Recently, programmable logic control manufacturers introduced device-level buses (networks) to integrate sensors and low-level control devices with various automation systems including motion control systems.
Customers, particularly automotive customers, are looking for ways to reduce the amount of wiring needed to integrate various devices in their plants. They also want to monitor and operate production systems from central locations, and easily modify and re-arrange their manufacturing processes as new product needs dictate. Local area networks (LANs) meet these goals for many systems. But they are not fast enough to transfer data from sensors and small control devices, such as motor starters and single axis controls, to host PCs or PLCs.
To connect these devices to a manufacturing process controller or a motion control system, engineers typically run yards or miles of discrete wiring to each of these devices. This year, however, a few control manufacturers introduced a new type of bus — the device-level network. With these networks, such as SDS, DeviceNet, or other CAN-based bus, one four-wire, control cable connects sensors, transducers, and motor starters, as well as drives, controllers, and other devices into an integrated system.
Device-level networks meet the special data transmission needs of sensors and small control devices. Offering limited amounts of transmitted data but extremely fast throughput, these networks send one to eight bits of data at typical throughput rates of 100 to 200 Kbits/sec, rates most larger networks can’t match.
In addition, these device-level networks, be they bit or byte based, reduce the amount of wiring. With one control cable, a device on the network sends status updates, position information, and operational data and receives commands from a control. Because installation involves only one cable, connection costs plus installation and debug time are cut by at least half. Moreover, users still have the flexibility to place inputs and outputs where needed. In addition, users see other benefits of:
• Improved maintenance and fault detection.
• Increased system reliability and up time.
(For more on device-level networks, see PTD, “PLCs bus into the future” 7/95).
Motion control specific sensors and devices
Most photoelectric and proximity sensor manufacturers already have products that are compatible with all the various networks. Now, manufacturers of more complex control products — ones that transmit more than a few bits of data — are making their products compatible with device-level networks. One example is the BTL-2-F captive magnet linear transducer from Balluff. It will be compatible with any network, from DeviceNet to Profibus DP.
This sensor is one of the first transducers compatible with device-level networks, and offers the benefit of less complex wiring between the transducer and control. “We’re getting many requests from companies wanting to incorporate transducers onto device-level networks,” says Jeff Smith, manager of transducer products at Balluff. “While a position transducer sends a continuous stream of data, customers want this feedback coming through one set of cabling, as opposed to the separate wiring done before. Several of the device-level networks now available can handle the data load of these sensors as well as the requirements of simpler devices.”
Through this integrated arrangement, engineers can program set points to a transducer through a programmable logic controller (PLC) rather than walking to the device.
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Manufacturers are also making motor protection devices compatible with these new communication networks. Connected to a device-level bus, these devices let engineers gather real-time and historical information on motor productivity and equipment capacity. Solidstate overload relays, for example, monitor motor currents and phase balance, as well as identify the cause of an overload relay trip. The SMP relay from Allen- Bradley lets users attach an operator panel that has start, stop, and reset buttons for local control and a liquid crystal display for monitoring.
You can also control servo and step motors using PLCs, personal computers, or other controls connected to a devicelevel network. Engineers can program motion parameters including acceleration, deceleration, velocity, and position directly from the device-level network. Kollmorgan Motion Technologies Group and Pro-Log Corporation are two of the companies that make device-level compatible products.
Several drive manufacturers like the increased system flexibility and modularity offered by device-level networks.
MagneTek Drives and Systems, for example, offers its GPD 333 adjustable-frequency drive. “We see these networks making every vendor’s drive look the same on the network,” says Lisa Roever, design engineer at MagneTek. “So if a drive goes down, you can replace it with another manufacturer’s drive you have readily available, and the network will not notice the difference. The network will still send and receive data in the same manner. This flexibility saves much user programming and setup time.”
Agrees Ken Deken, marketing manager for ac drives, Reliance Electric, “These device-level networks simplify connectivity and interoperability of many intelligent devices. They eliminate hard wiring, dedicated interface cards and complex software drives. Plus, such networks increase the amount and rate of information flowing from plant-floor devices to control systems.”
“We also find our customers are looking for an easy way to set up a monitor and operate one, or several, drives from a central station,” adds Mr. Deken. “DeviceNet or other device-level networks give you that capability. It also means products are interchangeable, giving users more options in system design.”
Use of drives with device-level networks also makes it easier to connect to any PLC that’s compatible with a network. This capability gives drive manufacturers access to many PLC lines and gives users more choices in drive selection for their motion control systems.
Like the servo and step motor controls, PLCs can control many motion parameters of drives compatible with devicelevel networks. They can start and stop the drive; give it a frequency reference, alter accel and decel rates, and access all the programming concepts.
Some companies are also making axis position controllers compatible with the new networks. A single-axis position controller from Yaskawa Electric America Inc., for example, uses the DeviceNet network to upload and download setup and configuration data. This arrangement eliminates discrete inputs and outputs for this controller.
One network or more?
Many of these motion control devices are compatible with several networks. Most manufacturers feel that if the customer wants compatibility with a particular bus or network, they will comply with that request. “This is where you have to be today,” says Mr. Smith. “Network features that are an advantage for one person are not necessarily an advantage for someone else. There are very few absolutes in this.”
However, a few manufacturers feel it is important to support only one network, in order to achieve the benefits offered by an accepted standard product. Check with your manufacturer to ensure its products meet your communication needs.
For more information
To find out more about the Smart Motor Protectors, contact Allen-Bradley Response Center, 10701 Hampshire Avenue South, Bloomington, MN 55438, Tel: (800) 338-4235, Fax: 800-500-0329.
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For more on the BTL transducers and other network compatible products, contact Balluff Inc., 8125 Holton Drive, Florence, KY 41042, Tel: (800) 543-8390, Fax: 606-727-7745.
For more on the Smart or Fast Drives, contact Kollmorgen Motion Technologies Group, 201 RTock Road, Radford, VA 24141, Tel: (703) 639-2495, Fax: 703-731-0847.
For more on the GPD 333 drive, contact MagneTek, 16555 W. Ryerson Road, New Berlin, WI 53151, Tel: (414) 782-0200, Fax: 414-782-8737.
For more on ProxBlox sensor products, contact Namco Controls Corp., 5335 Avion Park Drive, Highland Heights, OH 44143, Tel: (216) 460-1360, Fax: 216-460-3800.
For more on the GV3000 ac drives or FlexPak 3000 dc drives, contact Reliance Electric, 24701 Euclid Ave., Cleveland, OH 44117, Tel: (216) 266-7000.
For more on the HR position controller, contact Yaskawa Electric America Inc., 2942 MacArthur Boulevard, Northbrook, IL 60062- 2028, Tel: (800) 633-5756, Fax: 708-291-3457.