Safety PLCs Provide Space Savings For OEMs

With a growing focus on safety and an increasing number of safety products installed on machinery to protect personnel, end users are finding a greater number of safety relays in their control panels. There is a great desire to reduce panel space and wiring, improve communications and increase the automation of all control systems — including safety. This has piqued the interest in safety programmable logic controllers (PLCs) in safety-related systems.

Safety PLCs provide all of the same functionality of traditional safety relays, but offer space savings and improved communications, while also providing the safety levels needed for the protection of personnel. Used primarily in large systems, safety PLCs can provide a greater concentration of safety I/O in a smaller footprint than safety relays, saving control panel space and related interwiring. All of the functionality of safety relays, from emergency stops to light curtains to zero speed control, are provided in safety-certified function blocks. While safety relays are typically rated up to category 4 per EN/ISO 954-1 only, safety PLCs generally include this rating, along with ratings up to performance level “e” per EN/ISO 13849-1 and SIL 3 per IEC 61508. These ratings will allow safety PLCs to be used in most safety circuits.

A variety of communications options are available with safety PLCs. Some communicate safety-related information via the backplane of the PLC rack and through the cables connecting the various PLC racks, but external communications are typically not safety rated. Others provide safety communications only between the safety PLC processor and remote I/O via a certified safety communications network, and external communications are also non-safety rated. Still others have communications networks that can carry safety and non-safety information on the same cable at the same time. The latter systems can either be used for safety only, non-safety only, or a combination of safety and non-safety-rated communications simultaneously. This allows the user to choose between using one network for both safety and standard control system communications, or separate networks for safety and control running independently from each other – in short, whichever method is the best fit for their application. All safety PLCs have communications networks available that are not rated for safety but are used for non-safety-rated communications such as diagnostics, allowing them to communicate to other standard PLCs in the system.

This flexibility is important, as many times a user will want to upgrade their safety systems but not disturb the existing control system which is running well. The control system may or may not communicate seamlessly with the safety communications of the safety PLC chosen for the upgrade. They may want to choose a safety PLC that can run an independent safety network amongst all of the safety components and then communicate the data and diagnostics separately to the system to keep the two systems separate. All of these networks can allow improved communications between the safety and control systems, as well as to other supervisory controls. Improved communications along with advanced diagnostics make these safety PLC systems easier to troubleshoot and monitor.

The safety PLCs’ software provides users opportunities as well. Some safety PLCs utilize the same software to program the standard control system as well as the safety-related portions of the control system. Users can appreciate the convenience of being able to program all of the control with the same programming language and software, as there is no new software for technicians to master. The same programming also allows the embedding of safety-related functionality into the rest of the automation and control system. The user does, however, need to make sure the hardware is “non-interfering” and does not have any negative impact on the safety-related components and instructions.

Some safety PLCs have programming software that operates separately from the rest of the standard control system, and for many users and OEMs, this is the preferred method for the safety system. They want to minimize interactivity between the safety system and the rest of the control system, and also want to make sure that if someone gains access to the standard control system and is able to make changes to it, they will be unable to make any changes to the safety system. Once they have designed the safety system, they feel there should not be any changes made to it, and a separate software system with a different software package helps ensure this.

While many are interested in safety automation and safety PLCs, implementing them can present challenges. Some small- to medium-sized systems may have safety circuits that are not large enough to justify a full safety PLC due to the number of I/O and installed cost – for these systems a safety relay or safety controller may be the most appropriate, cost-effective solution. For others, the standard control system is running well, and they cannot justify the cost of replacing all of the existing control with a safety PLC solution that integrates safety and control with the same software package and communications network. The solution here is a separate safety PLC system just for the safety function. Still, there are other PLC automation systems that can run standard control, but to which the safety component can be added at a later time while using the same software and some of the same hardware.

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.