As safety control systems become more common in machine/industrial environments, different solutions are becoming available. This allows for more precise safety control systems in specific applications, but it also makes selecting a safety control system more difficult. At issue are an understanding of what architectures are available, what applications best suit those architectures, and what costs are involved. This article explores each of these criteria to help the reader make more informed decisions when designing a safety control system.
A variety of architectures are available with which to build an appropriate safety control system:
* Component systems
The most basic system consists of a control device and an actuating device (example: an e-stop button that opens a safety relay coil). These systems offer the most rudimentary functionality and are the most cost-effective solutions for low-risk applications.
* Dedicated safety relay systems
So called because they are hardwired, application-specific controls, dedicated safety relay systems are designed to interface with particular safety components. Some are intended for use with e-stops, for example, while others are created to work specifically with light curtains.
Most dedicated safety relay systems perform simple logic functions, like timing and muting, while monitoring the performance of the safety system. They provide basic diagnostics via front panel LEDs and auxiliary contacts connected to a PLC or indicator lamp. Dedicated relay systems typically offer a very small number of I/O points - all of them digital - and are ideal for medium- to high-risk safety applications.
* Expandable safety relay systems
This relatively new architecture helps bridge the gap between dedicated safety relay systems and safety PLCs by offering plug and play expandable digital I/O connected to a single base relay module for additional flexibility. Because they are microprocessor based, they also boast enhanced diagnostics and communication functionality. This means that they can deliver output and error status over a fieldbus network to an operator panel or other device.
As with dedicated systems, expandable safety relay systems are used in medium to high-risk safety applications.
* Safety PLCs
Safety PLCs offer additional benefits to a safety system architecture. Perhaps one of the biggest benefits is the flexibility afforded by the safety PLC's inherent programmability. Modifying an application is as easy as changing the program on one's laptop and uploading it to the controller. Safety PLCs offer the highest I/O count of all - up to several hundred digital I/O points, if necessary. They are also the only solution if analog I/O is required as part of the system. Safety PLCs are a cost-effective solution for systems requiring large numbers of I/O points, whether they be centralised or distributed (fully distributed safety systems are achievable today through the use of safety networks). Safety PLCs can be applied in a variety of ways, but are most often used in complex, high-risk applications.
* Safety networks
These specialised industrial networks are highly scalable, and allow for high-speed, high response safety cells to be connected via a high-speed backbone such as Ethernet/IP Safety.
Selecting the right system for the application
Regardless of the size and complexity of the application, it is important to perform a thorough evaluation before settling on a safety control architecture. Very rarely is there only one system that suits an application, but through some investigation, the field of options can be narrowed significantly. Ultimately, some safety control systems are not suited for certain applications, while others can address a broader range.
For example, if a safety system only requires an e-stop and a safety gate on a single machine, then most likely any safety relay system will do. When the number of I/O points is not yet established or will be increased in the foreseeable future, an expandable relay system is well suited, as input and output modules can be easily added. However, when functionality such as muting control, two-hand control or timed outputs is required, expandable relays are not a viable option because they do not currently offer such functionality. As a result, one is left to choose between dedicated safety relays and safety PLC systems.
Sequential shutdown applications, which require stopping a machine or process in a series of steps over a predetermined period of time, can likewise force a choice between safety relays and safety PLCs. Safety relay systems, for instance, can be well suited to single or two-step sequential shutdown applications, such as those in which access to a hazard must be prevented using a guard locking device controlled by a dedicated, timed-delay safety relay. However, for multistep shutdown or ramp-down sequences like transfer line applications, in which material is queued up before a cell is shut down, safety PLCs offer significant benefits due to the fact that they can provide the necessary logic through software, as opposed to the hard-wired logic of relays.
