Programmable logic controllers (PLCs) are a great example of industrial technology that technicians have appropriated for theatre. The first PLC was created at the request of General Motors in 1968 to replace the complicated arrangement of electrical relays and mechanical sequencers used to automate the assembly line at a GM transmission plant. Prior to the installation of the PLC, changing the assembly line processes for each new model year required a team of electricians rewiring the entire system; with the PLC, programmers could change the programming without changing the physical connections--a significant improvement in efficiency. Just as in major massive assembly line operations, a PLC can be used to control and automate onstage effects.

A programmable logic controller is a specialized computer that can receive input signals and based on its programming manipulate output devices in a variety of ways. Very simple PLCs send and receive digital (i.e, "on" or "off") signals: a push button is depressed, a limit switch is hit, a light sensor is tripped, etc., and this change in state from on to off or off to on is recognized by the PLC; similarly, the program in the PLC will turn an output on or off, which might actuate an indicator light, a solenoid, a pneumatic valve, etc. More complicated PLCs can send and receive analog signals (i.e, a range of voltage or current which is proportionally related to something in the physical world, like velocity, volume, amount of light, etc.), count thousands of on and off signals in fractions of a second. Additionally, in more complex PLCs, programming can be used to develop sequenced operations, timed operations, complicated mathematical operations (for example, PID process control loops), and more.

Though they are technically computers, programmable controllers differ from desktop or laptop computers in a number of significant ways (beyond the vastly different size and shape). Programmable controllers are generally constructed to withstand a wider variety of environmental changes than typical consumer machines; programmable controllers in industry may be exposed to vibration, changes in temperature, humidity, and other variables that a typical household machine may not be able to withstand. Additionally--jokes about Mac versus Windows aside--because PLCs have a specific, focused use, their operating systems are significantly more stable than consumer machines, making them more reliable.

PLCs are typically programmed using a specialized programming language called "ladder logic," which is similar in structure to the functionality of the older relay control systems PLCs replaced. Additionally, PLCs are designed to cycle through their programming repeatedly, many times per second, updating input and output states each cycle. This can make programming a PLC seem daunting to the uninitiated.

As with many theatrical technologies, as costs have gone down the use and availability of PLCs has begun to spread beyond large-budget productions to academic, regional and summer-stock productions. A simple yet flexible PLC set up can cost as little as a couple of hundred dollars; a careful initial purchase can provide for years of expansion and adaptation for a variety of applications.

Over the next few months, we'll look more deeply into the use and functionality of PLCs.