Build a safer machine block by block

The concept of safety implementation in function blocks isn't new, but according to industry writer and CONTROL DESIGN contributor Mark Lamendola, its actual use in machine automation is hard to find.

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Mark Lamendola

the sheer number of safety-related standards makes it difficult for an industrial OEM to build a machine that will comply with all that seem to apply to it. One way to make compliance easier is to address safety from the very beginning of the development cycle, and programming with function blocks might become an important part of the solution. There’s a good case to be made that function block implementation advantages include ease of design, implementation cost-savings, operational cost-savings, and performance advantages. These are compelling motivations for any machine builder.

In many ways, a function block (FB) drawing (See Figure 1) resembles an integrated circuit (IC). FBs essentially are reusable software ICs. We implement the same logic in FB configurations that we would if we had used hardware. We can tap into libraries of standard FBs, which have been tested and verified, to create new applications. This has naturally led designers to explore safety implementation in FBs.

Champion the Cause
The use of function blocks for safety application is something PLCopen is working on. PLCopen (www.plcopen.org), Zaltbommel, The Netherlands, is a worldwide, vendor and product-independent association and says its mission is “to be the leading association resolving topics related to control programming that supports the use of international standards.”

FIGURE 1: FUNCTIONALITY FEEDBACK         
         
With functin block programming in programmable safety controllers, a machine user can figure out why the machine won't start. If the left-hand rear guard door switch is not closed, a programmable controller can tell you, but a safely relay can't. 

       
A PLCopen task force is working on a function block library for safety-critical environments. The group’s goals include providing an easy-to-use interface to the safety functionality and reusability in a wide range of applications. It also seeks acceptance and/or approval of functionality and related concepts by certification bodies, thus providing the basis for certifiable FBs. It wants to provide a common basis, terminology, references, and “style” guide for FBs. If successful, it is likely these efforts will speed certification and reduce mistakes and costs.

Progress is being made. “PLCopen Technical Committee Two has identified 14 safety functions, which are represented by 14 FBs,” says Eelco van der Wal, managing director of PLCopen. “These include ‘Mode Selector,’ ‘Safety Door Monitoring,’ and ‘Two-Hand Control’.” PLCopen also identified the four operating modes as manual, automatic, maintenance and semi-automatic.

Why Use Function Blocks?
Metso (www.metso.com), Helsinki, Finland, is a global supplier of process industry machinery, systems and aftermarket services. Metso serves customers primarily in the fiber and paper technology, rock and minerals processing, and automation and control technology industries. “Safety FBs would help standardize the implementation of safety functionality in a program,” says Bob Bettendorf, controls engineer and technical documentation manager at Metso. “This would make programs easier to write and easier to troubleshoot because there would be less variation from programmer to programmer.”

It’s important then to start with safety in mind. “Safety often is included at the end, and not integrated into the system philosophy,” says, van der Wal. “This does not contribute to the overall safety aspects,” he says, but that is changing. “A parallel can be drawn in the movement from hard-wired relays to PLCs,” explains van der Wal.

To others this evolutionary path looks like it is headed in the right direction as well. “The logical progression for machine code implementation—safety or otherwise—is FBs,” says Joe Biondo, manager, market planning for PLCopen member Bosch Rexroth (www.boschrexroth.com). “The stability and compact nature of the FBs will make usage more widespread.”

In the FB environment, a user can provide error detection, damage assessment and confinement, error recovery, fault treatment and continued system service. Function Blocks make it possible to test safety equipment remotely, without impacting productivity. In addition, the need for manual testing should drop dramatically — as do safety risks to maintenance personnel. Function Block implementation eliminates duplicate training, maintenance, and inventory costs because an industrial OEM can use the same control device for both basic machine control and safety systems. Essentially, it takes much of the safety monitoring and control out of the hardware and puts it into software.

Safety implementation in FBs isn’t new. Designers have been using it for years. But advances in PLC design, control device intelligence and digital communications have made it increasingly attractive to implement. Object-oriented programming of FBs plays a key role. Today, you can use drag-and-drop techniques to do in minutes what took hours creating pages of coding.

“FBs tend to have parameters that must be defined and programmed for the intended function,” says Jeff Gellendin, product marketing manager, SafetyPLCs, Rockwell Automation (www.rockwellautomation.com), a PLCopen member. “One advantage of function block programming is that the ‘wiring’ of these parameters is visually obvious and easy to follow, as opposed to ladder logic programming, where the parameter bits must appear on several rungs of logic.”

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