, Honeywell, and Invensys. Also included is ABB, Siemens, and Allen Bradley (part of Rockwell Automation). Some older names are Foxboro, Bristol Babcock, Fischer Porter, and Bailey. Fisher Controls and MicroMotion are part of Emerson. Trade names include PLC5, MicroLogix, and ProcessLogix, and Simatic,  Opto 22 and Eaton also make PLC equipment. Modicon, now Schneider Electric, was an early maker of PLC equipment that produced the Modbus protocol.

Wago makes PLC processor and I/O modules, as does Phoenix and Omron. Texas Instruments TI made PLC equipment, as did Square-D and Westinghouse. Fuji, Mitsubishi and Yokagawa make industrial control equipment including PLC processor and remote I/O. Koyo makes PLC equipment for other vendors.

GE Fanuc is a PLC and industrial control company. IDEC and Automation Direct supply PLC equipment including power supply, I/O, and network communication adapter and interface.

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