By the middle of the twentieth century, factories had become increasingly electrical, mechanized, and complex.
Motors powered machinery. Conveyors moved materials. Sensors detected machine conditions. Switches, timers, relays, and electrical circuits determined what equipment should do next.
These systems could automate remarkably complicated processes—but they had a major weakness.
Much of their control logic was physically wired into electrical cabinets. Changing how a machine operated could require electricians and technicians to redesign and rewire large relay systems.
Manufacturing needed something more flexible.
The answer was the programmable logic controller, or PLC—a rugged industrial computer designed specifically to control machines and processes.
Before PLCs: The Relay-Controlled Factory
Electrical relays were essential to industrial automation long before PLCs existed.
A relay allowed an electrical signal to control another electrical circuit. By combining many relays with switches, timers, counters, sensors, and motor starters, engineers could create automatic sequences.
One relay might energize a motor. Another could prevent that motor from starting until a guard was closed. A timer could delay the next operation. Limit switches could indicate that a mechanism had reached the correct position.
Complex machines could require hundreds or even thousands of interconnected devices.
The Problem With Hard-Wired Logic
Relay control worked, but large systems could become difficult to modify.
Imagine an automobile manufacturer introducing a new vehicle model. The production sequence might change. Sensors could move. New equipment might be added. Timers and interlocks might require different behavior.
With hard-wired control, those changes could require extensive physical rewiring.
Large control cabinets contained rows of relays connected by enormous numbers of wires. Troubleshooting could require technicians to trace circuits manually through electrical drawings and cabinets.
Manufacturers wanted the reliability of industrial electrical control with the flexibility of programming.
The Central Problem
Factories had already automated machines. The next challenge was making the control system itself easier to change. Instead of rebuilding electrical logic every time production changed, manufacturers wanted to change the instructions.
General Motors Searches for a Better Controller
The automobile industry was especially affected by the limitations of hard-wired relay systems because production lines had to be modified as vehicle designs changed.
In the late 1960s, General Motors' Hydra-Matic division sought an electronic replacement for relay-based machine control.
The new system needed to survive the harsh conditions of a factory environment, operate reliably, work with industrial inputs and outputs, and be understandable to the people responsible for maintaining production equipment.
Several organizations pursued solutions to this industrial control problem.
Bedford Associates was among the companies that responded to the demand for a programmable industrial controller.
The team associated with Richard Morley developed a controller that became known as the Modicon 084. Modicon—derived from “Modular Digital Controller”— became one of the most important names in early PLC history.
Morley is frequently described as a major figure in the development of the PLC, but the technology should be understood as the result of broader engineering efforts involving multiple people and competing systems.
The breakthrough was larger than one device: industrial control logic could now increasingly exist as programmable instructions rather than only as physical wiring.
Logic Becomes Software
The PLC changed where the machine's decision-making logic existed.
In a relay-controlled system, the relationships between inputs and outputs were largely created through physical electrical connections.
In a PLC-controlled system, many of those relationships could be represented inside a program.
If the production sequence changed, engineers and technicians could modify the logic without completely rebuilding the control cabinet.
This dramatically increased the flexibility of industrial automation.
Change the Program, Change the Machine
The Jacquard loom demonstrated that machine behavior could be altered through encoded instructions.
PLCs brought that principle directly into industrial control.
The mechanical structure of a production machine might remain largely unchanged while its timing, sequence, conditions, and responses could be modified through programming.
Automation was becoming software-defined.
Ladder Logic: A Bridge From Relays to Software
A new controller would have been far less useful if factory personnel could not understand how to work with it.
One important solution was ladder logic.
Ladder diagrams visually resembled the electrical relay diagrams already familiar to electricians and industrial technicians.
Two vertical rails formed the sides of the diagram while horizontal “rungs” represented logical conditions and actions.
Contacts could represent input conditions. Coils could represent outputs or internal logic. Timers and counters could perform functions that once required dedicated physical devices.
This familiar structure helped bridge the world of electrical control and computer programming.
The PLC Scan Cycle
At the heart of PLC operation is a repeating control cycle.
In simplified terms, the controller examines its inputs, executes the program, updates outputs, and repeats the process continuously.
This occurs rapidly enough that the controller can coordinate industrial machinery in real time.
A sensor may detect a product. The PLC evaluates the programmed conditions. A conveyor stops. A cylinder extends. Another sensor confirms position. The PLC then allows the next operation to begin.
Thousands of such decisions can occur throughout an automated facility.
Read → Decide → Act → Repeat
A PLC continuously monitors the machine and responds according to programmed logic.
