The BTL linear position transducer, from Balluff Corp., is compatible with DeviceNet, SDS, and any other devicelevel network to send precise position information to the PLC for monitoring and position control. The BTL-2-F comes in stroke lengths of 2 to 142 in. A ball-joint connection on the magnetic slide permits 360 deg rotation around the vertical axis and ±18 deg from horizontal.

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

The SMC Dialog Plus solid-state motor controller uses DeviceNet to send information on productivity and equipment capacity. From Allen-Bradley Co., it has a built-in programmable motor overload and it protects motors from over and under current and voltage, jam, stall, phase reveral, and phase unbalance conditions. It can also rebalance unbalanced power.

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.

Continue on page 2