In fact, any application requiring advanced logic is best solved with a safety PLC. For instance, zone control - the ability to shut down part of an application while other parts continue to operate - requires AND/OR logic. AND/OR logic is difficult to implement with relays, due to the hard wiring that is necessary to create that logic. Safety PLCs, however, can be easily programmed to achieve AND/OR logic, thus saving time in configuring the logic while reducing wiring errors and conserving panel space.
Does the application have specific communication requirements? Dedicated safety relays can deliver basic status information via front panel LEDs or by auxiliary contacts connected to indicator lights, which may be all that is needed for a basic safety system. More complex applications requiring more advanced diagnostics may be better served by expandable safety relay systems, which have the flexibility to send input, output, monitoring and error status information over a fieldbus network (such as DeviceNet) to an operator interface device. As one might expect, safety PLCs offer the highest degree of communication functionality, producing diagnostic status, controller status and communication status information. Additionally, they boast two-way communication capability (peer-to-peer) and can accept information from other devices over their communication network.
Understanding cost
Because the ultimate goal is getting as much value from the safety system as possible, one must understand the impact each solution will have on the overall cost of the project.
First, consider the up-front costs. Costs will typically increase as one moves from component systems to safety PLC architectures. For example, dedicated and expandable safety relay systems usually cost only as much as the purchase of equipment plus the personnel costs to hard-wire the system and create the necessary safety logic. Safety PLC systems, though, can be more expensive at the outset because in addition to equipment costs, there is also the cost of programming software to consider, as well as the costs of training personnel to use that software and program the PLC correctly. Initial costs are only one consideration, however. Selecting an apparently inexpensive solution can cost more later on if it inhibits one's future plans. Here are some questions to ask when evaluating the safety system for the long term:
Will I have to expand the system?
Expansion is likely; it may ultimately be more cost-effective to invest in an expandable relay system for the plug-and-play attributes, or in a safety PLC system that provides for easy expansion via a safety network or additional I/O cards.
If changes to program logic are anticipated due to alterations in the process or expansion of the system, the programming capability of a safety PLC would lead to optimal cost-effectiveness and flexibility. Program modifications can be done while the system continues to run (no downtime), and take only a fraction of the time needed to make changes to a hard-wired relay system.
What is my I/O count and where are my I/O points located?
I/O count and location play a major role in the selection of a safety control architecture. The modular nature of expandable relay systems allows the user to selectively add inputs and outputs to create a cost-effective solution sized specifically for the application. Systems with higher I/O counts and those requiring communication between the I/O for safety control purposes are best addressed with networked or distributed safety PLC architecture. These architectures can also be the most cost-effective in terms of wiring and conservation of panel space.
Do I need zone control?
Will it be desirable to shut down only portions of the line in the event of an emergency or during maintenance, leaving the rest of the line operational and productive? While zone control can be accomplished with a relay-based system, the number of relays and added hardwiring required to implement the logic would be cumbersome. Here the programmability of a safety PLC is particularly advantageous and is one's best solution for reducing downtime and costs in zone control applications.
What impact does downtime have to my bottom line?
If it is crucial to minimise downtime, it makes sense to invest in a system that is as easy as possible to troubleshoot. Expandable relay systems can communicate some diagnostic information to an operator interface panel, with safety PLCs offering the most comprehensive diagnostic functionality available. Both solutions allow for remote troubleshooting, and remote HMI devices can be incorporated and programmed to assist operators in gathering information and troubleshooting the system more efficiently.
In conclusion
Selection of a safety control system can be approached from many different angles and with many different solutions. Available architectures, application requirements and cost constraints all play a role in the decision-making process. It is not mandatory, though, to settle on a single architecture for an entire application. The most efficient approach in some cases will be the hybrid - a solution that blends the best of several safety architectures. In fact, it is often difficult to choose a single architecture - exclusive of all others - that will meet all application requirements.
The key to achieving success in developing a safety control system is in understanding all of the variables and making decisions that maximise functionality while reducing long term costs.
For more information contact, Jeff Sandison, Rockwell Automation, 011 654 9700, [email protected], www.rockwellautomation.co.za
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