Evolving control systems

Gary A. Mintchell, Senior Editor -- Control Engineering, 1/1/2000

Control systems are in another evolutionary period. Usually this remark is taken to mean a technology leap in the controller. The complete system, however, is more than just the controller, and changes are occurring in every part of the system. As Control Engineering covers these changes during the year, it is useful to look at each product and subsystem in contex.

Almost everyone learns basic systems theory today. Briefly, each system has inputs, a decision-making element, outputs, and feedback to close the loop. Control system architecture includes input devices to the controller that serves as the decision-making element, and output devices that make things happen. Feedback is accomplished through the controller to advise operators, technicians, and engineers of abnormal behavior and gather data.

Control systems must know the status of the process and its environment. Input devices are the eyes, ears, and touch that the controller needs to determine necessary actions. Typical input devices range from pushbuttons and selector switches, through proximity and photoelectric sensors, to temperature and pressure sensors.

Today's sensors provide more than simple binary (on or off) inputs or varied current or voltage (4-20 mA or 0-10 V dc), although these are still very important. Vision systems providing complex information have become stable and affordable. Bar-code equipment is also part of the information providing system as well as status to the system.

Modern networks enable economical and fast data communication from sensors and other input devices to the controller. Since manufacturing management software now demands more and better information from the lowest level in the plant, these advances help power complete manufacturing enterprise progress.

CE reports and analyzes both types of manufacturing systems-process and discrete. Where it was once believed that these were distinct types of manufacturing never to merge, that border has now been breached. It is becoming increasingly difficult to tell process controllers (DCS) from discrete manufacturing controllers (PLC). To add to the confusion, computer numerical controllers (CNCs) are now blending with PLCs to handle automation as well as machining. Robotic controllers are also now able to handle more automation and data management chores.

The newer system architectures (see diagrams) highlight separating I/O modules from the controller chassis. This means a single cable serves in long wiring runs, rather than bundles of parallel wiring. Wiring from sensors and actuators to the I/O modules is reduced to the shortest possible route. Tracing wires becomes much easier, and common wiring problems are reduced.

To a large part, this is because of market and technology penetration of personal computers (PCs). PCs now are used as controllers, operator interface, communications interface, programming terminal, and data concentrator. As a result, such things as the computer operating system, control software, and other components have become topics of discussion. Few people knew, or cared, what the operating system, chip set, or backplane bus structure was in a PLC or DCS. Now engineers openly debate the merits of Microsoft Windows NT, or embedded operating systems like Windows NT Embedded or CE.

Once the controller scans input status ascertaining the environment and makes decisions based on programmed logic, it turns outputs on or off as required. Output devices are usually denoted as coils in programs because the typical response is to energize a coil in a motor starter, relay, or solenoid. Other outputs can be an analog signal to a variable frequency drive controlling a pump or fan or a message sent to a display.

What's in the future? Look for controller functions to move out to I/O modules then to devices. Intelligent devices will form loops of local control networked to a supervisory controller. Communication loss to the controller will not immediately shut down the process or machine unless a safety has been triggered. Networks and software will become more important and embedded control will reside at the device level. The system will be monitored by Web browser technology, often on a wireless pocket computer.