Understanding Decentralized Motor Control

07/25/2007




Decentralized
Motor Control

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July 25, 2007

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Understanding Decentralized Motor Control

For material handling chores, the old saying of divide and conquer was never truer. In the case of motor control, it's decentralize and conquer. Decentralized control systems are helping companies achieve big gains in productivity and even bigger cost savings.

In a decentralized control system, motor control functions are removed from a central control cabinet and distributed out on a machine, close to the motors. Savings of typically one-third are realized due to reduced material and labor costs. In addition to saving money, decentralized control systems save panel and floor space. They also reduce startup time and increase overall uptime. For these reasons, any application with 10 or more motors, as a rough rule of thumb, would benefit from a decentralized drive system.

A decentralized motor control system combines a variable frequency drive or motor starter, I/O, disconnect switch, integrated brake and overload control, fieldbus, power, and frequently a safety stop, into a unit that's either integrated with or mounted near a motor. The savings in money, space, and time arise due to that proximity.

This is an animated image, click on it to see it in action.

A decentralized motor control system combines a fieldbus, I/O, a disconnect switch, brake and overload control, and power into a unit that's either integrated with or mounted near a motor. Source: SEW-EURODRIVE. This is an animated image, click on it to see it in action.


Savings at the panel
Those economies begin at the panel. In a conventional centralized setup, the drive panel houses the controls. The panel has to be sized to accommodate the electronics, including expensive heat dissipating systems, with an increase that scales with the number of motors. Estimates are that up to half the space in a typical drive panel is consumed this way, which makes the panels both larger and more expensive. A decentralized drive system eliminates the need for many of these components, including fans, heat sinks, terminal blocks, junction boxes and remote I/O modules. This shrinks the panel size and costs.

With a traditional approach, expensive cable and tray are required between the panel and motor. The more motors, and the longer the distance between the panel and motor, the more cable and tray are needed. In contrast, a decentralized system requires minimal wiring due to the close proximity of the motor to its control devices. Fewer wires mean less potential for cross-talking noise and interference, a major source of machine downtime. The cost of adding motors is also reduced because a decentralized system relies on branch feeders.

An independent study by Applied Engineering Solutions estimated total cost savings of decentralized versus centralized methods could run 20 to 60 percent, depending upon the exact configuration and size of the project. According to the study, most of the savings are achieved from reduced labor costs for installation, startup and panel construction, as well as cable. A real-world example involving a continuous curving conveyor system with 112 motors found savings of about $150,000, largely in the form of reduced installation costs. The benefits of decentralization, however, aren't confined to installation.

Savings in people and time
Decentralization also benefits system startup and day-to-day operation. Since the controlling electronics sit next to the motor, a technician has easy access to the information needed for troubleshooting and system startup. A single technician can commission the system, or diagnose a problem and make repairs once the system in operation.

Decentralized control systems, where islands of control surround motors and other operating devices, also significantly reduce engineering time. Each island needs only to control a discrete number of functions, simplifying programming and debugging. This also allows the use of smaller, less expensive PLCs. By connecting these islands of control to communication networks, entire processes can be monitored easily without disrupting local functionality. As a result, you gain all the advantages of centralized control in monitoring system health without the complexity and cost.

Automotive manufacturers were among the earliest adopters of decentralized control, using it to make faster production line changes and reduce vehicle manufacturing costs. Decentralized control systems allow them to eliminate the long cable runs that are expensive to install, easy to damage and time-consuming to change.

As stated earlier, decentralized control shows most promise for applications involving ten or more motors. These include the large conveyor systems used in material handling applications such as packaging, food and beverage processing, logistics and warehouses, as well as automotive and other assembly processes.

It should be noted that not all decentralized control systems are equal, and certain factors should be carefully considered. For example, it's important to have modular components. This allows a failed component to be quickly replaced, rather than replacing an entire unit, which reduces downtime. Just a few spare parts need to be kept on hand rather than entire systems. The system should also allow a wide range of fieldbus network choices.

A number of vendors offer decentralized motor control components and configurations for material handling. SEW-EURODRIVE, for example, has introduced its next generation decentralized control solution, a modular system called MOVIFIT , which provides a flexible, scalable platform for both wet and dry environments. The company also offers a whitepaper on frequently asked questions on the topic.

With material and labor costs typically reduced by one third with the use of decentralized motor control, builders of material handling systems owe it to themselves—and their customers—to investigate the possibilities.

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