Control Systems Are Increasingly Replacing PLCs in Automation Solutions

Small, fast and cheap PLC-solutions usually offer few engineering options and often do not provide convenient visualisation. In contrast, distributed control systems integrate a host of components such as controls, engineering tools, HMIs (human machine interfaces) and numerous peripheral devices and tools.

The segment between PLCs and the world of control systems – the hybrid market – is currently being targeted by both sides. PLC components are becoming more powerful and they can therefore also process more complex tasks. ‘Lite’ control systems are being introduced and they are increasingly being installed in small to medium-sized applications with less complex automation tasks.

For small and less complex automation tasks with often only few signals standalone PLCs have been and are still used, because until now process control systems have been too expensive for these tasks.

In the process industry many control tasks, eg compressors, centrifuges or steam generators, have been automated as PLC-based package units, leading to variety of different control systems and tools.

Diminishing Returns
The use of different PLCs brings serious disadvantages for the users: different tools increase the training budget and lead to more complexity, but without adding value. Particularly during maintenance, minor changes can cause considerable expenses, as cross-influences from different systems require manual adjusting.

The differences in visualisation and operation as well as the individual alarm concepts used by the various manufacturers can, in extreme cases, even affect the availability and safety of the application and the total plant negatively.

Additionally, the flexibility of the maintenance personnel is reduced because not all users can be familiar with all tools. Often older PLCs can only be maintained by a small number of specialists, many of whom are due to retire over the next few years.

Procuring and storing spare parts for many different systems and products cause additional work and expenses. All these reasons result in higher costs for servicing and maintaining the plants.

In contrast to PLCs, control systems are primarily based on analogue control loops with slow monitoring and control functions but less on fast positioning or switching operations.

The process is operated and monitored in the control room. The systems and plants generally run continuously and often have very high demands when it comes to availability. Consequently, implementing changes must be possible in online project configuration.

Additionally, repairing and changing of components while operating the plant is essential. Applications are often specifically configured for one project and the processes concerned and therefore require powerful and efficient engineering-tools with extensive integration possibilities.

Compact Alternative
Driven by technological developments in the last years, manufacturers of process automation technology are nowadays able to offer control systems with higher scalability as an alternative to PLCs in process-oriented applications. The advantages are obvious: efficient engineering, easy operating and maintaining as well as increased productivity due to intelligent diagnosis.

It is now possible for smaller automation solutions, which to date have been dominated by PLCs, to benefit from the advantages of process control engineering, particularly in the process industry. There are opportunities for use in many industries, including chemicals, petrochemicals, oil and gas, metalworking, cement production, glass production, etc.

PCs and PLCs Have Grown Up

The question is whether or not I believe PC-based controls will replace PLCs, PACs or CNCs.

I’ve been around in the industry long enough to remember when both PCs and PLCs were in their infancy, and the “old-timers” still were designing their control systems around relay logic.

It still sends a shiver down my spine when I recall sitting in front of a multi-door control cabinet chock full of relays and motor contactors, trying to figure out which contact was bad, while I looked at a dog-eared, incomplete set of “as built” blueprints. Ah, those certainly were memorable days.

So, now that I’ve established my bona-fides, along with a few credentials in both age and senility, let’s address the question at hand: PC-based controls or PLCs?

This question has been around for a while, too, but it really got to be a hot topic in the ’90s. Prior to that, any process that required higher-order mathematics—trigonometry functions, for example—was handled by a PC because the PLCs at that time did not have the math co-processing chips. Also, PCs were networkable and could handle a lot of database applications.

However, PCs were notorious for failing in industrial conditions because their hardware wasn’t designed to handle heat, dust and vibration. Also, you were required to have specialized programming skills that the run-of-the-mill industrial electrician/technician did not possess, so if there was a breakdown that required troubleshooting beyond checking the I/O, it often required bringing in outside resources to fix the problem—good for OEMs and integrators; bad for production people.

PLCs were made specifically for industrial control applications. The hardware was ruggedly designed, and the programming wasn’t complicated as it was based on the electrical control wiring ladder diagram.