Read the inputs. Execute the logic. Update the outputs. Repeat.
This simple cycle forms the control foundation for an enormous range of industrial equipment.
Timers, Counters, and Sequences
PLCs could replace many physical control devices with programmable functions.
A timer could delay an operation. A counter could track completed cycles. Logic could determine whether several conditions were satisfied before allowing a machine to move.
Sequences that once required large networks of physical relays could be organized within software.
This reduced some wiring complexity while making sophisticated automation far easier to modify and expand.
Sensors Give the Controller Awareness
A PLC cannot control a process effectively without information about what is happening.
Industrial sensors provide that information.
Limit switches detect position. Photoelectric sensors detect objects. Proximity sensors identify nearby metal components. Pressure sensors monitor fluid systems. Temperature devices monitor heat. Encoders measure position and motion.
The PLC receives these signals and uses them to determine what the machine should do next.
This created an increasingly important automation architecture: sense the process, evaluate the information, and control the response.
From Digital Decision to Physical Action
The controller's decision ultimately has to affect the physical world.
PLC outputs can command contactors, solenoid valves, indicator lights, drives, actuators, and other devices.
A single logical decision can therefore produce real mechanical movement.
The controller may determine that conditions are safe, energize a valve, extend a pneumatic cylinder, verify completion through another sensor, and then advance the process.
Software has become physical action.
PLCs and Industrial Robots
Industrial robots and PLCs developed into complementary technologies.
A robot controller manages the detailed motion of the robot itself, while the PLC often coordinates the robot with the surrounding production system.
The PLC may tell the robot that a part is ready. The robot performs its task and reports completion. The PLC then starts a conveyor, activates another machine, or releases the next product.
This coordination allows many specialized machines to operate as one automated production system.
From Individual Controllers to Connected Factories
Early controllers largely focused on individual machines and processes.
As industrial networking developed, PLCs increasingly communicated with other controllers, operator interfaces, drives, robots, remote input/output systems, supervisory computers, and plant information systems.
The isolated machine controller became part of a much larger digital network.
Operators could view machine status through human-machine interfaces. Production data could be collected. Alarms could identify faults. Supervisory systems could monitor entire processes.
Automation was becoming connected.
The PLC Is Still Everywhere
Despite enormous advances in computing, PLCs remain fundamental to industrial automation.
They control conveyors, packaging machines, manufacturing lines, material-handling systems, pumps, water-treatment equipment, processing systems, automated warehouses, and countless other applications.
Modern PLC platforms can communicate across industrial networks, exchange information with robots and drives, collect process data, support motion control, and integrate with higher-level computer systems.
Artificial intelligence and cloud computing may analyze enormous amounts of information, but reliable real-time control of physical machinery still frequently happens close to the equipment.
That is where the PLC remains one of automation's most important technologies.
From Water Clocks to Digital Logic
The PLC belongs to a technological story that began long before electronics.
Ancient mechanisms used floats and valves to regulate water. Medieval clocks used escapements to regulate motion. Steam engines used governors to regulate speed. Relay systems used electrical logic to coordinate machines.
PLCs transformed those centuries-old ideas of sequencing, regulation, feedback, and automatic action into programmable digital control.
The Digital Brain of Industrial Automation
The programmable logic controller represents one of the defining transitions in automation history.
Factories had already learned how to mechanize production. Assembly lines organized flow. Electric motors provided flexible power. Relays allowed machines to execute automatic electrical sequences.
PLCs made those sequences programmable.
Instead of defining every machine decision through physical wiring, engineers could increasingly define behavior through logic stored inside an industrial controller.
That change made automated systems more flexible, scalable, and easier to modify.
Sensors could provide information. Programs could make decisions. Outputs could control physical equipment. Networks could connect machines together.
Industrial automation had gained something remarkably close to a digital brain.
The next transformation would give those controlled machines increasingly flexible mechanical bodies: industrial robots.
References & Further Reading
Morley, Richard E. Out of the Barn. New York: McGraw-Hill, 2002.
Bolton, William. Programmable Logic Controllers. 6th ed. Oxford: Newnes, 2015.
Stenerson, Jon. Industrial Automation and Process Control. Upper Saddle River, NJ: Prentice Hall, 2003.
Parr, E. A. Programmable Controllers: An Engineer's Guide. 3rd ed. Oxford: Newnes, 2003.
Groover, Mikell P. Automation, Production Systems, and Computer-Integrated Manufacturing. 4th ed. Boston: Pearson, 2015.