Machines are becoming more motion-centric; as axes increase, so does the potential for machines to be more flexible and productive. Distributed intelligence is designed around motion-centric automation, so more intelligent digital drive axes can be added without having to upgrade or change the control platform. The diagrams above compare a central control architecture to distributed intelligence.

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Al Morin
Bosch Rexroth Corp.
Hoffman Estates, Ill.

It's always the same. Packaging end users want higher product throughput out of smaller, lower cost machines, without sacrificing one iota of product quality. Next-gen machines must also be more flexible and scalable to meet changing market demands and simplify plant integration. From a controls perspective, the answer is quite clear. By integrating motion and logic in scalable hardware packages, designers have a fighting chance to meet these ever tougher demands.

Command central

The advent of centralized architecture for motion control and logic has provided several advantages. For one, the integration of motion control into rack-based PLCs helped reduce component counts in the control panel enclosure, making it possible to program motion and logic from a single point in a single program. This delivered an initial round of cost savings; ultimately, however, this was only true when a single processor was used with a medium axis count.

Centralized control has an inherent limitation because a fixed amount of microprocessor resources are available for all required functions: motion, logic, overhead, communications, and other tasks. In any operation, top priority is given to the motion task. Whenever an axis is added, a new burden is placed on the centralized processor.

Hitting the control ceiling

At a certain point, the processor hits its limit and starts reducing performance to accommodate the additional axis. This reduction might be in the form of a slower response to registration inputs, not being able to run complicated cams or programmable limit switches, or not being able to run the system to the machine's full potential. This in turn can set up the need to add more processors, so the machine can run at full capacity. Once this becomes necessary, there is practically no cost advantage — or operational advantage — when a design engineer is forced to install complex PLCs for simple, low-axes count applications.

The disadvantage to PLC-based motion controllers is the centralized control architecture. In a number of situations, it has proven to be the limiting factor in providing low-cost, high-performance solutions. On simple machines like fillers, augers, infeeds, wrappers, and cartoners, using a PLC for motion control can be overkill, not to mention prohibitively expensive.

In addition, centralized control can limit a designer's ability to optimize machine performance. Packaging machines are highly motion-centric, which makes the motion control critical to maximizing efficiency and throughput. For example, a vertical form fill and seal machine that can mechanically run at 200 pieces-perminute (PPM) might only be able to do 145 PPM, due to limited controls performance. In some cases, using a centralized control architecture can double the price of the control system. As an alternative, design engineers would be wise to consider distributed intelligence.