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Pepperl+Fuchs Blog

The Pros and Cons of AS-Interface Safety Devices

Posted by Helge Hornis on Thu, Aug 02, 2012

Considering all the issues

as interface safety at work resized 600I get this question a lot from users of AS-Interface safety at work: When should I use a Safety at Work device with integrated AS-Interface connection vs. a conventional safety device connected to a safe coupling module?

If you are not familiar with AS-Interface Safety at Work, perhaps a short introduction will help.  The idea behind this technology is to utilize the installation flexibility and simplicity of AS-Interface in conjunction with safety devices.  Instead of hardwiring e-stops, gate switches, and light curtains to safety relays, it is now possible to connect them to AS-Interface and then transmit their data safely to a safety controller (a.k.a. the SafetyMonitor).  The image below shows such a setup.

Connect safety devices to AS-Interface

 

Since safety devices, being an integral part to keeping operators safe, must be constructed to always fail safe they must work differently than non-safe products.  In this case, safety is accomplished by transmitting dynamic safety code sequences.  With an e-stop, the presence of this safety code sequence indicates that the e-stop is in the released state (i.e., pulled out) whereas its absence means that the e-stop is in the safe state (i.e., pushed in.)

In order for such safe hardware to be compatible with AS-Interface, certain rules must be met.  For instance, the number of data bits transmitted must be the same as in the case of their non-safe brethren. It is also necessary for such devices to operate on the two-conductor, AS-Interface wire.  And users certainly expect this technology to be equally as easy as non-safe AS-Interface.

One way of setting up such a system is to connect the dry-contact safe outputs from the e-stop or door switch (or any other safety device) to safe inputs on a coupling module.  Another way is to transfer these AS-Interface electronics into the safety device itself.  Both solutions accomplish exactly the same thing and can result in exactly the same level of safety.

Getting back to the original question of whether to use a Safety at Work intelligent device or a conventional safety device with a safe coupling module, there unfortunately is no clear-cut answer, you have to look at the application.  Personal preferences will also factor in.  Allow me to list pros and cons of either approach.

Integrated safety devices -- the CONS

  • The market for these single-purpose devices is limited, and that makes them costly because the usage is lower
  • Only a small number of manufacturers offer such devices, limiting the user's options.  This is especially obvious when looking at e-stops

Integrated safety devices -- the PROS

  • Installation simplicity and speed are second to none.  Most typically, these intelligent devices come with an M12 network connection.  Combined with a passive flat cable to M12 adapter, the electrical installation takes perhaps 20 seconds and is error free
  • By moving the electronics into the safety device itself, it is also easy to take advantage of the output bits transmitted through AS-Interface.  Controlling illumination (e-stop), solenoids (locking door switches) or a number of diagnostic LEDs does not increase installation complexity one bit
  • Space may be a minor concern, but since all the electronics are mounted inside the device, only the tiny passive splitter needs to be added

Safe coupling modules and conventional safety devices -- the CONS

  • While the simplicity of AS-Interface is still a given, the user must now terminate wires coming from the safety device
  • In addition to mounting the safety device, the safe coupling module must also be installed
  • Making use of conventional AS-Interface outputs is still possible, but adds extra leads

Safe coupling modules and conventional safety devices -- the PROS

  • Cost is a powerful driver when it comes to making decisions.  Conventional safety devices are available from a large number of suppliers, offering many different levels of quality.  Consequently, they tend to be priced attractively.  Similarly, safe coupling modules, not being single purpose, are used in much larger quantities further reducing their street price
  • Installation limitations are now being addressed with newer developments.  While safe coupling modules of the past were always based on available module housings originally designed to address the need to connected 4 or even 8 I/O devices, there are a few developments that utilize optimized, smaller housings
  • Machine builders like using the same part over and over because familiarity reduces installation time and eliminates errors.  Combining a safe coupling module with conventional safety devices helps them do that.  As a consequence, the necessary stocking levels in both the machine builder's and their customer's warehouses are reduced

Considering all the issues in your application

Hopefully, this information helps you make a better informed decision.  Personally, I like looking at this question more from a device and application-specific point of view, instead of making a black-and-white decision for or against a particular approach.   Let me discuss five common types of safe input devices, list the issues I considered and the suggestions derived from them.

E-stops

The issue: E-stops are particularly polarizing as they are highly visible with a certain feel when actuated.  Add the fact that buttons can be large or small, push or pull release, plastic or metal, suitable for IP65 or IP67.  The list goes on and on.  

My suggestion: With all this in mind using a safe coupling module with a conventional output used to control the button illumination is the right choice; especially when the safe coupling module is just as little as the passive splitter needed to connect an e-stop with integrated electronics.

Rope pull switches

The issue: Rope pull switches are nearly as "bad" as e-stops.  The number of different designs is mind boggling.  Rope tension, release options, single- or double-rope connections are factors that drive the selection process.

My suggestion:  Just as with e-stops, the sheer number of possible mechanical designs makes the safe coupling module approach the better solution.

Interlocking door switches

The issue: The number of mechanical designs is still significant but as users do not directly interact with these door switches it is a lot simpler for a machine designer to make changes.  Interlocking door switches frequently are controlled by several conventional outputs.  The solenoid plus a number of diagnostics LEDs (e.g., door locked, door unlocked) come to mind.

My suggestion: In this case I would go with the AS-Interface integrated design mainly because the increasing number of leads -- eight in the above example -- is getting to the point where working within a small wiring compartment is no longer any fun.

Simple door switches

The issue:  These kinds of devices have only two contacts, and just like with their interlocking counterpart, users are not directly interacting with them.

My suggestion: In this case I will again advise an AS-interface safe coupling solution.

Magnetic door switches

The issue: Safety magnet switches are typically small, encapsulated and come with a cable pigtail.  The good news is that the number of common housings is limited and as they are usually mounted out of the way, users don't have strong feelings about them.
   
My suggestion: Go with the AS-Interface integrated solution.  The installation is simpler and, with the limited number of housing designs used, prices are quite attractive.

Consider all the issues

A similar analysis can be made for any other safety device.  While users will put more emphasis on one issue and less on another, and favor one design approach over another, the procedure is still valid.  Consider the issues at hand.  In the end, the biggest advantage comes from AS-Interface itself and the connection, internal or via a safe coupling module, is just the icing on the cake.

Topics: Applications, AS-Interface

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