Programmable Logic Controllers

Background

Machines have existed in various forms for thousands of years. Simple machines such as levers or wheels stretch back to time immemorial, while more sophisticated machines, such as catapults or water-driven mills, are contemporary to civilization (James & Thorpe, 1994). With the Industrial Revolution, the combination of higher quality materials for machine construction and improved power sources (such as the steam engine) changed the nature of manufacturing. Very rapidly, manufacturing processes shifted from artisan construction to machine-assisted manufacturing (Lane, 1978).

Despite the advantages provided by these new machines, the lack of automatic control systems remained a significant barrier. Where an artisan once made the product directly by hand, his labor was now employed in constantly managing the machine that made the product. Industrialists noted that if machines could somehow manage themselves, manufacturing would be more efficient, uptime would be increased, and human labor could be shifted to more value-added activities. Inventors began exploring possibilities for automatic regulation, starting with mechanical systems, such as the centrifugal governor, and expanding into other methods, such as thermal, hydraulic, or pneumatic systems (Bennett, 1996).

In the 19^th and 20^th Centuries, electrical engineering and technology had become sufficiently mature that electrically-based systems were a preferred technology for controlling manufacturing equipment (Bennett, 1996). The core of these controls systems was the electro-mechanical relay, or simply “relay.” In it's first stage (called a “coil”), this device uses the current from an electrical signal to generate a magnetic field. This magnetic field then causes a mechanical switch to complete another electrical circuit (the “contact”) in the second stage (Gurevich, 2006). To disconnect the contact, the first stage is deactivated and a spring returns the contact to a resting state. The relay is important because its design performs two important functions. First, it electrically isolates the activating signal from the output signal, allowing information to be reproduced without loss and with potentially different electrical properties (voltage, current, or power levels). Second, it provides a method for controlling a circuit based on the behavior of not just one, but multiple components connected to the coil. Different connections create different behavior, including logical functions.

This final feature creates the potential to add not only control (on/off activation) but logic (control coordinated with other elements of the system) to the functioning of a machine. Very rapidly, machine designers began using “relay logic,” or interconnected relays emulating logical functions, to control their machines (Wakerly, 2001). Banks of relays were installed on metal sheets called “backplanes” and mounted within metal enclosures called “cabinets.” These cabinets both protected the array of delicate relays from harsh manufacturing environments, and protected users from the relays' electrical energy.