Distributed control can cut system reaction time by 95%.
Enrico De Carolis
Director of TechnologyDevelopment
DeviceNet is a workhorse communications network with many fine capabilities, but it sometimes has limitations when systems have a large number of nodes and fast response is critical. For instance, in high-speed photoelectric detection and sorting operations, DeviceNet systems are often too slow. Distributed control that does not rely on network connections to resolve logic provides an ideal solution. It can significantly reduce the time required to process sensor inputs and actuate pneumatic cylinders, as well as simplify networks, improve reliability, and speed installation.
A proven distributed-control method embeds DeviceLogix technology from Rockwell Automation, Milwaukee, Wis., in a Numatics pneumaticvalve manifold. The heart of DeviceLogix is a DeviceNet node that can act as a sequence controller. It is not a PLC, but it can be programmed to execute a sequence of commands independent of any PLC or industrial PC. Such DeviceLogix systems can operate with standard DeviceNet networks to provide a simple distributed-control architecture. Or they can be used in stand-alone applications without a network connection or PLC to sequence pneumatic valves and control I/O.
Valve manifolds with integrated I/O and DeviceLogix deliver fast network response because local on-board logic offers sense-to-activation times otherwise unattainable through centralized DeviceNet control architecture.
For instance, in a conventional setup, the valveinput module receives sensor signals and transfers them onto the network via a DeviceNet node. A scanner card picks up the information and sends it to the PLC where the logic is resolved. Then the appropriate output data travels back to the scanner, onto the network, back to the DeviceNet node, and to the output module, finally energizing valves and actuating cylinders. Each of these steps takes some time. And more nodes on the network increase the amount of data traveling back and forth, which tends to further slow the overall response of the system.
DeviceLogix significantly reduces communication time. Input signals still travel to the input module. But because program logic resides at the DeviceLogix node, data is processed and outputs generated much quicker. Local processing time is a fraction of that required for data to travel back and forth through the network connection. It can cut the 30 to 40-msec cycle times typical with DeviceNet to 2 to 4 msec a 90 to 95% time reduction to sense and actuate.
DeviceLogix also offers a number of additional benefits for machine builders. Less communications traffic reduces network bandwidth delays and permits smaller PLC programs to run faster. Machine sections can be built in a modular fashion, speeding installation and reducing diagnostic and troubleshooting times. And fewer potential failure points increase system and device reliability. DeviceLogix can also offer safe-shutdown sequencing, the ability for an internal parameter to be set up that detects network failure and initiates a controlled shutdown or other user-defined "safe" condition.
Nodes are programmed with RSNetWorx for DeviceNet commissioning software. The program's graphical interface lets users simply "drag and drop" logic-function blocks onto a page and connect them in the required sequence. Logic functions include AND, NAND, OR, NOR, XOR, and XNOR, as well as latches, counters, and timers.
Programs are downloaded through a standard DeviceNet communication connection into nonvolatile flash memory, so no backup battery is needed. And users can visually follow the program sequence online and force I/O for troubleshooting.
The system is compatible with Rockwell Automation's Auto-Device Replacement (ADR), a timesaving feature in the event a node fails and must be replaced. ADR retains a copy of the logic program in the scanner memory and automatically reprograms a new node. This eliminates the need to manually download a program to commission a new node.
DeviceLogix can be used as part of DeviceNet networks for low-cost distributed control, as simple stand-alone systems, or in hybrid systems where the PLC and local logic share control responsibilities. As a standalone system, it is ideal for small machines with a relatively high number of valves requiring sequential operations, such as conveyors, lift-assist manipulator arms, small clamp-and-weld fixtures, vacuum systems, and simple repetitive pick-and place operations.
In general, if a system uses air logic, a DeviceLogix valve manifold will simplify the machine. It dramatically cuts hardware and labor costs associated with air-logic circuits and eliminates the slight time delay typical of these systems. It can also reduce overall costs when expensive, high-performance distributed processors are used for localized I/O and valve control.