Joe the Maintenance Electrician easily could learn to program the PLC and then use it as a troubleshooting tool. Curiously, it was about this time that the maintenance mechanic’s life got a little harder. He no longer could just look at a machine for two seconds, declare it to be an electrical problem and walk away to drink coffee and play cards, while the electrician was left to prove that the solenoid valve or cylinder seals were bad.

In the past decade though, PLCs evolved such that they can perform the same functions as the PCs, and the PCs have become more rugged. The advent of soft logic-type programming for PCs virtually eliminated the need for specialized custom programming for most industrial control applications. Both the PC and the PLC CPU still require an I/O interface of some type, whether a PC chassis or PLC rack.

The Logical Platform

The PLC has moved well beyond its original discrete control function to become a multi-discipline platform for plant-wide automation. Mogan Swamy reports.

Customers increasingly need to make good business decisions based on information obtained from the plant floor. But the devices or methods used to enable this must be at an optimal cost, with the minimum ofdisruption in its execution or implementation.

Interestingly, these were the exact driving forces that gave birth to the programmable logic controller (PLC): it was envisioned as a controller to replace troublesome relay panels, to replace costly minicomputers and reduce programming time for various machine tool applications. In its conception, overall, it provided a simpler interface betweencomputers and machines at a relatively low cost.

The concepts upon which the PLC’s development was based, has not diminished, but has been further heightened with a need to define a vision for the factory of the future. This demands the provision of an architectural roadmap, based on hardware and software. Conceptually, the PLC design should depend on prevailing business drivers and emerging technologies and all evidence suggests that the PLC has been up to the task, without slacking on the goalsfor which it was originally envisioned.

In this sense, the PLC continues to play an important role in integrated automation. Theintegration of a PLC into the factory floor makes it not just a single controller, but an extension of thewhole enterprise computer system.

In the current context, it would most likely to be part of the technology mix in more collaborative discrete control systems, designed and built for a distributed manufacturing process and supply chain, but sensitive to a demand-driven market, and the need for real-time collaboration and response acrossthe manufacturing enterprise.

The fact is the PLC has become the workhorse of factory automation. Other technologies such as CNC, motor drives, motion control, robotics, automatic ID systems, and vision systems are now factoryfunctional because they are hitched to a PLC system. This implies that information from all these other domains are the engine that runs the production line, with the PLC taking an event-driven approach that allows for optimization of the production processes,by providing access to real-time events.

Event-driven information, potentially, can be the cornerstone of an effective production-to-business strategy. As this information is captured, as it occurs, it can be moved to enhance production management, improve manufacturing process visibility and streamline supply chain applications. But this can only be effective if the information is shared across theproduct lifecycle and the manufacturing enterprise.

The trend towards a unified and flattened, tiered, and a hierarchical discrete manufacturing environment, has helped the PLC define its own relationship between the different domains in the manufacturing environment, and move towards aproduction-to-business based architecture.This has resulted in PLC trends towards increased communication capability, smaller sizes, better software and implementation tools, and diagnostics. In addition, these developments have tried to address customer demands for open standards, multi-control disciplines, modular architecture, and comprehensiveautomation solutions software.

Quick & Simple PLC Programming

THE use of function blocks for programming of programmable logic controllers (PLCs) is gaining wider acceptance. Rather than the classic “contact and coil” representation of ladder diagram or relay ladder logic programming, function blocks present a graphical image to the programmer with underlying algorithms already defined. The programmer simply completes needed information within the “block” to complete that phase of the program.

The dual benefits of reduced downtime, and ease of use and training, have quickened the adoption of function blocks and the use of libraries of function block components. The adoption of function blocks has also been encouraged by the development of standards that make their use easier. The Internatiional Electrotechnical Commission’s IEC 61131 makes it easier to use function blocks, though some say the standard has reached the end of its useful life. Many manufacturers have turned to IEC 61499, which builds on its predecessor by providing an open architecture for distributed control systems.

The result is that function blocks are becoming widely used and simple enough for a non-programmer to use. Even so, some detractors claim that function blocks can sometimes slow down production development when the blocks allow developers to move ahead without thinking through the process completely.

Risk-averse

Function block technology actually goes back a few decades, but the process control industry was slow to pick it up because of an initial lack of standards and because the manufacturing industry typically doesn’t move quickly. Unlike most areas of technology, plant automation moves at a slow pace. “The controls industry is very, very risk averse. The attitude is, ‘If it isn’t broken, don’t fix it,’ ” says Ron Bliss, software manager of Logix, for Rockwell Automation Inc., of Milwaukee. “I never got fired for keeping things the way they were.”

