The Zone vs Distributed Cost Debate: What’s the Best Assembly Line Control Architecture for Assembly Systems?
by George Jewell, on Sep 8, 2015 4:40:07 AM
There has been an ongoing debate over “zone” vs “distributed” control architecture for assembly line systems. It is the old-school “zone” approach verse the “distributed” approach, which takes advantage of Moore’s law.
Moore’s Law (excerpt from Wikipedia): “The law is named after Intel co-founder Gordon E. Moore, who described the trend in his 1965 paper that the number of components in integrated circuits had doubled every year from the invention of the integrated circuit in 1958 until 1965 and predicted that the trend would continue "for at least ten years. His prediction has proven to be uncannily accurate….”
So how does Moore’s law factor in to assembly line systems?
Like everything based in electronics, the cost and size of components has decreased and the performance has greatly increased for industrial control. PLC costs have dropped, while memory and performance have increased. But probably the most important development has been in packaging.
Over the last 5-6 years, automation manufactures have been re-packaging their products with an IP67 rating, which has allowed for a “more panel / less control” design and has also helped reduce overall costs and clean up the machine designs.
Distributed Control for Assembly Line Systems
Distributed control is nothing new; it’s been part of control systems for decades. What’s changed is that distributed control is now possible at the scale that assembly systems use control hardware (with hundreds of stations).
In the past, when you made the comparison of zone vs. distributed control, zone always won from a cost standpoint, but the industry consensus is that from a design and engineering perspective, distributed control is better.
Fast forward to the current crop of IP67 panel less control products with a much lower price and distributed control for assembly is now possible. However, there is still a smaller debate on cost. When you compare both control approaches as basic standalone systems (so that neither one has a higher level system managing them), they use only the products available from the automation manufactures (i.e. HMI’s not PC’s with management software). At eFlex Systems, we believe distributed still comes out ahead when you look at the overall lifecycle costs for the end user of the assembly line.
Zone Control for Assembly Line Systems
Zone control is a pretty traditional controls approach. It consolidates as much of the process into one PLC / panel and then you write the necessary code to control everything connected to it. The code is written as standard as possible but each station is coded uniquely for the process tasks that are required for each station. As a result, there is a very large custom program with some areas reused, wherever possible.
What the customer ends up with is a large, complex program that is time consuming and costly to change, especially when faced with major line rate changes or new model introduction. The result is contracting outside resources (at great expense) to tackle the huge volume of work that needs to completed in a relatively short period of time. This is a big reason why cost starts moving the advantage toward distributed architecture. And when distributed architecture is implemented with standardization as the guiding principle, overall costs are much less as well.
Debate to be continued in future posts…