I recently completed an interesting industrial system requiring a fair amount of safety logic because of its different zones. Here we’ll look at the three separate physical iterations (we actually purchased and wired the components), one theoretical (hindsight is 20/20, after all), and the manufacturer list prices of each.
The system used standard, common, dual-channel E-Stop buttons and guard switches. Each device also had an auxiliary contact used for PLC monitoring purposes. Initially, they daisy-chained into a single master safety relay: Push a button or open a guard door, and the master safety relay disables everything. The master relay fed into three secondary relay banks (left, center, and right). The secondary banks give zones of safety that don’t affect the others. The left and right banks each had a light curtain feeding into them (as well as the master signal from the switches and buttons). Thus, if a light curtain on the right were broken, the left side would continue to function.
The bill of material for this first logic iteration included:
1x Master Safety Relay
3x Configurable Safety Base Unit
5x Input Safety Modules
7x Output Safety Modules
Cost: $4,157.85
The prime contractor then asked me to integrate its safety with ours (we shared a common danger zone, so this made a lot of sense). However, the prime contractor required the safety zones be broken out further: E-Stop buttons and guards reported through separate channels. I could no longer do this with a single-function safety relay, and had to exchange it for more of the modular devices as used in the secondary zones. That made sense; it meant simply using more of the common modules already in use.
This second iteration bill of materials removed the master safety relay and added these additional units to those already noted above:
1x Configurable Safety Base Unit
3x Input Safety Modules
5x Output Safety Modules
The total cost of this second bill of materials of 24 modules was $6,358.28.
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.
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).
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.
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.
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.
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.
Excellent point, Kenish. If the systems could be evaluated for risk reduction, false alarm occurrence and other operational performance, that would probably factor as the greatest cost indicator. That evaluation may be a tad difficult, though.
The concept of safety zones and limited area E-Stops would undoubtedly let production run more efficiently, but have you ever heard about an operator on one side of the line hitting an E-Stop when they saw an operator on the other side of the line get in trouble? It sounds a bit like the "It's not MY job" syndrom there. I can see the validity of having area stop buttons but for an emergency, unless the machine designer can predict exactly where and how accidents will happen, having a "MASTER Stop" makes a lot more sense.
What you say, William, makes a lot of sense. I would think that zone e-stops would only work if the work zones were obvious - say a particular machine that does not appear to be connected to the rest of the system would have its own stop.
Robotic safety zones is an interesting technology area where there has been ongoing innovation. Robotic programmable safety systems are providing new technology that may enable reducing the footprint of robotic systems used in packaging applications, for example. With that premium on floor space, this is a potentially key feature. Plus, by implementing the safety solution within the framework of the robotic controller, these solutions are eliminating the need and cost of external safety devices such as light curtains, limit switches and other safety-related systems.
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