Bliss notes that the slow pace of advancement in controls programming matches the long lifetime of plant equipment. “If you put in a piece of production equipment, it will be there for 20 years, and those things around it will be there for a long time as well,” says Bliss. “So when you have something there for a long time and you don’t have large budgets, and you have to train someone on new technology, you’re going to push back against that new technology.”

That attitude began to change as manufacturers faced globalization. With international competition becoming increasingly intense, fewer companies today view the status quo as acceptable. They are becoming interested in achieving “continuous process improvement.” While that’s an overused phrase, the concept does represent a shift in attitude at manufacturers, and that shift has encouraged the adoption of new technology.

The controls industry has also seen a shift from the concept of hard wire to flowcharts. Function blocks have been part of that transition. “One of the reason function blocks have begun to take over is that they replace the relay ladder logic that has been around for 40 years,” says Greg Dixson, product manager for Automation Worx at Phoenix Contact, in Middletown, Pa. Ladder logic mimics hard wire. According to Dixson, ladder logic is being replaced by flow logic. “When we think in yeses and noes, we’re thinking in flowcharts. It’s much easier to understand.” Dixson notes that young programmers take much more quickly to flow logic and that it is much more adaptable to personal computers (PCs).

For many plant operators, the continual pressure to improve operations while reducing downtime is partly solved by the use of function blocks for PLC programming. “Function blocks give you time savings and code portability,” says David Stall, electrical engineer at Grob Systems Inc., in Bluffton, Ohio. “Once you have a working function block and test it on a project, you can take it to the next project.”

While function blocks don’t eliminate the need to write code, they do significantly reduce the need to rewrite code. “Function blocks help because any time I have a piece of code, I throw it into a function block and I only have to write it one time,” says Stall. “Any time I have to interface into another piece of equipment, I do it with the function block.”

In some cases, the function blocks can represent actual pieces of equipment. “With a functional block, you’re encapsulating your process; you’re putting an object wrapper around a whole machine with its inputs and outputs,” says Bob Nelson, manager of PLC and I/O (input/output) products in the Discrete Automation Business at Siemens Energy & Automation Inc., in Norcross, Ga. “You can easily tie together pieces of equipment, and the end user doesn’t need to know what’s happening in the machine.”

That makes integration simple, because the function blocks can be put together like physical equipment. “We have customers who are building code that has a physical relationship to hardware architectures. You have a physical representation and code representation,” says Barney Keeton, business development executive at Schneider Electric, in Palatine, Ill. “You may have an OEM (original equipment manufacturer) who builds conveyors with barcode readers at transfer stations. The company could build a directive function block that matches how it links the conveyers together.” So, when the OEM puts the function blocks in the same connective arrangement as the conveyers, the conveyers are connected by software just as they are physically.

One of the biggest advantages of using function blocks is that they are quick, easy to use, and easy to learn. “The benefits of blocks include a more efficient use of time. They are easy to understand, easy to troubleshoot and easy to train people on,” says Dave Woll, vice president of consulting at ARC Advisory Group Inc., in Dedham, Mass.

Function blocks also appeal because they are stable, they reduce complexity, and they are reusable. As for stability, function blocks do not change significantly. In many cases, programmers can use the same function blocks in a wide range of applications.

Function blocks can greatly reduce complexity. A programmer can work with a function block without understanding how it works internally. Some manufacturers have marveled that you don’t need a programmer to make use of function blocks.

Function blocks are also popular because they are reusable. This attribute comes in handy whether a manufacturer is changing a product or migrating production from one plant to another. Once a function block has been developed and tested, it can become part of the manufacturer’s library of functions. “The directive function blocks are reusable from project to project,” says Schneider’s Keeton.

Legacy integration

A benefit not often mentioned in discussions of function blocks is the ability to integrate legacy equipment. “We’ve used function blocks to integrate legacy products in an open network,” says Joe Biondo, e-business manager at Bosch Rexroth Corp., in Hoffman Estates, Ill. “We build a software library that tells our software how to handle this legacy product.”

