Contributed editorial appearing in
Scientific Computing & Instrumentation 18:8, July 2001, pg. 16.
Adaptation is pivotal for the survival of any organism. Be it biological, electro-mechanical, or conceptual, a system’s ability to adjust and compensate for inevitable change correlates strongly with longevity. Many of the flashy “must-have” toys in our household eventually sink to the bottom of the toy box, while the LEGO bucket receives constant attention. LEGO’s modular components afford the creation of vehicles, buildings, monsters, and the occasional “death ray,” depending on the mood. Of course, we could buy a specialized toy for each of these categories (trust me, we’ve tried), but it is a hassle to find the exact item and all of its accessories.Near the turn of the last century, Henry Ford demonstrated the feasibility of mass production through the interchangeability of parts and labor. By modularizing the task at hand, semi-skilled labor wielding low-technology tools could assemble high - technology automobiles at a rate of 6,000 cars per day, while competitive companies of master craftsmen labored to produce 700 per year. Unfortunately for Ford, once the megalithic production line was constructed, he was unwilling to make changes, as epitomized by his famous statement, “they can have any color as long as it is black.” Alfred Sloan of General Motors seized the opportunity to dominate the automotive industry by introducing a “product-centric” organizational view emphasizing customer choice and product customization. Sloan’s adaptation spurred the development of production lines that could be retooled rapidly. This paradigm continues to evolve at the MIT Sloan School of Management.
In addition to the physical tool heads changed in the retooling of a product line, the electro-mechanical control systems must be modified to insure proper execution of the tasks. Early control systems were entirely based on electromagnetic switches or “relays.” Similar in concept to its all - electronic cousin, the transistor, the flow of control current through the electromagnetic coil of a relay causes its metal contact to open or close, modulating the flow of current in the circuit containing the relay. The circuit is ultimately connected to devices such as motors, valves, alarms, and other relays. Only when all of the relays leading up to the final device are switched “on” does the control circuit provide the current necessary for actuation. Physical relays produce heat, have a finite lifetime, and must be physically wired into place. As manufacturing tools became more complex, the time required to hardwire and troubleshoot hundreds of control circuit relays became costly.
While working under contact for General Motors in the late 1960s, a company called Bedford Associates in Bedford, Massachusetts developed a substitute for the relay-based control circuit known as the Modular Digital Controller or MODICON. The company later changed its name to Modicon and is now owned by AEG Schneider Automation. The MODICON concept views all of the relays upstream of the final “output relay” as logical switches represented by ones and zeros in computer memory. Using a repeating loop known as a “scan,” the logical state of input lines connected to switches and sensors on the machine are stored into memory, manipulated by an analysis program, and the output relays are set to the appropriate state. This device was such a success that it soon became known generically as a Programmable Logic Controller or PLC. Since general-purpose computers were unwieldy in the late 1960’s, the PLC was constructed as a small modular device containing input and output circuitry, a central processing unit (CPU), and memory. To ease acceptance by the plant engineers required to infuse the new technology into existing product lines, a programming language was developed that resembles the wiring or “ladder” diagrams utilized to hardwire relay panels. The language, known as “ladder logic,” involves the creation of logical truth tables that are loaded into the PLC and result in the specified output relay switching “on” when all of the appropriate upstream virtual relays are “true”.
PLCs were soon connected via proprietary communication busses to increase the complexity and size of the manufacturing equipment and permit the remote uploading of control programs. Influenced by the standardization of network communication protocols, modern PLCs are increasingly plugged into Ethernet and wireless plant floor networks. Technological advances in the hardening and miniaturization of data acquisition circuits are extending the number and type of input lines that can be utilized by the PLC for process control. Owing to size decreases and speed increases, computer-based data acquisition (DAQ) is encroaching on the realm currently dominated by the PLC. While the PLC acts autonomously, computer-based DAQ permits process conditions to be logged and analyzed. As we enhance our ability to store and process large volumes of data, we improve the opportunities for process refinement and adaptation.
