Build a safer machine block by block

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.”Beckhoff (www.beckhoff.com), another PLCopen member, implements open automation systems based on PC-compatible control technology. Gerd Hoppe, a member of Beckhoff’s management team, says efforts to implement safety on a machine or in a plant will move “from pure electric solutions to those using a mix of electric, network, and software means. This allows users to achieve the same or higher standards in safety, but with less effort, simpler structures and more flexibility.” He argues that traditional safety installations easily exceed 10-15% of the total electric machine equipment, and when installed as a separate system they add significantly to the complexity of installations. “Simplifying the architecture and the engineering of automation systems positively influences the manufacturing industry,” says Hoppe.Some machine builders agree. “I see several justifications for using a safety PLC,” says Jason Christopher, controls engineer with Liberty Precision Industries (www.libertypi.com), Rochester, N.Y. Liberty builds and commissions machining systems to a wide variety of industries (See Figure 2).       

FIGURE 2: A SOFTWARE IC 

       

In many ways, a function block (FB)resembles an integrated circuit (IC). Function blocks essentially are reusable software ICs in that we implement the same logic in FB configurations that we would if we used hardware. 

Christopher has reservations about programmable safety systems, but concedes that, “First, you have the increased functionality of safety circuits — no longer just monitoring the on/off state of guards, but also limiting speed or range of motion. Second, you have ease of implementation. It’s easier and cheaper to program [software] than [it is] to create hardwired solutions.” Christopher adds that there is also the advantage of diagnostics. “OEM customers have to be able to figure out why the machine won't start,” he says.” If the left-hand rear guard door switch is not closed, a PLC can tell you, but a safety relay can't.”But, how do you get these FBs to attain the higher levels of integrity that safety demands? “At the safety level, the software tool should help the user as much as possible, says van der Wal. "We have a newly defined data type, called SafeBool, which refers to a safe Boolean signal in the system that can act as input and output. This provides the development tool to identify safety-critical program parts and guide the user with the allowed connections, while inhibiting faulty connections. In this way, support for the different levels of the safety standards can be implemented.”Standardize or Fail
Standardization is another key to this whole process. Some people argue that standardizing limits engineers from employing superior methods as needed, but in reality, that argument doesn’t hold up.“Standardization is essential in this field,” says Hoppe. “If screws could tighten by rotating randomly right or left, how would mechanics [know which way to] tighten a screw? The solution was the standardization of the screw thread. In the same way, safety functionality and field use benefit dramatically from standardization.”Biondo agrees. “Standardization will help the technology supplier, the machine builder and the end user,” he says. “Although machine builders and technology suppliers can implement their own safety FBs, standardization will improve re-sales by the end-user — and international sales in all respects.”It’s important to keep the role of standardization in proper perspective, however. “The standardization effort on function blocks will not drive the usage of safety PLCs,” argues Thomas Pilz, CEO, Pilz Automation Safety (www.pilz.com). “What will drive it is the cost-saving potential that safety PLCs and safety networks offer in medium to large automation applications.”So it seems users aren’t going to adopt safety FBs because of standardization. Standardization is not a product benefit for the end user. Standardization lowers costs, and that primarily is what the user sees as the benefit.But, this can lead to a “chicken-or-egg” argument that goes something like this: Until enough people implement safety in function blocks, there’s little economic justification to support standardization efforts. But without standardization, people won’t implement the technology. This argument, however, ignores the larger picture.“I'm not sure the need to standardize safety function blocks is [either] greater or less than the need to standardize ‘standard’ PLC function blocks,” says Gellendin, referring to end users who deploy PLCs from several vendors in their plants. He says standardized FBs would allow these facilities to have identical programs for each brand of PLC.Standardizing for safety purposes is only one part of a larger movement toward standardization. PLCopen is creating standards across the board, not just in safety. There is no reason to leave safety out of the standardization process because including it will bring real benefits.Machine builders tend to view standardization as a good reason for the implementation. “There is the risk of confusion for the programmer or technician when they switch between different manufacturers using proprietary function blocks,” says Christopher. “Having different approaches to safety is risky business.”Community Concerns
There’s plenty of opinion that says PLCopen has to do more than just agree on methods of implementation. They also need to address concerns that currently block acceptance. “From what I have seen thus far, there [will have to be] a lot more ‘real’ engineering involved to get me comfortable with a programmable safety system,” says Christopher. “With safety relays, the assumption is that they are going to fail in a safe state as a single-point failure. With a solid-state system, the failure mode is unknown, so you really have to know what you are doing and know all of the relevant product data to feel comfortable that you are creating a safe installation.”Ladder logic is simple and cost-effective; that’s why it’s so common in control systems. “Ladder logic is not going to go away, because there are so many good reasons to use it,” says Christopher. With this in mind, Bettendorf says, “I would recommend that the task force consider the structure of the safety FBs so the same functionality can be implemented in ladder logic and structured text.” Fortunately, PLCopen addresses these concerns with IEC 61131-3, the only global standard for industrial control programming. The standard includes the definition of the Sequential Function Chart (SFC) language, which is used to structure the internal organization of a program, and four interoperable programming languages: Instruction List (IL), Ladder Diagram (LD), Function Block Diagram (FBD) and Structured Text (ST).Yet, there are some other questions that PLCopen must address before their work is done. "How will safety FBs affect legal liability?” asks Bettendorf. ”Will they lessen or increase the machine manufacturers’ liability? Will they lessen or increase the PLC manufacturers’ liability? Will the standards group have any liability just because they developed the standard?”PLCopen says it is working within the community to address such concerns, but it’s a two-way street. The community also needs to work with PLCopen to educate each other and end users. “Beckhoff is a member of PLCopen, so I am not neutral about this,” says Hoppe. “However, PLCopen has developed very good skills to define a standard. I recommend that end users build their understanding and get excited about it. This technology opens doors to new, flexible and safe protection systems that allow for operational improvements.” Hoppe exhorts manufacturers to communicate the end user value rather than focusing on the cost saving nature of this technology.More Standards Wars Ahead?
Should machine builders be concerned that PLCopen will simply deliver yet another standard that will fight with other standards for dominance? And if so, how do they prepare for what happens when the dust settles?“Actually, there are no other standards — just proprietary solutions,” says van der Wal. “And they can be very expensive, since they are normally stand alone, and not integrated into the same software and hardware platform. Biondo agrees, saying, “The IEC61131-3 standard is the most accepted by the automation community. The only other ‘standards’ I'm aware of in the discrete industry are brand-specific.”So, where does this leave machine builders and their customers? “Today’s choices for safety add additional systems to the core machine installation,” says Hoppe. Integrated safety would represent a huge step forward .The installation and programming would have a simpler architecture with more flexibility, at the same safety levels. PLCopen represents IEC61131-3 as a user group, promoting the only vendor-independent worldwide-accepted programming standard for PLCs and automation controllers. All international automation providers have more or less implemented IEC61131.”

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Mark Lamendola is a freelance writer and frequent contributor to CONTROL DESIGN with many years of experience working in and writing about industrial automation issues. You can reach Mark at [email protected].