Introduction to Automated Control Systems and PLC History

What Is an Automated Control System?

An automated control system monitors inputs—sensors, switches, pushbuttons—and switches outputs such as motors, valves, and lights according to a set of logical rules, without a person making each decision manually. In commercial buildings and industrial facilities, automated control handles everything from HVAC compressor sequencing to conveyor belt coordination. Understanding how these systems evolved, and why the programmable logic controller (PLC) became the dominant technology, is essential knowledge for any electrician working in modern construction.

Relay-Based Control: Where It Began

Before PLCs existed, control logic was built entirely from electromechanical relays, timers, and contactors wired together on large steel panels. Each relay is a simple device: an electromagnetic coil pulls in a set of contacts, completing or interrupting a circuit. Complex logic—start/stop interlocks, time delays, motor sequencing—required dozens or hundreds of relays wired in precise combinations.

Relay panels work and are still appropriate in some simple, stable applications. Their advantages include straightforward troubleshooting with a voltmeter, no software required, and the ability to function in high-noise electrical environments. However, relay control has real limitations:

• Inflexibility. Changing the logic means physically rewiring the panel—expensive and time-consuming.
• Size and weight. A complex relay panel can occupy an entire wall.
• Maintenance burden. Relay contacts wear out mechanically and must be replaced on a maintenance schedule.
• Troubleshooting time. Tracing a fault through hundreds of wires and contacts can take hours.

The Birth of the PLC

In the late 1960s, the US automotive industry needed a way to retool assembly lines faster without rebuilding entire relay panels each model year. General Motors issued a specification for a solid-state, reprogrammable replacement for relay logic. Dick Morley and his colleagues at Bedford Associates responded with the Modicon 084 in 1969, widely recognized as the first commercial PLC.

Early PLCs simply replaced relays with solid-state logic stored in memory—the program could be changed without rewiring. Through the 1970s and 1980s, microprocessor advances made PLCs faster, smaller, and capable of handling analog signals, data networking, and more sophisticated math functions. By the 1990s, PLCs were standard equipment in virtually every industrial sector and had moved into commercial building systems as well.

How a PLC Works: The Basics

A PLC has three functional sections that work in a continuous loop called a scan cycle:

1. Input section. The processor reads the current state of every connected input device—limit switches, pushbuttons, temperature sensors, current transformers—and stores the states in an input image table in memory.

2. Logic execution. The processor runs through the user program (usually written in ladder diagram language) and determines what outputs should be ON or OFF based on current input states.

3. Output section. The processor updates the output image table and drives physical outputs—motor starters, solenoid valves, indicator lights—accordingly.

This scan cycle repeats continuously, typically completing in a few milliseconds, giving the system a real-time response that relay panels cannot match for complex logic.

Ladder Diagram: A Language Electricians Already Know

The most common PLC programming language is ladder diagram (LD), intentionally designed to resemble relay-logic schematics. Horizontal rungs represent logical conditions; normally-open and normally-closed contacts represent input conditions; coil symbols represent output actions. An electrician who can read a relay schematic can learn to read and write basic ladder diagrams with modest additional training—a key reason PLCs were adopted so readily by the electrical trade.

Major PLC Families in US Industry

Three manufacturers dominate the US market and appear most frequently on construction and industrial job sites:

• Allen-Bradley / Rockwell Automation. The MicroLogix, CompactLogix, and ControlLogix families are among the most widely installed PLCs in North America. Rockwell's Studio 5000 Logix Designer software is a standard skill in many industrial facilities.
• Siemens. The S7-1200 and S7-1500 series are common in process industries and facilities with European equipment ties. Siemens uses the TIA Portal programming environment.
• AutomationDirect. The Do-more and Productivity series offer a lower cost of entry and are frequently found in smaller commercial and light industrial applications. Their DirectSOFT and Do-more Designer software packages are well regarded for learning.

NCSS standards and NFPA 79 (Electrical Standard for Industrial Machinery) govern the installation and wiring practices for PLC panels; always consult the applicable edition adopted in your jurisdiction.

Safety Considerations When Working Around PLC Panels

PLC enclosures contain live AC and DC voltages. Before opening any panel or making any wiring connection, follow your facility's lockout/tagout (LOTO) procedure in accordance with OSHA 29 CFR 1910.147. Verify absence of voltage with a properly rated meter before touching any conductor. Wear appropriate PPE: safety glasses and insulated gloves rated for the voltage present at minimum. Never assume a PLC output is de-energized just because the program shows it OFF—field wiring faults can impose voltage independently of the controller. When a PLC controls a motor or actuator, coordinate with all affected workers before energizing the system during testing; unexpected machine motion is a leading cause of injury during commissioning.