Oddly enough, programmable logic controllers (PLCs) and I have a few things in common. We're both over 40 years old, and we've both increased our intelligence to handle a greater number and variety of tasks.
OK, perhaps that latter observation is more than a bit subjective as it applies to me, but there's no question that the humble PLC has evolved in myriad ways to overcome challenges in controlling automated assembly lines. Like desktop computers, PLCs have become significantly smaller, smarter, faster, cheaper and easier to use during the past decade. Indeed, pico- and nano-sized PLCs are virtual commodities today, and some models cost less than the relays they're designed to replace.
And, like good manufacturing engineers, PLCs are accepting more responsibility for managing the assembly line. High-end controllers can now provide motion control, interface with vision systems, collect high-speed measurement data, and communicate with enterprise networks. In fact, high-end PLCs are doing so much more that they've even got a new name-programmable automation controllers (PACs).
PACs are available from several suppliers, including Control Technology Corp. (Hopkinton, MA), GE Fanuc Automation Inc. (Charlottesville, VA), National Instruments Corp. (Austin, TX) and Rockwell Automation Inc. (Milwaukee).
For example, Control Technology's Blue Fusion 5100 PAC provides fully configurable I/O, motion control, human-machine interface (HMI) support, and direct device-to-enterprise connectivity. Servomotors, analog transducers, switches, sensors, touch screens and other "real-world" devices are connected directly to terminal blocks on the controller. The controller itself is connected to an intranet, the Internet or other Ethernet-based network, which obviates the need for an intermediary computer to collect and distribute data.
Engineers can monitor, control and program the PAC via Internet browsers and enterprise servers. The device can be configured for various applications through six internal function module bays. The controller can accommodate 50 digital and analog I/O points and 6.5 axes of stepper or servomotor control. Two serial ports are available for HMIs, a programming interface or other serial devices.
While some suppliers have been adding intelligence and responsibilities to PLCs, others have introduced stripped-down models to help engineers distribute control, speed up network response time, and put some intelligence in otherwise "dumb" devices. One such product is DeviceLogix from Rockwell Automation. Though not a PLC, DeviceLogix enables a DeviceNet node to be programmed to execute a sequence of commands independently of a PLC or computer. A DeviceLogix-enabled DeviceNet node can be used in conjunction with a standard DeviceNet network, providing simple distributed control. Additionally, it can be used in a standalone application, without a network connection, to sequence pneumatic valves and control I/O.
Rockwell has integrated DeviceLogix into I/O blocks, push buttons and motor starters. Numatics Inc. (Highland, MI) has integrated the technology into modular valve manifolds. In an automated assembly system with many pneumatic grippers and cylinders, a manifold with some ability to "think" on its own can greatly reduce the time from when a sensor signal is received to when a component is actuated.
In a conventional pneumatic setup, the valve input 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, which processes the signal and outputs a command. That signal goes back to the scanner, onto the network, back to the DeviceNet node, and to the output module, which energizes the valves and actuates the cylinders. Each step takes time, and that time increases with the number of nodes on the network.
Valve manifolds with integrated I/O and DeviceLogix provide faster response. Sensor signals still travel to the input module. But, because the device can process the signal on its own, the command signal is generated much quicker.
