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.
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.
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.
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.
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