I love this real-world example of how one simple design decision and component change can have real impact on the cost of the BOM and the ultimate performance of the product. This is a classic example of what happens every day in design iteration. It really shines a spotlight on how a more holistic view and big-picture thinking can really have impact on design results not to mention, costs. Thanks for sharing, TJ.
Beth, I have to write a correction here. I realize I neglected to SUBTRACT the cost of the standard PLC from my safety PLC figure (the safety PLC performs both the safety function and the regular PLC function in one unit and thus replaces the standard PLC).
This would adjust the total safety PLC cost down to around $7000. The safety PLC decision becomes even more attractive for the next system.
Interesting piece, TJ. With safety becoming more an integral part of the integrated architecture/product line-ups for many automation control vendors, it will be interesting to see how the pricing trends play out over the next few years. Would expect that the premium will be heading downward but it's impossible to know how fast.
The purchase price is declining, and safety PLC options are even more attractive at installation because the long cable home runs get decreased. Instead of having all safety device cables return to the central location where the safety relays reside, the devices can land locally at distributed Ethernet IO locations which communicate via regular Ethernet cable. The Ethernet IO cable already must be pulled back to the central PLC; this gives a significant cost savings for cabling and installation time.
The drawback to SafetyPLC option is the complexity for post-acceptance troubleshooting. The skill level required to maintain and troubleshoot such a system is much higher. Trouble-shooting a system using regular safety relays can be much simpler.
Apologies for quoting Yoda, but it does seem like we (engineers) are expected to predict future events, and be right every single time (OR THOUSANDS COULD DIE!!!)
While it was theoretically possible that I could have discovered the manufacturer flaw prior to purchasing the hardware, It would not have been practical. This would mean searching the manufacturer's knowledgebase for every single part used, with multiple searches and good keyword guesses for each part.
I suppose engineers should expect the customer to change their mind several times through the life of a project (we did charge a change fee), planning too much for such events simply drives the cost up unnecessarily.
OK, writing that helped gel the answer to your question. A logic-based (software) solution to problems may cost more up front but is likely to be the lower cost choice over the life of a design. The fact that it is likely the more complex solution as well means the work force needs more education (and should be compensated better for having acquired that education).
About two years ago, Design News published the article, "Carefully Analyze Safeguard Benefits and Tradeoffs," which you can find here: http://www.designnews.com/document.asp?doc_id=228714. Unfortunately, the figures and illustrations no longer appear with the article. A reference to a paper about an ISO standard is out of date. Use this link to locate the paper, "A New Approach to Machine Safety: EN ISO 13 849-1:2006 – Safety-related Parts of Control Systems,"http://www.schmersal.com/kasbase/bilddata/broschue/k-info/b_prepp2.pdf.
TJ, thanks for this post. You've taken the argument to where it really counts--cost. You've also noted that foresight could have saved time and money. Design News will try to come back and take a deeper look at this, along the lines of the article Jon Titus mentions, about analyzing the benefits and tradeoffs of safety in the context of safety-rated PLCs.
A number of factors are affecting the cost of safety including suppliers moving to provide "right-sized" solutions based on the requirements and the complexity of systems. Plus there is the move to networked safety systems, especially on the Industrial Ethernet side, where there has been an increase in the number of devices available and infrastructure which allows large amounts of safety data to be passed over the network in a failsafe way. A second major trend affecting costs is the combination of failsafe operation, machine and motion control in one controller. Many systems in the past had separate controllers for motion and safety. But increasingly all of these functions are available in a single controller on one network.
The dollar amounts are really "price", not "cost". Cost requires a holistic view- what are the consequences of the safety system failing to react? How often will a false alarm occur and what is the cost in downtime and product/ ingredient scrap? Also the price range of the various options is ~$6k. If that's part of a $10M system with a few sales per year, the project delays might far exceed any price savings. Liability cost exposure might need to be considered too.
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