A winning combination: Integrating motion and logic helps reduce costs
October 16, 2007 by John Wenzler
Today’s fast-changing, highly competitive global marketplace is driving many system and machine designers–both end-users and OEMs–to aggressively try to achieve one very demanding goal: greater machine performance at a lower cost.
For packaging end-users such as plant operators, this means a push to get more product throughput out of smaller, low-cost machines without sacrificing one iota of product quality. OEMs are driven by parallel requirements to offer highly flexible solutions at lower costs.
They must deliver system/machine scalability, meet changing market demands and support simplified integration with the rest of the line. Their primary goal is to offer solutions where end-users pay only for what is needed. From a controls perspective, integrating motion and logic in a scalable hardware package can help fulfill this need.
The advent of a centralized architecture for motion control and logic has provided some advantages. The integration of motion control into rack-based PLCs helped reduce the component count in the control panel enclosure, and made 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 axes count.
A centralized control has an inherent limitation: there is a fixed amount of microprocessor resources available for all required functions–motion, logic, overhead tasks and communications.
In any operation, top priority is always given to the motion task. Whenever an axis is added, a new burden is placed on the centralized processor.
At a certain point, the processor hits its limit and starts reducing performance to accommodate the additional axes. This reduction might be in the form of a slower response to registration inputs, not being able to run complicated cams, programmable limit switches, or not being able to run the system as fast as the machine is capable of running. This, in turn, can result in the need to add more processors so the machine can run at full capacity. Once this becomes necessary, there is little or no cost or operational advantage if 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, scalable, high-performance solutions. On simple machines like fillers, augers, infeeds, wrappers and cartoners, using a PLC for the motion control can be overkill. It can add prohibitive costs that make it difficult to create a machine that fully meets an end-user’s cost-performance requirements.
In addition, centralized control can limit an OEM’s ability to optimize machine performance. Packaging machines are very motion-centric, which makes motion control critical to maximizing efficiency and throughput. For example, a vertical form fill and seal machine that can mechanically run at 200 pieces-per-minute (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.
Centralized control has reached the limit in the value it can offer. With today’s fast-changing markets calling for much more production flexibility and scalability, the limitations of PLC-based centralized motion control are more evident. New technology and new approaches to motion control and logic have created a powerful alternative: distributed intelligence.
Distributed versus centralized control is defined by the location of processing power for the motion control. With a centralized architecture, a fixed amount of PLC processing power is divided among all the axes. As axes are added, the available processing power is reduced.
Distributed intelligence (DI) solves the problem in a simpler way. It moves the burden of controlling an individual axis out to the drive. Thanks to advances in microelectronics, intelligence can be distributed throughout a machine to the sensors, motors, drives and other components.
In a DI system, each drive is capable of closing the feedback loop and can handle such advanced functions as cam tables, absolute feedback, electronic line shafting (ELS), diagnostics and high-speed registration. It is even possible to add safety and predictive maintenance functionality at the drive.
The processing power that can be built into the drive with today’s low-cost processors and memory allows the drive to be quite intelligent. Most importantly, when you add a drive, you add more intelligence to the system. This is the exact opposite of centralized control, where every additional axis drains processing performance.
Distributed intelligence not only reduces the processing load on the controller, it changes the controller’s role in motion control to a supervisory one.
Distributed intelligence is a modular, responsive architecture. It supports the scalability that is an absolute requisite in current operating environments. Adding an axis is greatly simplified: just add a new servo axis. There is no need for additional expansion cards or functionality for the controller. The intelligence is in the drive itself.
Adding functionality and intelligence in a drive-by-drive, distributed fashion frees design engineers to create machines that serve end-user demands for more convenience and flexibility.
Because processing power has ceased to be a limitation, more servo-controlled axes are practical. Other advantages include faster setup, greater precision and higher reliability.
DI architecture can also enhance operational uptime and flexibility by supporting integrated safety and predictive maintenance at the drive level. It is made easier because of the quicker response and data monitoring inherent in a distributed intelligence platform.
Implementing a DI system requires several components engineered to work in a decentralized architecture. These include intelligent drives and a DI-ready controller.
Some may think an intelligent drive is one that can simply handle the position loop and receive inputs. However, this type of drive still places a heavy burden on the processor. For true distributed intelligence, a drive should be able to handle such tasks as closing the position loop, absolute positioning, high-speed registration, cam tables and diagnostics.
As more tasks are handled by the drive, the load on the controller is reduced. A perfect example is the provision of safety and predictive maintenance tasks at the drive level. These tasks do not necessarily have to be managed from a central location. Plus, by making them drive specific, problems can be quickly isolated, downtime can be reduced and machine throughput optimized.
The motion controller is the next component in this architecture. A DI-ready controller must take full advantage of intelligent drives. Its key tasks include running logic, overseeing drive communications, I/O peripherals, HMIs and system networks. Involvement in the motion is at a supervisory level.
Integrated logic and motion control in a drive
Integration of the logic and motion control in a drive implements the distributed intelligence model without sacrificing machine performance and ultimate value. This is ideal for packaging systems such as carton erecting, flow wrappers, smart belts, infeeds,cartoners and labellers.
Integrating motion and logic in a drive is the way to achieve the flexibility and scalability todayÃs fast-changing production environment requires. As OEMs strive to create high-performance, low-cost machines, and end-users in the packaging industry push to keep a lid on capital expenditures, the distributed intelligence solution provides an innovative path forward. It leverages the advantages offered by todayÃs advanced microelectronics, and supports a complete, high-performance system at the lower cost end-users require.
John Wenzler is a corporate account executive for the food and packaging industry at Bosch Rexroth Corp.