Not everyone believes function blocks are the answer to more efficient controls programming. ARC’s Woll notes that function blocks can sometimes create more problems than they solve, by designing automation incorrectly. “The biggest problem process manufacturers in North America have is operational failure. That comes from operators doing the wrong thing, or from automation designed the wrong way,” says Woll. “One thing I’m seeing is a trend toward state logic control and away from block-oriented languages.” He notes that with state logic control, engineers are “forced to give a lot of thought to the process and the control it needs.”

Some companies specifically avoid using function blocks, according to Woll, because it is more precise than using function blocks. “At Dow Chemical, they do their programming using state logic, and Boeing does the same thing,” says Woll. “It’s to ensure precision in the design and operation. It’s been a philosophy they’ve had for a long time.”

Other drawbacks to function blocks include the manual effort involved in copying and pasting code, the difficulty in either copying each tag individually or creating new ones from scratch, the need to rewrite code to utilize new tags, and the need to edit the new tags to contain the appropriate preset values.

Even with these limitations, it’s clear that for most manufacturers, the time-savings that comes from using function blocks far outweighs their drawbacks. The trend is still weighed heavily in favor of adopting function blocks and building libraries of this object-oriented code that can be used and reused with little time and effort.

PROGRAMMABLE LOGIC CONTROLLERS

A programmable logic contoller (PLC) is a solid-state device with digital processing capabilities designed for real-time control of residential applications. A PLC comprises of input and output units (I/O) and a central processing unit (CPU).The input and output components are built into the same physical box with the CPU. The package provides a small, light-weight, low-cost, and self-contained solution for a wide range of control applications.


Input and Output Units (I/O)

Inputs are wired to sensing devices such as button keypads, selector switches, photocells, motion detectors, etc. If an input senses that a sensor is closed, the input converts the voltage to a logic 1 signal understood by the CPU to be ON. A logic 1 signal indicates an On or CLOSED state, and a logic 0 signal indicates an OFF or OPEN state.

Ouputs are wired to switching devices such as lights, garage doors, heating systems, and other household appliances. Outputs switch the supplied control voltage that energizes or de-energizes the switching devices. If an output is turned ON by the CPU, the control voltage is switched to activate the switching device.

Central Processing Unit (CPU)

Within the CPU are the digital processor, memory, and power supply. These components interact to solve application logic and pass control signals to the outputs. The CPU reads the converted input signals, executes the user logic program stored in its memory, then writes the appropriate output signals to the switching devices. The application program is written in a language called ladder logic.


System Block Diagram

The block diagram shows the major components of a PLC. The PLC monitors the state of the sensing devices by receiving signals from its inputs, solves a user logic program stored in its CPU, and then directs switching device activity by sending control signals to its outputs.

The Details of the Safety Standards

While changing safety standards are always part of the industry environment for plant machine manufacturers, the standards that will go into affect for Europe at the end of next year provide a shift in how safety is assessed.

The new standards, ISO 13849-1 and IEC 62061 add a quantitative calculation to machine design. These standards will replace EN 954-1, which became the machine safety standard in Europe and throughout most of the world after being released in 1992.

Many in the industry view 954-1 as overly simplistic, because it does not require the assessment of safety components in relation to time or lifecycle. The new standards require machine builders to add quantitative calculations to the design. This will result in a more methodical assessment of the machine’s performance, reliability and availability.

EN 954-1. European Norm (EN) 954-1—titled “Safety of Machinery, Safety Related Parts of Control Systems”—was developed for safety of plant machinery. The standard has two parts: 1. General principles of design and 2. Validation, testing and fault lists. The standard sets out procedures for the selection and design of safety measures. It also provides a list of typical safety functions such as stops, manual re-sets, starts and re-starts and more. While appropriate for its day, 954-1 is now considered appropriate mostly for low-complexity systems.

ISO 13849-1. This standard developed by the International Organization for Standardization builds on EN 954-1. The standard specifies system reliability based on hardware-oriented structure, calculated mean time to dangerous failure, and diagnostic coverage of the safety function. The standard applies beyond electric and electronic systems to include mechanical, hydraulic and pneumatic parts of the control system.

