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Motion: how to decide when to distribute control

Although automation vendors tend to urge users to consider decentralizing drives and distributing as much intelligence as possible throughout their machinery, most readily admit that it’s not for every application. The centralized approach is still valid for many applications.

The natural question to ask is, how does one go about making a decision? Here is some advice from Tom Jensen, technology evangelist at Lenze Americas Corp.

• Start with a white sheet of paper. “Don’t copy and paste code and concepts for new mechatronic machines,” advises Jensen. “Copying old concepts locks you into old hardware concepts.”

• Think functionality. Ask, what does the machine need to do? In general, complex motion tends to benefit the most from centralized control. “If the machine can be simplified, gains might be realized with remote motor-mounted frequency drives,” notes Jensen.

• Consider the machine’s footprint. Small machines with short cable runs normally have a cabinet.

• Count the motors. “A low motor count works against decentralized drives,” says Jensen. “Generally, decentralized motion control is great for a single axis, but syncing multiple axes requires programming and wiring between controllers that can be done easily in a centralized controller.”

An example of an application that can benefit from distributed drives is the machinery used in warehouses and distribution centers. These facilities can have more than 12,000 motors and drives actuating various palletizing and wrapping machines, conveyors, automated storage-and-retrieval units, and robots.

“A traditional motor would have a cabinet-mounted motor control device, such as soft start, breaker or drive,” says Jensen. “Such solutions would prove to be more costly and hard to maintain because of the motor structure.”

For this reason, he and his colleagues work with manufacturers of warehouse equipment to reduce the size of motor-control centers by installing motor-mounted induction drives. Driving many motors with one three-phase power supply can greatly reducing the branch protection needed to support a line, according to Jensen. Distributing drives and daisy chaining motors also can reduce the need for cabinet and cable by as much as 80 percent, he says.

>> Read Automation World's complete coverage, "Decentralized Motion, Driven by Intelligence"

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