Three Different Approaches to Safety PLC’s

Streat Automation has completed a number of safety PLC projects using Rockwell, SICK and Siemens PLC hardware.  Each of these suppliers has a different approach to implementing safety in a PLC controller.

Compact GuardLogix

Rockwell offer an ‘integrated’ solution called GuardLogix with both the process software and safety software integrated into a single project.  The safety IO is modular and can be connected to the GuardLogix processor over standard networks like Ethernet or ControlNet.

It’s possible to build a very large and complicated GuardLogix system with thousands of IO.  However the main disadvantage is cost, the ControlLogix version of the safety controller is reasonably expensive.  Rockwell have addressed this by releasing Guardlogix in the CompactLogix range of processors, which Rockwell calls a mid-range control system.

SICK Safety Controller

SICK offer a programmable safety controller called Flexi Soft, it is programmed with their Flexi Soft designer software which is available as a free download from the SICK website.  The SICK Flexi Soft safety controller isn’t a fully functioned PLC but it’s more than a programmable relay.  Programming a Flexi Soft is fairly simple and intuitive, its function block only with a limited range of safety only blocks. It’s possible to build a small to mid-sized system with SICK Flexi Soft, it’s a modular plug together system with a range of processors, communications modules and IO modules.  SICK have also implemented a distributed IO network called Flexi Line.

We’ve used the SICK safety PLC on a number of machine safety upgrade projects where the safety PLC has been retrofitted to the machine in addition to the main process PLC.  SICK Flexi Soft is a good fit for this type of project.

It’s worth mentioning the limitations of the Flexi Soft Ethernet communications module, it can be used for programming and you can read values from it via a PLC message instruction but you can’t talk to it directly from an HMI.

Siemens ET200S

The Siemens ET200S safety PLC is similar to the Rockwell Compact GuardLogix in terms of functionality.  It also offers integration of the process and safety code and has a range of IO modules available that can connect to the processor via PROFINET (industrial Ethernet).  The ET200S is programmed using Step 7 and is a very cost effective safety PLC solution.

If you want to know more about safety PLC’s please feel free to contact us.