IEC 62061. The International Electrotechnical Commission developed this standard for “functional safety of safety-related electrical electronic and programmable electronic control systems.” The standard determines the amount of risk that needs to be reduced in a machine in terms of safety integrity levels (SIL). The machine industry uses three SILs that determine the level of risk. The standard sets the SIL levels for the machine system and subsystems, though with flexibility.

Machine Safety Incorporates Relays, PLCs, Risk Assessment and Standards

What’s in your bottle? The SPF of your sunscreen lotion might protect your hide, but achieving appropriate safety integrity levels (SILs) will protect your operators and machines. This is especially true if you precede SILs with a hazard identification and risk assessment (RA) and follow them with performance requirements, consistent implementation, thorough training and continuous revaluation.

If you don’t apply protection, however, you could wake up at the metaphorical beach with a lobster-red sunburn. Likewise, most machine safety programs are inspired by a painful wake-up call. These events can be deadly serious destructions of life, limb and equipment, or only slightly less serious near misses that could turn tragic next time if changes aren’t made.

Enough Was Enough

After several years of poor safety performance, Goodyear Tire & Rubber's plant in Gadsden, Ala., had two major injuries, which occurred when employees were caught in the facility’s let-off shear machinery in 2006. In one event, a machine had been left in automatic mode, and it seized and injured an operator’s hands when he patted down the roll of rubber on it.

“We had a huge need for improved safety,” says Charles Skaggs, Goodyear’s health and safety manager. “We had 300 new staff this year, and for the past four or five years, we’ve had to tell them that Gadsden was at or near the bottom of all Goodyear plants in terms of safety. After 2006, our corporate management said it wasn’t going to put up with these incidents any more and asked us to study ways for our machines to achieve first-class safety ratings.”

Goodyear’s subsequent study included input from the Rubber Manufacturer Assn, which reported that the most dangerous place in the Gadsden facility—and in most tire-making applications—is the wind-up and let-off areas in their fabric bias cutter and sheet calendar machines. RMA recommended that Goodyear focus on improving the wind-up and let-off safety at all its global facilities. Consequently, Goodyear’s management ordered mandatory safety release (MSR) capabilities, so its machines could attain a Level 1 safety rating, and budgeted $3 million for the project.

To help improve the safety of its machines, many of which were very old, Goodyear used a Rockwell Automation modular kit-based solution that could be implemented and reproduced among multiple machines. Goodyear began installing the presence-sensing equipment and light-activated barriers from August to December 2007. These devices prevent the wind-up and let-off machines from running if an operator puts his or her hand in it. The kit also includes new e-stop equipment, replacing the former safety cables and belly bars, as well as new safety interlocks and fencing.

“Because the kits are modular, we could implement them in 67 wind-up/let-off applications in 20 weeks,” says Skaggs. “The kits were so successful that Goodyear plans to spread them across all of our plants.” Besides completing its MSR project on time, Skaggs reports that Gadsden improved its safety performance and record by 61% in the approximately 12 months that it’s been in place. The plant had 34 fewer OSHA-reportable incidents during the same period, and its safety project also reduced downtime by 34%. The $885,000 worth of safety equipment that Goodyear has installed so far paid for itself in just four months. More specifically, Gadsden’s OSHA-reportable incidents dropped from 148 in 2004 to just 29 in 2007 and 27 as of October 2008.

Skaggs believes the main requirements for a successful safety improvement project include 100% commitment from management, sufficient training and awareness, thorough understanding of the production system and coming to realize that safety is not a technology problem, but is about educating people and overcoming traditional resistance to change.

To further encourage and ensure safety, last year Goodyear started a rapid improvement activity (RIA) program, in which company participants spend one day in a safety class and then go through their facilities and applications, seek out safety-related items that need to be improved and try to complete 80% of those fixes in three days. So far, Skaggs says Goodyear’s employees have found 262 items and have improved 219 of them.

“After doing so badly in 2003-04, Gadsden’s plant management and Goodyear’s corporate management said we just had to do safety differently,” adds Skaggs. “Before that, we just didn’t have enough of a focus on it. So, we revised our whole safety structure and also began to drive more safety responsibility to our safety teams on the plant floors. We also work very closely with our union’s safety representatives, and we have a very good relationship with them because we both have the same goal of no one getting hurt. I think this kind of relationship is something you must have to improve safety and maintain it.”