Safe Decentralization and Enable Principle

As explained already, in many cases safety technology follows the developments made in standard control technology. The benefits from transferring the input/output level to the field via decentralization have resulted in the same process being applied to safety related inputs and outputs. This was followed by the development of a safety bus system, which not only allows field inputs and outputs but also a safety related connection between safety control systems.

The diagram below illustrates a typical application in which the enable principle has been implemented.

Circuit diagram for the enable principle

The safety control system switches the safety-related outputs, and the standard PLC transfers the switch command for the corresponding output to the safety control system via fieldbus.

Essentially it is a really simple principle, if you ignore the disadvantage that the switch command from the standard control system must be considered in the program for the safety control system. Graphically speaking the situation is this: The standard control system must place the switch command on the fieldbus, from where the failsafe control system retrieves it before inserting it into the output's control program as an AND function.

Programming becomes unclear, because the control task and safety function are mixed within the safety control system. A further development of the field transfer principle helps to simplify this case.

The diagram below illustrates the extension of the enable principle. The enable for the control command from the standard control system now takes place directly at input/output level. Handling is simplified tremendously as a result; both control systems can be programmed and tested independently. Performing the enable in the I/O system means there are no delay times from processing within the safety control system, and it's no longer necessary to pass on the control commands via the fieldbus.

Extending the enable principle

Today's safety control systems: Overview of safety control systems

Safety control systems essentially came about because of the desire to connect safety through programming, in a similar way to that of a PLC control system. It's no surprise then, that safety control systems are following the example of the PLC world. Centralized systems came first, followed by decentralized systems in conjunction with safebus systems. Programming followed the same formula, except that the instruction set was drastically reduced from the start to just a few languages, such as IL (Instruction List) or LD (Ladder Logic/Ladder Diagram). These measures were taken for reasons of safety, for the opinion was that limiting the programming
options would minimize the errors made in generating the program. Initial systems clearly focused on processing safety functions. Although even at the start it was possible to program the safety control system for standard automation, in practice this application found very limited use.

Safety-related features aside, there is little to distinguish safety control systems from standard automation control systems in terms of their actual function. Essentially a safety control system consists of two PLC control systems which process the application program in parallel, use the same process I/O image and continuously synchronize themselves. It sounds so simple, but the detail is quite complex: Cross-comparisons, testing of the input/output level, establishing a common, valid result, etc. are all multi-layer processes, which
illustrate the internal complexity of such systems. Ultimately, of course, the user is largely unaware of this; with the exception of some specific features, such as the use of test pulse signals to detect shorts across the contacts, modern systems behave in the same way as other PLC control systems.

Structure of a safe control system:

• Two separate channels
• Diverse structure using different hardware
• Inputs and outputs are constantly tested
• User data is constantly compared
• Voltage and time monitoring functions
• Safe shutdown in the event of error/danger

Tipper Tie Chooses the PNOZ Multi for Safety

As the premier supplier of packaging, clipping, and clip machinery throughout the world, Tipper Tie is committed to sustaining a leading edge position in automation and safety for its customers. Recognizing the importance of safety for both customers and its employees, Tipper Tie has chosen to partner with Pilz Automation Safety, also a world leader in its field, to achieve a high level of safety integrity that Tipper Tie’s customers and employees have come to expect.

The challenges in meeting today’s safety standards are more difficult than ever. And meeting those standards in the most effective and efficient manner can be particularly complex.  Rising to this challenge, Pilz Automation Safety’s PNOZ Multi safety controller is being used and was selected as the optimal safety solution for monitoring devices such as safety gate interlocks and emergency stops buttons.  Pilz Automation Safety’s PNOZ Multi family provides the ultimate in flexibility and simplicity and facilitates a very powerful safety positioning for Tipper Tie’s target markets.

The flexibility and expandability of a programmable safety relay controller, such as the Pilz PNOZ Multi, translates into a common and scalable safety solution that is capable of meeting a variety of safety challenges on any machine design. It also affords a platform on which Tipper Tie could design changes to accommodate changing market requirements.  Moreover, Pilz’s PNOZ Multi features useable diagnostic information which is unmatched by any other product offering.  Thus, Tipper Tie is assured that its customers have the maximum safety flexibility and machine availability.  Lastly, the PNOZ Multi’s ability to communicate over an Ethernet based network is a feature that helps Tipper Tie to distribute diagnostic information from the PNOZ Multi to a variety of HMIs and PLCs.  This capability gives machinery operators the ability to easily determine machine safety status.  It also optimizes troubleshooting processes to minimize engineering and technician resources.

Tipper Tie is the premier packaging, clipping, and clip machinery company throughout the world and it has chosen Pilz’s PNOZ multi as an important safety controller in its machine design.  Attached are two photos highlighting Tipper Tie’s use of this controller in a panel and an HMI image.

Motion Control for Packaging Machines

In today's competitive atmosphere, shorter innovation cycles and flexibility are vital to packaging companies wanting to rise above the competition. We are continually hearing from machine builders and Original Equipment Manufacturers (OEMs) about these challenges and the complexities they bring to machine operator safety and the pressure to do this efficiently.


Further, this demand has resulted in the decrease of batch build and multi-pack runs and now requires machinery to produce multiple packaging variations from the same machine. Adapting to this means the ability to perform quick product changeovers while maintaining a high level of throughput productivity.

Ultimately this impacts the machine operator's ability to perform quickly and safely. This reality has driven the industry to look into alternative solutions. Automation, coupled with motion control is the key to accomplishing this. However, the increase in motion requires a robust safety system to monitor and control these added moving components. For safety processes to run quickly and smoothly systems require multiple axis controllers, additional motion control devices and they must meet industry standards.

When these modified systems are deployed properly you will achieve the required flexibility and observe an increase in the throughput of products. One last issue to accomplishing this is choosing the right safety products to fit system needs.

When looking for motion control products it is important to look at the overall picture. First of all, motion control is needed to manage the movement of any highly dynamic drives and camshafts, synchronization of a wide range of decentralized drives to keep manufacturing safe at a low cost and at a consistent quality. Second, quick setup and maximum flexibility, as well as the ability for automatic adjustment to product variances are essential. Finally, the handling and robotics of a packaging machine is as an integral part of a complete automation solution.

Any and all motion control products must include the following: PLC to control the machine, optimum movement management on decentralized drives for motion control and an individual safety solution customized with integrated software and tools that meet universal, local and federal legislations.

Packaging specific applications and motion control components should also be adaptable for individual requirements. The ability to join together safety and industrial standards is a crucial element in any motion application. Equally imperative, safety must be integrated within the drive to make sure that every law of motion is available, optional real or virtual master axes and all additional tools able to suit the task of a specific packaging machine.

It is important to note, that a control solution should also be platform-independent. Integrated soft PLC in accordance with IEC 61131-2, can fast scan time from < 50us for 1000 instructions and no adjustments are required to change platforms. Thus, all of the automation in one project with support for modular structures, large selection of field buses, extensive libraries, motion control, interpolation and OPC Server, can positively impact both safety and productivity.

In further examining comprehensive motion control functionality, always consider this indispensible checklist to discern the most important features within any motion control product:

• Virtual main shaft and cam synchronization

• Integral "flexible cam"

• Register control

• Web tension control

• Linear and circular interpolation

• Electronic camshaft and safe motion within the drive

When choosing safety products for your packaging machines choose a company that is aligned with your priorities and use the above checklist to ensure you're making the best choice possible.