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.
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.
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.
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.
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.
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