Siemens vs. Rockwell…from a Safety Perspective

Apples_iStock_000010657118SmallMore than hardware or software, it’s their approach to safety and redundancy that differentiates vendors’ from protocols, controllers, I/O and networks.

Choosing an industrial equipment supplier is a complex decision, one that can have implications for years. It’s important to have an understanding of the many tradeoffs that come with this selection process.

The criterion must cover the myriad aspects of a total solution: global support can be as important as hardware and software concerns. Each of these three categories has layers of considerations. For many companies, Siemens and Rockwell are the two primary choices in this selection process.

Based on U.S. customer feedback, Siemens is ahead on the hardware side and Rockwell Automation has lead on the software side.

Many properties must be examined when product lines are being compared. In recent years, regulators have made safety an important element in the selection process. In 2008, Rockwell ramped up its focus on safety with Common Industrial Protocol (CIP) protocol compatibility.

Currently, software V20 version is shipping for Rockwell’s GuardLogix line. It communicates with GLX controllers and other Ethernet/IP devices via unicast, transmitting safety I/O using DeviceNet.

Rockwell’s safety – line includes modular safety relays. The next step up is the Smart Guard 600, which uses only DeviceNet. Rockwell does not have local safe I/O with GLX or cGLX, so all safety communications must be distributed. Drives and safety PLCs remain separate entities, Safety PLCs with CIP can support up to 64 CIP connections with CIP safety support, using produce/consume to talk to safety I/O. Transferring data from one processor to the next requires creating a produce/consume tag every time there is a connection.

The Siemens SIMATIC line has offered PROFIsafe since 1999, enough development time to result in safety solutions such as PC-based safety, wireless safety and ET200 iSP fail-safe , to name a few. The wireless capabilities have become increasingly important in recent years as tablets and smart phones make their way into facilities that also use wireless to put sensors and other gear in hard-to-reach locations.

The backbone of safety-oriented SIMATIC controllers are standard controllers and standard networks with safety built into the firmware. Safety signals and control communications both travel over the same networks, eliminating the need for dedicated networks. The operating system and hardware components have been extended by various protection mechanisms allowing the user to mix and match standard and safety functionalities in one system.

I/O is another important factor for most industrial environments. Rockwell’s Guard I/O detects failures at the I/O and field device level, while helping enhance operator protection. Compact Block guard I/O comes in three flavors: a relay out, input/output card and an input-only card.

Rockwell’s CGLX I/O modules have no onboard communications, users must buy Ethernet-IP scanners separately. They are limited to 64 connections. Each safety module takes one connection, and some safety output modules take up two connections.

Siemens provides flexibility with the ET 200 distributed I/O line. A modular concept lets customers build configurations that meet their current needs while simplifying expansion as needs change. PROFINET connections are built in and also a range of Profibus and Profinet cards can be added quickly for further expansion as required, minimizing downtime.

Fail Safe Operations

The global focus on safety also brings redundancy into more system designs. Many functional safety requirements call for eliminating single-point failures. Duplicating key elements is an easy way to avoid problems when a component fails.

SIMATIC controllers also make it simple to add redundancy to ensure unexpected downtime is minimized. Siemens also provides redundancy support when new versions of its software ship. That’s a contrast to Rockwell, which typically does not add redundant capabilities with initial software revs, instead waiting a few months before adding that feature.

Testing is another critical aspect for safe operations. Siemens lets plant operators test standard and safety software with the PLCSIM simulator tool. It can handle two runtime groups, so one cycle time can run at different scan time than the other cycle time. For example, an application with100 door switches can run at 30 ms scan time, while a second with two sets of light curtains can run at 10ms scan time.

Rockwell has only instance data for add on instructions, so using AOI makes the safety program cumbersome. There’s no dedicated Rockwell simulation tool. Users must buy the Emulation RSLogix Emulate 5000 module, which requires a chassis monitor, test stand and Linx Lite for communication.

Ease of configuration is another difference between the two companies/ architectures. Siemens has safety signatures for all safety FBs. They are safety certified for more precise safety traceability. Newer modules have no DIP switch settings, which help simplify module replacement, increasing uptime.

For Rockwell when a project is created, a safety signature is created for the initial download. If another project is created or a module is changed, the processor’s safety signature has to be changed to add that module. Users can’t just swap the module out to get running again. With Rockwell systems, standard tags have to be mapped to safety tags to permit use with safety equipment.

Many other factors will come into play when companies pick their equipment supplier. The difficulty of configuring and revising systems are important considerations, as is the ability to simulate elements before they’re implemented. It’s important for companies to have a good understanding of the available offerings and support services before making decisions that will determine much of the corporation’s strategic focus for years.

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