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Leuze and Sick Safety PLCs

I like simplicity, but sometimes complexity is unavoidable.  Recently, I had to re-design a safety system to meet SEMI S2 requirements without destroying the customer’s parts.  Unfortunately, meeting both requirements required a more complex safety system than a traditional fixed safety controller, so I had a choice: add complex wired logic to our current safety controller or move to a programmable safety PLC.

The choice was easy: go for an expandable safety PLC because:

  1. The safety PLC is much more flexible.
    1. If someone finds a problem in the safety logic, it’s easy to update: just send out the update to certified techs to update, instead of re-wiring every machine.
    2. If we need to add a new feature, again it’s an easy update instead of laborious re-wiring.
    3. If a new feature requires additional inputs or outputs, we can always add a new I/O module, and keep the wiring simple.
    4. We can handle different customer safety requirements by changing the Safety PLC software, while the base hardware stays the same.
  2. The safety PLC is much easier to build: it’s straightforward wiring to inputs and outputs, instead of criss-crossing wires trying to implement safety logic.  My feeling is that the safety PLC probably saves money, too, because although the initial cost may be higher, we will have far less wiring problems (and they’ll be easier to troubleshoot) and troubleshooting problems always takes a lot of time.

On the other hand, the price of most safety PLCs is really scary: I don’t need a super-fancy, networked über-safety PLC.  So I checked out my list of usual suspects plus did a lot of searching, and came up with two candidates I really liked: the Leuze MSI-202 and the Sick Flexi Soft.  (Actually, maybe I found 4, since the Phoenix PSR-TRISAFE looks just like the Leuze, and the Mitsubishi WS is the same as the Flexi Soft, except it costs more.)

Despite their list prices, both models are <$1000 when configured with 8 outputs.  Both have free software (big kudos to Leuze and Sick!).  Both software programs include a simulator (which is really helpful both for both evaluation and developing).  Both are expandable, which is essential (I’ve been burned too many times by non-expandable systems).

The Leuze MSI 202’s advantages include:

  • It has more inputs.  The base configuration is 20 safe  inputs, 4 safe outputs, and 4 monitoring (non-safe) outputs; each expansion module adds 8 inputs and 4 outputs (or, if you want, 12 inputs / 0 outputs) and 2 monitoring outputs.  The Flexi Soft’s controller has no inputs; each XTIO module add 8 safe inputs and 4 safe outputs.
  • Monitoring is much easier: you can install the software (for free), open the project, enter the project password, and then monitor the project’s internal state (downloading a project requires knowing the PLC’s password).  I like this feature because it can make troubleshooting in the field much easier.  The Flexi Soft doesn’t have an equivalent.

The Flexi Soft’s advantages include:

  • More flexible communications.  You can read and write from a block of memory (not the safety area!), while the Leuze only uses standard digital I/O.
  • Better network support.  The Leuze only supports Profibus-DP via an add-on module, while the Flexi Soft supports Sick’s Flexi Line protocol on some models (which could handy if you’re using other safety equipment from Sick), and,via 7 different add-on modules, CANOpen, DeviceNet, Ethernet/IP, EtherCAT, Modbus/TCP, Profinet, and Profibus-DP.  Note that neither PLC supports networked safety.
  • The Flexi Soft has a greater range of expansion modules.  The Leuze only has one type, while the Flexi Soft has four types.

For my application, the Leuze MSI 202’s advantages were important (I needed 20 inputs), while the Flexi Soft’s advantages were nice, but not necessary.

Some final notes based on using the MSI 202:

  • Overall, the project has gone well.
  • The simulation feature has been very useful trying to get my software written before the machine was wired.
  • The software seems a little slow, and has its quirks, but overall it’s been very usable.
    • I’ve especially learned to be patient when downloading the project to the PLC.  I’ve found it works better to wait until the CFG light blinks before pressing the Confirm button.
  • Safe Function Blocks such as EDM take up a lot of memory, as does the base configuration.  However, even with a reasonably long program, I’m only at ~50% memory used.
  • Communications is via a standard 5-pin mini-USB connector.
  • I choose the version with spring clamp terminal blocks; I really like Leuze’s choice of terminal blocks.  They’re easy to operate terminal blocks with two connections per terminal — really nice!
  • For some reason, Leuze USA is behind the times; if you want the MSI 202 manual and the latest software, go to the Leuze’s European web site (which I linked to at the top).
  • SEMI S2 requires an EMO switch with no software involved, not even a safety PLC.  I still wanted to do EDM (external device monitoring) on the EMO circuit, but the standard EDM Function Block is designed for the safey PLC to control the contactor’s coil, while I have to have EMO switch control the coil.  I was able to work around this with my own EDM logic.


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